Scientists find oceans of water on Mars: It's just too deep to tap

Using seismic activity to probe the interior of Mars, geophysicists have found evidence for a large underground reservoir of liquid water -- enough to fill oceans on the planet's surface.

The data from NASA's Insight lander allowed the scientists to estimate that the amount of groundwater could cover the entire planet to a depth of between 1 and 2 kilometers, or about a mile.

While that's good news for those tracking the fate of water on the planet after its oceans disappeared more than 3 billion years ago, the reservoir won't be of much use to anyone trying to tap into it to supply a future Mars colony. It's located in tiny cracks and pores in rock in the middle of the Martian crust, between 11.5 and 20 kilometers below the surface. Even on Earth, drilling a hole a kilometer deep is a challenge.

The finding does pinpoint another promising place to look for life on Mars, however, if the reservoir can be accessed. For the moment, it helps answer questions about the geological history of the planet.

"Understanding the Martian water cycle is critical for understanding the evolution of the climate, surface and interior," said Vashan Wright, a former UC Berkeley postdoctoral fellow who is now an assistant professor at UC San Diego's Scripps Institution of Oceanography. "A useful starting point is to identify where water is and how much is there."

Wright, alongside colleagues Michael Manga of UC Berkeley and Matthias Morzfeld of Scripps Oceanography, detailed their analysis in a paper that will appear this week in the journal Proceedings of the National Academy of Sciences.

The scientists employed a mathematical model of rock physics, identical to models used on Earth to map underground aquifers and oil fields, to conclude that the seismic data from Insight are best explained by a deep layer of fractured igneous rock saturated with liquid water. Igneous rocks are cooled hot magma, like the granite of the Sierra Nevada.

"Establishing that there is a big reservoir of liquid water provides some window into what the climate was like or could be like," said Manga, a UC Berkeley professor of earth and planetary science. "And water is necessary for life as we know it. I don't see why [the underground reservoir] is not a habitable environment. It's certainly true on Earth -- deep, deep mines host life, the bottom of the ocean hosts life. We haven't found any evidence for life on Mars, but at least we have identified a place that should, in principle, be able to sustain life."

Manga was Wright's postdoctoral adviser. Morzfeld was a former postdoctoral fellow in UC Berkeley's mathematics department and is now an associate professor of geophysics at Scripps Oceanography.

Manga noted that lots of evidence -- river channels, deltas and lake deposits, as well as water-altered rock -- supports the hypothesis that water once flowed on the planet's surface. But that wet period ended more than 3 billion years ago, after Mars lost its atmosphere. Planetary scientists on Earth have sent many probes and landers to the planet to find out what happened to that water -- the water frozen in Mars' polar ice caps can't account for it all -- as well as when it happened, and whether life exists or used to exist on the planet.

The new findings are an indication that much of the water did not escape into space but filtered down into the crust.

The Insight lander was sent by NASA to Mars in 2018 to investigate the crust, mantle, core and atmosphere, and it recorded invaluable information about Mars' interior before the mission ended in 2022.

"The mission greatly exceeded my expectations," Manga said. "From looking at all the seismic data that Insight collected, they've figured out the thickness of the crust, the depth of the core, the composition of the core, even a little bit about the temperature within the mantle."

Insight detected Mars quakes up to about a magnitude of 5, meteor impacts and rumblings from volcanic areas, all of which produced seismic waves that allowed geophysicists to probe the interior.

An earlier paper reported that above a depth of about 5 kilometers, the upper crust did not contain water ice, as Manga and others suspected. That may mean that there's little accessible frozen groundwater outside the polar regions.

The new paper analyzed the deeper crust and concluded that the "available data are best explained by a water-saturated mid-crust" below Insight's location. Assuming the crust is similar throughout the planet, the team argued, there should be more water in this mid-crust zone than the "volumes proposed to have filled hypothesized ancient Martian oceans."

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First-ever atomic freeze-frame of liquid water

In an experiment akin to stop-motion photography, scientists have isolated the energetic movement of an electron while "freezing" the motion of the much larger atom it orbits in a sample of liquid water.

