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The second ExoMars mission will be launched in 2020 to target an ancient location interpreted to have strong potential for past habitability and for preserving physical and chemical biosignatures (as well as abiotic/prebiotic organics). The mission will deliver a lander with instruments for atmospheric and geophysical investigations and a rover tasked with searching for signs of extinct life. The ExoMars rover will be equipped with a drill to collect material from outcrops and at depth down to 2 m. This subsurface sampling capability will provide the best chance yet to gain access to chemical biosignatures. Using the powerful Pasteur payload instruments, the ExoMars science team will conduct a holistic search for traces of life and seek corroborating geological context information. Key Words: Biosignatures—ExoMars—Landing sites—Mars rover—Search for life. Astrobiology 17, 471–510.

Home Plate is a layered plateau in Gusev crater on Mars. It is composed of clastic rocks of moderately altered alkali basalt composition, enriched in some highly volatile elements. A coarsegrained lower unit lies under a finer-grained upper unit. Textural observations indicate that the lower strata were emplaced in an explosive event, and geochemical considerations favor an explosive volcanic origin over an impact origin. The lower unit likely represents accumulation of pyroclastic materials, whereas the upper unit may represent eolian reworking of the same pyroclastic materials.

[1] The selection of Meridiani Planum and Gusev crater as the Mars Exploration Rover landing sites took over 2 years, involved broad participation of the science community via four open workshops, and narrowed an initial $155 potential sites (80-300 Â 30 km) to four finalists based on science and safety. Engineering constraints important to the selection included (1) latitude (10°N-15°S) for maximum solar power, (2) elevation (less than À1.3 km) for sufficient atmosphere to slow the lander, (3) low horizontal winds, shear, and turbulence in the last few kilometers to minimize horizontal velocity, (4) low 10-m-scale slopes to reduce airbag spin-up and bounce, (5) moderate rock abundance to reduce abrasion or strokeout of the airbags, and (6) a radar-reflective, load-bearing, and trafficable surface safe for landing and roving that is not dominated by fine-grained dust. The evaluation of sites utilized existing as well as targeted orbital information acquired from the Mars Global Surveyor and Mars Odyssey. Three of the final four landing sites show strong evidence for surface processes involving water and appear capable of addressing the science objectives of the missions, which are to determine the aqueous, climatic, and geologic history of sites on Mars where conditions may have been favorable to the preservation of evidence of possible prebiotic or biotic processes. The evaluation of science criteria placed Meridiani and Gusev as the highest-priority sites. The evaluation of the three most critical safety criteria (10-m-scale slopes, rocks, and winds) and landing simulation results indicated that Meridiani and Elysium Planitia are the safest sites, followed by Gusev and Isidis Planitia.

[1] The cumulative fractional area covered by rocks versus diameter measured at the Pathfinder site was predicted by a rock distribution model that follows simple exponential functions that approach the total measured rock abundance (19%), with a steep decrease in rocks with increasing diameter. The distribution of rocks >1.5 m diameter visible in rare boulder fields also follows this steep decrease with increasing diameter. The effective thermal inertia of rock populations calculated from a simple empirical model of the effective inertia of rocks versus diameter shows that most natural rock populations have cumulative effective thermal inertias of 1700-2100 J m À2 s À0.5 K À1 and are consistent with the model rock distributions applied to total rock abundance estimates. The Mars Exploration Rover (MER) airbags have been successfully tested against extreme rock distributions with a higher percentage of potentially hazardous triangular buried rocks than observed at the Pathfinder and Viking landing sites. The probability of the lander impacting a >1 m diameter rock in the first 2 bounces is <3% and <5% for the Meridiani and Gusev landing sites, respectively, and is <0.14% and <0.03% for rocks >1.5 m and >2 m diameter, respectively. Finally, the model rock size-frequency distributions indicate that rocks >0.1 m and >0.3 m in diameter, large enough to place contact sensor instruments against and abrade, respectively, should be plentiful within a single sol's drive at the Meridiani and Gusev landing sites.

[1] The cratered plains of Gusev traversed by Spirit are generally low-relief rocky plains dominated by impact and eolian processes. Ubiquitous shallow, soil-filled, circular depressions, called hollows, are modified impact craters. Rocks are dark, fine-grained basalts, and the upper 10 m of the cratered plains appears to be an impact-generated regolith developed over intact basalt flows. Systematic field observations across the cratered plains identified vesicular clasts and rare scoria similar to original lava flow tops, consistent with an upper inflated surface of lava flows with adjacent collapse depressions. Crater and hollow morphometry are consistent with most being secondaries. The sizefrequency distribution of rocks >0.1 m diameter generally follows exponential functions similar to other landing sites for total rock abundances of 5-35%. Systematic clast counts show that areas with higher rock abundance and more large rocks have higher thermal inertia. Plains with lower thermal inertia have fewer rocks and substantially more pebbles that are well sorted and evenly spaced, similar to a desert pavement or lag. Eolian bed forms (ripples and wind tails) have coarse surface lags, and many are dust covered and thus likely inactive. Deflation of the surface $5-25 cm likely exposed two-toned rocks and elevated ventifacts and transported fines into craters creating the hollows. This observed redistribution yields extremely slow average erosion rates of $0.03 nm/yr and argues for very little long-term net change of the surface and a dry and desiccating environment similar to today's since the Hesperian (or $3 Ga).

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-