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Mapping the Shapes of Vesta and Ceres

Carol A. Raymond
Dawn Deputy Principal Investigator, JPL

Dawn was to carry a laser altimeter built by Goddard Space Flight Center (D. Smith and M. Zuber, PIs) but this instrument was descoped in Phase B. The laser altimeter was on the descope list because there was a viable alternate means to obtain topographic measurements, albeit with lower resolution, using image data from the framing cameras. Traditional stereo imaging does not produce height data in an absolute reference frame. While relative topography is useful for many applications, it is inadequate for analyzing the relationship of gravity to topography to derive information on the interior structure of the two protoplanets. In order to overcome this deficiency, the Dawn topographic mapping investigation will be unique in several regards. Our team combines the expertise and experience of MOLA investigators and Dawn co-Is D. Smith and M. Zuber with that of co-Is at Max Planck (U. Keller) and DLR (R. Jaumann and S. Mottola) who bring a wealth of experience with stereo photogrammetry, including the HRSC on Mars Express. Our goal is to produce geodetically accurate topographic data that may be used to understand the relationship between gravity and topography (at long wavelengths), and geologic processes. This will be accomplished through joint analysis of radiometric tracking data with landmark control points derived from image pairs. Dawn will also use techniques that combine stereo photogrammetry with photoclinometry, which were developed and applied by R. Gaskell at JPL to derive the topography of Eros using NEAR data. These techniques may improve the height accuracy, especially at short wavelengths.

Ultimately, the height accuracy depends on the number and quality of the image data used in the reconstruction. We will collect at least three global mosaics (nadir and two off-nadir views) in the High Altitude Mapping Orbit as well as global mosaics in Survey orbit. More images from LAMO, and a lower altitude for LAMO, will improve the height accuracy at short wavelengths. It will also improve the higher order gravity field, allowing a more complete understanding of the geophysics of the two bodies.