Maps of magnesium/silicon (left) and thermal neutron absorption (right) across Mercury's surface (red indicates high values, blue low). These maps, together with maps of other elemental abundances, reveal the presence of distinct geochemical terranes. Volcanic smooth plains deposits are outlined in white.

Two new papers from members of the MESSENGER Science Team provide global-scale maps of Mercury's surface chemistry that reveal previously unrecognized geochemical terranes -- large regions that have compositions distinct from their surroundings. The presence of these large terranes has important implications for the history of the planet.

The MESSENGER mission was designed to answer several key scientific questions, including the nature of Mercury's geological history. Remote sensing of the surface's chemical composition has a strong bearing on this and other questions. Since MESSENGER was inserted into orbit about Mercury in March 2011, data from the spacecraft's X-Ray Spectrometer (XRS) and Gamma-Ray Spectrometer (GRS) have provided information on the concentrations of potassium, thorium, uranium, sodium, chlorine, and silicon, as well as ratios relative to silicon of magnesium, aluminum, sulfur, calcium, and iron.

Until now, however, geochemical maps for some of these elements and ratios have been limited to one hemisphere and have had poor spatial resolution. In "Evidence for geochemical terranes on Mercury: Global mapping of major elements with MESSENGER's X-Ray Spectrometer," published this week in Earth and Planetary Science Letters, the authors used a novel methodology to produce global maps of the magnesium/silicon and aluminum/silicon abundance ratios across Mercury's surface from data acquired by MESSENGER's XRS.

These are the first global geochemical maps of Mercury, and the first maps of global extent for any planetary body acquired via the technique of X-ray fluorescence, by which X-rays emitted from the Sun's atmosphere allow the planet's surface composition to be examined. The global magnesium and aluminum maps were paired with less spatially complete maps of sulfur/silicon, calcium/silicon, and iron/silicon, as well as other MESSENGER datasets, to study the geochemical characteristics of Mercury's surface and to investigate the evolution of the planet's thin silicate shell.

The most obvious of Mercury's geochemical terranes is a large feature, spanning more than 5 million square kilometers. This terrane "exhibits the highest observed magnesium/silicon, sulfur/silicon, and calcium/silicon ratios, as well as some of the lowest aluminum/silicon ratios on the planet's surface," writes Shoshana Weider, a planetary geologist and Visiting Scientist at the Carnegie Institution. Weider and colleagues suggest that this "high-magnesium region" could be the site of an ancient impact basin. By this interpretation, the distinctive chemical signature of the region reflects a substantial contribution from mantle material that was exposed during a large impact event.

A second paper, "Geochemical terranes of Mercury's northern hemisphere as revealed by MESSENGER neutron measurements," now available online in Icarus, presents the first maps of the absorption of low-energy ("thermal") neutrons across Mercury's surface. The data used in this second study were obtained with the GRS anti-coincidence shield, which is sensitive to neutron emissions from the surface of Mercury.

"From these maps we may infer the distribution of thermal-neutron-absorbing elements across the planet, including iron, chlorine, and sodium," writes lead author Patrick Peplowski of The Johns Hopkins University Applied Physics Laboratory. "This information has been combined with other MESSENGER geochemical measurements, including the new XRS measurements, to identify and map four distinct geochemical terranes on Mercury."

According to Peplowski, the results indicate that the smooth plains interior to the Caloris basin, Mercury's largest well-preserved impact basin, have an elemental composition that is distinct from other volcanic plains units, suggesting that the parental magmas were partial melts from a chemically distinct portion of Mercury's mantle. Mercury's high-magnesium region, first recognized from the XRS measurements, also contains high concentrations of unidentified neutron-absorbing elements.

"Earlier MESSENGER data have shown that Mercury's surface was pervasively shaped by volcanic activity," notes Peplowski. "The magmas erupted long ago were derived from the partial melting of Mercury's mantle. The differences in composition that we are observing among geochemical terranes indicate that Mercury has a chemically heterogeneous mantle."

"The consistency of the new XRS and GRS maps provides a new dimension to our view of Mercury's surface," Weider adds. "The terranes we observe had not previously been identified on the basis of spectral reflectance or geological mapping."

"The crust we see on Mercury was largely formed more than three billion years ago," says Carnegie's Larry Nittler, Deputy Principal Investigator of the mission and co-author of both studies. "The remarkable chemical variability revealed by MESSENGER observations will provide critical constraints on future efforts to model and understand Mercury's bulk composition and the ancient geological processes that shaped the planet's mantle and crust."

