The entire universe in blog form

Sept. 22 2014 11:23 AM

Two Impacts, One Landslide… on Mercury

When I was a kid, I used to love looking through my telescope at craters on the Moon. Their size, shape, and subtle (and not-so-subtle) differences fascinated me, and still do.

Those differences still pull at me. Some craters are just odd, and that oddness is generally a clue to how they formed… or what happened after they formed.

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Mercury is Moon-like in many ways: It has a solid, rocky surface, and no atmosphere to speak of. That means it’s covered in craters, and that in turn means some will have that fun weirdness. Here’s one for your impactful enjoyment:

Mercury crater
Crater upon crater upon crater.

Photo by NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

That image above was taken on July 19, 2014 by the ever-vigilant MESSENGER spacecraft, which has been orbiting the closest planet to the Sun for the past three years. The image is about nine km (5.5 miles) across, and shows part of a largish crater. The Sun was only about 20° above the local horizon (shining from the left), so the crater rim shadow covers most the floor. You can just see part of a central mound peeking out from the shadow (common in larger craters, due in part to material in the center flowing upwards, rebounding, after the initial impact).

You can see smaller impact craters dotting the crater, but right in the middle of the big crater wall is the weirdly shaped crater. Here’s a close-up:

crater close up
Don't worry about it — we all go through a slump sometimes.

Photo by NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

At first you might think that feature in the middle with the wide, humped rim is the crater, but it’s not. In fact, the impact is the overexposed round feature just upslope a bit on the big crater’s wall. Impacts inside previously-established craters are common on airless worlds like Mercury, since they get hit a lot. But this one is different because it looks like the event triggered a massive landslide.

The impact was huge; when the asteroid or comet slammed into the older crater’s wall it exploded with a yield far larger than any human-made nuclear weapon, leaving a pit a kilometer cross. The energy must have shaken the ground mightily, dislodging the material around it. That caused the huge runoff (what geologists call a “mass wasting event”) that flowed down the bigger crater’s wall, creating that wide crescent-shaped feature when it finally stopped.

You can also see a much wider fan of material spreading out around the crescent, getting wider toward the big crater’s floor. I suspect that’s lighter debris that flowed around the other debris. That very upper edge of the fan is interesting; it appears to be bright on the inside (toward the bottom of the frame) and darker on the outside. Given the Sun shining from the left, that makes it seem to me that the edge is actually a raised ridge; the inside edge lit by the Sun, the outside edge slightly in shadow. The resolution of the picture on that scale is low, so I can’t be sure. But that all seems to hang together.

What an interesting spot! Geologists like craters because they don’t have to dig down to sample older material; the impacting asteroid did that work for them. And here we have another crater exposing the ground underneath that, and a huge landslide on top that tells them even more about the consistency and constitution of the surface material. It’s a scientific bonanza.

Now all we need to do is get a geologist there to poke around. I wonder how long it will be until a human wanders the surface of this tiny and terribly hot world, exploring the amazing sights there…?

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Sept. 22 2014 5:30 AM

MAVEN Arrives at Mars

Mars has acquired a new moon: the Mars Atmosphere and Volatile EvolutioN spacecraft entered orbit around the planet at 02:26 UTC Monday morning.

Man, I never get tired of writing a sentence like that.

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MAVEN spent just over ten months plying the vacuum of interplanetary space after its launch in November of 2013. As it approached Mars, it fired its engines for a total of just over a half hour, changing velocity enough to transfer from a heliocentric orbit into one about Mars. The scale of such an accomplishment is hard to overstate; it’s an incredible achievement, and one we’ve done over and again.

And now MAVEN’s real job will begin. Oh, there’ll be a systems checkout for a while, making sure everything is working up to snuff. But then the science starts: MAVEN will analyze the atmosphere and space environment of Mars, its primary mission to understand just why Mars used to have a thick atmosphere, and no longer does. It’s understood that over the eons, the solar wind has eroded away the planet’s atmosphere — Mars lacks a strong magnetic field, which can deflect away most of the onslaught of fast subatomic particles from the Sun; Earth's relatively strong magnetic field is why its atmosphere has managed to stick around — but the details aren’t well determined.

