The Moon's meteor history

During its 4.6 billion year history, the Earth has been bombarded by comets, asteroids and, on one occasion, even a planet smashed into us. The product of this latter collision was the Moon! And the Moon can reveal a lot more about what’s hit the Earth than the planet itself can, because, unlike Earth, the Moon doesn’t have weather, tectonic plates, or Ice Ages constantly remodelling its landscape. Instead, the lunar surface carries a near-pristine record of the sorts of objects that have been slamming into us over the aeons.Adam Murphy spoke to Thomas Gernon, from Southampton University, who was on the team who has worked out how to interpret the lunar landscape to decode what’s been arriving, and when...

Thomas - Our team used data collected by NASA's lunar reconnaissance orbiter which is the spacecraft which has been orbiting the moon for the past decade or so gathering data. What we found was a strong link between the rock abundance surrounding these big impact craters and their age.

What that means is the younger craters tended to have much larger blocks exposed around them than the older craters which were surrounded by finer material. This correlation was substantiated by craters that we have dated quite well using samples, for example, from the Apollo mission. Using this really strong correlation we were able to actually date over 100 of the moon's largest impact craters and effectively produce the first chronology of these large craters on the moon. So really interestingly both the Earth and the moon reveal a strong increase in the impact flux around 290 million years ago, so just before the age of the dinosaurs. We think that there were two to three times more rocks colliding with the planet, with Earth, just before the dinosaurs had evolved. So it's possible that this spike made the impact thought to have wiped out the dinosaurs on earth more likely.

Adam - How do things weather on the moon. Because there's no atmosphere there, so there's no kind of wind and weather. How did things actually weather down?

Thomas - What we think is the large rocks are effectively breaking down over hundreds of millions of years. And we think that the rocks are breaking down due to two things. First is a bombardment of micrometeorites, these are tiny fragments which are constantly pummeling the moon's surface. And the other reason is that we have thermal cycling. The lunar day is on the order of 28 Earth days and so we switch between 14 days of daylight and heating the rocks up and 14 days of nighttime when the rocks are cooling down.

Adam - And that rapid heating and cooling and the extremes is what breaks them down? 

Thomas - Absolutely. So they are, we think, gradually breaking down and I guess over a billion-year timescales your big blocks are going to turn into soil or what they call lunar regolith.

Adam - What implications does your data have for Earth then?

Thomas -  We compared the age distribution on the moon with those of the earth. What we discovered was surprising, that the actual impact fluxes were almost identical on the moon and the earth over the past six hundred and fifty million years. This was surprising because we often assume that the earth's impact record is biased due to erosion and so on.

Adam - Was there any period of earth that doesn't have craters that you were able to find did have bombardment?

Thomas - Startlingly what we found was the similarity between the Earth and the moon stopped at 650 million years ago. What we see is an abrupt cutoff on the earth of the impact craters which otherwise we would have expected to be present using the moon as an analogy. This lines up really well with a period in which the earth experienced really severe global glaciation, dubbed “snowball earth”, which is thought to have lasted for tens of millions of years. And in another recent paper, we demonstrated that as compelling evidence that there was really high levels of erosion during this snowball phase. And what we suggest is erosion on the order of three to five vertical kilometers, on average, across the continents. And this is unlike anything ever seen during Earth history. This is effectively a period in which we've we've scraped all of the really big impact craters off the face of the earth surface almost like a Brillo pad.

Adam - What's the next step then what's the next thing to look at?

Thomas - We're excited about pushing the limits of this technique and maybe seeing if it applies further back through history over several billion years of time. What we've done is use the moon as a catalogue in order to effectively understand the Earth process so we can learn a lot about for example erosion and plate tectonics through earth's history by looking at other bodies within our solar system and that's really exciting for me.