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Saturn moon's ocean may harbor hydrothermal activity, spacecraft data suggest

This cutaway view of Saturn's moon Enceladus is an artist's rendering that depicts possible hydrothermal activity that may be taking place on and under the seafloor of the moon's subsurface ocean, based on recently published results from NASA's Cassini mission.

NASA's Cassini spacecraft has provided scientists the first clear evidence that Saturn's moon Enceladus exhibits signs of present-day hydrothermal activity which may resemble that seen in the deep oceans on Earth. The implications of such activity on a world other than our planet open up unprecedented scientific possibilities.

"These findings add to the possibility that Enceladus, which contains a subsurface ocean and displays remarkable geologic activity, could contain environments suitable for living organisms," said John Grunsfeld, astronaut and associate administrator of NASA's Science Mission Directorate in Washington. "The locations in our solar system where extreme environments occur in which life might exist may bring us closer to answering the question: are we alone in the universe."

Hydrothermal activity occurs when seawater infiltrates and reacts with a rocky crust and emerges as a heated, mineral-laden solution, a natural occurrence in Earth's oceans. According to two science papers, the results are the first clear indications an icy moon may have similar ongoing active processes.

The first paper, published this week in the journal Nature, relates to microscopic grains of rock detected by Cassini in the Saturn system. An extensive, four-year analysis of data from the spacecraft, computer simulations and laboratory experiments led researchers to the conclusion the tiny grains most likely form when hot water containing dissolved minerals from the moon's rocky interior travels upward, coming into contact with cooler water. Temperatures required for the interactions that produce the tiny rock grains would be at least 194 degrees Fahrenheit (90 degrees Celsius).

"It's very exciting that we can use these tiny grains of rock, spewed into space by geysers, to tell us about conditions on -- and beneath -- the ocean floor of an icy moon," said the paper's lead author Sean Hsu, a postdoctoral researcher at the University of Colorado at Boulder.

Cassini's cosmic dust analyzer (CDA) instrument repeatedly detected miniscule rock particles rich in silicon, even before Cassini entered Saturn's orbit in 2004. By process of elimination, the CDA team concluded these particles must be grains of silica, which is found in sand and the mineral quartz on Earth. The consistent size of the grains observed by Cassini, the largest of which were 6 to 9 nanometers, was the clue that told the researchers a specific process likely was responsible.

On Earth, the most common way to form silica grains of this size is hydrothermal activity under a specific range of conditions; namely, when slightly alkaline and salty water that is super-saturated with silica undergoes a big drop in temperature.

"We methodically searched for alternate explanations for the nanosilica grains, but every new result pointed to a single, most likely origin," said co-author Frank Postberg, a Cassini CDA team scientist at Heidelberg University in Germany.

Hsu and Postberg worked closely with colleagues at the University of Tokyo who performed the detailed laboratory experiments that validated the hydrothermal activity hypothesis. The Japanese team, led by Yasuhito Sekine, verified the conditions under which silica grains form at the same size Cassini detected. The researchers think these conditions may exist on the seafloor of Enceladus, where hot water from the interior meets the relatively cold water at the ocean bottom.

The extremely small size of the silica particles also suggests they travel upward relatively quickly from their hydrothermal origin to the near-surface sources of the moon's geysers. From seafloor to outer space, a distance of about 30 miles (50 kilometers), the grains spend a few months to a few years in transit, otherwise they would grow much larger.

The authors point out that Cassini's gravity measurements suggest Enceladus' rocky core is quite porous, which would allow water from the ocean to percolate into the interior. This would provide a huge surface area where rock and water could interact.

The second paper, recently published in Geophysical Research Letters, suggests hydrothermal activity as one of two likely sources of methane in the plume of gas and ice particles that erupts from the south polar region of Enceladus. The finding is the result of extensive modeling to address why methane, as previously sampled by Cassini, is curiously abundant in the plume.

The team found that, at the high pressures expected in the moon's ocean, icy materials called clathrates could form that imprison methane molecules within a crystal structure of water ice. Their models indicate that this process is so efficient at depleting the ocean of methane that the researchers still needed an explanation for its abundance in the plume.

In one scenario, hydrothermal processes super-saturate the ocean with methane. This could occur if methane is produced faster than it is converted into clathrates. A second possibility is that methane clathrates from the ocean are dragged along into the erupting plumes and release their methane as they rise, like bubbles forming in a popped bottle of champagne.

The authors agree both scenarios are likely occurring to some degree, but they note that the presence of nanosilica grains, as documented by the other paper, favors the hydrothermal scenario.

