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 103 km. Sputtering erosion estimates at Europa's surface range from 0.02-2 µm·yr1 (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·yr1 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·yr1, and a gardening estimate (63), based on a lunar analogy, of 1-10 cm over a mean europan surface age of 10 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 1 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, 0.02 µm·yr1, and that the gardening depth over 107 yr is 1 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·yr1, 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·yr1 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 107 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·yr1, solid line; 0.2 µm·yr1, long dashes; 0.02 µm·yr1, 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 cm2 (0.13% of 4.3 × 1024 molecules cm2 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) 0.1 (55). The net radiation energy flux at Europa is 7.8 × 1013 eV cm2·s1, most of which is due to electrons (61). For t = 107 yr, these values give n = 2.5 × 1025 molecules H2O2 cm2. This represents 6 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 60% 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) 1.0 (64) and G(CO) 9.0 (69). For 0.08% CO2 in Europa's ice, we find the column density of CO to be N(CO) [G(CO)/G(H2O2)]N(H2O2) × 0.08% 4 × 1020 molecules CO, or 10% the abundance of CO2. This in turn gives N(HCHO) [G(HCHO)/G(H2O2)]N(H2O2) × (CO/H2O) 5 × 1018 molecules HCHO cm2 mixed through the upper 1.3 m.