1. Biological Investigations and the Search for Life

This article, Martian Biological Investigators and the Search for Life, is Part 5 of the results of the Human Exploration of Mars Science Analysis Group (HEM-SAG) which are presented in five papers in this issue of the Journal of Cosmology (Levine et al., 2010a,b,c,d,e).

Several recent papers have addressed different aspects and problems associated with the search for extinct and extant life on Mars, including the interpretation of 35-year old data based on automated measurements on robotic probes, e.g., the biology experiments on the Viking 1 and 2 Landers in 1976. Several of these recent papers point out the likelihood of the existence of extinct and/or extant extremophiles on Mars. These papers point to the important role of in situ human exploration in the detection of potential extinct and extant life on Mars.

In 1976, the Labeled Release (LR) experiment on the Viking Landers gave results that met the pre-mission criteria for the detection of life. However, the consensus of the Mars community favored a chemical or physical explanation, rather than a biological explanation for the LR results. In the more than three decades since the original LR measurement, the LR Team has continued their analysis and interpretation of the 1976 LR results. Levin (2010) points out that since Viking, no life detection experiments have been sent to Mars and that new information about the environment of Mars and the discoveries of extremophiles on Earth have re-energized interest in the possibility of living microorganisms on Mars. Levin (2010) proposes a new "Chiral LR" (CLR) for a future Mars mission.

Houtkooper and Schulze-Makuch (2010) suggested the presence of microbial organisms on Mars that use a mixture of hydrogen peroxide and water as an intercellular solvent as an adaption to the harsh surface environment of Mars. The recent discovery by the Phoenix Lander of large amounts (0.4-0.6 wt%) of perchlorate salts in the Martian soil sheds a new light on these interpretations. An interesting property perchlorate salts share with hydrogren peroxide is their effectiveness as anti-freeze, which improves the habitability of the surface of Mars, particularly for xerophilic organisms employing a mixture of hydrogen peroxide and water in their intracellular fluid (Houtkooper and Schulze-Makuch, 2010).

Sephton (2010) has speculated that the harsh environmental conditions on Mars will ensure that any extant life will have adaptations similar to those found in the extremophilic organisms on Earth. Extant life, if present, is also likely to exploit the subsurface environment, which provides more equable conditions than those at the surface. Sephton (2010) also speculates that life may be present in fossil form, entombed in sedimentary rocks.

Leuko et al. (2010) speculate that Mars may be an intriguing location to search for Halophilic Archea (or their remnants) dating back to its early, wetter and warmer environment. Halite has been shown to preserve living organisms for more than 400 million years and recent data suggests the presence of halite on Mars.

Mahaney and Dohm (2010) speculate on possible Mars fossil and extant extremophiles in Mars-like Antarctic environments. They propose that paleosols in the Antarctic Dry Valleys are excellent terrestrial analogues for paleosols on Mars, including weathered regolith located close to the polar caps on Mars.

Yung et al. (2010) have investigated the significance of the discovery of methane (CH₄) on Mars and its relationship to life. They conclude that the abiotic and biotic pathways for methanogensis of Mars are surprisingly similar. Both mechanisms require carbon dioxide (CO₂) and hydrogen (H₂) as starting materials for the production of methane.

2. Biology/Life Investigations on Mars

Human enabled biological investigations on Mars would focus on taking samples and making measurements to Determine if Life Ever Arose on Mars. This goal is consistent with the 2006 Mars Exploration Program Analysis Group (MEPAG) goals and priorities, and we do not see this goal changing in the next 30-year period.

The search for life on Mars can be generally broken into two broad categories: 1) the search for past (extinct) life on Mars and 2) the search for present (extant) life. Both have been, and will continue to be based on a search for water, since all life on Earth requires water for survival.

Abundant evidence on the Martian surface of past water activity (e.g. rivers, lakes, groundwater discharge) has lead to Mars becoming a strong candidate as a second planet in our solar system with a history of life. With increasing knowledge of the extremes under which organisms can survive on Earth, especially in the deep subsurface, whether Martian life is still present today has become a compelling and legitimate scientific question.

The National Research Council (NRC) was recently commissioned to do a study to develop "an up-to-date integrated astrobiology strategy for Mars exploration that brings together all the threads of this diverse topic into a single source for science mission planning" (Jakosky et al 2007). This report did not consider how to do science with humans, but we rely heavily on it and earlier MEPAG documents here as snapshots of the current community thinking on astrobiological investigations on Mars.