And, for free, we get some very cool science by coincidence. On Oct. 19 the comet C/2013 A1 (Siding Spring) will pass very close to Mars, and may very well interact with its atmosphere. MAVEN will have a front row seat to this event, and since it’s designed to study Mars’s air, it will be the perfect spectator for this event. What will we learn?

I’ll note that in just a day or so we’ll get to go through this again: India’s Mars Orbiter Mission is expected to arrive at the planet on Wednesday at about 02:00 UTC. Amazing, that we have two spacecraft arriving at a planet within days of each other.

Any time we send a probe into orbit around another planet, it’s cause for celebration. And with MAVEN we’ll get insight into Earth’s sister-that-isn’t. Why did Mars lose its air? What happened to the water on the planet? How does this affect what we know of Earth’s history, present, and future?

These are not small, trifling questions. These are among the biggest we can posit — we’re asking about the history of an entire planet — and they can be answered thanks to human curiosity, to human spirit, and, of course, to science.

I love this stuff.

Congratulations to NASA, to Lockheed Martin, and to everyone on the MAVEN team.

Sept. 21 2014 8:00 AM

An Astronaut’s Guided Video Tour of Earth

I love astronaut photography of Earth, especially the dizzying and psychedelic time-lapse videos. I’ve wondered, though, why we don’t see more straight-up plain old video of the Earth as the International Space Station passes over?

Well, here you go: I love this short tour of the Earth, narrated by astronauts Mike Hopkins and Rick Mastracchio as they pass over some pretty familiar landmarks:

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It’s fun to see places I’ve visited (or even lived, like Houston and San Francisco) slowly slide past the view. Of course, “slowly” is just perspective; the ISS orbits at eight kilometers per second—18,000 miles per hour! But it’s also several hundred kilometers up, and distance does tend to change frame of reference.

And it’s not hard to see that video like this puts to rest the silly urban legend that the Great Wall of China is the only human-made structure visible from space. Cities, farm lands, dams, and even bridges are easily spotted, even without the zoom lens. Even better: The Great Wall is actually pretty hard to spot from orbit! It’s not actually that wide, and doesn’t have a lot of contrast with the surrounding land, making it difficult to see.

Great Wall
Well, you can see the Great Wall from space ... if you use radar, like in this shot from the Spaceborne Imaging Radar flown on Endeavour.

Photo by NASA

Don’t believe everything you hear. Or see. Or anything, actually: Always look for evidence. What you find might disappoint you at first, but it’s always good to learn something new. And you might stumble on something really cool … like photos taken by space-traveling humans as they fall endlessly around a blue-green world.

Sept. 20 2014 7:00 AM

The Shaggy Sun

The Sun has been pretty active lately, popping off a series of fairly powerful X-class flares; they've been generating aurorae and other magnetic phenomena on Earth.

But even before it started spouting off it was still busily doing Sunny things. Recently it was almost as if it was posing for photographers, and when you get someone like Alan Friedman—who knows his way around a solar photo—you get beauty and majesty.

alanfriedman_sun_sep12014
Yeah baby, yeah!

Photo by Alan Friedman

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That shot was taken on Sept. 1, 2014, and shows a portion of the Sun. I know, it looks weird, doesn’t it? He uses a filter that only allows through a very narrow slice of color emitted by warm hydrogen, and this tends to emphasize the gas under the influence of magnetic fields. He also does something tricky: He uses the negative of the image to enhance details.

So the bright spots are actually dark sunspots! The few you see in this image are roughly the size of the Earth, if you want your sense of self-importance vaporized today.

The other features are prominences, filaments, and, apparently, a shag carpet covering the Sun that it still has from the 1970s. All of these features are magnetic in nature. The Sun’s extremely complicated magnetic behavior starts deep within, with magnetic field lines connected to moving cells of ionized gas (called plasma). As these huge packets of gas rise to the surface through convection, their magnetic fields lines move plasma around on the surface. The lines can also get tangled up and then snap, releasing their energy as solar flares.

