The Magellan space probe found many puzzling crater-like features on Venus when it scanned the surface with radar. Now an American scientist believes he has solved the puzzle. He says the features are not the result of meteorite strikes but were caused by immense explosions in Venus's lower atmosphere when a body disintegrated on entry.
Most craters on Venus are either volcanic in origin or were caused by the impact of meteorites, but the Magellan team reported finding plenty of craters with strange shapes. Some seemed to have hardly any central crater at all, but consisted of concentic discs which appeared dark in the orbiter's radar images. In contrast, a typical crater caused by the impact of a meteorite is surrounded by bright discs of debris.
Kevin Zahnle of NASA Ames Research Center, California, has modelled the motion of meteorites travelling hypersonically through the thick Venusian atmosphere. He presented the results at the 23rd Planetary Science Center in Houston, Texas.
'Some meteorites explode because of frictional heating,' says Zahnle. 'A spherical blast wave expands, and I've worked out what happens when that hits the ground.' Zahnle says the blast wave, which is equivalent to that from a million megaton nuclear explosion, shocks the surface rocks to a depth of a kilometre and to a radius of between 15 and 30 kilometres. 'Everything is shattered to fine rubble,' says Zahnle. Because this does not reflect Magellan's radar, this explains why the Venus orbiter registers a dark disc.
Magellan has found about 400 of these young 'airblast' craters. Zahnle thinks that Venus probably has more than 1000. Further study of them could also sharpen up estimates of how frequently objects of this kind hit the Earth, he says.
Zahnle has included various kinds of incoming objects in the models: dead comets, asteroids, and metallic meteorites. His studies have caused him to speculate on the nature of the so-called Tunguska event of 1908, in which an object entered the atmosphere above Siberia. It was too small to have been a comet, and it would have hit the ground if it were a metallic object, says Zahnle. 'It was probably a dense stony object, such as an asteroid,' he concludes. 'It probably entered the atmosphere at low velocity and exploded 10 kilometres above the ground.'
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