Because the planets are so terribly old, and impacts so rare, I still have this (very slight) prejudice that craters are old too. The Moon was bombarded billions of years ago, and the craters on Earth are mostly so old that they’ve eroded away. Heck, even a "new" crater like the one in Arizona is tens of thousands of years old.
Getting the age of a crater can be tricky. But sometimes it’s so easy it’s literally a matter of keeping your eyes on one spot. Like this spot on Mars:
That image (highly color enhanced; click here for a grayscale version) shows a crater seen by a camera on the Mars Reconnaissance Orbiter in 2011. We can tell it’s young because it’s still surrounded by the ejecta blanket; material that blasted out of the crater and settled around it. That stuff tends to erode away (or get covered in dust and sand by Martian winds) relatively quickly.
But in this case, we know just how young it is: it wasn’t seen in images taken of the same spot by a camera on board the Odyssey Mars probe… in 2009! In other words, this crater is less than three years old!
That’s so cool. And it speaks to the power of having multiple, sustained missions to other worlds. Things change. If we take one picture and then walk away, we’ll miss a lot.
Image credit: NASA/JPL/University of Arizona
Large impacts are fascinating. There’s the thriller-movie aspect of them, of course, spiced with enough reality to make them legitimately scary. But the physics of them is equally enthralling, and complex enough that it will be a rich field for scientists to study for years to come.
The good news for both these aspects is our Moon. Seriously! There are enough craters there for anyone to be happy studying them, and since the Moon is a giant lifeless chunk of rock, impacts there seem less urgently threatening.
I want to show you two craters on the Moon that are very different, and therefore very interesting.
First up, Copernicus. Or more accurately, a small part of this 90+ km (55 mile) wide impact feature: its central peaks.
[Click to enselenate.]
This image was taken by NASA’s wonderful Lunar Reconnassance Orbiter. Copernicus is a big crater, and easy to spot even with binoculars since it sits in a vast lava plain; the surrounding material is darkish grey, while the crater is far brighter. It’s also surrounded by a gorgeous system of rays: linear streaks caused by the collapsed plumes of material after the asteroid or comet smacked into the Moon to form the crater itself.
Copernicus has a series of mountains in its center, the tallest over a kilometer high. These weren’t created in tectonic events like on Earth, though! Giant impacts that cause big craters have weird physics. The pressure upon impact can be so high that the rock in the surface flows like a liquid. It splashes outward, then flows back in, surging upwards in the middle of the impact point. This video showing water dropping into various surfaces might help:
Someday, Mars will stop surprising me.
Today is not that day.
The image below was taken by the HiRISE camera on the Mars Reconnaissance Orbiter, which has been taking devastatingly high-res pictures of the Red Planet for many years. While passing over the edge of the Tharsis Shield — a huge uplifted region of Mars home to its four gigantic volcanoes –it saw this bizarre fieldof craters:
[Click to hephaestenate.]
First, you may think these are mounds and not craters, but that’s an illusion. Our brain uses illumination to gauge up and down in pictures like these, and assumes the sunlight is coming from above. However, these really are craters, but the illumination is coming from below — north is roughly toward the top of the picture and the crater field is at a northern latitude of about 50°. Flip the picture over if it helps (I’ll be honest, even doing that makes it hard for me to see these as other than mounds; confounded brain!). You can see more examples of this illusion here, here, and here.
But that’s not the weirdest thing about these craters. What’s really odd is they aren’t circular! Impacts are generally round unless 1) the impact is at a very shallow angle, b) the terrain suddenly goes from one kind of material to another, creating a discontinuity, or γ) something happened after the crater was formed to distort it.
A shallow-angle impact is almost certainly not the case here, since there are so many craters spread out over the region that an incoming object would’ve had to break up into a gazillion pieces, all of which came in at that angle. Not impossible, but it seems unlikely.
The changing terrain idea doesn’t work, since again the craters are spread out over the area. You might see one crater with a sudden break in its rim or change in shape, but dozens? Spread out in all directions? Nope.
That leaves after effects, and in this case we have two more clues. Read More
I give talks about asteroid impacts quite often, and sometimes people ask me why we should worry about them. I reply, "Go outside and look at the Moon. Then tell me we don’t need to worry about asteroid impacts!" The Moon is covered in craters, and it really brings home — literally — the fact that we need to understand impacts better.
