Life signs could survive near surfaces of Enceladus and Europa

Europa, a moon of Jupiter, and Enceladus, a moon of Saturn, have evidence of oceans beneath their ice crusts. A NASA experiment suggests that if these oceans support life, signatures of that life in the form of organic molecules (e.g. amino acids, nucleic acids, etc.) could survive just under the surface ice despite the harsh radiation on these worlds. If robotic landers are sent to these moons to look for life signs, they would not have to dig very deep to find amino acids that have survived being altered or destroyed by radiation.

"Based on our experiments, the 'safe' sampling depth for amino acids on Europa is almost 8 inches (around 20 centimeters) at high latitudes of the trailing hemisphere (hemisphere opposite to the direction of Europa's motion around Jupiter) in the area where the surface hasn't been disturbed much by meteorite impacts," said Alexander Pavlov of NASA's Goddard Space Flight Center in Greenbelt, Maryland, lead author of a paper on the research published July 18 in Astrobiology. "Subsurface sampling is not required for the detection of amino acids on Enceladus -- these molecules will survive radiolysis (breakdown by radiation) at any location on the Enceladus surface less than a tenth of an inch (under a few millimeters) from the surface."

The frigid surfaces of these nearly airless moons are likely uninhabitable due to radiation from both high-speed particles trapped in their host planet's magnetic fields and powerful events in deep space, such as exploding stars. However, both have oceans under their icy surfaces that are heated by tides from the gravitational pull of the host planet and neighboring moons. These subsurface oceans could harbor life if they have other necessities, such as an energy supply as well as elements and compounds used in biological molecules.

The research team used amino acids in radiolysis experiments as possible representatives of biomolecules on icy moons. Amino acids can be created by life or by non-biological chemistry. However, finding certain kinds of amino acids on Europa or Enceladus would be a potential sign of life because they are used by terrestrial life as a component to build proteins. Proteins are essential to life as they are used to make enzymes which speed up or regulate chemical reactions and to make structures. Amino acids and other compounds from subsurface oceans could be brought to the surface by geyser activity or the slow churning motion of the ice crust.

To evaluate the survival of amino acids on these worlds, the team mixed samples of amino acids with ice chilled to about minus 321 Fahrenheit (-196 Celsius) in sealed, airless vials and bombarded them with gamma-rays, a type of high-energy light, at various doses. Since the oceans might host microscopic life, they also tested the survival of amino acids in dead bacteria in ice. Finally, they tested samples of amino acids in ice mixed with silicate dust to consider the potential mixing of material from meteorites or the interior with surface ice.

The experiments provided pivotal data to determine the rates at which amino acids break down, called radiolysis constants. With these, the team used the age of the ice surface and the radiation environment at Europa and Enceladus to calculate the drilling depth and locations where 10 percent of the amino acids would survive radiolytic destruction.

Although experiments to test the survival of amino acids in ice have been done before, this is the first to use lower radiation doses that don't completely break apart the amino acids, since just altering or degrading them is enough to make it impossible to determine if they are potential signs of life. This is also the first experiment using Europa/Enceladus conditions to evaluate the survival of these compounds in microorganisms and the first to test the survival of amino acids mixed with dust.

The team found that amino acids degraded faster when mixed with dust but slower when coming from microorganisms.

"Slow rates of amino acid destruction in biological samples under Europa and Enceladus-like surface conditions bolster the case for future life-detection measurements by Europa and Enceladus lander missions," said Pavlov. "Our results indicate that the rates of potential organic biomolecules' degradation in silica-rich regions on both Europa and Enceladus are higher than in pure ice and, thus, possible future missions to Europa and Enceladus should be cautious in sampling silica-rich locations on both icy moons."

A potential explanation for why amino acids survived longer in bacteria involves the ways ionizing radiation changes molecules -- directly by breaking their chemical bonds or indirectly by creating reactive compounds nearby which then alter or break down the molecule of interest. It's possible that bacterial cellular material protected amino acids from the reactive compounds produced by the radiation.

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Astronomers uncover methane emission on a cold brown dwarf

Using new observations from the James Webb Space Telescope (JWST), astronomers have discovered methane emission on a brown dwarf, an unexpected finding for such a cold and isolated world. Published in the journal Nature, the findings suggest that this brown dwarf might generate aurorae similar to those seen on our own planet as well as on Jupiter and Saturn.

More massive than planets but lighter than stars, brown dwarfs are ubiquitous in our solar neighborhood, with thousands identified. Last year, Jackie Faherty, a senior research scientist and senior education manager at the American Museum of Natural History, led a team of researchers who were awarded time on JWST to investigate 12 brown dwarfs. Among those was CWISEP J193518.59-154620.3 (or W1935 for short) -- a cold brown dwarf 47 light years away that was co-discovered by Backyard Worlds: Planet 9 citizen science volunteer Dan Caselden and the NASA CatWISE team. W1935 is a cold brown dwarf with a surface temperature of about 400° Fahrenheit, or about the temperature at which you'd bake chocolate chip cookies. The mass for W1935 isn't well known but it likely ranges between 6-35 times the mass of Jupiter.

After looking at a number of brown dwarfs observed with JWST, Faherty's team noticed that W1935 looked similar but with one striking exception: it was emitting methane, something that's never been seen before on a brown dwarf.

"Methane gas is expected in giant planets and brown dwarfs but we usually see it absorbing light, not glowing," said Faherty, the lead author of the study. "We were confused about what we were seeing at first but ultimately that transformed into pure excitement at the discovery."

Computer modeling yielded another surprise: the brown dwarf likely has a temperature inversion, a phenomenon in which the atmosphere gets warmer with increasing altitude. Temperature inversions can easily happen to planets orbiting stars, but W1935 is isolated, with no obvious external heat source.

"We were pleasantly shocked when the model clearly predicted a temperature inversion," said co-author Ben Burningham from the University of Hertfordshire. "But we also had to figure out where that extra upper atmosphere heat was coming from."

To investigate, the researchers turned to our solar system. In particular, they looked at studies of Jupiter and Saturn, which both show methane emission and have temperature inversions. The likely cause for this feature on solar system giants is aurorae, therefore, the research team surmised that they had uncovered that same phenomenon on W1935.

Planetary scientists know that one of the major drivers of aurorae on Jupiter and Saturn are high-energy particles from the Sun that interact with the planets' magnetic fields and atmospheres, heating the upper layers. This is also the reason for the aurorae that we see on Earth, commonly referred to as the Northern or Southern Lights since they are most extraordinary near the poles. But with no host star for W1935, a solar wind cannot contribute to the explanation.

There is an enticing additional reason for the aurora in our solar system. Both Jupiter and Saturn have active moons that occasionally eject material into space, interact with the planets, and enhance the auroral footprint on those worlds. Jupiter's moon Io is the most volcanically active world in the solar system, spewing lava fountains dozens of miles high, and Saturn's moon Enceleadus ejects water vapor from its geysers that simultaneously freezes and boils when it hits space. More observations are needed, but the researchers speculate that one explanation for the aurora on W1935 might be an active, yet-to-be discovered moon.

"Every time an astronomer points JWST at an object, there's a chance of a new mind-blowing discovery," said Faherty. "Methane emission was not on my radar when we started this project but now that we know it can be there and the explanation for it so enticing I am constantly on the look-out for it. That's part of how science moves forward."

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Mimas' surprise: Tiny moon of Saturn holds young ocean beneath icy shell

Hidden beneath the heavily cratered surface of Mimas, one of Saturn's smallest moons, lies a secret: a global ocean of liquid water. This astonishing discovery, led by Dr. Valéry Lainey of the Observatoire de Paris-PSL and published in the journal Nature, reveals a "young" ocean formed just 5 to 15 million years ago, making Mimas a prime target for studying the origins of life in our Solar System.

"Mimas is a small moon, only about 400 kilometers in diameter, and its heavily cratered surface gave no hint of the hidden ocean beneath," says Dr Nick Cooper, a co-author of the study and Honorary Research Fellow in the Astronomy Unit of the School of Physical and Chemical Sciences at Queen Mary University of London.

"This discovery adds Mimas to an exclusive club of moons with internal oceans, including Enceladus and Europa, but with a unique difference: its ocean is remarkably young, estimated to be only 5 to 15 million years old."

This young age, determined through detailed analysis of Mimas's tidal interactions with Saturn, suggests the ocean formed recently, based on the discovery of an unexpected irregularity in its orbit.

As a result, Mimas provides a unique window into the early stages of ocean formation and the potential for life to emerge.

"The existence of a recently formed liquid water ocean makes Mimas a prime candidate for study, for researchers investigating the origin of life," explains Dr Cooper.

