impact excavation and the search for subsurface life on mars

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Icarus 155, 340–349 (2002) doi:10.1006/icar.2001.6725, available online at http://www.idealibrary.com on Impact Excavation and the Search for Subsurface Life on Mars Charles S. Cockell NASA Ames Research Center, MS 245-3, Moffett Field, California 94035-1000; and British Antarctic Survey, Cambridge, United Kingdom E-mail: [email protected] and Nadine G. Barlow Department of Physics, University of Central Florida, Orlando, Florida 32816-2385 Received April 29, 2001; revised July 8, 2001 Because of the ubiquity of subsurface microbial life on Earth, ex- amination of the subsurface of Mars could provide an answer to the question of whether microorganisms exist or ever existed on that planet. Impact craters provide a natural mechanism for ac- cessing the deep substrate of Mars and exploring its exobiological potential. Based on equations that relate impact crater diameters to excavation depth we estimate the observed crater diameters that are required to prospect to given depths in the martian subsurface and we relate these depths to observed microbiological phenomena in the terrestrial subsurface. Simple craters can be used to examine material to a depth of 270 m. Complex craters can be used to reach greater depths, with craters of diameters 300 km required to reach depths of 6 km or greater, which represent the limit of the terres- trial deep subsurface biosphere. Examination of the ejecta blankets of craters between 17.5 and 260 km in diameter would provide in- sights into whether there is an extant, or whether there is evidence of an extinct, deep subsurface microbiota between 500 and 5000 m prior to committing to large-scale drilling efforts. At depths <500 m some crater excavations are likely to be more important than others from an exobiological point of view. We discuss examples of impacts into putative intracrater paleolacustrine sediments and regions as- sociated with hydrothermal activity. We compare these depths to the characteristics of subsurface life on Earth and the fossil microbi- ological record in terrestrial impact craters. c 2002 Elsevier Science (USA) Key Words: impacts; Mars; life; excavation; critical depth; dril- ling; paleolacustrine. 1. INTRODUCTION The question of microbial life on Mars, extinct or extant, remains a field of much discussion. The reports of possible bi- ologic markers preserved in the ancient ALH84001 meteorite (McKay et al. 1996) have revived a subject that many scientists had thought was answered after the Viking Lander investiga- tions in the late 1970s. There is evidence that liquid water in the form of rivers, lakes, and possibly oceans existed on the martian surface in the past (Baker et al. 1991, Parker et al. 1993, Head et al. 1999, Cabrol and Grin 1999, Malin and Edgett 2000a) and that subsurface water may still exist today (Malin and Edgett 2000b). These observations reopen the question of whether Mars has provided an environment favorable for the evolution and maintenance of life. Current conditions on the martian surface are extremely hos- tile to life, based on our knowledge of the terrestrial limits to life. The low atmospheric pressure (6 hPa), low surface tem- perature (average of 240 K), lack of liquid water, UV radiation fluxes three orders of magnitude more damaging to DNA than those on the Earth (Cockell et al. 2000), and the oxidized soil chemistry (Bieman et al. 1977) all make the surface of Mars extremely unfavorable as an abode of life. This conclusion is consistent with the results of the Viking Lander biology experi- ments (Klein et al. 1992). However, recent discoveries have led to the speculation that the martian subsurface might provide a more hospitable envi- ronment for a martian biota. Recently biologists have found terrestrial bacteria existing at depths to 5.3 km in igneous rocks (Pedersen 2000). Because the martian substrate is believed to contain substantial amounts of H 2 O, including liquid in some locations (Barlow and Bradley 1990, Clifford 1993, Carr 1996, Barlow et al. 2001), such subsurface oases may provide an environment for biota to escape the hostile conditions at the surface. To investigate this hypothesis, samples of materials derived from the subsurface must be obtained and analyzed. Deep dril- ling is a method for obtaining subsurface samples to depths of up to 3 km (Mancinelli 2000). Another potential approach to accessing the substrate is to use “nature’s drill”: impact craters. Impact excavation of subsurface material might be a useful exo- biological search strategy prior to committing large resources into deep drilling. Impact craters excavate materials to a depth proportional to the size of the crater. Material excavated during the impact event is emplaced on the surface within easy access 340 0019-1035/02 $35.00 c 2002 Elsevier Science (USA) All rights reserved.

