The Mars Orbiter Laser Altimeter Archive: Final Precision Experiment Data Record Re- Lease and Status of Radiometry

A final release (Version L) of the Mars Orbiter Laser Altimeter (MOLA) Precision Experiment Data Record (PEDR) has been submitted to the Planetary Data System (PDS). Additional gridded data record products are forthcoming. These products have evolved since their original description, owing in part to improved gravity modeling and cartographic reference frames, and in part to refinements in calibration. An additional component, the 1064 nm narrowband radiometry data, is also being archived. These data will be invaluable for future studies by Mars explorers and scientists. MOLA altimetry [7] commenced in northern latitudes shortly after orbit insertion in September, 1997, and continued during the Science Phasing Orbits starting in March, 1998. By March, 1999 the Mars Global Surveyor spacecraft (MGS) attained a 400-km circular mapping orbit and MOLA operated nearly continuously for 1.25 Mars years [5], obtaining near-global altimetric coverage. Data products were released at roughly 6 month intervals after acquisition, allowing time for calibration and crossregistration. In the elliptical aerobraking orbits it soon became apparent that orbital and pointing uncertainty were often greater than mission requirements in the face of limited tracking. An extensive crossover adjustment has been applied to correct the majority of errant tracks to within 1-m accuracy, providing near-perfect registration. Shaded-relief digital terrain models from MOLA now bridge the gap between historical and MGS wide-angle images and MOC narrow-angle images, and provide context for Mars Odyssey investigations. The PEDR profile data are now being re-released in their entirety, with several changes described below. Initial Experiment Gridded Data Records were generated at resolutions of up to 32 pixels per degree, with few gaps. The final Mission Experiment Gridded Data Records (MEGDR’s) are now provided at 64 and 128 pixels per degree globally, with lower resolution products as well. Densities as high as 512 pixels per degree are available in a polar stereographic projection. The most noticeable change in the current release is the adoption of the IAU2000 rotation model and cartographic frame [4] recommended by the Mars Cartography Working Group, based on Viking and Pathfinder tracking [1], as well as a determination of the position of landmark crater Airy-0 by Viking Cameras, MOC and MOLA.The orientation of the prime meridian (W0) at the epoch J2000 (Ephemeris Time) went from 176.868 to 176.630 , a change of 0.238 in longitude. Adoption of the current standard will allow registration of images and profiles globally with an uncertainty of < 100 m. The second significant change involves the topographic reference datum, subtracted from planetary radii to yield equipotential topography. Originally described by a degree-4 potential model [6] for an earlier USGS topographic model, we now understand that the equipotential surface that corresponds to terrestrial "sea level" has over 1 km of lumpiness with respect to an ellipsoid of revolution. While this change does not affect the fundamental planetary radius data, accurate knowledge of height with respect to this surface is required to assess the influence of topography on dynamic processes. The current potential model GMM3, updated from GMM2-B[2] is based on tracking from the entire MGS primary mission as well as extended mission data through July 2001, and selective incorporation of altimetric crossovers, with tracking equations generated in the IAU2000 reference frame. One effect of this change has been to reduce slightly the measured topographic height of Olympus Mons by 20 m, an artifact of better resolution of its gravitational bulge by mgm1025, but overall, the topographic changes shouldn’t warrant geological concern.