The discovery was made possible by analysing data from NASA's Cassini spacecraft, which meticulously studied Saturn and its moons for over a decade.

By closely examining the subtle changes in Mimas's orbit, the researchers were able to infer the presence of a hidden ocean and estimate its size and depth.

Dr Cooper continues: "This has been a great team effort, with colleagues from five different institutions and three different countries coming together under the leadership of Dr Valéry Lainey to unlock another fascinating and unexpected feature of the Saturn system, using data from the Cassini mission."

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NASA's Hubble watches 'spoke season' on Saturn

This photo of Saturn was taken by NASA's Hubble Space Telescope on October 22, 2023, when the ringed planet was approximately 850 million miles from Earth. Hubble's ultra-sharp vision reveals a phenomenon called ring spokes.

Saturn's spokes are transient features that rotate along with the rings.

Their ghostly appearance only persists for two or three rotations around Saturn.

During active periods, freshly-formed spokes continuously add to the pattern.

In 1981, NASA's Voyager 2 first photographed the ring spokes.

NASA's Cassini orbiter also saw the spokes during its 13-year-long mission that ended in 2017.

Hubble continues observing Saturn annually as the spokes come and go. This cycle has been captured by Hubble's Outer Planets Atmospheres Legacy (OPAL) program that began nearly a decade ago to annually monitor weather changes on all four gas-giant outer planets.

Hubble's crisp images show that the frequency of spoke apparitions is seasonally driven, first appearing in OPAL data in 2021 but only on the morning (left) side of the rings.

Long-term monitoring show that both the number and contrast of the spokes vary with Saturn's seasons.

Saturn is tilted on its axis like Earth and has seasons lasting approximately seven years.

"We are heading towards Saturn equinox, when we'd expect maximum spoke activity, with higher frequency and darker spokes appearing over the next few years," said the OPAL program lead scientist, Amy Simon of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

This year, these ephemeral structures appear on both sides of the planet simultaneously as they spin around the giant world.

Although they look small compared with Saturn, their length and width can stretch longer than Earth's diameter!

"The leading theory is that spokes are tied to Saturn's powerful magnetic field, with some sort of solar interaction with the magnetic field that gives you the spokes," said Simon.

When it's near the equinox on Saturn, the planet and its rings are less tilted away from the Sun.

In this configuration, the solar wind may more strongly batter Saturn's immense magnetic field, enhancing spoke formation.

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Can signs of life be detected from Saturn's frigid moon?

As astrophysics technology and research continue to advance, one question persists: is there life elsewhere in the universe? The Milky Way galaxy alone has hundreds of billions of celestial bodies, but scientists often look for three crucial elements in their ongoing search: water, energy and organic material. Evidence indicates that Saturn's icy moon Enceladus is an 'ocean world' that contains all three, making it a prime target in the search for life.

During its 20-year mission, NASA's Cassini spacecraft discovered that ice plumes spew from Enceladus' surface at approximately 800 miles per hour (400 m/s). These plumes provide an excellent opportunity to collect samples and study the composition of Enceladus' oceans and potential habitability.

However, until now it was not known if the speed of the plumes would fragment any organic compounds contained within the ice grains, thus degrading the samples.

Now researchers from the University of California San Diego have shown unambiguous laboratory evidence that amino acids transported in these ice plumes can survive impact speeds of up to 4.2 km/s, supporting their detection during sampling by spacecraft.

Their findings appear in The Proceedings of the National Academy of Sciences (PNAS).

Beginning in 2012, UC San Diego Distinguished Professor of Chemistry and Biochemistry Robert Continetti and his co-workers custom-built a unique aerosol impact spectrometer, designed to study collision dynamics of single aerosols and particles at high velocities.

Although not built specifically to study ice grain impacts, it turned out to be exactly the right machine to do so.

"This apparatus is the only one of its kind in the world that can select single particles and accelerate or decelerate them to chosen final velocities," stated Continetti.

"From several micron diameters down to hundreds of nanometers, in a variety of materials, we're able to examine particle behavior, such as how they scatter or how their structures change upon impact."

In 2024 NASA will launch the Europa Clipper, which will travel to Jupiter.

Europa, one of Jupiter's largest moons, is another ocean world, and has a similar icy composition to Enceladus.

There is hope that the Clipper or any future probes to Saturn will be able to identify a specific series of molecules in the ice grains that could point to whether life exists in the subsurface oceans of these moons, but the molecules need to survive their speedy ejection from the moon and collection by the probe.

Although there has been research into the structure of certain molecules in ice particles, Continetti's team is the first to measure what happens when a single ice grain impacts a surface.

To run the experiment, ice grains were created using electrospray ionization, where water is pushed through a needle held at a high voltage, inducing a charge that breaks the water into increasingly smaller droplets.

The droplets were then injected into a vacuum where they freeze.

The team measured their mass and charge, then used image charge detectors to observe the grains as they flew through the spectrometer.

A key element to the experiment was installing a microchannel plate ion detector to accurately time the moment of impact down to the nanosecond.

The results showed that amino acids -- often called the building blocks of life -- can be detected with limited fragmentation up to impact velocities of 4.2 km/s.

"To get an idea of what kind of life may be possible in the solar system, you want to know there hasn't been a lot of molecular fragmentation in the sampled ice grains, so you can get that fingerprint of whatever it is that makes it a self-contained life form," said Continetti.

"Our work shows that this is possible with the ice plumes of Enceladus."

Continetti's research also raises interesting questions for chemistry itself, including how salt affects the detectability of certain amino acids.

It is believed that Enceladus contains vast salty oceans -- more than is present on Earth.

Because salt changes the properties of water as a solvent as well as the solubility of different molecules, this could mean that some molecules cluster on the surface of the ice grains, making them more likely to be detected.

"The implications this has for detecting life elsewhere in the solar system without missions to the surface of these ocean-world moons is very exciting, but our work goes beyond biosignatures in ice grains," stated Continetti.

"It has implications for fundamental chemistry as well. We are excited by the prospect of following in the footsteps of Harold Urey and Stanley Miller, founding faculty at UC San Diego in looking at the formation of the building blocks of life from chemical reactions activated by ice grain impact."

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Studies of geologic faulting on icy moons aid exploration of extraterrestrial watery worlds

On the surface of many of the icy moons in our solar system, scientists have documented strike-slip faults, those that occur when fault walls move past one another sideways, as is the case at the San Andreas fault in California. Two recently published studies led by University of Hawai'i at Manoa earth and space scientists document and reveal the mechanisms behind these geologic features on the largest moon of Saturn, Titan, and Jupiter's largest moon, Ganymede.

"We are interested in studying shear deformation on icy moons because that type of faulting can facilitate the exchange of surface and subsurface materials through shear heating processes, potentially creating environments conducive for the emergence of life," said Liliane Burkhard, lead author of the studies and research affiliate at the Hawai'i Institute of Geophysics and Planetology in the UH Manoa School of Ocean and Earth Science and Technology.

When an icy moon moves around its parent planet, the gravity of the planet can cause tidal flexing of the surface of the moon, which can drive geologic activity such as strike-slip faulting. Tidal stresses vary as the moon changes distance from its planet because the moon's orbit can be elliptical rather than circular.

Titan, a frozen ocean world

The extremely cold temperatures on the surface of Titan mean that water ice acts as rock that can crack, fault, and deform. Evidence from the Cassini spacecraft suggests that tens of miles below the frozen surface, there is a liquid water ocean. Further, Titan is the only moon in our solar system with a dense atmosphere, which, uniquely, supports an Earth-like hydrological cycle of methane clouds, rain, and liquid flowing across the surface to fill lakes and seas, placing it among a handful of worlds that could potentially contain habitable environments.

The NASA Dragonfly mission will launch in 2027, with a planned arrival on Titan in 2034. The novel rotorcraft lander will conduct several flights on the surface, exploring a variety of locations to search for the building blocks and signs of life.

In their investigation of the Selk crater area on Titan, the designated initial landing site for the Dragonfly mission, Burkhard and her co-author explored the potential for shear deformations and strike-slip faulting. To do this, they calculated the stress that would be exerted on Titan's surface due to tidal forces as the moon orbits Saturn and tested the possibility of faulting by examining various characteristics of the frozen ground.

"While our prior research indicated that certain areas on Titan might currently undergo deformation due to tidal stresses, the Selk crater area would need to host very high pore fluid pressures and a low crustal coefficient of friction for shear failure, which seems improbable," said Burkhard. "Consequently, it's safe to infer that Dragonfly won't be landing in a strike-slip ditch!"

