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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun, 15 Mar 2015 23:26:50 -0700 (PDT)
Message-ID: <201503160626.t2G6QoRS013086_at_zagami.jpl.nasa.gov>

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.
-----------------------------
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.
Received on Mon 16 Mar 2015 02:26:50 AM PDT