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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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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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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Saturn moon, Enceladus, could support life in its subsurface ocean

 Using data from NASA's Cassini spacecraft, scientists at Southwest Research Institute (SwRI) modeled chemical processes in the subsurface ocean of Saturn's moon Enceladus. The studies indicate the possibility that a varied metabolic menu could support a potentially diverse microbial community in the liquid water ocean beneath the moon's icy facade.

Prior to its deorbit in September of 2017, Cassini sampled the plume of ice grains and water vapor erupting from cracks on the icy surface of Enceladus, discovering molecular hydrogen, a potential food source for microbes. A new paper published in the planetary science journal Icarus explores other potential energy sources.

"The detection of molecular hydrogen (H2) in the plume indicated that there is free energy available in the ocean of Enceladus," said lead author Christine Ray, who works part time at SwRI as she pursues a Ph.D. in physics from The University of Texas at San Antonio. "On Earth, aerobic, or oxygen-breathing, creatures consume energy in organic matter such as glucose and oxygen to create carbon dioxide and water. Anaerobic microbes can metabolize hydrogen to create methane. All life can be distilled to similar chemical reactions associated with a disequilibrium between oxidant and reductant compounds."

This disequilibrium creates a potential energy gradient, where redox chemistry transfers electrons between chemical species, most often with one species undergoing oxidation while another species undergoes reduction. These processes are vital to many basic functions of life, including photosynthesis and respiration. For example, hydrogen is a source of chemical energy supporting anaerobic microbes that live in the Earth's oceans near hydrothermal vents. At Earth's ocean floor, hydrothermal vents emit hot, energy-rich, mineral-laden fluids that allow unique ecosystems teeming with unusual creatures to thrive. Previous research found growing evidence of hydrothermal vents and chemical disequilibrium on Enceladus, which hints at habitable conditions in its subsurface ocean.

"We wondered if other types of metabolic pathways could also provide sources of energy in Enceladus' ocean," Ray said. "Because that would require a different set of oxidants that we have not yet detected in the plume of Enceladus, we performed chemical modeling to determine if the conditions in the ocean and the rocky core could support these chemical processes."

For example, the authors looked at how ionizing radiation from space could create the oxidants O2 and H2O2, and how abiotic geochemistry in the ocean and rocky core could contribute to chemical disequilibria that might support metabolic processes. The team considered whether these oxidants could accumulate over time if reductants are not present in appreciable amounts. They also considered how aqueous reductants or seafloor minerals could convert these oxidants into sulfates and iron oxides.

"We compared our free energy estimates to ecosystems on Earth and determined that, overall, our values for both aerobic and anaerobic metabolisms meet or exceed minimum requirements," Ray said. "These results indicate that oxidant production and oxidation chemistry could contribute to supporting possible life and a metabolically diverse microbial community on Enceladus."

"Now that we've identified potential food sources for microbes, the next question to ask is 'what is the nature of the complex organics that are coming out of the ocean?'" said SwRI Program Director Dr. Hunter Waite, a coauthor of the new paper, referencing an online Nature paper authored by Postberg et al. in 2018. "This new paper is another step in understanding how a small moon can sustain life in ways that completely exceed our expectations!"

The paper's findings also have great significance for the next generation of exploration.

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NASA's Cassini reveals New Sculpting in Saturn Rings

A false-color image mosaic shows Daphnis, one of Saturn's ring-embedded moons, and the waves it kicks up in the Keeler gap. Images collected by Cassini's close orbits in 2017 are offering new insight into the complex workings of the rings.

As NASA's Cassini dove close to Saturn in its final year, the spacecraft provided intricate detail on the workings of Saturn's complex rings, new analysis shows.

Although the mission ended in 2017, science continues to flow from the data collected. A new paper published June 13 in Science describes results from four Cassini instruments taking their closest-ever observations of the main rings.

Findings include fine details of features sculpted by masses embedded within the rings. Textures and patterns, from clumpy to strawlike, pop out of the images, raising questions about the interactions that shaped them. New maps reveal how colors, chemistry and temperature change across the rings.

Like a planet under construction inside a disk of protoplanetary material, tiny moons embedded in Saturn's rings (named A through G, in order of their discovery) interact with the particles around them. In that way, the paper provides further evidence that the rings are a window into the astrophysical disk processes that shape our solar system.

The observations also deepen scientists' understanding of the complex Saturn system. Scientists conclude that at the outer edge of the main rings, a series of similar impact-generated streaks in the F ring have the same length and orientation, showing that they were likely caused by a flock of impactors that all struck the ring at the same time. This shows that the ring is shaped by streams of material that orbit Saturn itself rather than, for instance, by cometary debris (moving around the Sun) that happens to crash into the rings.

"These new details of how the moons are sculpting the rings in various ways provide a window into solar system formation, where you also have disks evolving under the influence of masses embedded within them," said lead author and Cassini scientist Matt Tiscareno of the SETI Institute in Mountain View, California.

Enduring Mysteries

At the same time, new puzzles have arisen and old mysteries have deepened with the latest research. The close-up ring images brought into focus three distinct textures -- clumpy, smooth and streaky -- and made it clear that these textures occur in belts with sharp boundaries. But why? In many places the belts aren't connected to any ring characteristics that scientists have yet identified.

"This tells us the way the rings look is not just a function of how much material there is," Tiscareno said. "There has to be something different about the characteristics of the particles, perhaps affecting what happens when two ring particles collide and bounce off each other. And we don't yet know what it is."

The data analyzed were gathered during the Ring Grazing Orbits (December 2016 to April 2017) and the Grand Finale (April to September 2017), when Cassini flew just above Saturn's cloud tops. As the spacecraft was running out of fuel, the mission team deliberately plunged it into the planet's atmosphere in September 2017.

Cassini's Visible and Infrared Mapping Spectrometer (VIMS) uncovered another mystery. The spectrometer, which imaged the rings in visible and near-infrared light, identified unusually weak water-ice bands in the outermost part of the A ring. That was a surprise, because the area is known to be highly reflective, which usually is a sign of less-contaminated ice and thus stronger water ice bands.

The new spectral map also sheds light on the composition of the rings. And while scientists already knew that water ice is the main component, the spectral map ruled out detectable ammonia ice and methane ice as ingredients. But it also doesn't see organic compounds -- a surprise, given the organic material Cassini has discovered flowing from the D ring into Saturn's atmosphere.

"If organics were there in large amounts -- at least in the main A, B and C rings -- we'd see them," said Phil Nicholson, Cassini VIMS scientist of Cornell University in Ithaca, New York. "I'm not convinced yet that they are a major component of the main rings."

The research signals the start of the next era of Cassini science, said NASA's Ames Research Center's Jeff Cuzzi, who's been studying Saturn's rings since the 1970s and is the interdisciplinary scientist for rings on the Cassini mission.

"We see so much more, and closer up, and we're getting new and more interesting puzzles," Cuzzi said. "We are just settling into the next phase, which is building new, detailed models of ring evolution -- including the new revelation from Cassini data that the rings are much younger than Saturn."

The new observations give scientists an even more intimate view of the rings than they had before, and each examination reveals new complexities, said Cassini Project Scientist Linda Spilker, based at NASA's Jet Propulsion Laboratory in Pasadena, California.

"It's like turning the power up one more notch on what we could see in the rings. Everyone just got a clearer view of what's going on," Spilker said. "Getting that extra resolution answered many questions, but so many tantalizing ones remain."