Ganymede, a moon with a checkered past

In a second publication, Burkhard and her co-authors investigated the geologic history of Ganymede, Jupiter's largest moon, in the area of Nippur/Philus Sulci by examining high-resolution data available for this region and conducting a tidal stress investigation of Ganymede's past.

Ganymede has documented strike-slip faults on the surface, but its current orbit is too circular, as opposed to elliptical, to cause any tidal stress deformation.

The researchers found that several crosscutting bands of light terrain in the Nippur/Philus Sulci site show varying degrees of tectonic deformation, and the chronology of tectonic activity implied by mapped crosscutting relationships revealed three eras of distinct geologic activity: ancient, intermediate and youngest.

"I investigated strike-slip faulting features in intermediate-aged terrain, and they correspond in slip direction to the predictions from modeling stresses of a higher past eccentricity. Ganymede could have undergone a period where its orbit was much more elliptical than it is today," said Burkhard.

Other shear features found in younger geologic units in the same region do not align in slip direction with typical first-order shear indicators.

"This suggests that these features might have formed through another process and not necessarily due to higher tidal stresses," Burkhard added. "So, Ganymede has had a tidal 'mid-life crisis', but its youngest 'crisis' remains enigmatic."

The recent studies along with space exploration missions create a positive feedback of knowledge.

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Hundred-year storms? That's how long they last on Saturn

The largest storm in the solar system, a 10,000-mile-wide anticyclone called the Great Red Spot, has decorated Jupiter's surface for hundreds of years.

A new study now shows that Saturn -- though much blander and less colorful than Jupiter -- also has long-lasting megastorms with impacts deep in the atmosphere that persist for centuries.

The study was conducted by astronomers from the University of California, Berkeley, and the University of Michigan, Ann Arbor, who looked at radio emissions from the planet, which come from below the surface, and found long-term disruptions in the distribution of ammonia gas.

The study was published today (Aug. 11) in the journal Science Advances.

Megastorms occur approximately every 20 to 30 years on Saturn and are similar to hurricanes on Earth, although significantly larger. But unlike Earth's hurricanes, no one knows what causes megastorms in Saturn's atmosphere, which is composed mainly of hydrogen and helium with traces of methane, water and ammonia.

"Understanding the mechanisms of the largest storms in the solar system puts the theory of hurricanes into a broader cosmic context, challenging our current knowledge and pushing the boundaries of terrestrial meteorology," said lead author Cheng Li, a former 51 Peg b Fellow at UC Berkeley who is now an assistant professor at the University of Michigan.

Imke de Pater, a UC Berkeley professor emerita of astronomy and of earth and planetary sciences, has been studying gas giants for over four decades to better understand their composition and what makes them unique, employing the Karl G. Jansky Very Large Array in New Mexico to probe the radio emissions from deep inside the planet.

"At radio wavelengths, we probe below the visible cloud layers on giant planets. Since chemical reactions and dynamics will alter the composition of a planet's atmosphere, observations below these cloud layers are required to constrain the planet's true atmospheric composition, a key parameter for planet formation models," she said. "Radio observations help characterize dynamical, physical and chemical processes including heat transport, cloud formation and convection in the atmospheres of giant planets on both global and local scales."

As reported in the new study, de Pater, Li and UC Berkeley graduate student Chris Moeckel found something surprising in the radio emissions from the planet: anomalies in the concentration of ammonia gas in the atmosphere, which they connected to the past occurrences of megastorms in the planet's northern hemisphere.

According to the team, the concentration of ammonia is lower at midaltitudes, just below the uppermost ammonia-ice cloud layer, but has become enriched at lower altitudes, 100 to 200 kilometers deeper in the atmosphere. They believe that the ammonia is being transported from the upper to the lower atmosphere via the processes of precipitation and reevaporation. What's more, that effect can last for hundreds of years.

The study further revealed that although both Saturn and Jupiter are made of hydrogen gas, the two gas giants are remarkably dissimilar. While Jupiter does have tropospheric anomalies, they have been tied to its zones (whitish bands) and belts (darkish bands) and are not caused by storms like they are on Saturn. The considerable difference between these neighboring gas giants is challenging what scientists know about the formation of megastorms on gas giants and other planets and may inform how they're found and studied on exoplanets in the future.

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New image from James Webb Space Telescope reveals astonishing Saturn and its rings

Saturn's iconic rings seem to glow eerily in this incredible infrared picture, which also unveils unexpected features in Saturn's atmosphere. See link to image below.

This image serves as context for an observing program that will test the telescope's capacity to detect faint moons around the planet and its bright rings. Any newly discovered moons could help scientists put together a more complete picture of the current system of Saturn, as well as its past.

Methane gas absorbs almost all the sunlight falling on the atmosphere at this picture's specific infrared wavelength (3.23 microns). As a result, Saturn's familiar striped patterns aren't visible because the methane-rich upper atmosphere blocks our view of the primary clouds. Instead, Saturn's disk appears dark, and we see features associated with high-altitude stratospheric aerosols, including large, dark, and diffuse structures in Saturn's northern hemisphere that don't align with the planet's lines of latitude. Interestingly, researchers previously spotted similar wave-like in early JWST NIRCam observations of Jupiter.

Unlike the atmosphere, Saturn's rings lack methane, so at this infrared wavelength, they are no darker than usual and thus easily outshine the darkened planet. This new image of Saturn also reveals intricate details within the ring system, showcasing several of the planet's moons like Dione, Enceladus, and Tethys.

"We are very pleased to see JWST produce this beautiful image, which is confirmation that our deeper scientific data also turned out well," said Dr. Matthew Tiscareno, a senior research scientist at the SETI Institute who led the process of designing this observation. "We look forward to digging into the deep exposures to see what discoveries may await."

Over the past few decades, missions like NASA's Pioneer 11, Voyagers 1 and 2, the Cassini spacecraft and the Hubble Space Telescope have observed Saturn's atmosphere and rings. The image captured by JWST is just a taste of what this observatory will uncover about Saturn in the coming years as scientists. This image is part of a suite of deeply exposed images where researchers hope to identify new ring structures and perhaps even new moons of Saturn.

Moving from the inner to the outer features of Saturn's rings, we can observe the dark C ring, the bright B ring, the narrow and dark Cassini Division, and the medium-bright A ring with the dark Encke Gap near its outer edge. Additionally, off the outer edge of the A ring, we can see the narrow strand known as the F ring. The rings cast a shadow on the planet and vice versa, creating intriguing visual effects.

In-depth exposures not shown in this image will allow scientists to investigate Saturn's fainter rings, including the thin G ring and diffuse E ring, which are not visible here. Saturn's rings consist of an assortment of rocky and icy fragments, ranging in size from smaller than a grain of sand to as large as mountains on Earth. Recently, researchers used JWST to explore Enceladus and discovered a substantial plume emanating from the moon's southern pole. This plume contains particles and copious amounts of water vapor, contributing to Saturn's E ring.

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New study puts a definitive age on Saturn's rings -- they're really young

A new study led by physicist Sascha Kempf at the University of Colorado Boulder has delivered the strongest evidence yet that Saturn's rings are remarkably young -- potentially answering a question that has boggled scientists for well over a century.

The research, to be published May 12 in the journal Science Advances, pegs the age of Saturn's rings at no more than 400 million years old. That makes the rings much younger than Saturn itself, which is about 4.5 billion years old.

"In a way, we've gotten closure on a question that started with James Clerk Maxwell," said Kempf, associate professor in the Laboratory for Atmospheric and Space Physics (LASP) at CU Boulder.

The researchers arrived at that closure by studying what might seem like an unusual subject: dust.

Kempf explained that tiny grains of rocky material wash through Earth's solar system on an almost constant basis. In some cases, this flux can leave behind a thin layer of dust on planetary bodies, including on the ice that makes up Saturn's rings.

In the new study, he and his colleagues set out to put a date on Saturn's rings by studying how rapidly this layer of dust builds up -- a bit like telling how old a house is by running your finger along its surfaces.

"Think about the rings like the carpet in your house," Kempf said. "If you have a clean carpet laid out, you just have to wait. Dust will settle on your carpet. The same is true for the rings."

It was an arduous process: From 2004 to 2017, the team used an instrument called the Cosmic Dust Analyzer aboard NASA's late Cassini spacecraft to analyze specks of dust flying around Saturn. Over those 13 years, the researchers collected just 163 grains that had originated from beyond the planet's close neighborhood. But it was enough. Based on their calculations, Saturn's rings have likely been gathering dust for only a few hundred million years.

