We describe the calibration and performance of the Mars Odyssey spacecraft's Thermal Emission Imaging System visible imaging subsystem (THEMIS VIS) and present comparisons with other instruments in order to validate the results. The main challenge to the THEMIS VIS calibration process is the significant amount of stray light that accumulates during both integration and readout. The stray light is influenced by scene elements outside of the field of view of the THEMIS VIS detector, and so its magnitude can only be estimated. As a result, residual stray light artifacts are common in calibrated THEMIS VIS images and are especially prominent when the exposure time is short, or the scene contrast is high. Nevertheless, our absolute 2σ calibration uncertainty for the central region of the most frequently used THEMIS VIS channel, the 654 nm band, is better than 5% for all but the shortest exposures times, and our comparisons with Hubble Space Telescope and Mars Exploration Rover measurements show no evidence of systematic calibration inaccuracies.

The polar layered deposits (PLD) of Mars have attracted considerable attention since their identification in Mariner 9 images, largely due to the possibility that these finely layered, volatile-rich deposits hold a record of recent eras in Martian climate history. The PLD have been a target of imaging and other sensors in the last several decades, but only recently has it been possible to obtain a moderately high resolution image map, using the visible sensor on 2001 Mars Odyssey s Thermal Emission Imaging System (THEMIS). We report here on the acquisition of a 36 meter/pixel contiguous single-band visible image data set of the south polar layered deposits (SPLD), obtained during early southern spring in 2003. The data will undoubtedly be applied to many problems in Mars polar studies. We will discuss here, and in more detail at the Conference, the use of these images to further characterize the population of impact craters on the SPLD, and the implications of the observed population for the age and evolution of the SPLD.

Introduction: The NASA Mars Odyssey spacecraft began its primary orbital mapping mission in February 2001. Odyssey carries an imaging instrument called THEMIS, which has both infrared and visible wavelength imaging capabilities. The THEMIS Visible Imaging Subsystem (VIS; Figure 1) is a 5-color, 1024×1024 interline transfer CCD camera that acquires high spatial resolution 425 to 860 nm multispectral images from Mars orbit. This abstract describes the data reduction and calibration methods used to process VIS data, and presents some initial results on surface color properties at high spatial resolution. Motivation: Multispectral imaging at visible wavelengths can provide information on the composition, distribution, and physical properties of ferric (Fe) and ferrous (Fe) iron-bearing rocks and minerals, surface frosts and ices, and atmospheric aerosols on Mars. Previous telescopic [1-3] and spacecraft [4-6] multi-spectral observations at visible wavelengths have shown there to be distinct color units at a variety of spatial scales on Mars, and that the variability of these units is related to variations in the crystallinity of highly altered ferric minerals, ranging from regions spectrally similar to terrestrial palagonites to regions exhibiting spectral signatures consistent with more coarsely-crystalline hematite. The five VIS filters have central bandpasses of 425, 540, 654, 749, and 860 nm, bandwidths of ~50 nm (Figure 2), and are bonded in ~1000×200 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 ~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 January 2003, just over 3.4% of the surface of Mars has been imaged by VIS during daytime (between about 3:00 pm to 4:30 pm local solar time), with about 40% of that coverage at 18 m/pixel and 60% of that coverage at 36 or 72 m/pixel. About half of the VIS image sequences are monochrome (654 nm) for high resolution geomorphology studies and some serve as context for higher resolution MGS/MOC images; the rest are multispectral sequences in 2 to 5 colors. VIS data are calibrated using a combination of pre-flight radiometric calibration measurements and inFigure 1. The Odyssey THEMIS VIS instrument. The color filters can be seen overlying the CCD at center.

