Habitats and taphonomy of Europa

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Icarus 177 (2005) 515–527www.elsevier.com/locate/icaru

Habitats and taphonomy of Europa

Jere H. Lippsa,b,∗, Sarah Rieboldta,b

a Department of Integrative Biology, 3140, 3060 VLSB, University of California, Berkeley, CA 94720, USAb Museum of Paleontology, University of California, Berkeley, CA 94720, USA

Received 17 August 2004; revised 29 March 2005

Available online 13 June 2005

Abstract

Jupiter’s moon Europa possesses an icy shell kilometers thick that may overlie a briny ocean. The inferred presence of watevolcanic energy, and nutrients suggests that Europa is potentially inhabited by some kind of life; indeed Europa is a primary tarsearch for life in the Solar System although no evidence yet exists for any kind of life. The thickness of the icy crust would imposelife, but at least 15 broad kinds of habitats seem possible for Europa. They include several on the sea floor, at least 3 in the water cmany in the ice itself. All of these habitats are in, or could be transported to, the icy shell where they could be exposed by geologicimpacts so they might be explored from the surface or orbit by future planetary missions. Taphonomic processes that transport, prexpose habitats include buoyant ice removing bottom habitats and sediment to the underside of the ice, water currents depositing cof water column habitats on the ice bottom, cryovolcanoes depositing water on the surface, tidal pumping bringing water columhabitats to the near-surface ice, and subice freezing and diapiric action incorporating water column and bottom ice habitats intoparts of the icy shell. The preserved habitats could be exposed at or near the surface of Europa chiefly in newly-formed ice, tiltedice blocks, ridge debris, surface deposits, fault scarps, the sides of domes and pits, and impact craters and ejecta. Future exploratiofor life must consider careful targeting of sites where habitats are most likely preserved or exist close to the surface. 2005 Elsevier Inc. All rights reserved.

Keywords:Europa; Astrobiology; Habitats; Taphonomy; Exploration

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1. Introduction: Europa may or may not harbor life,but we should explore for it anyway

1.1. Life may exist or have existed on Europa because tlife support structure is mostly present

Europa may harbor or have harbored life below and inicy shell because of the probable presence of a briny ocenergy sources, and nutrient supplies(Chyba, 2000; Chybaand Phillips, 2001, 2002; Greenberg, 2002, 2005; Greenet al., 2000, 2002b; Schulze-Makuch and Irwin, 2002). In-deed, for these reasons Europa is a primary target insearch for life in the Solar System(National Research Council, 2003). All of the evidence—the geophysical measu

* Corresponding author.E-mail addresses:[email protected], [email protected]

(J.H. Lipps),[email protected](S. Rieboldt).

0019-1035/$ – see front matter 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.icarus.2005.04.010

,

ments and surface features—indicate a saline ocean ulying an icy shell on Europa(Greeley et al., 2004; Greenbeet al., 1998; Kivelson et al., 2000; Pappalardo et al., 19Rathbun et al., 1998). Underlying the ocean, the moon hasrocky core and mantle(Carr et al., 1998; Castillo et al., 200Kivelson et al., 2000), and overlying it, ice to a thickness etimated at a few to many kilometers(Greeley et al., 2004Greenberg and Geissler, 2002; Greenberg et al., 20Pappalardo and Head, 2001; Turtle and Pierazzo, 20.The ocean itself is estimated to be perhaps 100 km t(Greenberg et al., 2002b)and contains far more water thaEarth’s oceans. Possible energy sources that may sulife are tidal flux, hydrothermal vents and volcanism,diation (Chyba, 2000), chemical disequilibria(Chyba andHand, 2001; Chyba and Phillips, 2001), and, possibly, toa minimal extent, sunlight near the surface(Greeley et al.,2004). In addition to these assumptions of an ocean, eneand nutrients, life would have had to appear on Europa

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http://dx.doi.org/10.1016/j.icarus.2005.04.010

516 J.H. Lipps, S. Rieboldt / Icarus 177 (2005) 515–527

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one or more of these assumptions fail, then life is probanot present on the moon. No data exist, however, to rethese assumptions, nor are they likely to be obtainedsome time; so we accept them in order to facilitate furtresearch and planning for future exploration for life.

Life on Europa would be subject to extreme conditioparticularly of very low temperatures in the icy shell, vehigh radiation at the surface, high pressures at the seaand variable salinity of the oceans(Marion et al., 2003). In-deed the chemistry of the hydrosphere of Europa couldquite inhospitable, including low pH(Kargel et al., 2000).While we recognize these factors as important considtions, life on Earth can exist at high pressures, low temptures, unusual salinities, chemically difficult, and, with ptection, in high-radiation environments(Dalton et al., 2003).Calculations of the salinity, for example, indicate a ranthat life can tolerate on Earth. However, these conditimay or may not attain on Europa.

Although speculation on the kinds of life that mightfound at Europa has ranged from bacteria-like to compmulticellular forms, these ideas may mislead interpretatand exploration goals, for we have no idea what life wobe like even if it were carbon-based. A photosyntheticadriven ecosystem is doubtful(Chyba and Hand, 2001Gaidos et al., 1999), but irradiation of the surface may produce simple to complex hydrocarbons that could supsome kind of life(Chyba, 2000; Chyba and Hand, 200Cooper et al., 2001; Gaidos et al., 1999). Theoretical ecosystems at the bottom of the water column and in the wter might harvest energy by chemosynthetic or ionotropmeans which would support additional trophic levels(Irwinand Schulze-Makuch, 2003). Life on Europa thus remaina possibility, and while we do not qualify statements beabout the presence of life on Europa, we recognize thanature and, indeed, its very past or present existence remuncertain.

