Tag: crater

A penetrating, double-ringed crater on Mars

By Phil Plait | September 27, 2012 6:55 am

Mars is weird. Right? I mean, it’s a whole other planet. So you expect it to be weird.

But then I see pictures like this one from the Mars Reconnaissance Orbiter’s HiRISE camera, and I am reminded just how weird it is:

[Click to chicxulubenate.]

Most craters you see are pretty simple: something impacts the ground at high speed, BOOM!, and you get a crater like a dish tossed into soft sand. But this one has two rings, one inside the other. That can happen with huge impacts producing craters hundreds of kilometers across, but this one is small, only 230 meters from side to side – an American football stadium would just fit inside this crater.

The most likely explanation for the double ring is that the Martian landscape here is layered. There’s rock and sand on the surface, but underneath that is a layer of ice. The big rim is from the displaced rock, and the inner, smaller ring is from the impactor plowing through the ice. Each layer has a different strength – rock is harder than ice – so it’s as if two craters were formed, one inside the other. Radar observations of Mars from orbit have indicated there’s ice under the surface in this region, so that fits.

Similar double-ringed craters have been seen on Mars – though the structure and history is by no means well understood! – and some have been found on the Earth’s Moon as well. Those tend to be big, as I mentioned, though they don’t have to be.

By the way, the image above is color enhanced to show details. The blue may be from carbon dioxide frost, which can be seen in similar color-enhanced HiRISE images. The ripples in the center are sand dunes, sculpted into parallel waves by the ceaseless Martian wind.

Craters this small on Earth are extremely unlikely to form; the impactor would be maybe 20 meters or so across, and objects that size tend to break up when they ram through our thick atmosphere at high speed. Mars has much thinner air, so rocks that size can hit intact. Studying craters on Mars is a chance to see what these hypervelocity impacts are like under very different conditions, which helps us understand them. The physics of extremely high-speed collisions is hard to study experimentally – accelerating large objects to that kind of speed is both difficult and more than slightly dangerous – so it’s nice to have a lab like Mars where we can observe these effects.

Tip o’ the lens cap to HiRISE on Twitter. Image credit: NASA/JPL/University of Arizona.

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CATEGORIZED UNDER: Cool stuff, Pretty pictures

Excavating a long-dead lunar fire fountain

By Phil Plait | September 18, 2012 7:00 am

Although it may seem rather obvious now, for a long time there was a debate over craters on the Moon. Were they from impacts, or big volcanoes?

We now know that the vast majority are from objects slamming into the Moon. There’s a lot of junk floating around out there in space, and without an atmosphere the Moon suffers impacts from even the smallest chunk of rock.

However, you can’t completely disregard lunar volcanoes! Although long dead, they’ve been seen as well, though in far, far smaller numbers. And then, sometimes, you get a crater and a volcano just to mix it up. And I mean that literally.

Here is a very cool image from the Lunar Reconnaissance Orbiter showing just such a thing:

[Click to vulcanimpactenate.]

This odd-looking feature took me a few minutes to figure out, even after reading the description page. What you’re seeing is an impact crater about 170 meters (185 yards) across – the whole image is 450 meters (500 yards) in width. But what’s all that black stuff in the middle?

The picture inset here is an overview of the region (about 200 km across), and you can see the big blanket of darker material on the somewhat lighter lunar surface. The asterisk marks the location of the above picture. The dark stuff is pyroclastic – ash and other material that’s been blown out of volcanic vents in what are called fire fountains. In this inset picture it looks dark, but when it was fresh it was even darker. Over the eons, blasting by subatomic particles in the solar wind – think of it as cosmic erosion – has lightened it. Either that, or nearby impacts blew lighter-colored material over it.

Either way, it’s lightened up. But then something roughly the size of a tennis court, probably an asteroid, slammed into the middle of that region. It churned up pyroclastic material under surface, excavating it. This material still retains its fresh dark look, so the surrounding landscape looks positively gray in comparison.

Mind you, this is only one small part of a much larger image showing this region in high-resolution. I really suggest you take a look at the full image, where you can scan and zoom and really see what’s what. Seriously, it’s an amazing swath of lunar real estate. It’s very flat – as you might expect from an area inundated with volcanic flow – but peppered to near-saturation with small craters. This one crater with the dark material in it really stands out, as much due to its size as to its color and composition. It’s quite lovely.

Oh, that Moon of ours. We’ve sent twelve humans there, and dozens of various probes, and there’s still a whole lot left to explore.

Image credit: NASA/GSFC/Arizona State University.

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CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

A lunar crater is graben the spotlight

By Phil Plait | September 11, 2012 7:00 am

I am endlessly fascinated by the Moon. There may be an inherent bias there because it is, after all, the closest astronomical object in the sky. Still, it has an amazingly varied surface with lots of really odd features.

One of my favorite types of things to look at are overlapping features. It can produce a very complicated terrain, difficult to understand. Or can also create a lovely tableau that cleanly separates the two features, like this very pretty shot from Lunar Reconnaissance Orbiter (LRO) showing a fresh crater near a graben:

[Click to enlunenate.]

A graben is a crack or fracture. They form on the Moon when the crust is stretched, splitting the surface. They look like long, relatively straight and narrow valleys with steep sides. You can only see a part of it on the right side of the image above; the Sun is shining from the right and illuminating the left-hand side of the graben. The picture below is zoomed out and should help you see the situation.