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NASA's Cassini reveals surprises with Titan's lakes

This near-infrared, color view from Cassini shows the sun glinting off of Titan's north polar seas.

On its final flyby of Saturn's largest moon in 2017, NASA's Cassini spacecraft gathered radar data revealing that the small liquid lakes in Titan's northern hemisphere are surprisingly deep, perched atop hills and filled with methane.

The new findings, published April 15 in Nature Astronomy, are the first confirmation of just how deep some of Titan's lakes are (more than 300 feet, or 100 meters) and of their composition. They provide new information about the way liquid methane rains on, evaporates from and seeps into Titan -- the only planetary body in our solar system other than Earth known to have stable liquid on its surface.

Scientists have known that Titan's hydrologic cycle works similarly to Earth's -- with one major difference. Instead of water evaporating from seas, forming clouds and rain, Titan does it all with methane and ethane. We tend to think of these hydrocarbons as a gas on Earth, unless they're pressurized in a tank. But Titan is so cold that they behave as liquids, like gasoline at room temperature on our planet.

Scientists have known that the much larger northern seas are filled with methane, but finding the smaller northern lakes filled mostly with methane was a surprise. Previously, Cassini data measured Ontario Lacus, the only major lake in Titan's southern hemisphere. There they found a roughly equal mix of methane and ethane. Ethane is slightly heavier than methane, with more carbon and hydrogen atoms in its makeup.

"Every time we make discoveries on Titan, Titan becomes more and more mysterious," said lead author Marco Mastrogiuseppe, Cassini radar scientist at Caltech in Pasadena, California. "But these new measurements help give an answer to a few key questions. We can actually now better understand the hydrology of Titan."

Adding to the oddities of Titan, with its Earth-like features carved by exotic materials, is the fact that the hydrology on one side of the northern hemisphere is completely different than the that of other side, said Cassini scientist and co-author Jonathan Lunine of Cornell University in Ithaca, New York.

"It is as if you looked down on the Earth's North Pole and could see that North America had completely different geologic setting for bodies of liquid than Asia does," Lunine said.

On the eastern side of Titan, there are big seas with low elevation, canyons and islands. On the western side: small lakes. And the new measurements show the lakes perched atop big hills and plateaus. The new radar measurements confirm earlier findings that the lakes are far above sea level, but they conjure a new image of landforms -- like mesas or buttes -- sticking hundreds of feet above the surrounding landscape, with deep liquid lakes on top.

The fact that these western lakes are small -- just tens of miles across -- but very deep also tells scientists something new about their geology: It's the best evidence yet that they likely formed when the surrounding bedrock of ice and solid organics chemically dissolved and collapsed. On Earth, similar water lakes are known as karstic lakes. Occurring in in areas like Germany, Croatia and the United States, they form when water dissolves limestone bedrock.

Alongside the investigation of deep lakes, a second paper in Nature Astronomy helps unravel more of the mystery of Titan's hydrologic cycle. Researchers used Cassini data to reveal what they call transient lakes. Different sets of observations -- from radar and infrared data -- seem to show liquid levels significantly changed.

The best explanation is that there was some seasonally driven change in the surface liquids, said lead author Shannon MacKenzie, planetary scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. "One possibility is that these transient features could have been shallower bodies of liquid that over the course of the season evaporated and infiltrated into the subsurface," she said.

These results and the findings from the Nature Astronomy paper on Titan's deep lakes support the idea that hydrocarbon rain feeds the lakes, which then can evaporate back into the atmosphere or drain into the subsurface, leaving reservoirs of liquid stored below.

Cassini, which arrived in the Saturn system in 2004 and ended its mission in 2017 by deliberately plunging into Saturn's atmosphere, mapped more than 620,000 square miles (1.6 million square kilometers) of liquid lakes and seas on Titan's surface. It did the work with the radar instrument, which sent out radio waves and collected a return signal (or echo) that provided information about the terrain and the liquid bodies' depth and composition, along with two imaging systems that could penetrate the moon's thick atmospheric haze.

The crucial data for the new research were gathered on Cassini's final close flyby of Titan, on April 22, 2017. It was the mission's last look at the moon's smaller lakes, and the team made the most of it. Collecting echoes from the surfaces of small lakes while Cassini zipped by Titan was a unique challenge.

"This was Cassini's last hurrah at Titan, and it really was a feat," Lunine said.

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New surprises from Jupiter and Saturn

Jupiter.

The latest data sent back by the Juno and Cassini spacecraft from giant gas planets Jupiter and Saturn have challenged a lot of current theories about how planets in our solar system form and behave.

The detailed magnetic and gravity data have been "invaluable but also confounding," said David Stevenson from Caltech, who will present an update of both missions this week at the 2019 American Physical Society March Meeting in Boston.

"Although there are puzzles yet to be explained, this is already clarifying some of our ideas about how planets form, how they make magnetic fields and how the winds blow," Stevenson said.

Cassini orbited Saturn for 13 years before its dramatic final dive into the planet's interior in 2017, while Juno has been orbiting Jupiter for two and a half years.

Juno's success as a mission to Jupiter is a tribute to innovative design. Its instruments are powered by solar energy alone and protected so as to withstand the fierce radiation environment.

Stevenson says the inclusion of a microwave sensor on Juno was a good decision.

"Using microwaves to figure out the deep atmosphere was the right, but unconventional, choice," he said. The microwave data have surprised the scientists, in particular by showing that the atmosphere is evenly mixed, something conventional theories did not predict.

"Any explanation for this has to be unorthodox," Stevenson said.

Researchers are exploring weather events concentrating significant amounts of ice, liquids and gas in different parts of the atmosphere as possible explanations, but the matter is far from sealed.

Other instruments on board Juno, gravity and magnetic sensors, have also sent back perplexing data. The magnetic field has spots (regions of anomalously high or low magnetic field) and also a striking difference between the northern and southern hemispheres.

"It's unlike anything we have seen before," Stevenson said.

The gravity data have confirmed that in the midst of Jupiter, which is at least 90 percent hydrogen and helium by mass, there are heavier elements amounting to more than 10 times the mass of Earth. However, they are not concentrated in a core but are mixed in with the hydrogen above, most of which is in the form of a metallic liquid.

The data has provided rich information about the outer parts of both Jupiter and Saturn. The abundance of heavier elements in these regions is still uncertain, but the outer layers play a larger-than-expected role in the generation of the two planets' magnetic fields. Experiments mimicking the gas planets' pressures and temperatures are now needed to help the scientists understand the processes that are going on.

For Stevenson, who has studied gas giants for 40 years, the puzzles are the hallmark of a good mission.

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Waves in Saturn's rings give precise measurement of planet's rotation rate

This image of Saturn's rings was taken by NASA's Cassini spacecraft on Sept. 13, 2017. It is among the last images Cassini sent back to Earth.

Saturn's distinctive rings were observed in unprecedented detail by NASA's Cassini spacecraft, and scientists have now used those observations to probe the interior of the giant planet and obtain the first precise determination of its rotation rate. The length of a day on Saturn, according to their calculations, is 10 hours 33 minutes and 38 seconds.

The researchers studied wave patterns created within Saturn's rings by the planet's internal vibrations. In effect, the rings act as an extremely sensitive seismograph by responding to vibrations within the planet itself.

