The low‐degree shape of Mercury

The shape of Mercury, particularly when combined with its geoid, provides clues to the planet's internal structure, thermal evolution, and rotational history. Elevation measurements of the northern hemisphere acquired by the Mercury Laser Altimeter on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, combined with 378 occultations of radio signals from the spacecraft in the planet's southern hemisphere, reveal the low-degree shape of Mercury. Mercury's mean radius is 2439.36 \plusmn 0.02 km, and there is a 0.14 km offset between the planet's centers of mass and figure. Mercury is oblate, with a polar radius 1.65 km less than the mean equatorial radius. The difference between the semimajor and semiminor equatorial axes is 1.25 km, with the long axis oriented 15ẹg west of Mercury's dynamically defined principal axis. Mercury's geoid is also oblate and elongated, but it deviates from a sphere by a factor of 10 less than Mercury's shape, implying compensation of elevation variations on a global scale.

[1]  Daniel J. Scheeres,et al.  Characterizing and navigating small bodies with imaging data , 2006 .

[2]  W. M. Kaula.  The gravity and shape of the Moon , 1975 .

[3]  Jürgen Oberst,et al.  The equatorial shape and gravity field of Mercury from MESSENGER flybys 1 and 2 , 2010 .

[4]  Martin A. Slade,et al.  Shape and Orientation of Mercury from Radar Ranging Data , 1995 .

[5]  H. Jay Melosh,et al.  Planetary Surface Processes: References , 2011 .

[6]  U. Fink,et al.  New upper limits for atmospheric constituents on Io , 1976 .

[7]  David E. Smith,et al.  Initial observations from the Lunar Orbiter Laser Altimeter (LOLA) , 2010 .

[8]  P. Esposito,et al.  The occultation of Mariner 10 by Mercury , 1976 .

[9]  S. Murchie,et al.  The distribution and origin of smooth plains on Mercury , 2013 .

[10]  Donald B. Campbell,et al.  Radar altimetry of Mercury: A Preliminary analysis , 1986 .

[11]  Erwan Mazarico,et al.  The gravity field, orientation, and ephemeris of Mercury from MESSENGER observations after three years in orbit , 2014 .

[12]  Jürgen Oberst,et al.  Measurement of the radius of Mercury by radio occultation during the MESSENGER flybys , 2011 .

[13]  M. Kobrick,et al.  Venus topography:a harmonic analysis , 1985 .

[14]  M. Zuber,et al.  Support of long‐wavelength topography on Mercury inferred from MESSENGER measurements of gravity and topography , 2015 .

[15]  David E. Smith,et al.  Topography of the Northern Hemisphere of Mercury from MESSENGER Laser Altimetry , 2012, Science.

[16]  J. T. Ratcliff,et al.  Lunar rotational dissipation in solid body and molten core , 2001 .

[17]  Donald E. Gault,et al.  Preliminary imaging results from the second Mercury encounter , 1975 .

[18]  I. Matsuyama,et al.  Gravity and tectonic patterns of Mercury: Effect of tidal deformation, spin‐orbit resonance, nonzero eccentricity, despinning, and reorientation , 2009 .

[19]  David E. Smith,et al.  The Mercury Laser Altimeter Instrument for the MESSENGER Mission , 2007 .

[20]  Jean-Luc Margot,et al.  Thickness of the crust of Mercury from geoid‐to‐topography ratios , 2015 .

[21]  David E. Smith,et al.  The curious case of Mercury's internal structure , 2013 .

[22]  David E. Smith,et al.  Gravity Field and Internal Structure of Mercury from MESSENGER , 2012, Science.

[23]  David E. Smith,et al.  Mercury's 2nd-Degree Shape and Geoid: Lunar Comparisons and Thermal Anomalies , 2014 .

[24]  K. L. Edmundson,et al.  Global Controlled Mosaic of Mercury from MESSENGER Orbital Images , 2011 .

[25]  J. Laskar,et al.  Mercury's capture into the 3/2 spin–orbit resonance including the effect of core–mantle friction , 2009, 0901.1843.

[26]  D. Tholen,et al.  Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009 , 2011 .

[27]  Jürgen Oberst,et al.  Mercury׳s global shape and topography from MESSENGER limb images , 2014 .

[28]  Georges Balmino,et al.  Gravitational potential harmonics from the shape of an homogeneous body , 1994 .

[29]  David E. Smith,et al.  The global topography of Mars and implications for surface evolution. , 1999, Science.

[30]  F. S. Turner,et al.  Radius and limb topography of Mercury obtained from images acquired during the MESSENGER flybys , 2011 .