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Drilling in ancient permafrost on Mars for evidence of a second genesis of life
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Planetary and Space Science 53 (2005) 1302–1308 www.elsevier.com/locate/pss
Drilling in ancient permafrost on Mars for evidence of a second genesis of life H.D. Smitha,b,, C.P. McKayc a
SETI Institute/NASA Ames, Mail Stop 245-3, Moffett Field, CA 94035, USA b International Space University, Strasbourg, France c Space Science Division, NASA/Ames Research Center, Mail Stop 245-3, Moffett Field CA 94035, USA Received 20 June 2004; received in revised form 30 June 2005; accepted 7 July 2005 Available online 26 August 2005
Abstract If life ever existed on Mars, a key question is the genetic relationship of that life to life on Earth. To determine if Martian life represents a separate, second genesis of life requires the analysis of organisms, not fossils. Ancient permafrost on Mars represents one potential source of intact, albeit probably dead by radiation, Martian organisms. Strong crustal magnetism in the ancient heavily cratered southern highlands between 60 and 801S and at about 1801W indicates what may be the oldest, best preserved icerich permafrost on Mars. Drilling to depths of 1000 m would reach samples unaffected by possible warming due to cyclic changes in Mars’ obliquity. When drilling into the permafrost to retrieve ancient intact Martian organisms, it is necessary to take special precautions to avoid the possibility of contamination. Earth permafrost provides an analog for Martian permafrost and convenient sites for instrument development and field testing. r 2005 Elsevier Ltd. All rights reserved. Keywords: Mars; Exobiology; Ice; Search for life; Experimental techniques
1. 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 Corresponding author. NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035, US. Tel.: +1 650 604 2045; fax: +1 650 604 6779. E-mail address: [email protected] (H.D. Smith).
0032-0633/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2005.07.006
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
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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 1C (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, o90 1C, so thermal decay would not limit the longterm survival of life in permafrost (Kanavarioti and Mancinelli, 1990; Bada and McDonald, 1995). Lowlevel 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.
2. 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 Carr (1986) suggested the presence of deep ice poleward of 301 on Mars based on crater morphologies. This is probably the best indication of the presence of deep ice underlying the near-surface ice
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Fig. 1. Crustal magnetism, crater distribution and ground ice on Mars. Each green dot represents a crater with diameter greater than 15 km. The boundary between the smooth northern plains and the cratered southern highlands is shown with a green line. The crustal magnetism is shown as red for positive and blue for negative. Full scale is 1500 nT. The typical strength of Earth’s magnetic field at the surface is 5000 nT. The solid blue lines show the extent of near surface ground ice as determined by Odyssey mission. Ground ice is present near the surface poleward of these lines. Crater morphology indicates deep ground ice poleward of 301 (Squyres and Carr, 1986), shown here by dark blue lines and arrows. The region between 60 and 801S near 1801W is heavily cratered, preserves crustal magnetism, and has ground ice present. This is our suggested target site for drilling. This figure is adapted from Acun˜a et al. (1999), based on the crater distribution in Barlow (1997). The distribution of near-surface ground ice is from Feldman et al. (2002).
mapped out by Odyssey. These ice distributions are shown in Fig. 1. The ice-rich permafrost in the Northern polar region may contain frozen water but the low density of craters on the surface indicates that it is young, and therefore less likely to hold ancient Martian life. The southern hemisphere of Mars, however, contains heavily cratered terrain that presumably dates back to early in Martian history when water was prevalent and life was more probable. The surprise discovery of strong magnetic fields in the ancient cratered highlands of Mars (Acun˜a et al., 1999; Connerney et al., 1999) provides a possible way to determine the oldest undisturbed permafrost on Mars. As shown in Fig. 1, the crustal magnetism in the southern hemisphere of Mars occurs in heavily cratered terrain indicating that this area dates back to the heavy bombardment at the end of the formation of the planet (Acun˜a et al., 1999). It is important to note that there is no crustal magnetism in the vicinity of the large craters of Hellas and Argyre. Presumably these impacts erased the magnetic properties of the surrounding crust, possibly by shock pressure (Acun˜a et al., 1999; Hood et al., 2003). If Hellas and Argyre occurred after the magnetic features were formed, this indicates that the magnetic
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features predate the heavy bombardment, predate the crustal dichotomy between the northern and southern hemisphere, and predate the formation of the large geological features such as Tharsis and Valles Marineris. Thus the magnetic crustal features may be sites of the oldest undisturbed terrains on Mars providing a window into the early environments that may contain material dating back to an earlier wetter Mars. These locations experienced the heavy bombardment, but not impacts large enough to erase magnetism. Sites in the southern
Fig. 2. Image of a possible location in the ancient ice-rich terrain for drilling, the landing site for Mars Polar Lander near 761S, 1951W. Shown here is the landing error ellipse (90 km by 5 km) for the Mars Polar Lander. Image courtesy of NASA/JPL.