The planet's rings, in other words, are new phenomena, arising (and potentially even disappearing) in what amounts to a blink of an eye in cosmic terms.

"We know approximately how old the rings are, but it doesn't solve any of our other problems," Kempf said. "We still don't know how these rings formed in the first place."

From Galileo to Cassini

Researchers have been captivated by these seemingly-translucent rings for more than 400 years. In 1610, Italian astronomer Galileo Galilei first observed the features through a telescope, although he didn't know what they were. (Galileo's original drawings make the rings look a bit like the handles on a water jug). In the 1800s, Maxwell, a scientist from Scotland, concluded that Saturn's rings couldn't be solid but were, instead, made up of many individual pieces.

Today, scientists know that Saturn hosts seven rings comprised of countless chunks of ice, most no bigger than a boulder on Earth. Altogether, this ice weighs about half as much as Saturn's moon Mimas and stretches nearly 175,000 miles from the planet's surface.

Kempf added that for most of the 20th Century, scientists assumed that the rings likely formed at the same time as Saturn.

But that idea raised a few issues -- namely, Saturn's rings are sparkling clean. Observations suggest that these features are made up of roughly 98% pure water ice by volume, with only a tiny amount of rocky matter.

"It's almost impossible to end up with something so clean," Kempf said.

Cassini offered an opportunity to put a definitive age on Saturn's rings. The spacecraft first arrived at Saturn in 2004 and collected data until it purposefully crashed into the planet's atmosphere in 2017. The Cosmic Dust Analyzer, which was shaped a bit like a bucket, scooped up small particles as they whizzed by.

Engineers and scientists at LASP designed and built a much more sophisticated dust analyzer for NASA's upcoming Europa Clipper mission, which is scheduled to launch in 2024.

The team estimated that this interplanetary grime would contribute far less than a gram of dust to each square foot of Saturn's rings every year -- a light sprinkle, but enough to add up over time. Previous studies had also suggested that the rings could be young but didn't include definitive measures of dust accumulation.

Stroke of luck

The rings might already be vanishing. In a previous study, NASA scientists reported that the ice is slowly raining down onto the planet and could disappear entirely in another 100 million years.

That these ephemeral features existed at a time when Galileo and the Cassini spacecraft could observe them seems almost too good to be true, Kempf said -- and it begs an explanation for how the rings formed in the first place. Some scientists, for example, have posited that Saturn's rings may have formed when the planet's gravity tore apart one of its moons.

"If the rings are short lived and dynamical, why are we seeing them now?" he said. "It's too much luck."

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Hubble captures the start of a new spoke season at Saturn

New images of Saturn from NASA's Hubble Space Telescope herald the start of the planet's "spoke season" surrounding its equinox, when enigmatic features appear across its rings. The cause of the spokes, as well as their seasonal variability, has yet to be fully explained by planetary scientists.

Like Earth, Saturn is tilted on its axis and therefore has four seasons, though because of Saturn's much larger orbit, each season lasts approximately seven Earth years. Equinox occurs when the rings are tilted edge-on to the Sun. The spokes disappear when it is near summer or winter solstice on Saturn. (When the Sun appears to reach either its highest or lowest latitude in the northern or southern hemisphere of a planet.) As the autumnal equinox of Saturn's northern hemisphere on May 6, 2025, draws near, the spokes are expected to become increasingly prominent and observable.

The suspected culprit for the spokes is the planet's variable magnetic field. Planetary magnetic fields interact with the solar wind, creating an electrically charged environment (on Earth, when those charged particles hit the atmosphere this is visible in the northern hemisphere as the aurora borealis, or northern lights). Scientists think that the smallest, dust-sized icy ring particles can become charged as well, which temporarily levitates those particles above the rest of the larger icy particles and boulders in the rings.

The ring spokes were first observed by NASA's Voyager mission in the early 1980s. The transient, mysterious features can appear dark or light depending on the illumination and viewing angles.

"Thanks to Hubble's OPAL program, which is building an archive of data on the outer solar system planets, we will have longer dedicated time to study Saturn's spokes this season than ever before," said NASA senior planetary scientist Amy Simon, head of the Hubble Outer Planet Atmospheres Legacy (OPAL) program.

Saturn's last equinox occurred in 2009, while NASA's Cassini spacecraft was orbiting the gas giant planet for close-up reconnaissance. With Cassini's mission completed in 2017, and the Voyager spacecrafts long gone, Hubble is continuing the work of long-term monitoring of changes on Saturn and the other outer planets.

"Despite years of excellent observations by the Cassini mission, the precise beginning and duration of the spoke season is still unpredictable, rather like predicting the first storm during hurricane season," Simon said.

While our solar system's other three gas giant planets also have ring systems, nothing compares to Saturn's prominent rings, making them a laboratory for studying spoke phenomena. Whether spokes could or do occur at other ringed planets is currently unknown. "It's a fascinating magic trick of nature we only see on Saturn -- for now at least," Simon said.

Hubble's OPAL program will add both visual and spectroscopic data, in wavelengths of light from ultraviolet to near-infrared, to the archive of Cassini observations. Scientists are anticipating putting these pieces together to get a more complete picture of the spoke phenomenon, and what it reveals about ring physics in general.

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Researchers complete first real-world study of Martian helicopter dust dynamics

Mars is a dusty planet. From tiny dust devils to vast storms that shroud the planet, dust is a constant challenge for research missions. That was especially true for Ingenuity, the rotorcraft that since February 2021 has been exploring Mars alongside NASA's Perseverance rover. Now, researchers at Stevens Institute of Technology, the Space Science Institute, and the Jet Propulsion Laboratory have completed the first real-world study of Martian dust dynamics based on Ingenuity's historic first flights on the Red Planet, paving the way for future extraterrestrial rotorcraft missions.

The work, reported in the December 2022 issue of Journal of Geophysical Research: Planets, could support NASA's Mars Sample Return Program, which will retrieve samples collected by Perseverance, or the Dragonfly mission that will set course for Titan, Saturn's largest moon, in 2027.

"There's a reason that helicopter pilots on Earth prefer to land on helipads," said Jason Rabinovitch, a co-author and assistant professor at Stevens. "When a helicopter lands in the desert, its downdraft can stir up enough dust to cause a zero-visibility 'brownout' -- and Mars is effectively one big desert."

Rabinovitch has been working on the Ingenuity program since 2014, joining the Jet Propulsion Laboratory soon after the concept was first pitched to NASA and creating the first theoretical models of helicopter dust lifting in the dusty Martian environments. At Stevens, Rabinovitch continues to work with JPL and investigates plume-surface interactions during powered descent of a spacecraft. He also models supersonic parachute inflation and geophysical phenomena, such as plumes on Enceladus.

Studying dust dynamics on another planet isn't easy, explained Rabinovitch. "Space is a data-poor environment. It's hard to send videos and images back to Earth, so we have to work with what we can get."

To overcome that challenge, Rabinovitch and colleagues at JPL used advanced image-processing techniques to extract information from six helicopter flights, all low-resolution videos captured by Perseverance. By identifying tiny variations between video frames, and the light intensity of individual pixels, the researchers were able to calculate both the size and the total mass of dust clouds kicked up as Ingenuity took off, hovered, maneuvered, and landed.

The results were within striking distance of Rabinovitch's engineering models -- itself a remarkable achievement, given the limited information available to the team way back in 2014, when Rabinovitch and his colleagues were writing back-of-the-envelope calculations intended to support the original design of Ingenuity.

The research shows that, as predicted, dust is a significant consideration for extra-terrestrial rotorcraft, with Ingenuity estimated to have kicked up about a thousandth of its own mass (four pounds) in dust each time it flew. That's many times more dust than would be generated by an equivalent helicopter on Earth, though Rabinovitch cautions that it's tricky to draw direct comparisons.

"It was exciting to see the Mastcam-Z video from Perseverance, which was taken for engineering reasons, ended up showing Ingenuity lifting so much dust from the surface that it opened a new line of research," said Mark Lemmon, senior research scientist at the Space Science Institute Mars Science Laboratory and first author of the study.

"When you think about dust on Mars, you have to consider not just the lower gravity, but also the effects of air pressure, temperature, air density -- there's a lot we don't yet fully understand," Rabinovich said. Still, he added, that's what makes studying Ingenuity's dust clouds so exciting.

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Evidence that Saturn's moon Mimas is a stealth ocean world

When a Southwest Research Institute scientist discovered surprising evidence that Saturn's smallest, innermost moon could generate the right amount of heat to support a liquid internal ocean, colleagues began studying Mimas' surface to understand how its interior may have evolved. Numerical simulations of the moon's Herschel impact basin, the most striking feature on its heavily cratered surface, determined that the basin's structure and the lack of tectonics on Mimas are compatible with a thinning ice shell and geologically young ocean.

