Box 2.2 Redox Chemistry and Its Role in Biological Systems

Redox is the chemical process by which a reaction between two chemical species can result in the oxidation of one of the reactants and, simultaneously, the reduction of the other. For example, H2 and O2 are out of redox equilibrium in the modern terrestrial environment, and they can be driven to react chemically by a small input of energy (from, for example, a spark) to form H2O, releasing energy in the process. The hydrogen is said to have become oxidized and the oxygen reduced. The essence of the redox process is the transfer of electrons from the oxidized species to that which is reduced (it is the charge on the electron receptor that is being "reduced"). The energy released in such reactions can be used to drive other chemical reactions. As such, redox chemistry plays an important role in most biological systems.

Any species out of equilibrium can, in principle, react by exchanging electrons until they are in equilibrium. Moreover, in the proper situation, just about any chemical species can act as either an oxidizing (electron acceptor) or reducing (electron donor) agent. Common electron donors include H2, CO, Fe, Mn, CH4, S, NO2, H2S, and NH3. Common electron acceptors include O2, CO2, CO, S, NO2, Mn, and Fe. The diversity of donors and acceptors means that a wide variety of different redox systems can power life.

photosynthesis is thought to have developed later than chemosynthesis, organisms in a europan ocean might have access to sunlight in regions of recent eruption of liquid water to the surface. Some organisms might be able to survive by metabolizing organic molecules that arrive in meteoritic or cometary debris and become entrained into an ocean or are remnants of organic molecules previously produced in situ or of prior living organisms. Still others might utilize H2 and CO 2 in the formation of acetogen.61 Organisms utilizing each of these metabolic strategies are found on Earth and might be able to survive in an oceanic environment on Europa.

Recent calculations have cast doubt on the idea that sufficient energy is available to sustain life in a europan ocean. Researchers have argued that the water in a closed europan ocean would rapidly become chemically reduced due to interactions with hot rocks in hydrothermal systems.62 If this view is correct, a europan ocean would not be an energetically favorable environment for life. This view has, however, been challenged. Additional calculations have indicated that even if the chemical nature of the ocean is reducing, abundant redox chemistry can still take place and, thus, provide an energy source for metabolism. 63 Nevertheless, it is still far from clear whether or not a europan ocean contains sufficient energy to support an origin of life. Moreover, if life did originate at some time, it is unclear if it survived to the present day.

SUMMARY OF OPEN ISSUES REGARDING EUROPA

The major outstanding questions about Europa that remain unanswered at the close of the Galileo Europa Mission are whether Europa has a liquid-water ocean beneath its icy surface and whether there is the potential for the existence of life there. The former is amenable to investigation by spacecraft in the immediate future, whereas the latter will require development of new technological approaches and implementations.

The question of the presence or absence of a europan ocean is central to our understanding of the satellite as a whole. It provides the intellectual underpinning for our understanding of the geology, geophysics, atmosphere, and history of the satellite. Though Europa's gravity field may be known to spherical-harmonic-degree three or better through analysis of Galileo tracking data, knowledge of the properties of this field will not tell us if there is liquid water beneath the surface.

The two most intriguing arguments in favor of a subsurface liquid-water ocean on Europa at present or in the recent geologic past are the geologic evidence for rafting of blocks of ice floating on an underlying fluid and the detection by the Galileo magnetometer of an electromagnetic induction response in Europa that appears to be



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