Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p11b..13b&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P11B-13
Other
6055 Surfaces And Interiors, 6061 Remote Sensing, 6225 Mars
Scientific paper
The Visible Imaging Subsystem (VIS) on the Mars Odyssey spacecraft THEMIS instrument is a 5-color, 1024x1024 interline transfer CCD camera that is currently acquiring high spatial resolution multispectral images from Mars orbit. The five VIS filters have central bandpasses of 425, 540, 654, 749, and 860 nm, bandwidths of approximately 50 nm, and are bonded in 1000x200 pixel strips directly onto the VIS CCD. Odyssey is in a near-polar orbit, traveling southward on the dayside of the planet, and VIS acquires multispectral images by using along-track motion to step the ground footprint through each desired filter. Nominal ground surface resolution is approximately 18 meters per pixel, and summing modes are available that can provide 36 m or 72 m resolution for increased surface coverage. As of early September, just over 1.1% of the surface of Mars has been imaged by VIS during daytime (between about 3:00 pm to 4:15 pm local solar time), with about 80% of that coverage at 18 m/pixel and 20% of that coverage at 36 or 72 m/pixel. About 55% of the VIS image sequences are monochrome (650 nm) for geomorphology studies; the rest are multispectral sequences in 2 to 5 colors. VIS data are calibrated using a combination of pre-flight radiometric calibration measurements and in-flight flatfield and bias data. We have developed a VIS calibration pipeline that performs a bias subtraction, removes CCD frame transfer smear, and applies a flatfield correction for pixel-to-pixel nonuniformities. In addition, raw VIS data contain a substantial stray light component that is modeled and removed as part of our pipeline process using data collected in flight. VIS images corrected for these instrumental effects are then converted to radiances using pre-flight integrating sphere measurements. Division by the solar spectrum at Mars convolved to the VIS bandpasses results in a set of PDS-format image cubes calibrated to radiance factor (I/F). We validate our derived radiances by comparing regions observed by VIS and HST/WFPC2 over the same wavelengths and during the same martian season. The contribution of the martian atmosphere to the observed VIS radiance is a function of the amount of aerosol particles (dust, ice) suspended in the atmosphere and on their microphysical properties (e.g., size, shape, and composition). The situation is complicated by the limited viewing geometry of the VIS imaging sequences and the general lack of aerosol absorption features in the VIS bandpasses. We plan to characterize and subsequently "remove" the effects of the atmosphere by modeling our VIS radiances combined with thermal IR radiances from the THEMIS IRS subsystem and existing knowledge/assumption about the properties of martian aerosols. In this presentation we report initial results on color properties and visible to short-wave near-IR spectral variability of the surface at high spatial resolution, concentrating on potential MER landing sites and other regions that have been identified from previous Viking, HST, and MGS data as being spectrally anomalous. We also provide details of our atmospheric removal algorithm.
Bell Jon F.
Bender Kelly C.
Caplinger Michael
Cherednik L. L.
Christensen Per Rex
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