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Page 1: Impact Excavation and the Search for Subsurface Life on Mars

Icarus 155, 340–349 (2002)

doi:10.1006/icar.2001.6725, available online at http://www.idealibrary.com on

Impact Excavation and the Search for Subsurface Life on Mars

Charles S. Cockell

NASA Ames Research Center, MS 245-3, Moffett Field, California 94035-1000; and British Antarctic Survey, Cambridge, United KingdomE-mail: [email protected]

and

Nadine G. Barlow

Department of Physics, University of Central Florida, Orlando, Florida 32816-2385

Received April 29, 2001; revised July 8, 2001

Because of the ubiquity of subsurface microbial life on Earth, ex-amination of the subsurface of Mars could provide an answer tothe question of whether microorganisms exist or ever existed onthat planet. Impact craters provide a natural mechanism for ac-cessing the deep substrate of Mars and exploring its exobiologicalpotential. Based on equations that relate impact crater diametersto excavation depth we estimate the observed crater diameters thatare required to prospect to given depths in the martian subsurfaceand we relate these depths to observed microbiological phenomenain the terrestrial subsurface. Simple craters can be used to examinematerial to a depth of ∼270 m. Complex craters can be used to reachgreater depths, with craters of diameters ≥300 km required to reachdepths of 6 km or greater, which represent the limit of the terres-trial deep subsurface biosphere. Examination of the ejecta blanketsof craters between 17.5 and 260 km in diameter would provide in-sights into whether there is an extant, or whether there is evidenceof an extinct, deep subsurface microbiota between 500 and 5000 mprior to committing to large-scale drilling efforts. At depths <500 msome crater excavations are likely to be more important than othersfrom an exobiological point of view. We discuss examples of impactsinto putative intracrater paleolacustrine sediments and regions as-sociated with hydrothermal activity. We compare these depths tothe characteristics of subsurface life on Earth and the fossil microbi-ological record in terrestrial impact craters. c© 2002 Elsevier Science (USA)

Key Words: impacts; Mars; life; excavation; critical depth; dril-ling; paleolacustrine.

1. INTRODUCTION

surface in the past (Baker et al. 1991, Parker et al. 1993, Head

40

The question of microbial life on Mars, extinct or extant,remains a field of much discussion. The reports of possible bi-ologic markers preserved in the ancient ALH84001 meteorite(McKay et al. 1996) have revived a subject that many scientistshad thought was answered after the Viking Lander investiga-tions in the late 1970s. There is evidence that liquid water in theform of rivers, lakes, and possibly oceans existed on the martian

3

0019-1035/02 $35.00c© 2002 Elsevier Science (USA)

All rights reserved.

et al. 1999, Cabrol and Grin 1999, Malin and Edgett 2000a) andthat subsurface water may still exist today (Malin and Edgett2000b). These observations reopen the question of whether Marshas provided an environment favorable for the evolution andmaintenance of life.

Current conditions on the martian surface are extremely hos-tile to life, based on our knowledge of the terrestrial limits tolife. The low atmospheric pressure (∼6 hPa), low surface tem-perature (average of ∼240 K), lack of liquid water, UV radiationfluxes three orders of magnitude more damaging to DNA thanthose on the Earth (Cockell et al. 2000), and the oxidized soilchemistry (Bieman et al. 1977) all make the surface of Marsextremely unfavorable as an abode of life. This conclusion isconsistent with the results of the Viking Lander biology experi-ments (Klein et al. 1992).

However, recent discoveries have led to the speculation thatthe martian subsurface might provide a more hospitable envi-ronment for a martian biota. Recently biologists have foundterrestrial bacteria existing at depths to 5.3 km in igneous rocks(Pedersen 2000). Because the martian substrate is believed tocontain substantial amounts of H2O, including liquid in somelocations (Barlow and Bradley 1990, Clifford 1993, Carr 1996,Barlow et al. 2001), such subsurface oases may provide anenvironment for biota to escape the hostile conditions at thesurface.

To investigate this hypothesis, samples of materials derivedfrom the subsurface must be obtained and analyzed. Deep dril-ling is a method for obtaining subsurface samples to depths ofup to 3 km (Mancinelli 2000). Another potential approach toaccessing the substrate is to use “nature’s drill”: impact craters.Impact excavation of subsurface material might be a useful exo-biological search strategy prior to committing large resourcesinto deep drilling. Impact craters excavate materials to a depthproportional to the size of the crater. Material excavated duringthe impact event is emplaced on the surface within easy access

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IMPACT EXCAVATION

of many of the planned rover missions and ultimately humanexplorers.

Much of the ejecta created during an impact is melted, butthere is a substantial proportion of material that is in a state oflow shock (<5 GPa) or that otherwise is ejected intact (Melosh1989). Within this material there is the chance of finding evi-dence for biological activity. Horneck et al. (2001) showed thatBacillus subtilis spores could survive a simulated impact whenthe samples experienced peak pressures of 32 GPa and temper-atures of 250◦C. They concluded that bacteria could survive ameteorite impact. They were primarily interested in interplan-etary transfer of life and so the high shock pressures and tem-peratures they studied were related to the requirement to reachescape velocity. We suggest therefore that it is very likely thatextant bacteria would survive the much lower pressures and tem-peratures needed merely to excavate them to the surface and it isalso likely that signatures associated with fossiliferous biologi-cal remains would also survive excavation to the surface.

In this paper we quantify the required crater diameters neededto prospect the subsurface to defined depths and we relate thisto terrestrial subsurface biological phenomena.

