conductivity of salt water is high (~ 1 S/m), and a wave driven by a changing magnetic field with a period of just over 11 hours would have a penetration skin depth of ~ 100 km. Thus, if the ocean thickness is in the range of tens of kilometers or larger, it would be able to produce an induction response to the varying background field. In this limit, the strength of the induced response depends only on the depth of the conducting layer below the surface.

A second type of response to the field and plasma conditions occurs when Europa is at the center of the plasma sheet. There, the much higher fluxes of energetic particles produce a large escaping flux of sputtered and ionized material; this flux can be greater than 50 kg/s. Europa is similar to a comet in such a situation, and the newly picked-up plasma affects the background magnetic field at distances as great as 8 Europa radii from Europa. In this situation, the magnetic field drapes around the cloud of plasma and its strength increases upstream of Europa and decreases downstream. Measurements from Galileo's E-12 orbit showed that the field indeed increased by more than 400 nT upstream of Europa. The expected induction response was of the order of 40 nT during the E-12 flyby and could not be separated from the very large comet-like response.52

POTENTIAL FOR BIOLOGICAL ACTIVITY

If liquid water exists beneath the surface ice layer on Europa, then one of the environmental requirements for life will have been met. If, in addition, the satellite has provided a source of energy for metabolism and access to the requisite biogenic elements, then it is possible that life may have originated on Europa independently of life on Earth, and even that it may exist now.

On Earth, organisms use either sunlight (via photosynthesis) or chemical reactions (via chemosynthesis) as energy sources for their metabolic processes. However, plausibility arguments based on the phylogenetic tree of all life on Earth suggest that chemosynthesis likely predates photosynthesis.53,54 The chemosynthetic microorganisms that branch most deeply in the tree are autotrophs; they gain energy from inorganic chemical reactions such as reduction of sulfur to hydrogen sulfide or formation of methane (methanogenesis) from carbon dioxide and hydrogen. Because these microorganisms do not require sunlight as a source of energy and carry out reduction reactions involving inorganic compounds, they suggest both the type of life that might thrive beyond Earth and the first kind of organism that might form in an energetic extraterrestrial environment. On Earth, many of these microorganisms are "hyperthermophiles" that require temperatures above 70°C for growth and live in hot springs and hydrothermal systems.55,56 It is not yet known whether high temperatures are a necessary condition for the origin or early evolution of life, but there are many indications that hydrothermal systems are ideally suited for providing geochemical sources of metabolic energy and may be sites of organic synthesis. 57,58

Chemosynthesis is possible on Earth owing to numerous environments that are not in a state of chemical equilibrium. In many of these environments, the chemical interaction of water with rocks, and the movement and circulation of the resulting solutions between regions of differing temperatures, establish disequilibrium states by bringing together compounds that are in different oxidation states. For example, water-rock reactions in rocks containing ferrous silicates (like basalts, peridotites, and other igneous rocks) can lead to a small amount of H2 production from H2O as some of the ferrous iron is oxidized to ferric iron. The H2 can be generated in solutions that contain bicarbonate, leading to a mixture that is thermodynamically unstable and that should react to form methane. However, the inorganic reaction by which methane forms is extremely slow, providing a situation in which methanogenesis by organisms becomes a viable energy-producing metabolic strategy.59 Autotrophic methanogenesis illustrates how chemosynthetic biological systems can be fueled by geochemically generated reduction-oxidation (redox) disequilibria. If there are sources of redox disequilibria on Europa, then energy-producing chemical reactions may occur there and may have the potential to support life (see Box 2.2).

Water-rock interactions on Europa seem plausible given the possible presence of liquid water surrounding an underlying rocky mantle that is likely to contain ferrous silicates near the boundary. In addition, the rocky interior by itself would be nearly comparable in size to the Moon and may have been volcanically active in the past, and may even be so at the present; circulation of water through volcanically heated rock in the form of hydrothermal systems can provide access to energy.60 Finally, a geologically active interior could provide access to the biogenic elements in a form that would allow their utilization in prebiological or biological chemical reactions.

Other sources of energy might also be available on Europa to support metabolism. For example, although



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