SETI public: Life on Venus and its implications for other worlds

[Larry Klaes <lklaes@bbn.com>]

From: "Bruce Moomaw" <moomaw@jps.net>
To: "Europa Icepick Group" <europa@klx.com>
Subject: Venus as an abode of life? -- important news
Date: Wed, 1 Mar 2000 05:38:06 -0800
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-----Original Message-----
From: Larry Klaes <lklaes@bbn.com>
To: europa@klx.com <europa@klx.com>; Europa Icepick Group <europa@klx.com>
Date: Monday, February 28, 2000 6:34 PM
Subject: Life on Venus? (was Re: To Slavko Orsolic)

>At 08:13 AM 02/26/2000 -0800, Bruce Moomaw wrote:
>
>[SNIP]
>
>>Venus' surface rocks.  (David Grinspoon and several others have recently
>>proposed that  repeated periods of major volcanic activity on Venus --
>>instead of heating the air as you believed -- may dump enough sulfuric
acid
>>haze into the atmosphere to significantly lower its temperature.   But
their
>>view is that actually it is currently in one of its cooler periods, and
that
>>in a few tens of millions of years the temperature will rise again by
about
>>another 100 deg C.  And in any case, they are not talking about anywhere
>>remotely near a reduction in temperature sufficient to allow life to
>>reevolve on Venus.)
>
>Speaking of David Grinspoon, in his 1997 book Venus Revealed,
>he speculates on life forms that could exist in the less harsh
>layers of Venus' otherwise thick and nasty cloudy atmosphere.
>Grinspoon does this in part to help us look beyond our paradigms
>of where extraterrestrial life could live and how it could exist.
>You can read it here:
>
>http://sunra.colorado.edu:80/david/book.html
>
>If life could exist somewhere on present-day Venus, just imagine
>how well that bodes for Europa.
>
>Larry


        Well, Dr. Clark Chapman said in his review of Grinspoon's book in
the July-Aug. 1997 "Planetary Report" that "I cringed as I began reading the
final, 50-page chapter entitled 'Life on Venus'.  But I should not have
worried: it is a wonderful philosophical ramble about life in the universe,
written by a dabbler in biology, with thankfully few references to Venus as
a potential habitat.  After all, Venus is the epitome of the inhospitable
planet, and even were silicon-based organisms to exist, there is little to
be said about them -- not enough to fill even a fraction of 50 pages.
Fortunately, halfway through the chapter Grinspoon moves beyond its title
topic to discuss future robotic exporation (and terraforming) of Venus and
other planets."

And Grinspoon himself said during an interview: "My ruminations about
possible Venusian life are more of a fantasy than a theory".

        HOWEVER:  I've just found the first genuinely serious and
scientifically detailed piece I've ever seen on the possibility of life on
ancient Venus -- and even in its present-day cloud layer.  It's Charles S.
Cockell's essay "Life on Venus" in the Dec. 1999 "Planetary and Space
Science":

http://www.elsevier.com/cas/tree/store/pss/sub/1999/47/12/1051.pdf

        Cockell (who actually is an exobiologist) concludes that, as I've
always thought, there's a serious chance that life evolved on ancient Venus
before its oceans had boiled away -- although, as he says, there is still
debate over whether Venus ever had enough liquid water to allow significant
oceans to exist.  "If Venus did receive a similar volatile inventory to
early Earth during accretion, then models suggest the possibility of a
wet-warm Venus with oceans that may have been close to boiling -- a 'moist
greenhouse' Venus.  Such a scenario may have existed for several hundred
million years until the increase in solar luminosity led to the evaporation
of the water, contributing to a greenhouse effect."  (pg. 1488)  And early
Venusian microbes might very well have been able to evolve to fit such
high-temperature liquid water oceans.

        However, he goes farther than that -- to my amazement, he thinks
that there's a slim chance that Venus' current sulfuric acid cloud layer
might be able to support a microbe population today!  As he points out, the
extreme concentarion of sulfuric acid in the cloud droplets is a problem:
"Because acid springs in surface regions on Earth and volcanic lakes are
routinely diluted by precipitation, organisms in sulfuric-acid water bodies
with comparable concentrations to the Venusian cloud layer (81-98%) have not
been identified.  Certainly most unadapted terrestrial microorganisms would
be killed by such concentrations, and this has tangential implications for
planetary protection." (pg. 1495)  However: "For present-day Venus, the pH
of the surface may not be a constraint to life."  (pg. 1492 and 1497)