2. METHODS—IMPACT EXCAVATION

Simple versus Complex Craters

The transition between simple bowl-shaped craters and com-plex craters which display more complicated interior morpholo-gies (such as central peaks and wall terraces) is related primarilyto the gravitational field of the body and to a lesser extent thetarget material (Melosh 1989). Craters on the Earth, Moon, andon Mercury display a simple-to-complex transition diameter thatis a function of 1/g, where g is the surface acceleration due togravity (Pike 1988).

Martian impact craters change from the simple to the com-plex morphologies at a diameter less than that predicted by the1/g relationship (average of about 7 km as opposed to ∼9 km).Volatiles in the martian substrate are believed to be responsiblefor this discrepancy (Melosh 1989), which could also explainthe variation in simple-to-complex transition diameter values re-ported by the Mars Global Surveyor (MGS), Mars Orbiter LaserAltimeter (MOLA) instrument between the north polar region(∼6 km in an area believed to contain substantial quantities ofsubsurface ice) (Garvin et al. 2000) and the equatorial regions(∼8 km in an area believed to contain subsurface ice at greaterdepths) (Garvin and Frawley 1998). In this paper we take theaverage transition diameter to be 7 km.

Simple Craters

Simple craters on Mars undergo little post-impact modifi-cation except for that caused by depositional or erosional pro-cesses. The diameter of the crater undergoes little expansion due

to wall collapse; hence the final crater diameter is approximatelythe same as the diameter of the crater formed during the impact

AND MARTIAN LIFE 341

process (i.e., the transient diameter). The depth of the pristinesimple craters on Mars has been determined from the topographymeasurements made by the MGS MOLA instrument. Analysisof these results indicates that simple crater depths (d) can beestimated from their diameters (D) by the following equation(Garvin and Frawley 1998):

d = 0.14 D0.90±0.14. (1)

Thus simple craters up to 7 km in diameter excavate up to depthsof ∼800 m. The material in the ejecta blankets surrounding thesecraters is typically derived from the top third of this crater depth(Melosh 1989), which constitutes approximately the top 270 mof the surface of Mars.

Complex Craters

Complex craters typically undergo substantial modificationin the first few minutes after the impact crater has formed. Thewalls slump under the influence of gravity, enlarging the finalcrater diameter beyond that of the initial transient diameter. Acentral peak often forms in the crater center, the result of reboundof the highly shocked material underlying the crater floor. Cen-tral pits also are common in martian impact craters, sometimesforming in place of the central uplift while other times occur-ring at the summit of the central peak. These central pits arebelieved to result from the release of gases produced during im-pact into subsurface volatiles (Wood et al. 1978, Barlow andBradley 1990). The final depth of a complex crater is shallowerin relation to its diameter compared to a simple crater (Garvinand Frawley 1998):

d = 0.25 D0.49±0.15. (2)

However, this relationship only provides the depth relative to theobserved crater diameter, which is different from the transientdiameter in complex craters due to expansion of the depressionthrough wall collapse and recession. Croft (1985) empiricallyderived a relationship between the observed rim diameter (Dr)and the excavation (transient) diameter (De)

De = D0.15±0.04q D0.85±0.04

r , (3)

where Dq is the simple-to-complex transition diameter. How-ever, when one calculates the +0.04 error values, one ends upwith transient crater diameters larger than the current diameter,a physically impossible situation. Croft (1985) notes that theactual uncertainty is probably less than 0.04 but maintains thisvalue to account for the observed variation in individual datapoints. This equation was derived using data from lunar andterrestrial craters—it may need modification for martian cratersdue to differences in target properties (such as the presence ofsubsurface volatiles) and impact parameters (generally lower

impact velocities). To avoid the problem of overestimating thesize of the transient cavity, we set the maximum transient crater
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342 COCKELL AN

TABLE IThe Final Diameter of Craters Required to Reach Certain Depths

on Mars to Prospect for Subsurface Biological Activity

Exobiological Final craterprospecting diameter Type of Crater Candidatedepth (m) required (km) crater number craters

≤270 ≤7 Simple 1 18.5◦N, 51.6◦W1000 38 Complex 2 23.08◦S, 168.06◦W2000 88 Complex 3 9.6◦S, 276.1◦W

(Jarry–Desloges)5800 305 Complex 4 40.8◦S, 157.7◦W

(Newton)

Note. Examples are given for each depth. Complex crater depths are calculatedassuming a transition from simple to complex craters at a diameter of 7 km. Thecrater numbers correspond to the numbers in Fig. 1.

diameter equal to the observed crater diameter in all of our cal-culations.

The depth of excavation for complex craters is approximately

de = De/10. (4)

The ejecta blanket is primarily derived from the top third of thecrater depth (Melosh 1989).

3. RESULTS

Using the equations in Section 2 we calculate that the maxi-mum depth of ejecta excavated by simple craters is 270 m. Toreach greater depths, complex craters must be used.

