One of my favorite astrophotographers, Alan Friedman, spied something odd on the Moon.

Flying reindeer I’ll buy. But an inertialess propulsion system? C’mon.

Happy holidays everyone!

Credit: Alan Friedman

Last July, the spacecraft Dawn slipped into orbit around Vesta, one of the largest asteroids in the solar system — the first time a probe had ever orbited a main-belt asteroid. From its height of 16,000 km (almost 10,000 miles), it started mapping the 500 km (300 mile) wide rock, returning the first close-up pictures in amazing detail.

Over time, the height of the spacecraft over the surface was lowered, and it has now attained its lowest altitude orbit: a mere 200 km (120 miles) over the asteroid’s cratered, battered terrain. I mean asteroidain. Whatever. Anyway, it’s now sending back higher-resolution images than ever before, including this very cool one:

[Click to asteroidenate.]

This shows a region of Vesta about 18 km (11 miles) on a side, dominated by a ginormous impact crater. You can see how the crater’s central floor is flat, and you get just a hint of a slightly raised rim around the edge of the crater. The shadow of the rim falling into the crater also suggests variations in the elevation of the rim top (though craters in the floor of the big crater distort the shadow’s edge a little too). I like all the small craters inside the big one; they come in a variety of shapes, some deep, some shallow, and one (near the rim at the bottom of the picture) appears to be sliced in half; I suspect material flowing down the crater wall in a landslide half buried it. Light colored streaks pointing down the crater wall indicate slides do occur. Triggered by other impacts, maybe?

We’ll be seeing lots of amazing images and science coming from this spacecraft over the next few months. Be sure to check the mission’s Image of the Day pages to stay on top of what we’re seeing on Vesta… but be quick, because time’s running out. In May, Dawn will leave Vesta and start a new journey for a new target: the largest asteroid in the solar system, Ceres. It arrives there in 2015.

Image credit: NASA/ JPL-Caltech/ UCLA/ MPS/ DLR/ IDA

The Moon is packed with all sorts of interesting features that only come to light — literally, in some cases — when very high-resolution imaging is done. For example, the lunar far side has a bunch of small volcanoes, some only a few hundred meters across, like this one:

[Click to enlunenate.]

The image is about 500 meters across, so this is a hill you could climb pretty easily, even though the low Sun angle implies the slope is greater than 13° (remember, the Moon has 1/6th the Earth’s gravity so that would be a pretty easy hike). Those boulders on the top are weird; they only appear to be on one side, and there doesn’t seem to be anything in that direction that would be a source of them. There are none on the plains around it, or at the bottom of a nearby crater, either. The source must be the volcano itself, I’d wager. Note the crater at the top of the mound, too – you might think that’s the volcanic vent, but in fact it’s not centered on the dome, indicating it’s a coincidental impact crater.

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I know I just posted a MESSENGER photo of craters, but this one is different and spectacular enough that I figure, why not? I love a big, splashy, wide-angle shot of a rayed crater! So here’s the lovely, 80-km wide impact crater Debussy on the surface of Mercury:

[Click to haphaestenate.]

Craters make rays when the ejected material blasted out forms long plumes which fall across the surface. On airless worlds, those trajectories are ballistic, heading straight out from the center of the impact. Deeper material tends to be a lighter shade than surface material, so the interior of the crater and the rays are lighter than surrounding surface stuff. You can also see what’s called the apron, the layer of material that falls immediately around the crater, surrounding it (that’s more clear in an earlier image of the crater looking more straight down on it).

Rayed craters are common (even on our Moon; take a look at Tycho!), and usually indicate the impact was recent (geologically), since the rays eventually get eroded by the solar wind, cosmic rays, and subsequent meteorite impacts. Debussy is therefore one of the younger features on Mercury. It still has that youthful shine.

Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The Dawn spacecraft entered orbit around the main belt asteroid Vesta just a few weeks ago, and images are coming back in dribs and drabs. NASA just released this fantastic one, taken by Dawn’s wide-angle camera:

[Click to protoplanetate.]

Pretty cool, eh? Vesta is about 500 km (300 miles) across, so you’re seeing about half the rock from top to bottom here. The most obvious thing are the two ginormous craters. Note the scale bar; the bottom crater is about 70 km across, and the top one about 50. The fact that they nearly overlap, and are clearly the two biggest features for a big area around them, makes me think it was a double impact. Many asteroids are binary, so two objects a few kilometers in size and orbiting each other 50 or more kilometers apart would do the trick^*. [Update: Emily Lakdawalla agrees.]

The bottom crater is weird; the bottom right edge looks like it’s collapsed a bit, marring the near perfect circle of the rim. That feature itself looks like an arc of a circle; might there have been a third rock that hit? That seems unlikely, and I have a hard time believing even a piece of the rim of a previously existing crater would’ve survived the impact!

Also, look around the two craters. See how far away from them, the surface is saturated with smaller impact craters? Near the big two, though, there are fewer. It’s a sure bet the impacts threw out a lot of debris which blanketed the area. The escape velocity of Vesta is a meager 350 meters/sec (about 750 mph); a lot of the stuff blown out on impact would’ve been moving faster than that! So some would’ve escaped the asteroid entirely, but some would’ve settled down over hundreds of square kilometers of area around the site.

Both crater floors have a filled-in appearance. No doubt a lot of energy of the impact went into melting the surface, which flowed inward. Some of that might be slumped material from the crater edges, too. My knowledge of how craters form is limited, especially on asteroids. But I’d love to see high-resolution images of this! That would answer a lot of these questions straight away.

And of course, we’ll be seeing those soon. Dawn is slowly moving down toward Vesta, gradually lowering its height until it settles into its final orbit. At that point, we’ll be getting really high-res shots, and maybe a few enigmas will be solved… only to be replaced, no doubt, by ten times as many. Fun!

[Edited to add: there is that biggish third crater at the top, of course, but I'm not sure it's related to the other two. It's much smaller, for one thing, and there are several other craters that size nearby that appear unrelated. It has a softer rim, implying greater age due to erosion (meteorite impacts), and isn't aligned with them -- though the sharp curvature of the asteroid makes that difficult to verify; in some images they look more colinear (that is, aligned).]

Image credit: Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

^* At first the size of the impactors was a guess, but then I poked around and found this crater diameter calculator. I put in values for Vesta, and found that the impactors would’ve been roughly 5 km across. My instincts, sometimes, are good.

Nature imitates art! Kinda!

I am endlessly fascinated by impact craters. You might think they all look alike, but they don’t. They have different shapes, structures, shading, even (sometimes) colors. And these features can tell us a lot about the object that caused the impact as well as the structure of the surface they hit.

For example, here’s a nifty crater on the Moon as seen by the Lunar Reconnaissance Orbiter:

That image is 500 meters (.3 miles) across, so this is a decent-sized hole [click it to enlunanate]. Note the rubble; that’s a clear indication that the surface of the Moon where the object hit was rocky as opposed to sandy. You only get fractured boulders like that when the impacted surface has some cohesion. Most of the Moon is covered with a layer of dust called regolith, but here, under that powdery surface, was rock.

I’m not a geologist, so that wasn’t the first thing I thought of when I saw this picture. What I did think of was how familiar this crater looked to me. And it didn’t take me long to figure out why…

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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. (more…)