1.2. Search strategies should target habitats transportedpreserved in, and exposed in surface ice features

As far as life is concerned, much critical physical achemical evidence is missing and is unlikely to be acquuntil a future mission to Europa. The next mission will likeaim to explore for life at the same time it acquires the dataquired for efficient exploration; such a mission must be cafully prepared. A fundamental aim here is to continueevaluation process to enable detailed planning in the sefor life at Europa by focusing on plausible habitats basepart on ice-associated communities on Earth, on proceby which these habitats and their included organismsbecome incorporated into the icy shell, and on exposurethe surface by geologic or impact processes. The visibleologic and assumed biotic heterogeneity of Europa suggexploration for life should be focused at particular sites,the surface is hardly homogeneous. While more data,ticularly high resolution images and spectral and molec

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analyses, are required, this paper evaluates classes of sthe icy shell that may sequester life or its remains. Sucstrategy may aid the search for life on Europa in three w(1) stimulation of additional focused research on potenastrobiological sites, (2) identification and prioritizationastrobiological targets, and (3) development of instrumfor the detailed characterization and exploration of thesegets.

2. Life lives in habitats

2.1. Life is distributed in habitats because resources areheterogeneously distributed

Life has resource requirements no matter what kinmight be, indicating that it will be distributed into particlar habitats—ecologic settings where specific associationorganisms live because of the availability of those resou(energy, nutrient supplies, substrate, prey organisms, cupatterns, and many more). Models assuming homogenelife on Europa are unrealistic, but they may set constraintinterpretations and hence are useful, even though life istainly heterogeneously distributed. The high probability tlife, if it exists or existed, is or was distributed in habitaprovides the first element of a search strategy and the bfor more research.

The second and third elements are the processes of iporation of these habitats into the icy shell and their expoby tectonic or other activity in geologic outcrops at the sface of Europa. The icy shell has remarkable lineamedeformations, chaotic terrains, and impact craters(Carr etal., 1998; Greeley et al., 2004; Greenberg et al., 1998; 1Pappalardo et al., 1999)indicating a young and dynamic suface, which, based on the paucity of impact craters, isthe order of∼60 myr old or less(Schenk et al., 2004). It isprobably tectonically active now. The origins and historof all of these tectonic features, except impact craters,uncertain and several alternative hypotheses exist forone (Greeley et al., 2004). Nevertheless,Figueredo et al(2003) provided a preliminary assessment of the potenfor detecting biosignatures in broad categories of Eurogeologic features. We follow a similar strategy here but cobine it with an assessment of which potential life habitmight be preserved in particular geologic features as anditional aid to exploration.

2.2. Past and present biosignatures may be preserved ihabitats

Although we focus on potential habitats for life in ordertarget searches, the discovery and definition of those habultimately depends on the detection of biosignatures, therect consequences of life’s past or present activities, of ssort once the habitats are explored. Biosignatures incluwide range of biological products (Table 1): whole or parts

Habitats and taphonomy of Europa 517

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Table 1Possible biosignatures on Europa

Biosignatures Nature Example

Extant life Organisms, whole or partsthereof

Single- or multiple-celled organisms orcolonies

Microbes, small multicellular organisms

Biochemicals Chemicals released by biological processes CO2, CH4, O2, structural biomoleculesExtant or past life Biostructures Structures built by organisms Stromatolites, mats, filaments, layers i

sedimentBiotextures Structures caused by organisms Laminations caused by mat buildersBioturbation Disrupted substrata caused by action of

organismsSediment or ice reworked by past or presorganisms

Fossils: preserved life Fossils, whole or parts of pastor present organisms preservedin rocks or ice

Whole organisms or colonies Microbes, small multicellular organisms

Trace fossils Tracks, trails, predation, and other marks madeby organisms

Holes in ice made by organisms

Biomarkers Biochemicals diagenetically altered andpreserved in geological materials

Steranes on Earth

Isotopes Isotopic values indicative of biological activity Carbon isotopes indicative of photosynt

We use “biosignatures” to indicate any direct consequence of life’s activities, past or present, that could be detected on a planetary body(Figueredo et al.,2003).

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of organisms, biomolecules generated by life, biostructumade by organisms, bioturbation, and biotextures causeorganisms, body fossils, trace fossils, biomarkers, andtopes formed by organisms. For our purposes, fossils incboth extinct and extant organisms or their products presein ice. Except for biochemicals, biomarkers, and isotopall of these biosignatures may be detected by microscopmacroscopic imaging. If microbes are present, they madetected, for example, by microscopic observation of thindividually or by macroscopic imaging of their accumutions, as in mats, layers, or filaments. Likewise, fossils malso be detected at various resolutions. Biomoleculesby organisms, biomarkers changed diagenetically fromoriginal biomolecules, and isotopes may be detectedspectrometers, molecular techniques, or other instrumAs on Earth, fossil biosignatures may not be as numeas extant life due to taphonomic alterations. On the ohand, biosignatures may be preserved in ice more reathan in rocks on Earth, except in ice where radiation or glogic processes may degrade organic materials.

