Possible ecosystems and the search for life on Europa

Introduction

Possible ecosystems and the search for life on Europa

Christopher F. Chyba* and Cynthia B. Phillips

Center for the Study of Life in the Universe, SETI Institute, Mountain View, CA 94043; and Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305


No broadly accepted definition of life exists. Most proposed definitions (1-5) face severe objections (3, 6, 7). Nevertheless, one working definition of life has become influential in the origins-of-life community: "life is a self-sustained chemical system capable of undergoing Darwinian evolution" (8). The notion that "the origin of life is the same as the origin of evolution" is a popular corollary. But however valuable this Darwinian definition may be for guiding laboratory experiments, it is unlikely to prove useful to a remote in situ search for life (3, 6). In a search for extraterrestrial life in our solar system, we instead fall back on a less ambitious notion of "life as we know it," meaning life based on a liquid water solvent, a suite of "biogenic" elements (most famously carbon, but others as well), and a source of free energy (7). The availability of these on a given world would suggest life to be possible, so that further exploration may be warranted.

There is now great excitement over Jupiter's moon Europa as a possible location for extraterrestrial biology (9). Here we examine Europa's suitability for life as we know it and consider candidate ecosystems that seem plausible in light of current knowledge. We then sketch life detection experiments that could be conducted with a spacecraft lander.

Source: Proc. Natl. Acad. Sci. USA  vol. 98, n3, pp. 801-804, January 30, 2001


On the Habitability of Europa

The idea of habitability was introduced by Dole (10, 11) to refer to those planetary conditions suitable for human life. The word has since come to imply requirements both less stringent and less anthropocentric, referring instead to the stability of liquid water at a world's surface. A circumstellar habitable zone is the volume of space around a single or multiple-star system within which an Earth-like world could support surface liquid water (12, 13).

The historical emphasis on surface liquid water is easy to understand. First, life on Earth---still our sole example of a biology---utterly depends on liquid water (7, 14). Second, primary production of organic matter is dominated by sunlight-driven photosynthesis at Earth's surface (15). In the traditional view, a planet's mass must be large enough to maintain sufficient geological activity to power the climate-stabilizing carbonate-silicate feedback cycle (16). For surface liquid water to persist longer than approx1 Gyr, a planetary mass greater than approx0.1 Earth masses seems required, by analogy to Mars (12). Similar constraints have been derived for satellites of giant planets (17).

Europa's putative subsurface ocean suggests that the traditional view of planetary habitability should be broadened (7, 11, 18). This suggestion is strengthened by the elucidation of the terrestrial subsurface biosphere (19), the microbial biomass of which appears comparable to Earth's entire surface biomass, although subsurface biological turnover times are long (20). If some terrestrial life exists or could exist independently of surface photosynthesis, then the possibilities for extraterrestrial biospheres greatly expand. If life originated on Mars during its apparent early clement period (21), it is possible that its progeny remain in subsurface hydrothermal niches (22).

A more fundamental question is whether life can originate at depth, independently of the sun. If not, then only worlds that have clement surfaces (Earth) or that once did (Mars) could host endemic biologies, although interplanetary transfer of microorganisms might still introduce life to previously sterile worlds (23). But if the origin of life could occur at depth, then worlds like Europa could host their own biologies. Processes at hydrothermal vents may have been important in Earth's origin of life (24, 25), but it remains unclear whether the entire origin of life could have been independent of sunlight-driven surface conditions and photochemistry.



Liquid Water and Biogenic Elements

A subsurface "ocean" of liquid water on Europa was suggested in the early 1970s (26), and further considered subsequent to the Voyager spacecraft flybys (27). The ground-based spectroscopic signature of Europa is dominated by water ice (28). The paucity of craters on Europa's surface, combined with estimates of the impact flux, suggest a geological resurfacing timescale approx10 million years (29, 30). Galileo spacecraft gravity measurements indicate that Europa has a combined ice/liquid water shell approx80-170 km thick overlying a metallic and rocky core and mantle (31, 32). Models indicate sufficient geothermal and tidal heating to maintain much of the ice shell as liquid water beneath an outer ice layer approx10 km thick (26, 27, 33, 34).

