Biology
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p53b..02c&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P53B-02
Biology
[5200] Planetary Sciences: Astrobiology, [6221] Planetary Sciences: Solar System Objects / Europa
Scientific paper
Galileo’s discovery of the jovian moons was a crucial step in the process, completed by Newton, that overthrew the Aristotelian dichotomy between the physics of the terrestrial realm and the physics of the heavens. Now, 400 years later, we know of one kind of biology, Earth biology (DNA-protein life) and have glimpses of other possibilities more closely or distantly related (e.g., the RNA world). The galilean satellite Europa is one of the most likely venues in our solar system for presenting us with another example of life, and life likely from an entirely separate origin. Europa therefore gives us a chance to extend our understanding of biology beyond Earth biology to a more generalized biology, providing a biological counterpart to the galilean/newtonian revolution. This possibility is the reason that Europa is one of the highest priorities in solar system exploration. It is a still entirely speculative but credible possibility, because of Europa’s extraordinary geophysics and chemistry. First, radiogenic decay and tidal energy appear sufficient to maintain a subsurface liquid water ocean on Europa that resides between an ice shell and a rocky mantle. Gravity measurements confirm this differentiation, and magnetometer measurements seem to confirm the liquidity of the ocean. Magnetometer measurements further put strong limits on the thickness of the ice shell overlying the ocean and on the salinity of the ocean itself. Because the ocean is covered by kilometers of ice, the enormous free energy of sunlight is rarely available for chemistry or possible biology, but radiolytic chemistry at the surface ice may provide a powerful oxidizing arrow for the ocean that, coupled with deep hydrothermal activity, maintains a supply of electron acceptor and donor pairs that could be used by life. The details of this scenario depend on surface impact gardening and sputtering rates, and on the interaction of the ice shell with the ocean. Current estimates based on cratering rates suggest an ice shell age 1-2 orders of magnitude less than the age of the solar system; sufficiently frequent melting into the ocean could lead to an oxidized, rather than a reduced ocean. Europa likely formed with a chondritic composition and, in addition, accumulated some material (while losing much in erosion) from impacts over the age of the solar system, so that the biologically essential elements should all be present. It is this combination of liquid water, essential elements, and available energy, all in contact with potentially catalytic mineral surfaces, that makes Europa so biologically attractive and gives its ocean the appearance of habitability, at least for some potential Earth-analog organisms. Of course, habitability for life and prospects for the origin of life are two distinct issues.
Chyba Christopher F.
Hand Kevin Peter
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