June 19, 2006

Part V: Astrobiology

Sympathy for the Devil:
The Case for Life on Venus
David Grinspoon, Southwest Research Institute, Boulder
33 min. (slideshow requires QCShow Player)
Audio only (mp3 format)
View as a webpage (quicktime, real player) (notes)

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 have 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 lecture: 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, 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.

— Wirt Atmar

About the Speaker

David Grinspoon is an internationally known planetary scientist, funded by NASA to study the evolution of Earth-like planets elsewhere in the universe, as well as an award-winning writer. He is currently Principal Scientist in the Department of Space Studies at the Southwest Research Institute in Boulder, Colorado, and Adjunct Professor of Astrophysical and Planetary Science at the University of Colorado.

Dr. Grinspoon serves as an advisor to NASA on space exploration strategy, and has lectured and published widely. His first book, Venus Revealed, was a Los Angeles Times Book Prize finalist. His latest book, Lonely Planets: The Natural Philosophy of Alien Life won the 2004 PEN literary award for nonfiction. His popular writing has appeared in Slate, Scientific American, Natural History, The Sciences, Astronomy, the Boston Globe and the New York Times. His technical papers have been published in Nature, Science, and numerous other journals.

Dr. Grinspoon has been featured on numerous television and radio shows, including PBS’s “Life Beyond Earth,” BBC’s “The Planets,” NPR’s Science Fridays and Weekend Edition, and BBC World Service. He is also an award-winning musician who has played guitar and sung in several great bands destined for obscurity. Grinspoon lives in Denver.

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