highlands between 60 and 801S, near 1801W containing the previously mentioned magnetic crustal features may be the oldest permafrost on Mars and the best target when searching for Martian life. One particular site of interest in the region now known to contain magnetic anomalies is the landing site of the Mars Polar Lander. The Polar Lander crashed on Mars in December 1999; its target site was extensively imaged both before and after the crash. The landing eclipse is shown in Fig. 2 together with a high-resolution image of a representative area (Fig. 3) of the terrain in this region. This site was certified for landing as part of the Mars Polar Lander mission and would be suitable for a drilling mission as well. Similar to the mammoths extracted from the ice in Siberia (Willerslev et al., 2004b), Martian microbes extracted from the permafrost would be dead but some of their biomolecules would be intact. From samples of this ancient Martian permafrost it would be possible, in principle, to determine the biochemical composition of Martian life, and any phylogenetic relationship between that life and Earth life.
Fig. 3. A series of images taken by the MOC camera on the Mars Global Surveyor. They show the variety of terrains and textures present within Mars Polar Lander’s landing zone. Each of the six boxes shows an area of about 0.6 km2. Knobs, pits, ridges, gullies, and smooth intervening surfaces are all seen. These images show that the terrain at the expected site would be suitable for a drilling mission. Image by NASA/JPL Malin Space Science Center.
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3. 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 the surface environment on Mars was hostile to Earth life, particularly due to the strong UV. For missions with deep subsurface access the planetary protection requirements are certain to be more severe (see eg. Mancinelli, 2003). This includes not just the borehole and the samples, but also the surface where the drill is operating. For robotic drill missions to Mars an overall assessment of the planetary protection requirements and the need not to contaminate the samples at depth may necessitate that the entire drill be sterilized in a way similar to Viking and in contrast to Pathfinder and the Mars Exploration Rovers that had bioload reduction but not sterilization.
4. 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 other than air is used. The drilling apparatus is often dirty when brought out of the hole. Due to the size of the equipment and field locations, it is usually impossible to sterilize the drill before each run. As a result, it is necessary to measure the extent of contamination. When drilling the temperature of the ground must also be considered. In ground that is not frozen it is harder to ensure sample integrity due to the liquefaction of the sample. It is easier for contaminants to penetrate a non-frozen core than a frozen core. Both air and ground temperatures affect the ability to drill. Layering and core structure are compromised when melting occurs. The low pressure of the Martian environment compared to the pressure on Earth offers two benefits. When drilling on Earth a drilling fluid is used to help clear the borehole of loose cuttings. In a recent experiment (Zacny et al., 2004), drilling in frozen
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samples was tested in a Mars pressure chamber without drilling fluid. At this low pressure, ice within the permafrost vaporized from the sample and the flow of this vapor naturally cleared the borehole of loose cuttings. With the elimination of a drilling fluid, the chance of contamination is decreased. In addition, if a core is unattainable, these loose cuttings could be collected for research purposes. The results of Zacny et al. (2004) indicate that drilling in permafrost on Mars may be easier than drilling in permafrost on Earth. With current drilling methods, it is clear that the exteriors of samples from virtually all drilling methods are contaminated by the drilling environment. To determine the amount of contamination by the drilling process, a tracer can be used. There are three main types of tracers: biological, non-biological, and naturally occurring tracers. Biological tracers are typically made of genetically marked bacteria. Non-biological tracers include dyes, spheres, and other additives of nonbiological origin. Biological and non-biological tracers are added to the drilling system and then the sample is searched for their presence to determine sample integrity. In contrast, a naturally occurring tracer is any property of the environment or the subsurface material that varies with depth in a known way. Contamination from the surface and cross contamination between layers can be determined from tracking this naturally occurring tracer. Examples include stable isotopes and mineral phases. On Earth, biological tracers are the most widely used type of tracer because they are easy to use and to detect. Since the research being conducted on the sample is often microbiological in nature; the same equipment and techniques used for sample analysis can be used for detecting the tracers. Some of the most common microbes used as tracers are Serratia