Venus, with its boiling-hot surface, doesn't seem a likely
place to find ET. 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.
Skeptics
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.
Too heavy?
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. 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. 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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