Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p23d..01s&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P23D-01
Other
[5400] Planetary Sciences: Solid Surface Planets, [5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [5440] Planetary Sciences: Solid Surface Planets / Magnetic Fields And Magnetism, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties
Scientific paper
MESSENGER is the first spacecraft to visit the planet Mercury in more than 30 years. Images from MESSENGER’s first Mercury flyby, in January 2008, provided evidence for widespread volcanism, and candidate sites for volcanic centers were identified. Newly imaged lobate scarps and other tectonic landforms confirmed that Mercury contracted globally in response to interior cooling. The ~1500-km-diameter Caloris basin, viewed in its entirety for the first time by MESSENGER, was the focus for concentrations of volcanic centers, some with evidence of pyroclastic deposits, and widespread contractional and extensional deformation. Smooth plains interior and exterior to the basin are demonstrably younger than the basin-forming event. Reflectance spectra of Mercury’s surface showed no evidence for FeO in surface silicates. Reflectance and color imaging observations provided fresh support for earlier inferences that Mercury’s surface material consists dominantly of iron-poor, calcium-magnesium silicates with an admixture of spectrally neutral opaque minerals. MESSENGER’s second flyby, in October 2008, revealed the presence of neutral Mg and Ca in Mercury’s anti-sunward tail and documented strongly differing distributions of Mg, Ca, and Na in the nightside exosphere, the result of different combinations of time-variable source, transfer, and loss processes. A southward interplanetary magnetic field (IMF) was accompanied by multiple indications of magnetic reconnection at rates ~10 times typical at Earth; in combination with the more quiescent conditions under northward IMF seen during the first flyby, the results indicate that Mercury’s magnetosphere is more responsive to IMF direction than those of other planets. The internal magnetic field is dominantly dipolar with a vector moment closely aligned with the spin axis. The nearly global observations of Mercury surface units distinguishable by color and composition enforce the importance of the largely volcanic smooth plains, which occupy ~40% of the surface area, and of low-reflectance material, occupying ~15% of the surface area and primarily in deposits excavated by impact, consistent with having formed at depth. The second flyby also revealed the ~700-km-diameter Rembrandt basin, less volcanically infilled than Caloris, but likewise a focus for concentrated magmatic and deformational activity. On 29 September, MESSENGER flew by Mercury for a third time, an encounter that featured targeted observations of the surface, exosphere, and tail optimized from the experiences of the first two flybys.
Anderson Benjamin J.
Bedini Peter D.
Evans Larry G.
Gold Robert E.
Head James W.
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