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K. Aksnes (Institute of Theoretical Astrophysics, University of Oslo, Norway), F. A. Franklin (Center for Astrophysics, Cambridge, MA)
In a recent paper (Astron. J. 122:2734,2001) the authors have used 520 observations of mutual events of the four Galilean satellites between 1908 and 1997 to solve for the main constants in Sampson's theory, including the mean motions ni and their rates dni/dt. Because of strong correlations, it is not possible to obtain meaningful least-squares solutions for both ni and dni/dt, except for Io for which we obtain a mean motion correction dn1 = (0.4±1.1)10-6deg/day and (dn1/dt)/n1 = 3.6±1.0 in units of 10-10yr-1. This dn1 value is comparable to the values determined for dn2, dn3 and dn4. Although the data can be satisfied nearly as well by correcting only the ni's, this leads to a ten times larger dn1 value, which seems unacceptably large. Furthermore, dn1/dt = 0 would require an unphysically large tidal torque from Jupiter to overcome Io's orbital decay because of the observed energy dissipation in Io. Our determination of (dn1/dt)/n1 is in reasonable agreement with the values 3.3±0.5 and 4.54±0.95 obtained by de Sitter (1928) and Goldstein & Jacobs (1995) but conflicts with the value -0.074±0.087 found by Lieske (1987), mainly from timings of ordinary eclipses. Our results imply that Io is now spiraling slowly inward, losing more orbital energy from dissipation than it gains from Jupiter's orbital tidal torque. These results suggest that the specific dissipation function of Jupier, Q, is around 200,000 for an Io heat flux of 1.3x1014W.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.