Prospects For Earth-Based Measurements Of Europa's Librations

Details Prospects For Earth-Based Measurements Of Europa's Librations Prospects For Earth-Based Measurements Of Europa's Librations

Oct 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.2608m&link_type=abstract

American Astronomical Society, DPS meeting #42, #26.08; Bulletin of the American Astronomical Society, Vol. 42, p.1026

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The exploration of Europa is of great interest because it may be hospitable to certain life forms [1]. Several lines of evidence suggest that a subsurface ocean exists beneath an icy shell [2,3], but there is debate about the thickness of the shell [4], which impacts Europa's astrobiological potential. As in the case of Mercury, it may be possible to determine whether an outer shell is decoupled from the interior and to evaluate the shell thickness by measuring the amplitude of forced longitude librations [5,6]. In the simplest configuration of a rigid shell decoupled from a spherically symmetric interior, the libration amplitude is amplified from the nominal value of 18" by C/Cs, where C is the polar moment of inertia of the body and Cs is that of the outer shell that participates in the librations. For a 100-km thick shell, the libration amplitude would reach 200", an estimate that remains valid even in the presence of gravitational coupling between asymmetrical layers [7]. If there are significant departures from rigid behavior, the shell may deform with the ocean underneath and exhibit a libration amplitude of 52" [8]. Europa reaches closest approach in October 2011, offering a once-in-a-decade opportunity to measure spin rate variations by tracking radar speckles, as advocated by Holin [9,10]. Librations of a rigid shell thinner than 100 km would be detectable. We will describe the experimental design and expected sensitivity.
References:
[1] NRC, Europa Science Strategy, 1999.
[2,3] Kivelson et al, Greeley et al, in Jupiter, CUP, 2004.
[4] Greenberg, Unmasking Europa, Praxis, 2008.
[5] Peale, Nature 262, 1976.
[6] Margot et al, Science 316, 2007.
[7] van Hoolst et al, Icarus 195, 2008.
[8] Goldreich and Mitchell, Icarus, in press.
[9] Green, in Radar Astronomy, McGraw-Hill, 1968.
[10] Holin, Radiophys. Quant. Elec. 31, 1988.

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