Abstract. The Curiosity rover discovered fine--grained sedimentary rocks, inferred to represent an ancient lake, preserve evidence of an environment that would have been suited to support a Martian biosphere founded on chemolithoautotrophy. This aqueous environment was characterized by neutral pH, low salinity, and variable redox states of both iron and sulfur species. C, H, O, S, N, and P were measured directly as key biogenic elements, and by inference N and P are assumed to have been available. The environment likely had a minimum duration of hundreds to tens of thousands of years. These results highlight the biological viability of fluvial--lacustrine environments in the post--Noachian history of Mars.

X-ray diffraction analysis of the Rocknest scoop sample is described in (23); similar analyses were performed for John Klein and Cumberland. John Klein and Cumberland were the first two drill samples collected by Curiosity. All scooped or drilled samples pass through the Collection and Handling for In situ Martian Rock Analysis (CHIMRA) sample collection and processing system (10). All powders for X-ray diffraction are processed through a 150-m sieve before delivering a portion to the CheMin inlet funnel.The sieved drill powders were placed into sample cells with 6 μm thick Mylar® windows. Mylar® contributes a minor, broad scattering signature in diffraction patterns that is generally "swamped" by diffraction from the loaded sample. In addition, an aluminized light shield also contributes "peaks" to the observed diffraction patterns. Only ~10 mm 3 of material is required to fill the active volume of the sample cell, which is a disc-shaped volume 8 mm in diameter and 175 m thick. A collimated ∼70 μm diameter X-ray beam illuminates the center of the sample cell. A piezoelectric vibration system on each cell pair shakes the material during analysis, causing grains in the cell to pass through the X-ray beam in random orientations.CheMin measures XRD and XRF data simultaneously using Co radiation in transmission geometry (11). The instrument operates in single-photon counting mode so that between each readout the majority of CCD pixels are struck by either a single X-ray photon or by no photons. In this way, the system can determine both the energy of the photons striking the CCD (XRF) and the two-dimensional (2-D) position of each photon (XRD). The energy and positional information of detected photons in each frame are summed over repeated 10-sec measurements into a "minor frame" of 30 min of data (180 frames). The 2-D distribution of Co K X-ray intensity represents the XRD pattern of the sample. Circumferential integration of these rings, corrected for arc length, produces a conventional 1-D XRD pattern. For conversion of the 2-D CCD pattern to a 1-D pattern we have used FilmScan © software from Materials Data, Inc.CheMin generally operates for only a few hours each night, when the CCD can be cooled to its lowest temperature, collecting as many minor frames as possible for the available analysis time, usually five to seven per night. XRD data were acquired over multiple nights for the John Klein and Cumberland drill samples to provide acceptable counting statistics. Total data collection times were 33.9 hr for John Klein and 20.2 hr for Cumberland. The data for individual minor frames and for each night's analysis were examined separately, and there was no evidence of any changes in instrumental parameters as a function of time over the duration of these analyses. Before sample delivery and analysis, the empty cell was analyzed to confirm that it was indeed empty before receiving the sample. The flight instrument was calibrated on the ground before flight using a quartz-beryl standard, and measurement of this st...