Scientific Colloquium
January 15, 2010
SEAN SOLOMON
CARNEGIE INSTITUTION OF WASHINGTON
"Exploring
Mercury by Spacecraft: the MESSENGER Flybys"
The MErcury Surface, Space
ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft,
developed under NASA’s Discovery Program, is the first space probe to
visit the planet Mercury in more than 30 years. MESSENGER flew by the
innermost planet twice in 2008 and once last fall. MESSENGER’s flybys
confirmed that Mercury’s internal magnetic field is primarily dipolar
and documented that the magnetosphere is more responsive to variations
in solar winds conditions than that of any other planet. MESSENGER
detected magnesium in Mercury’s exosphere, demonstrated that Mercury’s
anti-sunward neutral tail contains multiple species, and revealed that
the distributions of sodium, calcium, and magnesium in the exosphere
and tail vary differently with latitude, time of day, and Mercury’s
position in orbit, signatures of multiple source processes. MESSENGER’s
laser altimeter showed that the equatorial topographic relief of
Mercury exceeds 5 km and documented the relief of numerous impact
craters and fault scarps. MESSENGER’s images provided evidence for
widespread volcanism, and candidate sites for volcanic centers were
identified. Also revealed were newly imaged lobate scarps and other
tectonic landforms supportive of the hypothesis that Mercury contracted
globally in response to interior cooling. Reflectance spectra show no
evidence for FeO in surface silicates, and reflectance and color
imaging observations support earlier inferences that Mercury’s surface
material consists dominantly of iron-poor, calcium-magnesium silicates
with an admixture of spectrally neutral opaque minerals, but
MESSENGER’s neutron spectrometer showed that the surface abundance of
iron plus titanium is comparable to that of several lunar mare regions.
MESSENGER is now on course for insertion into orbit about Mercury in
March 2011, and a full Earth-year of orbital observations is planned
for the remainder of the nominal mission.