Venus possibly habitable for billions of years

The hellish climate of Venus may have arisen far more recently than previously supposed, suggests new research. If so, pleasant Earth-like conditions probably persisted for two billion years after the planet’s birth – plenty of time for life to have developed.

Venus is virtually the same size as Earth and, on average, is our nearest neighbour. Today, its atmospheric temperatures are hot enough to melt lead and concentrated sulfuric acid continuously drizzles down from thick sulphurous clouds that completely block out the Sun.

But the planet once had a climate similar to Earth’s and vast oceans of water. Planetary scientists agree that period ended when Venus lost its water due to a runaway greenhouse effect, but the question is when.

Until now, the best estimate, calculated 15 years ago by James Kasting, of the Pennsylvania State University, was four billion years ago – just 600 million years after the Solar System’s birth.

But new work by David Grinspoon, at the Southwest Research Institute in Boulder, Colorado, suggests the momentous transition may have occurred much later. He points out the Kasting’s estimate was just a lower limit on when the change happened, because it did not include the effect of clouds in the Venusian atmosphere.

Reflected sunlight

Clouds reflect sunlight back to space and therefore cool a planet’s surface, and Grinspoon’s preliminary calculations indicate that the effect can be dramatic – keeping the atmosphere 100 Kelvin cooler than without them.

Although more detailed modeling remains to be done, Grinspoon says the difference could mean that oceans and pleasant temperatures may have persisted on Venus for at least two billion years.

This also suggests that another global transformation on Venus about 700 million years ago, in which the whole planet’s surface appears to have melted and reformed, may actually have been a continuation of the same greenhouse warming that dried out the planet.

Once the water was lost, Grinspoon says, plate tectonics would have stopped completely, and with it the most efficient way for the planet to shed its internal heat. This could have led to a buildup that eventually caused the whole crust to melt and then reform.

More generally, if this analysis is right, it means that the “habitable zone” for planets around other stars may be much wider than has been assumed, since Venus had been thought to be far outside it.

Grinspoon presented his work at the American Astronomical Society’s Division for Planetary Sciences meeting in Monterey, California on Saturday.