Similar to Earth's vibrations from an earthquake, Saturn responds to perturbations by vibrating at frequencies determined by its internal structure. Heat-driven convection in the interior is the most likely source of the vibrations. These internal oscillations cause the density at any particular place within the planet to fluctuate, which makes the gravitational field outside the planet oscillate at the same frequencies.

"Particles in the rings feel this oscillation in the gravitational field. At places where this oscillation resonates with ring orbits, energy builds up and gets carried away as a wave," explained Christopher Mankovich, a graduate student in astronomy and astrophysics at UC Santa Cruz.

Mankovich is lead author of a paper, published January 17 in the Astrophysical Journal, comparing the wave patterns in the rings with models of Saturn's interior structure.

Most of the waves observed in Saturn's rings are due to the gravitational effects of the moons orbiting outside the rings, said coauthor Jonathan Fortney, professor of astronomy and astrophysics at UC Santa Cruz. "But some of the features in the rings are due to the oscillations of the planet itself, and we can use those to understand the planet's internal oscillations and internal structure," he said.

Mankovich developed a set of models of the internal structure of Saturn, used them to predict the frequency spectrum of Saturn's internal vibrations, and compared those predictions with the waves observed by Cassini in Saturn's C ring. One of the main results of his analysis is the new calculation of Saturn's rotation rate, which has been surprisingly difficult to measure.

As a gas giant planet, Saturn has no solid surface with landmarks that could be tracked as it rotates. Saturn is also unusual in having its magnetic axis nearly perfectly aligned with its rotational axis. Jupiter's magnetic axis, like Earth's, is not aligned with its rotational axis, which means the magnetic pole swings around as the planet rotates, enabling astronomers to measure a periodic signal in radio waves and calculate the rotation rate.

The rotation rate of 10:33:38 determined by Mankovich's analysis is several minutes faster than previous estimates based on radiometry from the Voyager and Cassini spacecraft.

"We now have the length of Saturn's day, when we thought we wouldn't be able to find it," said Cassini Project Scientist Linda Spilker. "They used the rings to peer into Saturn's interior, and out popped this long-sought, fundamental quality of the planet. And it's a really solid result. The rings held the answer."

The idea that Saturn's rings could be used to study the seismology of the planet was first suggested in 1982, long before the necessary observations were possible. Coauthor Mark Marley, now at NASA's Ames Research Center in Silicon Valley, subsequently fleshed out the idea for his Ph.D. thesis in 1990, showed how the calculations could be done, and predicted where features in Saturn's rings would be. He also noted that the Cassini mission, then in the planning stages, would be able to make the observations needed to test the idea.

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Surprising chemical complexity of Saturn's rings changing planet's upper atmosphere

During Cassini's 'Grand Finale' plunge into Saturn's innermost ring and upper atmosphere in 2017, the mass spectrometer aboard the probe sampled chemicals at altitudes between Saturn's rings and atmosphere.

Political humorist Mark Russell once joked, "The scientific theory I like best is that the rings of Saturn are composed entirely of lost airline luggage."

Well, there's no luggage, it turns out. But a new study appearing in Science based on data from the final orbits last year of NASA's Cassini spacecraft shows the rings of Saturn -- some of the most visually stupendous objects in the universe -- are far more chemically complicated than previously was understood.

Furthermore, the paper shows the innermost D ring of the gas giant is hurling dust grains coated in its chemical cocktail into the planet's upper atmosphere at an extraordinary rate as it spins. Over long timescales, the researchers say this infalling material may change the carbon and oxygen content of the atmosphere.

"This is a new element of how our solar system works," said Thomas Cravens, professor of physics & astronomy at the University of Kansas and a co-author of the new paper. "Two things surprised me. One is the chemical complexity of what was coming off the rings -- we thought it would be almost entirely water based on what we saw in the past. The second thing is the sheer quantity of it -- a lot more than we originally expected. The quality and quantity of the materials the rings are putting into the atmosphere surprised me."

Cravens is a member of Cassini's Ion and Neutral Mass Spectrometer (INMS) team. During Cassini's "Grand Finale" plunge into Saturn's innermost ring and upper atmosphere in 2017, the mass spectrometer aboard the probe sampled chemicals at altitudes between Saturn's rings and atmosphere.

More than simply water, the INMS found the rings to be composed of water, methane, ammonia, carbon monoxide, molecular nitrogen and carbon dioxide.

"What the paper is describing is the environment in the gap between the inner ring and upper atmosphere, and some of the things found were expected, such as water," Cravens said. "What was a surprise was the mass spectrometer saw methane -- no one expected that. Also, it saw some carbon dioxide, which was unexpected. The rings were thought to be entirely water. But the innermost rings are fairly contaminated, as it turns out, with organic material caught up in ice."

A further new finding from Cassini's mass spectrometer showed large amounts of the chemical brew from Saturn's D ring is flung into the planet's upper atmosphere by the ring spinning faster than the planet's atmosphere itself.

"We saw it was happening even though it's not fully understood," the KU researcher said. "What we saw is this material, including some benzine, was altering the uppermost atmosphere of Saturn in the equatorial region. There were both grains and dust that were contaminated."

Cravens said the findings could cast new light on mechanisms underpinning our solar system as well as other solar systems and exoplanets -- and also prompt a host of new scientific questions.

"This could help us understand, how does a planet get rings? Some do, some don't," he said. "What's the lifetime of a ring? And what's replenishing the rings? Was there a time when Saturn didn't have rings? How did that composition get into there in the first place? Is it left over from the formation of our solar system? Does it date back to proto pre-solar nebula, the nebula that collapsed out of interstellar media that formed the sun and planets?"

According to Cravens, the higher-than-expected rate of material being expelled from Saturn's D Ring into the planet's upper atmosphere, or ionosphere, is sufficient that astronomers now think the lifespan of the ring may be briefer than previously estimated.

"Because of this data, we now have shortened the lifetime of inner rings because of the quantity of material being moved out -- it's much more than we thought before," Cravens said. We know that it's bumping material out of the rings at least 10 times faster than we thought. If it's not being replenished, the rings aren't going to last -- you've got a hole in your bucket. Jupiter probably had a ring that evolved into the current wispy ring, and it could be for similar reasons. Rings do come and go. At some point they gradually drain away unless somehow they're getting new material."

Assisted by KU graduate and undergraduate students, a first stage of Cravens' work involved sorting and cleaning raw data from Cassini's INMS instrument.

"The raw data came through from our instrument on Cassini to deep-space antennas to NASA's Jet Propulsion Laboratory and then to computers at the Southwest Research Institute in San Antonio where Hunter Waite, the first author, is based," he said.

But Cravens' main contribution involved interpreting that data with a focus on how materials from the rings are altering Saturn's ionosphere. Cravens and his colleagues report the influx of chemicals from the rings change Saturn's equatorial ionospheric chemistry by converting the hydrogen ions and triatomic hydrogen ions into heavier molecular ions, depleting the planet's ionospheric density.

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Electrical and chemical coupling between Saturn and its rings

Spacecraft Cassini with an instrument from the Swedish Institute of Space Physics on board (in red circle) passed through Saturn’s atmosphere.

A Langmuir probe, developed in Sweden and flown to Saturn on the Cassini spacecraft, has made exciting discoveries in the atmosphere of the planet. Jan-Erik Wahlund at the Swedish Institute of Space Physics in Uppsala and his colleagues show that there is a strong coupling, both chemically and electrically, between the atmosphere of Saturn and its rings. These research results have now been published in the journal Science.

