THE ISOSTATIC STATE OF MEAD CRATER

We have analyzed high-resolution Magellan Doppler tracking data over M ead crater, using both line-of-sight and spherical harmonic methods, a nd have found a negative gravity anomaly of about 4-5 mgal (at spacecr aft altitude, 182 km). This is consistent with no isostatic compensati on of the present topography; the uncertainty in the analysis allows p erhaps as much as 30% compensation at shallow depths (similar to 25 km ). This is similar to observations of large craters on Earth, which ar e not generally compensated, but contrasts with at least some lunar ba sins which are inferred to have large Moho uplifts and corresponding p ositive Bouguer anomalies. An uncompensated load of this size requires a lithosphere with an effective elastic lithosphere thickness greater than 30 km. In order for the crust-mantle boundary not to have partic ipated in the deformation associated with the collapse of the transien t cavity during the creation of the crater, the yield strength near th e top of the mantle must have been significantly higher on Earth and V enus than on the Moon at the time of basin formation. This might be du e to increased strength against frictional sliding at the higher confi ning pressures within the larger planets. Alternatively, the thinner c rusts of Earth and Venus compared to that of the Moon may result in hi gher creep strength of the upper mantle at shallower depths. (C) 1994 Academic Press,Inc.