"In the waning days of NASA's Cassini mission to Saturn, the spacecraft identified a curious libration, or oscillation, in Mimas' rotation, which often points to a geologically active body able to support an internal ocean," said SwRI's Dr. Alyssa Rhoden, a specialist in the geophysics of icy satellites, particularly those containing oceans, and the evolution of giant planet satellite systems. She is the second author of a new Geophysical Research Letters paper on the subject. "Mimas seemed like an unlikely candidate, with its icy, heavily cratered surface marked by one giant impact crater that makes the small moon look much like the Death Star from Star Wars. If Mimas has an ocean, it represents a new class of small, 'stealth' ocean worlds with surfaces that do not betray the ocean's existence."

Rhoden worked with Purdue graduate student Adeene Denton to better understand how a heavily cratered moon like Mimas could possess an internal ocean. Denton modeled the formation of the Hershel impact basin using iSALE-2D simulation software. The models showed that Mimas' ice shell had to be at least 34 miles (55 km) thick at the time of the Herschel-forming impact. In contrast, observations of Mimas and models of its internal heating limit the present-day ice shell thickness to less than 19 miles (30 km) thick, if it currently harbors an ocean. These results imply that a present-day ocean within Mimas must have been warming and expanding since the basin formed. It is also possible that Mimas was entirely frozen both at the time of the Herschel impact and at present. However, Denton found that including an interior ocean in impact models helped produce the shape of the basin.

"We found that Herschel could not have formed in an ice shell at the present-day thickness without obliterating the ice shell at the impact site," said Denton, who is now a post-doctoral researcher at the University of Arizona. "If Mimas has an ocean today, the ice shell has been thinning since the formation of Herschel, which could also explain the lack of fractures on Mimas. If Mimas is an emerging ocean world, that places important constraints on the formation, evolution and habitability of all of the mid-sized moons of Saturn."

"Although our results support a present-day ocean within Mimas, it is challenging to reconcile the moon's orbital and geologic characteristics with our current understanding of its thermal-orbital evolution," Rhoden said. "Evaluating Mimas' status as an ocean moon would benchmark models of its formation and evolution. This would help us better understand Saturn's rings and mid-sized moons as well as the prevalence of potentially habitable ocean moons, particularly at Uranus. Mimas is a compelling target for continued investigation."

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Saturn's rings and tilt could be the product of an ancient, missing moon

Swirling around the planet's equator, the rings of Saturn are a dead giveaway that the planet is spinning at a tilt. The belted giant rotates at a 26.7-degree angle relative to the plane in which it orbits the sun. Astronomers have long suspected that this tilt comes from gravitational interactions with its neighbor Neptune, as Saturn's tilt precesses, like a spinning top, at nearly the same rate as the orbit of Neptune.

But a new modeling study by astronomers at MIT and elsewhere has found that, while the two planets may have once been in sync, Saturn has since escaped Neptune's pull. What was responsible for this planetary realignment? The team has one meticulously tested hypothesis: a missing moon.

In a study appearing in Science, the team proposes that Saturn, which today hosts 83 moons, once harbored at least one more, an extra satellite that they name Chrysalis. Together with its siblings, the researchers suggest, Chrysalis orbited Saturn for several billion years, pulling and tugging on the planet in a way that kept its tilt, or "obliquity," in resonance with Neptune.

But around 160 million years ago, the team estimates, Chrysalis became unstable and came too close to its planet in a grazing encounter that pulled the satellite apart. The loss of the moon was enough to remove Saturn from Neptune's grasp and leave it with the present-day tilt.

What's more, the researchers surmise, while most of Chrysalis' shattered body may have made impact with Saturn, a fraction of its fragments could have remained suspended in orbit, eventually breaking into small icy chunks to form the planet's signature rings.

The missing satellite, therefore, could explain two longstanding mysteries: Saturn's present-day tilt and the age of its rings, which were previously estimated to be about 100 million years old -- much younger than the planet itself.

"Just like a butterfly's chrysalis, this satellite was long dormant and suddenly became active, and the rings emerged," says Jack Wisdom, professor of planetary sciences at MIT and lead author of the new study.

The study's co-authors include Rola Dbouk at MIT, Burkhard Militzer of the University of California at Berkeley, William Hubbard at the University of Arizona, Francis Nimmo and Brynna Downey of the University of California at Santa Cruz, and Richard French of Wellesley College.

A moment of progress

In the early 2000s, scientists put forward the idea that Saturn's tilted axis is a result of the planet being trapped in a resonance, or gravitational association, with Neptune. But observations taken by NASA's Cassini spacecraft, which orbited Saturn from 2004 to 2017, put a new twist on the problem. Scientists found that Titan, Saturn's largest satellite, was migrating away from Saturn at a faster clip than expected, at a rate of about 11 centimeters per year. Titan's fast migration, and its gravitational pull, led scientists to conclude that the moon was likely responsible for tilting and keeping Saturn in resonance with Neptune.

But this explanation hinges on one major unknown: Saturn's moment of inertia, which is how mass is distributed in the planet's interior. Saturn's tilt could behave differently, depending on whether matter is more concentrated at its core or toward the surface.

"To make progress on the problem, we had to determine the moment of inertia of Saturn," Wisdom says.

The lost element

In their new study, Wisdom and his colleagues looked to pin down Saturn's moment of inertia using some of the last observations taken by Cassini in its "Grand Finale," a phase of the mission during which the spacecraft made an extremely close approach to precisely map the gravitational field around the entire planet. The gravitational field can be used to determine the distribution of mass in the planet.

Wisdom and his colleagues modeled the interior of Saturn and identified a distribution of mass that matched the gravitational field that Cassini observed. Surprisingly, they found that this newly identified moment of inertia placed Saturn close to, but just outside the resonance with Neptune. The planets may have once been in sync, but are no longer.

"Then we went hunting for ways of getting Saturn out of Neptune's resonance," Wisdom says.

The team first carried out simulations to evolve the orbital dynamics of Saturn and its moons backward in time, to see whether any natural instabilities among the existing satellites could have influenced the planet's tilt. This search came up empty.

So, the researchers reexamined the mathematical equations that describe a planet's precession, which is how a planet's axis of rotation changes over time. One term in this equation has contributions from all the satellites. The team reasoned that if one satellite were removed from this sum, it could affect the planet's precession.

The question was, how massive would that satellite have to be, and what dynamics would it have to undergo to take Saturn out of Neptune's resonance?

Wisdom and his colleagues ran simulations to determine the properties of a satellite, such as its mass and orbital radius, and the orbital dynamics that would be required to knock Saturn out of the resonance.

They conclude that Saturn's present tilt is the result of the resonance with Neptune and that the loss of the satellite, Chrysalis, which was about the size of Iapetus, Saturn's third-largest moon, allowed it to escape the resonance.

Sometime between 200 and 100 million years ago, Chrysalis entered a chaotic orbital zone, experienced a number of close encounters with Iapetus and Titan, and eventually came too close to Saturn, in a grazing encounter that ripped the satellite to bits, leaving a small fraction to circle the planet as a debris-strewn ring.

The loss of Chrysalis, they found, explains Saturn's precession, and its present-day tilt, as well as the late formation of its rings.

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Why Jupiter doesn't have rings like Saturn

Because it's bigger, Jupiter ought to have larger, more spectacular rings than Saturn has. But new UC Riverside research shows Jupiter's massive moons prevent that vision from lighting up the night sky.

"It's long bothered me why Jupiter doesn't have even more amazing rings that would put Saturn's to shame," said UCR astrophysicist Stephen Kane, who led the research.

"If Jupiter did have them, they'd appear even brighter to us, because the planet is so much closer than Saturn." Kane also had questions about whether Jupiter once had fantastic rings and lost them. It is possible for ring structures to be temporary.

To understand the reason Jupiter currently looks the way it does, Kane and his graduate student Zhexing Li ran a dynamic computer simulation accounting for the orbits of Jupiter's four main moons, as well as the orbit of the planet itself, and information about the time it takes for rings to form. Their results are now online, soon to be published in the Planetary Science journal.

Saturn's rings are largely made of ice, some of which may have come from comets, which are also largely made of ice. If moons are massive enough, their gravity can toss the ice out of a planet's orbit, or change the orbit of the ice enough so that it collides with the moons.

"We found that the Galilean moons of Jupiter, one of which is the largest moon in our solar system, would very quickly destroy any large rings that might form," Kane said. As a result, it is unlikely that Jupiter had large rings at any point in its past.