The findings, reported today in the journal Science, provide a new window into the electronic structure of molecules in the liquid phase on a timescale previously unattainable with X-rays. The new technique reveals the immediate electronic response when a target is hit with an X-ray, an important step in understanding the effects of radiation exposure on objects and people.

"The chemical reactions induced by radiation that we want to study are the result of the electronic response of the target that happens on the attosecond timescale," said Linda Young, a senior author of the research and Distinguished Fellow at Argonne National Laboratory. "Until now radiation chemists could only resolve events at the picosecond timescale, a million times slower than an attosecond. It's kind of like saying 'I was born and then I died.' You'd like to know what happens in between. That's what we are now able to do."

A multi-institutional group of scientists from several Department of Energy national laboratories and universities in the U.S. and Germany combined experiments and theory to reveal in real-time the consequences when ionizing radiation from an X-ray source hits matter.

Working on the time scales where the action happens will allow the research team to understand complex radiation-induced chemistry more deeply. Indeed, these researchers initially came together to develop the tools needed to understand the effect of prolonged exposure to ionizing radiation on the chemicals found in nuclear waste. The research is supported by the Interfacial Dynamics in Radioactive Environments and Materials (IDREAM) Energy Frontier Research Center sponsored by the Department of Energy and headquartered at Pacific Northwest National Laboratory (PNNL).

"Members of our early-career network participated in the experiment, and then joined our full experimental and theoretical teams to analyze and understand the data," said Carolyn Pearce, IDREAM EFRC director and a PNNL chemist. "We couldn't have done this without the IDREAM partnerships."

From the Nobel Prize to the field

Subatomic particles move so fast that capturing their actions requires a probe capable of measuring time in attoseconds, a time frame so small that there are more attoseconds in a second than there have been seconds in the history of the universe.

The current investigation builds upon the new science of attosecond physics, recognized with the 2023 Nobel Prize in Physics. Attosecond X-ray pulses are only available in a handful of specialized facilities worldwide. This research team conducted their experimental work at the Linac Coherent Light Source (LCLS), located at SLAC National Accelerator Laboratory, in Menlo Park, Calif, where the local team pioneered the development of attosecond X-ray free-electron lasers.

"Attosecond time-resolved experiments are one of the flagship R&D developments at the Linac Coherent Light Source," said Ago Marinelli from the SLAC National Accelerator Laboratory, who, together with James Cryan, led the development of the synchronized pair of X-ray attosecond pump/probe pulses that this experiment used. "It's exciting to see these developments being applied to new kinds of experiments and taking attosecond science into new directions."

The technique developed in this study, all X-ray attosecond transient absorption spectroscopy in liquids, allowed them to "watch" electrons energized by X-rays as they move into an excited state, all before the bulkier atomic nucleus has time to move. They chose the liquid water as their test case for an experiment.

"We now have a tool where, in principle, you can follow the movement of electrons and see newly ionized molecules as they're formed in real-time," said Young, who is also a professor in the Department of Physics and James Franck Institute at the University of Chicago.

These newly reported findings resolve a long-standing scientific debate about whether X-ray signals seen in previous experiments are the result of different structural shapes, or "motifs," of water or hydrogen atom dynamics. These experiments demonstrate conclusively that those signals are not evidence for two structural motifs in ambient liquid water.

"Basically, what people were seeing in previous experiments was the blur caused by moving hydrogen atoms," said Young. "We were able to eliminate that movement by doing all of our recording before the atoms had time to move."

From simple to complex reactions

The researchers envision the current study as the beginning of a whole new direction for attosecond science.

To make the discovery, PNNL experimental chemists teamed with physicists at Argonne and the University of Chicago, X-ray spectroscopy specialists and accelerator physicists at SLAC, theoretical chemists at the University of Washington, and attosecond science theoreticians from the Hamburg Centre for Ultrafast Imaging and the Center for Free-Electron Laser Science (CFEL), Deutsches Elektronen-Synchrotron (DESY), in Hamburg, Germany.

During the global pandemic, in 2021 and into 2022, the PNNL team used techniques developed at SLAC to spray an ultra-thin sheet of pure water across the X-ray pump pulse path.

