Tag Archives: #water

NASA Study Highlights Importance of Surface Shadows in Moon Water Puzzle (Planetary Science)

The shadows cast by the roughness of the Moon’s surface create small cold spots for water ice to accumulate even during the harsh lunar daytime.

Scientists are confident that water ice can be found at the Moon’s poles inside permanently shadowed craters – in other words, craters that never receive sunlight. But observations show water ice is also present across much of the lunar surface, even during daytime. This is a puzzle: Previous computer models suggested any water ice that forms during the lunar night should quickly burn off as the Sun climbs overhead.

“Over a decade ago, spacecraft detected the possible presence of water on the dayside surface of the Moon, and this was confirmed by NASA’s Stratospheric Observatory for Infrared Astronomy [SOFIA] in 2020,” said Björn Davidsson, a scientist at NASA’s Jet Propulsion Laboratory in Southern California. “These observations were, at first, counterintuitive: Water shouldn’t survive in that harsh environment. This challenges our understanding of the lunar surface and raises intriguing questions about how volatiles, like water ice, can survive on airless bodies.”

This illustration zooms in on the area of detail indicated in the previous photo, showing how shadows enable water ice to survive on the sunlit lunar surface. When shadows move as the Sun tracks overhead, the exposed frost lingers long enough to be detected by spacecraft. Credit: NASA/JPL-Caltech

In a new study, Davidsson and co-author Sona Hosseini, a research and instrument scientist at JPL, suggest that shadows created by the “roughness” of the lunar surface provide refuge for water ice, enabling it to form as surface frost far from the Moon’s poles. They also explain how the Moon’s exosphere (the tenuous gases that act like a thin atmosphere) may have a significant role to play in this puzzle.

Water Traps and Frost Pockets

Many computer models simplify the lunar surface, rendering it flat and featureless. As a result, it’s often assumed that the surface far from the poles heats up uniformly during lunar daytime, which would make it impossible for water ice to remain on the sunlit surface for long.

So how is it that water is being detected on the Moon beyond permanently shadowed regions? One explanation for the detection is that water molecules may be trapped inside rock or the impact glass created by the incredible heat and pressure of meteorite strikes. Fused within these materials, as this hypothesis suggests, the water can remain on the surface even when heated by the Sun while creating the signal that was detected by SOFIA.

But one problem with this idea is that observations of the lunar surface show that the amount of water decreases before noon (when sunlight is at its peak) and increases in the afternoon. This indicates that the water may be moving from one location to another through the lunar day, which would be impossible if they are trapped inside lunar rock or impact glass.


Click on this interactive visualization of the Moon and take it for a spin. The “HD” button in the lower right offers for more detailed textures. The full interactive experience is at Eyes on the Solar System.

Davidsson and Hosseini revised the computer model to factor in the surface roughness apparent in images from the Apollo missions from 1969 to 1972, which show a lunar surface strewn with boulders and pockmarked with craters, creating lots of shady areas even near noon. By factoring this surface roughness into their computer models, Davidsson and Hosseini explain how it’s possible for frost to form in the small shadows and why the distribution of water changes throughout the day.

Because there is no thick atmosphere to distribute heat around the surface, extremely cold, shaded areas, where temperatures may plummet to about minus 350 degrees Fahrenheit (minus 210 degrees Celsius), can neighbor hot areas exposed to the Sun, where temperatures may reach as high as 240 Fahrenheit (120 Celsius).

As the Sun tracks through the lunar day, the surface frost that may accumulate in these cold, shaded areas is slowly exposed to sunlight and cycled into the Moon’s exosphere. The water molecules then refreeze onto the surface, reaccumulating as frost in other cold, shaded locations.

“Frost is far more mobile than trapped water,” said Davidsson. “Therefore, this model provides a new mechanism that explains how water moves between the lunar surface and the thin lunar atmosphere.”

A Closer Look

While this isn’t the first study to consider surface roughness when calculating lunar surface temperatures, previous work did not take into account how shadows would affect the capability of water molecules to remain on the surface during daytime as frost. This new study is important because it helps us to better understand how lunar water is released into, and removed from, the Moon’s exosphere.

“Understanding water as a resource is essential for NASA and commercial endeavors for future human lunar exploration,” Hosseini said. “If water is available in the form of frost in sunlit regions of the Moon, future explorers may use it as a resource for fuel and drinking water. But first, we need to figure out how the exosphere and surface interact and what role that plays in the cycle.”

To test this theory, Hosseini is leading a team to develop ultra-miniature sensors to measure the faint signals from water ice. The Heterodyne OH Lunar Miniaturized Spectrometer (HOLMS) is being developed to be used on small stationary landers or autonomous rovers – like JPL’s Autonomous Pop-Up Flat Folding Explorer Robot (A-PUFFER), for example – that may be sent to the Moon in the future to make direct measurements of hydroxyl (a molecule that contains one hydrogen atom and an oxygen atom).

