Lecture of the Week: Part V: Astrobiology

The Evolutionary Biology Lecture of the Week is now available at:

http://aics-research.com/lotw/

The talks center primarily around evolutionary biology, in all of its
aspects: cosmology, astronomy, planetology, geology, astrobiology,
ecology, ethology, biogeography, phylogenetics and evolutionary biology
itself, and are presented at a professional level, that of one scientist
talking to another. All of the talks were recorded live at conferences.

This is the fifth lecture in a summer-long series on the new science of
astrobiology.

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June 19, 2006

Part V: Astrobiology

Sympathy for the Devil:
The Case for Life on Venus
David Grinspoon, Southwest Research Institute, Boulder
33 min.

Venus favors the bold.
-- Ovid (43 BC - 17 AD)

The next four lectures will ask the question: "Why did things go so
right for life on Earth?" To answer that question, we ask, "Why did
things go so badly on Earth's nearest neighbors, Venus and Mars?"

Mars is half the size of Earth, but Venus is nearly Earth's identical
twin. All three planets likely had substantial oceans, but Venus and
Mars lost their oceans some time ago. Mars' oceans apparently evaporated
almost immediately, but Venus may have kept its oceans for 600 million
years, as estimated by Jim Kasting of Penn State in 1988, or for as long
as two billion years, as suggested by David Grinspoon in this week's
lecture.

For all of its history, Mars has lain outside the "habitable zone,"
where planetary temperatures make long-term surface liquid water
possible. Worse, because of Mars' small size, the internal heat engine
of Mars sputtered off sometime ago, thus the world stopped evolving
geologically, no longer replenishing its atmospheric gases.

Venus, on the other hand, lies interior to the habitable zone and has
received too much solar flux. While its oceans were evaporating, UV
radiation was dissociating its water into its constituent components,
providing sufficient energy for the escape of hydrogen into space. For a
bit of time, perhaps a billion years, Venus may had water oceans
overlain by an oxygen-rich atmosphere before any other planet. But
that's not Venus today.

Grinspoon outlines the two great transitions in the geological history
of Venus in this lectu the initial loss of its oceans and the later
overturn of its crust, which may in fact be the consequence of a single
continuous dehydrating evolution. With the dessication of Venus'
surface, water-borne tectonic subduction ceased and the planetary
surface became in effect one large plate. The result was that about 700
million years ago, internal heat built to the point that the entire
surface of Venus may have melted in one global event.

Venus' present may well be Earth's future. As the Sun continues to
brighten, it is expected that the Earth will also lose its oceans in 500
million to one billion years, thus in appproximately the same short
period of time that it has taken life on Earth to progress from
trilobite to astronaut, life will come to an end on Earth as well — at
least on its surface.

Venus has not traditionally been considered a promising target for
astrobiological exploration, yet Grinspoon proposes that Venus should be
central to such an exploration program for several reasons. All of our
ideas about extraterrestrial biochemistry are, of necessity,
extrapolations from the single example of life which we have been able
to study. Planetary exploration, with an increasing focus on
astrobiology, has been designed to "follow the water." This is a
reasonable strategy but it is based, at best, on an educated guess about
life’s universals.

If we think beyond the specifics of a particular chemical system
required to build complexity and heredity, we can ask what general
properties a planet must possess in order to be considered a possible
candidate for life. Grinspoon argues that the answers might include an
atmosphere with signs of chemical disequilibrium and active, internally
driven cycling of volatile elements between the surface, atmosphere and
interior, what he calls his "Living Worlds Hypothesis." At present, the
only two planets we know of which possess these characteristics are
Earth and Venus.

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