"We needed a nice, flat, thin sheet of water where we could focus the X-rays," said Emily Nienhuis, an early-career chemist at PNNL, who started the project as a post-doctoral research associate. "This capability was developed at the LCLS." At PNNL, Nienhuis demonstrated that this technique can also be used to study the specific concentrated solutions that are central to the IDREAM EFRC and will be investigated at the next stage of the research.

From experiment to theory

Once the X-ray data had been collected, theoretical chemist Xiaosong Li and graduate student Lixin Lu from the University of Washington applied their knowledge of interpreting the X-ray signals to reproduce the signals observed at SLAC. The CFEL team, led by theoretician Robin Santra, modelled the liquid water response to attosecond X-rays to verify that the observed signal was indeed confined to the attosecond timescale.

"Using the Hyak supercomputer at the University of Washington, we developed a cutting-edge computational chemistry technique that enabled detailed characterization of the transient high-energy quantum states in water," said Li, the Larry R. Dalton Endowed Chair in Chemistry at the University of Washington and a Laboratory Fellow at PNNL. "This methodological breakthrough yielded a pivotal advancement in the quantum-level understanding of ultrafast chemical transformation, with exceptional accuracy and atomic-level detail."

Principal Investigator Young originated the study and supervised its execution, which was led on-site by first author and postdoc Shuai Li. Physicist Gilles Doumy, also of Argonne, and graduate student Kai Li of the University of Chicago were part of the team that conducted the experiments and analyzed the data. Argonne's Center for Nanoscale Materials, a DOE Office of Science user facility, helped characterize the water sheet jet target.

Together, the research team got a peek at the real-time motion of electrons in liquid water while the rest of the world stood still.

"The methodology we developed permits the study of the origin and evolution of reactive species produced by radiation-induced processes, such as encountered in space travel, cancer treatments, nuclear reactors and legacy waste," said Young.

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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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Some icy exoplanets may have habitable oceans and geysers

A NASA study expands the search for life beyond our solar system by indicating that 17 exoplanets (worlds outside our solar system) could have oceans of liquid water, an essential ingredient for life, beneath icy shells. Water from these oceans could occasionally erupt through the ice crust as geysers. The science team calculated the amount of geyser activity on these exoplanets, the first time these estimates have been made. They identified two exoplanets sufficiently close where signs of these eruptions could be observed with telescopes.

The search for life elsewhere in the Universe typically focuses on exoplanets that are in a star's "habitable zone," a distance where temperatures allow liquid water to persist on their surfaces. However, it's possible for an exoplanet that's too distant and cold to still have an ocean underneath an ice crust if it has enough internal heating. Such is the case in our solar system where Europa, a moon of Jupiter, and Enceladus, a moon of Saturn, have subsurface oceans because they 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. On Earth, entire ecosystems thrive in complete darkness at the bottom of oceans near hydrothermal vents, which provide energy and nutrients.

"Our analyses predict that these 17 worlds may have ice-covered surfaces but receive enough internal heating from the decay of radioactive elements and tidal forces from their host stars to maintain internal oceans," said Dr. Lynnae Quick of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Thanks to the amount of internal heating they experience, all planets in our study could also exhibit cryovolcanic eruptions in the form of geyser-like plumes." Quick is lead author of a paper on the research published on October 4 in the Astrophysical Journal.

The team considered conditions on 17 confirmed exoplanets that are roughly Earth-sized but less dense, suggesting that they could have substantial amounts of ice and water instead of denser rock. Although the planets' exact compositions remain unknown, initial estimates of their surface temperatures from previous studies all indicate that they are much colder than Earth, suggesting that their surfaces could be covered in ice.

The study improved estimates of each exoplanet's surface temperature by recalculating using the known surface brightness and other properties of Europa and Enceladus as models. The team also estimated the total internal heating in these exoplanets by using the shape of each exoplanet's orbit to get the heat generated from tides and adding it to the heat expected from radioactive activity. Surface temperature and total heating estimates gave the ice layer thickness for each exoplanet since the oceans cool and freeze at the surface while being heated from the interior. Finally, they compared these figures to Europa's and used estimated levels of geyser activity on Europa as a conservative baseline to estimate geyser activity on the exoplanets.