Hydroxyl, which is a molecular cousin of water (a molecule with two hydrogen atoms and one oxygen atom), can serve as an indicator of how much water may be present in the exosphere. Both water and hydroxyl could be created by meteorite impacts and through solar wind particles hitting the lunar surface, so measuring the presence of these molecules in the Moon’s exosphere can reveal how much water is being created while also showing how it moves from place to place. But time is of the essence to make those measurements.

“The current lunar exploration by several nations and private companies indicates significant artificial changes to the lunar environment in the near future,” said Hosseini. “If this trend continues, we will lose the opportunity to understand the natural lunar environment, particularly the water that is cycling through the Moon’s pristine exosphere. Consequently, the advanced development of ultra-compact, high-sensitivity instruments is of critical importance and urgency.”

The researchers point out that this new study could help us better understand the role shadows play in the accumulation of water ice and gas molecules beyond the Moon, such as on Mars or even on the particles in Saturn rings.

The study, titled “Implications of surface roughness in models of water desorption on the Moon”, was published in the Monthly Notices of the Royal Astronomical Society on August 2, 2021.

Featured image: The Moon is covered with craters and rocks, creating a surface “roughness” that casts shadows, as seen in this photograph from the 1972 Apollo 17 mission. Image Credit:  NASA

Provided by Jet Propulsion Laboratory

Mars Bright South Pole Reflections May Be Clay – Not Water (Planetary Science)

Bright reflections observed at Mars’ south pole serve as evidence for water. But, seeing may be deceiving.

After measuring the area’s electrical properties with orbiting, ground-penetrating radar, an international group of scientists now say that reflections of the red planet’s south pole may be smectite, a form of hydrated clay, buried about a mile below the surface, according to a July 29 report in the journal Geophysical Research Letters.

The research, led by Isaac B. Smith of York University, Toronto, with major contributions by second author Dan Lalich, research associate in the Cornell Center for Astrophysics and Planetary Science in the College of Arts and Sciences, said the presence of liquid water requires implausible amounts of heat and salt.

“Those bright reflections have been big news over the last few years because they were initially interpreted as liquid water below the ice,” Lalich said. “That interpretation is inconsistent with other observations that imply the ice isn’t warm enough to melt, given what we know about conditions on Mars.”

Even on Earth, Lalich said, it is rare to see subsurface reflections from radar that are brighter than the surface reflection.

The reflection is about a mile below the planet’s surface, where “you don’t expect as bright of a reflection,” he said. “We were getting radar reflections that were much brighter than the surface. And that’s really weird. It’s not something that we had really seen before and it’s not something we expected.”

The group had pored over data from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument – a radar that examines the Martian subsurface with a 130-foot antenna via the European Space Agency’s Mars Express orbiter. The MARSIS instrument, jointly developed by the Italian Space Agency and NASA’s Jet Propulsion Laboratory, can probe the planet to a depth of three miles.

Lalich and the other scientists used a diagnostic physical property in ground-penetrating radar called dielectric permittivity, which measures the ability to store electric energy. The group used the reflection strength to estimate the permittivity difference between the ice and the base of the polar cap, and then compared that estimate to lab measurements of smectite.

“Smectites are very abundant on Mars, covering about half the planet, especially in the Southern Hemisphere,” said York University’s Smith. “That knowledge, along with the radar properties of smectites at cryogenic temperatures, points to them being the most likely explanation to the riddle.”

Lalich said the data to confirm the hydrated clay was easily reproduced from the observed data, meaning that liquid water is not necessary to generate bright reflections. The scientists were hoping to find lakes and other geologic forms.

“Unfortunately, that’s a bit of a downer,” he said, “because lakes below the ice cap would have been very exciting. We believe the smectite hypothesis is more likely and it’s more consistent with other observations.”

In addition to Smith and Lalich, the co-authors on “A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice,” are Craig Rezza, graduate student, York University; Briony Horgan, associate professor, Purdue University; Jennifer L. Whitten, assistant professor, Tulane University; and Stefano Nerozzi, postdoctoral research associate and Jack Holt, professor, University of Arizona.

Featured image: Mars’ south pole – which looks like creamy swirls in cappuccino – is an icy cap with carbon dioxide and other geologic traits.  About a mile below the cap is smectite, a hydrated version of clay. © ESA/Mars Express

Reference:  I. B. Smith et al, A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice, Geophysical Research Letters (2021). DOI: 10.1029/2021GL093618

Provided by Cornell University

Water As A Metal (Physics)

Under normal conditions, pure water is an almost perfect insulator. Water only develops metallic properties under extreme pressure, such as exists deep inside of large planets. Now, an international collaboration has used a completely different approach to produce metallic water and documented the phase transition at BESSY II. The study is published now in Nature.