The gas packets cool when they reach the surface, but sometimes the magnetic field acts like a net, trapping that gas. It can’t sink, and it cools more than the surrounding gas, so it looks dark in comparison. Sunspots!

All this information is hidden unless you look at the Sun in just the right way, examine what it’s doing by slicing up and dissecting its light as it reaches Earth (or space-based observatories). When you do, you get knowledge, as well as profoundly beautiful (and, sometimes, profoundly odd) portraits of our nearest star.

So go take a look at more of Alan's photos. Trust me: You'll be glad you did.

Sept. 19 2014 11:33 AM

The Curious Incident of the Supernova in the Nighttime

Detective Gregory: “Is there any other point to which you would wish to draw my attention?”
Holmes: “To the curious incident of the dog in the night-time.”
Gregory: “The dog did nothing in the night-time.”
Holmes: “That was the curious incident.”

— “Silver Blaze,” by Arthur Conan Doyle

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Sometimes, you learn more about something when it isn’t there.

In January 2014, the light from a distant supernova reached Earth. It caused some excitement because the star that exploded was in the nearby galaxy M82 and that meant it was within reach of even small telescopes—in fact, it was discovered using an amateur-sized 35 cm telescope being used by an undergraduate astronomy class. Using my own telescope, I saw this supernova at the eyepiece as well.

Even better, the supernova, called SN 2014J (the 10th one discovered in 2014), was a special one: Type Ia, the kind used to measure the expansion of the Universe. We can see them very far away, and they’re used as benchmarks to calibrate distances to extremely remote galaxies. Seeing one nearby allows us to better understand them, and therefore better understand the size and expansion of the Universe itself.

Because the supernova was in a nearby, well-studied galaxy, we have lots of observations of the area before and after the explosion. The mighty orbiting Chandra X-Ray Observatory was pointed at M82 after the supernova went off, and found something surprising: nothing.

The image shows the galaxy in X-rays—extremely high-energy light emitted by black holes, very hot gas, stars being born, and, sometimes, supernovae. The little box marks the location of the star, and inset are enlargements; on the left is centered on the region before the star blew up, and on the right is the same area after the supernova. As you can see, there’s nothing there.

That’s interesting! We know that this type of explosion is caused by a white dwarf, the extremely dense core of a star that was once much like the Sun but has since shed its outer layers (much like the Sun will do in about 6 billion years after it becomes a red giant). There are several ideas about what happens next. It’s possible the white dwarf siphons material off a nearby companion star; that stuff piles up, gets very hot, and then fuses like a gigantic nuclear bomb, shredding the star and creating a very large bang indeed.

Another model is that two white dwarfs circle each other and, over billions of years, eventually merge. They collapse into an even-denser neutron star, and again you get a very large explosion. There’s even a third idea that there are three stars involved, two white dwarfs and a “normal” star, and the interaction between them causes a direct, head-on collision between the two dwarfs, causing the explosion. All three ideas have their merits, and astronomers are still arguing over them.

Chandra image of supernova.
Then you don’t see it, now you don’t.

Photo by NASA/CXC/SAO/R.Margutti et al.

What makes SN 2014J critical to this is that if it were a white dwarf sucking material off a binary companion star, we’d expect some of that gas to get flung out into space; white dwarfs, apparently, are sloppy eaters. But when the dwarf explodes, the blast wave would slam into that material, and the interaction should generate copious X-rays. Yet none is seen.

And that’s why this is so interesting. It would appear the normal star binary companion model doesn’t work for 2014J … at least, without some other event going on, like perhaps a whole bunch of smaller pre-supernova eruptions that cleared the region of gas. That’s possible, but I prefer not to have to resort to special circumstances when other explanations are also likely.

The next step is to continue taking more observations. The debris from the explosion is still screaming outward, and will continue to do so for years. As it expands, any gas out there will get plowed, and Chandra should see it. Until then, though, we must be much like Sherlock Holmes, looking at the evidence that isn’t there as well as that which is.

