[meteorite-list] New MESSENGER Maps of Mercury's Surface Chemistry Provide Clues to the Planet's History

[Ron Baalke]

http://messenger.jhuapl.edu/news_room/details.php?id=273

MESSENGER Mission News
March 13, 2015

New MESSENGER Maps of Mercury's Surface Chemistry Provide Clues to the 
Planet's History

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."

[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.
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MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) 
is a NASA-sponsored scientific investigation of the planet Mercury and 
the first space mission designed to orbit the planet closest to the Sun. 
The MESSENGER spacecraft was launched on August 3, 2004, and entered orbit 
about Mercury on March 18, 2011, to begin a yearlong study of its target 
planet. MESSENGER's first extended mission began on March 18, 2012, and 
ended one year later. MESSENGER is now in a second extended mission, which 
is scheduled to conclude in March 2015. Dr. Sean C. Solomon, the Director 
of Columbia University's Lamont-Doherty Earth Observatory, leads the mission 
as Principal Investigator. The Johns Hopkins University Applied Physics 
Laboratory built and operates the MESSENGER spacecraft and manages this 
Discovery-class mission for NASA.