Venus: older and more interesting that we thought?

A mission to Venus might be able to find out more about the planet's history than previously thought, as new evidence casts doubt on the theory that the planet was totally resurfaced during a cataclysmic bout of vulcanism 500m years ago.

Scientists at the University of Minnesota have carried out a new analysis of the surface and have concluded that this massive, destructive event probably never happened, New Scientist reports.

The age of a planet's surface is traditionally estimated by counting craters. The more pock-marked the rock, the older the surface. By this method of reckoning, Venus, at 4.5bn years old, ought to have around 5,000 craters. Any fewer than this, and processes such as weathering and geological activity have to be invoked.

But Venus only has around 1,000 visible craters, all of which are very well preserved. This led scientists to conclude that a single, massive volcanic event must have wiped all the other craters from the planet's surface, leaving a coating of fresh lava between one and three kilometres deep over most of the planet.

Vicki Hansen of the University of Minnesota and her colleagues revisited data from the Magellan mission. They analysed areas where smooth flat plains are punctuated by islands of older terrain. By considering how each island sloped off, they could estimate roughly where neighbouring islands would meet, that is, where the base of the valley lay.

They found that the older terrain was covered by a much thinner layer of lava than had previously been thought. Less than a kilometre. This suggested that the "catastrophic eruption" scenario could not account for the missing craters.

Instead, the researchers concluded that Venus must have been resurfaced much more gradually, and that this process has left plenty of the older surface intact. Hansen says parts of surviving surface could be as much as a billion years old.

"The implication is that Venus on its surface preserves an extremely long record of rich geological history," Hansen told New Scientist. "That's what's so exiting about it. It says Venus actually has a lot of secrets to tell us."

If this is so, future missions to Venus will be able* to collect and analyse samples of rock from various periods throughout the planet's history, providing us with a much broader picture of the evolution of our twin planet. ®

*Providing they do not melt or get crushed, or corroded, or otherwise destroyed by Venus' rather inhospitable environment. ®