"Massive planets form massive moons, which prevents them from having substantial rings," Kane said.

All four giant planets in our solar system -- Saturn, Neptune, Uranus and also Jupiter -- do in fact have rings. However, both Neptune and Jupiter's rings are so flimsy they're difficult to view with traditional stargazing instruments.

Coincidentally, some of the recent images from the newly commissioned James Webb Space Telescope included pictures of Jupiter, in which the faint rings are visible.

"We didn't know these ephemeral rings existed until the Voyager spacecraft went past because we couldn't see them," Kane said.

Uranus has rings that are aren't as large but are more substantial than Saturn's. Going forward, Kane intends to run simulations of the conditions on Uranus to see what the lifetime of that planet's rings might be.

Some astronomers believe Uranus is tipped over on its side as the result of a collision the planet had with another celestial body. Its rings could be the remains of that impact.

Beyond their beauty, rings help astronomers understand the history of a planet, because they offer evidence of collisions with moons or comets that may have happened in the past. The shape and size of the rings, as well as the composition of the material, offers an indication about the type of event that formed them.

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How elliptical craters could shed light on age of Saturn's moons

A new SwRI study describes how unique populations of craters on two of Saturn's moons could help indicate the satellites' age and the conditions of their formation. Using data from NASA's Cassini mission, SwRI postdoctoral researcher Dr. Sierra Ferguson surveyed elliptical craters on Saturn's moons Tethys and Dione for this study, which was co-authored by SwRI Principal Scientist Dr. Alyssa Rhoden, Lead Scientist Dr. Michelle Kirchoff and Lead Analyst Dr. Julien Salmon.

"Our work aims to answer the broader question of how old these moons are. To get at this question, my colleagues and I mapped elliptical craters on the surfaces of these moons to determine their size, direction and location on the moon," Ferguson said.

Circular craters are very common and can be formed from a wide range of impact conditions. However, elliptical craters are rarer and form from slow and shallow impacts, which make them especially useful in determining an object's age because shape and orientation also indicate their impactor's trajectory.

"By measuring the direction these craters point, we can get an idea of what the impactors that made these craters looked like in a dynamical sense and from which direction they might have hit the surface," she said.

At first, Ferguson was not expecting to find a pattern among the directions of the elliptical craters, but she eventually noticed a trend along the equator of Dione, one of Saturn's small moons. There, elliptical craters were overwhelmingly oriented in an east/west pattern, while the directions were more random close to the moon's poles.

"We initially interpreted this pattern to be representative of two distinct impactor populations creating these craters," she said. "One group was responsible for creating the elliptical craters at the equator, while another, less concentrated population may be more representative of the regular background population of impactors around Saturn."

Ferguson also mapped elliptical craters on Tethys, Saturn's fifth largest moon, and found that a similar size-frequency distribution of craters is unusual for objects orbiting the Sun, but curiously matches estimates for the impactor population that appears to be present on Neptune's moon, Triton. Because that population is thought to be planetocentric, or drawn in by the ice giant's massive gravity, Ferguson's results point to the importance of considering planetocentric impactors when examining the age of objects in the Saturnian system.

"It was really astonishing to see these patterns," she said.

Ferguson believes the equatorial craters could have formed from independent disks of debris orbiting each moon or potentially a single disk that affected both moons.

"Using Triton as a guide, Tethys could reasonably be billions of years old. This age estimate is dependent on how much material was available for impacting the surface and when it was available" Ferguson said. "To be certain, of course, we will need more data, but this research tells us a lot. It can give us an idea of what the formation conditions of these moons were like. Was this a system that was completely chaotic with materials hitting these satellites every which way, or was there a neat and orderly system?"

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Experiments measure freezing point of extraterrestrial oceans to aid search for life

Researchers from the University of Washington and the University of California, Berkeley have conducted experiments that measured the physical limits for the existence of liquid water in icy extraterrestrial worlds. This blend of geoscience and engineering was done to aid in the search for extraterrestrial life and the upcoming robotic exploration of oceans on moons of other planets.

The results were recently published in Cell Reports Physical Sciences.

"The more a liquid is stable, the more promising it is for habitability," said co-corresponding author Baptiste Journaux, an acting assistant professor of Earth and space sciences at the UW. "Our results show that the cold, salty, high-pressure liquids found in the deep ocean of other planets' moons can remain liquid to much cooler temperature than they would at lower pressures. This extends the range of possible habitats on icy moons, and will allow us to pinpoint where we should look for biosignatures, or signs of life."

Jupiter and Saturn's icy moons -- including Europa, Ganymede and Titan -- are leading candidates within our solar system for hosting extraterrestrial life. These ice-encrusted moons are thought to harbor enormous liquid oceans, up to several dozen times the volume of oceans on Earth.

"Despite its designation as the 'blue marble,' Earth is remarkably dry when compared to these worlds," Journaux said.

The oceans on these moons may contain various types of salts and are expected to range from about 100 miles deep, on Europa, to more than 400 miles deep, on Titan.

"We know that water supports life, but the major part of the oceans on these moons are likely below zero degrees Celsius and at pressures higher than anything experienced on Earth," Journaux said. "We needed to know how cold an ocean can get before entirely freezing, including in its deepest abyss."

The study focused on eutectics, or the lowest temperature that a salty solution can remain liquid before entirely freezing. Salt and water are one example -- salty water remains liquid below the freezing temperature of pure water, one of the reasons people sprinkle salt on roads in winter to avoid the formation of ice.

The experiments used UC Berkeley equipment originally designed for the future cryopreservation of organs for medical applications and for food storage. For this research, however, the authors used it to simulate the conditions thought to exist on other planets' moons.

Journaux, a planetary scientist and expert on the physics of water and minerals, worked with UC Berkeley engineers to test solutions of five different salts at pressures up to 3,000 times atmospheric pressure, or 300 megapascals -- about three times the pressure in Earth's deepest ocean trench.

"Knowing the lowest temperature possible for salty water to remain a liquid at high pressures is integral to understanding how extraterrestrial life could exist and thrive in the deep oceans of these icy ocean worlds," said co-corresponding author Matthew Powell-Palm, who did the work as a postdoctoral researcher at UC Berkeley, also co-founder and CEO of the cryopreservation company BioChoric, Inc.

Journaux recently started working with NASA's Dragonfly mission team, which will send a rotorcraft in 2027 to Saturn's largest moon, Titan. NASA also is leading the Europa Clipper mission in 2024 to explore Europa, one of the many moons orbiting Jupiter. Meanwhile, the European Space Agency in 2023 will send its JUICE spacecraft, or Jupiter Icy Moons Explorer, to explore three of Jupiter's largest moons: Ganymede, Callisto and Europa.

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Scientists model landscape formation on Titan, revealing an Earth-like alien world

Saturn's moon Titan looks very much like Earth from space, with rivers, lakes, and seas filled by rain tumbling through a thick atmosphere. While these landscapes may look familiar, they are composed of materials that are undoubtedly different -- liquid methane streams streak Titan's icy surface and nitrogen winds build hydrocarbon sand dunes.

The presence of these materials -- whose mechanical properties are vastly different from those of silicate-based substances that make up other known sedimentary bodies in our solar system -- makes Titan's landscape formation enigmatic. By identifying a process that would allow for hydrocarbon-based substances to form sand grains or bedrock depending on how often winds blow and streams flow, Stanford University geologist Mathieu Lapôtre and his colleagues have shown how Titan's distinct dunes, plains, and labyrinth terrains could be formed.

Titan, which is a target for space exploration because of its potential habitability, is the only other body in our solar system known to have an Earth-like, seasonal liquid transport cycle today. The new model, published in Geophysical Research Letters April 25, shows how that seasonal cycle drives the movement of grains over the moon's surface.

"Our model adds a unifying framework that allows us to understand how all of these sedimentary environments work together," said Lapôtre, an assistant professor of geological sciences at Stanford's School of Earth, Energy & Environmental Sciences (Stanford Earth). "If we understand how the different pieces of the puzzle fit together and their mechanics, then we can start using the landforms left behind by those sedimentary processes to say something about the climate or the geological history of Titan -- and how they could impact the prospect for life on Titan."

A missing mechanism

In order to build a model that could simulate the formation of Titan's distinct landscapes, Lapôtre and his colleagues first had to solve one of the biggest mysteries about sediment on the planetary body: How can its basic organic compounds -- which are thought to be much more fragile than inorganic silicate grains on Earth -- transform into grains that form distinct structures rather than just wearing down and blowing away as dust?