Every child knows that water conducts electricity—but this refers to “normal” everyday water that contains salts. Pure, distilled water, on the other hand, is an almost perfect insulator. It consists of H2O molecules that are loosely linked to one another via hydrogen bonds. The valence electrons remain bound and are not mobile. To create a conduction band with freely moving electrons, water would have to be pressurized to such an extent that the orbitals of the outer electrons overlap. However, a calculation shows that this pressure is only present in the core of large planets such as Jupiter.

Providing electrons

An international collaboration of 15 scientists from eleven research institutions has now used a completely different approach to produce a aqueous solution with metallic properties for the first time and documented this phase transition at BESSY II. To do this, they experimented with alkali metals, which release their outer electron very easily.

Avoiding explosion

However, the chemistry between alkali metals and water is known to be explosive. Sodium or other alkali metals immediately start to burn in water. But the team found a way to keep this violent chemistry in check: They did not throw a piece of alkali metal into water, but they did it the other way round: they put a tiny bit of water on a drop of alkali metal, a sodium-potassium (Na-K) alloy, which is liquid at room temperature.

Metallic water prepared for first time under terrestrial conditions
The first image shows a pure drop of sodium-potassium alloy; in the next images, we see the drop exposed to the action of the water vapor at 10-4 mbar. A layer of water forms on the drop, in which electrons liberated from the metal dissolve, giving it a golden metallic sheen. Credit: Phil Mason / IOCB Prague

Experiment at BESSY II

At BESSY II, they set up the experiment in the SOL3PES high vacuum sample chamber at the U49/2 beamline. The sample chamber contains a fine nozzle from which the liquid Na-K alloy drips. The silver droplet grows for about 10 seconds until it detaches from the nozzle. As the droplet grows, some water vapor flows into the sample chamber and forms an extremely thin skin on the surface of the droplet, only a few layers of water molecules. This almost immediately causes the electrons as well as the metal cations to dissolve from the alkali alloy into the water. The released electrons in the water behave like free electrons in a conduction band.

Golden water skin

“You can see the phase transition to metallic water with the naked eye! The silvery sodium-potassium droplet covers itself with a golden glow, which is very impressive,” reports Dr. Robert Seidel, who supervised the experiments at BESSY II. The thin layer of gold-colored metallic water remains visible for a few seconds. This enabled the team led by Prof. Pavel Jungwirth, Czech Academy of Sciences, Prague, to prove with spectroscopic analyses at BESSY II and at the IOCB in Prague that it is indeed water in a metallic state.

Fingerprints of the metallic phase

The two decisive fingerprints of a metallic phase are the plasmon frequency and the conduction band. The groups were able to determine these two quantities using optical reflection spectroscopy and synchrotron X-ray photoelectron spectroscopy: While the plasmon frequency of the gold-colored, metallic ‘water skin’ is about 2.7 eV (i.e. in the blue range of visible light), the conduction band has a width of about 1.1 eV with a sharp Fermi edge. “Our study not only shows that metallic water can indeed be produced on Earth, but also characterizes the spectroscopic properties associated with its beautiful golden metallic luster,” says Seidel.

Featured image: In the sample chamber, the NaK alloy drips from a nozzle. As the droplet grows, water vapor flows into the sample chamber and forms a thin skin on the drop’s surface. Credit: HZB

Reference: Philip E. Mason et al, Spectroscopic evidence for a gold-coloured metallic water solution, Nature (2021). DOI: 10.1038/s41586-021-03646-5

Provided by Helmholtz Association of German Research Centres

Mini Radar Could Scan The Moon For Water And Habitable Tunnels (Planetary Science)

A miniature device that scans deep below ground is being developed to identify ice deposits and hollow lava tubes on the Moon for possible human settlement.

The prototype device, known as MAPrad, is just one tenth the size of existing ground penetrating radar systems, yet can see almost twice as deeply below ground – more than 100 metres down – to identify minerals, ice deposits, or voids such as lava tubes.

Local start-up CD3D PTY Limited has now received a grant from the Australian Space Agency’s Moon to Mars initiative to further develop the prototype with RMIT University, including testing it by mapping one of Earth’s largest accessible systems of lava tubes.  

CD3D CEO and RMIT Honorary Professor, James Macnae, said their unique geophysical sensor had several advantages over existing technology that made it more suitable for space missions.

“MAPrad is smaller, lighter and uses no more power than existing ground penetrating radar devices, yet can see up to hundreds of meters below the surface, which is around twice as deep as existing technology,” Macnae said. 

