Venus Once Had Abundant Water, Space Probe Indicates

Venus, the arid and intensely hot “twin planet” of Earth, once was temperate and covered by perhaps 75 feet of water--conditions that may have accommodated life, National Aeronautics and Space Administration scientists said Wednesday.

But researchers at the NASA-Ames Research Center in Mountain View, Calif., stressed they have no evidence that even simple organisms actually did evolve on Venus before a runaway greenhouse effect boiled off the water, leaving the planet almost certainly uninhabitable today.

“All the data we have indicates that Venus started out just like Earth,” said Tom Donahue, the University of Michigan professor of planetary sciences in charge of analyzing data from the venerable Pioneer-Venus satellite.

Bad luck put Venus just slightly too close to the sun, Donahue and his team of scientists concluded. Although Venus is relatively close to Earth--it is the nearest of all planets--it orbits just beyond that area in the solar system in which life is most likely to evolve.

Mars is in the zone, Donahue said, but was too small to maintain a hot core or to generate enough gravity to hold onto its atmosphere. If Venus had Mars’ orbit, he added, some form of life likely would be found there today.

Putting aside speculation about extraterrestrial life, scientists said that evidence of primordial water on Venus--and the earlier discovery of moisture on Mars--can help explain how Earth evolved and where it may be headed.

Despite blistering heat, crushing atmospheric pressure and deadly poisonous air, Venus in particular intrigues scientists because its size and seismically active, volcano-pocked surface seem so similar to Earth’s.

It also has “strong, persistent lightning in the atmosphere,” said Robert Strangeway of UCLA.

Until now, however, abundant water was missing from the comparison.

Researchers estimated how much water existed on Venus billions of years ago by measuring the amount of hydrogen in the atmosphere and the current ratio of hydrogen to deuterium, a slightly heavier isotope.

Pioneer counted hydrogen isotopes very early in its 14-year mission, which ended last fall when it exhausted fuel in the rockets that had prevented it from tumbling into the dense Venusian air and burning up from friction.

Scientists know that electrical forces in the planet’s atmosphere are ejecting both kinds of atoms into space at different rates. By plugging in the newfound ratio of hydrogen and deuterium atoms, they were able to calculate backward and determine that several billion years ago, Venus was covered by perhaps 75 feet or more of water.

“Venus once was a very wet planet,” Donahue said, though even at its peak it probably never had more than 0.4% of the water on Earth.

Liquid water probably disappeared from Venus at least 3 billion years ago--a more precise date will have to wait for further calculations, Donahue said--but it may have been around for 1.5 billion years, long enough to nurture bacteria and other simple organisms.

No one can say if that happened, and Donahue said it would be difficult to see fossils or other evidence of life now. Volcanoes apparently have blanketed Venus with lava, to the point where some scientists say no spot on its surface is more than 400 years old.

“I doubt we’ll ever be able to close the circle and say if the conditions (for life) did exist or did not exist,” Donahue said. “The planet has been completely altered since those days.”

Scientists said they cannot yet be sure if all that water was in liquid form or steam--a vital element in the speculation over the possibility of life--but they suspect it was probably liquid at some point because the sun billions of years ago was not as intense as it today, so Venus was cooler.

When the sun did brighten, it touched off the catastrophic series of events that ended virtually all hope of life on Venus by stripping the planet of its water and then locking up most of its carbon in atmospheric gases.

Solar energy caused water to evaporate faster than it could rain back down, researchers said. Since water vapor is the most efficient greenhouse gas, this began a self-feeding cycle of rapidly rising temperatures that caused water to essentially boil off the upper atmosphere and be lost to space.

The hot, dry atmosphere left behind then began chemically extracting carbon from rocks, scientists said, creating the poisonous carbon dioxide atmosphere now cloaking the planet.

Finally, ultraviolet radiation from the sun began ionizing the atmosphere, stripping electrons off atoms. This created an electrical field that is slowly attracting the few remaining hydrogen isotopes and blasting them into space.