In April the American space agency NASA put the Cassini spacecraft into an orbit that took it right through the narrow gap between the innermost visible ring (the D-ring) and at the same time very close to the Saturn, so close that it passed through the outer parts of the planet's atmosphere. Cassini made 22 such orbits, and on the 15 September, according to plan, Cassini was sent down into the gas masses of Saturn and burned up. During all of these orbits most of the instruments on board made detailed measurements.

Now the scientific results are starting to take shape, and the results from the Swedish instrument are the first to be published in the well-known journal Science. The instrument, a so-called Langmuir proble, was developed at the Swedish Institute of Space Physics in Uppsala. The upper atmosphere of Saturn is charged and consists primarily of hydrogen and hydrogen ions. The Langmuir probe can be compared with a weather station for electrically charged gas; it measures its density, temperature and velocity. It also measures particles' energy and moreover gives a rough estimate of what the gas consists of.

"The first results are surprising," says Jan-Erik Wahlund, IRF, principle investigator and responsible for the Langmuir probe on Cassini.

Strong variations in density indicate that the electrically charged part of Saturn's atmosphere (the so-called ionosphere) has a strong coupling to the visible rings that consist primarily of ice particles. The ice particles are also electrically charged.

"It is as though the small ice particles in the D-ring suck up electrons from the ionosphere," says Jan-Erik Wahlund. "As a result of the coupling, electrical flows of gas to and from the rings along the magnetic field of Saturn cause the greatest variations in density."

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Cassini's Final Days Produced a Burst of Fresh Science

This view looks toward the sunlit side of the rings from about 7 degrees above the ring plane. The image was taken in visible light with the wide-angle camera on NASA's Cassini spacecraft on June 7, 2017.

While the Cassini spacecraft is dead — engineers deliberately plunged it into Saturn's atmosphere on Sept. 15, ending its 13-year mission around the planet — analyzing the reams of data from the mission will take decades, NASA said in a statement, as new insights were released this week from the spacecraft's final days.

"There are whole careers to be forged in the analysis of data from Cassini," Linda Spilker, the mission's project scientist at NASA's Jet Propulsion Laboratory in California, said in a statement. "In a sense, the work has only just begun."

Cassini's main goal was to learn more about the nature of Saturn and its moons, as the planet moved through its 29-year orbit around the sun. The spacecraft returned data in droves, yielding insights into a mysterious hexagon storm at one of Saturn's poles, water geysers that spurt on the moon Enceladus, and a possible prebiotic chemistry on the moon Titan.

With the mission low on fuel, Cassini plunged into Saturn a month ago after making several daring dives between the planet and its famous rings. On Oct. 16 at a meeting of the American Astronomical Society Division for Planetary Science, scientists revealed new insights from those final days.

Activity in Saturn's rings resembles how planets form

While showing pictures and videos of Saturn's rings, researchers said there are some features in those rings that have similar processes to those that birth exoplanets. For example, ring features called propellers — wakes in the rings generated by moonlets — are similar to baby planets growing in the gas discs surrounding young stars. The day before Cassini died, said Cassini participating scientist Matt Tiscareno of the SETI Institute, the spacecraft captured images of all six propellers that it tracked during its mission: Blériot, Earhart, Santos-Dumont, Sikorsky, Post, and Quimby.

Methane is raining down from Saturn's rings

Cassini's ion and neutral mass spectrometer captured the first-ever measurements of Saturn's upper atmosphere, which extends almost as far as the planet’s rings. The team, which included Cassini participating scientist Mark Perry from the Johns Hopkins University Applied Physics Laboratory, not only discovered water raining down from the rings, but also methane, which was a surprise, since the team expected the gas was too volatile to survive within Saturn’s rings or its atmosphere.

We may soon know the length of a day on Saturn

Since Cassini's final orbits brought it closer to the planet, its magnetic field measurements are much more sensitive. The more detailed data could be a boon for researchers such as Michele Dougherty, leader of Cassini's magnetometer team from Imperial College London, who is trying to figure out the length of a day on Saturn. Scientists expect that by learning the magnetic field's tilt, they'll be able to determine the planet's internal rotation — an indication of a day’s length. Scientists said if the magnetic field tilt is greater than 0.016 degrees, they should be able to "nail down the true length of the planet's day."

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NASA’s Cassini Spacecraft Signs Off After Marathon of Scientific Discovery

A model of the Cassini spacecraft is seen at NASA's Jet Propulsion Laboratory (JPL) September 13, 2017 in Pasadena, California.

After 20 years in space, NASA's famed Cassini spacecraft made its final death plunge into Saturn on Friday, ending a storied mission that scientists say taught us nearly everything we know about Saturn today and transformed the way we think about life elsewhere in the solar system.

Cassini, an international project that cost $3.9 billion and included scientists from 27 nations, disintegrated as it dove into Saturn's atmosphere at a speed of 75,000 miles (120,700 kilometers) per hour.

"The signal from the spacecraft is gone," said Cassini program manager Earl Maize of NASA's Jet Propulsion Laboratory.

"I hope you are all as deeply proud of this amazing accomplishment," he told colleagues at mission control. "This has been an incredible mission, an incredible spacecraft and you are all an incredible team."

Cassini's final contact with Earth came at 7:55 am EDT (1155 GMT). Its descent into Saturn's atmosphere began about an hour and a half earlier, but the signal took that long to reach Earth because of the vast distance.

Cassini's plunge into the ringed gas giant — the furthest planet visible from Earth with the naked eye — came after the spacecraft ran out of rocket fuel after a journey of some 4.9 billion miles (7.9 billion kilometers).

Its well-planned demise was a way to prevent any damage to Saturn's ocean-bearing moons Titan and Enceladus, which scientists want to keep pristine for future exploration because they may contain some form of life.

"There are international treaties that require that we can't just leave a derelict spacecraft in orbit around a planet like Saturn, which has prebiotic moons," said Maize.

Cassini science team members await the final loss of signal from the Cassini spacecraft, indicating Cassini's destruction in Saturn's atmosphere and the end of Cassini's 20-year mission to gain a better understanding of the ringed planet and its icy moons, at the Jet Propulsion Laboratory in Pasadena, California on September 15, 2017.

Three other spacecraft have flown by Saturn — Pioneer 11 in 1979, followed by Voyager 1 and 2 in the 1980s.

But none have studied Saturn in such detail as Cassini, named after the French-Italian astronomer Giovanni Domenico Cassini, who discovered in the 17th century that Saturn had several moons and a gap between its rings.

"This is the final chapter of an amazing mission, but it's also a new beginning,” said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate. "Cassini's discovery of ocean worlds at Titan and Enceladus changed everything, shaking our views to the core about surprising places to search for potential life beyond Earth."

Discoveries

Cassini launched from Cape Canaveral, Florida in 1997, then spent seven years in transit followed by 13 years orbiting Saturn.

In that time, it discovered six more moons around Saturn, three-dimensional structures towering above Saturn's rings, and a giant storm that raged across the planet for nearly a year.

The 22-by-13-foot (6.7-by-4 meter) spacecraft is also credited with discovering icy geysers erupting from Enceladus, and eerie hydrocarbon lakes made of ethane and methane on Saturn's largest moon, Titan.

In 2005, the Cassini orbiter released a lander called Huygens on Titan, marking the first and only such landing in the outer solar system, on a celestial body beyond the asteroid belt.