“It was able to achieve this improved performance, even after being shrunken to a hand-held size, because it operates in a different frequency range: using the magnetic rather than the electric component of electromagnetic waves.”

Lab technician holds MAPrad prototype in the Micro Nano Research Facility clean rooms at RMIT.
Lab technician holds MAPrad prototype in the Micro Nano Research Facility clean rooms at RMIT. © RMIT

The magnetic waves emitted and detected by the device measure conductivity and electromagnetic wave reflections to identify what lies underground. Voids and water-ice provide strong reflections, while various metal deposits have high conductivity at unique levels.

From mining to Moon mission

The specialised radar system was developed by RMIT University and Canadian company International Groundradar Consulting in a collaborative research project funded through the AMIRA Global network.

Successful field tests have since been carried out in Australia and Canada using a backpacked prototype for mining and mineral prospecting.

“MAPrad’s initial development was specifically focussed on facilitating drone surveys for mining applications, but it has obvious applications in space where size and weight are at a premium, so that’s where we’re now focusing our efforts,” Macnae said.

To further prove the technology’s usefulness for a range of Moon missions, the researchers will be seeking permission to scan one the world’s largest accessible systems of lava tubes at the spectacular Undara caves in Far North Queensland, Australia.

Undara is an Aboriginal word meaning ‘long way’, in reference to the unusually long system of lava tubes that are located within the park. The tubes have diameters of up to 20 metres and some are several hundred metres in length.

Inside one of the vast lava tubes at Undara Volcanic National Park, in Far North Queensland. Getty Images
Inside one of the vast lava tubes at Undara Volcanic National Park, in Far North Queensland. Getty Images

RMIT University engineer, Dr Graham Dorrington, said they would traverse the park above the caves to detect the voids below, some of which have not been completely mapped yet. 

“We know the dimensions of the main tubes, so comparison with surface scans to check accuracy should be possible,” he said.

“Undara will be an excellent testing site for us since it’s the closest thing on Earth to the lava tubes thought to exist on the Moon and Mars.” 

The search for water and shelter in space

Massive tunnels left by ancient volcanic lava flows may exist at shallow depths below the surface of the Moon and Mars. 

It’s thought these enclosures could be suitable for the construction of space colonies as they provide protection from the Moon’s frequent meteorite impacts, high-energy ultra-violet radiation and energetic particles, not to mention extreme temperatures.

On the Moon’s surface, for example, daytime temperatures are often well above 100°C, dropping dramatically to below -150°C at night, while the insulated tunnels could provide a stable environment of around -22°C.

The team hopes to qualify MAPrad for space use so it can help uncover the resources available on the Moon (pictured) and Mars to support life. Credit: NASA.
The team hopes to qualify MAPrad for space use so it can help uncover the resources available on the Moon (pictured) and Mars to support life. Credit: NASA.

But of more immediate concern is mapping ice-water deposits on the Moon and getting a clearer picture of the resources available there to support life. 

Dorrington said their system could be mounted on a space rover, or even attached to a spacecraft in low orbit, to monitor for minerals in near-future missions and for lava tubes in later missions.

“After the lava tube testing later this year, the next step will be optimising the device so as not to interfere or interact with any of the space rover or spacecraft’s metal components, or cause incompatible electromagnetic interference with communications or other instruments,” Dorrington said. 

“Qualifying MAPrad for space usage, especially for use on the Moon, will be a significant technical challenge for us, but we don’t foresee any showstoppers.”

The team will use the unique capabilities of the RMIT Micro Nano Research Facility and the Advanced Manufacturing Precinct and are also looking to collaborate on later stages of development with specialists in spacecraft integration or organisations with payload availability.

The research team at RMIT University includes Honorary Professor James Macnae, Professor Pier Marzocca, Professor Gary Bryant, Professor Arnan Mitchell, Dr Gail Iles and Dr Graham Dorrington

Provided by RMIT University

Study Looks More Closely At Mars’ Underground Water Signals (Planetary Science)

A new paper finds more radar signals suggesting the presence of subsurface ‘lakes,” but many are in areas too cold for water to remain liquid.

In 2018, scientists working with data from ESA’s (the European Space Agency’s) Mars Express orbiter announced a surprising discovery: Signals from a radar instrument reflected off the red planet’s south pole appeared to reveal a liquid subsurface lake. Several more such reflections have been announced since then.

In a new paper published in the journal Geophysical Research Letters, two scientists at NASA’s Jet Propulsion Laboratory in Southern California describe finding dozens of similar radar reflections around the south pole after analyzing a broader set of Mars Express data, but many are in areas that should be too cold for water to remain liquid.

