The habitability conditions of possible Mars landing sites for life exploration

A major focus of Mars exploration is the search for life. Since the Viking landers in the 1970's, no mission has searched for evidence of modern life. Future landed missions with this objective will require a landing site that optimizes the potential for current or geologically-recent habitable conditions. While the definition of habitability is broad, we concentrate on two conditions for habitability of temperature and water activity. In addition, we consider the access to subsurface ice (e.g., via a lander drill) as a major requirement. To examine potential landing sites, we utilize a standard model of temperature and ground-ice stability to explore a range of latitudes for the past 2.5 Myrs of orbitally induced climate change. We find that in the current epoch, habitable conditions do not occur within the ice-rich permafrost, where stable ice is limited to poleward of similar to 47(degrees)N. Habitable conditions defined as favorable to metabolic activity in Earth-like potential Mars organisms did occur seasonally during periods of high obliquity >33(degrees), when ground ice would have been more ubiquitous. The most recent occurrence was about 510 kyrs ago when temperatures and water activities seasonally exceeded the threshold for metabolic activity of -40 C-degrees and 0.6, respectively. Periods such as these last 10s of kyrs at each occurrence. At no time or location do habitability conditions favor propagation (cell replication above -18 C-degrees) in Earth-like Mars organisms. We present a model of subsurface stratigraphy based on the occurrence of ground ice, the age of the ice, and history of habitable conditions. From this stratigraphy we find that all latitudes offer some potential for life exploration. However, middle-latitude sites (47(degrees)N to 61(degrees)N) offer access to 100-kyr-old ice that experienced past habitable conditions, while high-latitudes sites (>61(degrees)N) offer this plus access to "ancient ice" > > 1 Myrs old. Access to, and comparison between, multiple stratigraphic layers with differing ages and histories will maximize the science results of a life exploration mission. These latitude limits may vary slightly with surface conditions of slope and soil properties.