Amid Doubters, Making a Case for Lively Venus

By Guy Gugliotta

MOFFETT FIELD, Calif. -- Compared with Mars, Earth's other neighbor is not a very nice place. The surface temperature is 900 degrees Fahrenheit, and lava covers most of the landscape. It's always overcast, but it never rains. The clouds are made of sulfuric acid.

"When they found the sulfur, that was the nail in the coffin," said planetary scientist David Grinspoon. The consensus: "Fire and brimstone," he said. "Venus is Hell."

But Grinspoon, a researcher at the Southwest Research Institute and author of a book called "Lonely Planets," has another idea. At the recently concluded Astrobiology Science Conference 2004 here, he braved an auditorium full of colleagues abuzz over the Mars rovers to deliver a lecture on "The Case for Life on Venus."

Or, as he subtitled it, "Sympathy for the Devil."

"There are other planets besides the Earth and Mars," he told the audience. "I'd like to remind you that studying Venus is vital to understanding life elsewhere. Comfortable notions about Venus are not good things."

Grinspoon is a leading advocate for the thesis that Venus's battery-acid clouds might very well support microbial life -- like the "extremophile" microorganisms that Earth scientists have found thriving near volcano outflows.

"We are fairly sure Venus once had an ocean," said University of Texas at El Paso geobiologist Dirk Schulze-Makuch, a Grinspoon collaborator who studies acidic hot springs. "As the ocean slowly evaporated, the organisms would have had enough time to adapt to the cloud environment."

Or not. "When life can survive, it tends to dominate," said astrobiologist Christopher P. McKay, of NASA's Ames Research Center, sponsor of the astrobiology conference. "When I look out my window, everything's green," he said, but Venus's clouds show no such marker. "Why aren't the clouds green?"

Grinspoon, interviewed by telephone after the conference, acknowledged that McKay had a fair point, but maintained that life on Venus "is just as plausible" as life in Mars's chill, stony badlands. "We shouldn't be so quick to rule out environments that haven't been properly explored," he said.

So far, exploration has not been kind to Venus. Known since ancient times because of its brightness, the planet was named for the Roman goddess of love and beauty, and for much of recorded history enjoyed a reputation as "the jewel of the sky," "the morning star" or "the evening star." (It is currently the bright evening star in the west.)

The onset of modern astronomy only added luster. "There was a long history of people believing there was life on Venus," Grinspoon said. "It was about the same size as Earth. It had clouds. It was commonly believed it was tropical -- wet, hot and steamy." Venus is 67 million miles from the sun and has an almost perfectly circular orbit. A Venusian year lasts 225 days.

The bloom began to fade in the late 1950s when radiation measurements suggested that Venus was considerably hotter than tradition had taught. This view was confirmed in 1962 by Mariner 2, the first Earth spacecraft ever to embark on a voyage of exploration.

And after more than 20 subsequent flybys, probes and orbitings, the news is still all bad. In an image transformation worthy of Dr. Jekyll, Venus in a few short decades fell from its perch as the solar system's darling to become "the example of how a planet couldn't possibly have life," Grinspoon said.

But maybe it can. Scientists generally accept that Venus had large, warm liquid water oceans for at least several hundred million years, and Grinspoon said they could have lasted as long as 2 billion years. "Conditions for the origin of life were present," Grinspoon said. At least for a while.

At some point, however, the oceans heated up and eventually boiled away. "Venus is much closer to the sun, and eventually it got too hot," Schulze-Makuch said. The result was a "runaway greenhouse effect," in which water vapor from the oceans trapped the sun's heat on the Venusian surface, causing the temperature to rise higher and higher as more vapor boiled off.

If microorganisms were present, they would have begun migrating to the clouds at this stage. "Probably the clouds were not so acidic at first," Grinspoon said. "It would have been a gradual transition, the kind life handles best."

Once the ocean had disappeared, Venus's land masses would have ceased moving over the planet's surface, the way continents do on Earth. "For plate tectonics to work, you must have water," Schulze-Makuch said. "It acts like grease."

With the plates jammed, pressure inside the planet began to build, Schulze-Makuch continued, culminating in an apocalyptic outburst of volcanic activity about 700 million years ago. Venus's surface today is covered with craters, lava flows and volcanic mountain escarpments.

"The surface of the planet was repaved, and it's almost all the same age, something you can't say of Earth, Mars or the moon," Grinspoon said. "Because of this, we don't know much about Venus's early environment."

Over time the clouds' acidity has probably increased dramatically because of sulfur dioxide and other sulfur compounds spewing from the volcanoes. The clouds start about 30 miles above the surface and top out at 42 miles. They are wind-driven but unbroken, cloaking the entire planet.

The Venusian atmosphere below the clouds is 90 times as dense as Earth's -- about the same density as seawater on Earth at a half-mile depth -- and composed almost entirely of carbon dioxide, which maintains the greenhouse effect that keeps Venus's surface at furnace temperatures.

Venus does offer a few mitigating features. The cloud temperature drops from 206 degrees Fahrenheit at the base to minus 46 at the top, and the middle is "like room temperature on Earth," Grinspoon said.

The cloud cover is also uniform and stable, unlike Earth's, and dust particles float in Venus's clouds for months, rather than a day, as they do on Earth, plenty of time for microbes to reproduce.

If they are there. "In some of the hot springs [on Earth], microorganisms thrive by oxidizing hydrogen sulfide" in environments not unlike Venusian clouds, Schulze-Makuch said. "On Venus, the microbes would eventually come down and burn up, but would have plenty of time to reproduce."

McKay, however, said Grinspoon and Schulze-Makuch have made the mistake of "basing their speculations on the properties of individual organisms." Instead, he said that "you need a community of organisms -- because individual organisms can't live in isolation."

It is hard to imagine how a community could form aboveground, McKay added. "There's something about the transient nature of clouds. I can't prove it, but I would argue that life needs a pond."