Table I shows the results of calculations on the size of a com-plex crater required to reach given depths in the subsurface. Toprospect to 1 km requires a crater of 38-km diameter. To reacha depth 6 km, which corresponds to the some of the deepestreports of a subsurface biosphere on Earth (Pedersen 2000), re-quires craters of ∼300-km diameter. Examples of craters thatwould provide ejecta from these depths are also shown.

In Figs. 1a and 1b the relationship between both simple andcomplex craters, observed diameter, and excavation depth hasbeen plotted and the depths at which certain biological phe-nomena observed in the terrestrial subsurface have been foundare shown as horizontal lines. In Fig. 1a the horizontal linescorrespond to the depths of paleolacustrine sequences that havebeen studied in terrestrial impact structures (discussed inSection 4.1.1). In Fig. 1b they correspond to the biological at-tributes of the terrestrial deep subsurface.

4. DISCUSSION

From an exobiological point of view the subsurface can bedivided into two regions of potential biological interest. For thepurposes of this paper we define the first region as the subsurface

down to a 500-m depth. We call this the “near-surface region.”Within this region there are subsurface targets with exobiologi-

D BARLOW

cal interest, but the potential biota contained within them is notof subsurface origin. These include buried intracrater paleola-custrine sediments and buried volcanic or impact hydrothermalfeatures. The attractiveness of examining excavation material

FIG. 1. (a) The relationship between the observed diameter of martiansimple craters and the depth of excavation. The horizontal dotted lines are thedepths corresponding to paleolacustrine sequences in terrestrial impact cratersdiscussed in the text. (b) The relationship between the observed diameter ofmartian complex craters and the depth of excavation. The horizontal dottedlines are the depths corresponding to regions in the Earth’s subsurface, wherebiological activity or biomarkers have been reported (see text for discussion). Thesolid horizontal line at 500 m is used in this paper to separate the “near-surfaceregion” (<500 m), where intracrater paleolacustrine sequences and other buried,but not strictly subsurface, biological targets might be sought and the “deepsubsurface region” (>500 m), where on Earth biological activity is primarilyrestricted to organisms that have tolerance or specific adaptations to survive and

grow in the subsurface. The numbers on the curve (1–4) correspond to the cratersin Table I.
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from craters that penetrate to these depths is therefore very de-pendent upon location.

Below 500 m is the region that corresponds to the deep subsur-face biosphere on Earth. Organisms such as anaerobic chemoli-thautotrophic and heterotrophic bacteria that have specificmetabolic characteristics required to survive and grow in thissubsurface environment inhabit this region. We call this the“deep subsurface region.” The craters one chooses to prospect inthis region are less influenced by location compared to cratersused to prospect the near-surface region if one assumes thatthe subsurface biosphere is essentially a planetary-wide phe-nomenon.

The true nature of the Martian subsurface is still a matter ofsome conjecture. The porosity of the martian substrate is be-lieved to be largely due to brecciation from impact events andis estimated to range from 20 to 50% for the megaregolith re-gion above the basement rock (typically estimated to lie about10 km below the surface) (Clifford 1993). The geothermal gra-dient for Mars is not well constrained at the present time, butconsideration of various models for the thermal evolution ofMars led Clifford (1993) to adopt a value of 30 mW m−2 for thepresent day heat flux (compared to an average of 82 mW m−2

for Earth). The heat flow certainly was higher in the martian pastand Schubert and Spoln (1990) estimate a value of 150 mW m−2

at the end of the heavy bombardment period (about 3.8 × 109

years ago).The presence of subsurface H2O on Mars would enhance the

probability of subsurface life. The existence of outflow channelswhich are postulated to form from outbursts of subsurface water(Baker 1982), lobate layered ejecta morphologies (Barlow andBradley 1990), and other geologic features suggestive of groundice (Carr 1996) have led to models of how much ice and liquidwater could be contained within the near-surface region of Mars.Using the porosity and heat flow estimates noted above, Clifford(1993) has argued that the average thickness of the present-day martian cryosphere (the permanently frozen near-surfacezone) ranges from 2.27 km at the equator to 6.53 km at thepoles. Underlying the cryosphere, the porosity and heat flowvalues indicate that liquid water can exist in a zone varying inthickness from 4 km near the poles to 8 km at the equator (for50% porosity and assuming the bedrock layer lies about 10 kmbelow the surface).

Thus the martian substrate probably contains zones whereconditions are comparable to those in the near surface and deepsurface biospheres on the Earth. We now discuss our results inthe context of biological information from these regions on Earthand their possible implications for Mars.

4.1. The Near-Surface Region

The near-surface region can be explored by either using sim-ple craters up to a diameter of 7 km that excavate to 270 m orusing complex craters up to 17.5 km in diameter that access the

subsurface from 270 to 500 m. Although craters of the appro-priate sizes can be found in most areas of the martian surface,

AND MARTIAN LIFE 343

some near surface regions of the planet are more interesting thanothers from an exobiological standpoint. This is because theyhave been associated with past episodes of water ponding (suchas crater paleolakes), they are regions of potential hydrothermalactivity (such as near volcanoes), or they are expected to containsubsurface permafrost.