They predict that surface temperatures are colder than previous estimates by up to 60 degrees Fahrenheit (16 degrees Celsius). Estimated ice shell thickness ranged from about 190 feet (58 meters) for Proxima Centauri b and one mile (1.6 kilometers) for LHS 1140 b to 24 miles (38.6 kilometers) for MOA 2007 BLG 192Lb, compared to Europa's estimated average of 18 miles (almost 29 kilometers). Estimated geyser activity went from just 17.6 pounds per second (about 8 kilograms/second) for Kepler 441b to 639,640 pounds/second (290,000 kilograms/second) for LHS 1140b and 13.2 million pounds/second (six million kilograms/second) for Proxima Centauri b, compared to Europa at 4,400 pounds/second (2,000 kilograms/second).

"Since our models predict that oceans could be found relatively close to the surfaces of Proxima Centauri b and LHS 1140 b, and their rate of geyser activity could exceed Europa's by hundreds to thousands of times, telescopes are most likely to detect geological activity on these planets," said Quick, who is presenting this research December 12at the American Geophysical Union meeting in San Francisco, California.

This activity could be seen when the exoplanet passes in front of its star. Certain colors of starlight could be dimmed or blocked by water vapor from the geysers. "Sporadic detections of water vapor in which the amount of water vapor detected varies with time, would suggest the presence of cryovolcanic eruptions," said Quick. The water might contain other elements and compounds that could reveal if it can support life. Since elements and compounds absorb light at specific "signature" colors, analysis of the starlight would let scientists determine the geyser's composition and evaluate the exoplanet's habitability potential.

For planets like Proxima Centauri b that don't cross their stars from our vantage point, geyser activity could be detected by powerful telescopes that are able to measure light that the exoplanet reflects while orbiting its star. Geysers would expel icy particles at the exoplanet's surface which would cause the exoplanet to appear very bright and reflective.

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Gullies on Mars could have been formed by recent periods of liquid meltwater, study suggests

A study led by Brown University researchers offers new insights into how water from melting ice could have played a recent role in the formation of ravine-like channels that cut down the sides of impact craters on Mars.

The study, published in Science, focuses on Martian gullies, which look eerily similar to gullies that form on Earth in the Dry Valleys of Antarctica and are caused by water erosion from melting glaciers. The researchers, including Brown planetary scientist Jim Head, built a model that simulates a sweet spot for when conditions on Mars allow the planet to warm above freezing temperatures, leading to periods of liquid water on Mars when ice on and beneath the surface melts.

The scientists found that when Mars tilts on its axis to 35 degrees, the atmosphere becomes dense enough for brief episodes of melting to occur at gully locations. They then matched the data from their model to periods in Mars history when the gullies in the planet's Terra Sirenum region are believed to have expanded rapidly downhill from high elevation points -- a phenomenon that could not be explained without the occasional presence of water.

"We know from a lot of our research and other people's research that early on in Mars history, there was running water on the surface with valley networks and lakes," said Head, a professor of geological sciences at Brown. "But about 3 billion years ago, all of that liquid water was lost, and Mars became what we call a hyper-arid or polar desert. We show here that even after that and in the recent past, when Mars' axis tilts to 35 degrees, it heats up sufficiently to melt snow and ice, bringing liquid water back until temperatures drop and it freezes again."

The findings help fill in some of the missing gaps on how these gullies formed, including how high they start, how severe the erosion is and how far they extend down the side of craters.

Previous theories suggest Martian gullies were carved by carbon dioxide frost, which evaporates from soil, causing rock and rubble to slide down slopes. The height of the gullies made many scientists theorize that meltwater from glaciers had to be involved because of the distance they traveled down the slopes and how eroded the gullies looked. Proving liquid water could exist on Mars since it disappeared so long ago has been difficult because temperatures typically hover about 70 degrees below freezing.

The results from the new study suggest that gully formation was driven by periods of melting ice and by CO2 frost evaporation in other parts of the year. The researchers found this has likely occurred repeatedly over the past several million years with the most recent occurrence about 630,000 years ago.