“We’re not certain whether these signals are liquid water or not, but they appear to be much more widespread than what the original paper found,” said Jeffrey Plaut of JPL, co-principal investigator of the orbiter’s MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) instrument, which was built jointly by the Italian Space Agency and JPL. “Either liquid water is common beneath Mars’ south pole or these signals are indicative of something else.”

Study looks more closely at Mars’ underground water signals
The European Space Agency’s Mars Express flies over the red planet in this illustration. Credit:  ESA/NASA/JPL-Caltech

Frozen Time Capsule

The radar signals originally interpreted as liquid water were found in a region of Mars known as the South Polar Layered Deposits, named for the alternating layers of water ice, dry ice (frozen carbon dioxide), and dust that have settled there over millions of years. These layers are believed to provide a record of how the tilt in Mars’ axis has shifted over time, just as changes in Earth’s tilt have created ice ages and warmer periods throughout our planet’s history. When Mars had a lower axial tilt, snowfall and layers of dust accumulated in the region and eventually formed the thick layered ice sheet found there today.

By beaming radio waves at the surface, scientists can peer below these icy layers, mapping them in detail. Radio waves lose energy when they pass through material in the subsurface; as they reflect back to the spacecraft, they usually have a weaker signal. But in some cases, signals returning from this region’s subsurface were brighter than those at the surface. Some scientists have interpreted these signals to imply the presence of liquid water, which strongly reflects radio waves.

Plaut and Aditya Khuller, a doctoral student at Arizona State University who worked on the paper while interning at JPL, aren’t sure what the signals indicate. The areas hypothesized to contain liquid water span about 6 to 12 miles (10 to 20 kilometers) in a relatively small region of the Martian south pole. Khuller and Plaut expanded the search for similar strong radio signals to 44,000 measurements spread across 15 years of MARSIS data over the entirety of the Martian south polar region.

Study looks more closely at Mars’ underground water signals
The colored dots represent sites where bright radar reflections have been spotted by ESA’s Mars Express orbiter at Mars’ south polar cap. Credit:  ESA/NASA/JPL-Caltech

Unexpected “Lakes’

The analysis revealed dozens of additional bright radar reflections over a far greater range of area and depth than ever before. In some places, they were less than a mile from the surface, where temperatures are estimated to be minus 81 degrees Fahrenheit (minus 63 degrees Celsius) – so cold that water would be frozen, even if it contained salty minerals known as perchlorates, which can lower the freezing point of water.

Khuller noted a 2019 paper in which researchers calculated the heat needed to melt subsurface ice in this region, finding that only recent volcanism under the surface could explain the potential presence of liquid water under the south pole.

“They found that it would take double the estimated Martian geothermal heat flow to keep this water liquid,” Khuller said. “One possible way to get this amount of heat is through volcanism. However, we haven’t really seen any strong evidence for recent volcanism at the south pole, so it seems unlikely that volcanic activity would allow subsurface liquid water to be present throughout this region.”

What explains the bright reflections if they’re not liquid water? The authors can’t say for sure. But their paper does offer scientists a detailed map of the region that contains clues to the climate history of Mars, including the role of water in its various forms.

“Our mapping gets us a few steps closer to understanding both the extent and the cause of these puzzling radar reflections,” said Plaut.

Featured image: The bright white region of this image shows the icy cap that covers Mars’ south pole, composed of frozen water and frozen carbon dioxide. Credit: ESA/DLR/FU Berlin/Bill Dunford

Reference: Khuller, A. R., & Plaut, J. J. (2021). Characteristics of the Basal Interface of the Martian South Polar Layered Deposits. Geophysical Research Letters, 48, e2021GL093631. https://doi.org/10.1029/2021GL093631

Provided by NASA

How Much Water Was Delivered From The Asteroid Belt To The Earth After Its Formation? (Planetary Science)

How much water could have been brought to the Earth through asteroid collisions after the formation of earth? Rebecca Martin and Mario Livio now answered this question by using N-body simulations and by comparing the relative impact efficiencies of 3 radially narrow regions of the asteroid belts. Their study recently appeared in Arxiv.

As we all know, the planets in the inner solar system are relatively dry. The precise amount of water in and on the Earth is unknown, but is thought to be between one and ten “oceans”, where one ocean is about 2.5 × 10¯4 M or the mass of water on the Earth’s surface. The question is, how the Earth could have formed with its current amount of water, as it is not possible for Earth to form water by its own. Thus, there have been several suggestions: through adsorption of hydrogen molecules on to silicate grains, delivery of water through meteorites and icy pebbles etc. But, the current leading scenario for the majority of the water delivery is by external pollution from the outer parts of the asteroid belt.

