Elevations measured by the Mars Orbiter Laser Altimeter have yielded a high-accuracy global map of the topography of Mars. Dominant features include the low northern hemisphere, the Tharsis province, and the Hellas impact basin. The northern hemisphere depression is primarily a long-wavelength effect that has been shaped by an internal mechanism. The topography of Tharsis consists of two broad rises. Material excavated from Hellas contributes to the high elevation of the southern hemisphere and to the scarp along the hemispheric boundary. The present topography has three major drainage centers, with the northern lowlands being the largest. The two polar cap volumes yield an upper limit of the present surface water inventory of 3.2 to 4.7 million cubic kilometers.

Loading of the lithosphere of Mars by the Tharsis rise explains much of the global shape and long-wavelength gravity field of the planet, including a ring of negative gravity anomalies and a topographic trough around Tharsis, as well as gravity anomaly and topographic highs centered in Arabia Terra and extending northward toward Utopia. The Tharsis-induced trough and antipodal high were largely in place by the end of the Noachian Epoch and exerted control on the location and orientation of valley networks. The release of carbon dioxide and water accompanying the emplacement of approximately 3 x 10(8) cubic kilometers of Tharsis magmas may have sustained a warmer climate than at present, enabling the formation of ancient valley networks and fluvial landscape denudation in and adjacent to the large-scale trough.

Abstract. Detailed analysis of the distribution of craters on Venus using Mth nearest neighbor analysis, coupled with models based upon surface morphology constraints, indicates that the hypothesis of complete spatial randomness (CSR) cannot be rejected, but is not a unique model of the observed crater distribution. Based on morphologic mapping, the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0.5 T (the mean surface age of the planet). This four-unit plains model, along with its derivatives, produce test statistics that indicate such models also cannot be rejected. Further, the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis (model corresponds to Venus) is true is lowest for the CSR model. There is no particular reason to pick a CSR model (along with its implications for catastrophic resurfacing) as a constraint on the evolution of Venus, and there are geological reasons to choose the multiage models. We find that we cannot distinguish statistically among models that have two, three, or four distinct production ages within the plains. However, the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons. First, the binomial probability that such a process could exist within each of the plains units is < 0.05 except the smallest and youngest unit, PL 1. Second, using a chi-squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10 '4, indicating confident rejection of the hypothesis. Thus CSR cannot be used as a constraint on models of resurfacing or planetary evolution of Venus because of the nonuniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age. Geological and geophysical constraints provide our best clues for understanding Venus.

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