[1] Rocks on the floor of Gusev crater are basalts of uniform composition and mineralogy. Olivine, the only mineral to have been identified or inferred from data by all instruments on the Spirit rover, is especially abundant in these rocks. These picritic basalts are similar in many respects to certain Martian meteorites (olivine-phyric shergottites). The olivine megacrysts in both have intermediate compositions, with modal abundances ranging up to 20-30%. Associated minerals in both include low-calcium and highcalcium pyroxenes, plagioclase of intermediate composition, iron-titanium-chromium oxides, and phosphate. These rocks also share minor element trends, reflected in their nickel-magnesium and chromium-magnesium ratios. Gusev basalts and shergottites appear to have formed from primitive magmas produced by melting an undepleted mantle at depth and erupted without significant fractionation. However, apparent differences between Gusev rocks and shergottites in their ages, plagioclase abundances, and volatile contents preclude direct correlation. Orbital determinations of global olivine distribution and compositions by thermal emission spectroscopy suggest that olivine-rich rocks may be widespread. Because weathering under acidic conditions preferentially attacks olivine and disguises such rocks beneath alteration rinds, picritic basalts formed from primitive magmas may even be a common component of the Martian crust formed during ancient and recent times.

dark parts of the two-toned rocks, the height of the largest of the bright (light-toned) rocks, and the perched rocks would suggest local deflation of 5 to 60 cm. Thus, there must have been previous deposition on this order. The precise location and relative elevation of Spirit during its traverses from the Columbia Memorial station to Bonneville crater were determined with bundleadjusted retrievals from rover wheel turns, suspension and tilt angles, and overlapping images. Physical properties experiments show a decrease of 0.2% per Mars solar day in solar cell output resulting from deposition of airborne dust, cohesive soil-like deposits in plains and hollows, bright and dark rock coatings, and relatively weak volcanic rocks of basaltic composition. Volcanic, impact, aeolian, and water-related processes produced the encountered landforms and materials. References and NotesDuring the first few Mars solar days (sols) (1) of operations, we determined the landed location in inertial coordinates by analyzing Spiritto-Earth two-way X-band Doppler transmissions and two passes of ultrahigh-frequency two-way Doppler between Spirit and the Mars Odyssey orbiter. The equivalent location in the International Astronomical Union (IAU) 2000 body-centered reference frame is 14.571892°S, 175.47848°E. The location with respect to surface features was derived by the correlation of hills and craters observed in images taken by the Pancam, the Entry Descent and Landing (EDL) Camera, and the Mars Orbital Camera.On the basis of these analyses, the landing site is located at 14.5692°S, 175.4729°E in IAU 2000 coordinates, ϳ300 m north-northwest of the radiometric solution. This offset is consistent with the map tie errors between inertially derived coordinate systems and those derived from image-based coverage of the planet. Localization experiments during traverses focused on systematic acquisition of forwardand backward-looking overlapping images, onboard inertial measurement unit (IMU) observations to derive rover tilt, and tracking the number of wheel turns to provide wheel-based odometry. These observations were employed in a least-squares bundle adjustment to solve for the position and orientation of Spirit in local Cartesian coordinates at discrete locations during traverses (Fig. 1 and Plate 14). In addition, measurements of differential rocker and bogie angles in the suspension system, together with IMU data, were used to reconstruct the elevation of each wheel at a 2-to 8-Hz sample rate relative to the start of each traverse (Fig. 1).Localization results were extracted for 33 traverse segments from the Columbia Memori-