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Surface-Ocean Exchange
- Possible ecosystems and the search for life on Europa

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.

For a radius of 1,565 km, Europa's surface area is 3.1 × 1017 cm2. If the upper 1.3 m of Europa's ice is recycled into the ocean in approx107 yr, approx8 × 1013 g HCHO and approx7 × 1017 g H2O2 would enter Europa's ocean every 10 million years. The H2O2 will decompose into H2O via 2H2O2right-arrow 2H2O + O2 with an activation energy of 71 kJ·mol-1 and an upper limit for the Arrhenius preexponential factor of A = 1 × 105·s-1 in the absence of catalysis (65), giving a half life  at 273 K.

A putative microbial ecology on Europa then could be powered by the reaction HCHO + O2right-arrow H2O + CO2. The soil bacterium Hyphomicrobium can live on HCHO as its sole carbon source (66). Taking the dry mass of an aquatic cell to be 2 × 10-14 g (28) of which 50% is carbon (66), if 8 × 1013 g HCHO were incorporated with 100% efficiency in cell biomass, this would correspond to 3 × 1027 cells. If Europa's crust is recycled into the ocean over 107 yr, average cell synthesis would be dn/dt approx 3 × 1020 cells·yr-1. The steady-state biomass n is given by multiplying dn/dt by the biological turnover time tau. Adopting tau approx 1 × 103 yr, appropriate for Earth's deep biosphere (28), n approx 3 × 1023 cells.

A different estimate relies on the total chemical energy available over 107 yr from the reaction HCHO + O2right-arrow H2O + CO2. Terrestrial methanotrophs oxidize CH4 to HCHO, and then on to HCO3-. Oxidation of HCHO by these organisms yields 4.7 eV per molecule (66), giving 7.3 × 1029 eV·yr-1 = 2.8 × 107 kcal·yr-1. We estimate the efficiency, phi, for microbial biomass (dry weight) production by dividing the dry mass that can be produced per mole of ATP, YATP, by the energy required for ATP production, EATP (67). For a variety of microorganisms growing anaerobically or aerobically, YATP approx 10 g·mol-1 (68). Typically, EATP approx 10 kcal·mol-1 (69), giving phi approx 1 g·kcal-1. Were all of the available energy used by microorganisms, this value for phi would give approx1 × 1024 cells. Thus both estimates---one assuming biomass to be carbon-limited, the other energy-limited---yield close to the same result.

A Europan ocean 100 km deep (31, 32, 35) has a volume about twice that of Earth's oceans. Were approx1023-1024 cells distributed evenly throughout Europa's ocean, average cell densities would be about 0.1-1 cell·cm-3. Even if this water reached the surface and froze, such low cell densities would render life detection extremely difficult. For example, for an instrument (perhaps fluorescent HPLC) with a sensitivity of approx105 cells, approx102-103 liters of ice would need to be melted and filtered (or evaporated) to yield sufficient sample for a detection. This requirement could be greatly lessened if organisms were strongly concentrated in nutrient-rich regions near the ice-water interface, as might be expected by analogy to the variable distribution of terrestrial microbes (20, 66). If the microorganisms maintained themselves within the upper 100 m of the ocean, ice derived from this layer could have concentrations approx102-103 cells·cm-3, requiring approx0.1-1 liter of meltwater to be processed.

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