Huygens was a joint project of the European Space Agency, Italian Space Agency, and NASA.

"The mission has changed the way we think of where life may have developed beyond our Earth," said Andrew Coates, head of the Planetary Science Group at Mullard Space Science Laboratory at University College London.

"As well as Mars, outer planet moons like Enceladus, Europa, and even Titan are now top contenders for life elsewhere," he added. "We've completely rewritten the textbooks about Saturn."

Linda Spilker, Cassini project scientist, likened Cassini's mission to a marathon.

"For 13 years we have been running a marathon of scientific discovery, and we are on the last lap," she said early Friday.

Eight of the spacecraft 12 scientific instruments were on, capturing data, in Cassini's last moments, before it disintegrates like a meteor, she said.

"We are flying more deeply into Saturn than we have ever flown before," she said. "Who knows how many Ph.D. theses might be in just those final seconds of data?"

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Cassini Prepares to Crash Into Saturn as NASA Reflects on Mission's Discoveries

Saturn and its rings.

After 14 years of exploration, the Cassini spacecraft is preparing to write its final chapter on the Saturn system.

No other spacecraft in history has come to know a single planetary system as intimately as Cassini knows Saturn: Cassini is the first spacecraft to visit Saturn up close since the Voyager probes flew by in 1980 and 1981, and the mission has given scientists their best-ever views of the gas giant.

The probe has spent more than a decade observing Saturn, studying storms in its cloud tops, learning about its strange, striped atmosphere, probing its cloaked interior and zipping through its more than 60 moons and a ring system stretching more than eight times the planet's radius.

"The mission has exceeded all of our expectations, done better than we could have ever dreamed," Curt Niebur, Cassini's program scientist from NASA headquarters in Houston, said during a press teleconference in August. "The Saturn system is absolutely chock-full of amazing worlds of all sizes, and Cassini has been exploring them for the past 13 years.

"Since our arrival in 2004, we've watched the seasons change on Saturn, which is just an incredible opportunity, considering a year on Saturn lasts 29 Earth years," he added. "We've watched the particles and the rings around Saturn collide and glide during their gravitational dance, and we've confirmed things that we suspected might exist in the Saturn system, but even more pleasantly, we've been shocked by things that we never predicted we would find."

Cassini's scientists have watched a Saturn storm erupt and run over its own tail after circling the entire planet. They've probed the mysteries of the Earth-size, hexagonal jet stream on the planet's pole that persists in all seasons, but changes color over time. Researchers have sent a probe down to Saturn's largest moon, Titan, to see the lakes, seas and rivers of methane on its surface, and have flown Cassini through the geyser jets of the tiny moon Enceladus to probe its newfound ice-covered ocean.

"These two new worlds, Titan and Enceladus, which were so completely revealed to us by Cassini, have changed the idea that ocean worlds like Earth and [Jupiter's moon] Europa are rare in the universe," Niebur said. "And this in turn is changing our views about finding [habitable worlds] and about how prevalent and common habitable environments and even life beyond Earth might truly be."

The mission has continually adapted to the mysteries it uncovered in Saturn's system. The probe used flybys of Titan to adjust its orbit and perform 162 targeted flybys of Saturn's many moons, including 127 of Titan itself.

Saturn's moons Enceladus and Tethys line up above Saturn's rings in this 2015 photo from the Cassini spacecraft, which has been exploring Saturn's system since 2004.

"Cassini's had a long-distance, you might say, long-term relationship with Titan," Earl Maize, Cassini's project manager at NASA's Jet Propulsion Laboratory in California, said during the teleconference. "[During] each of these flybys, Titan has shared some of its secrets with us, and at the same time shaped Cassini's trajectory."

Via an international collaboration, Cassini brought Europe's Huygens lander to the Saturn system and dropped it down on Titan, giving Earthlings a stunning view of the moon's liquid oceans and complex organic atmosphere. Maize said that moment is his pick for the most amazing part of the Cassini mission.

"The collaboration… between three space agencies and all these thousands of people on the ground… [to] put a probe onto Titan, capture signal on the way down, land it softly on the surface and play those images back — I still give myself goose bumps just seeing that first image," Maize said. The Cassini mission is a cooperative project between NASA, the European Space Agency (ESA) and the Italian Space Agency.

For Linda Spilker, a Cassini project scientist at JPL, the probe's investigation of the tiny moon Enceladus was the most exciting part of the mission.

"To actually see this plume of water vapor and waterized particles coming out of the south pole of a moon that's only 300 miles [480 kilometers] across was absolutely astonishing," Spilker said at the teleconference. "And then to take instruments built for other purposes and turn them toward sampling and flying through the plumes and actually measuring the constituents — finding a salty global ocean containing organics, the possibility of hydrothermal ventsand just revealing a world that we thought was completely frozen solid when we first got to Saturn."

"Enceladus has no business existing," Niebur added, "and yet there it is — practically screaming at us, 'Look at me! I completely invalidate all of your assumptions about the solar system!'"

To protect those Enceladus and Titan from contamination with Earth life, Cassini is going to dive down into Saturn's atmosphere before the probe runs out of fuel, which could have left it drifting on a collision course with the planet's moons. Although the Huygens mission met planetary-protection requirements back in 2005, when it landed on Titan, scientists' new information about that moon's potential habitability has made researchers keen to protect it from further exposure, Cassini research scientists have said. Saturn's "Grand Finale" dive is primarily aimed to protect Enceladus, which has a higher planetary-protection standard — Titan is just a bonus, the scientists said.

For the final dive, researchers will turn their focus back to the planet itself and its rings for a dramatic, fact-finding atmospheric crash.

"The mission has been insanely, wildly, beautifully successful," Niebur said. "But Cassini will not go quietly."

The spacecraft's Grand Finale orbits have brought it closer to the gas giant than any spacecraft has traveled, traversing the gap between the planet and its rings and diving into the unknown to learn as much as possible about the planet.

"Some of our key science goals during the Grand Finale are trying to understand Saturn from the inside out, to figure out the length of a Saturn day and to determine the mass of the rings and the composition of Saturn's atmosphere," Spilker said. "Our understanding of this fascinating new data is still evolving for me and the science team. There are so many puzzles at Saturn.

"Scientists love mysteries, and the Grand Finale is providing mysteries for everyone," she added. And surely that will continue with its final dive, she said.

At the briefing, researchers described how the probe would eventually stop sending photos, but would be sending back data gathered with its instruments right up until the end. The probe's first protective blankets will burn off first as it pierced the planet's upper atmosphere — "just like you see when you re-enter the atmosphere on Earth," Julie Webster, a Cassini operations manager at JPL, said at the conference — and then the craft will reach the aluminum melting point within about 20 seconds. The probe's iridium will be the last to melt, occurring about 30 seconds after the aluminum; everything will be melted away within a minute.

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Peculiar Waves in Saturn's Rings Spotted by NASA's Cassini Probe

Wave-like images captured by Cassini as it orbits Saturn.

NASA's Cassini spacecraft has captured a spectacular photo of a perplexing wave structure in one of Saturn's rings as the probe heads into its final days at the gas giant.

The rings of Saturn are embedded with billions of water-ice particles ranging in size from grains of sand to monstrous chunks. Saturn's rings also feature waves that propagate outward in spiral patterns.