Clearly, an adequate assessment of life on another prequires multiple instruments and techniques. While checal detection is a powerful tool, optical detection of paspresent life has enormous public relations value as weadditional science values. Because public support is essefor outer Solar System exploration, imaging is a powetool that should not be omitted from life detection search

3. Many habitats are possible on Europa

A consideration of potential habitats is essential forderstanding if life exists or ever existed on Europa anddeveloping a search strategy to discover it. Habitatsstudied on Earth by ecologists sometimes without detaknowledge of the organisms contained therein. A similar

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proach to Europa may be equally fruitful, as we haveevidence whatsoever of the kinds of organisms, if any,live or lived on Europa. Potential habitat definition on Eropa may be guided by comparison to Earth’s icy habitWhile these Earth habitats are surely not identical or esimilar to any occupied by life on Europa, they do suggseveral broad classes of habitats that could reasonablcur on Europa (Table 2). Within these classes, more specioccurrences are also possible. The broadest classes inthose on the sea floor, in the water column, associatedice or in the ice crust, and possibly even near the surfacthe ice itself, and each has potential energy resourcesciated with it.

3.1. The sea floor provides many habitats

Benthic habitats likely exist on the floor of Europaocean. Hard substrates are surely present due to extrusvolcanic material on the sea floor and exposure by eroor sediment bypassing by rotationally- or tidally-inducbottom currents. Soft substrates are probable becauseticles would accumulate on the bottom and in depressor basins on the sea floor. The ocean floor is unlikelybe smooth; instead basins and ridges can be expectedrocky interior swells and deflates with distance from igneactivity. Hydrothermal vents and possibly submarine volnoes, also likely present, would cause depth differencethe bottom too. All of these factors would provide differehabitats for life, based on varying depth, substrate, and inactions of water masses and current patterns with the botEven within single biotopes, life on Earth has adaptedmany ways and evolved over long times(Lipps and Hick-man, 1982), and life on Europa may well have done tsame.

On Earth, these habitats under Antarctic ice contain averse assemblage of organisms (Fig. 1) utilizing transported


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Table 2Possible habitats, modes of life, transport mechanisms, and location in surface exposures with an estimate of probability of occurrence (if life is or was present)inferred for Europa

Possible Habitats Mode of life Transport to the surface Surface exposures (occurr

Ice Surface overhangs, caves,ledges, cracks, fissures,cavities

Biofilms, attached, motile,possibly photosynthetic

Present at surface whereshielded by>2 m of ice

Areas protected from radiatioand chemicals. (Unlikely?)

Fissures and cracks belowsurface level in water or ice

Biofilms, attached or motile onsides, drifting or swimming

Accreted to sides by pumpingor tidal action

Ridges and troughs, bandchaos, craters, ejecta. (Probabl

Pores and channels nearsurface in ice

Biofilms, attached, motilebelow surface in water or slush

Migrate to surface. Tectonictilting or rotating of ice

Blocks and matrix in chaostroughs and ridges, edgesbands, craters, ejecta. (Possibl

Pores, channels and cracks onbottom of ice

Biofilms, attached, motile,cryptic

Tectonic tilting or rotation ofice, impact craters

Blocks in chaos, craters, eject(Probable)

Impact craters Biofilms, attached, motile,cryptic

Pores, cracks, water films, inand near craters

Craters and their disturbed ic(Unlikely)

Impact melt water Floating, attached on icesurfaces, motile, biofilms

Tectonic tilting or rotation ofice blocks, impact craters

Blocks in chaos, craters, eject(Possible)

Lakes in the icy shell Floating, benthic or attachedon ice surfaces, motile

Tectonic tilting or rotation ofice blocks, impact craters

Blocks in chaos, craters, eject(Possible)

Water column Water–ice interface Plankton, nekton Accreted to ice, erupted, orflowed

Overturned, tilted blockscraters, flows or eruptions owater or slush. (Probable)

Throughout water column Plankton, nekton Accreted to ice, trapped inrising anchor or other ice,water eruptions or flows,diapiric action

Flows or eruptions of water oslush. (Probable)

In different water layers instratified ocean

Plankton, nekton Trapped in rising ice from theseafloor, water eruptions orflows, diapiric action

Flows or eruptions of water oslush; possible. (Possible)

Near bottom water Plankton, nekton Trapped in anchor or other ice Matrix of chaos, in andridges. (Possible)

Benthos Soft substrata (mud, sand) onbottom of ocean

Epibiotic, motile, burrowing,interstitial, tiered

Trapped in anchor or groundedice, ice gouging

Blocks and matrix in chaos, inand near ridges. (Probable)

Hard substrata (rock, ice, otherorganisms)

Epibiotic, motile, attached,tiered

Trapped in anchor or groundedice, ice gouging

Matrix in chaos, in and nearidges, cryovolcanoes. (Probble)

Hydrothermal vents Epibiotic, motile, attached,tiered, symbiosis

Trapped in anchor or groundedice, ice gouging, or carried inbuoyant water

Matrix in chaos, in and nearidges, cryovolcanoes. (Probble)

Submarine volcanoes Epibiotic, motile, attached,tiered, symbiosis

Trapped in bottom ice, icegouging, or carried in warmbuoyant or erupted water

Matrix in chaos, at ridges, cryovolcanoes. (Possible)

Modified from Lipps et al. (2004).

