Washington University in St. Louis

New Model Explains Venusian Land Forms

March 5, 1998

Because Earth and Venus have nearly the same size, scientists long have called them planetary twins.

But scientists theorize the processes that form the geological features of the planets are different. Earth forms its continents and physical features and sheds its interior heat by plate tectonics. Though Venus might be expected to do the same, its surface shows scant evidence for plate tectonics, and planetary researchers long have debated, often heatedly, just what the corresponding process is on our sister planet.

Now a new model of Venus, derived largely from the highly successful Magellan Mission early in this decade, shows that two of the planet's most predominant features, crustal plateaus and volcanic rises, were formed by a mechanism similar to hot spot plumes, a process still active on Earth today and evident in the Hawaiian Islands. Hotspots are thermal plumes of hot rock originating deep within the Earth and rising buoyantly upward over millions of years. They eventually surface in dramatic, lava-spewing displays geologists call flood basalts.

The new interpretation comes from the mapping of very subtle geological faults on the surfaces of the crustal plateaus, and was published March 6, 1998, in Science magazine.

Roger J. Phillips, Ph.D., professor of earth and planetary sciences at Washington University in St. Louis, and his colleague Vicki L. Hansen, Ph.D., professor of geological sciences at Southern Methodist University in Dallas, analyzed recent data and hypothesize that a thickening of the Venusian lithosphere, the outer strong shell of a rocky planet, approximately one billion years ago largely shut down the creation of crustal plateaus and led to the formation of volcanic rises instead.

Phillips and Hansen suggest that the thickening occurred rapidly in geological time, in 100 to 200 million years. The thickening prevented the plumes from melting substantially and creating new crustal material to form crustal plateaus. Volcanic rises form when there is not massive melting in the plumes.

Phillips and Hansen estimate the Venusian lithosphere is about 60 miles thick today, compared with about 24 miles thick at the end of the 'thin lid' era, which they suggest lasted up until about one billion years ago.

"To maintain the thin lithosphere you have to have some sort of recycling going on, which on Earth is plate tectonics, but that's not the case with Venus over the past billion years," said Phillips. "Our calculations also show that you get plenty of plains volcanism during the thin lid era, almost an embarrassment of riches, with formation going on almost constantly up to the point where the lithosphere thickens.."

That conclusion clashes with another popular planetary theory that holds that plains formation was an abrupt episode on Venus.

Phillips said the lack of water on Venus no doubt contributed to the formation of the thickened lithosphere.

"The rocks are stronger on Venus probably because of a lack of water," he said. " It probably got to the point where the stresses induced by the interior just couldn't break the strong rocks, and the process of lithospheric recycling, which maintained the thin lid, just quit.

"Water is the basic difference between Venus and Earth in this context Water makes the lithosphere of Earth relatively weak, and lack of water makes this structure relatively strong on Venus. Whether you have recycling of the lithosphere into the mantle comes down to a competition between how strong the lithosphere is and how much force from the convecting mantle can be applied to break the lithosphere. This competition basically operates differently on the two planets."

Because the Phillips and Hansen model indicates a recycling process during the thin lid era, it's conceivable that Venus was using plate tectonics during its geological heyday.

"Prior to the thickening of the lithosphere, Venus could have had plate tectonics, as far as we're concerned," Phillips said. "There was some kind of lithospheric recycling. It may have been plate tectonics or something else, but it was there."

The model also couples the climate and interior evolution of Venus, the planet that suffers from the runaway greenhouse effect to give a present-day surface temperature of nearly 900 degrees Fahrenheit. All of the volcanism that went on during the early years of the planet pumped so-called greenhouse gases sulfur dioxide and carbon dioxide in the Venusian atmosphere, so that the surface temperature was even hotter then.

"The gases increased the greenhouse effect, which in turn raised surface temperatures, and that in turn led to more interior melting, which resulted in more greenhouse gases being released," Phillips said. "There is a very tight coupling between climate evolution and interior evolution on Venus over much of its history, and that is something we're just beginning to take seriously on Earth.

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