BOULDER, Colorado -- 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 SPACE.com, "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 SPACE.com, "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 SPACE.com.
"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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