The crater is clearly younger than the graben feature. The radial streaks around the crater are called rays, and are formed when plumes of material ejected from the impact fall back down to the ground. They’re common around young craters; solar wind, later impacts, and even thermal compression and expansion of rocks over the Moon’s day-night cycle eventually erode them away.

You can see the rays extended over and into the graben, so the crater must be younger. It’s hard to say just how much younger, but even relative ages can help geologists understand the lunar surface better. And detailed images like this – you can see individual blocks of rock inside the crater itself – are crucial for study. Someday, I think, human geologists will be investigating places like this in person, and mapping missions like LRO will make that possible.

Image credit: NASA/GSFC/Arizona State University

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MORE ABOUT: crater, graben, LRO, Moon, rays

The faces of Licinia

By Phil Plait | September 6, 2012 6:35 am

In April, 2012, the Dawn spacecraft was a mere 272 kilometers (170 miles) from the surface of the asteroid Vesta when it took this wonderful picture of the crater Licinia:

[And oh yes, you want to click to envirgingoddessenate – it links to a picture of the area around the crater as well.]

Licinia is about 25 km (15 miles) across – too big to fit in Dawn’s field-of-view from that height. But it does show spectacular detail, including what look like landslides into the bowl from the crater rim; you can see them as dark streaks running down the crater wall. Mounds of material at the base of the crater wall indicate bigger landslides, too. Vesta’s gravity is far weaker than Earth’s – it’s about 1/40th what we experience here – but even then, it’s a force that won’t be denied.

While I was inspecting the crater floor, I saw something that made me laugh out loud. The floor is lit by the distant Sun, but a sharp shadow of the crater rim is cast on it as well. The inky black shadow is irregular due to the uneven crater rim. Inset here is a piece of that shadow line. Do you see the dark shadow "face" looking to the left at the top? It jumped right out at me… and then I saw another face just below it, this time bright and looking to the right and slightly up!

Once you see it…

Man. Pareidolia is a force almost as strong as gravity.

Anyway, Dawn’s visit to Vesta has come to an end. It left the asteroid on September 5, and began the long two-and-a-half year voyage to visit Ceres, the largest of the main belt asteroids between Mars and Jupiter. Vesta is a fascinating place, but so is Ceres, and we know very little about it.

That’s all about to change. But then, that’s what exploration is for.

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MORE ABOUT: crater, Dawn, Licinia, Vesta

Mickey Mouse MESSENGER Mercury

By Phil Plait | June 15, 2012 8:58 am

MESSENGER is a spacecraft that’s been orbiting Mercury since early 2011, sending back to Earth huge amounts of data about the tiny planet, including incredible high-resolution close-up images. It’s an amazing mission…

… but I wonder what kind of Mickey Mouse outfit would put up this kind of image for display?

[Click to enmusculate.]

Well, at least I know to whom NASA can turn if the current budget cuts get through Congress.

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

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Desktop Project Part 1: A weird Moon crater

By Phil Plait | March 26, 2012 7:00 am

[Over the past few weeks, I’ve collected a metric ton of cool pictures to post, but somehow have never gotten around to actually posting them. Sometimes I was too busy, sometimes too lazy, sometimes they just fell by the wayside… but I decided my computer’s desktop was getting cluttered, and I’ll never clean it up without some sort of incentive. I’ve therefore made a pact with myself to post one of the pictures with an abbreviated description every day until they’re gone, thus cleaning up my desktop, showing you neat and/or beautiful pictures, and making me feel better about my work habits. Enjoy.]

First up in my Desktop Project is a weird crater on the Moon, seen by the Lunar Reconnaissance Orbiter:

What a strange little thing! [Click to enlunenate.]

It’s about 140 meters across the rim, and it’s located in Plato, a big, relatively flat walled plain — basically, a crater that got mostly filled in with lava long ago — about 110 km (70 miles) across. You can see rubble and other debris scattered around it (in this image, sunlight is coming from below and to the left), and the interior is just odd.

This is called a bench crater, where you get roughly concentric features inside the crater itself. It’s probably from a high-velocity impact by a small (5-meter or so) asteroid, and the terrain where it hit probably has a thin layer of compacted regolith — the powdery surface material covering a lot of the Moon. This loose material blasted out more than the harder rock below, so you get this weird two-tiered structure.

Craters can be pretty complex; you might think you just get bowl, but in fact the impact speed, angle, the terrain, and the overall size of the impactor make a huge difference in crater structures.

Also? The first thing I thought of when I saw this picture was that it looked like the plaster cast they made of the giant ant footprint in one of my favorite movies of all time, "Them!" And that makes me a bigger dork than you can ever hope to be.

Image credit: NASA/GSFC/Arizona State University

CATEGORIZED UNDER: Astronomy, Pretty pictures

The Moon sleighs me

By Phil Plait | December 24, 2011 5:16 pm

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

Dawn dips down to Vesta

By Phil Plait | December 21, 2011 10:43 am

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

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MORE ABOUT: Ceres, crater, Dawn, Vesta

Tiny lunar volcanoes

By Phil Plait | December 12, 2011 11:19 am

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.

Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures

Clair de Mercury

By Phil Plait | September 30, 2011 6:30 am

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

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