High-resolution images of Europa seem consistent with this picture (35). The orientation and relative age relationships of lineaments is consistent with nonsynchronous rotation of an ice shell decoupled from a synchronously rotating interior by liquid water or ductile ice (36). There are regions of chaotic terrain, where broken pieces of the surface seem to have "rafted" into new positions (35, 37, 38), cracks and extensional bands, which likely were filled in with new, fluid material (39), and cycloidal cracking explicable in terms of changing diurnal stress (40). Such features could have been formed in a thin (approx1 km thick) frozen crustal layer overlaying liquid water (41), but solid-state formation mechanisms also have been suggested. The latter typically involve diapirism within a thick (tens of kilometers thick) ice shell, possibly including bodies of melt or partial melt, overlying a liquid water ocean (35, 42-44).

Perhaps the most compelling evidence for a subsurface liquid water layer on Europa comes from magnetic field results (45) that show the signal of an induced field. This field requires a near-surface global conducting layer, for which the most probable explanation is a salty ocean. All of this evidence, however, remains indirect in nature (46). A definitive answer must await the arrival of the Europa Orbiter spacecraft.

The abundance of most biogenic elements on Europa is not known. It is common to assume Europa's composition to be that of a carbonaceous chondrite meteorite (47), in which case biogenic elements would be abundant. Little is known observationally. Spectral evidence reveals certain organic functional groups (C---H, Ctriple-bondN) on Jupiter's moons Ganymede and Callisto, and hints at their presence on Europa (48). Comet impacts over solar system history should have provided Europa with a supply of biogenic elements irrespective of its initial inventory. If comets have typical densities of 1 g·cm-3, the quantity of biogenic elements accreted by Europa over 4 Gyr is quite substantial (49). However, more material would be lost in impact ejecta if comets are highly porous objects, and cometary porosity is poorly constrained.


Sources of Free Energy

Along with liquid water and suitable chemical elements, life requires a source of free energy. Photosynthesis would be extremely constrained by Europa's ice cover (50). Gaidos et al. (51) argue that because of this, most metabolic pathways operating on Earth would be denied to putative europan organisms. Methanogenesis at hydrothermal vents at the bottom of Europa's approx100 km-deep ocean could supply similar amounts of energy to that which supports ecosystems at terrestrial vents, although the potential annual biomass production would be approx108-109 times below terrestrial primary production based on photosynthesis (52). It is also possible that niches might exist within Europa's ice shell where transient near-surface liquid water environments could permit photosynthesis or other metabolic processes (41, 53).


A Radiation-Driven Ecosystem?

A Radiation-Driven Ecosystem?

Radiation due to charged-particle acceleration in the jovian magnetosphere should simultaneously produce oxidants (54, 55) and simple organics (56, 57) at Europa's surface. Chyba (58, 59) suggested that these molecules, if delivered to the liquid water layer, could provide a source of free energy sufficient to sustain a europan ecosystem.

The radiation also destroys exposed molecules, leading to steady-state concentrations (56, 57). Erosion due to sputtering occurs when charged particles eject material (60, 61). This material can be lost entirely, or redistributed over length scales as long as approx103 km. Sputtering erosion estimates at Europa's surface range from approx0.02-2 µm·yr-1 (60-62). Simultaneously, impact gardening occurs due to small micrometeorites impacting the surface. Gardening is predominantly a vertical mixing process, whereas sputtering's major result is a steady removal of material from the uppermost part of the surface. Gardening is nonlinear, with initial mixing rates at Europa as high as 1.2 µm·yr-1 for a fresh surface (61), and slowing as a regolith develops.

Gardening and sputtering thus compete in the creation, destruction, and preservation of important compounds on Europa's surface. Chyba (58) used an estimate of sputtering at the europan surface (60) of 0.2 µm·yr-1, and a gardening estimate (63), based on a lunar analogy, of 1-10 cm over a mean europan surface age of approx10 Myr (29, 30). Chyba (58, 59) therefore took oxidants and organic molecules to be lost through sputtering before they were gardened down to depths at which they would be protected against further radiation processing or sputtering loss. He took the relevant radiation-processed depth at Europa's surface to be approx1 mm, the stopping depth of incident electrons (56, 57), but the results of Cooper et al. (61) suggest that substantial radiation processing extends to depths >1 cm for a surface age of 10 million years.

However, more recent estimates (61) suggest that the sputtering rate at Europa is more than an order of magnitude lower, approx0.02 µm·yr-1, and that the gardening depth over 107 yr is approx1 m, rather than 1-10 cm. In this case, oxidants and organics created by irradiation of Europa's surface can be efficiently buried by gardening, and therefore protected. Here we re-evaluate the model of Chyba (58, 59) for these new estimates. Our conclusions will in turn need to be reconsidered as our quantitative understanding of impact gardening at Europa further improves.

