Byline: Jennifer Nagorka Dallas Morning News
Encircled in a thick mantle of clouds, Venus presents a bland, smooth face to light telescopes.
But some spacecraft data and Earth-based observations have hinted that Venus' surface is more bizarre than beautiful.
Magellan, a spacecraft that uses radar to map the planet, is peering through the clouds to study Venus' outer skin. As scientists suspected, Venus' true face is a blistered, pockmarked, wrinkled expanse of rock.
Magellan's finely detailed maps may help scientists finally understand the forces that sculpt the planet.
Most important, researchers examining the Magellan data now declare that Venus is not dead. The scientists see more and more evidence suggesting that active volcanoes dot its landscape, although they haven't caught one erupting yet. The high resolution of Magellan's images also tells scientists the chemical composition of some rock formations, because only certain types of lava can create certain volcanic structures.
Magellan was launched in October 1989 and began mapping Venus in August. Magellan scientists shared information and speculations this month at the fall meeting of the American Geophysical Union in San Francisco.
Many researchers had received new data only a few weeks before the meeting. They prefaced their talks by warning that further analysis might change some of their interpretations. By last week,
Magellan had mapped one- third of Venus. Scientists' reports at the meeting were based on data that covered about 15 percent of the planet.
"We're still fumbling in the dark for an overall, global view of what's going on," said project scientist Stephen Saunders of the Jet Propulsion Laboratory in Pasadena, Calif., which coordinates the mission.
For decades, Mercury, Venus and Mars have been considered planetary siblings. All the so- called terrestrial planets have solid, rocky surfaces. The giant planets such as Jupiter and Saturn consist of increasingly dense layers of gas and liquid encasing a relatively small kernel of solid material.
Venus orbits the sun between Mercury and Earth. In size, Venus is also between Mercury and Earth - more than twice the size of Mercury but 400 miles smaller in diameter than Earth. A day on Venus lasts 243 Earth days. Venus rotates in the opposite direction from Earth, so the sun would appear to rise in the west to someone standing on the planet's surface.
Venus' ultraheavy, carbon dioxide-laden atmosphere screens out much of the sun's radiation.
"Venus ... has less energy reaching its surface than Earth does," said Gordon Pettengill of the Massachusetts Institute of Technology.
But the planet shows how well the so-called greenhouse effect works. Its surface temperature broils at about 900 F because its carbon dioxide atmosphere seals in heat.
Scientists have long assumed that the planet's outer skin, like Earth's, is a thin layer of rock wrapped around an iron-nickel core. A thick, churning middle layer, or mantle, of semimolten rock is sandwiched between the core and the skin. The planet's molten interior leaks heat to the crust, which also keeps the planet's surface unpleasantly warm.
No liquid water can exist on Venus. Because of its high surface temperature, rocks remain softer and weaker than the same rocks would be on Earth. Its crust warps easily.
"It is a very alien environment from terrestrial standards," Pettengill said.
Magellan specializes in high- resolution radar mapping. From its elliptical orbit, which dips as close as 155 miles above the planet, the spacecraft can distinguish surface features as large as the length of a football field.
By learning where features exist, scientists can deduce a bit about why they exist. The detail of Magellan's maps also allows scientists to distinguish between similar features, such as volcanic craters and meteorite-impact craters. Layered lava flows, also visible in the images, help scientists understand the sequence of events that shaped the planet's surface.
Venus' surface features appear pristine, as delicate and fine as frost on a window. The lack of water and weather on Venus slows erosion to an infinitesimal pace. "It doesn't erase mountains," said Raymond Arvidson of Washington University in St. Louis. He estimated that chemical and wind erosion may wear down surface features at a rate of a millionth of a meter per year.
Scientists know that two processes - meteorites striking the surface and various styles of volcanism - created most Venusian topography. They have not yet decided whether Venus also experiences Earth-style plate tectonics, in which pieces of crust migrate slowly around the globe. Magellan is mapping an area that should help settle the debate over plate tectonics, said Roger Phillips of Southern Methodist University.
