VENUS RECONSIDERED

The Magellan imagery shows that Venus has a crater abundance equivalen t to a surface age of 300 million to 500 million years and a crater di stribution close to random. Hence, the tectonics of Venus must be quie scent compared to those of Earth in the last few 100 million years. Th e main debate is whether the decline in tectonic activity on Venus is closer to monotonic or episodic, with enhanced tectonism and volcanism yet to come. The former hypothesis implies that most radioactive heat sources have been differentiated upward; the latter, that they have r emained at depth. The low level of activity in the last few 100 millio n years inferred from imagery favors the monotonic hypothesis; some ch emical evidence, particularly the low abundance of radiogenic argon, f avors the episodic. A problem for both hypotheses is the rapid decline of thermal and tectonic activity some 300 million to 500 million year s ago. The nature of the convective instabilities that caused the decl ine, and their propagation, are unclear.