ONE OF THE PRIMARY GOALS of the NASA As-trobiology program is to “determine how to

Recovery of prokaryotic body fossils from methane seep carbonates such as those of the Cre-taceous Tepee Buttes of Colorado serves as a model for sampling in future astrobiological missions. The fossils, found primarily at the interface between paragenetic fabrics, suggest a sharp physicochemical gradient. Evidence of these microbial fossils occurs at a variety of scales. In the field, microbialite is found as meter-scale thrombolitic zones and centimeter-scale stromatolitic crusts lining voids inferred to be the sites of ancient methane seepage. Pet-rographic fabrics suggestive of microbialite include indistinct peloids (0.1–1 mm in diame-ter) and crusts of authigenic micrite. Primary evidence obtained from scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy analysis comprises pinnate bacteria (0.3 m in diameter and 1–1.5 m long), sheaths (2–4 m in diameter), coccoids (0.5–1 m in diameter, up to 40 per cluster), and the presence of framboidal pyrite (6–8 m in di-ameter). These results are in agreement with studies of other ancient and modern seeps and suggest a morphological conservatism of microbial form that can be incorporated into stud-ies of extraterrestrial environments where it is presumed that reduced gases drive the meta-bolic activity of prokaryote-like organisms. Target areas that could serve as conduits for re-duced gas seeps include tectonic or impact-driven faulting, zones of cryosphere melting, or other disruptions in crustal coherence. Ancient seeps, preserved as localized anomalous evap-orite deposits in the sedimentary cover, could be detected by remote sensing. Key Words