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resources derived from photosynthetically-driven commuties beyond the ice(De Laca and Lipps, 1976; Lipps et a1977, 1979). Likewise on Europa, a diverse (relative to tobiodiversity on Europa) biota might also be expected in bthe soft and hard substrate habitats, with trophic resoucoming from the ice and water column above, self-generwithin each habitat, or carried by currents from nearby htats, such as vents.

Hydrothermal vent habitats may be present at Europso they would provide their own energy resources(McCol-lom, 1999). Io, the innermost Galilean moon, is volcancally very active due to the tidal stressing by Jupiter, atidal forces stress Europa as well. While no evidence exto support the presence of hydrothermal vents, they squite likely and would be a primary habitat for life. OEarth, hydrothermal vents occur chiefly along the spre

ing centers of the world’s oceans. The biota varies frsite to site but is fueled by chemosynthetic bacteria. Svents have been postulated as possible sites for the oof life (Wächtershäuser, 1988), and the same reasoningenergy and chemical supplies for life’s origins would aply to Europa. Earth’s vent systems are characterizedhigh biomass, abundant organisms, rapid growth, andmetabolism(Lutz and Kennish, 1993). The vent commu-nities include bacteria, various protists, and many unumetazoans. These associations are documented backSilurian (Little et al., 1997)and probably existed for muclonger than that. The extant communities however conof evolutionarily young taxa at least amongst the metazo(Little and Vrijenhoek, 2003). They have clearly evolvethrough time, as might be expected of a biota living in anvironment which changes frequently. On Europa, hydroth


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Fig. 1. Soft benthic habitat below the Ross Ice Shelf, Antarctica, at 62depth. The known biota consists of foraminifera, ostracods, mollusks,phipods, isopods, and fish (shown here). Fish is about 15 cm long.though this biota is more than 500 km from the open ocean, it is depenon transport of photosynthetically-supported organisms under the icehence is unlike Europa(Lipps et al., 1979). On Europa, a benthic assocation could be supported by a food web starting in cracks, channelspores in the icy shell or in the overlying water column or nearby hydrotmal vents. Photo of video monitor by J.H. Lipps, 1979.

mal vents may have operated in similar fashion, producan ever-changing local environment of chemically-rich,water that promotes the growth of cellular chemoautotropThe biota might also be expected to have high biomass, rgrowth, fast metabolism, and evolve significantly over glogic time given the energy and variability possible in vesystems. Vents are a strong candidate target for astrobiocal exploration but would be hard to reach directly on futmissions.

3.2. The water column provides at least three habitats aprobably more

Europa’s water column might provide a variety of habitrelated chiefly to water mass distribution, oceanic structand tidal and current patterns. As on Earth, the waterumn would probably be differentiated at least into habinear the sea floor, near the overlying ice, and in the invening water. If the ocean is stratified as some havegested(Thompson and Delaney, 2001), then more habitatmay exist in each stratum. Habitats near the sea floorfer by having an important resource available: suspenmaterials from the bottom(Cartes et al., 2001). Life mightalso utilize the water adjacent to the lower surface ofice because of organic matter and organisms releasedice-bottom habitats (seeFig. 2) and nutrients manufactureat the ice surface penetrating through ice cracks to theter immediately below. The mid-water habitat might be ratively depauperate compared to other two water coluhabitats because production of organic material mighconsumed or lost before reaching the lower levels, andpended material from the bottom may not reach that h

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Fig. 2. Krill grazing the bottom of Antarctic sea ice for diatoms and otfood items. Individual krill are approximately 3–8 cm long. PhotographNicolas Temnikow, 1975, on the Antarctic Peninsula.

unless thermal plumes occur in the water. These upperter column habitats would differ considerably from Eartphotosynthetically-driven ones, and may contain far feindividuals and species. However, they may be powereleakage of nutrients or organic matter through the ice coor from submarine vents.

3.3. The icy shell may provide numerous habitats

Ice on Earth includes a large variety of habitats (Table 2).These are usually inhabited by abundant photosyntheticmary producers and a host of bacteria, algae, protozoand metazoans(Ackley and Sullivan, 1994; Krebs et a1987; Lipps and Krebs, 1974; Lizotte and Arrigo, 199Thomas and Dieckmann, 2002), a condition unlikely on Eu-ropa. However, other mechanisms for production mighpossible that would allow life to utilize cyrohabitats. Sufaces commonly provide advantages for organisms in Eaice and would likely do so on Europa. On Earth, life ulizes cracks and pores in the lower surfaces of sea ice fogrowth of algae, protection of juveniles and populations,tachment, and even feeding on the organisms within thOn Europa, cracks and fissures that penetrate from thetom of the ice upwards are likely habitats for life(Greenberg,2002; Greenberg et al., 2000), even if they do not penetrate all the way to the surface. The bottom of the ice mialso contain abundant brine channels and pores that cbe occupied. Organisms in these habitats might be fuby nutrients and organic matter leaking down from higplaces in the ice or from the surface, unlike ice communion Earth driven by photosynthesis. Because such leakalikely to be local, these habitats would be spatially disctinuous as well.

