Blob tectonics on Venus.

Blob tectonics on Venus

Planetary scientists have debated for more than a decade whether some form of the plate tectonics that shaped Earth also molded the surface of Venus. Now two researchers suggest that some of the most dramatic features on Venus result not from plate tectonics, but from a process they call "blob tectonics."

On Earth, the Hawaiian Islands and some other island chains represent one classic manifestation of plate tectonics. Such island chains apparently formed when a "hot spot" -- a plume of hot rock rising from the planet's interior -- broke through the crustal plate overhead. Over millions of years, towering volcanic peaks arose, which got carried away by the moving plate. As proposed by Robert R. Herrick and Roger J. Phillips of Southern Methodist University in Dallas, blob tectonics would involve gigantic, single "blobs" of hot materials rising like bubbles. These deformed and sometimes punched through a stationary Venusian surface, the Dallas scientists suggest.

Four examples of such blobs may underlie Aphrodite Terra, a hilly region that stretches at least 10,000 kilometers along Venus' equator, Herrick and Phillips say. Beta Regio, one of the first features on the planet identified as volcanic in origin, may kfber another candidate site for blob tectonics.

"A basic tenet of this model is that tectonism and magmatism [the formation of molten material] at Beta Regio and western Aphrodite are dominated by the evolution of plume heads or blobs as they rise to the base of the lithosphere and spread laterally," the authors write in the November GEOPHYSICAL RESEARCH LETTERS. The lithosphere contains the planet's crust and the uppermost mantle.

One clue to the presence of four separate blobs at Aphrodite Terra comes from variations in the planet's gravity, which indicate regions of lower-density materials in the mantle at different depths. The density measurements -- made by tracking the rises and dips in altitude of the orbiting U.S. satellites Pioneer Venus (which reached Venus in 1978) and Magellan (which arrived last August) -- show one less-dense area about 15 km down in a circular area roughly 2,500 km across. A second area lies 70 km down in an elliptical shape some 3,500 km long; a third appears as a 2,500 km circle at a depth of 80 km, and a fourth lies about 100 km below the surface, forming a circular feature 3,000 km across.

If the theory is correct, Magellan's sharp radar images will reveal specific surface features resulting from rising, individual blobs, Phillips says. He and other Magellan scientists are looking for elevated regions cracked by faults in more than one direction and areas that lack meteorite impact craters, as if existing craters have been covered by massive volcanic flows.

Phillips says processing of Magellan's radar images of western Aphrodite Terra should begin later this month at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Magellan will not image Beta Regio until next spring.