By Eugenie Samuel Reich
A tortured, volcanic wasteland, baked by a runaway greenhouse effect, the surface of Venus has clearly had an unpleasant history. But just how unpleasant has become the subject of renewed debate among planetary scientists trying to understand the planet's enigmatic topography.
Ever since NASA's Magellan spacecraft radar-mapped Venus twenty years ago, researchers have been struck by the relative sparseness and random distribution of its impact craters. The pattern, completely unlike that found on other terrestrial planets, suggests a surface that is uniformly young. A leading theory advanced to explain this is 'catastrophic resurfacing', the idea that between 300 million and 1 billion years ago, volcanoes covered most of the surface of Venus with molten flows and buried or destroyed any craters that existed then. Since that time--or so the story goes--Venus has been quiescent, its unchanging surface altered only by the random impacts of occasional asteroids.
But new research, some of which is due to be discussed at the 2010 VEXAG International Workshop in Madison, Wisconsin, this week, is refining this picture of Venus. One line of evidence emerged earlier this year, with the publication of thermal emission data from the European Space Agency's Venus Express spacecraft in Science. Those data showed increased infrared radiation coming from several volcanic regions and was interpreted by a NASA-funded team as evidence that the rocks there formed from lava flows within the past million years--recently, on a geological timescale.
Fred Taylor, an atmospheric physicist at the University of Oxford who is part of the Venus Express team, says that the catastrophic resurfacing hypothesis can be adapted to the discovery of active volcanism, as it was never necessary to assume that the planet has been utterly dead since the resurfacing event. But the picture is under direct attack from other quarters. Vicki Hansen, a structural geologist at the University of Minnesota, Duluth, says that Monte Carlo simulations by her former master's student, Emily Bjonnes, show that the destruction of ancient craters could have occurred gradually. Rather than catastrophic resurfacing that would take as little as 10 million years, Venus may have had gradual turnover of a thin crust stirred by mantle plumes or asteroid collisions that renewed the surface 1 percent at a time over billions of years. "The data [are] pointing in that direction," says Hansen, who will present her results next week at the VEXAG conference.
She points to work by Robert Herrick of the University of Alaska Fairbanks, showing that most of Venus's craters show signs of modification from lava flows after their formation, suggesting that gradual processes have been at work. "I think the idea of a short catastrophe is incorrect," agrees Herrick, adding that he leans towards the picture of Venus as a more "normal" planet, with episodes of volcanism wiping out craters over a long time period.
Under a cloud
David Grinspoon, part of the Venus Express team and a curator of astrobiology at the Denver Museum of Nature & Science in Colorado, says that the hypothesis that Venus was catastrophically resurfaced was never meant to represent the entire story of Venus. "I think it's a straw man to say we're overturning this," he says; "the idea has evolved but most scientists would still say that the processes that are occurring now are not those that were occurring in the past."
Herrick says that Venus is understudied compared to Mars, the Moon and even Mercury. It's been harder to map than Mars because of its 25-kilometre-thick cloud layer, while its apparent inhospitability to life has made it less of an attractive research target. Suzanne Smrekar of the Jet Propulsion Laboratory in Pasadena, California, the first author of the Science paper, says that as we begin to find Earth-like planets in other solar systems, some of which may turn out to be similar to Venus, it's becoming urgent to understand why the planet took such a different path from the Earth in its evolution. "Venus is [almost] the same size as Earth, with the same density and composition. Why did the two planets evolve so differently?" she asks.
Taylor says that the high levels of sulphur dioxide in the venusian upper atmosphere are also almost certainly due to recent volcanic eruptions. At the conference this week, he intends to suggest that bands seen by Venus Express in the lowest layer of cloud, about 45 km above the surface, could be volcanic ash, a striking signature that future balloon missions could confirm. "This year there have been long discussions about how important volcanoes are for climate," he says, "I conclude that they make a major contribution." Over the next few billion years, Taylor says, Venus's volcanism will subside, and the planet will begin to lose much of its heavy atmosphere, leading to a lower surface temperature more like that of Earth.
Aside from Venus Express, the Japanese orbiter Akatsuki is expected to arrive at Venus this December. The next generation of missions is still in the proposal stages. It includes the European Venus Explorer (EVE), which would be launched in the 2016-2018 time frame and would include a balloon to float in the atmosphere, a probe that would descend to the surface, and an orbiter; and the Venera-D mission by Russia, to launch in 2016, which could include a lander.