SEARCHING FOR LIFE ON MARS

Although present surface conditions on Mars appear unfavorable for life, orbital images of Mars show numerous water-carved channels and possible paleolake basins where water may have once ponded. Geological relationships suggest that during the early history of the planet, liquid water was widespread over the surface. Some scientists have even suggested that during this time a large ocean existed on the northern plains of Mars. Indications are that liquid water disappeared from the surface of Mars about 3 billion years ago, perhaps as a result of gradual losses of the atmosphere by crustal weathering processes (which sequester CO₂ in rocks and soils) and losses to space. If surface life developed on Mars during an early Earth-like period, it quite likely left behind a fossil record. As on Earth, this record should be preserved in ancient, water-formed sedimentary rocks.

Given the complexity and scale of the problem, one cannot expect to land just anywhere on Mars and find evidence of past or present life. The astrobiology community has recommended a phased approach in which global reconnaissance is combined with preliminary surface missions to target the best sites for detailed surface investigations and sample return. The basic goal is to locate sites where there is evidence of past or present water activity and geologic environments that were favorable for the capture and preservation of fossil biosignatures.

In exploring for extant life-forms, there is an interest in finding habitable zones of liquid water in the shallow subsurface that can be accessed by drilling from robotic platforms. This may prove challenging given that models for a groundwater system on Mars suggest that if present, it should be located at a depth of several kilometers, requiring deep drilling technologies that are currently undeveloped. It may actually be simpler to discover a record of ancient life by targeting water-formed sedimentary deposits laid down by ancient hydrothermal systems or in paleolake basins. A key step in implementing this approach is to better understand the mineralogy of the Martian surface. The Thermal Emission Spectrometer instrument began mapping from Mars orbit in 1999 and in 2000 discovered coarse-grained ("specular") hematite deposits at Sinus Meridiani. Hematite is a form of iron-oxide, which in a coarse-grained form strongly suggests the past activity of water. This site has been targeted for possible landed missions in the future.