"We didn't expect that our study of clathrates in the Enceladus ocean would lead us to the idea that methane is actively being produced by hydrothermal processes," said lead author Alexis Bouquet, a graduate student at the University of Texas at San Antonio. Bouquet worked with co-author Hunter Waite, who leads the Cassini Ion and Neutral Mass Spectrometer (INMS) team at Southwest Research Institute in San Antonio.

Cassini first revealed active geological processes on Enceladus in 2005 with evidence of an icy spray issuing from the moon's south polar region and higher-than-expected temperatures in the icy surface there. With its powerful suite of complementary science instruments, the mission soon revealed a towering plume of water ice and vapor, salts and organic materials that issues from relatively warm fractures on the wrinkled surface. Gravity science results published in 2014 strongly suggested the presence of a 6-mile- (10-kilometer-) deep ocean beneath an ice shell about 19 to 25 miles (30 to 40 kilometers) thick.

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Engineers create chameleon-like artificial 'skin' that shifts color on demand

Developed by engineers from the University of California at Berkeley, this chameleon-like artificial "skin" changes color as a minute amount of force is applied.

Borrowing a trick from nature, engineers from the University of California at Berkeley have created an incredibly thin, chameleon-like material that can be made to change color -- on demand -- by simply applying a minute amount of force.

This new material-of-many-colors offers intriguing possibilities for an entirely new class of display technologies, color-shifting camouflage, and sensors that can detect otherwise imperceptible defects in buildings, bridges, and aircraft.

"This is the first time anybody has made a flexible chameleon-like skin that can change color simply by flexing it," said Connie J. Chang-Hasnain, a member of the Berkeley team and co-author on a paper published today in Optica, The Optical Society's (OSA) new high-impact journal.

By precisely etching tiny features -- smaller than a wavelength of light -- onto a silicon film one thousand times thinner than a human hair, the researchers were able to select the range of colors the material would reflect, depending on how it was flexed and bent.

A Material that's a Horse of a Different Color

The colors we typically see in paints, fabrics, and other natural substances occur when white, broad spectrum light strikes their surfaces. The unique chemical composition of each surface then absorbs various bands, or wavelengths of light. Those that aren't absorbed are reflected back, with shorter wavelengths giving objects a blue hue and longer wavelengths appearing redder and the entire rainbow of possible combinations in between. Changing the color of a surface, such as the leaves on the trees in autumn, requires a change in chemical make-up.

Recently, engineers and scientists have been exploring another approach, one that would create designer colors without the use of chemical dyes and pigments. Rather than controlling the chemical composition of a material, it's possible to control the surface features on the tiniest of scales so they interact and reflect particular wavelengths of light. This type of "structural color" is much less common in nature, but is used by some butterflies and beetles to create a particularly iridescent display of color.

Controlling light with structures rather than traditional optics is not new. In astronomy, for example, evenly spaced slits known as diffraction gratings are routinely used to direct light and spread it into its component colors. Efforts to control color with this technique, however, have proved impractical because the optical losses are simply too great.

The authors of the Optica paper applied a similar principle, though with a radically different design, to achieve the color control they were looking for. In place of slits cut into a film they instead etched rows of ridges onto a single, thin layer of silicon. Rather than spreading the light into a complete rainbow, however, these ridges -- or bars -- reflect a very specific wavelength of light. By "tuning" the spaces between the bars, it's possible to select the specific color to be reflected. Unlike the slits in a diffraction grating, however, the silicon bars were extremely efficient and readily reflected the frequency of light they were tuned to.

Flexibility Is the Key to Control

Since the spacing, or period, of the bars is the key to controlling the color they reflect, the researchers realized it would be possible to subtly shift the period -- and therefore the color -- by flexing or bending the material.

"If you have a surface with very precise structures, spaced so they can interact with a specific wavelength of light, you can change its properties and how it interacts with light by changing its dimensions," said Chang-Hasnain.

Earlier efforts to develop a flexible, color shifting surface fell short on a number of fronts. Metallic surfaces, which are easy to etch, were inefficient, reflecting only a portion of the light they received. Other surfaces were too thick, limiting their applications, or too rigid, preventing them from being flexed with sufficient control.

The Berkeley researchers were able to overcome both these hurdles by forming their grating bars using a semiconductor layer of silicon approximately 120 nanometers thick. Its flexibility was imparted by embedding the silicon bars into a flexible layer of silicone. As the silicone was bent or flexed, the period of the grating spacings responded in kind.

