REPORT Laser Altimeter Observations from MESSENGER’s First Mercury Flyby

spacecraft spans ~20 % of the near-equatorial region of the planet. Topography along the profile is characterized by a 5.2-kilometer dynamic range and 930-meter root-mean-square roughness. At long wavelengths, topography slopes eastward by 0.02°, implying a variation of equatorial shape that is at least partially compensated. Sampled craters on Mercury are shallower than their counterparts on the Moon, at least in part the result of Mercury’s higher gravity. Crater floors vary in roughness and slope, implying complex modification over a range of length scales. Topography is a fundamental measurementto characterize quantitatively the surfacesof solid planetary bodies at length scales ranging from the long-wavelength shape to such local and regional processes as impact cratering, volcanism, and faulting. During the first flyby of Mercury by the MESSENGER spacecraft on 14 January 2008 (1), the Mercury Laser Altimeter (MLA) (2, 3) successfully ranged to the planet’s surface, providing the first altimetric observations of the planet from a spacecraft