By Paul Rincon BBC News science reporter, in Houston, Texas

Scientists age Solar System surfaces by counting craters

Accepted views of how the planet Venus evolved are challenged by new age dates for its surface.

Massive volcanism 500 million years ago was thought to have covered over much of the planet's ancient features.

But work carried out at Imperial College London, UK, suggests a "volcanic catastrophe" is not needed to explain the look of Venus's surface.

The British team presented details of its research to a major science conference in Texas, US.

Scientists will have an early opportunity to examine the new ideas - Europe's Venus Express spacecraft is due to arrive at the planet next month for a two-year investigation of Earth's near-neighbour.

Crater counters

Researchers date planetary surfaces by looking at the distribution of their impact craters.

On most planets and moons, impact craters tend to be clustered on very old parts of the surface, due to the heavy bombardment that is believed to have taken place in the early Solar System.

But craters on Venus are distributed randomly over the whole planet. This has led some scientists to the conclusion that most of the surface is of similar age.

One way to arrive at this result is by rapid resurfacing - the model long accepted by planetary scientists.

Timothy Bond and Mike Warner of Imperial College London have now thrown that theory into doubt.

Using computer modelling, they came up with a suite of possible scenarios that were compatible with the planet's cratering record and surface features.

They concluded that there was no need to invoke massive outpourings of lava over a short period. Instead, the planet's present-day surface could be compatible with a slow decline of volcanic activity, they argue.

Heat calculations

"The transition from a high rate of resurfacing to a low rate could have lasted as long as two billion years," Timothy Bond told the BBC News website.

Mission guide: Venus Express

Professor Warner added: "We haven't shown that a very short event isn't possible, we've just shown that there are a much wider range of possibilities.

"A very short event is, a priori, quite unlikely given that there is a much wider range of likely realities."

Previous work suggests the volcanic upheaval 500 million years ago covered up "almost all" of the ancient surface.

The models developed at Imperial College suggest about 26% of the planet's surface could be older than 700 million years.

The findings agree with new models of heat loss from the interior of Venus produced by Dr Richard Ghail, also of Imperial College.

'Steady state'

Earth's surface is divided into many plates that move relative to one another on convection currents in the mantle below.

At a type of boundary called a subduction zone, one plate is dragged down below an adjacent plate and destroyed in the mantle. At another, called a spreading ridge, two plates move apart and grow as volcanism adds new material at their edges.

These processes, called plate tectonics, continually cool the Earth and keep it in balance - what scientists call a "steady-state".

There is little evidence of plate tectonics on Venus. Therefore, some scientists think heat might build up below the Venusian crust, leading to occasional catastrophic releases of magma along with rapid resurfacing of the planet.

However, Dr Ghail believes the surface features of Venus do not necessarily reflect the rate of plate tectonics on the planet.

Instead, he thinks high temperatures in the interior create a weak zone between the crust and the mantle which essentially decouples, or separates, them from each other. This would allow more continual plate tectonic activity that would leave little evidence on the surface.

"I think we're moving closer towards a steady-state model for Venus," Dr Ghail told the BBC News website.

The researchers presented their results here at the Lunar and Planetary Science Conference in Houston, Texas.