A strange gaseous molecule has been discovered lurking in the atmospheres of both Mars and Venus, scientists announced today, adding that it could affect Venus's hyperactive greenhouse effect.

The molecule's signature was first noticed in Venus's atmosphere in April 2006, when the European Space Agency's Venus Express arrived at the planet and began to measure the composition of the atmosphere.

The Infrared Atmospheric Spectrometer instrument aboard the spacecraft watched the sun set behind the planet and measured the wavelengths of light absorbed by the planet's atmosphere. Because different gases absorb at different wavelengths, scientists can infer the composition of the atmosphere from the wavelengths that are strongly absorbed.

While observing Venus, scientists noted an unusual signature in the mid-infrared region of the spectrum that they couldn't identify.

"It was conspicuous and systematic, increasing with depth in the atmosphere during the occultation, so we knew it was real," said study leader Jean-Loup Bertaux of the Service d'aeronomie of France's National Center for Scientific Research (CNRS).

Later that year, NASA scientists observing Mars using telescopes in Hawaii notified Bertaux's team that they had found the same unusual signature.

Because the atmospheres of both Mars and Venus are composed of 95 percent carbon dioxide (as compared to Earth's atmosphere which has only 0.04 percent carbon dioxide and is composed primarily of nitrogen), the researchers thought the strange molecule could be an isotope of carbon dioxide. (Isotopes have the same number of protons, but a different number of neutrons than the main form of an element.)

This exotic form of carbon dioxide has one "normal" oxygen attached to its carbon atom, while the other attached oxygen atom has 10 neutrons, instead of the usual eight.

The differently-weighted oxygen atoms let the isotope absorb more energy than normal carbon dioxide molecules, which could mean that it contributes more to the greenhouse effect on stifling-hot Venus, the researchers said. (Because the isotope only accounts for about 1 percent of carbon dioxide molecules on Earth, its contribution to our greenhouse effect is likely very small.)