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…)

When an asteroid or comet impacts a planet, the explosion ejects huge amounts of material, sending it flying in all directions. But there are also plumes of material, long fingers of rock and dust that stream out as well. The boulders and such inside this plume then fall back to the ground, making linear chains of secondary craters. We see lots of these on our Moon, moons in the outer solar system, and Mercury, too.

If these features are long enough, it’s inevitable two chains from two different primary craters would cross somewhere. And it turns out this has been seen… but where?

Well, X marks the spot!

This MESSENGER image of Mercury shows exactly that: two crater chains from two separate impacts crossing over each other (and a third, shorter chain is at the bottom, too). They’re almost exactly perpendicular to each other, which is cool, and the intersection happens to lie in a big, shallow crater about 120 km (72 miles) across that fills this image. Unfortunately, MESSENGER hasn’t been orbiting Mercury long enough to have surveyed the whole planet yet, so I wasn’t able to find the source craters of these two chains.

Interestingly, both chains have elongated craters at their ends, one on the upper left and the other at the top. That indicates a very low-angle impact; anything hitting the ground from an angle above about 10° tends to make a circular crater. However, the one on the left appears to be right on the big crater’s rim, so the elongation may be due to the ground angle changing. The other may be coincidence; both are far too small to have been the source craters for the chains.

I’m not sure there’s any real scientific value in knowing these crater chains intersect or examining the intersection in detail. Still. They’re fun to look at, fun to explore, and they’re just seriously nifty.

UNLESS… hmmm.

This is a pretty neat picture taken by the Lunar Reconnaissance Orbiter: two craters, side by side:

[Click to impactenate.]

What’s cool about it is the obvious age discrepancy between the two craters. The Moon lacks water and air, but it has erosion nonetheless: micrometeorite impacts, solar wind, and even thermal stress cause by the month-long day night cycle slowly wears away at the surface. Old craters have a rounded look to them, while fresher craters are sharp-edged, and show the debris from impact.

The full-res image has a scale of just a meter per pixel, so a lot of the smaller boulders you see around the younger crater on the right are the size of cars. Both craters are roughly 300 or so meters across; you could walk briskly across them in a couple of minutes.

I noticed the young crater has an odd shape, non-circular, almost diamond-shaped. Then I looked at other, nearby craters, and saw the same thing, so it must mostly be due to lighting. However, there is a funny hillock just to the right of the crater, and the boulder field around it is not symmetric; there are more above and below it. I wonder if there is a density change in the underlying rock just to the right of the crater, which helped shape the crater…?

That area is mostly flat lava flood plains, and in the zoomable and pannable larger-area context image there are some interesting features that look like very old crater rims poking up through the plain. Check it out! One of my favorite things about LRO is the pile of high-res pictures like this one you can zoom in an out of. It really helps give you a feel for what you’re seeing.

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Speaking of weird impact craters on Mars…

Mars Express is a European Space Agency orbiter that’s been snapping away at the Red Planet since late 2003. In August 2010 it took this picture of a bizarre feature on Mars:

[Click to impactenate.]

I would’ve thought this was a canyon of some sort, but in fact it’s an elongated crater! Most likely some large object broke up as it entered the atmosphere of Mars, striking the surface at a low angle and creating a series of craters that merged to form this strange thing. Unlike the triple crater I mentioned last time, this one is pretty frakkin’ big: it’s 78 kilometers (almost 50 miles!) long, 10 km (6 miles) wide at one end and 25 km (15.5 miles) wide at the other. Whatever hit here was pretty big, certainly over a kilometer across before it broke up. Probably several.
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When you look at the Moon, you see a surface covered in craters. Yet the Earth, which is bigger and has more gravity — and therefore, you’d think, be hit more often than the Moon — hardly has any craters!

The difference, of course, is that the Earth has weather and tectonic activity. Craters erode, and over time go away^*. But not all of them do. Some are so big they take hundreds of millions of years to erode, while others are in dry climates where erosion is limited… like, say, in Algeria. Where the Tin Bider crater lies!

This picture, from the Earth Observatory-1, shows the roughly 6 km (4 mile) wide crater, located in the high desert of northern Africa. It has a complicated terraced structure, indicating that the rock inside may have slumped after impact — a common feature in larger craters. It has undergone some erosion, too… not surprising, given its age of about 70 million years!

There’s an interesting thing about this crater. North is up in the picture, so the sunlight is coming from the south, from the bottom of the picture. Regular readers of this blog know that this induces the well-known crater/dome illusion (another example can be found here). Our brains expect light to come from above, so when it comes from below the shadows send mixed signals to our brains, and we interpret craters as domes and vice-versa.
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I have a Martian mystery for you today, and one that is writ quite large and dramatically. It seems weird at first, then simple next, but when you dig deeper — literally — things get very weird indeed.

It all starts with an out-of-control awesome picture that honestly made me reel back and say "Wow!"

I present to you out-of-control awesome:

Wow!

Click the pic to embiggen. This unnamed crater is about 700 meters (roughly half a mile) across, and sits in the northern mid-latitudes region of Mars. It’s interesting, isn’t it? The multiple concentric bowls of the crater are trying to tell us something, but what?

My first thought, also mentioned on the HiRISE blog, is that this is a coincidental double impact: the big terraced crater was the original impact, then a later, second object impacted almost exactly in the center of the older one, hitting the bulls-eye like William Tell splitting an arrow.

The topography seems to support that; the inner crater has a raised rim, as you might expect from a second impact, and that would be hard to explain in a single impact. The terracing — shelf-like structures sortof like an upside-down wedding cake layering — is seen sometimes when an impactor smacks into layered ground. Imagine a layer of dirt on top of ice on top of rocks: each layer reacts differently to the impact, leaving the circular, concentric shelves in the crater bowl.

Note too that the central crater doesn’t look exactly centered, supporting a second impact.

Case closed… but wait, Your Honor! We have a surprise witness!
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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. (more…)