Image: Venus' clouds
This false color image of Venus’ clouds was taken by NASA’s Pioneer Venus Orbiter during its mission circling the cloudy world from 1979 into 1992.
updated 2/21/2006 5:51:39 PM ET 2006-02-21T22:51:39

A special study group has advised NASA that Venus is far too hellish of a world for life to exist on or below the planet’s surface. Furthermore, while the potential for life in the clouds of Venus can’t be ruled out, the expert panel gauged this possibility as extremely low.

The assessment concluded that “no significant risks” exist in contaminating Venus with Earth organisms on any future landers or atmospheric probes, including balloons. Likewise, any surface materials shot back from Venus or whiffs of its atmosphere returned to Earth pose no significant risk to our planet in terms of “back contamination.”

A Washington, D.C.-based arm of the National Academies, the National Research Council’s Space Studies Board, formed a Task Group on Planetary Protection Requirements for Venus Missions under its Committee on the Origin and Evolution of Life.

The six-person study group was chaired by Jack Szostak of the Howard Hughes Medical Institute, Massachusetts General Hospital, with their findings released today in a letter report to NASA that requested the advice.

“In its deliberations, the task group examined planetary protection considerations affecting Venus missions. The known aspects of the present-day environment of Venus offer compelling arguments against there being significant dangers of forward or reverse biological contamination, regardless of the unknowns,” Szostak explained to John Rummel, NASA’s Planetary Protection Officer, in a Feb. 8 cover letter to the task force findings.

Far from friendly
Since the early years of spacecraft exploration, Venus has been repeatedly targeted for up-close inspection—by orbiters, balloons, atmospheric probes, as well as landers.

Venus Express, launched by the European Space Agency (ESA) and now en route, will swing into orbit around the planet in April.

In its report, the task force noted that despite Venus being Earth’s near twin in terms of its mass, radius, and other bulk properties, the planet’s surface is far from friendly and perhaps represents “the most hostile planetary environment ever explored by robotic spacecraft.”

Surface temperatures are hot enough to melt lead. The surface pressure on Venus is the crushing equivalent of being nearly a mile deep in Earth’s ocean. The landscape, covered in volcanic rock, is desolate and waterless, but rich in sulfur. Venus’s atmosphere is more than 96 percent carbon dioxide, with 3 percent nitrogen and traces of other gases.

Enshrouding Venus are three distinct cloud layers. Water vapor there ranges from a few parts per million at the top of the cloud deck to a few tens of parts per million at the base. Cloud droplets, however, are formed of extremely concentrated sulfuric acid. Now toss in for good measure a high flux of solar ultraviolet radiation that floods the cloud deck.

Cloudy life
The task group said that there is general agreement that surface conditions on early Venus were much more Earth-like and far more conducive to life than what is present today. Many of the processes on Venus in its past are thought to be relevant to the origin of life here on Earth.

Plausible theories have been posed, the task force stated, about how life may have occurred on early Venus. As the planet’s environment was overtaken by a runaway greenhouse effect, some scientists suggest, perhaps microbial life that existed fled to more clement temperatures and pressures in the clouds.

In its report, the task force observed that any life remaining in the cloud deck would have had to adapt to conditions that do not overlap the range of conditions inhabited by life on Earth.

“Consequently, considerations of a possible origin of life on Venus are not relevant to considerations of the possibility that life currently exists on the surface of Venus or that living organisms of Earth origin could survive there,” the report states. “The origin of life within the Venus cloud deck must be considered to be highly improbable.”

Hostile chemistry
The task force questioned whether or not the surface of Venus has been volcanically active on a constant basis. It is not clear whether or not clouds have lingered throughout the history of the planet.

“There may well have been periods when Venus was entirely cloud free. If this has occurred, any cloud-based microbial ecology would have been permanently extinguished,” the report explained.

However, the task force added that while it is impossible to completely rule out the possibility that life might exist in the cloud decks of Venus, they consider this possibility to be “extremely low because of the hostile chemical nature of the cloud environment.”

