Hydrothermal vents appear to be the tip of the subsurface biosphere in the
ocean crust. The primary prducers in this biosphere are prokaryotes that to
lerate a wide variety of physical and chemical conditions and are versatile
in their use of inorganic compounds to drive metabolism. A synthesis of ch
emical and mineralogical data from Martian meteorites and measurements of t
he Martian surface suggest that conditions similar to those that make life
possible in Earth's oceanic crust, namely, water, carbon, nutrients, approp
riate temperatures, and gradients in redox conditions, also occur within Ma
rs. Chemolithoautotrophic microorganisms capable of living below the seaflo
or on Earth would probably survive in some regions of the Martian subsurfac
e.