3.4. Surface life may be possible in features protected frhigh radiation and chemical fluxes

The very-near surface and surface ice has been dismas a life habitat because of the intense radiation trap


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in Jupiter’s magnetosphere at Europa’s orbit(Baumstark-Khan and Facius, 2002). On the other hand, this radiation of ions, protons, and electrons does not penetratemore than about a meter and a half(Baumstark-Khan andFacius, 2002). Thus, a potential habitat that should be eplored may exist on surface areas and features thaprotected from the radiation flux(Figueredo et al., 2003Greeley et al., 2004), such as cracks, caves, overhangs, hoand other topographically rough areas. If life ever exisanywhere else on Europa, it may well have reachedadapted to these near-surface environments. A microbiota living on the walls, ceilings and floors of these fetures, for example, may not experience high radiation doat all because the radiation would be attenuated or prevefrom reaching the organisms more than a meter and aor so below the surface. Habitats in shallow, transparenhighly reflective ice could even utilize sunlight or the raogenic nutrients generated on the uppermost surface icenergy(Chyba, 2000).

The chemical environment of Europa’s surface may abe inhospitable to life because of the production of sulfuacid and hydrogen peroxide(Carlson et al., 1999a, 1999b.Nevertheless, on a tectonically-mixed and topographicavaried surface like Europa’s, places may well exist thatprotected from such harsh chemicals. Many of these splaces are also likely to be protected from radiation. Evethese habitats are considered highly improbable, they shbe considered in any exploration strategy.

3.5. Other habitats may be present as well

Four other habitats may be possible on Europa for whEarth analogs are poorly known or non-existent: impcraters, subsurface water lakes created by impacts,in the icy shell formed in other ways, and cryovolcanoOn Earth, bacteria rapidly occupy impact areas(Cockelland Lee, 2002; Cockell et al., 2003)and proceed throughprocess of distinctive succession because of the disturbOn Europa, impact craters in the icy crust may providsimilar kind of habitat at least below the immediate surfaEuropa has suffered impacts for its entire history in spitethe fact that the current surface is young. If life adapted omillennia to impact disturbance or if impacts create speconditions that become occupied by a subset of organfrom other places, these craters may constitute a sephabitat. Like other icy habitats, they may utilize radiogeand chemogenic nutrients that percolate through or arepeded by the broken ice.

Subice lakes may form under craters as the impactergy melts subsurface ice(Turtle and Pierazzo, 2001). Theselakes may exist for thousands of years after formation. Tcould become populated by organisms that were either lethe ice as cysts, spores, or other vegetative stages, or thgrated there after impact through cracks to the ocean bor from the surface. As the lakes eventually froze, organi

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and their habitat would be preserved in the ice, which in tmay be tectonically emplaced at the surface.

A third possible habitat is subice lakes formed by trapwater or ice melted by diapiric warmer ice or water. Eaanalogues are the 100 or so subice cap lakes in Antarthat remain largely unexplored(Price et al., 2002; Siegert eal., 2001). One of these, Lake Vostok, is huge and may ctain viable microorganisms(Karl et al., 1999). Presumablysuch lakes in Europa’s icy shell might also freeze, presing fossil biosignatures. The liquid or frozen habitats cobe transported by geologic processes to the surfacelater time. If Europa has no lakes and only a single laocean, then the lake model will be imprecise for a numof reasons, including lack of connectivity with other wabodies, long-term isolation from the ocean, and a problimited biota not representative of the planetary biota awhole.

Cryovolcanoes could also create conditions sufficiedifferent to allow separate habitats to exist, such as difences in temperature, density, salinity, currents, and tracements, and liquid water in vesicles, pores, and cracks.on Earth responds to these kinds of differences and Eurlife could be expected to do likewise. Both benthic and wter column habitats might exist in the vicinity of long-lasticryovolcanism.

3.6. Habitats may vary depending on the thickness of thicy crust

Two chief models for Europa’s icy shell were proposthat have consequences for habitat modeling—a thin crua few kilometers(Greenberg, 2005; Greenberg et al., 19and one∼20–30 km thick(Giese et al., 2001; Greeley et a2004; Hussmann et al., 2002). The icy shell may also vary inthickness from place to place(Turtle and Pierazzo, 2001)orfrom time to time(Figueredo and Greeley, 2004). The thick-ness of the icy shell has different consequences for lifelife’s habitats, and for the preservation and exposure of bin the surface ice. If the icy crust is thin, thick, or variable,the habitats described above may still exist but vary partlarly in abundance and productivity. A thin shell would allobetter communication of the surface to the ocean below,enhancing nutrient supply and possibly even photosynthIn the thick-shell model, the distribution of nutrients or oganic matter produced at the surface to the ice or watebelow may well restricted. Indeed, a thick shell has beenferred to inhibit life, and while life might differ with a thickshell that isolated the ocean below, it is still a possibilFor example, thick-shelled models invoke diapiric actionproduce some surface features(Pappalardo and Barr, 2004,and these may create physical avenues for transporting nents to the ocean below, thus supporting life associatedparticular places at the bottom of the ice, in the water cumn, and even at the bottom of the ocean. Other sourcnutrients would be available as well, such as those fromdrothermal vents on the sea floor. These nutrients coul