Therefore, any hitchhiking terrestrial organisms having survived the trip from Earth to Venus on a spacecraft plowing through that planet’s atmosphere would be quickly destroyed.

Planetary protocol
But what about a spacecraft headed back to Earth with a cargo of Venus specimens?

“It is not possible to demonstrate conclusively that a spacecraft returning to Earth after collecting samples of Venus’s surface and atmosphere will not come into contact with hypothetical aerial life forms and inadvertently carry them back to Earth; however, this has to be considered an extremely unlikely scenario,” the task force reported. “At any rate, any life forms that had adapted to living in the extremely acidic environment of Venus’s cloud layer would not be able to survive in the environmental conditions found on Earth.”

In light of its findings, the task group advised NASA that future Venus spacecraft don’t need to undergo stringent planetary protection protocol. Only simple documentation is appropriate.

“I am satisfied with the conclusions,” NASA’s John Rummel told, “and hope that Venus science will proceed at a rate that will make another update necessary before another 30 years have passed. Venus Express is a good step in that direction!”

Venus-life connection
While agreeing with the report on certain points, Dirk Schulze-Makuch, an associate professor in the Department of Geology at Washington State University in Pullman, Washington remains open-minded that the clouds of Venus may be a safe harbor for microbial life.

Not a task force member, Schulze-Makuch has extensively looked into the Venus-life connection, and also briefed the task force on his research.

“I agree with the task force that the risk of forward and backward contamination is very low,” Schulze-Makuch told, “because of the very different living conditions in the Venusian clouds compared to basically all known Earth environments.”

However, Schulze-Makuch said he was disappointed that the study group did not suggest any scientific investigation for the explicit purpose of reducing uncertainty with respect to planetary protection issues.

“I wonder if they would come to the same conclusion if we would have confirmation that oceans on the surface of Venus existed for billions of years until fairly recently,” Schulze-Makuch said, and that life originated there and later found a refuge in the Venusian atmosphere. Even as hostile conditions gradually increased within the atmosphere, life may have been able to hang on and could still be present in the cloud layers, he suggested.

“For me this is an entirely plausible scenario,” Schulze-Makuch added.

Rare niches
As the task force explained, there shouldn’t be any significant interaction between putative Venusian cloud microbes and Earth organisms, Schulze-Makuch said. However, there is some uncertainty because most Earth microbes are still unknown and there are some known organisms that come close to living in Venus-like conditions, he suggested.

“We do not know [about] and thus obviously cannot estimate capabilities of any alien organism,” Schulze-Makuch said. “Perhaps, if they originated in an earlier Venus ocean they may have still retained the capability to quickly adapt to their earlier environment. Thus, they might be capable of competing in selected, rare niches on Earth, such as volcanic vents.”

ESA’s Venus Express will not resolve the question of possible Venusian life, Schulze-Makuch said. “But its measurements of water, chemical species, and volcanic activity will shed some light on the viability of our hypothesis of possible microbial life in the Venusian atmosphere.”

The chances of an indigenous microbial community floating around in the Venusian atmosphere, Schulze-Makuch concluded, “are not remote but are significant in my mind!”

Alien to Earth
“I am satisfied with the letter report,” said David Grinspoon, a planetary scientist here at the Southwest Research Institute and a task force member.

“The point isn’t that there is definitely no life on Venus,” Grinspoon told “In our present ignorance, restricted to studying the life of one planet, we don’t know enough about the true biodiversity of the universe to make such a claim.”

But Grinspoon added that if there is life in the clouds of Venus, “then it exists in conditions which do not overlap conditions in which even extremophile terrestrial organisms can survive.” This life, he pointed out, would be so alien to Earth life that it and Earth life could not threaten each other.

ESA’s Venus Express, in Grinspoon’s view, is unlikely to turn up results that change informed views on the life question. “Of course we should always be ready to revisit this if we find something truly surprising about Venus, or about life, that challenges our ideas.”

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