Published online 13 January 2009 | Nature | doi:10.1038/news.2009.24


Venus may have had continents and oceans

Granite highlands point to past water — and perhaps life.

Venus's atmosphere is 96% carbon dioxide, with clouds of sulphuric acid.NASA

The planet Venus, now hellishly hot and dry, may have once have been far more like Earth, with oceans and continents. That is the implication of new research claiming to see evidence for granite highlands on the planet in data almost two decades old.

In 1990, NASA's Galileo spacecraft detected nighttime infrared emissions coming from Venus' surface. Analysing these data, an international team led by planetary scientist George Hashimoto, now at Okayama University, Japan, found that Venus's highland regions emitted less infrared radiation than its lowlands.

One interpretation of this lower infrared emission from the highlands, say the authors, is that they are composed largely of 'felsic' rocks, particularly granite. Granite, which on Earth is found in continental crust, requires water for its formation. The results are published in the Journal of Geophysical Research1.

"This is the first direct evidence that early in the history of the Solar System, Venus was a habitable planet with plenty of water," says Dirk Schulze-Makuch, an astrobiologist at Washington State University in Pullman, who was not involved in the study. "The question is how long Venus remained habitable. But this gives new impetus for the search for microbial life in Venus's lower atmosphere."

Cloudy picture

Before Galileo, researchers had believed that only radar could see through the dense clouds of sulphuric acid in Venus's atmosphere to the surface, says co-author Kevin Baines, a scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. "Detecting the surface in the infrared is a breakthrough," he says.

The interpretation of such measurements, however, hinges on disentangling the effects of Venus's thick atmosphere. Not everyone is convinced that Hashimoto's team has achieved this.

David Crisp, also at NASA's Jet Propulsion Laboratory, has studied the atmospheres of various planets. Crisp, who was not involved in this latest analysis, spent several years attempting such detections using similar data. These new conclusions aren't supported either by the available data or the team's own models, he says.

"We understand our paper doesn't resolve everything," responds co-author Seiji Sugita, a planetary scientist at the University of Tokyo. Sugita says the next step is to apply their models to data from the European Space Agency's Venus Express spacecraft, which is already orbiting Venus, and the Japanese Space Agency's Venus Climate Orbiter, scheduled for launch in 2010.

Tectonic plates

The possible presence of granite also suggests that tectonic plate movement and continent formation may have occurred on Venus, as well as recycling of water and carbon between the planet's mantle and atmosphere. The implication of continent formation is "quite significant", says geophysicist Norm Sleep of Stanford University in California.

Venus might have once been almost entirely underwater, says Sleep — although without further geochemical data, he adds, we don't know whether this early ocean's temperature was 30 ºC or 150 ºC.

Whether tepid or boiling, he says, any ocean on Venus would have lasted only a few hundred million years. As the Sun became hotter and brighter, the planet experienced a runaway greenhouse effect. Nowadays, the planet is a paragon of the uninhabitable, with an atmosphere of 96% carbon dioxide and a surface temperature of around 460 ºC.

"Any life on Venus that hadn't figured out how to colonize the cloud tops a billion years after the planet's formation would have been in big trouble," says Sleep. 

  • References

    1. Hashimoto, G. L., et al. J. Geophys. Res. 113, E00B24 (2008). | Article |
Commenting is now closed.