The new image from Cassini captures an up-close view of a spiral density wave visible in Saturn's B ring. The wave structure is a buildup of material that has formed from the gravitational pull of Saturn's moons, NASA officials said.

The density wave visible in Saturn's B ring originates 59,796 miles (96,233 kilometers) from the planet, where the "ring particles orbit Saturn twice for every time the moon Janus orbits once, creating an orbital resonance," according to a statement from NASA.

In the new image, the wave structure — aptly named the Janus 2:1 spiral density wave — appears to ricochet outward, away from Saturn and toward the upper-left corner of the photo, creating hundreds of bright wave crests.

The density wave is generated by the gravitational pull of Saturn's moon Janus. However, Janus and one of Saturn's other moons, Epimetheus, share practically the same orbit and swap places every four years, creating a new crest in the wave, according to the statement.

As a result, the distance between any pair of crests corresponds to four years' worth of wave oscillations. This pattern represents the orbital history of Janus and Epimetheus, much like the rings of a tree reveal information about its growth.

Based on this idea, the crests of the wave at the very upper left of the new Cassini image correspond to the positions of Janus and Epimetheus during the Saturn flybys of NASA's twin Voyager probes in 1980 and 1981, according to the statement.

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Saturn’s Icy Moons Are a Little Less Mysterious Thanks to Cassini’s Long Mission

An artist rendering of the Cassini spacecraft entering orbit around Saturn.

The Cassini spacecraft will take a death plunge into Saturn next on September 15 after more than a decade observing the planet and its moons. Perhaps its greatest contribution to science is helping us learn about the many icy moons circling Saturn and its elegant rings. Scientists are interested in finding life outside of Earth, to see if it's similar to what we have on our own planet.

Icy moons are likely our best shot at finding microbes. That's because they have a ready source of heat — in this case, tidal interactions with Saturn — as well as abundant water in the form of oceans and lakes. Cassini's observations from orbit showed us geysers, oceans, and other signs of liquid. Future missions on the drawing board might use submarines or little rovers to take a close-up look at the moons.

Here are some of Cassini's key discoveries.

Enceladus

Enceladus is perhaps the most famous icy moon of Saturn, as Cassini has tracked at least 101 geysers spouting water into space from "tiger stripes" or cracks in the ice. Late in Cassini's mission, in April 2017, researchers announced that Cassini detected hydrogen in Enceladus’s plumes, which suggests that there are hydrothermal vents in the ocean below. These vents are warm spots that, on Earth, are places where creatures tend to congregate as it is a ready-made source of energy and food.

Photograph of Enceladus, the sixth-largest moon of Saturn

Perhaps the most notable discovery was in September 2015, when researchers found evidence of a global ocean, based on how the moon wobbles as it orbits Saturn. Other key discoveries included evidence that Enceladus’s spouting water lands in Saturn's atmosphere and that the south polar area changes over time, hinting at evidence of Earth-like plate tectonics. It also recorded multiple observations of the geysers’ composition, structure, and eruption frequency.

This view of Titan, the largest of Saturn's 56 known moons, was taken on 26 December 2005 and reveals structure in the moon's complex atmosphere.

Titan

Scientists knew very little about the surface of Titan because observations showed only an orange blob. Cassini changed that forever, using radar to reveal a world of hydrocarbon-filled lakes that shift with the changing seasons.

Its mission began with the deployment of the European Space Agency’s Huygens lander, which flew to the surface in January 2005. Huygens made the first measurements from the lower atmosphere, finding — to the surprise of investigators — methane, even though the gas is typically broken down by the sun. This means that there is likely a renewable source of methane somewhere on Titan.

Cassini also made several science discoveries of its own, such as finding a disappearing and reappearing island on Titan's surface, watching the evolution of Titan's seasons, and finding evidence of an underground ocean.

Rhea, moon of the planet Saturn, assembled from a composition of multiple photos taken by the Voyager 1 spacecraft, 1980

Rhea

While Rhea is not reported on as much as Titan and Enceladus, it is notable because of its size — it's the second-largest of Saturn’s moon, although at 475 miles (764 kilometers) it is only a third of the radius of Titan. Its surface is likely made of water ice because it is so reflective of light. High reflectivity is also apparent on the small moons Dione and Tethys.

Cassini showed that Rhea likely has a mixture of ice and rock underneath its surface. Cassini also revealed canyons on Rhea — showing that the moon must have had tectonic movement long ago — as well as a wispy atmosphere of oxygen and carbon dioxide. Rings were also found in 2008.

This NASA Cassini Spacecraft image obtained 24 November, 2004 shows Saturn's icy moon Tethys

Tethys

Tethys, a small icy moon of Saturn, has mysterious red arcs on its surface that Cassini first detected in 2004. The scientists are not sure about the source of the arcs, but speculation includes chemical impurities in the ice, or perhaps fractures that are just a little bit smaller than what Cassini's cameras were able to resolve.

Cassini also found a heat signature on Titan that resembles the shape of the 1980s video game character Pac-Man. The approximately V-shaped structure likely happens when electrons, traveling at high speed around Saturn, crash onto the moon. The electron collision turns the soft surface into hard ice, which would not heat up as fast during the day, or cool down as quickly during the night, compared to the surrounding surface. A similar heat signature is present on Mimas.

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NASA Probe Cassini Enters Last Weeks as Historic Saturn Mission Comes to an End

Saturn and its rings.

It might as well be the mission motto: "Saturn continues to surprise us."

That's what Earl Maize, project manager for the Cassini mission at Saturn, said during a news conference on Aug. 29. He was referring to the fact that the inner ring system where the Cassini probe is spending some of its final weeks doesn't have as much dust as scientists anticipated, and is therefore not as harsh on the spacecraft's instruments. If the mission could last longer — if the probe weren't running out of fuel, that is — Maize said he'd love to have Cassini explore that region longer.

This trend of finding surprising or unexpected physical characteristics — of Saturn and its moons — has characterized Cassini's entire 13-year stint in the Saturnian system, the scientists said, and continues to prove true in the probe's very last weeks. On Sept. 15, Cassini will complete its "Grand Finale" set of maneuvers and crash into Saturn's atmosphere, transmitting data back to Earth right up until the spacecraft breaks apart.

On its current trajectory, Cassini has also taken the first-ever in-situ samples of Saturn's atmosphere, and Linda Spilker, the Cassini project scientist, said those early results suggest that the chemical and dynamic interactions between particles from the planet's rings and the planet's upper atmosphere are "more complex … than we had both anticipated."

That's good news, she said, because "scientists love mysteries, and the Grand Finale is providing mysteries for everyone."

A final tally of some notable numbers from Cassini’s long mission.

Ongoing Mysteries

Cassin began its "Grand Finale" in April, and the first fruits of new science from that curtain call are already sprouting, NASA scientists said during today's news briefing.

For the last few months, Cassini has been looping through Saturn's inner ring system and sampling the top of the planet's atmosphere, which is "like dipping our toe in Saturn's atmosphere, in preparation for the final plunge," Spilker said during the news briefing.

From those initial tastes of Saturn's atmosphere, Cassini scientists are already finding "incredible, intriguing information" about how the material from Saturn's ring system mixes with the upper layers of the planet's atmosphere. On Sept. 15, the probe will plummet to depths of up to 9,300 miles (15,000 kilometers).

"By having in-situ sampling of the atmosphere, we can directly measure things like the hydrogen-to-helium ratio," Spilker said. Both Jupiter and Saturn consist largely of hydrogen and helium, but the ratios of these two elements can help scientists learn about how and when those planets formed, as well as the nature of the solar system at that time.