Sept. 18 2014 7:30 AM

Red and Green Ghosts Haunt the Stormy Night

Randy Halverson is a photographer and has quite a gift for time-lapse video (I’ve featured his work many times on the BA blog).

He recently sent me a video he took in central South Dakota that is quite astonishing. While photographing a storm at night, he caught two very rare events at the same time: sprites, and gravity waves rippling through airglow!

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First, here’s the video, because it’s amazing.

Did you see the sprite? It happens just before the six-second mark in the video. Look to the right, just above the storm cloud. The red flash is obvious once you spot it.

Sprites are a phenomenon associated with lightning storms; they’re electrical discharges from the top of the storm cloud, not the bottom. They’re not really understood, but they occur simultaneously with lightning between clouds or from cloud to ground, and glow eerily red. They were only first discovered in 1989 (officially, that is; pilots have been reporting them for decades, but no one outside that cadre really took them seriously) so not a huge amount is known about how they operate.

red sprite
Close-up on the red sprite. Click to enfairyenate.

Photo by Randy Halverson, used by permission

On a larger scale, you can also see green ripples moving across the sky. The green is airglow, molecules in the upper atmosphere that are energized by the Sun during the day, and give off that energy as light at night. This occurs via chemoluminescence; a process where the excited nitrogen and oxygen atoms molecules bump each into other and form bonds, giving off that light.

The rippling is due to gravity waves. This is simply an up-and-down oscillation of something under the influence of gravity. For example, waves on the surface of the water in your bathtub are gravity waves; the water gets pushed up a little bit (maybe when you plop your rubber ducky into the water), and then gravity pulls that crest of water back down. But the water itself pushes back, and you get oscillatory motion.

This can happen in air, too (air is a fluid, after all). Currents of air in the upper atmosphere bob up and down pretty often, similar to the water in your tub. This motion can disturb the process that creates airglow, so you get those rippling waves moving across the sky. I only recently learned about this phenomenon, when I saw a time-lapse video taken in Chile, a world away from South Dakota.

Several years ago, Randy sent me an email asking about a weird rippling glow he saw in some footage he had taken. I wasn’t sure at that time what it was (though I was sure it wasn’t an aurora), and we agreed it could be airglow. It’s funny that he would send me this new video right after I finally learned what that rippling was! If Randy had asked me two weeks ago I wouldn’t have known. He found out on his own, and now we both understand. And I hope now you do too.

The sky above us is just incredible. There’s still so much to discover, so much to figure out, so much to explore. And it’s all right there, just over our heads.

Sept. 17 2014 11:18 AM

A Bridge Across the Sky

A couple of months ago I spent a few days in western Colorado, far away from city lights. I was with a group of science enthusiasts for Science Getaways, taking a week long vacation chock full o’ science.

That part of the country is blessed with dark skies and fantastic weather, and I took my telescope out every night to view the heavens. But just standing there and looking up was surpassingly moving. The Milky Way loomed large over the land, stretching across the sky, so bright it was a palpable thing.

A half a world away, that feeling was captured perfectly by astrophotographer Amirreza Kamkar, who took this incredible picture in Abyaneh, Iran:

Sept. 17 2014 7:30 AM

Ring Around the Rainbow

The basic physics of a rainbow is pretty simple, really. Sunlight enters the front of a raindrop. It hits the back side of the drop, reflects, and then heads back out the front side again. The light bends a little bit when it enters and exits the drop—scientists get all fancy and call this refraction—and different colors bend by different amounts. The light bends change direction by a total of about 138°, but red light bends a little less, blue light a little more.

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So, to see a rainbow, you face away from the Sun (180°), then look about 42° away from that point (180°–138°). The drops in an arc along that angle will then bend the light back toward you, and you get a rainbow, with the colors spread out a bit because they bend by different amounts.

Oh, wait. Did I say “arc”? Because technically, any raindrop 42° away from the anti-solar point (ooh, fancy science-speak again) will bend the light back to you. We see rainbows in the sky because in general the ground is close to you. When we look up toward the sky we see for a long way, and there are lots of raindrops along your eyeline that can add their light together to make the rainbow. When you look down, the ground gets in the way, there aren’t as many drops, and you don’t see a rainbow.

