Greenhouse on Venus

A rare form of carbon dioxide in which one oxygen atom contains ten neutrons instead of the usual eight could be to blame for the searing greenhouse effect on the planet Venus. The findings were discussed by planetary scientists speaking at the annual meeting of the American Astronomical Society’s Division of Planetary Sciences in Orlando, Florida, in October.

Jean-Loup Bertaux, of the CNRS, France, and Ann-Carine Vandaele, Institut d’Aeronomie Spatiale de Belgique, used the infrared atmospheric spectrometer (SOIR) instrument to measure solar occultations in which a planet appears directly between the earth and the sun and the sunlight passes through the planet’s atmosphere on its way to earth.

Using this technique, the team detected the chemical signature of the so-called isotopomer carbon dioxide, which is present on earth at just 1%, in the atmospheres of both Mars and Venus. The discovery finally solves a puzzle regarding observations of Venus made in April 2006 by the European Space Agency’s Venus Express.

A conspicuous spectral line at 3.3 micrometers in the mid-infrared region of the spectrum was observed, explains Bertaux. Intriguingly the spectral line systematically increases the deeper into the planet’s atmosphere you look. Initially, the team kept quiet about their results, suspecting the presence of an organic molecule in the atmosphere of Venus. However, the chemical signature did not fit the usual pattern seen for organic molecules.

Then, in December 2006, Mike Mumma of NASA’s Goddard Space Flight Center, Maryland, enquired whether the SOIR team was seeing anything special on Venus at 3.3 micrometers. He had discovered an unidentified spectral signature at that very wavelength using telescopes on Hawaii pointing at Mars. The two teams compared the absorption signatures: they were identical.

The atmospheres of Mars and Venus are composed of 95% carbon dioxide, although the Venusian atmosphere is much thicker. The American team suggested that the signature could be coming from an isotopomer of carbon dioxide. However, a spectral line for this molecule had not been seen at 3.3 micrometers before.

An investigation by three independent groups, one led by Mumma in America; Sergei Tashkun and Valery Perevalov at Tomsk State University, Russia; and Richard Dahoo at Service d’Aeronomie du CNRS, France, all came to the same conclusion. The signature could be caused by a rare energy transition only possible in the isotope.

This rare transition allows the isotopomer of carbon dioxide to absorb even more energy and so contribute even more to the greenhouse effect on Venus. Thankfully, on Earth, there is 250,000 times less carbon dioxide in the atmosphere, so the presence of just 1% of the isotopomer will not contribute significantly more to the greenhouse effect here.