Impact craters on Venus reveal several qualities about the planet. There are few craters smaller than two miles in diameter, Phillips said.
Pettengill of MIT said Venus' dense atmosphere slows and perhaps breaks up the small meteors that would create diminutive craters.
"The object as it comes in has to bore through something about one-tenth the density of sea water," Pettengill said.
By the time the small meteorite finally reaches the surface, it carries little force. The atmosphere doesn't impede larger objects as much, so they hit the planet's surface hard enough to dig craters.
The craters that do pepper the landscape have several puzzling features, scientists said. Unlike impact craters on Mars and Mercury, few of the Venusian craters form neat bull's-eye patterns of a round pit surrounded by circles of ejected material. Instead, the ejected material slops around many craters in a lopsided, butterflylike shape.
Scientists suspect that the atmosphere may help explain the unusual crater form. Big meteorites won't plunge straight through the atmosphere, as they do on Earth, scientists said. Instead, the meteorites lose energy and strike the planet at a shallow angle. It's the difference between dropping a rock straight into a pond, and trying to skip the rock on the water. This "angle of incidence" causes meteors to throw up material asymmetrically, Pettengill said.
Large, dark horseshoe shapes hundreds of miles wide surround a fraction of the craters mapped so far, recent images show. The actual crater pit sits in the same place a nail would be if someone were hanging up a horseshoe.
These dark parabolas "all open to the west," even though the ejected material from some craters is skewed in another direction, said Don Campbell, a professor of astronomy at Cornell University.
Most impact craters on Venus appear as bright and sharp as freshly minted coins. This has indicated to scientists that some process, almost certainly volcanism, periodically erases older craters.
"The surface is relatively young compared to the moon or Mars," said James Head of Brown University, estimating it at less than 800 million years old. The moon's and Mars' surfaces are billions of years old.
So far, researchers have only been able to find one crater "caught in the act of being resurfaced," Pettengill said.
Because volcanic activity creates most of Venus' topography, and because those volcanic structures are not erosion-damaged, Head called Venus "a volcanologist's dream."
Just about any volcanic feature scientists can imagine has been found on Venus - small domes, mountains, collapsed structures called calderas, lava plains, craters, channels, large and small flows.
Among the more curious features are "pancake domes," flat, steep-sided structures. Their shape suggests that they were created by very thick lava "probably ... like toothpaste," Head said. That, in turn, suggests that there are many types of lava, created when molten rock mixes and changes beneath the planet's surface, he said. More fluid lava created long, sinuous channels and broad volcanic plains on the planet, he said.
Scientists don't yet know whether violently eruptive volcanoes, such as Mount St. Helens, exist on Venus, Head said. On Earth, volcanoes erupt explosively when gas escapes from magma and pressure builds beneath a thin layer of crust. Pressure eventually ruptures the rock cap, ejecting ash and steam and sometimes lava.
Venus' dense atmosphere may keep tiny bubbles of gas trapped in molten rock as it moves up to the surface of the planet. When magma finally reaches the surface, it slinks more than it spews. Magmas with high gas concentrations may be able to build up sufficient pressure to cause a violent eruption even in the dense Venusian atmosphere, Head said.
Phillips of SMU said he believes that most Venusian topography - from jagged, steep mountains to broad, low mounds - can be explained by volcanism and subsurface hot spots. Earth has hot spots, created by a rising plume of extremely hot mantle material. The superheated material creates ocean ridges and volcanoes such as those in Hawaii.
A similar phenomenon probably works on Venus, Phillips said. By carefully mapping and comparing surface features on Venus, scientists may be able to locate the planet's hot spots.
With only a small percentage of the planet mapped, all scientists said it was too soon to conclude that they understood how Venus works. Future data may contain surprises, they said, just as the most recent data have.
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