4.1.1. Crater paleolakes. The exobiological importance ofcrater paleolakes is seen by their high priority ranking amongpossible landing sites for 2003 Mars exploration rovers (Barlow2001, Bridges 2001, Cabrol and Grin 2001, Newsom 2001). TheIsidis Basin, the proposed landing site for the Mars Express Bea-gle 2 lander, may also have preserved lucustrine deposits fromwetter periods in martian history (Chicarro 2001). Cabrol andGrin (1999) have catalogued 179 martian impact craters, whichappear to have contained lakes during periods of warmer andwetter climatic conditions. Subsequent impacts into these in-tracrater sedimentary deposits have excavated materials, whichcould be examined for fossil evidence of a biota that lived inthese lakes. Figure 2 shows DaVinci crater (crater 55 in theCabrol and Grin catalog), which contains one example of animpact into a possible intracrater paleolake environment. Chan-nels have breached the rim of DaVinci crater and the interiorappears to contain smooth floor deposits that may be lacustrinesediments.

The ejecta blanket of the 13.4-km diameter crater superposedon DaVinci’s floor could have excavated material from a depth of∼400 m, based on calculations using the equations in Section 2.At depths of 400 m or less, fossilized remains of life are found in

FIG. 2. Impacts into possible paleolake environment. This image showsDaVinci crater, a 107-km diameter crater which Cabrol and Grin (1999) iden-tified as a possible crater paleolake. The 13.4-km diameter crater superposedon the putative lacustrine floor deposits is located at 0.8◦N 39.14◦W. Using the

equations in the text, one can estimate that the ejecta surrounding this craterwere derived primarily from within the upper 405 m (+42 m, −62 m).
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344 COCKELL AN

terrestrial impact structures. The Tswaing impact crater in SouthAfrica (24◦34′S, 28◦04′E) is a simple crater with a diameter of1.1 km. Within the crater is found an 89.95-m paleolacustrine se-quence that was formed by evaporitic processes (Partridge et al.1993). In the top 34 m of the sequence are muds and evapor-ites dominated by halite, followed by 30 m of biological debrisand chemical precipitates dominated by calcium carbonate. Theprimary contributors to the organic carbon deposits in the se-quence are cyanobacteria that are known to produce a variety ofbiomolecules (Wynn-Williams et al. 1999).

Research at the crater (Cockell et al. 2001) resulted in an es-timation of the total biomass of recalcitrant carotenoids and thecyanobacterial UV-screening compound, scytonemin, producedby the intracrater microbial mat population. It is approximately1.45 metric tons in the top 0.25 cm of the southern quadrantof the crater alone, constituting a huge (potentially ∼50,000metric tons in the entire lacustrine sequence) biomolecular recordin the crater. The record covers a period of ∼200,000 years(Partridge et al. 1993). Although these molecules have structuresthat probably make them quite specific to the course of terres-trial evolution, and they come from photosynthetic cyanobac-teria, for which the same criticism could apply, the paleola-custrine sequence in the crater (Partridge et al. 1993) and itsenormous biomolecular record (Cockell et al. 2001) demon-strate that craters like DaVinci on Mars that have superimposedcraters which have impacted into the intracrater lacustrine se-quences are a good method for determining whether any biotaever existed in these types of water-rich environments. Becausethese environments are so conducive to terrestrial microbial life(primarily because of the ponding of water), the lack of any sig-nature provided by the ejecta of an intracrater impact event likethat in DaVinci would be compelling evidence for the lack oflife on Mars.

The Ries impact structure in Germany (54◦24′N, 44◦00′E) isanother example of a crater containing a substantial paleolacus-trine sequence. The crater, which has a diameter of 20–35 km,contains within it a paleolacustrine sequence 400 m deep (Riding1979). Within the surface of this sequence are fossilized remainsof algal tufas formed by the green alga Cladophorites. Althoughpaleontological focus has been given to the macrofauna, therest of the sequence contains other carbonate remains of thebiota trapped within the lake including unicellular components(Riding 1979).

Not all paleolcaustrine sequences from impact structures ofthis size are as thick as those of Ries. The sequence within the23-km diameter Haughton impact structure (75◦22′N, 89◦41′W)is 48 m deep, but contains within it a substantial biological recordof the Miocene Arctic (Hickey et al. 1988).

Other examples of terrestrial craters with examined paleo-lake deposits, some of which are submerged at present, includethe Brent crater, Canada (46◦05′N, 78◦29′W) (Lozej and Beals1975), the Lake Kaali structure, Estonia (58◦24′N, 22◦40′E)(Saarse et al. 1991), the Steinheim crater, Germany (48◦41′N,

10◦04′E) (Schweigert 1993), the Bosumtwi crater, Ghana(6◦30′N, 1◦25′W) (e.g., Hall et al. 1978), and the New Quebec

D BARLOW

crater, Canada (61◦17′N, 73◦40′W) (Richard et al. 1991). Al-though these craters have not had their paleolacustrine sequencesanalyzed for fossil prokaryotes trapped at the time of deposition(which is difficult to do because the sequences are likely to havebeen microbially colonized since deposition), they illustrate thepotential for biological preservation at depths of less than 500 mand often within the near-surface environment that correspondsto the excavation depths of intracrater simple craters on Mars(<270 m).

