The big bombardment
Ken Tapping, April 24, 2013
Our Solar System was born about 4.5 billion years ago from the collapse of a big cloud of cosmic gas and dust. In the collapse particles collided, forming bigger and bigger lumps, producing the Sun, moons and planets we see in the Solar System today. However, there is still a lot of dust and other lumps orbiting the Sun today. Collisions are still happening, but far less often than they used to.
When something hits a planet or moon, it makes a crater, with a size related to how big the impacting body is and how fast it is moving. What is interesting is that we can count the numbers of craters of various sizes and put together the history of the impacting process.
Imagine the new, young planet Ping Pong. It is utterly smooth, with no mountains or anything else marring its surface. The something hits it, making a crater. Then there is another impact, and another. Initially most of the planet is unmarked, so the chances are much higher that the next object will not hit an existing crater. However, as the cratering continues, the uncratered fraction of the planet’s surface decreases, so more and more of the new impacts fall on existing ones. Finally, on the average, for every new crater, an old one is obliterated. The total number of craters then ceases to change much; the planet is saturated.
However, it gets interesting if the size range of the bodies ploughing into Ping Pong changes with time. For example, if over time the percentage of collisions with really big bodies declines. Then we will get an increasing population of smaller craters inside older, big craters. If the size range remains the same, we do not see this; so the crater record can tell us the impact history.
Unfortunately we cannot do this on the Earth because plate tectonics is continuously recycling the surface. There remain only a few isolated places on Earth where there are rocks dating back to the Earth’s youth. However the Moon is a different thing entirely. There is no plate tectonics, and most of the surface rocks are ancient.
A small telescope will show the Moon’s surface to be covered with craters of all sizes, many overlapping and partially obliterating others. There are huge, darker plains, with fewer craters, and lighter, mountainous regions that are heavily cratered. Moreover, careful examination shows these mountain ranges are the walls of large craters that have been partially destroyed by later impacts. The plains are immense lava flows, the results of collisions with small asteroids, which flooded old craters and presented new, clean surfaces, resetting the crater counter. These plains were subsequently cratered by later impacts. Thanks to the samples brought back by the Apollo astronauts we have an idea of rock ages, so know how old those plains are.
The current inferences are that between 4.1 and 3.8 billion years ago the bombardment was particularly fierce. Then it greatly subsided. It’s interesting that it was after this happened that life became widespread here on Earth.
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