Mathematics – Logic
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p23d..02p&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P23D-02
Mathematics
Logic
[5417] Planetary Sciences: Solid Surface Planets / Gravitational Fields, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [5450] Planetary Sciences: Solid Surface Planets / Orbital And Rotational Dynamics, [6235] Planetary Sciences: Solar System Objects / Mercury
Scientific paper
Two MESSENGER flybys have provided new insights into the geophysics of Mercury. Contractional lobate scarps are globally distributed, tend to be isolated rather than concentrated, and formed over a period extending from the end of heavy bombardment to after formation of the youngest smooth plains. Such characteristics are most consistent with formation as a result of slow cooling and contraction of the interior. This view is supported by new models based on MESSENGER Mercury Laser altimeter (MLA) observations that demonstrate substantial strain accumulation on faults by brittle localization (fault formation) or velocity weakening (sliding on mature faults). These processes are only weakly localizing but permit slip accumulation over long timescales, consistent with the geological observations. The range of shallow thermal gradients that yield this situation under the assumption of modest fault weakening is consistent with previous models for Mercury’s lithospheric structure and early thermal history. Data from MLA provided new information on the equatorial shape of Mercury, and Doppler tracking of MESSENGER through the flyby encounters provided new gravity field data. The two MLA passes, on opposite hemispheres, place a strong constraint on the elliptical (2nd-degree) shape of the equator, showing that its long axis is oriented close to 0° longitude. Rotational dynamics requires that the 2nd-degree geoid shape be oriented in the same direction, and a full 4×4 spherical harmonic gravity field expansion of the Doppler data confirms this. Adopting the effective elastic thickness inferred from depth of faulting, the correlation of gravity and shape constrains the allowable crustal thickness to be less than an upper limit of 140 km required for a non-melting lower crust. Improvement in the estimation of the C22 gravity coefficient strengthens the earlier conclusion that Mercury’s outer core is fluid. The gravitational oblateness, J2, is poorly resolved from the two near-equatorial flybys, but the mass of the planet has been more precisely determined than from Mariner 10. The third flyby, which occurred on 29 September, provides further constraints on the geophysical nature of Mercury.
Farmer Grant T.
Hauck Steven A.
Johnson Clifton L.
Lemoine Frank G.
Margot Joelle
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