Venus, with its boiling-hot surface, doesn't seem a likely place to find E.T.
But a new paper argues not only that Venusian clouds could harbor microbial life, but also that the life there could potentially hitch a ride aboard the solar wind to Earth.
The possibility for microbial life on Venusian clouds has been suggested before, though it's still not widely thought to be likely.
However, the assertion that this life could potentially float from Venus to Earth is novel, and contentious.
The clouds on Venus are thought to be the planet's best bet for life because the temperatures there are cooler than at the too-hot surface, and water vapor has been detected in the atmosphere.
"The temperature and pressure there are entirely congenial to the survival of certain types of microbes," said researcher Chandra Wickramasinghe of the Cardiff Centre for Astrobiology at Cardiff University in Wales. "Microbes are known to survive in similar environments on Earth."
In particular, bacteria that have been found in extreme conditions in sulfurous hot springs on Earth would also thrive in the Venusian clouds, he said.
Wickramasinghe, writing with co-author Janaki Wickramasinghe in the June 2008 issue of the journal Astrophysics and Space Science, further suggests that these microbes could potentially be transferred from Venus to Earth by the solar wind, the stream of charged particles that is continuously ejected from the sun.
This stream is known to sometimes carry charged particles, called ions, from Venus' upper atmosphere off the planet, though no one has ever suggested it could carry heavier dust particles or microbes.
"We point out that Venus and Earth are very close in terms of proximity," Chandra Wickramasinghe told SPACE.com. "There are occasions where Venus and Earth are aligned, which would be the best possible time for any exchange of material from Venus to Earth."
The last such alignment took place in 2004 and the next will happen in 2012, he said.
But other scientists are skeptical that the solar wind would be able to carry these particles between the two planets.
First of all, the possibly life-harboring clouds are much too low, said David Grinspoon, an astrobiologist at the University of Arizona who was on the science team for the European Space Agency's Venus Express satellite mission.
"It's a much higher altitude where the solar wind is sweeping ions away from the planet," he said. "There's a huge physical disconnect between the clouds and the material that's being swept away by the solar wind."
But Wickramasinghe contends that convection in the atmosphere could be enough to lift these particles to the heights needed to be picked up by the solar wind.
Another issue is whether the dust particles and microbes, if they did manage to be lofted to the upper atmosphere, would be too heavy to be carried away by the solar wind, which has only been observed to lift up light ions.
"Dust particles are orders of magnitude heavier than these ions and there's not enough energy in the solar wind to be knocking them out of the atmosphere," Grinspoon said.
"I'm skeptical that you could really get them up to that height but even if you could you would still need a way to eject them. That's a whole different ballgame," he added. "It's like saying because I've got a window open and there's air leaking out, therefore my bed might somehow fly out the window into my backyard."
Janet Luhmann, a geophysicist at the University of California, Berkeley, calculated that the heavy microbes would need vastly more energy to escape Venus' gravity than the ions do, which is probably beyond the capability of the solar wind to provide.
"While this idea is interesting, it doesn't look too good when the numbers are applied," she said.
Wickramasinghe maintains that the process is reasonable though, if the force of the solar wind combines with pressure from the sun's radiation to pull the particles off Venus.
"I concede that the processes are speculative and need to be discussed more carefully, but the fact remains that if there is microbial life on Venus, being lifted to high enough altitudes through convective motion, then the transfer is a possibility."
Grinspoon countered that while speculations are fine, it's important to back it up with reasonable science.
"It's OK to pursue speculative ideas because we don't want to be too cozy and safe and assume that we know everything about life in the universe," he said.
"However, we have to be rigorous and careful and honest and logical and scientifically meticulous when we speculate. They have not done that" Grinspoon added. "When somebody does this and calls it astrobiology, it risks discrediting the entire field and associating this necessary speculative edge of astrobiology with complete pseudoscience."
Evidence or drawing?
He took special issue with one figure in Wickramasinghe's paper. The caption reads, "Evidence of solar wind excavating the atmosphere of Venus."
"Evidence? There's no evidence there, it's just a drawing!" he said. "I can't believe, if this was peer reviewed, that somebody didn't point this out."
Indeed, David Brain, a University of California, Berkeley scientist on the Venus Express team who released the image, said it was "definitely an illustration."
But Wickramasinghe defended calling the picture evidence.
"The color picture is a representation of data," he said. "I wouldn't call it an artists' depiction. If it was an artist's depiction, it was based on data from the satellite."
While Brain said the image itself wasn't evidence for solar wind excavation of the Venus atmosphere, he did confirm that the Venus Express mission found ample proof that this process is going on.
"[Wickramasinghe's] paper was fun to read and demonstrates thinking outside of the box on some big picture science related to life in the solar system and on Earth," he said. '[The paper's thesis] relies on several assumptions and steps, each of which would need to be individually verified before this idea was broadly accepted."
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