marcescens (Willerslev et al., 2003, 2004a; Phelps and Frederickson, 2002; Christner et al., 2005), Bacillus globigii, (Eiswirth and Hotzel, 1995; Phelps and Frederickson, 2002), Saccharomyces cerevisiae (bakers yeast), Escherichia coli, (Harvey and Harms, 2002; Phelps and Frederickson, 2002), and Chromobacterium violacem (Eiswirth and Hotzel, 1995). Biological tracers are usually introduced into the drilling process by spiking the drilling fluid. The use of biological tracers on Mars would not be allowed due to planetary protection issues. Non-biological tracers include chemical substances, ionic tracers, dyes, and fluorescent microspheres. Chemical tracers are often used for environmental assessment of hazardous waste treatment. In general, chemical tracers are not as sensitive as biological tracers. Specific examples of tracers used are: sodium chloride, bromide, potassium sulfate, and iodide. (Eiswirth and Hotzel, 1995; Phelps and Frederickson, 2002). The introduction of chemical tracers into the environment is often achieved by the use of the drilling fluid, but can
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also be applied by pouring a solution of water and chemical tracer down the borehole. Fluorescent dyes and microspheres, are becoming more popular as they do not change the biological or chemical structure of the sample. Some examples of fluorescent dyes are eosine, naphtionate, pyranine, and rhodamine (Eiswirth and Hotzel, 1995). Fluorescent microspheres come in two basic types, carboxylated and latex, (Eiswirth and Hotzel, 1995; Harvey and Harms, 2002; Phelps and Frederickson, 2002) and variety of sizes, as small as 0.05 mm (Juck et al., 2004). Microspheres of a suitable size can mimic the penetration of biological tracers without adding a biological substance or causing a chemical reaction. When a drilling fluid is not used, tracers have been dropped down the borehole and painted on the drill bit (Juck et al., 2004). Naturally occurring isotopes, or passive tracers, are also a way to check the cleanliness and sequencing of the samples. Some naturally occurring isotopic ratios studied on Earth include: 18O/16O, 2H/1H, 15N/14N, 14 C/13C, 40Ar/14N, 40Ar/36Ar (Gibson et al., 2003). Other elements such as strontium, boron, sulfur, chloride, tritium, and deuterium are also used to infer sample integrity on Earth (Bogard et al., 2001; Gibson et al., 2003). On Mars the exact geological origin of the polar caps and thus the exact isotopic ratios analogous to Earth are unknown. However, based on the monotonic changing pattern of 15N/14N ratio in Mars atmosphere Doran et al. (2000) suggests using the 15 /14 N N ratio as an isotopic tracer for Mars.
5. 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 451 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 501, the total sunlight received at the pole over the entire year is 1.4 times higher than for the present obliquity of 251. At these high values of the obliquity the permafrost temperatures may increase. The depth to which cycling warming will reach can be determined from the thermal damping depth, d, d2 ¼ (KT/p), where T is the period and K is the thermal diffusivity. Fig. 4 shows a plot of d for typical thermal properties of permafrost for periods from 1 to 106 yr. Note that this depth is an overestimate since it does not take into account the effect of latent heat which significantly retards temperature rises above freezing in permafrost. From Fig. 4 it is clear that depths of 1000 m
Fig. 4. Dampening depth in meters in Martian permafrost for periods up to 106 yr.
should contain samples unaltered by thermal cycles due to obliquity with periods of less than 106 yr. If obliquity cycles removed deep ground ice then subsequent recharge by diffusion from the atmosphere would only result in ground ice layers to depths of a few tens of meters at most due the blockage of pore spaces by ice (Mellon et al., 2004). The crater morphologies surveyed by Squyres and Carr (1986) indicate the persistence of deep ground ice in the polar regions arguing against complete removal by past obliquity cycles. However, even if there has been periodic removal and replacement of ground ice at depth, these locations may still preserve the organic record of past life. The sublimation of the ice would leave any organic material behind. Only if the ice melted to form liquid would the organics be removed. Subsequent replacement of the ice by diffusion would also not alter the organic record. The presence of ice is important because it is likely to help protect organic material against decomposition due to soil oxidants. Thus, the best record of organic, and possibly biological material is probably preserved in locations which have been ice-rich for most of, if not all, their post depositional history, and have never experienced conditions leading to formation of liquid water. Deeper layers may have the least thermal variation but they may also contain less biological material depending on the mode of formation of these deep layers. Deeply buried sediments are preferred over deeply buried volcanic rocks. These considerations further indicate that South polar permafrost is the likely target for a search for samples of ancient Martian biology.