"We can directly measure composition of… constituents at a very, very low level in the atmosphere — things that would be much harder to see from a distance with remote sensing and spectroscopy," she said.

This image shows Cassini’s last two Grand Finale orbits, followed by a distant flyby of Titan that pushes the spacecraft into Saturn (final half orbit, in orange).

Spilker said scientists also hope that Cassini's plunge will help them understand the nature of Saturn's magnetic field source, the mass of its rings, and the exact length of its day (the time it takes the planet to spin once on its axis).

Getting that data back to Earth before Cassini is destroyed has required a change in the probe's basic data-transmission system, the scientists said. Typically, Cassini stores its data on a hard drive and transmits the information back to Earth much later, but during its final plummet, Cassini will sop up information from the planet and transmit it back to Earth almost immediately. That direct transmission will start about 3 hours before Cassini hits Saturn's atmosphere.

"A 2- to 3-second latency is all we're expecting," Maize said (referring to the time between Cassini collecting data and transmitting it). "So, we will have repurposed Cassini into an atmospheric probe, and we will have it broadcasting data back down to the very last minute."

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Cassini says goodbye to a true Titan

These two views of Saturn's moon Titan exemplify how NASA's Cassini spacecraft has revealed the surface of this fascinating world.

Mere weeks away from its dramatic, mission-ending plunge into Saturn, NASA's Cassini spacecraft has a hectic schedule, orbiting the planet every week in its Grand Finale. On a few orbits, Saturn's largest moon, Titan, has been near enough to tweak Cassini's orbit, causing the spacecraft to approach Saturn a bit closer or a bit farther away. A couple of those distant passes even pushed Cassini into the inner fringes of Saturn's rings.

Titan will be waiting once again when the road runs out in September. A last, distant encounter with the moon on Sept. 11 will usher Cassini to its fate, with the spacecraft sending back precious science data until it loses contact with Earth.

But this gravitational pushing and shoving isn't a new behavior for Titan. It's been doing that all along, by design.

The True Engine of the Mission

Repeated flybys of Titan were envisioned, from the mission's beginning, as a way to explore the mysterious planet-size moon and to fling Cassini toward its adventures in the Saturn system. Scientists had been eager for a return to Titan since NASA's Voyager 1 spacecraft flew past in 1980 and was unable to see through the dense, golden haze that shrouds its surface.

Titan is just a bit larger than the planet Mercury. Given its size, the moon has significant gravity, which is used for bending Cassini's course as it orbits Saturn. A single close flyby of Titan could provide more of a change in velocity than the entire 90-minute engine burn the spacecraft needed to slow down and be captured by Saturn's gravity upon its arrival in 2004.

The mission's tour designers -- engineers tasked with plotting the spacecraft's course, years in advance -- used Titan as their linchpin. Frequent passes by the moon provided the equivalent of huge amounts of rocket propellant. Using Titan, Cassini's orbit could be stretched out, farther from Saturn -- for example, to send the spacecraft toward the distant moon Iapetus. With this technique, engineers used Titan flybys to change the orientation of Cassini's orbit many times during the mission; for example, lifting the spacecraft out of the plane of the rings to view them from high above, along with high northern and southern latitudes on Saturn and its moons.

What We've Learned

Over the course of its 13-year mission at Saturn, Cassini has made 127 close flybys of Titan, with many more-distant observations. Cassini also dropped off the European Space Agency's Huygens probe, which descended through Titan's atmosphere to land on the surface in January 2005.

Successes for Cassini during its mission include the revelation that, as researchers had theorized, there were indeed bodies of open liquid hydrocarbons on Titan's surface. Surprisingly, it turned out Titan's lakes and seas are confined to the poles, with almost all of the liquid being at northern latitudes in the present epoch. Cassini found that most of Titan has no lakes, with vast stretches of linear dunes closer to the equator similar to those in places like Namibia on Earth. The spacecraft observed giant hydrocarbon clouds hovering over Titan's poles and bright, feathery ones that drifted across the landscape, dropping methane rain that darkened the surface. There were also indications of an ocean of water beneath the moon's icy surface.

Early on, Cassini's picture of Titan was spotty, but every encounter built upon the previous one. Over the course of the entire mission, Cassini's radar investigation imaged approximately 67 percent of Titan's surface, using the spacecraft's large, saucer-shaped antenna to bounce signals off the moon's surface. Views from Cassini's imaging cameras, infrared spectrometer, and radar slowly and methodically added details, building up a more complete, high-resolution picture of Titan.

"Now that we've completed Cassini's investigation of Titan, we have enough detail to really see what Titan is like as a world, globally," said Steve Wall, deputy lead of Cassini's radar team at NASA's Jet Propulsion Laboratory in Pasadena, California.

Scientists now have enough data to understand the distribution of Titan's surface features (like mountains, dunes and seas) and the behavior of its atmosphere over time, and they have been able to begin piecing together how surface liquids might migrate from pole to pole.

Among the things that remain uncertain is exactly how the methane in Titan's atmosphere is being replenished, since it's broken down over time by sunlight. Scientists see some evidence of volcanism, with methane-laden water as the "lava," but a definitive detection remains elusive.

Cassini's long-term observations could still provide clues. Researchers have been watching for summer rain clouds to appear at the north pole, as their models predicted. Cassini observed rain clouds at the south pole in southern summer in 2004. But so far, clouds at high northern latitudes have been sparse.

"The atmosphere seems to have more inertia than most models have assumed. Basically, it takes longer than we thought for the weather to change with the seasons," said Elizabeth Turtle, a Cassini imaging team associate at Johns Hopkins Applied Physics Laboratory, Laurel, Maryland.

The sluggish arrival of northern summer clouds may match better with models that predict a global reservoir of methane, Turtle said. "There isn't a global reservoir at the surface, so if one exists in the subsurface that would be a major revelation about Titan." This points to the value of Cassini's long-term monitoring of Titan's atmosphere, she said, as the monitoring provides data that can be used to test models and ideas.

Results from the Last Close Pass

Cassini made its last close flyby of Titan on April 22. That flyby gave the spacecraft the push it needed to leap over Saturn's rings and begin its final series of orbits, which pass between the rings and the planet.

During that flyby, Cassini's radar was in the driver's seat -- its observation requirements determining how the spacecraft would be oriented as it passed low over the surface one last time at an altitude of 608 miles (979 kilometers). One of the priorities was to have one last look for the mysterious features the team dubbed "magic islands," which had appeared and then vanished in separate observations taken years apart. On the final pass there were no magic islands to be seen. The radar team is still working to understand what the features might have been, with leading candidates being bubbles or waves.

Most interesting to the radar team was a set of observations that was both the first and last of its kind, in which the instrument was used to sound the depths of several of the small lakes that dot Titan's north polar region. Going forward, the researchers will be working to tease out information from these data about the lakes' composition, in terms of methane versus ethane.

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Cassini Spacecraft Captures Breathtaking Image of Sunrise on Saturn

This is what dawn on Saturn looks like, from afar.

The ringed planet is partly hidden in darkness and partly illuminated by the faint light of a distant sun in a gorgeous photo taken by NASA's Cassini spacecraft.