Craters within craters can provide access to deep pre-impactsedimentary units of exobiological interest. Drill holes at theLappajarvi impact structure in Finland (63◦12′N, 23◦42′E) re-vealed the presence of microfossils at depths between 74 and92 m (Uutela 1990). The organic-walled microfossils, composedprimarily of sphaeromorphs belonging to the acritarch group, areat least 600 Ma old and could be as old as 1.2 Ga. Thus, althoughthe fossils are pre-impact, the work at Lappajarvi shows howimpact structures on Earth have provided easier access to sub-surface fossiliferous materials. Thus, intracrater impact craterson Mars that excavate to depths <500 m should be viewed asa potential source of subsurface materials in regions with ev-idence for liquid water (and therefore with potentially buriedsedimentary units), regardless of whether the crater itself con-tains a paleolacustrine sequence.

4.1.2. Volcanoes and possible hydrothermal systems. Othernear-surface locations with promise for exobiological studies arethe volcanoes of Mars. Such areas obviously provide a sourceof heat and, if water is present, hydrothermal systems can result.Volcanic hydrothermal systems are likely of longer duration thanthose associated with impact crater formation for all but perhapsthe largest basin-sized impacts. Some locations where water andinternal heat may have operated together can be seen in the formof small channels along the flanks of martian volcanoes, partic-ularly the old highland paterae (Gulick 1998). Figure 3 showssuch an area on Apollinaris Patera. The craters superposed onthe channeled volcano flanks have certainly excavated material,which could be of exobiologic interest (Gulick 2001). Miner-alogic evidence of possible hydrothermal reactions has also beenreported in the Terra Meridiani and Aram Chaos regions, wherethe MGS thermal emission spectrometer has detected large de-posits of crystalline hematite (Christensen et al. 2000a, b). Thepossibility of the Terra Meridiani hematite deposit containingevidence of biotic or prebiotic chemistry has made it a favoredlocation for the landing site of one of the 2003 Mars explorationrovers (Allen et al. 2001, Barlow 2001, Christensen et al. 2001,Gilmore and Tanaka 2001, Hynek et al. 2001, Noreen et al.2001).

Hydrothermal vents in terrestrial volcanic regions reveal awide variety of bacteria, which congregate in these warm, wet lo-cations. These include iron-oxidizing bacteria (Karl et al. 1989)and bacteria using the various oxidation states of the sulfurcycle, similarly to deep-ocean hydrothermal vents (Segerer et al.

1993). Hydrothermal venting within volcanic crater lakes hasbeen shown to provide a habitat for a great diversity of life
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FIG. 3. Impact into possible hydrothermal environment. This 8.8-kmdiameter crater is located on the flank of Apollinaris Patera at 9.25◦S 185.58◦W.Using the equations in the text, one can estimate that the ejecta surrounding thiscrater were derived primarily from within the upper 283 m (+10 m, −43 m).

(Dymond et al. 1989), similarly to surface geothermal regionswhose hydrothermal outflows support luxuriant microbial as-semblages (Nold and Ward 1995).

4.1.3. Layered ejecta blankets. What about the remains ofextant biological activity in permafrost or fossil remains of lifein near-surface cold environments? As noted previously, modelsof the near-surface of Mars suggest that a permanently frozenzone (the cryosphere) is located across Mars at depths greaterthan a meter in the equatorial region and in contact with thesurface at high latitudes.

The layered ejecta morphologies (previously called fluidizedor lobate (Barlow et al. 2000)) of martian craters probably resultfrom impact into subsurface volatiles. Single layer morpholo-gies are proposed to result from impact into ice while multiplelayer morphologies may represent excavation into liquid-richreservoirs (Barlow and Bradley 1990). Double-layer morpholo-gies are theorized to result from impact into layered materi-als with varying volatile concentrations (Mouginis-Mark 1981,Barlow and Bradley 1990)—These morphologies are commonlyfound in areas of lacustrine or fluvial deposition (Barlow et al.1999, Head et al. 1999). An example of a layered ejecta mor-phology can be seen surrounding the crater superposed on the

floor of DaVinci crater (Fig. 2)—the single layer ejecta mor-phology of this crater suggests that the target material is ice-rich

AND MARTIAN LIFE 345

(Barlow and Bradley 1990). Figure 4 shows two examples ofcraters surrounded by a double-layer ejecta morphology inAcidalia Planitia. These craters could represent impact into lay-ered sedimentary materials deposited by the putative OceanusBorealis (Baker et al. 1991). Heat generated during crater forma-tion could support hydrothermal activity in these areas for someperiod of time (Newsom et al. 1996), enhancing the environmentfor putative heat-loving microbes or providing a transient periodof liquid water availability by melting the cryosphere.