6. Aseptic permafrost drilling on Earth Microbiological studies in permafrost on Earth provide the most direct analog to permafrost drilling
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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 past years there have been at least two programs directed toward aseptic drilling in permafrost on Earth. The use of tracers in these studies to document the pristine nature of the samples is relevant to Mars. One study by Gilichinsky and colleagues (Gilichinsky et al., 1992; Willerslev et al., 2003) is based on a custom design drill that drives a coring tube with a gas motor and a clutch, and transmission system that can provide both torque and vertical force during the drilling process (Gilichinsky et al., 1992). Using this drill, samples have been obtained from ice-rich permafrost in Northern Siberia, New Zealand (Willerslev et al., 2003, 2004a), and Beacon Valley Antarctica up to 50 m in depth. The innovation that allows for aseptic sample collection is that the drilling is done completely dry without any lubricant or drilling fluid. Willerslev et al. (2003) introduced a biological tracer S. marcescens onto the drilling equipment and then looked for the presence of these bacteria in the sample and was thereby able to demonstrate the extent of contamination of the core. A second study was done on Ellesmere Island Nanavut Canada as reported in Juck et al. (2004). Fluorescent carboxylated microspheres with diameters 0.05 and 0.5 mm were used to test aseptic collection methods using a drill with compressed air as the fluid to remove cuttings and with a coring system that did not require any drilling fluid. Fluorescent microspheres were chosen to act as a biological surrogate having similar size to real biological organisms without altering the biological or chemical constituents of the sample. The semi-portable drilling rig used compressed chilled air as the drilling fluid. The drill could also operate without any fluid depending on the permafrost characteristics (Dickenson et al., 1999). Juck et al. (2004) concluded that the best method for tracer studies in this system consisted of covering the inside of the bit, core catcher, and first few centimeters of the core tube with a concentrated solution of microspheres. Of the methods tested this was the quickest, the most effective way of distributing the tracer, and could be done without interrupting the drilling process. Due to the many advantages, the ‘‘paint on bit’’ tracer introduction method was applied to the majority of the samples. The fluorescent microspheres were found to penetrate the core between 2 and 5 mm (average 371.7 mm) in the three intact soil permafrost samples, and between 2 and 6 mm (average 4.371.7 mm) in the four ice samples.
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7. Conclusions The ancient ice-rich permafrost in the southern highlands of Mars (60–801S, near 1801W) 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 independent genesis. Deep drilling on robotic or human missions could be focused on obtaining this material in ways that do not contaminate the Martian subsurface and provide positive controls ensuring that any biological material in the samples were not from Earthly contamination. A large Mars chamber simulating Martian conditions such as temperature and pressure would be ideal (possibly necessary) for testing aseptic drilling methods as a precursor to a Mars drilling mission. Since such a chamber does not exist yet, drilling in permafrost on Earth for microbiological studies provides the best analog to Martian drilling. Permafrost tracer studies used on Earth are of two types—fluorescent microspheres and selected microbial strains. Both have been shown to be effective in Earth applications. However, for Mars applications fluorescent microspheres are preferred over microbial strains. Further work on the development of automated drilling systems or simplified systems for human operators in space suites will be necessary before deep drilling into Martian permafrost is a reality.