"The light has traveled around 80 minutes since it left the sun's surface by the time it reaches Saturn," NASA officials wrote in a description of the image, which was released yesterday (July 10). "The illumination it provides is feeble; Earth gets 100 times the intensity, since it's roughly 10 times closer to the sun. Yet compared to the deep blackness of space, everything at Saturn still shines bright in the sunlight, be it direct or reflected."[Cassini's Saturn 'Grand Finale' Plan in Pictures]

Cassini took the photo on Feb. 25, at a distance of about 762,000 miles (1.23 million kilometers) from the ringed planet.

The $3.2 billion Cassini-Huygens mission — a joint effort of NASA, the European Space Agency, and the Italian Space Agency — has been orbiting Saturn since July 2004, but its days are numbered: The spacecraft is scheduled to plunge into the gas giant's cloud tops on Sept. 15, in an intentional death dive designed to ensure that Cassini doesn't contaminate the Saturn moons Titan or Enceladus with microbes from Earth. (Both Titan and Enceladus may be capable of supporting life, scientists have said.)

Huygens was a piggyback lander that separated from the Cassini mothership and made a historic touchdown on Titan in January 2005 — the first soft touchdown ever achieved on a body in the outer solar system.

From Seeker

Cassini Gets ‘Ringside Seat’ to Saturn's Changing Seasons

The hexagonal vortex at Saturn's north pole changed color significantly between June 2013 (left) and April 2017 (right), as seen in views from the Cassini spacecraft. For the left image, each frame occurs approximately 130 minutes after the previous one, and for the right, each frame follows after an average of 230 minutes. Researchers combined images taken with the spacecraft's red, green and blue filters for the natural-color views.

The Cassini spacecraft at Saturn watched over the ringed planet's solstice Wednesday (May 24), accomplishing the main goal of its second extended mission. A solstice occurs on Saturn roughly every 15 Earth years as its seasons change.

Cassini arrived at Saturn in 2004, and the spacecraft completed its primary mission to study the planet, its rings and its moons by 2008. Its first extended mission, which lasted until 2010, was to observe the system during the planet's equinox, when the sun strikes the rings edge-on and the days are of equal length on the north and south poles. The goal of its second extended mission — a seven-year plan called the Solstice Mission — was to observe all the way up to the north pole's summer solstice (when the days are longest at Saturn's north pole, and shortest at the planet's south pole) and investigate the system's seasonal changes.

"During Cassini's Solstice Mission, we have witnessed — up close for the first time — an entire season at Saturn," Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory (JPL) in California, said in a statement. "The Saturn system undergoes dramatic transitions from winter to summer, and thanks to Cassini, we had a ringside seat."

A giant storm passed by its own tail after circling Saturn in this true-color Cassini photo, taken Feb. 25, 2011, about 12 weeks after the storm started. Lightning deep in the planet's atmosphere produced significant radio noise during the storm. The storm formed and dispersed over the course of seven months; researchers said this type of Saturn storm happens only about every 30 Earth years.

During the Solstice Mission, researchers watched a huge storm encircle the planet and disperse over the course of seven months. They watched the hexagonal jet stream surrounding Saturn's north pole change from blue to yellow (except for the very center) over the course of the northern hemisphere's spring.

Cassini data suggests that the increased sunlight interacts with compounds in the upper atmosphere to form particles called photochemical aerosols, which accumulate into a yellowish haze. The very center may stay blue for one of two reasons, researchers said in an image caption: it hasn't been exposed to sunlight as long as the areas around it because it's at the very top of the planet, or the circulation in the whirling vortex pulls the compounds downward.

Cassini saw the seasonal changes come over the planet suddenly based on latitude, rather than gradually, the researchers said.

"Eventually, a whole hemisphere undergoes change, but it gets there by these jumps at specific latitude bands at different times in the season," Robert West, a member of Cassini's imaging team at JPL, said in the statement.

The solstice's intense sunlight heated Saturn's rings, letting Cassini's instruments better investigate ring particles' composition and the way they clump together. The planet's orientation compared to the sun and Earth, with its rings tipped maximally toward Earth, also meant that Cassini could easily send a radio signal through the densest part of the rings to investigate, according to the statement.

Storms on Saturn's moon Titan changed as Saturn moved through equinox and toward solstice. In this 2011 image, methane clouds can be seen concentrated near the moon's equator.

On Saturn's largest moon, Titan, methane storm clouds shifted up from the south toward the moon's equator from 2004 to 2010. Their corresponding shift northward as the solstice approached was surprisingly slow; according to the statement, cloud models had expected the activity to happen years earlier.

But some action was sudden: In 2013, haze and trace hydrocarbons formerly found only in the north suddenly built up in the Titan's south, indicating that its atmospheric circulation had changed direction due to the changing sun exposure, the statement said.

"Observations of how the locations of cloud activity change and how long such changes take give us important information about the workings of Titan's atmosphere and also its surface, as rainfall and wind patterns change with the seasons too," Elizabeth Turtle, a researcher on Cassini's imaging team at Johns Hopkins University's Applied Physics Laboratory in Maryland, said in the statement.

And on Saturn's moon Enceladus, which hosts a subsurface ocean and blasts geysers of material out through its plumes, the main seasonal change was its southern hemisphere's transition to winter darkness. This change let Cassini monitor the moon's temperature more easily, to investigate the intriguing moon.

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Video Shows Cassini's View During First Saturn Ring Dive

An amazing new video shows just what NASA's Cassini spacecraft saw during its first "Grand Finale" plunge between Saturn's cloud tops and the gas giant's rings last week.

The new Saturn dive video captures about an hour of Cassini observations on April 26, starting near the planet's north polar vortex, and the bizarre hexagonal jet stream that surrounds it, and heading south from there.

"I was surprised to see so many sharp edges along the hexagon's outer boundary and the eye-wall of the polar vortex," Cassini imaging team associate Kunio Sayanagi, who's based at Hampton University in Virginia, said in a statement.

"Something must be keeping different latitudes from mixing to maintain those edges," added Sayanagi, who helped produce the new video.

The view shifts over the course of the dive video in several different ways. For example, Cassini gets closer to Saturn's cloud tops, dropping in altitude from 45,000 miles to 4,200 miles (72,400 to 6,700 kilometers). As a result, image resolution changes from 5.4 miles (8.7 km) per pixel to 0.5 mile (0.8 km) per pixel, NASA officials said.

In addition, near the end of the movie, "the camera frame rotates as the spacecraft reorients to point its large, saucer-shaped antenna in the direction of the spacecraft’s motion," NASA officials wrote in the same statement. "The antenna was used as a protective shield during the crossing of Saturn's ring plane."

That shield didn't end up taking very many hits; Cassini's first dive revealed that the narrow gap between Saturn and its innermost rings is surprisingly empty.

The $3.2 billion Cassini-Huygens mission — a joint effort of NASA, the European Space Agency and the Italian Space Agency — launched in October 1997 and reached the Saturn system in July 2004. (Huygens was a lander that the Cassini mothership helped deliver to the surface of the ringed planet's largest moon, Titan, in January 2005.)

The Cassini orbiter is almost out of fuel, so mission controllers are leading the probe through its last few months of life. This Grand Finale phase consists of 22 daring dives through the Saturn rings' gap, with each one coming about 6.5 days after the last.

The first plunge occurred on April 26 and the second on Tuesday night (May 2). The third one will take place in the early morning hours of May 9 EDT.

There is still a lot more to be learned from these additional dives, mission team members said.

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