P53A-2103
The gravity field and orientation of Mercury after the MESSENGER mission

Friday, 18 December 2015
Poster Hall (Moscone South)
Erwan Mazarico1, Antonio Genova2, Sander J Goossens3, Frank G Lemoine4, Gregory A Neumann5, Maria T Zuber6, David E Smith2 and Sean C Solomon7, (1)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (2)Massachusetts Institute of Technology, Cambridge, MA, United States, (3)University of Maryland Baltimore County, Baltimore, MD, United States, (4)NASA Goddard SFC, Greenbelt, MD, United States, (5)NASA, Baltimore, MD, United States, (6)Massachusetts Inst Tech, Cambridge, MA, United States, (7)Columbia University of New York, Palisades, NY, United States
Abstract:
After more than four years in orbit about Mercury, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft impacted the planet’s surface north of Shakespeare crater (54.44° N, 210.12° E,) on 30 April 2015.

One of the main goals of the mission was to determine the gravity field of Mercury in order to learn about Mercury’s interior. Together with ground-based radar measurements of the obliquity and forced librations, MESSENGER-derived gravity models helped revise models of Mercury’s interior. Nevertheless, the refinement of Mercury’s orientation with the latest data from MESSENGER can further improve the interior modeling of the planet. The last eight months of the mission provided a special opportunity to conduct low-altitude measurements, with extensive radio tracking coverage below 200 km altitude north of ~30°N.

MESSENGER’s Mercury Laser Altimeter (MLA) mapped the topography of Mercury’s northern hemisphere with a sub-meter vertical precision, an along-track sampling of ~500 m, and a longitudinal resolution (~0.1°) limited by the number of spacecraft orbits (~4,000). The combination of gravity and topography helps determine crustal thickness and interior properties. Altimetric ranges provide geodetic constraints to improve the spacecraft orbit determination, and thus the gravity field model. In particular, whereas the MESSENGER spacecraft was not tracked at each periapsis passage, MLA operated nearly continuously (outside of thermally challenging periods). From an analysis of the entire radiometric and altimetric datasets acquired by MESSENGER, a new gravity field to degree and order 100 has been obtained, resolving features down to ~75 km horizontal scale. The altimetric data help reduce the uncertainties in the determination of the pole position. A reanalysis of the Mercury flybys also constrains the spin rate over the longest available time span.

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