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 Eureka Weather Station, and Operation Hurricane of the Canadian Military Base at Fort Eureka for their support in equipment and logistics. This project was funded by the NASA Astrobiology Science and Technology Instrument Development program through a grant to G. Briggs. References Acun˜a, M.H., Connerney, J.E.P., Ness, N.F., Lin, R.P., Mitchell, D., Carlson, C.W., McFadden, J., Anderson, K.A., Reme, H., Mazelle, C., Vignes, D., Wasilewski, P., Cloutier, P., 1999. Global
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Planetary and Space Science 53 (2005) 1302–1308 www.elsevier.com/locate/pss
Drilling in ancient permafrost on Mars for evidence of a second genesis of life H.D. Smitha,b,, C.P. McKayc a
SETI Institute/NASA Ames, Mail Stop 245-3, Moffett Field, CA 94035, USA b International Space University, Strasbourg, France c Space Science Division, NASA/Ames Research Center, Mail Stop 245-3, Moffett Field CA 94035, USA Received 20 June 2004; received in revised form 30 June 2005; accepted 7 July 2005 Available online 26 August 2005
Abstract If life ever existed on Mars, a key question is the genetic relationship of that life to life on Earth. To determine if Martian life represents a separate, second genesis of life requires the analysis of organisms, not fossils. Ancient permafrost on Mars represents one potential source of intact, albeit probably dead by radiation, Martian organisms. Strong crustal magnetism in the ancient heavily cratered southern highlands between 60 and 801S and at about 1801W indicates what may be the oldest, best preserved icerich permafrost on Mars. Drilling to depths of 1000 m would reach samples unaffected by possible warming due to cyclic changes in Mars’ obliquity. When drilling into the permafrost to retrieve ancient intact Martian organisms, it is necessary to take special precautions to avoid the possibility of contamination. Earth permafrost provides an analog for Martian permafrost and convenient sites for instrument development and field testing. r 2005 Elsevier Ltd. All rights reserved. Keywords: Mars; Exobiology; Ice; Search for life; Experimental techniques
1. 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 Corresponding author. NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035, US. Tel.: +1 650 604 2045; fax: +1 650 604 6779. E-mail address: [email protected] (H.D. Smith).
0032-0633/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2005.07.006
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
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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 1C (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, o90 1C, so thermal decay would not limit the longterm survival of life in permafrost (Kanavarioti and Mancinelli, 1990; Bada and McDonald, 1995). Lowlevel 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.
2. 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 Carr (1986) suggested the presence of deep ice poleward of 301 on Mars based on crater morphologies. This is probably the best indication of the presence of deep ice underlying the near-surface ice
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Fig. 1. Crustal magnetism, crater distribution and ground ice on Mars. Each green dot represents a crater with diameter greater than 15 km. The boundary between the smooth northern plains and the cratered southern highlands is shown with a green line. The crustal magnetism is shown as red for positive and blue for negative. Full scale is 1500 nT. The typical strength of Earth’s magnetic field at the surface is 5000 nT. The solid blue lines show the extent of near surface ground ice as determined by Odyssey mission. Ground ice is present near the surface poleward of these lines. Crater morphology indicates deep ground ice poleward of 301 (Squyres and Carr, 1986), shown here by dark blue lines and arrows. The region between 60 and 801S near 1801W is heavily cratered, preserves crustal magnetism, and has ground ice present. This is our suggested target site for drilling. This figure is adapted from Acun˜a et al. (1999), based on the crater distribution in Barlow (1997). The distribution of near-surface ground ice is from Feldman et al. (2002).
mapped out by Odyssey. These ice distributions are shown in Fig. 1. The ice-rich permafrost in the Northern polar region may contain frozen water but the low density of craters on the surface indicates that it is young, and therefore less likely to hold ancient Martian life. The southern hemisphere of Mars, however, contains heavily cratered terrain that presumably dates back to early in Martian history when water was prevalent and life was more probable. The surprise discovery of strong magnetic fields in the ancient cratered highlands of Mars (Acun˜a et al., 1999; Connerney et al., 1999) provides a possible way to determine the oldest undisturbed permafrost on Mars. As shown in Fig. 1, the crustal magnetism in the southern hemisphere of Mars occurs in heavily cratered terrain indicating that this area dates back to the heavy bombardment at the end of the formation of the planet (Acun˜a et al., 1999). It is important to note that there is no crustal magnetism in the vicinity of the large craters of Hellas and Argyre. Presumably these impacts erased the magnetic properties of the surrounding crust, possibly by shock pressure (Acun˜a et al., 1999; Hood et al., 2003). If Hellas and Argyre occurred after the magnetic features were formed, this indicates that the magnetic
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features predate the heavy bombardment, predate the crustal dichotomy between the northern and southern hemisphere, and predate the formation of the large geological features such as Tharsis and Valles Marineris. Thus the magnetic crustal features may be sites of the oldest undisturbed terrains on Mars providing a window into the early environments that may contain material dating back to an earlier wetter Mars. These locations experienced the heavy bombardment, but not impacts large enough to erase magnetism. Sites in the southern
Fig. 2. Image of a possible location in the ancient ice-rich terrain for drilling, the landing site for Mars Polar Lander near 761S, 1951W. Shown here is the landing error ellipse (90 km by 5 km) for the Mars Polar Lander. Image courtesy of NASA/JPL.