Fig. 1 shows a preliminary comparison of sputtering vs. gardening rates for Europa's surface. The curved line shows the gardening rate from Cooper et al. (61), derived from estimates of the interplanetary mass flux at Jupiter. The three straight lines show three different sputtering erosion rates, spanning the range of numbers in the literature (60-62). For the sputtering rate 2 µm·yr-1, sputtering dominates over gardening, so material is removed from Europa's surface before it has a chance to be buried and preserved. However, for the current best-estimate 0.02 µm·yr-1 case (61), gardening is the dominant process over Europa's entire surface age, and material is buried faster than most of it can be removed through sputtering. For a mean surface age of approx107 yr (29, 30), gardening should extend to a depth of 1.3 m (61). The radiation products produced over this time scale will be mixed through this layer.

Fig. 1.   Gardening (dotted line) vs. sputtering (2 µm·yr-1, solid line; 0.2 µm·yr-1, long dashes; 0.02 µm·yr-1, short dashes) rates on Europa.

 

Charged-particle interactions with water ice should produce molecular oxygen, hydrogen peroxide, and other oxidants (55-57, 60). Hydrogen peroxide has been detected on Europa at 0.13% by number relative to H2O (54). If this concentration holds through the entire 1.3-m gardening layer, there should be 5.6 × 1021 molecules H2O2 cm-2 (0.13% of 4.3 × 1024 molecules cm-2 H2O available) mixed down to 1.3 m.

This value may be compared with that from a simple production calculation based on radiation flux F, H2O2 G value (molecules produced per 100 eV), and irradiation time. The column density expected is given by n = FGt (56, 57, 61), mixed down to 1.3 m. For H2O2 in an H2O/CO2 ice mixture at 80 K, G(H2O2) approx0.1 (55). The net radiation energy flux at Europa is 7.8 × 1013 eV cm-2·s-1, most of which is due to electrons (61). For t = 107 yr, these values give n = 2.5 × 1025 molecules H2O2 cm-2. This represents approx6 times as much H2O2 produced as there were H2O molecules initially present in the upper 1.3 m. An analogous calculation for O2, using G(O2) = 0.01 (61) implies that approx60% of the water ice is converted to O2. If the upper 1.3 m of ice is all that is available to be radiation processed over 107 yr, production must be substrate-limited. The production quantities of H2O2 and O2 could be orders of magnitude higher than those we find here (61) if the upper meter of Europa's surface was recirculated downward, so that fresh material were regularly being exposed to the surface radiation flux.

Instead, we accept the observed H2O2 abundance and use relative G values to estimate the production of other species. We take CO2 to be present in Europa's ice at 0.2 wt% = 0.08% by number (58). Radiation will drive cycling among CO2, CO, and organics in the ice (56, 57); organic groups may have been observed (48). Scaling from G(H2O2), we use G values for the production of CO from CO2 ice (55) and the production of formaldehyde from H2O/CO ice (64) to estimate HCHO concentrations. G(HCHO) approx1.0 (64) and G(CO) approx 9.0 (69). For 0.08% CO2 in Europa's ice, we find the column density of CO to be N(CO) approx [G(CO)/G(H2O2)]N(H2O2) × 0.08% approx 4 × 1020 molecules CO, or approx10% the abundance of CO2. This in turn gives N(HCHO) approx [G(HCHO)/G(H2O2)]N(H2O2) × (CO/H2O) approx 5 × 1018 molecules HCHO cm-2 mixed through the upper 1.3 m.


Surface-Ocean Exchange

For near-surface creation of oxidants or organics to be relevant to a subsurface ecosystem, exchange with the subsurface water layer must occur. Models of Europa's geology remain contradictory. In the tidal-cracking ridge formation mechanism of Greenberg et al. (39), material could exchange between the ocean and the surface. Formation models for chaotic terrain, which include rafting blocks of crust in liquid water or a slushy matrix (37, 38), also would allow surface-ocean communication. Other models may be less favorable. If chaotic terrain and other disrupted regions of Europa's surface were instead the surface expressions of solid-state diapiric activity (35, 42), it would be important to understand the extent to which this mechanism allows exchange of surface material with the ocean.