I’m not being facetious, either. Looking at the Moon is a great way to learn about craters. By measuring their size, position, and shape, we can find out a lot about the history of impacts in the Earth-Moon system. The problem is there are so many craters — billions, if you look at high enough resolution. How on Earth — haha — can any scientist or team of scientists possibly look at them all?
Well, it depends on how big the team is. Enter citizen science: non-professional-science people who nevertheless love science. If you’re reading my blog — and you are — then that means you! CosmoQuest.org is a group of astronomers, run by my friend Dr. Pamela Gay, who have created a series of projects where people like you can perform needed tasks that are real science… in this case, measuring craters on the Moon! Using MoonMappers, you can identify and measure craters using images from the Lunar Reconnaissance Orbiter, a spacecraft currently circling our Moon and taking thousands of high-resolution pictures.
I signed up and started right in, and find it somewhat addicting. You’ll need to register first through the CosmoQuest forum, which takes one minute and is free. Once you’ve done that, just go back to Moonmappers and dive in. I was able to identify dozens of craters in just a few minutes. Here’s a typical scene:
The blue circles are craters found using automated software. The green ones mark craters I found. The task is really simple: you can mark craters with your mouse, dragging the circle to match its size. If you miss a bit, you can easily adjust the circle’s position to re-center it. You only need to find craters bigger than 18 pixels in size, so it’s not an impossible chore! You can also flag odd features like linear cliffs, boulders, and so on, if you happen to see any. Several of the images I went through had them. One had lovely striations in an old lava flow, so you never know what you’ll see.
Sound like fun? It is! But hurry: right now, CosmoQuest has issued a Million Crater Challenge, to get 1,000,000 craters identified by full Moon, which is on May 5, just days away. As I write this they’re still a long way from their goal. How many can you find?
And remember: this isn’t just fooling around, this is real science. How are craters made? Why are they different shapes? How many are 10 meters across versus 20 versus 30 versus 100? All these questions are important in understanding impacts… especially that last one. Getting the scales of impacts, and how the numbers of them increase as the size gets smaller, is critical in being able to predict how often they happen. At some point, we’ll see a small asteroid headed toward Earth, and we’ll have to decide if it’s big enough to worry about and spend hundreds of millions of dollars deflecting it. The work you do here, quite seriously, can help inform that decision.
The MESSENGER spacecraft, orbiting Mercury for nearly a year now, took this pretty nifty shot of the tiniest planet’s south polar region, showing deep, dark craters in the Goethe basin:
This region is about 300 km (180 miles) from the true south pole of the planet. On Earth that might be a cold spot, but on Mercury, cold spots are hard to come by.
… however, see how dark those craters are? Since they’re near the pole, the Sun never gets far above the horizon for them, and the crater floors are shrouded in perpetual darkness. That does make them cold! Well below the freezing point of water, it’s thought. Interestingly, radar observations of Mercury have indicated something in the crater floors is highly reflective, and water ice fits that bill. It’s not at all confirmed, but it’s entirely possible Mercury — a planet hot enough in the open Sun where zinc can exist as liquid lakes on the surface — might have frozen lakes of ice locked in crater bottoms near its poles!
While gazing idly at this picture, another thought popped into my head. Read More
Last night, at 02:56 UTC, it was the 42nd anniversary of humans putting a bootprint on another world. Before Apollo 11 touched down on the Moon, though, NASA and the USSR sent a fleet of unmanned probes there. Since that time we’ve sent many more, including the Lunar Reconnaissance Orbiter, one of my favorite spacecraft of all time. It takes amazing high-res images of the Moon… and to celebrate today’s anniversary, they released this mysterious picture:
Cooool. Click to enlunenate.
This image is about 400 meters across, and shows an impact site with two lobes of material laid down to the sides. This butterfly-shape is a clear indication of a low-angle impact; it’s seen on many bodies in the solar system including the Moon, Mars, and even Earth (though the physics of exactly how the bi-lobed patterns form is still not well understood). Features like this are very rare… but it’s known that when a satellite orbit decays, it will impact at a low angle.
