[1] Coronae are roughly circular tectonic features believed to form as a result of smallscale mantle upwellings. We examine the admittance signature for all the 32 Type 2 coronae (defined as having less than 50% complete fracture annuli) that are both well resolved in the gravity data and have fairly well constrained lithospheric parameters. We use top and bottom loading models to derive values of elastic thickness and crustal thickness or apparent depth of compensation. Both Cartesian and spatiospectral methods are used to calculate admittance. Overall we find reasonable agreement between the two methods. The shape of the filter used in the spatiospectral method [Simons et al., 1997] provides a means of isolating the signature of coronae that occur in close proximity to other geologic features. The narrow filters used in the Cartesian approach are useful for identifying transitions between different types of compensation, as well as for estimating error. We obtain a wide range of elastic thicknesses, from 25 to 80 km. Larger values are derived from bottom loading models, which have not been used in most prior studies of Venus. Fifteen of the coronae have elastic thickness values between 0 and 20 km and are probably isostatically compensated. Estimates of crustal and elastic thicknesses and apparent depth of compensation obtained for Type 2 coronae span the range obtained for Venus globally. Neither the thickness of the elastic lithosphere nor the crust appear to control whether Type 1 coronae, Type 2 coronae, or volcanoes form over small-scale mantle upwellings.

[1] Forced librations are periodic rotational rate variations due to gravitational interactions with an orbital partner. We have developed an analytic theory capable of calculating expected amplitudes of forced librations for nonresonant rotators as well as for bodies existing in a spin-orbit resonance. The theory has been applied to 34 solar system bodies, including terrestrial planets, planetary satellites, and the asteroid Eros. Parameters governing libration amplitude are the body's orbital eccentricity, moment difference, and the ratio of its spin rate to its orbital rate. In each case the largest libration amplitude is associated with the forcing frequency 2 ( p À 1) n, where n is the orbital mean motion and p is the spin/orbit rate ratio. This dominant frequency is simply semidiurnal as seen from the position of the torquing body. The maximum libration angular amplitude is 1.3 Â 10 À2 radians for Thebe, and the maximum mean equatorial displacement is 1.4 km for Mimas.

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We demonstrate that, under ideal circumstances, passive optical measurements can yield surface water depth estimates with an accuracy of a few centimeters. Our target area is the Salar de Uyuni, in Bolivia. It is a large, active salt flat or playa, which is maintained as an almost perfectly level and highly reflective surface by annual flooding, to a mean depth of 30-50 cm. We use MISR data to estimate spatial and temporal variations in water depth during the waning portion of the 2001 flooding cycle. We use a single ICESat laser altimetry profile to calibrate our water depth model. Though the salt surface is probably the smoothest surface of its size on Earth, with less that 30 cm RMS height variations over an area of nearly 10 4 km 2 , it is not completely featureless. Topography there includes a peripheral depression, or moat, around the edge of the salt, and several sets of prominent parallel ridges, with 5 km wavelength and 30 cm amplitude. The process by which these features form is still not well characterized.

[1] The spatial and temporal patterns associated with motion of the shadow of Phobos across the surface of Mars are quite different than those associated with solar eclipses on Earth. We present a simple analysis of variations in the position, velocity, size, and shape of the shadow. Simple expressions give reasonably accurate depictions of the shadow motion, which mainly consists of a subdiurnal longitude cycle and an annual latitude cycle. Over most of each year, there are an average of 3.22 shadow transits per day. The duration of the shadow transit depends on latitude. It is maximum at the equator and is then 11.8% of the orbital synodic period. As the subsolar point moves north, the shadow moves south, and vice versa. There is a narrow band, centered on the equator of Mars, within which every point is eclipsed at least once during each semiannual eclipse season. Outside that band, the density of coverage decreases slowly with increasing distance from the equator, until the limiting latitudes are reached. During epochs, like the present, when the obliquity of Mars is in excess of 21.2°, there are portions of each year during which no eclipses occur. As the obliquity increases beyond that transition value, the durations of the eclipse seasons decrease. The minimum possible eclipse season duration, expressed as a fraction of the Mars year, is the same as the maximum shadow transit duration, expressed as a fraction of the Phobos synodic period, since both ratios depend on the same geometry, which is essentially just the radius of the orbit of Phobos, compared to the radius of Mars.