Search

The search for life on Mars is a priority for NASA’s Science Mission Directorate, a pivotal question of the Astrobiology Program, and the ultimate goal of the Mars Exploration Program (MEPAG, 2008). Also, assessment of the presence or absence of life on Mars is a prerequisite for human exploration in that it will allow mitigation of potential threats to planetary protection. Nevertheless, the Viking missions remain the first, and only, attempt to search for life on the planet (or anywhere else beyond the confines of Earth). Since Viking, and specially in the last decade, robotic missions to Mars have focused on characterizing the physical, chemical, and geological environment as a necessary step to an efficient and systematic search for life. These missions have provided an unprecedented amount of data, and our knowledge of Mars has grown vastly, with respect to its current conditions and geological evolution. In the wake of 15 years of extraordinary efforts, and at a time when the next Decadal Survey is being drafted with recommendations and plans for the exploration of the Solar System, it would seem that the search for life on Mars would be a high priority. However, based on a preliminary disclosure of the main points included in the Decadal Survey during the Astrobiology Science Conference (AbSciCon, April 26–29, 2010, League City, Texas), in situ life-detection missions to Mars will not be recommended. Instead, a Mars Sample Return mission (2018–2022) will play a preeminent role as the only mission in the next 10 years, excluding the Mars Science Laboratory (2011), with the specific task of searching for evidence of past life on Mars. This programmatic strategy is in line with the general consensus reached after the Viking missions, which was that conditions on, or near, the surface of Mars are prohibitive for microbial activity. This assumption has been reinforced with each follow-up mission and is largely based on the fact that Viking, and more recently Phoenix, failed to detect any organic compounds in the surface soils. The constantly low temperatures and hyper-arid conditions on the surface, together with high doses of radiation and the formation of reactive oxidants in the atmosphere, conspire against an extant martian biota in surface soils. Given these observations, the search for extant life has received little support and has not been considered a programmatic priority, and it is on the verge of being removed altogether from the next Decadal Survey. We argue that the strategy for Mars exploration should center on the search for extant life. By extant life, we mean life that is active today or was active during the recent geological past and is now dormant. As we discuss below, the immediate strategy for Mars exploration cannot focus only on past life based on the results of the Viking missions, particularly given that recent analyses call for a re-evaluation of some of these results. It also cannot be based on the assumption that the surface of Mars is uniformly prohibitive for extant life, since research conducted in the past 30 years in extreme environments on Earth has shown that life is possible under extremes of cold and dryness. For these reasons, we recommend a new strategy for the next decade of robotic investigations on Mars, one in which the in situ search for extant life is the first priority. Given the most updated knowledge we have about Mars’ environmental evolution, we call for a long-term architecture of the Mars Exploration Program that is organized around three main goals in the following order of priority: (1) the search for extant life; (2) the search for past life; and (3) sample return. We argue that this is the most efficient approach by which to address, with a high degree of certainty, the question as to whether life exists on Mars.

We describe a viable microbial community in a subglacial lake within the Grímsvötn volcanic caldera, Iceland. We used a hot water drill to penetrate the 300-m ice shelf and retrieved lake water and volcanic tephra sediments. We also acquired samples of borehole water before and after penetration to the lake, overlying glacial ice and snow, and water from a nearby subaerial geothermal lake for comparative analyses. Lake water is at the freezing point and fresh (total dissolved solids = 260 mg L(-1)). Detectable numbers of cells were found in samples of the lake water column and tephra sediments: 2 x 10(4) ml(-1) and 4 x 10(7) g(-1), respectively. Plate counts document abundant cold-adapted cultivable organisms in the lake water, but not in the borehole (before penetration) or glacial ice. Denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments amplified from genomic DNA extracted from Grímsvötn samples indicates that the lake community is distinct from the assemblages of organisms in borehole water (before penetration) and the overlying ice and snow. Sequencing of selected DGGE bands revealed that many sequences are highly similar to known psychrophilic organisms or cloned DNA from other cold environments. Significant uptake of 14C-labeled bicarbonate occurred in dark, low-temperature incubations of lake water samples, indicating the presence of autotrophs. Acetylene reduction assays under similar incubation conditions showed no significant nitrogen fixation potential by lake water samples. This may be a consequence of the inhibition of diazotrophy by nitrogen in the lake.