University's Marc Parmentier and Paul Hess. In this scenario the absence of an asthenosphere may allow the low-density residuum to collect immediately beneath the lithosphere. Over time this buoyant material would spread out into a layer of fairly even thickness.

Smrekar is studying in particular what would happen if a hot spot formed beneath this residuum layer. Because of this layer, a hot plume of mantle material would not be able to reach the lithosphere, thus reducing the amount of volcanic activity. However, the presence of the residuum layer beneath a relatively thin lithosphere would not inhibit tectonic activity. And the high ADC values would be explained by the fact that buoyant mantle plumes would be stalled at depth. Thus, Smrekar believes, this hypothesis can explain not only the crater statistics but also the gravity signatures. Over the life of an individual hot spot, she notes, its ADC value would gradually decline as the hot mantle plume comes closer to the surface and finally dissipates. Smrekar and three colleagues are studying Venus' hot spots in the hope of determining their stage of evolution.

RETHINKING THE CRATERING RECORD

In the summer of 1992 Roger Phillips and geologists Ray Arvidson and Noam Izenberg went back to the work of studying the Magellan images and, once again, found evidence that the ''clumpiness" of the crater distribution is not simply random chance. Areas that are bright in the radar images have fewer craters than average. The craters that are present have been cut by fractures or are partly covered by lava flows. The prime example is the belt of tectonic and volcanic features that extends through the Aphrodite highlands and eastward toward the highland region called Beta. It is no surprise that craters should be missing here; as Jerry Schaber and others had pointed out earlier, this appears to be a region where the surface has been rifted apart and volcanism has taken place. These regions, Phillips believes, may be some of the youngest surfaces on Venus.

Conversely, as Phillips and his colleagues found, regions that appear radar dark have more craters than average. The most prominent dark areas lie in Venus' northern hemisphere, such as those north of Aphrodite. A closer look showed that the craters in these areas were surrounded by halos of radar-dark ground, each of which extends for many tens of kilometers. The halos are probably dark because they are places where the ground is covered with fine-grained dust, which scatters radar waves poorly. Specialists who study impact craters



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