As pointed out by Jakosky et al (2007), the search for life on Mars requires a very broad understanding of Mars as an integrated planetary system. Such an integrated understanding requires investigation of the following:

3. The Search for Extant Life

Jakosky et al (2007) suggested a number of high priority targets based on evidence for present-day or geologically recent water near the surface:

It has been noted that sites where recent water may have occurred would also include midlatitude deposits indicative of shallow ground ice.

Conditions in the top 5 m of the Martian surface are considered extremely limiting for life. Limiting conditions include high levels of ultraviolet radiation and purported oxidants as well as most of the surface being below the limits of water activity and temperature for life on Earth. For these reasons, finding evidence of extant life near the surface would likely be difficult and the search would almost certainly require subsurface access. This was also a key recommendation of Jakosky et al (2007).

4. The Search for Extinct Life

Jakosky et al (2007) list sites pertinent to geologically ancient water (and by association past life) include the following:

5. Human Science Reference Missions (HSRM): Biology/Life

As a demonstration of how HEM-SAG envisions carrying out the biological goals, a human science reference mission was designed to the Centauri Montes region. This region has drawn attention from astrobiologists as a result of the discovery by Malin et al (2006) that a flow feature (gully) inside a crater wall has apparently been active in the last decade, thereby providing the intriguing prospect of liquid water at or near the surface (Fig. 1). This region has also been well documented for its concentration of young, volatile-rich deposits and figures prominently in recent GCM simulations at different obliquities, which indicate that the eastern- Hellas region should be receiving significant amounts of water-ice from the south pole (Forget et al, 2006). Centauri Montes is also at the head of major Amazonian/Hesperian outflow channels.

The indicators of ice deposits and liquid water today, as well as the region being associated with outflow channels, provide ample local targets for the search for extant and extinct life. For geological investigations, this region has the attraction of all three epochs being represented in close proximity. We understand that the connection between the active gully features and liquid water is controversial (e.g., McEwen et al. 2007). This site was chosen because at the time of writing, it was one of the two most promising sites on the planet for finding liquid water near the surface. If other more promising sites are discovered, site selection for addressing biological goals should follow these new discoveries.

6. Science Goals To Be Addressed

Research at Centauri Montes would be focused on MEPAG goal number 1: "Determine If Life Ever Arose on Mars." Specific investigations are summarized in Table 1.

7. Other Disciplinary Science

Besides the biological targets, a number of sites of interest exist from a geological perspective within 100 km of the target crater. Of particular note is that geological units from all three major epochs are accessible from this site, including:

1. Noachian Hellas basin rim material

2. Hesperian Smooth Plains sedimentary deposits modified by fluvial, periglacial processes 3. Amazonian Debris Aprons (ice-rich material, rock glaciers)

4. Amazonian/Hesperian Pitted Plains deflated ice rich materials

5. Amazonian/Hesperian Outflow Channel material of Reull Vallis

Geophysical surveys could search for near surface liquid water. The close proximity of active gullies within the Centauri Montes region is suggestive of possible subsurface aqueous activity, which should be explored to provide a more complete understanding of the hydrologic system in this area. Such data may allow for the detection of a subsurface aquifer if it exists and we could map out its planar extent (Antol et al, 2005). The size of the aquifer could place constraints on the amount of water available to form the gullies. Such information is valuable for understanding the nature and extent of the hydrological cycle on Mars as well as selecting drill sites. This information would also be used to compare the amount of water potentially available in an aquifer (which is dependent upon the aquifer size) with combined modeling and geomorphology of predicted amounts of water that have run through the gullies to assess the consistency of such independent estimates of water volumes.

Climate and meteorology studies focused at Centauri Montes would provide insight into the unique process of water ice transport to this region from the South Pole. As the area shows a huge geological record spanning from Noachian to Hesperian, a long-term climatic evolution of the planet could be inferred through the integration of geological, sedimentological and geochemical data recovered from these old deposits. In addition, meteorology studies would likely be conducted at any site of human investigation on Mars and thus are not intrinsically sitespecific.

8. Location

Centauri Montes is located on the rim of Hellas Basin (Figs 2 and 3). The Centauri Montes and Hellas Montes regions are characterized by remnant massifs, interpreted to be crustal uplifts and ejecta from the Hellas impact which occurred early in Mars’ history (Lehmann et al, 2006). Subsequently, several geologic processes have worked to alter the landscape. Lobate debris aprons formed which are characterized by viscous creep and deformation and are attributed to the movement of rock glaciers (Squyres 1978, 1979; Squyres and Carr 1986; Crown and Stewart 1995). Volcanic processes formed several volcanic edifices in the region (Greeley and Crown 1990; Crown 1991). Outflow activity worked to move large volumes of water and debris in association with Dao, Niger, Harmakhis, and Reull Valles (Price 1992; Bleamaster and Crown 2004). Therefore the Centauri Montes region has a rich and complex geologic history.