They say that if ice was present at gully locations in the areas they looked at when Mars' axis tilted to about 35 degrees, the conditions would have been right for the ice to melt because temperatures rose above 273 degrees Kelvin, equivalent to about 32 degrees Fahrenheit.

"Our study shows that the global distribution of gullies is better explained by liquid water over the last million years," said Jay Dickson, the study's lead author and a former researcher at Brown who's now at California Institute of Technology. "Water explains the elevation distribution of gullies in ways that CO2 cannot. This means that Mars has been able to create liquid water in enough volume to erode channels within the last million years, which is very recent on the scale of Mars geologic history."

Despite doubts about meltwater being possible and scientists never being able to model the right conditions on Mars for ice to melt, the researchers were convinced that the meltwater theory was accurate because they had seen similar features firsthand in Antarctica. There, despite the cold temperatures, the sun is able to heat ice just enough for it to melt and for gully activity to occur.

The new study is a continuation of previous research the team started decades earlier looking at Martian gullies. In a 2015 study, for instance, the researchers showed it was possible that there may have been past periods on Mars when water was available to form gullies if Mars tilted on its axis enough. The findings encouraged them to model what that tilt was and match it with the locations and altitudes of gullies that have formed.

The paper raises anew the fundamental question of whether life could exist on Mars. This is because life, as it's known on Earth, goes hand in hand with the presence of liquid water. Mars will eventually tilt to 35 degrees again, the researchers said.

"Could there be a bridge, if you will, between the early warm and wet Mars and the Mars that we see today in terms of liquid water?" Head said. "Everybody's always looking for environments that could be conducive to not just the formation of life but the preservation and continuation of it. Any microorganism that might have evolved in early Mars is going to be in places where they can be comfortable in ice and then also comfortable or prosperous in liquid water. In the frigid Antarctic environment, for example, the few organisms that exist often occur in stasis, waiting for water."

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Pass the salt: This space rock holds clues as to how Earth got its water

Sodium chloride, better known as table salt, isn't exactly the type of mineral that captures the imagination of scientists. However, a smattering of tiny salt crystals discovered in a sample from an asteroid has researchers at the University of Arizona Lunar and Planetary Laboratory excited, because these crystals can only have formed in the presence of liquid water.

Even more intriguing, according to the research team, is the fact that the sample comes from an S-type asteroid, a category known to mostly lack hydrated, or water-bearing, minerals. The discovery strongly suggests that a large population of asteroids hurtling through the solar system may not be as dry as previously thought. The finding, published in Nature Astronomy, gives renewed push to the hypothesis that most, if not all, water on Earth may have arrived by way of asteroids during the planet's tumultuous infancy.

Tom Zega, the study's senior author and a professor of planetary sciences at the UArizona Lunar and Planetary Laboratory, and Shaofan Che, lead study author and a postdoctoral fellow at the Lunar and Planetary Laboratory, performed a detailed analysis of samples collected from asteroid Itokawa in 2005 by the Japanese Hayabusa mission and brought to Earth in 2010.

The study is the first to demonstrate that the salt crystals originated on the asteroid's parent body, ruling out any possibility they might have formed as a consequence of contamination after the sample reached Earth, a question that had plagued previous studies that found sodium chloride in meteorites of a similar origin.

"The grains look exactly like what you would see if you took table salt at home and placed it under an electron microscope," Zega said. "They're these nice, square crystals. It was funny, too, because we had many spirited group meeting conversations about them, because it was just so unreal."

Zega said the samples represent a type of extraterrestrial rock known as an ordinary chondrite. Derived from so-called S-type asteroids such as Itokawa, this type makes up about 87% of meteorites collected on Earth. Very few of them have been found to contain water-bearing minerals.

"It has long been thought that ordinary chondrites are an unlikely source of water on Earth," said Zega who is the director of the Lunar and Planetary Laboratory's Kuiper Materials Imaging & Characterization Facility. "Our discovery of sodium chloride tells us this asteroid population could harbor much more water than we thought."