highlands between 60 and 801S, near 1801W containing the previously mentioned magnetic crustal features may be the oldest permafrost on Mars and the best target when searching for Martian life. One particular site of interest in the region now known to contain magnetic anomalies is the landing site of the Mars Polar Lander. The Polar Lander crashed on Mars in December 1999; its target site was extensively imaged both before and after the crash. The landing eclipse is shown in Fig. 2 together with a high-resolution image of a representative area (Fig. 3) of the terrain in this region. This site was certified for landing as part of the Mars Polar Lander mission and would be suitable for a drilling mission as well. Similar to the mammoths extracted from the ice in Siberia (Willerslev et al., 2004b), Martian microbes extracted from the permafrost would be dead but some of their biomolecules would be intact. From samples of this ancient Martian permafrost it would be possible, in principle, to determine the biochemical composition of Martian life, and any phylogenetic relationship between that life and Earth life.
Fig. 3. A series of images taken by the MOC camera on the Mars Global Surveyor. They show the variety of terrains and textures present within Mars Polar Lander’s landing zone. Each of the six boxes shows an area of about 0.6 km2. Knobs, pits, ridges, gullies, and smooth intervening surfaces are all seen. These images show that the terrain at the expected site would be suitable for a drilling mission. Image by NASA/JPL Malin Space Science Center.
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3. 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 the surface environment on Mars was hostile to Earth life, particularly due to the strong UV. For missions with deep subsurface access the planetary protection requirements are certain to be more severe (see eg. Mancinelli, 2003). This includes not just the borehole and the samples, but also the surface where the drill is operating. For robotic drill missions to Mars an overall assessment of the planetary protection requirements and the need not to contaminate the samples at depth may necessitate that the entire drill be sterilized in a way similar to Viking and in contrast to Pathfinder and the Mars Exploration Rovers that had bioload reduction but not sterilization.
4. 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 other than air is used. The drilling apparatus is often dirty when brought out of the hole. Due to the size of the equipment and field locations, it is usually impossible to sterilize the drill before each run. As a result, it is necessary to measure the extent of contamination. When drilling the temperature of the ground must also be considered. In ground that is not frozen it is harder to ensure sample integrity due to the liquefaction of the sample. It is easier for contaminants to penetrate a non-frozen core than a frozen core. Both air and ground temperatures affect the ability to drill. Layering and core structure are compromised when melting occurs. The low pressure of the Martian environment compared to the pressure on Earth offers two benefits. When drilling on Earth a drilling fluid is used to help clear the borehole of loose cuttings. In a recent experiment (Zacny et al., 2004), drilling in frozen
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samples was tested in a Mars pressure chamber without drilling fluid. At this low pressure, ice within the permafrost vaporized from the sample and the flow of this vapor naturally cleared the borehole of loose cuttings. With the elimination of a drilling fluid, the chance of contamination is decreased. In addition, if a core is unattainable, these loose cuttings could be collected for research purposes. The results of Zacny et al. (2004) indicate that drilling in permafrost on Mars may be easier than drilling in permafrost on Earth. With current drilling methods, it is clear that the exteriors of samples from virtually all drilling methods are contaminated by the drilling environment. To determine the amount of contamination by the drilling process, a tracer can be used. There are three main types of tracers: biological, non-biological, and naturally occurring tracers. Biological tracers are typically made of genetically marked bacteria. Non-biological tracers include dyes, spheres, and other additives of nonbiological origin. Biological and non-biological tracers are added to the drilling system and then the sample is searched for their presence to determine sample integrity. In contrast, a naturally occurring tracer is any property of the environment or the subsurface material that varies with depth in a known way. Contamination from the surface and cross contamination between layers can be determined from tracking this naturally occurring tracer. Examples include stable isotopes and mineral phases. On Earth, biological tracers are the most widely used type of tracer because they are easy to use and to detect. Since the research being conducted on the sample is often microbiological in nature; the same equipment and techniques used for sample analysis can be used for detecting the tracers. Some of the most common microbes used as tracers are Serratia