A crater within this region located near 38.7°S, 263.3°W is the site of recent gully activity (Malin et al. 2006). Sometime between August 1999 and September 2005, a light-toned material was transported down-slope through a gully channel and deposited along the crater wall (Malin et al. 2006). The new deposit has extended branches, a digitate terminus, diverts around obstacles, and has low relief. These observations have been interpreted as suggestive of fluid (aqueous) flow (Malin et al. 2006). Due to the scientific interest in studying this site extensively with humans, the human landing site would be adjacent to this active gully crater.

9. Research Plan At Centauri Montes

Two modes of research would be carried out at Centauri Montes:

Mode 1 — Active gully investigations and local drilling This mode of research is primarily focused on assessing the recently active gully and other fresh gullies as potential sites of recently water activity, and hence extant life (Fig. 4).

1. Drilling. For this activity horizontal mobility would be minimal, largely dependent on how close to the active gully crater a suitable landing ellipse could be placed. The drill rig would need to be portable enough to be moved from the landing site, to the drill site. Alternatively, it could be moved from the landing site on the rover in pieces and assembled at the drill site. We envision the drill site and landing site to be close enough together that daily commuting could occur between the two. Drill samples (cores and or cuttings) would need to be acquired without drilling fluids (or with clean and sterile fluids) to protect against contamination and alteration and would require suitable on-site storage to keep them protected (as close to their ambient conditions as possible) until such time that they could be moved back to base (presumably the end of each work shift). Once back at base, cataloguing, sub sampling and analyses would be done in the habitat laboratory. If there is suitable interest from other disciplines (e.g., geophysics, geology), other (not necessarily as deep) holes could be drilled in the local area for specific goals of geology, geophysics and climate studies. Drill cores would be processed on site for analysis of organics or potential biota by sub-coring the main core and maintaining the sub-cores in sterile conditions. These sub-coring methods are similar to those used in deep biosphere research on Earth. Processing on site would minimize the chances of potential contamination by the human drillers during the transfer of the cores from the field site to the habitat or rover laboratory.

2. Direct measurements and sampling from the active gully. Based on available data, this seems to be achieved most easily by descending to the gully site from above. Samples would be gathered both inside and outside the gully to assess potential for organic/biological enrichment in and under the gully, particularly if it is determined that water was involved in gully formation.

3.. Sampling of sediments on the crater floor. The available imagery of the active gully crater suggests a history of fluid flow through this crater, possibly associated with the gullies. Drilling on the crater floor into some of these sediments, even to shallow depths would be useful for seeking out evidence of past life. Multiple samples would be collected from promising locations.

Mode 2 — Sampling Traverses The second half of the expedition would be spent traversing out to a radius of 50 km away from the landing site to access materials from the three different epochs and collect samples for investigation of past life. Samples would include short cores or grab samples of sediment or rock. For the astrobiology work we would only do minimal analysis in the field and would return many samples to the habitat laboratory for detailed analysis. The main focus is to look for samples which may have preserved bio-signatures (e.g., old ice, evaporites, etc.)

10. Horizontal Mobility Requirements

A comparison was done in the region of CM between the scientific benefits of traversing 50 km to a 100 km radial distance from the landing site (Fig. 5). In this region, 50 km horizontal mobility seems to be an adequate range given the great diversity of sample sites within this distance. Increasing the horizontal mobility to 100 km does not provide a dramatic increase in new types of terrain. The most significant new terrain type that could be accessed by extending the traverse distance greater than 50 km is the Hesperian Ridged Plains (HRPs). These materials outcrop north of the rim massifs shown in Fig. 5 and to access the HRPs the astronauts would likely need to drive around the massif blocks, requiring greater than 50 km driving distance. In addition, the preferred landing site shown in Fig. 5 is purposely centrally located. If this landing site is shifted in any direction, a traverse distance of greater than 50 km may be required to access the highest priority science sites. Nonetheless, due to the high concentration of scientifically compelling sites within the 50 km radius, this degree of horizontal mobility would be sufficient for this mission. Furthermore, only half of the mission would be dedicated to traversing, and so a 50 km range should provide ample sampling targets in the limited time.

We also recognize a safety constraint on the distance of the traverse. If there are no additional terrain types that are of high priority beyond 50 km then rescue and safety considerations make a 100 km traverse less desirable.