As the LRO site notes, in October 1967, the Lunar Orbiter 2 spacecraft impacted the lunar surface, possibly very near this spot. Could this be the final resting ground of an early NASA robotic explorer? It’s hard to say. When something hits hard enough to excavate material, it’s common to see ejected junk of different brightnesses, and here we see the dark patterns overlaid on a brighter surface. If that’s the impact area, though, the size of the impact looks too big for the mass and speed of the probe. Maybe it coincidentally hit a brighter area, but that stretches credulity, given the darker area all around.
So what happened here? The folks at LRO are planning follow-up observations to see if they can get pictures at a different Sun illumination angle, which will make any crater easier to spot. That might clear things up.
Or it might not. The Moon is the nearest astronomical object in the heavens by far, but it also has 38 million square kilometers of surface to explore! That’s four times the size of the Unites States… and LRO sees it at a resolution of roughly a half a meter. That’s a whole lot of pixels, and a whole lot of landscape in which to hide fun little mysteries. I hope there are many, many more.
The Dawn spacecraft is now in orbit around the main belt asteroid Vesta! Yay!
The spacecraft entered orbit around the main belt asteroid on Saturday, July 15. Two days later — today — it snapped this spectacular high-res image:
[Click to enprotoplanetate.]
Wow, what a mess! As expected, it’s littered with craters, but there are some interesting things to note. Some craters appear to be very deep, while others are shallow — that indicates a different type of terrain (asteroidain?) where the impactors hit (although in some cases it might be a lighting effect; a more direct sunlight angle makes craters look shallow). The grooves I mentioned in a previous post are everywhere, some looking more like scarps (cliffs) now. And look at that huge cliff on the upper right! I’ll be very curious to see that area at different angles. Is it part of a big basin, a collapse feature? Or is it a cliff caused by cracking in the surface? By the way, that lump in the center casting a shadow to the left is actually a mountain or mound of some kind well over 100 kilometers across.
The resolution is stunning; each pixel in the high-res version is about 1.4 km (0.9 miles) across — the asteroid itself is 530 km (330 miles) wide. Dawn is orbiting at a distance of 16,000 km (9900 miles; a bit more than the diameter of the Earth) and will slowly lower its orbit over time. Vesta’s mass is uncertain, so engineers played it safe and put it into a high orbit. This will allow an accurate mass to be determined, and then scientists and engineers can calculate how much thrust is needed to safely close in. That will take some time, about three weeks. During that time Dawn scientists will search the region around Vesta for tiny moons. None has ever been seen from Earth, but there’s nothing like being there.
… and I’ll add, we almost didn’t go. Back in 2005/6, this mission was actually canceled by NASA, causing quite the stir in the astronomy community. However, a strong voice was raised against this cutback, and Dawn was back on. After a long, long journey, it’s now where it belongs: in deep space, exploring, doing science, and expanding our frontiers.
I can hope the same will be true for JWST.
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.
What the heck hit Mars and made this?
[Click to barsoomenate.]
This image is from my favorite Red Planet paparazzo, the HiRISE camera on Mars Reconnaissance Orbiter. It shows three craters, each about 1.5 to 2 km (0.9 to 1.2 miles) across… and they all formed at the same time!
How can I tell? Well, for one thing, if this were a coincidence, with three impacts happening at very different times, then you’d see overlap in the crater rims; the earliest crater would be partially obscured by the later crater, and that in turn by the most recent impact. But that’s not the case here, since the rims aren’t overlaying each other. In fact, the straight walls between them are exactly what you’d expect if you have impact explosions happening simultaneously: the expanding shock waves smack into each other and create a linear feature.
Not only that, but let your eye follow the straight lines between craters up and down, above and below the craters themselves and onto the landscape. You can see that the hellish expanding wall of fire etched itself onto the Martian surface well beyond just the crater rims, and those linear features match the crater wall orientation. I annotated the image here to show you what I mean; the red lines are just outside the linear features.
I can picture what must have happened, millions of years ago over Mars…
Friday was the 25th anniversary of the loss of the Shuttle Orbiter Challenger, which I already wrote about as part of a post about Apollo 1 and Columbia. But I wanted to add that after that event in 1986, seven craters on the Moon were named after the astronauts:
This mosaic of LRO images is about 190 km wide, so these craters are actually quite large. Interestingly, these craters are themselves inside a much larger 524-km wide impact basin… named Apollo.
Image credit: NASA/GSFC/Arizona State University