marcescens (Willerslev et al., 2003, 2004a; Phelps and Frederickson, 2002; Christner et al., 2005), Bacillus globigii, (Eiswirth and Hotzel, 1995; Phelps and Frederickson, 2002), Saccharomyces cerevisiae (bakers yeast), Escherichia coli, (Harvey and Harms, 2002; Phelps and Frederickson, 2002), and Chromobacterium violacem (Eiswirth and Hotzel, 1995). Biological tracers are usually introduced into the drilling process by spiking the drilling fluid. The use of biological tracers on Mars would not be allowed due to planetary protection issues. Non-biological tracers include chemical substances, ionic tracers, dyes, and fluorescent microspheres. Chemical tracers are often used for environmental assessment of hazardous waste treatment. In general, chemical tracers are not as sensitive as biological tracers. Specific examples of tracers used are: sodium chloride, bromide, potassium sulfate, and iodide. (Eiswirth and Hotzel, 1995; Phelps and Frederickson, 2002). The introduction of chemical tracers into the environment is often achieved by the use of the drilling fluid, but can
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also be applied by pouring a solution of water and chemical tracer down the borehole. Fluorescent dyes and microspheres, are becoming more popular as they do not change the biological or chemical structure of the sample. Some examples of fluorescent dyes are eosine, naphtionate, pyranine, and rhodamine (Eiswirth and Hotzel, 1995). Fluorescent microspheres come in two basic types, carboxylated and latex, (Eiswirth and Hotzel, 1995; Harvey and Harms, 2002; Phelps and Frederickson, 2002) and variety of sizes, as small as 0.05 mm (Juck et al., 2004). Microspheres of a suitable size can mimic the penetration of biological tracers without adding a biological substance or causing a chemical reaction. When a drilling fluid is not used, tracers have been dropped down the borehole and painted on the drill bit (Juck et al., 2004). Naturally occurring isotopes, or passive tracers, are also a way to check the cleanliness and sequencing of the samples. Some naturally occurring isotopic ratios studied on Earth include: 18O/16O, 2H/1H, 15N/14N, 14 C/13C, 40Ar/14N, 40Ar/36Ar (Gibson et al., 2003). Other elements such as strontium, boron, sulfur, chloride, tritium, and deuterium are also used to infer sample integrity on Earth (Bogard et al., 2001; Gibson et al., 2003). On Mars the exact geological origin of the polar caps and thus the exact isotopic ratios analogous to Earth are unknown. However, based on the monotonic changing pattern of 15N/14N ratio in Mars atmosphere Doran et al. (2000) suggests using the 15 /14 N N ratio as an isotopic tracer for Mars.
5. 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 451 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 501, the total sunlight received at the pole over the entire year is 1.4 times higher than for the present obliquity of 251. At these high values of the obliquity the permafrost temperatures may increase. The depth to which cycling warming will reach can be determined from the thermal damping depth, d, d2 ¼ (KT/p), where T is the period and K is the thermal diffusivity. Fig. 4 shows a plot of d for typical thermal properties of permafrost for periods from 1 to 106 yr. Note that this depth is an overestimate since it does not take into account the effect of latent heat which significantly retards temperature rises above freezing in permafrost. From Fig. 4 it is clear that depths of 1000 m
Fig. 4. Dampening depth in meters in Martian permafrost for periods up to 106 yr.
should contain samples unaltered by thermal cycles due to obliquity with periods of less than 106 yr. If obliquity cycles removed deep ground ice then subsequent recharge by diffusion from the atmosphere would only result in ground ice layers to depths of a few tens of meters at most due the blockage of pore spaces by ice (Mellon et al., 2004). The crater morphologies surveyed by Squyres and Carr (1986) indicate the persistence of deep ground ice in the polar regions arguing against complete removal by past obliquity cycles. However, even if there has been periodic removal and replacement of ground ice at depth, these locations may still preserve the organic record of past life. The sublimation of the ice would leave any organic material behind. Only if the ice melted to form liquid would the organics be removed. Subsequent replacement of the ice by diffusion would also not alter the organic record. The presence of ice is important because it is likely to help protect organic material against decomposition due to soil oxidants. Thus, the best record of organic, and possibly biological material is probably preserved in locations which have been ice-rich for most of, if not all, their post depositional history, and have never experienced conditions leading to formation of liquid water. Deeper layers may have the least thermal variation but they may also contain less biological material depending on the mode of formation of these deep layers. Deeply buried sediments are preferred over deeply buried volcanic rocks. These considerations further indicate that South polar permafrost is the likely target for a search for samples of ancient Martian biology.
