Ancient seas and greenhouse gases

Scientists discover a new greenhouse gas on Venus, and learn the toasty planet's early oceans may have survived far longer than previously thought.
By | Published: October 11, 2007
Venus posed for NASA’s MESSENGER when this Mercury-bound probe flew past June 5. Spectral observations with another spacecraft, the European Space Agency’s Venus Express, have revealed a rare isotope of carbon dioxide that could contribute significantly to that world’s greenhouse effect.
NASA/JHUAPL/Carnegie Institution of Washington
October 11, 2007
Greenhouse gases in Earth’s atmosphere warm our planet just enough to keep it hospitable to life. If humans add too many such gases, however, they run the risk of boosting the temperature beyond the comfort level. On Venus, extreme levels of greenhouse gases long ago raised the planet’s temperature so high its oceans boiled away.

On Wednesday, scientists revealed the discovery of a new greenhouse gas that could play a significant role in Venus’ greenhouse effect, while other researchers suggested Venus’ early oceans lasted at least a billion years. Both announcements came at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences in Orlando, Florida.

If oceans survived for 1 or 2 billion years, Venus would have been habitable for a sizable fraction of the solar system’s history. And if Earth is any indication, that’s plenty of time for life to gain a foothold. It seems scientists may have to add Venus to the growing list of places to search for evidence of past life.

The new greenhouse gas is actually an old friend in disguise: a rare isotope of carbon dioxide. Carbon dioxide makes up 96 percent of the venusian atmosphere. A normal molecule of the gas contains one carbon atom (with an atomic weight of 12) and two oxygen atoms (each with an atomic weight of 16). A team of planetary scientists led by Jean-Loup Bertaux of CNRS in France has found a small fraction of Venus’ carbon-dioxide molecules have one oxygen atom with an atomic weight of 16 and the other with a weight of 18. The researchers suspect the isotope contributes to Venus’ greenhouse effect. How much is open to debate (estimates run as high as a few percent), which won’t be resolved until models accurately include the new ingredient.

Bertaux’s team found the isotope in observations made with the European Space Agency’s Venus Express spacecraft. The team observed the Sun through Venus’ atmosphere using the probe’s SOIR instrument. The resulting spectrum shows features where constituents in the planet’s atmosphere absorb sunlight. Oddly enough, the scientists were searching for the signature of hydrochloric acid near a wavelength of 3.3 micrometers. They found it, but also saw an adjacent band of unknown absorption features.

The band remained unidentified for several months. Then, out of the blue, planetary scientist Mike Mumma of NASA’s Goddard Space Flight Center contacted Bertaux and asked whether they saw anything special around 3.3 micrometers. Mumma and his colleague, Goddard’s Geronimo Villanueva, had seen unidentified features in the same wavelength region in spectra of Mars. When the two teams compared their spectra, the features were identical. The mystery was solved after the American team suggested the features could be the signature of the rare carbon-dioxide isotope. Calculations quickly confirmed it as the culprit.

The greenhouse effect eventually evaporated Venus’ oceans, but how long did it take? Earlier models suggested a lifetime of just 600 million years. Yet those models failed to take into account water clouds in the atmosphere, which would have slowed the oceans’ inevitable fate. More recent models by David Grinspoon of the Denver Museum of Nature & Science and Mark Bullock of the Southwest Research Institute show early oceans would have survived at least 1 billion years, and perhaps for as long as 2 billion years. Grinspoon said Wednesday that future Venus landers could resolve the debate, in part by analyzing how much of the mineral tremolite exists at various altitudes.