On Earth, silicate rocks and minerals on the surface erode into sediment grains over time, moving through winds and streams to be deposited in layers of sediments that eventually -- with the help of pressure, groundwater, and sometimes heat -- turn back into rocks. Those rocks then continue through the erosion process and the materials are recycled through Earth's layers over geologic time.

On Titan, researchers think similar processes formed the dunes, plains, and labyrinth terrains seen from space. But unlike on Earth, Mars, and Venus, where silicate-derived rocks are the dominant geological material from which sediments are derived, Titan's sediments are thought to be composed of solid organic compounds. Scientists haven't been able to demonstrate how these organic compounds may grow into sediment grains that can be transported across the moon's landscapes and over geologic time.

"As winds transport grains, the grains collide with each other and with the surface. These collisions tend to decrease grain size through time. What we were missing was the growth mechanism that could counterbalance that and enable sand grains to maintain a stable size through time," Lapôtre said.

An alien analog

The research team found an answer by looking at sediments on Earth called ooids, which are small, spherical grains most often found in shallow tropical seas, such as around the Bahamas. Ooids form when calcium carbonate is pulled from the water column and attaches in layers around a grain, such as quartz.

What makes ooids unique is their formation through chemical precipitation, which allows ooids to grow, while the simultaneous process of erosion slows the growth as the grains are smashed into each other by waves and storms. These two competing mechanisms balance each other out through time to form a constant grain size -- a process the researchers suggest could also be happening on Titan.

"We were able to resolve the paradox of why there could have been sand dunes on Titan for so long even though the materials are very weak, Lapôtre said. "We hypothesized that sintering -- which involves neighboring grains fusing together into one piece -- could counterbalance abrasion when winds transport the grains."

Global landscapes

Armed with a hypothesis for sediment formation, Lapôtre and the study co-authors used existing data about Titan's climate and the direction of wind-driven sediment transport to explain its distinct parallel bands of geological formations: dunes near the equator, plains at the mid-latitudes, and labyrinth terrains near the poles.

Atmospheric modeling and data from the Cassini mission reveal that winds are common near the equator, supporting the idea that less sintering and therefore fine sand grains could be created there -- a critical component of dunes. The study authors predict a lull in sediment transport at mid-latitudes on either side of the equator, where sintering could dominate and create coarser and coarser grains, eventually turning into bedrock that makes up Titan's plains.

Sand grains are also necessary for the formation of the moon's labyrinth terrains near the poles. Researchers think these distinct crags could be like karsts in limestone on Earth -- but on Titan, they would be collapsed features made of dissolved organic sandstones. River flow and rainstorms occur much more frequently near the poles, making sediments more likely to be transported by rivers than winds. A similar process of sintering and abrasion during river transport could provide a local supply of coarse sand grains -- the source for the sandstones thought to make up labyrinth terrains.

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Saturn’s high-altitude winds generate an extraordinary aurorae, study finds

Leicester space scientists have discovered a never-before-seen mechanism fuelling huge planetary aurorae at Saturn.

Saturn is unique among planets observed to date in that some of its aurorae are generated by swirling winds within its own atmosphere, and not just from the planet's surrounding magnetosphere.

At all other observed planets, including Earth, aurorae are only formed by powerful currents that flow into the planet's atmosphere from the surrounding magnetosphere. These are driven by either interaction with charged particles from the Sun (as at the Earth) or volcanic material erupted from a moon orbiting the planet (as at Jupiter and Saturn).

This discovery changes scientists' understanding of planetary aurorae and answers one of the first mysteries raised by NASA's Cassini probe, which reached Saturn in 2004: why can't we easily measure the length of a day on the Ringed Planet?

When it first arrived at Saturn, Cassini tried to measure the bulk rotation rate of the planet, that determines the length of its day, by tracking radio emission 'pulses' from Saturn's atmosphere. To the great surprise of those making the measurements, they found that the rate appeared to have changed over the two decades since the last spacecraft to have flown past the planet -- Voyager 2, also operated by NASA -- in 1981.

Leicester PhD researcher Nahid Chowdhury is a member of the Planetary Science Group within the School of Physics and Astronomy and corresponding author for the study, published in Geophysical Research Letters. He said:

"Saturn's internal rotation rate has to be constant, but for decades researchers have shown that numerous periodic properties related to the planet -- the very measurements we've used at other planets to understand the internal rotation rate, such as the radio emission -- tend to change with time. What's more, there are also independent periodic features seen in the northern and southern hemispheres which themselves vary over the course of a season on the planet.

"Our understanding of the physics of planetary interiors tells us the true rotation rate of the planet can't change this quickly, so something unique and strange must be happening at Saturn. Several theories have been touted since the advent of the NASA Cassini mission trying to explain the mechanism/s behind these observed periodicities. This study represents the first detection of the fundamental driver, situated in the upper atmosphere of the planet, which goes on to generate both the observed planetary periodicities and aurorae.

"It's absolutely thrilling to be able to provide an answer to one of the longest standing questions in our field. This is likely to initiate some rethinking about how local atmospheric weather effects on a planet impact the creation of aurorae, not just in our own Solar System but farther afield too."

Astronomers and planetary scientists based at the University of Leicester led a study alongside colleagues from NASA's Jet Propulsion Laboratory (JPL), the Japan Aerospace Exploration Agency (JAXA), and the Universities of Wisconsin-Madison, Boston, and Lancaster, plus Imperial and University Colleges, London, to resolve the decades-old question.

They measured infrared emissions from the gas giant's upper atmosphere using the Keck Observatory in Hawai'i and mapped the varying flows of Saturn's ionosphere, far below the magnetosphere, over the course of a month in 2017.

This map, when fixed against the known pulse of Saturn's radio aurorae, showed that a significant proportion of the planet's aurorae are generated by the swirling pattern of weather in its atmosphere and are responsible for the planet's observed variable rate of rotation.

Researchers believe the system is driven by energy from Saturn's thermosphere, with winds in the ionosphere observed between 0.3 and 3.0 kilometres per second.

Dr Tom Stallard, Associate Professor in Planetary Astronomy at the University of Leicester, added:

"The University of Leicester has long been involved in measuring the effects of this new discovery -- we've observed how the pulsing aurorae and the wobbling magnetic field lines stretching out into space highlight an apparently changing rotation rate. For two decades our researchers, along with the wider scientific community, have speculated about what might be driving these strange periodicities.

"Over the years, scientific meetings have had late-night discussions about whether the volcanic moon Enceladus might be the cause, or interactions with the thick atmosphere of the moon Titan, or perhaps interactions with Saturn's bright rings. But recently, many researchers have focused on the possibility that it is Saturn's upper atmosphere that causes this variability.

"This search for a new type of aurora harks back to some of the earliest theories about Earth's aurora. We now know that aurorae on Earth are powered by interactions with the stream of charged particles driven from the Sun. But I love that the name Aurora Borealis originates from the 'the Dawn of the Northern Wind'. These observations have revealed that Saturn has a true Aurora Borealis -- the first ever aurora driven by the winds in the atmosphere of a planet."

Dr Kevin Baines, a JPL-Caltech-based co-author of the study and a member of the Cassini Science Team, added:

"Our study, by conclusively determining the origin of the mysterious variability in radio pulses, eliminates much of the confusion into Saturn's bulk rotation rate and the length of the day on Saturn."

Because of the variable rotation rates observed at Saturn, scientists have been prevented from using the regular pulse of radio emission to calculate the bulk internal rotation rate. Fortunately, a novel method was developed by Cassini scientists using gravity-induced perturbations in Saturn's complex ring system, which now seems to be the most accurate means of measuring the planet's bulk rotational period, which was determined in 2019 to be 10 hours, 33 minutes and 38 seconds.

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Earth isn’t 'super' because the sun had rings before planets

Before the solar system had planets, the sun had rings -- bands of dust and gas similar to Saturn's rings -- that likely played a role in Earth's formation, according to a new study.

"In the solar system, something happened to prevent the Earth from growing to become a much larger type of terrestrial planet called a super-Earth," said Rice University astrophysicist André Izidoro, referring to the massive rocky planets seen around at least 30% of sun-like stars in our galaxy.

Izidoro and colleagues used a supercomputer to simulate the solar system's formation hundreds of times. Their model, which is described in a study published online in Nature Astronomy, produced rings like those seen around many distant, young stars. It also faithfully reproduced several features of the solar system missed by many previous models, including:
 

• An asteroid belt between Mars and Jupiter containing objects from both the inner and outer solar system.
• The locations and stable, almost circular orbits of Earth, Mars, Venus and Mercury.
• The masses of the inner planets, including Mars, which many solar system models overestimate.
• The dichotomy between the chemical makeup of objects in the inner and outer solar system.
• A Kuiper belt region of comets, asteroids and small bodies beyond the orbit of Neptune.