        I've always assumed that the real killer on present-day Venusian
cloud microbes would be the fact that they would be quickly swept into the
broiling lower atmosphere -- but Cockell says no.  "An important
consideration on life in Venusian clouds is the ability of organisms to be
horizontally transported in the clouds for sufficient lengths of time for a
population to be maintained... The extent to which such bacterial particles
can be maintained in the clouds will be determined by the drop rate
predicted by Stokes' law...  For an assemblage of 5-10 bacteria of average
size 1.1 micron, the time to drop through the lower cloud layer will exceed
200 days.  The salient conclusion here is that this time period exceeds by
three orders of magnitude, or even greater, the division time of many
bacteria in logarithmic growth, and by an order of mgnitude the division
time of many very slow-growing bacteria.  Thus, the ability of the Venusian
cloud droplets to sustain stable populations of bacteria and transport them
over great distances is undoubted."  (pg. 1495-96)

        However, he adds: "How these organisms would reach the cloud layer
in the first place is problematic, since there are no organisms on the
surface.  Volcanic activity, storms and impact events in Venus' early
history are potential mechanisms.  However, whether a cloud biota could be
maintained for a time period of eons is questionable, even if Stokes' law
does predict it is possible.  Large-scale impact events might disrupt
planetary atmospheric composition and circulation periodically to make such
a biota impossible to maintain over geologic time periods.  [Nevertheless]
the ideas presented above on the Venusian clouds provide an insight into the
fact that Venus is not so far away from possessing a potentially habitable
niche."  (pg. 1496)

        Cockell does think that it's worthwhile doing chemical experiments
on Venus' cloud droplets simply to see how close the place comes to being
habitable today:  "Coupled with better atmospheric models of early Venus, it
may be possible to put a more accurate date on when Venus itself was no
longer conducive to life.  This data will have extrasolar implications" --
that is, implications for the possible habitability of extrasolar planets
with roughly Venus-like conditions.  (pg. 1498)

        As for the tremendous problem of trying to locate fossil evidence of
ancient Venusian life, he agrees that it's extremely difficult -- but not
quite absolutely impossible:  "Like the search for life on Mars, the key
concern is the identification of Venusian biomarkers.  Because of the
buildup of the sulfur cycle on greenhouse Venus as volcanism continued,
sulfur biogeochemistry is an attractive possibility.  Isotopic fractionation
analysis might be insightful on Venusian rocks.  Sulfate-reducing bacteria
exhibit a slight preference for sulfur-32 over sulfur-34.  The isotopic
analysis of sulfide compounds and minerals allows for a determination of
biogenic involvement in the sulfur cycle as for Precambrian rocks on Earth.
Because of the long phase of active turnover and volcanism that has occurred
on the planet, the average surface age of Venus is quite young, perhaps
300-900 million years old.  Thus, identifying regions of very ancient
terrain is difficult and may even be impossible.  This is the key limitation
for biomineralogical studies of Venus.  There may be a small possibility
that some rocks from more ancient (possibly some highland) areas of Venus
exist, although identifying these could again be difficult, if not
impossible.  In the event that such candidates were found, an exobiology
sample return mission could be envisaged.  A much more intriguing
possibility would be the identification of a Venusian meteorite of great
age.  Such a rock might provide geochemical markers of early Venus which
would yield insights into the possibility of a biotic contribution to
geochemical cycles.  Other biogeochemical markers such as microfossil
remains or iron fractionation are also possible in analogy to Mars
exploration."  (pg. 1498)

        The possibility of finding an ancient Venusian meteorite on Earth
with biological evidence in it hadn't occurred to me -- but, again, we'd
have to be EXTREMELY lucky.  All calculations indicate that any Venusian
meteorites whatsoever are extremely rare on Earth if they exist at all --
it's far harder for meteor impacts to blast them off that planet than
Mars -- and if we did find one, it would very likely be igneous and thus
very unlikely to contain any convincing evidence of ancient Venusian
microbes (just look at the problems we've had trying to confirm these on
Mars meteorite ALH 84001!)  Still, people interested in exobiology are
nothing if not professionally hopeful, so why not hope about this someday?

        In any case, one aspect of Cockell's essay is directly relevant to
us: he makes it absolutely clear (in technical detail) that microbes are
known to be able to survive and thrive at pHs as high as 0 -- so, even if
Europa's ocean does turn out to be as concentrated a sulfuric acid solution
as some think, it is not a show-stopper for microbial life there.

                                                            Bruce Moomaw

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