Comets Crashed into the Moon

The Moon has clearly seen some stuff.  It’s visibly heavily cratered across it’s surface, which has remained unchanged since it’s surface solidified 4.2 Billion years ago.  Think about that – the Moon has been the same, with the exception of cratering, for 4 Billion years. This is a stark contrast to the Earth, whose erosion and tectonic activity cause the crust to change on scales of a few hundred million years.

Astronomers have worked hard to learn about the early solar system by looking at the Moon and its cratering patterns.  Most of the visible craters on the Moon are left over from the early periods of the Solar System, when Billions of tons of material was floating around colliding with the planets that were shepherding them.  But were all the craters from chunks of rock? In the early solar system there are many gases and ices present, what happened to them? Gases would likely drift into space due to the lack of lunar gravity, but what about ice?

A closer look at the Reiner Gamma. Credit: Courtesy of NASA/Lunar Reconnaissance Orbiter

Astronomers using modern computer models have simulated what comet impacts would have looked like on the lunar surface, based on our best knowledge of the composition of the Moon and the majority of early solar system comets.

Lunar swirls, like Reiner Gamma from the above image, have been puzzling scientists for decades.  Theories suggested that the brighter regions, sometimes extending thousands of kilometres across the lunar surface, were the result of leftover magnetic fields from the Moon’s ancient magnetism, which would block out the solar radiation that slowly darkened the Moon’s surface.  The moon would lose its magnetic field when its core solidified, so the timeline makes sense.

But Peter Schultz, a planetary geoscientist at Brown University, had a different idea from looking at the landing of the Apollo missions.  “You could see that the whole area around the lunar modules was smooth and bright because of the gas from the engines scoured the surface,” Schultz said. “That was part of what got me started thinking comet impacts could cause the swirls.”

Comets have a gaseous atmosphere called a coma, which would stay with it all the way to the rocky surface of the Moon. When the comets hit, their leftover gases would swirl and spin, carrying away the dark surface layers of lunar regolith.  This would expose the lighter rock below, in the same way the Lunar modules gases did.

Schultz’s simulations suggest that a small comet could swirl away the dust and create the light patterns spanning thousands of kilometres.  They also explain the magnetic anomalies found near these regions.  When a comet impacts, the heat generation can melt the iron-rich rocks.  When they heat up and cool rapidly they amplify and record the magnetic field at the time, in the same way rocks on Earth can be read to determine past magnetic fields in the Earth.

“This is the first time anyone has looked at this using modern computational techniques,” Schultz said. “Everything we see in simulations of comet impacts is consistent with the swirls as we see them on the Moon. We think this process provides a consistent explanation, but may need new Moon missions to finally resolve the debate.”

I love the fact that even though the Moon is so close to Earth, and has been studied by humans for thousands of years, we still have so much to learn.  It shows that the planets and Moons of our solar system are extremely complex, and that the Moon is much more than just a dead, heavily-cratered rock.


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