Today, scientists largely agree that Earth, along with other rocky planets such as Venus and Mars, formed in the inner region of the roiling, swirling cloud of gas and dust around the young sun, known as the solar nebula, where temperatures were very high -- too high for water vapor to condense from the gas, according to Che.

"In other words, the water here on Earth had to be delivered from the outer reaches of the solar nebula, where temperatures were much colder and allowed water to exist, most likely in the form of ice," Che said. "The most likely scenario is that comets or another type of asteroid known as C-type asteroids, which resided farther out in the solar nebula, migrated inward and delivered their watery cargo by impacting the young Earth."

The discovery that water could have been present in ordinary chondrites, and therefore been sourced from much closer to the sun than their "wetter" kin, has implications for any scenario attempting to explain the delivery of water to the early Earth.

The sample used in the study is a tiny dust particle spanning about 150 micrometers, or roughly twice the diameter of a human hair, from which the team cut a small section about 5 microns wide -- just large enough to cover a single yeast cell -- for the analysis.

Using a variety of techniques, Che was able to rule out that the sodium chloride was the result of contamination from sources such as human sweat, the sample preparation process or exposure to laboratory moisture.

Because the sample had been stored for five years, the team took before and after photos and compared them. The photos showed that the distribution of sodium chloride grains inside the sample had not changed, ruling out the possibility that any of the grains were deposited into the sample during that time. In addition, Che performed a control experiment by treating a set of terrestrial rock samples the same as the Itokawa sample and examining them with an electron microscope.

"The terrestrial samples did not contain any sodium chloride, so that convinced us the salt in our sample is native to the asteroid Itokawa," he said. "We ruled out every possible source of contamination."

Zega said tons of extraterrestrial matter is raining down on Earth every day, but most of it burns up in the atmosphere and never makes it to the surface.

"You need a large enough rock to survive entry and deliver that water," he said.

Previous work led by the late Michael Drake, a former director of the Lunar and Planetary Lab, in the 1990s proposed a mechanism by which water molecules in the early solar system could become trapped in asteroid minerals and even survive an impact on Earth.

"Those studies suggest several oceans worth of water could be delivered just by this mechanism," Zega said. "If it now turns out that the most common asteroids may be much 'wetter' than we thought, that will make the water delivery hypothesis by asteroids even more plausible."

Itokawa is a peanut-shaped near-Earth asteroid about 2,000 feet long and 750 feet in diameter and is believed to have broken off from a much larger parent body. According to Che and Zega, it is conceivable that frozen water and frozen hydrogen chloride could have accumulated there, and that naturally occurring decay of radioactive elements and frequent bombardment by meteorites during the solar system's early days could have provided enough heat to sustain hydrothermal processes involving liquid water. Ultimately, the parent body would have succumbed to the pummeling and broken up into smaller fragments, leading to the formation of Itokawa.

"Once these ingredients come together to form asteroids, there is a potential for liquid water to form," Zega said. "And once you have liquids form, you can think of them as occupying cavities in the asteroid, and potentially do water chemistry."

The evidence pointing at the salt crystals in the Itokawa sample as being there since the beginning of the solar system does not end here, however. The researchers found a vein of plagioclase, a sodium-rich silicate mineral, running through the sample, enriched with sodium chloride.

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One-third of galaxy's most common planets could be in habitable zone

Our familiar, warm, yellow sun is a relative rarity in the Milky Way. By far the most common stars are considerably smaller and cooler, sporting just half the mass of our sun at most. Billions of planets orbit these common dwarf stars in our galaxy.

To capture enough warmth to be habitable, these planets would need to huddle very close to their small stars, which leaves them susceptible to extreme tidal forces.

In a new analysis based on the latest telescope data, University of Florida astronomers have discovered that two-thirds of the planets around these ubiquitous small stars could be roasted by these tidal extremes, sterilizing them. But that leaves one-third of the planets -- hundreds of millions across the galaxy -- that could be in a goldilocks orbit close enough, and gentle enough, to hold onto liquid water and possibly harbor life.