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Fig. 3. A portion of the surface of Astypalaea Linea in the southern hemisphere of Europa showing the relative age relationships of selected ice struures fromoldest (A) to youngest (K). These structures (for geologic definitions, seeFigueredo and Greeley, 2004) include the older ridged plains (A), younger ridgplains (B, C), younger ridges (D, E, F), bands (G) perhaps related to strike slip faulting, younger ridges (H, J), and regional fractures (I, K). The wle areaimaged here has been subtly modified into 25-km folds more recently(Prockter and Pappalardo, 2000). Each of the lettered features may contain biosignatuin outcrops, hence the astrobiological study of these units and their time of formation may someday reveal evolutionary relationships of a biota, if present. Thetime represented from units A to K is probably about 50+ million years, based onFigueredo and Greeley (2004)scenario for resurfacing of the moon. Thimage is centered at latitude−66.94 and longitude 195.79 with a resolution of about 40 m/pixel acquired on orbit E 17 (17ESSTRSLPO1).

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aveite-04;

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-nos tofive

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further dispersed to other parts of Europa’s ocean by theconvection or plumes in the ocean(Melosh et al., 2004). Athick crust thus does not necessarily preclude the presof life, although it might restrict its abundance near the btom of the ice, in the ice or in the water for lack of sufficienutrient supplies.

A spatially or temporally thin crust would allow bettcommunication of the ocean with the surface. While tis not required for life to exist, it does increase probahabitat diversity, especially in the ice and water columnany one time and over time, as well as abundance of orisms within the habitats chiefly because of increased nutflows.

3.7. All habitats may have evolved through time and allmay have discrete biogeographies

Because different parts of the surface of Europa hdifferent ages, as evidenced by relative geologic crria (Figueredo et al., 2003; Figueredo and Greeley, 20Greeley et al., 2000, 2004), the potential to identify habitatof varying ages and evolutionary states is possible if suitsites can be determined in different age classes of ma

als (seeFig. 3). The crust of Europa is probably no oldthan about 60 my, but that would be sufficient time to shevolution at the rates commonly encountered on Earth ecially if that biota was fairly diverse at the time the geologfeatures were formed. Most modern groups of earthly vebrates, invertebrates, and protists have undergone draand obvious changes in far less than half that time. Evtionary rates depend on reproductive strategies and ratewell as environmental changes. We know nothing of the blogical factors involved in Europan evolution, of course,Europa has obviously undergone significant environmechange.

If the resurfacing of Europa in the last∼60 my was a sudden event, it would have significantly impacted a biota,matter where it lived. The resurfacing, however, appearhave been complex with different processes occurring inpostulated stages(Figueredo and Greeley, 2004). These in-cluded a thin crust dominated by tectonic processes formthe ridged plains, two subsequent periods of ridge fortion, followed by a cryovolcanic stage during which chaterrains and lineaments were formed, and a final stagregional fracturing and impact cratering. Because habmay well have been altered significantly, especially d


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ing the early stages with a thin crust, life may havesponded in several ways: mass or local extinctions, biographic shifts, reorganization of communities, and selecfor structures, functions and forms (“species”) among theganisms themselves. Although no way exists to define wthe changes may have been, the resurfacing processemajor geological events indicating a very dynamic cruperhaps even more so than Earth, and life should resmarkedly.

In more local situations, life may also have evolved. Fexample, hydrothermal vents on Earth are unstable locover short enough times to produce selection, and slar changes are likely on Europa. Nutrient supplies dring through the ice from the surface probably vary throutime as well, which would produce evolutionary change otime. Thus, the evolutionary biology of Europa is a posstopic for future study and even a preliminary assessmenit might aid in determining exploration goals. A more coplete study would require sampling multiple sites locatedice of different ages (Fig. 3) and would be far in the future.

On any planetary body as on Earth, habitats can bepected to be distributed differently, depending on resourcurrents, tidal effects, substrate variability, global factorsthe distribution of resources, and others. Most organion Earth, from Archaea and Bacteria(Castenholz, 1996Papke et al., 2003; Whittaker et al., 2003)to multicellularplants and animals are geographically differentiated. Baria living in sea ice at either pole(Staley and Gosink, 1999,for example, are disparate, although some may have uuitous distributions(Finlay, 2002). Benthic habitats existingon the sea floor may differ in their distributions dependon the presence or absence of particular conditions or bain the ocean floor. In particular, vent habitats likely vary,they do on Earth, with distance from one another. Whmany of these habitats could not be detected until ocegraphic exploration of Europa takes place, aspects of hadevelopment and maintenance, such as current patterdelivery of resources from the near-surface, might beferred. In the water column and ice, habitats could also hdifferent biogeographies, and this might complicate or eour task, depending on the degree of confidence we mattribute to their existence. If, for example, habitats inequatorial regions were judged to be more common, texploration for them might be restricted to the icy crustthose regions.

While diversity, at least on Earth, is not dependent onquantity of nutrients and trophic resources, total biomassabundance are. Habitats on Europa are not likely to hvery plentiful life because total nutrient supply is theorically low (Schulze-Makuch and Irwin, 2002). They may,however, contain rather diverse forms of life in low numbeas occurs on Earth in oligotrophic environments. Habiwith abundant and diverse organisms like those observeEarth may not exist except in special situations like ventlocal seepage through the icy shell.

re

s

tr

4. Habitat taphonomy and exposure in surface ice

4.1. Taphonomy

Habitats and live organisms are commonly preservegeologic materials and then transported to and exposea planetary surface. The study of these processes is ctaphonomy. It is a critical field of study for astrobiologsince planetary records (at least in the case of Earthlikely all life-hosting bodies) contain a long and commonabundant record of life in their geologic records. On bodlike Europa, it is also critical in targeting further investigtions, even for extant life in or below the surficial ice. Sapling the subsurface ice or ocean will be costly and of limiexploration value; however, sampling surface sites wheresubshell or within-shell habitats may be exposed providreasonable search strategy(Figueredo et al., 2003)with moreexploration and instrumental alternatives.

