Impacts: Fast and slow
Ken Tapping, 12th November, 2015
On Halloween, Earth had a close encounter with a dead comet: a ball of ice and dust some 400 m in diameter, and moving at 35 kilometres a second. It passed by at a safe distance of 490,000 km, further away than the Moon. Compared with the 12,756-km diameter of our world, that sounds a lot. However, by cosmic standards it is tiny.
Assume a really big archery target with a diameter of 980,000 km (2 times a radius of 490,000 km). In the middle is a bulls-eye with a diameter of 12,756 km. Arrows are fired randomly in the direction of the target by a really bad shot. On the basis of the fraction of the target occupied by the bulls-eye, on average one in six thousand of these random shots will hit it. Similarly, we can assume that on average, one in every six thousand comets or asteroids passing within 490,000 km of Earth might hit us. If one of these objects passes by every year, that does not guarantee 6000 years between impacts, but that maybe over 6 million years the Earth might be hit a total of 1000 times.
The Observer’s Handbook of the Royal Astronomical Society of Canada lists 60 significant impact craters still visible on the Earth, dating back as far as two billion years. With plate tectonics and weathering constantly renewing the Earth’s surface, the number of craters formed over the last two billion years by significant impacts must have been many times that. The most recent impacts were in 1908, when something came into the atmosphere and exploded over Siberia, and in 2013, when an object about 20m in diameter caused a large amount of damage in Russia.
These events fit our current ideas about the birth of the Solar System. It involves things smashing together at high speed, with the energy released by repeated impacts resulting in huge balls of molten rock that eventually cooled to form the planets. This picture has raised some serious questions. Firstly why does the Earth have so much water, and for that matter it looks as though in the past Mars did too? Why wasn’t it all boiled off? Secondly, we know that many of the carbon-based chemicals in cosmic clouds are the building blocks for life as we know it, and they seem to be present on most planets, even on Pluto. Molten rock temperatures would have destroyed them. Venus is a greenhouse-effect driven inferno and any organic chemicals it had must be long gone.
Now we are getting a hint of an answer. Not all impacts were high speed ones. In 2014 the Rosetta spacecraft went into low orbit around Comet 67p/Churyumov-Gerasimenko. Our first close-up view of the comet was a surprise, because it looks like two lumps of stuff stuck together. Analysis of the gases emitted by the comet has revealed something we had definitely not expected. The comet is giving off oxygen. This is really strange. Oxygen is a highly reactive gas, and given any opportunity it will combine with other elements. For example, it rusts iron to form iron oxide, and makes carbon-based molecules burn, turning the carbon into carbon dioxide. It is extremely unlikely there are plants on the comet, so it must have been there a long time – millions or billions of years – without reacting with anything. This could only happen if the oxygen remained very cold. If the comet had formed from high-energy collisions, the resulting high temperatures would have caused the oxygen to combine with the other materials making up the comet. Since we are pretty sure that the bodies in the solar system were formed by accretion, some collisions must have been slow and gentle, and not releasing lots of heat. For example some objects in space, including this comet, look as though they formed from two objects slowly coming together. This might be how the raw materials for life, including lots of water, came to be on Earth, and probably on Mars and other places too.
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