Sleuthing Around a Triple Impact on the Moon

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Jan. 5 2013 8:00 AM

Lunar Triple Whammy

The Lunar Reconnaissance Orbiter has been circling the Moon for years, taking amazing close-up pictures of our neighbor. While every shot is pretty cool, every now and again it sends one back that is particularly surprising. This one, just released by the folks who work with the main camera on LRO, really got me going:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

Triple impact crater on the Moon
A triple crater on the Moon, probably from a single object that broke up into three pieces. Click to enlunenate.

Image credit: NASA/GSFC/Arizona State University

[Click to embiggen, and you should since it helps you see the details I desribe. I'll also note I am not a selenologist—someone who studies the Moon—so what I say below is based on notes at the LRO page, as well as my experience reading and writing about the Moon. Some of this may prove to be incorrect, or superceded by better data. But that's science!]

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This crater triplet, which they’ve dubbed “Tres Amicis” (“Three Friends” in Latin), is pretty dang interesting. All three craters are about the same size—from right to left 125, 150, and 180 meters (135, 165, and 200 yards) across—and tightly clustered, spanning about 450 meters (a little over a quarter of a mile). Clearly, they’re related!

Most likely they formed when three asteroids or comets (or pieces thereof; see below) hit the lunar surface at very nearly the same time. Note the rays, the radial features extending away from each crater. Those are caused by plumes of material ejected by the impact, which then settle down on the surface. In a steep impact, one hitting nearly straight down, the rays are evenly distributed around the impact site (like a kid’s drawing of the Sun).

But here the rays aren’t like that. Look at the upper right crater; there are no rays extending to the upper right of it, which means the impact was probably at a shallow angle, heading from the upper right to the lower left—that tends to blow out the ejected material more in the direction of travel. The crater on the right is slightly elliptical, which is also expected in a shallow impact (though to be fair the other two are pretty circular, so it’s not clear to me what happened here).

Now look at the thicker rays from the big crater on the left. Those go out nearly perpendicular to the line connecting the craters, which to me is a clear indicator the three impacts happened very nearly simultaneously, certainly within seconds. In a single impact, the debris cloud would expand in a hemisphere, growing in size until the material fell to the surface. The debris would surround the crater in a circle. But if you get two impact events at very nearly the same time, right next to each other, the two expanding clouds will impact each other. That will push most of the debris to the side, just as we see here!

So the impacts forming the crater on the left and in the middle must have occurred close enough in time for their debris clouds to interact. Those clouds expand fast, so they must have been very close in time.

Closeup of the triple crater.
The same craters seen at late afternoon, when the Sun was low, which shows elevation features better. Click to embiggen.

Image credit: NASA/GSFC/Arizona State University

And there’s still more! Now look at the middle crater and the one to the right. See the ridge between them? That’s another feature that happens in near-simultaneous impacts. It’s similar to the expanding debris clouds forming a line, but this time it’s the material pushed outward in the crater bowls themselves itself interact, slamming into each other, piling up, and pushing sideways. This is a common feature in double impacts.

So the middle and left crater formed at nearly the same time, as did the middle and right crater. Therefore, all three formed at almost the same moment. The LRO page notes that there is ejected material from the middle crater that overlaps that of the one on the left, meaning it fell after the left crater formed. That may very well be true, and might also explain the rays from the left crater being pushed away a bit from the middle one. Given everything else, it looks like the impacts were mere seconds apart at most.

There’s still more to see, like the odd triangular feature between the middle and right craters. It almost looks like there was a much smaller fourth body that hit there, spraying material to the left. I’ll leave that to the experts.

But what’s particularly interesting to me is that this crater chain exists at all! It means there were three (at least) separate objects that hit here. That’s fine; we see that a lot: asteroids have moons, comets break apart, and there are tons of double and triple craters on the Moon (and other bodies, too).

But the thing is, the craters are in a line, and that line is also in the direction the impactors were traveling! If you had some asteroid out there that was made of three rocks orbiting each other, they have to be almost perfectly aligned to smack into the Moon and leave this chain. It’s far more likely that the craters would be lined up in a different way, at an angle to the incoming trajectory. That makes me wonder: was this an asteroid or comet that swung past the Moon and broke up? The gravity from the Moon would stretch the object (think of it as a tide, which is really what it is), and if it were particularly fragile, it would break apart, the pieces lining up naturally in the direction of travel. The low impact angle supports that; the object would have had to pass very close to the surface to break up. It either broke up and impacted, or swung around again at least once before it hit. Either way, it  would tend to lead to a shallow impact angle.

But again, I’m no expert here, and I’ll leave it to the pros to figure this the details. But I do enjoy puzzling over features like this. Looking at craters like these tell us about the Moon, but they also tell us about other objects in the solar system, too. What was it that hit here? Was it something unusual, or are small, fragile objects common in space?

It’s amazing what you can learn just by looking at the Moon, but what’s even more amazing to me are the further questions such investigations raise.