UF astronomy professor Sarah Ballard and doctoral student Sheila Sagear published their findings the week of May 29 in the Proceedings of the National Academy of Sciences. Ballard and Sagear have long studied exoplanets, those worlds that orbit stars other than the sun.

"I think this result is really important for the next decade of exoplanet research, because eyes are shifting toward this population of stars," Sagear said. "These stars are excellent targets to look for small planets in an orbit where it's conceivable that water might be liquid and therefore the planet might be habitable."

Sagear and Ballard measured the eccentricity of a sample of more than 150 planets around these M dwarf stars, which are about the size of Jupiter. The more oval shaped an orbit, the more eccentric it is. If a planet orbits close enough to its star, at about the distance that Mercury orbits the sun, an eccentric orbit can subject it to a process known as tidal heating. As the planet is stretched and deformed by changing gravitational forces on its irregular orbit, friction heats it up. At the extreme end, this could bake the planet, removing all chance for liquid water.

"It's only for these small stars that the zone of habitability is close enough for these tidal forces to be relevant," Ballard said.

Data came from NASA's Kepler telescope, which captures information about exoplanets as they move in front of their host stars. To measure the planets' orbits, Ballard and Sagear focused especially on how long the planets took to move across the face of the stars. Their study also relied on new data from the Gaia telescope, which measured the distance to billions of stars in the galaxy.

"The distance is really the key piece of information we were missing before that allows us to do this analysis now," Sagear said.

Sagear and Ballard found that stars with multiple planets were the most likely to have the kind of circular orbits that allow them to retain liquid water. Stars with only one planet were the most likely to see tidal extremes that would sterilize the surface.

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New ice is like a snapshot of liquid water

A collaboration between scientists at Cambridge and UCL has led to the discovery of a new form of ice that more closely resembles liquid water than any other and may hold the key to understanding this most famous of liquids.

The new form of ice is amorphous. Unlike ordinary crystalline ice where the molecules arrange themselves in a regular pattern, in amorphous ice the molecules are in a disorganised form that resembles a liquid.

In this paper, published in Science, the team created a new form of amorphous ice in experiment and achieved an atomic-scale model of it in computer simulation. The experiments used a technique called ball-milling, which grinds crystalline ice into small particles using metal balls in a steel jar. Ball-milling is regularly used to make amorphous materials, but it had never been applied to ice.

The team found that ball-milling created a novel amorphous form of ice, which unlike all other known ices, had a density similar to that of liquid water and whose state resembled water in solid form. They named the new ice medium-density amorphous ice (MDA).

To understand the process at the molecular scale the team employed computational simulation. By mimicking the ball-milling procedure via repeated random shearing of crystalline ice, the team successfully created a computational model of MDA.

"Our discovery of MDA raises many questions on the very nature of liquid water and so understanding MDA's precise atomic structure is very important" comments co-author Dr. Michael Davies, who carried out the computational modelling. "We found remarkable similarities between MDA and liquid water."

A happy medium

Amorphous ices have been suggested to be models for liquid water. Until now, there have been two main types of amorphous ice: high-density and low-density amorphous ice.

As the names suggest, there is a large density gap between them. This density gap, combined with the fact that the density of liquid water lies in the middle, has been a cornerstone of our understanding of liquid water. It has led in part to the suggestion that water consists of two liquids: one high- and one low-density liquid.

Senior author Professor Christoph Salzmann said: "The accepted wisdom has been that no ice exists within that density gap. Our study shows that the density of MDA is precisely within this density gap and this finding may have far-reaching consequences for our understanding of liquid water and its many anomalies."

A high-energy geophysical material

The discovery of MDA gives rise to the question: where might it exist in nature? Shear forces were discovered to be key to creating MDA in this study. The team suggests ordinary ice could undergo similar shear forces in the ice moons due to the tidal forces exerted by gas giants such as Jupiter.

Moreover, MDA displays one remarkable property that is not found in other forms of ice. Using calorimetry, they found that when MDA recrystallises to ordinary ice it releases an extraordinary amount of heat. The heat released from the recrystallization of MDA could play a role in activating tectonic motions. More broadly, this discovery shows water can be a high-energy geophysical material.

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