A flower bloom on the Moon

Contributed by
May 11, 2011
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If I ask you to close your eyes and picture a crater on the Moon, I bet what would come to your mind is a bowl-shaped depression, a raised rim, and maybe a central peak. You might also picture the surrounding area, which looks pretty featureless except for other craters.

I would also bet you wouldn't picture anything like this:

Isn't that lovely? [Click to enlunanate.] Looking like a kilometer-wide flower on the lunar surface, it's an unnamed crater just south of Mare Crisium, on the Moon's eastern limb near the equator. This image, from the Lunar Reconnaissance Orbiter, spans a distance of about 2.2 km (1.3 miles) across and the full-res image has a resolution of roughly 1.5 meters per pixel.

It's not your run-of-the-mill crater. It's surrounded by the material that was ejected when a small asteroid (or comet) slammed into the Moon. The impact excavated something on the order of a million tons of rock, blasting it off the surface and into the sky. The plume was thickest in the middle, right over the crater, and thinned with distance. It settled in those streaks, bright in the center where the material is thickest, and darkening farther away. This material is called the crater's ejecta blanket.

You can also see two black, smaller craters, one just below the main crater and one a bit farther to the left. They also display small ejecta blankets. Most likely they were formed from secondary impacts; boulders blasted up and out on high trajectories, which then impacted the surface after the plume had fallen. Their impact dug up the older, darker surface material underneath the blanket.

In fact, in the zoomable and pannable wide-angle view of this crater, you can see that the original surface surrounding the crater is quite dark compared to the ejecta blanket. And if you zoom in, you see those two black secondary craters aren't alone; there are hundreds of smaller, darker spots. Most if not all of them are from secondary impacts of smaller rocks that fell back to the lunar surface.

If you zoom out, you can see some of those light streaks (technically called rays) stretching over 4 km away. When the impactor slammed into the Moon it made a heckuva bang. I expect such an explosion would have been visible from Earth... though this crater probably formed well before there were humans striding the Earth.

Over time - lots and lots of time - erosion will take its toll. The solar wind, micrometeorite impacts, even thermal stress from the Moon's day/night cycle, all will weather away this crater. The blanket will fade, the crater features will dull, and eventually it will look like the countless other 100-meter-wide holes in the Moon. But who knows? By then, perhaps, a few more will have formed.

Image credit: NASA/GSFC/Arizona State University

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