The study by astronomers, astrophysicists and planetary scientists from Rice, the University of Bordeaux, Southwest Research Institute in Boulder, Colorado, and the Max Planck Institute for Astronomy in Heidelberg, Germany, draws on the latest astronomical research on infant star systems.

Their model assumes three bands of high pressure arose within the young sun's disk of gas and dust. Such "pressure bumps" have been observed in ringed stellar disks around distant stars, and the study explains how pressure bumps and rings could account for the solar system's architecture, said lead author Izidoro, a Rice postdoctoral researchers who received his Ph.D. training at Sao Paulo State University in Brazil.

"If super-Earths are super-common, why don't we have one in the solar system?" Izidoro said. "We propose that pressure bumps produced disconnected reservoirs of disk material in the inner and outer solar system and regulated how much material was available to grow planets in the inner solar system."

Pressure bumps

For decades, scientists believed gas and dust in protoplanetary disks gradually became less dense, dropping smoothly as a function of distance from the star. But computer simulations show planets are unlikely to form in smooth-disk scenarios.

"In a smooth disk, all solid particles -- dust grains or boulders -- should be drawn inward very quickly and lost in the star," said astronomer and study co-author Andrea Isella, an associate professor of physics and astronomy at Rice. "One needs something to stop them in order to give them time to grow into planets."

When particles move faster than the gas around them, they "feel a headwind and drift very quickly toward the star," Izidoro explained. At pressure bumps, gas pressure increases, gas molecules move faster and solid particles stop feeling the headwind. "That's what allows dust particles to accumulate at pressure bumps," he said.

Isella said astronomers have observed pressure bumps and protoplanetary disk rings with the Atacama Large Millimeter/submillimeter Array, or ALMA, an enormous 66-dish radio telescope that came online in Chile in 2013.

"ALMA is capable of taking very sharp images of young planetary systems that are still forming, and we have discovered that a lot of the protoplanetary disks in these systems are characterized by rings," Isella said. "The effect of the pressure bump is that it collects dust particles, and that's why we see rings. These rings are regions where you have more dust particles than in the gaps between rings."

Ring formation

The model by Izidoro and colleagues assumed pressure bumps formed in the early solar system at three places where sunward-falling particles would have released large amounts of vaporized gas.

"It's just a function of distance from the star, because temperature is going up as you get closer to the star," said geochemist and study co-author Rajdeep Dasgupta, the Maurice Ewing Professor of Earth Systems Science at Rice. "The point where the temperature is high enough for ice to be vaporized, for example, is a sublimation line we call the snow line."

In the Rice simulations, pressure bumps at the sublimation lines of silicate, water and carbon monoxide produced three distinct rings. At the silicate line, the basic ingredient of sand and glass, silicon dioxide, became vapor. This produced the sun's nearest ring, where Mercury, Venus, Earth and Mars would later form. The middle ring appeared at the snow line and the farthest ring at the carbon monoxide line.

Rings birth planetesimals and planets

Protoplanetary disks cool with age, so sublimation lines would have migrated toward the sun. The study showed this process could allow dust to accumulate into asteroid-sized objects called planetesimals, which could then come together to form planets. Izidoro said previous studies assumed planetesimals could form if dust were sufficiently concentrated, but no model offered a convincing theoretical explanation of how dust might accumulate.

"Our model shows pressure bumps can concentrate dust, and moving pressure bumps can act as planetesimal factories," Izidoro said. "We simulate planet formation starting with grains of dust and covering many different stages, from small millimeter-sized grains to planetesimals and then planets."

Accounting for cosmochemical signatures, Mars' mass and the asteroid belt

Many previous solar system simulations produced versions of Mars as much as 10 times more massive than Earth. The model correctly predicts Mars having about 10% of Earth's mass because "Mars was born in a low-mass region of the disk," Izidoro said.

Dasgupta said the model also provides a compelling explanation for two of the solar system's cosmochemical mysteries: the marked difference between the chemical compositions of inner- and outer-solar system objects, and the presence of each of those objects in the asteroid belt between Mars and Jupiter.

Izidoro's simulations showed the middle ring could account for the chemical dichotomy by preventing outer-system material from entering the inner system. The simulations also produced the asteroid belt in its correct location, and showed it was fed objects from both the inner and outer regions.

"The most common type of meteorites we get from the asteroid belt are isotopically similar to Mars," Dasgupta said. "Andre explains why Mars and these ordinary meteorites should have a similar composition. He's provided a nuanced answer to this question."

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Icequakes likely rumble along geyser-spitting fractures in Saturn's icy moon Enceladus

Tidal stresses may be causing constant icequakes on Saturn's sixth largest moon Enceladus, a world of interest in the search for life beyond Earth, according to a new study. A better understanding of seismic activity could reveal what's under the moon's icy crust and provide clues to the habitability of its ocean.

Enceladus is about 500 kilometers in diameter and almost entirely covered in ice. The moon is nearly 10 times as far away from the Sun as Earth and its bright surface reflects most sunlight, making it very cold, yet researchers have long speculated that the ice encases an underlying liquid ocean.

The moon likely experiences massive tidal forces caused by Saturn and the planet's other, larger moons -- similar to the way Earth's Moon causes tides on Earth. These tidal motions inside Enceladus warm its interior, crack the surface and sometimes squeeze tall geysers of water vapor through notable cracks called the tiger stripe fractures.

The new study used observations of Antarctic ice shelves to suggest tides on Enceladus may also cause small quakes in the ice at the moon's fractures, like icequakes observed on Antarctica's floating ice sheets.

"[Moons like] these are places that are exciting because they might have life," said Kira Olsen, a geophysicist at NASA's Goddard Space Flight Center. She said that since life is thought to have first developed in our oceans, liquid oceans under the ice of other worlds could be a good place to search for life. The icy crust of Enceladus might also protect the water below from radiation, making it more habitable.

The new study was published in the Journal of Geophysical Research: Planets, AGU's journal for research on the formation and evolution of the planets, moons and objects of our solar system and beyond.

"We have ideas of how thick the ice could be, but we don't have direct observation. Studying ice quakes is a way to get at that information," Olsen said.

Internal tides

To learn more about how Enceladus' tiger fractures might be moving, Olsen and her colleagues turned to floating ice shelves in Antarctica as the closest analogue on Earth for the types of activity they were seeing on Enceladus. They could then use their knowledge about how certain surface features on our planet produce seismic activity to estimate what kind of seismic activity is happening on the distant moon.

The researchers analyzed data collected by seismometers along the Ross Ice Shelf in the southern continent between 2014 and 2016 and compared these to satellite images of the area. They paid particular attention to two seismometers placed next to large rifts on the ice slab.

They related the seismic activity to the stress occurring along these rifts. The majority of icequakes on the Ross Ice Shelf occurred when the rifts were pulling apart, which happens when tides are falling.

We have no measurements of seismic activity on Enceladus, but Olsen and her colleagues created models that compared the types of fractures they saw on the moon's surface with those on the Ross Ice Shelf.

These models showed that the largest amount of seismic activity on Enceladus likely corresponded to the tides. Peak seismic activity there occurs when Enceladus is 100 degrees past the nearest approach to Saturn during its orbit. The ocean underneath the ice at this point acts something like water inside a sloshing balloon. The ice fractures are created at the points of highest stress, where the balloon would break apart.

The icequakes aren't massive along these cracks, even at the peak periods of stress. Olsen describes them more like "almost continuous little pops and fractures.

Mark Panning, a research scientist at NASA's Jet Propulsion Laboratory who was not involved in the new study, said that while the Cassini spacecraft revealed the moon is geologically active, it's difficult to tell how that translates to seismic activity. "The study represents a really key way of investigating what seismicity on Enceladus and other tidally activated icy worlds may look like by looking at the best analogs we can find on Earth," he said.

Olsen said scientists should aim to place seismometers within 10 kilometers of these fractures in any future missions to Enceladus to learn more about what's going on below.

"It's not a quiet out of the way place, but it's a pretty good place to study," she said.

More information about the seismic activity could then teach us more about the thickness of the ice crust on Enceladus. For now, no missions to Enceladus have been planned, but the European Space Agency is planning the JUICE mission to one of Jupiter's icy moons, Europa.

Olsen said that similar work could then be conducted on Titan, Saturn's largest moon, a world also covered with ice that may conceal liquid oceans and is another top pick for potential extraterrestrial life. NASA's Dragonfly mission is scheduled to visit Titan in 2036.

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