Drilling in ancient permafrost on Mars for evidence of a second genesis of life
Introduction
The search for life on Mars is currently focused on the detection of fossils. Fossilized remains of Martian life might be found in subsurface sediments in paleolake sites such as Gusev Crater (Cabrol et al., 1998) or in deposits from hydrothermal systems (Walter and DesMarais, 1993). Fossils would show that life was present on Mars, but they would not provide information on the biochemical nature of that life or any connection between life on Mars and life on Earth. There is a diversity of size, shapes, and environments for life forms on Earth, but they all share a common set of biomolecules and have a common origin. There is only one type of life on Earth. It has been postulated that Mars life and Earth life may share a common ancestor due to exchange of material (Mileikowsky et al., 2000). The Martian meteorites found on Earth are direct evidence that rocks from Mars can be carried to Earth without suffering sterilizing temperatures inside (Weiss et al., 2000) and could therefore have been carriers of microbial life. Thus, it cannot be assumed that finding fossil evidence for life on Mars demonstrates that life arose twice in our solar system, but only that conditions on Mars were favorable for life sometime in the past.
While the discovery of fossils on Mars would be of scientific interest, determining that Martian life as a second genesis would have more profound scientific, practical, and philosophical implications. Unfortunately, the nature of Martian life cannot be determined from fossils alone. Direct biochemical and genetic analysis of Martian organisms is necessary (McKay, 2001; Conrad and Nealson, 2001). The organisms need not be viable but the main part of their biomolecules must be intact—we seek corpses not fossils.
Perhaps the best place on Mars to search for intact Martian organisms is the ancient permafrost. Mars has extensive permafrost, some of which presumably dates back to the end of the Noachian, some 3.5 Gyr ago. On Earth, permafrost is a geologically transient phenomenon and the age of the oldest frozen ground here is a few tens of million years at most. In Siberia there is permafrost that is 5 Myr old, while in Antarctica ice-rich permafrost may be up to 8 Myr old (Sugden et al., 1995) although in locations it could theoretically be as old as 25 Myr. Studies in the Siberian permafrost have shown that microorganisms can remain viable after 3.5 million years, at temperatures of −10 °C (Gilichinsky et al., 1992). Preliminary results suggest that Antarctic permafrost ice that may possibly be as old as 8 Myr, contains viable microorganisms (Gilichinsky et al. in prep.).
Thermal decay and radiation both limit viability of microorganisms in permafrost. On Mars, the time spent frozen may be as much as 3–4 billion years, much longer than the age of the oldest permafrost on Earth. However the temperatures on Mars are also much lower, <−90 °C, so thermal decay would not limit the long-term survival of life in permafrost (Kanavarioti and Mancinelli, 1990; Bada and McDonald, 1995). Low-level radioactivity from U, Th, and K in permafrost in Siberia is equivalent to 0.2 rad/yr (2 mGy/yr) or about one million rads in 5 million years. Concentrations of U, Th, and K in the Martian soil are expected to be similar to the values for Earth soils based on the Martian meteorites (Stoker et al., 1993) and Odyssey measurements. While radiation might cause sufficient damage to frozen microorganisms to kill them, it would not destroy all their biomolecules. Therefore, organisms frozen in Martian permafrost could be used for biochemical and genetic analysis.
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Permafrost on Mars
It had long been surmised that the polar regions of Mars contained deep ice-rich permafrost (Squyres and Carr, 1986). Data collected by the neutron spectrometer on the Mars Odyssey spacecraft confirmed the presence of ground ice in the top meter of the Martian polar regions (Feldman et al., 2002). The near surface ice may be recent (Mellon et al., 2004) however, based on the total inventory of water on Mars (eg. Carr, 1996), this shallow ice presumably overlies deeper older ice. Squyres and
Planetary protection
On Mars, unlike the Earth, there are strict requirements controlling the introduction of any biological material into the environment. These planetary protection requirements are promulgated by the Committee on Space Research (COSPAR) and described by Rummel (2001). Previous missions to Mars have been surface vehicles with only minimal digging below ground. For surface missions the planetary protection guidelines have been relaxed since the time of the Viking mission due to the realization that
Drilling contamination and the search for life
When drilling to search for life, it is necessary to take special precautions to avoid the possibility of introducing life, or chemical contamination into the sample. Samples must be collected aseptically; we define aseptic as pristine, non-contaminated samples for biological analysis. With many current drilling methods on Earth, a fluid is in contact between the borehole and the bit so as to ensure proper flow and circulation while drilling. The problem of contamination is greater when a fluid
Martian permafrost over geological time
It is not certain that ground ice on Mars has been stable over geological time even deep below the surface in the polar regions. One factor that would influence the stability of ground ice is climate change due to variations in Mars’ orbit. Theoretical calculations show that Mars experiences strong obliquity cycles with values ranging up to 45° with periods of 105–106 yr (Laskar et al., 2002). At high obliquity, the polar regions of Mars receive more sunlight. For example at an obliquity of 50°,
Aseptic permafrost drilling on Earth
Microbiological studies in permafrost on Earth provide the most direct analog to permafrost drilling on Mars. The need to prevent contamination of the sample and the ability to document that such contamination has not occurred is perhaps even more severe on Earth where surface materials are rich with microbial life (Willerslev et al., 2004a; Juck et al., 2004). There has been considerable study of drilling in permafrost for oil and gas recovery (e.g., Kudryashov and Yakovlev, 1991) and over the
Conclusions
The ancient ice-rich permafrost in the southern highlands of Mars (60–80°S, near 180°W) could provide a source of intact Martian life. The strong crustal magnetism at these sites indicates their ancient, relatively undisturbed nature. At depths of ∼1000 m, the effect of the obliquity cycle is dampened and permafrost at this depth would be unaltered over geological time. Biological material, rather than a mineralized fossil, is needed if we are to determine if Martian life represents a second
Acknowledgements
The authors would like to thank T. Phelps at the Oak Ridge National Laboratory for invaluable discussions of tracer techniques and selection, Bain Webster and Tony Kingan, of Webster Drilling in New Zealand for their expertise in drilling, Alex Pyne and Warren Dickenson for their knowledge in aseptic drilling and core handling techniques, Wayne Pollard for field leadership and extensive geological knowledge of the Arctic, the Polar Continental Shelf Project for their support in logistics, the
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