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鈥檚 history is preserved on the planet鈥檚 surface.
Planetary scientists estimated the age of Venus鈥檚 surface after studying radar mapping data from NASA鈥檚 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鈥檚 surface.
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But they found only about 1000, suggesting that the planet鈥檚 surface is actually very young 鈥� 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 鈥渕issing鈥� 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 kilometre to 3 km deep 鈥� 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 鈥渃atastrophic resurfacing鈥� hypothesis.
Valley floor
Study leader Vicki Hansen of the University of Minnesota in Duluth, US, and colleagues analysed areas where islands of terrain poke up through flat 鈥減lains鈥�.
They looked at the slopes that lead down from these islands and disappear below the plains. By studying neighbouring islands, they were able to extrapolate where the slopes would reach a common base 鈥� the floor of a valley between them.
They found that this base was buried less than 1 km below the surface of the plains. The researchers say this is at odds with the catastrophic volcanism idea, which calls for a global blanket of lava up to 3 km deep.
They believe the islands are ancient terrain and the plains were laid down more recently 鈥� evidence that bits and pieces of the planet have been resurfaced at different times, leaving much of the planet鈥檚 older surface intact.
Gradual decline
That meshes well with a study presented at a conference in March by Timothy Bond and Michael Warner of Imperial College London, UK. They found a gradual decline in volcanic activity over a period as long as 2 billion years fit Venus鈥檚 crater statistics better than a single violent episode.
Ellen Stofan of University College London in the UK agrees that the resurfacing process on Venus was probably more drawn out than in the catastrophic volcanism scenario.
And Fred Taylor, a member of the Venus Express mission at Oxford University in the UK, says Hansen鈥檚 study adds to the evidence against a catastrophic resurfacing event, which now appears to be 鈥渢oo simplistic鈥�, he told New 杏吧原创. 鈥淧ersonally, I have always thought so.鈥�
Rich history
Based on their study, Hansen鈥檚 group argues that much of Venus鈥檚 surface is more than 1 billion years old. If this is correct, sending new missions to the planet could probe much further into its past than previously realised, Hansen says.
鈥淭he implication is that Venus, on its surface, preserves an extremely long record of rich geological history,鈥� Hansen told New 杏吧原创. 鈥淭hat鈥檚 what so exiting about it. It says Venus actually has a lot of secrets to tell us.鈥�
If the planet once had water on its surface, for example, a robotic lander might be able to find evidence in the form of water-related minerals, she says.
The study by Hansen鈥檚 group was presented on 24 October at a meeting of the Geological Society of America in Philadelphia, Pennsylvania, US.