So, by considering this, Martin and Livio now described their n-body simulations in which they modeled three radially narrow regions of the asteroid belt: the ν6 resonance (at about 2.1 au), the 2:1 mean motion resonance with Jupiter (at about 3.3 au) and the chaotic region outside of the asteroid belt. Next, they compared the relative impact
efficiencies between asteroids in these regions and the Earth.

“Our analysis assumed that the asteroid belt contained its primordial mass after the Earth had formed and the giant planets were on their current day orbits. Thus, we have skewed the assumptions to estimate an upper limit to the amount of water that could have been delivered to the Earth.”

They showed that the majority of asteroid collisions with the Earth originate from the ν6 resonance at the inner edge of the asteroid belt. About 2% of asteroids from the resonance collide with the Earth. While, the collision probability from the 2:1 mean motion resonance is about one hundred times smaller and from the chaotic region about a thousand times smaller.

(article continues below image)

Figure 1. The outcomes of the n-body simulations in time binned into 0.5 Myr intervals. Note that the points are slightly offset from the centre of the time bin so that they don’t completely overlap. The upper panels have the Earth radius of REarth = 1 R⊕ while the lower panels have Earth radius of REarth = 10 R⊕. The left panels show the simulation of the ν6 resonance in the range a = 2.0-2.1 au. The middle panels show the 2:1 resonance in the range a = 3.3 – 3.35 au. The right panels show the simulation in the region 4 – 4.1 au. The blue points show asteroids that are ejected. The red points show asteroids that hit the Sun. The green points show asteroids that hit the Earth. The magenta points show asteroids that hit Jupiter or Saturn. © Martin and Livio

They also estimated that, if the majority of asteroids in the primordial asteroid belt were moved into the ν6 resonance either through asteroid-asteroid interactions or gas drag, or the Yarkovsky effect, then at most, the asteroid belt could have delivered about eight oceans worth of water. Thus, the delivery of one ocean’s worth from the asteroid belt was certainly possible.

“However, the delivery of 10 oceans worth could have been difficult and if the Earth’s mantle contains such significant amounts of water then the Earth likely formed with a good fraction of it.”

— concluded authors of the study

Reference: Rebecca G. Martin and Mario Livio, “How much water was delivered from the asteroid belt to the Earth after its formation?”, Arxiv, pp. 1-5, 2021. https://arxiv.org/abs/2106.03999

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Scientists Reveal Effects of Water on 660 km Discontinuity in the Deep Earth (Earth Science)

The seismic discontinuity that occurs at an average depth of 647-654 km, usually called the 660 km seismic discontinuity, is the boundary between the transition zone and the lower mantle. It is considered to be caused by the transformation of ringwoodite to bridgmanite and MgO (post-spinel transition).

One important parameter that can potentially affect the post-spinel transition is hydration. According to recent studies, the transition zone may contain significant amounts of water, and thus understanding the effect of water on the post-spinel transition is important to determine its properties.

Dr. Joshua Muir, the postdoctoral researcher, and the team leader Prof. ZHANG Feiwu from the Institute of Geochemistry of the Chinese Academy of Sciences (IGCAS), in collaboration with Prof. John Brodholt from University College London (UCL), investigated the effect of water on the post-spinel transition at high temperatures and pressures.

The results indicated that the water only had a very small effect on the Clapeyron slope of the post-spinel transition. On the other hand, water had a moderate effect on its depth. 1 wt.% water increased the depth of phase transition onset by about 8 km. This variation on depth was relatively small compared to seismically observed variations in the 660 km discontinuity of around 35 km and so water alone could not account for the observed 660 km discontinuity topography.

Furthermore, this study demonstrated that the addition of water caused a large broadening of the three-phase loop (ringwoodite + bridgmanite + MgO) through the development of post-spinel transition. The width of the phase loop was negligible for water contents less than about 1000 ppm, but then grew rapidly to become about 10 km wide, before shrinking again as the water content approaches ringwoodite saturation.

The study, published in Earth and Planetary Science Letters on April 14, was supported by the National Natural Science Foundation of China, PIFI from Chinese Academy of Sciences (CAS) and NERC Grants.

Phase boundary for post-spinel transition (top), and the width of the three-phase loop as a function of water content at 1873 K (bottom) (Image by IGCAS)

Reference: Joshua M.R. Muir, Feiwu Zhang, John P. Brodholt, The effect of water on the post-spinel transition and evidence for extreme water contents at the bottom of the transition zone, Earth and Planetary Science Letters, Volume 565, 2021, 116909, ISSN 0012-821X, https://doi.org/10.1016/j.epsl.2021.116909. (https://www.sciencedirect.com/science/article/pii/S0012821X21001680)

Provided by Chinese Academy of Sciences

New Material Could Harvest Water All Day Long (Material Science)

Micro-engineered, bioinspired design allows the material to collect moisture from cool fog as well as generating and collecting steam under sunny conditions

Tiny structures inspired by the shape of cactus spines allow a newly created material to gather drinkable water from the air both day and night, combining two water-harvesting technologies into one.

