ONE traumatic event, lasting just a few hours, could have shaped the destiny of Venus.
In its early history, Venus shared many similarities with our planet. For example, it had lots more water than it does today. At some point, though, it took a radically different path and ended up parched and hellishly hot, earning it the nickname 鈥淓arth鈥檚 evil twin鈥�.
Now one planetary scientist is suggesting that Venus as we know it came into existence as the result of an almighty smash between two embryonic planets. According to J. Huw Davies of the University of Cardiff in the UK, this collision would have disposed of all its water and led to the newly formed planet鈥檚 extreme greenhouse atmosphere. What鈥檚 more, it could explain a few other mysteries, such as Venus鈥檚 curiously smooth surface and its odd rotation (Earth and Planetary Science Letters, ).
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The theory that Venus was once struck by another body is not new, but the idea had been shelved because, unlike Earth, Venus has no moon to provide an evidential fragment left over from such a collision. However, Davies points out that a head-on collision of two similar-sized planetary embryos would not create a moon 鈥� the two would fuse to form one big planet.
Davies reckons that such a smash would generate so much heat that large portions of each object would simply merge together. As a result, carbonate rocks would release huge volumes of carbon dioxide, which would linger and form the greenhouse atmosphere that helps heat the planet鈥檚 surface.
During the impact, water would react with iron to form iron oxides and release hydrogen. Unlike the heavier CO2 molecules, the hydrogen thrown out by the force of the blast would be carried away by the solar wind (see Diagram). Though many suggest the solar wind could have gradually removed any hydrogen in Venus鈥檚 atmosphere without the need to invoke a collision (快猫短视频, 28 November 2007, p 12), Davies argues this would have been too slow.
Moreover, the collision theory could answer a series of other questions. First, Venus鈥檚 surface has just a fraction of the craters that would be expected from billions of years of meteor impacts 鈥� a collision could have wiped the slate clean. Second, Venus rotates very slowly and, uniquely for solar system planets, . This could be explained if the two colliding bodies were rotating in different directions before impact. Finally, the planet鈥檚 apparent lack of plate tectonics could be explained by the loss of its water: on Earth, tectonic movement relies partly on water to create the shear zones where plates slip.
鈥淰enus鈥檚 surface has just a fraction of the craters expected of it 鈥� a collision could have wiped the slate clean鈥�
鈥淭he idea is amazingly simple,鈥� says Gareth Collins, a planetary impact specialist at Imperial College London. 鈥淚t鈥檚 incredible that no one thought of it this way before.鈥�
The best part, says Peter Grindrod at University College London, is that Davies has proposed a simple test: if Venus鈥檚 water was not lost in one big impact, then rocks at its surface will contain water locked into their structure.
