Pelted

A very long time ago, when our planet was young, it was hammered by rocks. Lots of huge rocks, for ages. Not a particularly enjoyable experience, you'd imagine, but character-forming, like camping. This prolonged period of galactic stoning has a name - The Late Heavy Bombardment. Seems a very long time ago for something to be called 'late', but it's considered late in the time frame of the formation of the solar system's planets. The Heavy Bombardment part you get, I'm sure.


Until recently, the bulk of the evidence for this event (if you can call something that lasted hundreds of millions of years an event - cricket seems to last hundreds of millions of years and that's called a game. Oh no. I've ended a sentence within parentheses. What's the correct way to get back into the sentence outside? Let's just ignore all this bit and carry on.) has come from collected lunar material. Rocks sampled on Apollo missions show a huge number of big impacts happened beginning about 4.1 billion years ago - some 427 million years after it was knocked from Earth and 'born'. If the moon was getting pounded, then Earth, Mars, Venus and Mercury were most likely getting the same treatment. It had been thought the bombardment continued until 3.7 billion years ago, but new clues have been found in terrestrial rocks which would suggest the brutality went on for as much as a couple of billion years.


Impactite horizons with tiny glassy beads - impact spherules - from large scale landings have been found from as 'recently' as 1.8 billion years ago. Large scale landings meaning really large in some cases - perhaps up to 70km across. Researchers think these show there's a case for an extension of the Late Heavy Bombardment, perhaps with a gradual decline in falls rather than a (relatively) sudden end. A complicating factor here, certainly with the earlier model, is the nature of the sampling. Although there were a few lunar landings, and a total of 380kg of rocks returned, the sample area isn't really a large percentage of the moon's surface area. We can't claim to have extensively explored the moon. Maybe other areas would/will show a different pattern of impact dates. In any case, we can be glad it's over. For now.