National Geographic : 1993 Feb
REWORKING THE SURFACE E verywhere on Venus is evidence of the com plex interplay of volca nic, tectonic, and impact processes (left). A large corona is rimmed by fractures and faults, while remnant pieces of jumbled tessera, at right, stand high above the plains. Low-lying areas here in the Lada Terra region of the south ern hemisphere have been filled in with molten rock. Like giant handprints in concrete, two impact craters leave signatures on a lava plain to the right of the corona. Also in Lada Terra, a flow of lava from the Ammavaru Cal dera has broken through a ridge of hills 185 miles to its east (right). Lava flowing through this small breach created a plain the size of Kentucky. The irony is this: Much of the surface of Venus is covered with lava, and tens of thousands of volcanoes have been identified. But planetary scientists have yet to find a "smoking volcano," evidence of current eruptive activity on the surface, in part because Magellan's radar was not designed to detect atmo spheric plumes. When was the last major epi sode of volcanism? Some scien tists say the long process of resurfacing is a constant cycle taking place at random spots around the planet as individual volcanoes rumble into action. But if this is so, then why has lava spilled into just 4 percent of the impact craters? Most experts now think that the crust of Venus may have been flooded during widespread volcanism that ended about 500 million years ago. Such global resurfacing may have occurred when crust became weakened by intense heat and was dragged down into the mantle. What theory will finally pre vail? Gravity studies by the Magellan team may help settle the question. In addition, Amer ican and Russian space scien tists are already huddling in the hope of putting another lander on Venus in a joint mission to try to find out. As usual in scientific endeav ors, new understanding has given rise to new, more intrigu ing questions.