Taphonomy involves several processes: death, prestion, transportation and emplacement, and exposure inological outcrops (Tables 2, 3). On icy bodies, organismand their biosignatures frozen in ice may not degrade,cept through tectonic and chemical activity (diagenesis)radiation in the uppermost surface. Once frozen in ice, tmay be exposed at the surface or near surface by geoprocesses. Because the ice on Europa is clearly of diffeages and much of it has undergone tectonic displacemimpacts, and perhaps outflows of water, exposures of dient sorts are common(Figueredo et al., 2003; Figueredo aGreeley, 2004; Greenberg et al., 2000, 2004; Greenbeal., 2002a; Prockter et al., 1999).

All of the habitats noted in Section2 have potential tobe transported to and entombed in the icy shell. For exple, ice grounding or ploughing and bottom ice formatiwhich occur at the sea floor in Earth’s polar regions (Fig. 4),may transport bottom materials to surface ice(Debenham,1919; Ishman and Webb, 2003; Kellogg and Kellogg, 19Reimnitz et al., 1993). These processes are possible onropa as well, depending on density and thickness of pof the icy shell and ocean and the temperature of the bowaters. If ice forms on the ocean floor, pieces of it may brloose, float to, and become incorporated in the bottom oice shell. However, in an ocean 100 km thick, transporsea floor material may be rare. Other transport mechanare more likely to move habitats to the icy shell. Cryovolcism ejecting subsurface water or icy slush onto the surfacto the bottom of the ice, tidal pumping of water or slushthrough cracks, emplacement of habitats by currents oron the bottom of the ice, as well as habitats in the ice itsmay embed organisms in the icy shell of Europa. Indeedspectrum of some of the debris adjacent to ridges, newimpact craters, and chaos terrains resembles that of cebacteria known from Earth(Dalton et al., 2003). While thatclaim is not made with surety, it poses exploration opportuties even from orbit. Most of the other emplacements worequire exposure in the ice by geologic events (Figs. 5–8).


rs,

s in

Table 3Geologic features and exposures on the surface of Europa that might contain biosignatures (seeTable 1)

Surface feature Exposures (occurrence) Possible biosignatures

Surface ice Caves, holes, overhangs, cracks in ice thicker than∼2 m.(Possibly common)

Organisms, biochemicals possible, fossils, biomarkeisotopes if frozen into ice

Colored areas adjacent toridges, in craters, at chaos

Surface deposit related to endogenic processes perhaps inthe ocean. Mg. (Common to abundant)

Fossils, organisms, biochemicals, biomarkers,isotopes, sediment?

New smooth ice Edges, surface. (Common) Fossils. traces, biomarkers, isotopesCryovolcanoes New erupted water or ice. (Uncommon) Extant organisms, fossils, biomarkers, isotopesNormal faults Scarps on uplifted side. (Uncommon) Fossils, traces, biotextures, biomarkers, isotope

fresh exposuresLateral slip faults Offset scarps, ridges, cracks. (Uncommon)Large ice blocks in chaos Edges, tilted edges, overturned ice, bottoms. (Common)Matrix in chaos Ice pieces, frozen water or slush. (Common)Domes Edges and sides of domes, broken ice on top. (Common)Pits Walls, floor. (Common)Ridges Debris in bilateral ridges, central crack. Exposed ends of

ridges. (Abundant)Other cracks and fractures Walls. (Abundant)Impact craters Walls, central peak, floor, overturned edges, ejecta. (Sparse)Impact melt water Melt water and frozen melt water below craters or tilted

or rotated blocks, edges and walls of other structures. (Un-known but theoretically possible)

Live organisms, biochemicals, fossils, biotextures,biostructures, trace fossils, biomarkers, isotopes

hownon-

ce on

sub-a,d de

d in

rted

on-

ice-,ice

Fig. 4. Sediment and fossils (a perfectly preserved large sponge is shere between the scientists) on the McMurdo Ice Shelf, Antarctica, demstrates transportation of bottom habitats and organisms to surface iEarth. These were trapped by anchor or grounded ice on the seafloor∼4 kyr,transported to the bottom of the ice, and then carried to the surface bylimation (Ishman and Webb, 2003). Sediments and fossils (foraminiferostracods, sponges, mollusks) of all sizes occur in these transporteposits. J.H. Lipps image by Ted Delaca, 1979.