11. Vertical Mobility Requirements

For biological investigations we would need vertical mobility to:

1. Investigate the groundwater hypothesis for recently reported active gullies on Mars (Malin et al. 2006). Several researchers have suggested that a shallow subsurface aquifer may be the source of the liquid water feeding the Martian gullies (Malin and Edgett 2000; Heldmann and Mellon 2004; Heldmann et al. 2007). In this scenario, a liquid water aquifer exists beneath the upslope plateau behind the gullies at a depth coincident with (or near to) the gully alcove base depth. The average alcove base depth in the southern hemisphere is ~200 meters poleward of ~40° and thus a drill capable of reaching several hundred meters depth would be required in order to reach the subsurface aquifer. To account for any uncertainties and/or heterogeneities in the subsurface as well as the natural variation in alcove depths within different gully systems, a drill capable of reaching greater than 250 m depth (preferably 400- 500 m) would be desirable.

2. Investigate other fresh gully sites near the landing site. Likely the same penetration depths as a) are required here.

3. Drill directly into gully sites to 5 m.

4. Drill (5-50 m) while doing longer traverses to collect samples from below the high radiation and oxidant surface region for ancient or dormant life in ground ice and other water lain deposits.

We envision two drills to achieve these goals. One would be heavy and not very portable (for 1 and 3). This drill requires further technological development. The second would be more portable and lightweight with the option to be mounted on the rover for the deeper penetrations, and detached and manipulated directly by astronauts for the shallower work in rough terrain. Examples of this type of arrangement already exist for terrestrial prospecting.

12. Science Capabilities Required

The investigation of field samples can be split broadly into two levels of analysis: 1) basic analyses in the field and measurements that need to be accomplished in-situ, and 2) more detailed analysis using a full suite of laboratory equipment.

To accomplish level 1 the rover should offer basic mobile laboratory capabilities. However, the logistics of operation in a confined rover space in the field probably militate against including large quantities of equipment. Further, from a purely logistical standpoint, field experiments should be limited to those required on site to select samples etc. Considerations of safety and limited time in the field mean that carrying out detailed laboratory investigations in a rover would likely to be undesirable.

Level 2 should be accomplished at the habitat. In a fixed location free of the constraints of EVA activity much more detailed and painstaking analysis of samples could be accomplished. For example, precise dating of samples, which would be extremely helpful in determining the geological and environment history of sampling sites and hence guiding sample selection, would be best done on isolated mineral phases physically separated from hand specimens. This separation can be most efficiently done by astronauts. The quality of data for some isotope systems would also be enhanced by sample preparation procedures that could be difficult to automate and package for robotic exploration.

Two laboratories would likely be needed. A medical/planetary protection laboratory would be used for human health monitoring and medical treatment as well as monitoring of the microbial ecology of the habitat and the region around it as part of a general planetary protection protocol/survey. However, to minimize the chances of cross-contamination this lab should be separated from a second laboratory used for extant/extinct life investigations. Separating the two laboratories would help prevent false positive detection of Martian life and it would reduce the chances of astronauts coming into contact with organic material from Mars.

The human laboratory could either be physically separated from the science laboratory within the habitat or alternatively a science laboratory might be erected separately from the habitat and accessed either in EVA suit or air-locked pressurized walkway. From a back contamination point of view this option may be attractive to reduce contact between Martian materials and people. Additionally, samples of interest should be subdivided so that a portion of each sample could be brought into the examination facility and the other portion could be stored outside or in the "shed" and not brought into an area where contamination may occur. These stored samples could then be reserved for return to Earth and further detailed examination.

13. Planetary Protection Issues at the HSRM Locality and Potential Mitigation

To achieve the biology goals, especially the search for extant life, we would almost certainly need to enter special regions (e.g., gully sites and the subsurface) with humans. We feel that a biologically focused mission would need to include a search for extant life, so technological developments would be needed to prevent forward contamination and provide a safe barrier for astronauts working on samples. Other potential mitigation:

1. Carry out repeated analysis of microbial populations in soils to quantify contamination around station for planetary protection analysis. This might be undertaken every 100 days, but this time period could be changed in response to detection of contaminants and/or during periods of intense EVA activity.

2. Monitor microbial presence in an ever-increasing perimeter around the station, or a set of perimeters to ascertain spread. Same tools used for microbial identification could be used for health monitoring and to check sterility of sampling tools if techniques to verify cleanliness and contamination control protocols are robust.

3. Any rover that would be developed needs a compartment, which would be open to the outside only and not to inside contamination. This compartment could be used to store samples for their return to the habitat base, thus minimizing the escape of microbial contaminants from the habitat.

4. The habitat laboratory should also be environmentally separate from crew habitation spaces.

5. The possible development or designation of special EVA suits with high biological containment specification and cleanliness protocols that would only be used to access special regions.