Bad Astronomy

The way to dusty death

The Lunar Reconnaissance Orbiter continues to send back very cool imagery of the Moon. Check it:

[Click to embiggen]

This is a classic crater chain, a linearly laid-out set of craters about 4.5 km (3 miles) from end to end. These appear all over the solar system, pretty much whenever you have a large object with no atmosphere like a moon (or Mercury). There are two potential causes of these: an asteroid or comet that broke up due to gravity prior to impact, or material ejected from a large impact some distance away.

In the first case you usually need something big to break up the incoming object, like the way Jupiter’s tides tore apart comet Shoemaker Levy 9 which slammed repeatedly into the big planet back in 1994. It’s possible this could happen with the Moon, but unlikely because its gravity is weak (and while the Earth could do it, the object would then be more likely to hit the Earth and not the Moon).

Giordano Bruno crater on the Moon
So this chain seen by LRO is probably the second kind, material splashed out when crater formed nearby. It turns out that there is a likely suspect: Giordano Bruno. It’s a medium-sized crater about 20 km (14 miles) across, located just over the limb of the Moon on the far side. As you can see by this picture, it’s surrounded by bright rays, linear features from plumes of material arcing out of the crater when it formed. That’s encouraging; you’d expect to see this kind of ejected material if the event formed crater chains farther away.

Moreover, the bright material around Giordano Bruno, and lack of craters superimposed on it, means it’s young, probably less than a few hundred million years old (which is downright sprightly for a crater on the Moon). The same can be said for the crater chain; the craters making it up appear relatively fresh.

While this isn’t conclusive, it’s interesting, and pretty good evidence that the two features are related. Either way, the chain is pretty interesting. The largest crater at the bottom is about 340 meters (1100 feet) across, so it’s roughly the size of a big mall’s parking lot. The blur of material below the chain in the picture may be due to material splashed out and disturbing the lunar surface when the craters were formed themselves, meaning they probably hit at a low angle.

Interestingly, Giordano Bruno is located north of this chain, up past the top of this picture. That means this chain formed roughly perpendicular to the direction of the primary event. And since the largest crater is to the right, and they get smaller as you go left, I’m thinking that a big chunk of rock was pulverized when Bruno was made, and went flying south at a low angle along with a pile of debris. They all splashed down here, 500 km (300 miles) south of Bruno, making this weird formation.

One of the most amazing things about our Moon is how the geology is laid out; even millions or hundreds of millions of years after the fact, like forensic scientists we can trace back what happened and put together a story of the events. Is this particular story correct? Maybe, maybe not. But the best way to find out is to keep looking, find more evidence and build up more tales. LRO is doing just that, and more missions down the line will give us even more narrative voices. Eventually, we may have a novel with thousands of chapters, and – if I may contradict the Bard – it will be full of sound and fury, signifying everything.

Image credits… Crater chain: NASA/Goddard Space Flight Center/Arizona State University. Giordano Bruno: Wikipedia. Title of the post: Billy Shakespeare.