Systematic geologic mapping of an area on Venus provides geologic unit and morphologic information about the area that was evaluated to provide a simple geologic history. In the study area, a large complexly deformed highland—Alpha Regio—was found to be among the oldest material units, but its relation to other nearby complexly deformed units was indeterminable. To better delineate the complex geologic and tectonic history of the area the authors expanded on the initial geologic map assessment by including tectonic unit mapping of the Alpha Regio highland and geophysical modeling of the highland to assess the possible effect of its formation on surrounding geologic units. Tectonic unit mapping identified five discrete periods of tectonic activity involved in the formation of the highland. Additionally, the tectonic assessment showed that late stage extensional features within the highland were related to a regional tectonic event that occurred after the formation of the highland. Geophysical models of highland formation by both upwelling and downwelling were assessed, and a downwelling model was applied to the Alpha Regio highland to calculate stresses related to its formation. Results of the modeling suggest that tectonic features mapped outside of the highland were not caused by stresses induced by the formation of Alpha Regio.

The Magellan spacecraft orbited Venus from August 10, 1990, until it plunged into the venusian atmosphere on October 12, 1994. Magellan had the objectives of (1) improving knowledge of the geologic processes, surface properties, and geologic history of Venus by analysis of surface radar characteristics, topography, and morphology and (2) improving knowledge of the geophysics of Venus by analysis of venusian gravity. The Magellan spacecraft carried a 12.6-cm radar system to map the surface of Venus. The transmitter and receiver systems were used to collect three datasets: synthetic aperture radar (SAR) images of the surface, passive microwave thermal emission observations, and measurements of the backscattered power at small angles of incidence, which were processed to yield altimetric data. Radar imaging and altimetric and radiometric mapping of the venusian surface were done in mission cycles 1, 2, and 3, from September 1990 until September 1992. Ninety-eight percent of the surface was mapped with radar resolution of approximately 120 meters. The SAR observations were projected to a 75-m nominal horizontal resolution; these full-resolution data compose the image base used in geologic mapping. The primary polarization mode was horizontal-transmit, horizontal-receive (HH), but additional data for selected areas were collected for the vertical polarization sense. Incidence angles varied from about 20° to 45°. High-resolution Doppler tracking of the spacecraft was done from September 1992 through October 1994 (mission cycles 4, 5, 6). High-resolution gravity observations from about 950 orbits were obtained between September 1992 and May 1993, while Magellan was in an elliptical orbit with a periapsis near 175 kilometers and an apoapsis near 8,000 kilometers. Observations from an additional 1,500 orbits were obtained following orbitcircularization in mid-1993. These data exist as a 75° by 75° harmonic field.

The Galileo mission has revealed remarkable evidence of mass movement and landform degradation on the icy Galilean satellites of Jupiter. Weakening of surface materials coupled with mass movement reduces the topographic relief of landforms by moving surface materials down-slope. Throughout the Galileo orbiter nominal mission we have studied all known forms of mass movement and landform degradation of the icy galilean satellites, of which Callisto, by far, displays the most degraded surface. Callisto exhibits discrete mass movements that are larger and apparently more common than seen elsewhere. Most degradation on Ganymede appears consistent with sliding or slumping, impact erosion, and regolith evolution. Sliding or slumping is also observed at very small (100 m) scale on Europa. Sputter ablation, while probably playing some role in the evolution of Ganymede's and Callisto's debris layers, appears to be less important than other processes. Sputter ablation might play a significant role on Europa only if that satellite's surface is significantly older than 108 years, far older than crater statistics indicate. Impact erosion and regolith formation on Europa are probably minimal, as implied by the low density of small craters there. Impact erosion and regolith formation may be important on the dark terrains of Ganymede, though some surfaces on this satellite may be modified by sublimation–degradation. While impact erosion and regolith formation are expected to operate with the same vigor on Callisto as on Ganymede, most of the areas examined at high resolution on Callisto have an appearance that implies that some additional process is at work, most likely sublimation-driven landform modification and mass wasting. The extent of surface degradation ascribed to sublimation on the outer two Galilean satellites implies that an ice more volatile than H2O is probably involved.