Cells can be preserved in permafrost. Work in Siberia sug-gests that viable populations of anaerobic bacteria can be main-tained in freezing permafrost for at least up to 2 myr (Rivkinaet al. 1998) and viable yeast were found in 3 myr-old permafrostsamples (Dmitriev et al. 1997). The survival of these microbeshas been recognized to have exobiological implications (Soinaet al. 1994; Ostroumov 1994). Metabolic activity of microbesin permafrost at temperatures below freezing has been demon-strated. Doubling times of ∼160 days were measured at −20◦C(Rivkina et al. 2000). Based on this work it is apparent that twofactors would influence our view of the martian permafrost asa potential record of extant life, if life ever had evolved on orbeen transferred to Mars. First, the presence of extant life in im-pact ejecta would depend on whether the subsurface permafrostwas conducive to the maintenance of a viable, actively growingpopulation of microbes. Second, in the absence of the appro-priate growth conditions in the present-day subsurface, it woulddepend upon whether a population of microbes ever receivedappropriate conditions for growth (for example, from impact-induced heating in the crater itself) during the past 2 myr, so thatviable members would still exist today in analogy to Siberiansamples (it is plausible that microbes could remain viable for

FIG. 4. Layered ejecta craters. The northern plains of Mars contain manyexamples of craters with double-layered ejecta morphologies, which have beeninterpreted as forming in sediments deposited in fluvial or lacustrine environ-ments. This figure shows a 15.1-km diameter crater located at 45.94◦N 13.66◦W,within the high-latitude mottled plains of Acidalia Planitia. The ejecta blanketsurrounding this crater is calculated to have been excavated from a maximum

depth of 449 m (+54, −69 m).
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even longer than this; this was simply the limit of the abovework).

Near-surface environments could still preserve the remainsof an extinct biota. As well as frozen remains of dead bacteriain permafrost (Soina et al. 1994), filamentous structures fromaqueous, low temperature (<100◦C), subsurface environmentshave been identified (Hofmann and Farmer 2000) and they are ofbiogenic origin. The fossils represent the remains of filamentousmicroorganisms that form mat-like or biofilm structures withfilaments typically 1–2 µm in diameter.

Another exobiological search strategy associated with com-plex craters is to examine the central peak inside the crater.Central peaks typically reach a height close to the rim heightsof martian craters (Melosh 1989). The depth from which thecentral uplift material is derived is unknown but spectroscopicanalysis of the central peaks of the lunar crater Copernicus indi-cate olivine-bearing minerals derived from the lower parts of thelunar crust (Pieters 1982). Hence the uplift brings buried mate-rial closer to the surface, where it may either be exposed or atleast more easily accessible than deep drilling into the substrate.

4.2. The Deep Subsurface

It has previously been recognized that the martian subsur-face might possess chemical conditions that are at least com-patible with our knowledge of the requirements of terrestrialsubsurface life (Boston et al. 1992, Stevens 1997, Fisk andGiovannonia 1999) and some argue that the subsurface of Marsis a potential refuge from impact events (Sleep and Zahnle 1998).We can perform a simple calculation to illustrate this potential.If we assume that martian rocks have a mean porosity of 2%(not dissimilar to terrestrial subsurface rocks) and material witha density of ∼1 g/cm3 filled 1% of this space (which is a reason-able approximation for some terrestrial rock dwelling microbes),then it is apparent that to a depth of 6 km we can hypothesize thatthe martian subsurface has the potential to support 3 × 1013 tonsof biomass provided it can provide energy and carbon for mi-crobial metabolic activity and growth. Using complex craterswith diameters greater than 17.5 km we can prospect the deepsubsurface regions of Mars to test this hypothesis.

Between 500 and 1000 m on Earth (which correspond to re-quired crater diameters between 17.5 and 38 km on Mars toexcavate between these depths), there is a great deal of evi-dence for subsurface biological activity. The microbial biomassat 500 m has been estimated to be equal to ∼10% of the totalsurface biosphere (Parkes et al. 2000). Groundwater that runsthrough subsurface rock environments can contain microbialpopulations with abundances up to 107 cells/mL at 600-m depth.The genotypic study of these bacteria suggests that a range ofgenera including iron and sulphate reducers is present (Pedersen1997). At depths of up to 900 m the presence of methanogenswas also demonstrated in groundwater and it is hypothesizedthat complex microbial interactions occur in deep groundwater,

including the production of acetate and methane from hydrogenand carbon dioxide by acetogens and methanogens, respectively

D BARLOW

(Kotelnikova and Pedersen 1998). The availability of hydrogenand carbon dioxide is suggested for the deep subsurface of Mars(Fisk and Giovanonnia 1999). Acetate in subsurface regions ofthe Earth may also become available by heating of organic mat-ter and at depths up to at least 600 m; this resource may help tofuel a substantial subsurface biosphere (Wellsbury et al. 1997).Because there is presently no large surface biosphere on Mars,burial of surface organics is irrelevant.