The material, a micro-architected hydrogel membrane (more on that later), can produce water through both solar steam-water generation and fog collection—two independent processes that typically require two separate devices. A paper about the material was published in Nature Communications on May 14.

Hydrogel trees
Images of representative fabricated PVA/PPy gel micro-tree array. Scale bar: 1 cm. © Caltech

Fog collection is exactly what it sounds like. At night, low-lying clouds along sea coasts are heavy with water droplets. Devices that can coalesce and collect those droplets can turn fog into drinking water.

Solar-steam generation is another water-collection technique. It works especially well in coastal areas because it is also capable of water purification, though it works during the day instead of at night. In the method, heat from the sun causes water to evaporate into steam, which can be condensed into drinking water.

Because the two technologies operate under such different conditions, they typically require different materials and devices to make them work. Now, a material developed at Caltech could combine them into a single device, working to generate clean water 24 hours a day.

“Water scarcity is a huge issue that humanity will need to overcome as the world’s population continues to grow,” says Julia R. Greer, the Ruben F. and Donna Mettler Professor of Materials Science, Mechanics and Medical Engineering and Fletcher Jones Foundation Director of the Kavli Nanoscience Institute. “Water covers three-quarters of the globe, but only about one half of one percent is available freshwater.”

Hydrogel tree
Images of an individual representative tree micro-topology. Scale bar: 1 mm. © Caltech

Greer has spent her career developing micro- and nano-architected materials; that is, materials whose very shapes (controlled at each length scale, nanoscopic and microscopic) give them unusual and potentially useful properties. In this case, Greer collaborated with Ye Shi, formerly a postdoctoral scholar at Caltech and now a postdoctoral scholar at UCLA, to create a membrane of arrayed tiny spines that resemble Christmas trees but are in fact inspired by the shape of cactus spines.

“Cacti are uniquely adapted to survive dry climates,” Shi says. “In our case, these spines, which we call ‘micro-trees,’ attract microscopic droplets of water that are suspended in the air, allowing them to slide down the base of the spine and coalesce with other droplets into relatively heavy drops that eventually converge into a reservoir of water that can be utilized.”

The spines are built out of a hydrogel; that is, a network of hydrophilic (water-loving) polymers that naturally attract water. Due to their tiny size, they can be printed onto a wafer-thin membrane. During the day, the hydrogel membrane absorbs sunlight to heat up water trapped beneath it, which becomes steam. The steam then recondenses onto a transparent cover, where it can be collected. During the night, the transparent cover folds up and the hydrogel membrane is exposed to humid air to capture fog. As such, the material can harvest water from both steam and fog.

In an operation test conducted during the night, samples of the materials ranging from 55–125 square centimeters in area were able to collect about 35 milliliters of water from fog. In tests during the day, the material was capable of collecting about 125 milliliters from solar steam.

The exact design of the membrane was created using the design program SolidWorks.

The hydrogel itself is a polyvinyl alcohol/polypyrrole (PVA/PPy) composite gel, a non-toxic and flexible material used in numerous applications including in capacitors, wearable strain and temperature sensors, and batteries.

To fine-tune the design of the micro-trees, Greer and Shi worked with Caltech’s Harry Atwater, Howard Hughes Professor of Applied Physics and Materials Science; and Ognjen Ilic, formerly a postdoctoral scholar at Caltech and now Benjamin Mayhugh Assistant Professor of Mechanical Engineering at the University of Minnesota.

Using computer modeling, Ilic computed the heat distribution within the micro-trees to help define the size and shape that would be most effective at drawing water from the air. With this successful proof-of-concept, the team now hopes to find a private partner capable of commercializing the technology for water-scarce regions.

“It is really inspiring that a relatively simple hydrophilic polymer membrane can be shaped in a morphology that resembles cacti spines and be capable of tremendous enhancement in water collection. I guess evolution really works,” Greer says.

The Nature Communications paper is titled “All-day Fresh Water Harvesting by Microstructured Hydrogel Membranes.” This work was supported by the Resnick Sustainability Institute, the Caltech Space Solar Power Project, the Joint Center for Artificial Photosynthesis (a U.S. Department of Energy Energy Innovation Hub), and served as a foundation for Greer’s subsequent Defense Advanced Research Projects Agency grant for the “Atmospheric Water Extraction” program, which is a collaboration with the University of Texas Austin and MIT.

