Search for life on Mars in surface samples: Lessons from the 1999 Marsokhod rover field experiment

The Marsokhod 1999 field experiment in the Mojave Desert included a simulat ion of a rover-based sample selection mission. As part of this mission, a t est was made of strategies and analytical techniques for identifying past o r present life in environments expected to be present on Mars. A combinatio n of visual clues from high-resolution images and the detection of an impor tant biomolecule (chlorophyll) with visible/near-infrared (NIR) spectroscop y led to the successful identification of a rock with evidence of cryptoend olithic organisms. The sample was identified in high-resolution images (3 t imes the resolution of the Imager for Mars Pathfinder camera) on the basis of a green tinge and textural information suggesting the presence of a thin , partially missing exfoliating layer revealing the organisms. The presence of chlorophyll bands in similar samples was observed in visible/NIR spectr a of samples in the field and later confirmed in the laboratory using the s ame spectrometer. Raman spectroscopy in the laboratory, simulating a remote measurement technique, also detected evidence of carotenoids in samples fr om the same area. Laboratory analysis confirmed that the subsurface layer o f the rock is inhabited by a community of coccoid Chroococcidioposis cyanob acteria. The identification of minerals in the held, including carbonates a nd serpentine, that are associated with aqueous processes was also demonstr ated using the visible/NIR spectrometer. Other lessons learned that are app licable to future rover missions include the benefits of web-based programs for target selection and for daily mission planning and the need for invol vement of the science team in optimizing image compression schemes based on the retention of visual signature characteristics.