Biology
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p53b..08s&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P53B-08
Biology
[5200] Planetary Sciences: Astrobiology, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [5475] Planetary Sciences: Solid Surface Planets / Tectonics, [6221] Planetary Sciences: Solar System Objects / Europa
Scientific paper
Europa possesses an outer icy shell; this much has been clear since Voyager. That Europa’s shell is also floating is now generally accepted as well, thanks to observations by Galileo. The existence of a low density outer “H2O” layer, 80-170 km in thickness, seems well established. Magnetic induction evidence strongly suggests a conducting near-surface layer and/or interior — a saline ocean. Cycloidal ridges, originating as tidally driven cycloidal fractures, apparently formed in a stress regime dominated by diurnal tides, but could not form in a tidally flexing ice shell grounded to the silicate interior — again supporting decoupling by an ocean. These points are not seriously in contention. Beyond this there is less agreement, especially as to the thickness of the shell overlying the ocean, the icy shell’s composition and rheology, and whether the icy shell more-or-less responds passively to tidal strains and heating from Europa’s interior, or whether it plays a more active role by means of solid state convection. Europa poses many important scientific questions, but in short, how have Europa’s icy shell and ocean and rocky interior evolved through geological time, and most fundamentally, what astrobiological potential do the icy shell and ocean below possess? Impact crater counts indicate that Europa’s surface is youthful, with a nominal age of just 40-90 Myr, based on cometary bombardment models. Thus, if geological evidence suggests an ocean at the time Europa’s surface features formed, the ocean is probably still there today. Improvements in modeling of ice rheology, of convection, and of tidal heating now suggest that a convecting ice shell is compatible with an underlying ocean. Thermal models and geological observations (such as pit, uplift and small chaos diameters and depths or heights) both point to an ice shell ~20 km thick, with observational evidence for both a change in tectonic style and a secular decrease in geological activity over the age of the surface. From a dynamical perspective, it is not implausible that Europa, like Io, is evolving away from a geologically recent state of higher eccentricity and greater tidal dissipation. While total shell thickness is unlikely to vary significantly over local or regional scales, the brittle lithosphere thickness certainly does. And it may simply have been the limited data return from Galileo, in type and quantity, that prevented the discovery of Enceladus-like activity there. We have come a long way from the criticisms of one of Galileo’s contemporaries, who argued that the moons of Jupiter could not even exist. Not only do they exist, but one of these moons, Europa, bears more than a passing resemblance to a smaller but more water-rich Earth. In time, this ocean world should offer a test of one of Science’s greatest questions: whether there was a second, independent origin of life.
Khurana Krishan K.
McKinnon William B.
Pappalardo Robert T.
Singer Kelsi N.
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