Between 1000 and 5000 m on Earth (which correspond torequired crater diameters between 38 and 260 km on Mars toexcavate between these depths), there is still evidence for vi-able populations of microbes. Anaerobic hyperthemophilic ar-chaea have been isolated at depths between 1799 and 2287 min oil wells where temperatures were between 60 and 84◦C(Miroshnichenk et al. 2001). Measurable microbial biomass wasshown at depths of 1997 and 2096 m in late Cretaceous and Ter-tiary rocks in the U.S. although at considerably lower abundancethan at shallower depths. These rocks had experienced tempera-tures in excess of 120◦C approximately 40 myr ago and so it is as-sumed that the present populations are entrained from shallowerdepths in recent geologic times (Colwell et al. 1997). As well asrock and subsurface oil environments (the latter probably beingless relevant for this discussion about Mars), viable microorgan-isms have been isolated from Antarctic ice at depths of ∼3600 mabove Lake Vostok (Price 2000). The minerals entrained in theice, which are of aeolian origin, have been postulated to becomeentrained into liquid veins in the intergrain boundaries in the iceand they are therefore suggested to provide carbon and energysources for metabolizing microorganisms (Price 2000).

From the point of view of our analysis, the point to emergefrom studies between 1000 and 5000 m is that in deep subsur-face environments on Earth that range from rock to oil to ice,microorganisms have been found. Because of this wide degreeof tolerance or adaptation to different subsurface environments,prospecting the martian subsurface using craters with diametersbetween 38 to 260 km, in almost any location, but particularlynear the equator where the putative zone of subsurface liquidwater might exist, could provide valuable insights into the chem-istry and exobiological potential of the deep subsurface.

At depths between 5000 and 7000 m (corresponding to re-quired crater diameters between 260 and 400 km on Mars, e.g.,Newton Crater, Table I), the presence of microbial populationsdepends upon location. Temperatures in the terrestrial subsur-face generally drop between 20 and 30◦C/km and so at depthsbetween 5 and 10 km temperatures exceed 113◦C, which is cur-rently the known upper limit for life. Viable microbes have beenreported in deep igneous rocks at a depths of 5.3 km (Pedersen2000), although in some locations the only evidence of life seemsto be in the form of biomarkers. At a depth of 6.7 km in the Siljanimpact structure in Sweden (61◦02′N, 14◦52′E) magnetite wasfound entrained in light hydrocarbon oil that may have biogenicorigins (Gold 1991). This magnetite sludge was found to be an

important component of the subsurface granite between 5.5- and6.7-km depth. As discussed earlier, at these depths subsurface
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water might still exist in the martian subsurface because of thelower geothermal heat gradient, but because the nature of thisregion of Mars is still quite speculative we limit our discussionin the potential limit to a martian subsurface biota to the pointsabove relating to Earth.

At all depths in the subsurface other biomarkers may bepresent. Kerogens from deep subsurface basalts and secondaryminerals within bacteria-like objects have also been proposedas signatures of deep subsurface life on Earth (McKinley et al.2000). Hopanoids, derived from bacterial hopanes are also foundin subsurface sedimentary samples at a range of depths. On Earththey are estimated to represent 1013 to 1014 tons of potential sub-surface biomarkers (Ourisson et al. 1984). And so in the absenceof an extant biota, it is clear from subsurface studies on Earth thatif Mars ever possessed a deep subsurface biosphere, the remainsof this biosphere should be present in the excavated material ofimpact craters with the diameters specified in this paper.

5. CONCLUSIONS

We do not know if Mars possesses or ever possessed life, butin order to answer this question successfully we need to use ourknowledge of the biogeographical distribution of microbial lifeon Earth to constrain strategies for examining various potentialhabitats on Mars. The ubiquity of microbial life in the terres-trial subsurface makes this habitat a significant target for theexploration of Mars. In this paper we have provided a quantita-tive examination of the relationship between crater diameters onMars and subsurface excavation depths with a special focus onexobiological questions. Simple craters generally provide accessto depths ≤270 m and complex craters to deeper regions. Be-tween the surface and 500 m there are a range of exobiologicallyinteresting targets, such as paleolacustrine and hydrothermal de-posits, which can be accessed by careful selection of craters inparticular locations. With excavation depths greater than 500 m,most complex craters with diameters greater than 17.5 km areexobiologically interesting because they eject materials that canprovide information on the subsurface environment and can beused to search for the presence of extinct or extant subsurface lifeon Mars. To reach depths of ≥5 km corresponding to deepest re-gions of the terrestrial subsurface biosphere, craters of diameter≥260 km must be used. The central peaks in complex craters canalso provide access to materials uplifted from great depths, mak-ing drilling to these materials logistically easier. Examination ofejecta will provide insights into whether the deep substrate sup-ports or has supported life and whether it is necessary to committo drilling into the martian subsurface.

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