4.2. Transported and preserved habitats can be exposegeological outcrops

Tectonic activity and impacts may expose transpohabitats and organisms at or near the surface (Table 3).On Europa, the many varied tectonic features may ctain material from any of the habitats in Section3 andTa-ble 2. The double ridges may result from water andpumped to the surface (Fig. 5), and these would likely contain indications of water column and ice(Greenberg et al.2000). The chaotic terrains contain blocks of surface

-

Fig. 5. Ridges, domes, chaos, and colored terrain at latitude 16 and longi-tude 268 on Europa. Geologic features on this image are varied and formpotential targets for astrobiology. The dark material on either side of thetriple ridge has a spectrographic signal nearly identical to known bacte-ria (Dalton et al., 2003). The chaotic material stands above the generaltopography and may have formed by diapiric action of warm ice or wa-ter penetrating to the surface and bringing biosignatures from the lower iceor water column with it. Image covers 80× 95 km at 160 m/pixel, Galileoorbit E6 (NASA/JPL image PIAOO852).


intervenintats

hnotssurean itsor i

Fig. 6. Conamara Chaos region of Europa showing broken, rotated (A), and tilted (B) ice blocks and broken ridges (C). These features and thegmatrix (D) of shattered ice, partially melted ice, or possibly frozen water from the ocean below may provide exposures of ice and water column habi. Imagetaken on Galileo orbit E 6 at 54 m/pixel (NASA/JPL image PIAOO591).

Fig. 7. Geological outcrops of ridged and rough areas in the Conamara Chaos region of Europa imaged at high resolution (9 m/pixel). The ridges and rougareas may harbor living organisms near the surface of the ice in protected cracks, crevices, caves, and cavities where radiation and chemicals canpenetrate.The ridges end in cliffs (A) several hundred meters high on the left side and in the right center of the image where the ridged plateaus terminate at fies. Thecliffs have icy debris and large boulders (B) at their bases. Cliffs and debris may provide samples of ice from depth in the icy crust or from the ocelf.The fractured and cratered (secondary impacts) areas in the lower image (C) also provide an array of outcrops that might yield fossils, biomarkerssotopicsignatures. Image shows an area 1.7× 4 km. Imaged on Galileo orbit E-12 (NASA/JPL photo identification PIA011820).

ex-ex-le

;f

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that have been tilted, rotated, or overturned, so thattensive outcrops of older and lower ice are perhapsposed (Figs. 6 and 7). These are important sites to sampfor astrobiology, as are domes and pits,(Greenberg, 2002Greenberg et al., 1999), which may be formed as diapirs owarmer ice or water from below. Diapiric action would prvide a mechanism for transporting habitats and life fromlower ice or water to places higher in the icy shell. Impcraters, as probes into the interior, offer many complexcrops in their walls, central peaks, and concentric ringswell as in overturned crater edges and ejecta nearby (Fig. 8).They excavate deeply and the central peaks, althoughtured, preserve stratigraphic information, at least on E(Melosh, 1989). The new smooth ice may provide excelent targets too. In addition to the high probability of fining biological materials(Figueredo et al., 2003), they offerless complicated landing opportunities than other topograically high sites.

-

5. Conclusions: Exploration for life on Europa shouldbe targeted on the most likely preserved and exposedhabitats

We conclude that many possible habitats might haveisted or do exist on Europa. If life ever appeared therewould be distributed into these ecologic situations and wolikely be preserved and transported to the icy shell. Ttonic and impact processes expose ice of different parthe shell and of different ages. These geologic featureon Earth, are primary places to search for indicators ofand present life. A reasonable search strategy for lifeEuropa would include orbital and landed studies of swhere habitats have been incorporated in the ice andposed at the surface. Such a strategy ideally would incluvariety of instruments capable of micro- and macroscoimaging, spectrometrically-determined chemical comptions, molecular detection, and life response experime


biocean mays, as well asu

Fig. 8. Impact craters, like Cilix, probe the icy shell and may expose biosignatures in their deposits and exposures created by the impact. Fossils,omarkers,and isotopes, in particular, may be exposed in the walls (A), central peaks (B), and floors (C) of the craters. In some cases, penetration to thebe possible and the crater floor may contain frozen sea water. The dark ejecta (D) and secondary impact craters may also contain biosignaturesthe exposures of the internal parts of preexisting structures like ridges (E). Larger impacts (Tyre Macula) may have caused multi-ring concentricbsidencefractures that expose interior ice in walls. This image of Cilix was taken on Galileo orbit E15 at 110 m/pixel (15ESCILIXSO1).

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Together, these would enable initial characterization ofropa’s life, if any, and capture the public interest necessto support expensive and complex outer solar systemsions.

Acknowledgments

We thank the Space Sciences Laboratory, UniversitCalifornia, Berkeley, and Lockheed Martin Advanced Tenology Center 2002 Mini-Grant Program for support of oinitial work on life on Europa. We also thank LockheMartin Advanced Technology Center and NASA High Cpability Instruments Concepts and Technology programits manager Dr. Curt Niebur for further support of thissearch. The Antarctic research by J.H.L. which providinsight to icy habitats was supported by the NSF Dsion of Polar Programs Grants OPP 74-12139 and DPP21735 to J.H.L. Mr. Joe Pitman, Dr. Greg DeLory, PrImke dePater, Dr. Brad Dalton, Dr. Ted E. DeLaca, aMr. Nicolas Temnikow and Prof. Ron Greeley and the Eropa Focus Group contributed to our work in various win the field, lab and discussions. Profs. Jack FarmerDirk Schulze-Makuch provided very helpful reviews of tmanuscript. We thank all of these people and organtions. This is a UC Museum of Paleantology publicatiNo. 1887.

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Habitats and taphonomy of Europa