6. Aseptic permafrost drilling on Earth Microbiological studies in permafrost on Earth provide the most direct analog to permafrost drilling
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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 past years there have been at least two programs directed toward aseptic drilling in permafrost on Earth. The use of tracers in these studies to document the pristine nature of the samples is relevant to Mars. One study by Gilichinsky and colleagues (Gilichinsky et al., 1992; Willerslev et al., 2003) is based on a custom design drill that drives a coring tube with a gas motor and a clutch, and transmission system that can provide both torque and vertical force during the drilling process (Gilichinsky et al., 1992). Using this drill, samples have been obtained from ice-rich permafrost in Northern Siberia, New Zealand (Willerslev et al., 2003, 2004a), and Beacon Valley Antarctica up to 50 m in depth. The innovation that allows for aseptic sample collection is that the drilling is done completely dry without any lubricant or drilling fluid. Willerslev et al. (2003) introduced a biological tracer S. marcescens onto the drilling equipment and then looked for the presence of these bacteria in the sample and was thereby able to demonstrate the extent of contamination of the core. A second study was done on Ellesmere Island Nanavut Canada as reported in Juck et al. (2004). Fluorescent carboxylated microspheres with diameters 0.05 and 0.5 mm were used to test aseptic collection methods using a drill with compressed air as the fluid to remove cuttings and with a coring system that did not require any drilling fluid. Fluorescent microspheres were chosen to act as a biological surrogate having similar size to real biological organisms without altering the biological or chemical constituents of the sample. The semi-portable drilling rig used compressed chilled air as the drilling fluid. The drill could also operate without any fluid depending on the permafrost characteristics (Dickenson et al., 1999). Juck et al. (2004) concluded that the best method for tracer studies in this system consisted of covering the inside of the bit, core catcher, and first few centimeters of the core tube with a concentrated solution of microspheres. Of the methods tested this was the quickest, the most effective way of distributing the tracer, and could be done without interrupting the drilling process. Due to the many advantages, the ‘‘paint on bit’’ tracer introduction method was applied to the majority of the samples. The fluorescent microspheres were found to penetrate the core between 2 and 5 mm (average 371.7 mm) in the three intact soil permafrost samples, and between 2 and 6 mm (average 4.371.7 mm) in the four ice samples.
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7. Conclusions The ancient ice-rich permafrost in the southern highlands of Mars (60–801S, near 1801W) 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 independent genesis. Deep drilling on robotic or human missions could be focused on obtaining this material in ways that do not contaminate the Martian subsurface and provide positive controls ensuring that any biological material in the samples were not from Earthly contamination. A large Mars chamber simulating Martian conditions such as temperature and pressure would be ideal (possibly necessary) for testing aseptic drilling methods as a precursor to a Mars drilling mission. Since such a chamber does not exist yet, drilling in permafrost on Earth for microbiological studies provides the best analog to Martian drilling. Permafrost tracer studies used on Earth are of two types—fluorescent microspheres and selected microbial strains. Both have been shown to be effective in Earth applications. However, for Mars applications fluorescent microspheres are preferred over microbial strains. Further work on the development of automated drilling systems or simplified systems for human operators in space suites will be necessary before deep drilling into Martian permafrost is a reality.
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 Eureka Weather Station, and Operation Hurricane of the Canadian Military Base at Fort Eureka for their support in equipment and logistics. This project was funded by the NASA Astrobiology Science and Technology Instrument Development program through a grant to G. Briggs. References Acun˜a, M.H., Connerney, J.E.P., Ness, N.F., Lin, R.P., Mitchell, D., Carlson, C.W., McFadden, J., Anderson, K.A., Reme, H., Mazelle, C., Vignes, D., Wasilewski, P., Cloutier, P., 1999. Global
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