The semiconductor material also allowed the team to create a skin that was incredibly thin, perfectly flat, and easy to manufacture with the desired surface properties. This produces materials that reflect precise and very pure colors and that are highly efficient, reflecting up to 83 percent of the incoming light.

Their initial design, subjected to a change in period of a mere 25 nanometers, created brilliant colors that could be shifted from green to yellow, orange, and red -- across a 39-nanometer range of wavelengths. Future designs, the researchers believe, could cover a wider range of colors and reflect light with even greater efficiency.

Chameleon Skin with Multiple Applications

For this demonstration, the researchers created a one-centimeter square layer of color-shifting silicon. Future developments would be needed to create a material large enough for commercial applications.

"The next step is to make this larger-scale and there are facilities already that could do so," said Chang-Hasnain. "At that point, we hope to be able to find applications in entertainment, security, and monitoring."

For consumers, this chameleon material could be used in a new class of display technologies, adding brilliant color presentations to outdoor entertainment venues. It also may be possible to create an active camouflage on the exterior of vehicles that would change color to better match the surrounding environment.

More day-to-day applications could include sensors that would change color to indicate that structural fatigue was stressing critical components on bridges, buildings, or the wings of airplanes.

"This is the first time anyone has achieved such a broad range of color on a one-layer, thin and flexible surface," concluded Change-Hasnain. "I think it's extremely cool."

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Underground ocean on Jupiter's largest moon, Ganymede

Observation of Aurorae on Ganymede. NASA's Hubble Space Telescope observed a pair of auroral belts encircling the Jovian moon Ganymede. The belts were observed in ultraviolet light by the Space Telescope Imaging Spectrograph and are colored blue in this illustration. They are overlaid on a visible-light image of Ganymede taken by NASA's Galileo orbiter. The locations of the glowing aurorae are determined by the moon's magnetic field, and therefore provide a probe of the moon's interior, where the magnetic field is generated. The amount of rocking of the magnetic field, caused by its interaction with Jupiter's own immense magnetosphere, provides evidence that the moon has a subsurface ocean of saline water.

NASA's Hubble Space Telescope has the best evidence yet for an underground saltwater ocean on Ganymede, Jupiter's largest moon. The subterranean ocean is thought to have more water than all the water on Earth's surface.

Identifying liquid water is crucial in the search for habitable worlds beyond Earth and for the search for life, as we know it.

"This discovery marks a significant milestone, highlighting what only Hubble can accomplish," said John Grunsfeld, assistant administrator of NASA's Science Mission Directorate at NASA Headquarters, Washington, D.C. "In its 25 years in orbit, Hubble has made many scientific discoveries in our own solar system. A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth."

Ganymede is the largest moon in our solar system and the only moon with its own magnetic field. The magnetic field causes aurorae, which are ribbons of glowing, hot electrified gas, in regions circling the north and south poles of the moon. Because Ganymede is close to Jupiter, it is also embedded in Jupiter's magnetic field. When Jupiter's magnetic field changes, the aurorae on Ganymede also change, "rocking" back and forth.

By watching the rocking motion of the two aurorae, scientists were able to determine that a large amount of saltwater exists beneath Ganymede's crust, affecting its magnetic field.

A team of scientists led by Joachim Saur of the University of Cologne in Germany came up with the idea of using Hubble to learn more about the inside of the moon.

"I was always brainstorming how we could use a telescope in other ways," said Saur. "Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon's interior."

If a saltwater ocean were present, Jupiter's magnetic field would create a secondary magnetic field in the ocean that would counter Jupiter's field. This "magnetic friction" would suppress the rocking of the aurorae. This ocean fights Jupiter's magnetic field so strongly that it reduces the rocking of the aurorae to 2 degrees, instead of 6 degrees if the ocean were not present.

Scientists estimate the ocean is 60 miles (100 kilometers) thick -- 10 times deeper than Earth's oceans -- and is buried under a 95-mile (150-kilometer) crust of mostly ice.

Scientists first suspected an ocean in Ganymede in the 1970s, based on models of the large moon. NASA's Galileo mission measured Ganymede's magnetic field in 2002, providing the first evidence supporting those suspicions. The Galileo spacecraft took brief "snapshot" measurements of the magnetic field in 20-minute intervals, but its observations were too brief to distinctly catch the cyclical rocking of the ocean's secondary magnetic field.

The new observations were done in ultraviolet light and could only be accomplished with a space telescope high above Earth's atmosphere, which blocks most ultraviolet light.

The team's results will be published online in the Journal of Geophysical Research: Space Physics on March 12.

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