Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p41e..01s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P41E-01
Mathematics
Logic
[5440] Planetary Sciences: Solid Surface Planets / Magnetic Fields And Magnetism, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [5475] Planetary Sciences: Solid Surface Planets / Tectonics, [5480] Planetary Sciences: Solid Surface Planets / Volcanism
Scientific paper
One of Earth's closest planetary neighbors, Mercury remained comparatively unexplored for the more than three decades that followed the three flybys of the innermost planet by the Mariner 10 spacecraft in 1974-75. Mariner 10 imaged 45% of Mercury's surface at about 1 km/pixel average resolution, confirmed Mercury's anomalously high bulk density and implied large fractional core size, discovered Mercury's internal magnetic field, documented that H and He are present in the planet's tenuous exosphere, and made the first exploration of Mercury's magnetosphere and solar wind environment. Ground-based astronomers later reported Na, K, and Ca in Mercury's exosphere; the presence of deposits in the floors of polar craters having radar characteristics best matched by water ice; and strong evidence from the planet's forced libration amplitude that Mercury has a fluid outer core. Spacecraft exploration of Mercury resumed with the selection for flight, under NASA's Discovery Program, of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. Launched in 2004, MESSENGER flew by the innermost planet three times in 2008-2009 en route to becoming the first spacecraft to orbit Mercury in March of this year. MESSENGER's first chemical remote sensing measurements of Mercury's surface indicate that the planet's bulk silicate fraction differs from those of the other inner planets, with a low-Fe surface composition intermediate between basalts and ultramafic rocks and best matched among terrestrial rocks by komatiites. Moreover, surface materials are richer in the volatile constituents S and K than predicted by most planetary formation models. Global image mosaics and targeted high-resolution images (to resolutions of 10 m/pixel) reveal that Mercury experienced globally extensive volcanism, including large expanses of plains emplaced as flood lavas and widespread examples of pyroclastic deposits likely emplaced during explosive eruptions of volatile-bearing magmas. Bright deposits within impact craters host fresh-appearing, rimless depressions or hollows, often displaying high-reflectance interiors and halos and likely formed through processes involving the geologically recent loss of volatiles. The tectonic history of Mercury, although dominated by near-global contractional deformation as first seen by Mariner 10, is more complex than first appreciated, with numerous examples of extensional deformation that accompanied impact crater and basin modification. Mercury's magnetic field is dominantly dipolar, but the field is axially symmetric and equatorially asymmetric, a geometry that poses challenges to dynamo models for field generation. The interaction between the solar wind and Mercury's magnetosphere, among the most dynamic in the solar system, serves both to replenish the exosphere and space weather the planet's surface. Plasma ions of planetary origin are seen throughout the sampled volume of Mercury's magnetosphere, with maxima in heavy-ion fluxes in the planet's magnetic-cusp regions. Bursts of energetic electrons, seen at most local times, point to an efficient acceleration mechanism operating within Mercury's magnetosphere on a regular basis that produces electrons with energies up to hundreds of keV on timescales of seconds.
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