* Astronomy

The colossal outpouring of lava thought to have almost totally resurfaced Venus 500 million years ago never happened, a new study says. If correct, it means that a much longer record of Venus's history is preserved on the planet's surface.
Planetary scientists estimated the age of Venus's surface after studying radar mapping data from NASA's Magellan spacecraft, which operated in the early 1990s.

Assuming Venus was exposed to the same rain of asteroids and comets that the other planets experienced, they expected Magellan would spot about 5000 craters on the planet's surface.
But they found only about 1000, suggesting that the planet's surface is actually very young – perhaps 500 million to 1 billion years old. And those craters appear remarkably well preserved, unaltered by erosion or other geological processes. That suggests that whatever erased the 4000 or so missing craters was an all-or-nothing process.
The most popular explanation is that a brief but enormous episode of volcanism blanketed most of the planet in a layer of lava 1 to 3 kilometres deep – thick enough to bury all of the craters made before that time.
Now, a new analysis of Magellan data suggests that such a deep layer of solidified lava cannot be present on the surface, casting doubt on the 'catastrophic volcanism' hypothesis.

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Back when Earth was very young, our home world was steadily pummelled by large solar system debris. While Earth withstood the barrage of hits like a prizefighter that wouldn't fall down, one blow nearly destroyed the world. A Mars-size body ploughed into us, completely disrupting both bodies and splashing massive amounts of debris into orbit which, most astronomers agree, coalesced to form our Moon.
But if something that large hit us, how did our nearest-neighbour planet, Venus, dodge the same fate? According to a new study, it didn't. Billions of years ago, according to work announced yesterday, Venus once had a moon that formed the same way Earth's did.
On Monday at the American Astronomical Society's Division of Planetary Sciences meeting in Pasadena, California, Caltech undergraduate Alex Alemi presented models created with David Stevenson of Caltech that suggest Venus was not only slammed with a rock large enough to form the Moon, the event happened at least twice.
According to Alemi and Stevenson, in models of the early solar system it is nearly impossible for Venus to avoid a big hit. Most likely, Venus was slammed early on and gained a moon from the resulting debris. The satellite slowly spiralled away from the planet, due to tidal interactions, much the way our Moon is still slowly creeping away from Earth.
However, after only about another million years Venus suffered another tremendous blow, according to the models. The second impact was opposite from the first in that it "reversed the planet's spin," says Alemi. Venus's new direction of rotation caused the body of the planet to absorb the moon's orbital energy via tides, rather than adding to the moon's orbital energy as before. So the moon spiralled inward until it collided and merged with Venus in a dramatic, fatal encounter.

"Not only have we gotten rid of the moon, but we've also done well to explain Venus's current slow rotation rate (and direction)," says Alemi. If a second moon formed from the second collision, it too would have been absorbed the way the first one was.

The models do allow for more than two impacts, but the probability of Venus enduring several massive collisions is low. "You can do this with multiple collisions, but the hypothesis is that (the net result) adds up to a negligible contribution" to the planet's final state, says Alemi.

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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.

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.

"The transition from a high rate of resurfacing to a low rate could have lasted as long as two billion years" - Timothy Bond.

"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" - Professor Warner.

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.
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 Richard Ghail.

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

Source: BBC