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 of Washington. 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."
See related figure. Caption: 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.
Mercury seen as never before
In its final weeks, the MESSENGER mission reveals fresh details about the planet's scorched surface.
Permanent shadows in craters on Mercury's surface allow ice to survive there.
NASA's MESSENGER spacecraft is set to plunge to its doom on 30 April, ending nearly four years of exploring Mercury. Before it goes, the mission is sending back the best images ever taken of the planet.
In the shots, released on 16 March, ice materializes in pits and swirls at the bottoms of craters, still frozen despite being so close to the Sun. Elsewhere on Mercury, short stairstep-like ridges appear, miniature versions of the huge ‘scarps’ the planet is famous for. And tiny hollows mark places where parts of the surface have been scoured away through some kind of powerful space weathering.
MESSENGER, which has been orbiting Mercury since 2011, has nearly run out of propellant to guide it. The spacecraft is currently about 15 kilometres above the planet's surface, the closest it has ever been.
“We’re able to see at close range portions of the planet we haven’t seen in such detail before,” says Sean Solomon, a geophysicist at the Lamont-Doherty Earth Observatory in Palisades, New York, and principal investigator for the mission. Solomon and other team members presented the new findings at the Lunar and Planetary Science Conference in The Woodlands, Texas.
Those include new perspectives on one of the mission’s biggest discoveries to date — the ice that lurks in permanently shadowed craters near Mercury’s poles. The ice remains frozen on sun-baked Mercury because it is never in direct sunlight, says Nancy Chabot, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
In the new low-altitude images, MESSENGER peers inside roughly a dozen craters near Mercury's poles. At first, photographs of the crater floors looked like nothing but black, because the bright crater rims oversaturate the images. But by processing the photographs differently, Chabot could see the dim crater floors pop into view. “We’re seeing into these regions where the sun never shines on Mercury,” she says.
Mercury's Fuller Crater may contain ice from a cosmic collision.
One crater, named Fuller after the architect Buckminster Fuller, shows patterned areas of light and dark — perhaps some sort of dark, carbon-rich material overlying the ice beneath. The boundary between light and dark areas is sharply defined, suggesting that they formed relatively recently. One possibility is that a space rock traveling from farther out in the Solar System slammed into Mercury, depositing water in the form of ice with darker material atop it.
Mercury is also famous for the long ridges, or scarps, that run across much of its surface. The biggest scarps, which can be hundreds of kilometres long, are probably cracks that formed as the planet cooled and shrank over time. Now, MESSENGER has spotted miniature versions of these scarps.
Long ridges, or 'scarps', run across Mercury's surface. Now researchers have found smaller versions of these formations on the planet.
They seem to appear in clusters, says Thomas Watters, a planetary scientist at the National Air and Space Museum in Washington DC. The small scarps also sometimes appear near dropped-down portions of Mercury’s crust, which look like portions of Earth’s crust that get moved around in active earthquake zones. “These scarps are exciting,” says Watters. “These faults are so young that they are probably forming today.”
Finally, MESSENGER has also seen new details in the mysterious ‘hollows’ first spotted when the spacecraft went into orbit. Strange bright areas inside some craters turned out to be irregularly shaped depressions, says David Blewett, a planetary scientist at the Laurel laboratory.
MESSENGER first spotted 'hollows' on Mercury in 2011, but the latest images show these depressions in greater detail.
The close-up images show that the hollows look younger than nearly anything else on Mercury. That also suggests that the planet is going through some kind of recent changes.
NASA’s Dawn spacecraft, which has just arrived at the water-rich asteroid Ceres, is beginning to see intriguing bright glints in crater bottoms. Researchers are anxious to see if these turn out to be like anything at Mercury.
Mission controllers will perform five more short engine burns to boost MESSENGER’s altitude, before the planet’s gravity drags it to its inevitable doom. A European Space Agency mission, BepiColumbo, is due to arrive at Mercury in 2024, to pick up where MESSENGER leaves off.