Featured image: Porous structure of gel matrix © Caltech

Reference: Shi, Y., Ilic, O., Atwater, H.A. et al. All-day fresh water harvesting by microstructured hydrogel membranes. Nat Commun 12, 2797 (2021). https://doi.org/10.1038/s41467-021-23174-0

Provided by Caltech

Deep Water On Neptune and Uranus May Be Magnesium-rich (Planetary Science)

While scientists have amassed considerable knowledge of the rocky planets in our solar system, like Earth and Mars, much less is known about the icy water-rich planets, Neptune and Uranus.

In a new study recently published in Nature Astronomy, a team of scientists re-created the temperature and pressure of the interiors of Neptune and Uranus in the lab, and in so doing have gained a greater understanding of the chemistry of these planets’ deep water layers. Their findings also provide clues to the composition of oceans on water-rich exoplanets outside our solar system. 

Neptune and Uranus are conventionally thought to have distinct separate layers, consisting of an atmosphere, ice or fluid, a rocky mantle and a metallic core. For this study, the research team was particularly interested in possible reaction between water and rock in the deep interiors.

“Through this study, we were seeking to extend our knowledge of the deep interior of ice giants and determine what water-rock interactions at extreme conditions might exist,” said lead author Taehyun Kim, of Yonsei University in South Korea.

“Ice giants and some exoplanets have very deep water layers, unlike terrestrial planets. We proposed the possibility of an atomic-scale mixing of two of the planet-building materials (water and rock) in the interiors of ice giants.”

To mimic the conditions of the deep water layers on Neptune and Uranus in the lab, the team first immersed typical rock-forming minerals, olivine and ferropericlase, in water and compressed the sample in a diamond-anvil to very high pressures. Then, to monitor the reaction between the minerals and water, they took X-ray measurements while a laser heated the sample to a high temperature.

The resulting chemical reaction led to high concentrations of magnesium in the water. Based on these findings, the team concluded that oceans on water-rich planets may not have the same chemical properties as the Earth’s ocean and high pressure would make those oceans rich in magnesium.

“We found that magnesium becomes much more soluble in water at high pressures. In fact, magnesium may become as soluble in the water layers of Uranus and Neptune as salt is in Earth’s ocean,” said study co-author Sang-Heon Dan Shim of Arizona State University’s School of Earth and Space Exploration.    

A diamond-anvil (top right) and laser were used in the lab on a sample of olivine to reach the pressure-temperature conditions expected at the top of the water layer beneath the hydrogen atmosphere of Uranus (left). In this experiment, the magnesium in olivine dissolved in the water. Credit: Shim/ASU

These characteristics may also help solve the mystery of why Uranus’ atmosphere is much colder than Neptune’s, even though they are both water-rich planets. If much more magnesium exists in the Uranus’ water layer below the atmosphere, it could block heat from escaping from the interior to the atmosphere.

“This magnesium-rich water may act like a thermal blanket for the interior of the planet,” said Shim.

Beyond our solar system, these high-pressure and high-temperature experiments may also help scientists gain a greater understanding of sub-Neptune exoplanets, which are planets outside of our solar system with a smaller radius or a smaller mass than Neptune.

Sub-Neptune planets are the most common type of exoplanets that we know of so far, and scientists studying these planets hypothesize that many of them may have a thick water-rich layer with a rocky interior. This new study suggests that the deep oceans of these exoplanets would be much different from Earth’s ocean and may be magnesium-rich.

An electron microscopy image of the olivine sample shows a large empty dome structure where magnesium under high-pressure water precipitated as magnesium oxide. Credit: Kim et al.

“If an early dynamic process enabled a rock–water reaction in these exoplanets, the topmost water layer may be rich in magnesium, possibly affecting the thermal history of the planet,” said Shim.

For next steps, the team hopes to continue their high-pressure/high-temperature experiments under diverse conditions to learn more about the composition of planets.

“This experiment provided us with a plan for further exploration of the unknown phenomena in ice giants,” said Kim.

Additional authors of this study include Stella Chariton and Vitali Prakapenka of the University of Chicago; Anna Pakhomova and Hanns-Peter Liermann of the Deutsches Elektronen Synchrotron, Germany; Zhenxian Liu of the University of Illinois, Chicago; Sergio Speziale of the German Research Center for Geosciences, Germany; and Yongjae Lee of Yonsei University, South Korea.

Featured image: Uranus, taken by the NASA spacecraft Voyager 2. This ice giant, which is the seventh planet from the sun in our solar system, is nearly four times larger than Earth, and most of its mass is a dense fluid above a rocky core. Image by NASA/JPL

Reference: Kim, T., Chariton, S., Prakapenka, V. et al. Atomic-scale mixing between MgO and H2O in the deep interiors of water-rich planets. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01368-2

Provided by Arizona State University