an equilibrium between resurfacing and crater production; he later rejected this model.)

But planetary scientists are hardly in agreement; each hypothesis has its supporters and skeptics among the Magellan scientists. Phillips' computer simulations of Venus' evolution, designed to show that a spatially random crater distribution could be preserved despite small scattered resurfacing events, met with mixed reviews. As Schaber says, "The cratering record is the easiest [aspect of Venus' history] to interpret, but, boy, it sure has caused a lot of controversy."

In part, the controversy stems from Venus' relatively small number of craters and the ambiguity that results from counting them. But there is another factor, namely the differing perspectives of the geologists, like Schaber, and geophysicists like Phillips. Two geophysicists who are attempting to unravel the mystery are Suzanne Smrekar of Caltech's Jet Propulsion Laboratory and Robert Grimm of Arizona State University. Officially, they are not members of the Magellan team, and being in their early thirties, they are considerably younger than most of those researchers; they belong to a new generation of planetary scientists. "It's important to realize," Grimm says, "that what we see on the surface isn't the whole story."

As a geophysicist, Grimm finds the global resurfacing model difficult to accept. "Here's Venus going like gangbusters, and suddenly it comes grinding, screeching, choking to a halt," he says, explaining that the current level of geologic activity is only a tiny fraction of what would have been required during the resurfacing event. "That's geophysically curious to me."

Grimm stresses that, while Schaber's global resurfacing hypothesis is the simplest geologically speaking, it challenges geophysicists' notions of how planets behave. From Grimm's point of view, the simplest hypothesis is that Venus—which had seemed so much like a twin sister before the spacecraft reconnaissances—really is like Earth on the inside, with a mantle that is vigorously convecting and causing geologic activity in the overlying lithosphere. But is this borne out by what we know about Venus? Though it is hidden from our view, geophysicists have developed some clever techniques to try to find out.

PROBING THE INTERIOR

Geophysicists like Smrekar and Grimm have relied primarily on seismic data to probe Earth's interior. Variations in the velocity of seismic waves picked up at widely spaced receiving stations reveal the



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