The Sciences

Lunar boulder hits a hole in one!

Bad AstronomyBy Phil PlaitMay 24, 2010 6:00 PM


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Y'know, I see a gazillion pictures of astronomical objects all the time, and I never get tired of them. But every now and again a picture comes along that's so wonderful I just have to share it. This is one such piece of wonderfulness: a lunar hole in one!

And you thought the windmill at the end of putt putt golf was hard. This picture -- click to enlunanate -- is from the Lunar Reconnaissance Orbiter, and shows a region of the Moon inside the crater Henry Frères. Taken on March 7, 2010, the image shows an area just 500 meters (550 yards) across -- if it were Earth, you could easily walk across it in less than ten minutes -- and shows objects down to less than a meter in size.

And it's just so cool! Look at the dashed trail going from left to right. See how it ends at the little crater, and even -- if you look closely -- can be seen to turn downwards? It suspiciously points right to the 10-meter (30+ foot) boulder sitting just inside the crater wall. Suspicious indeed. In fact, what you're seeing is the trail left by that boulder as it rolled and bounced downhill and stopped inside the crater! Look at the big picture. From the debris (small rocks) running up and down, you can tell that the terrain on the left side of the picture slopes down to the middle (in other words, if you started on the left side and walked to the center of the picture you'd be going downhill). The middle of the picture is relatively level ground. In my mind's eye, what happened here is clear. The boulder starts off at the left, and something -- perhaps a minor moonquake, or a nearby impact -- shakes the ground. The house-sized rock gets dislodged, and in the gentle gravity begins to roll downhill. It hits something and bounces, coming back down, skidding and rolling, only to be launched into the sky again and again. It slows a bit each time -- the ruts it digs get shorter as it moves left-to-right -- and by the time it gets to the end of the track it's barely moving, just enough to feel the change of slope due to the crater wall. It even rolled past the crater a bit (you can see the last groove is actually along the path a little beyond the crater), and almost slows to a stop... but then slowwwwwwly teeters backwards, back along the path it came. Just as it's about to come to a rest, it goes over the lip of the crater, slides into it, and lumbers to a halt halfway down the 60-meter (200 foot) crater's wall. I would give a lot to be able to see video of something like this happening on the Moon in real time. Wow! And that boulder's flight is just one of many scenes depicted here, which you can see if you let yourself explore. Just above the bouncing boulder's path is a trail of what looks like a dustslide, a bit more brightly colored than the moonscape around it. It slid downhill to the right as well, and partially buried some of the bigger debris. Obviously, this happened after the bigger rocks already slid down, since it buried some of them. And above that in the picture you can see fainter trails from other rocks sliding down. Those trails are harder to see, meaning they're older (millions of years of micrometeorite impacts and thermal flexing from the Moon's day/night cycle gradually erase features like that), which again is consistent with the picture I'm painting here.

Take a look at the crater at the bottom left. It's surrounded by a light-colored apron of ejected material. See how there's more of that ejecta to the right than to the left? That's what you'd expect if the slope goes downhill to the right; the material spreads out more as it falls downslope (the diagram here will help). And hmmm, there's a small crater about 10 meters across just to the right and below the big crater with the boulder in it. That crater is fresher; it still has a light apron as well. But what's that dark spot in the center? Beats me. Cool though, ain't it? There's so much to see and investigate, and this image is only about the size of a city block! It's a slice of a much longer 2.5 x 15 km strip, which you can interactively browse, too. WARNING: be prepared to lose a lot of your day if you click that link, but it's worth it, just like exploring the thousands of other pictures LRO has sent back is worth it as well. The LRO mission cost roughly $600 million. There are 300 million people living in the US right now... so play with those images for a few minutes, and then let me know if you got your two bucks' worth out of this mission. Image credit: NASA/GSFC/Arizona State University

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