In general. Not always. For example, you could be in an airplane when it’s raining. If the Sun is in the right spot, and you look out the opposite side, you could in fact see a rainbow making a full circle …  just like this:

circular_rainbow
Incredible full circle rainbow (and secondary, too) seen from an airplane.

Source: oskarslidums on reddit

Oh, wow! I knew scientifically this was possible, and I’ve heard pilots say they’ve seen them, but I’ve never actually seen a picture of one. That’s amazing.

Again, every raindrop in the air that’s 42° away from the spot in the sky opposite the Sun can bend the light back toward you. All those drops are the same distance from that point, and that defines the edge of a circle.

Even better, there’s a secondary bow, due to light reflecting twice inside each raindrop. That reverses the colors, so the secondary bow goes red to blue inside to outside. You can also see Alexander’s dark band in between them; drops located in that region bend light away from you, so it looks darker. Inside the primary bow it looks brighter because drops there bend light at you. The colors bent by drops there all overlap with each other, adding back together to create white light, so inside the primary bow there’s no color. I describe how all this works (with links) in a previous post. A little searching turned up another shot of a circular rainbow from the air, though it's not quite all in the single picture. It shows good supernumerary arcs, though.

If you look carefully, the shadows on the ground inside the rainbow all seem to point to its center. That’s just geometric perspective; since the center of the rainbow has to be 180° around the sky from the Sun, shadows have to point in that direction!

By the way, you can get higher order rainbows; I learned just recently that the tertiary bow is in the direction toward the Sun, the fourth just outside it (and is very faint), while the fifth and sixth are just outside and inside the primary arc, respectively. I’d love to see those!

Happily, in Boulder, Colorado, we do get more than our fair share of bright rainbows. I checked my pictures from the magnificent double rainbow we had the other day (the one where I got the super-lucky shot of lightning going across it!) and played with the contrast, brightness, and color saturation. I didn’t see the fifth and sixth order bows, but now that I know they can be there, I’ll keep trying for them. And at least I got this fun, if garish, shot out of it.

rainbow
Enhanced colors make this rainbow picture a bit gaudy, but I kinda like it.

Photo by Phil Plait

And the next time I’m in a plane looking out the anti-solar window, I’ll keep my eyes open for more than just glories

Sept. 16 2014 7:30 AM

A Galaxy of Tatooines

Using an old-style observing technique, astronomers have come to a very interesting conclusion: About half of all exoplanets in the galaxy are in binary star systems.

That’s astonishingly cool. Mind you, it’s not unexpected, but now we have some evidence for it.

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Here’s the deal. We know that roughly half the stars in the galaxy are in binary systems, with two stars physically orbiting each other. The circumstances that give rise to star formation make it pretty easy for two stars to form near each other (the Sun, of course, being an exception).

We also know that planets are pretty common around stars as well; we know of nearly 2,000 such exoplanets at this point, and thousands more are waiting to be confirmed. It’s natural to put these two ideas together, and ask: How many exoplanets orbit a star in a binary system?

You’d expect the ratio to be the same, but Nature is tricksy sometimes, so it’s best to check. However, that can be tough. Even though a binary companion might orbit a star billions of kilometers out, from light years away the two stars blend into one blob. Even Hubble might not separate the two.

To see what they could see, the astronomers relied on an old method called speckle interferometry. Basically, the Earth atmosphere roils over our heads, with little packets of air flying this way and that. When light from a star passes through them, it gets bent this way and that as well due to refraction. This happens many times per second, so when you take a long exposure the light blurs into a disk. Astronomers call this (confusingly) “seeing.” It’s also why stars appear to twinkle.

But there’s a way around this. Over time the light rays get smeared out, but if you take lots of extremely fast exposures, you freeze that motion out. It’s like taking super slo-mo video. Instead of a big disk, you get a bunch of near-perfect images of the star that jump around in location from image to image, but each frame is a nice, extremely high res shot of the star.

There are limitations to this technique; the star can’t be too faint, or else you won’t get enough light in each frame. There are also limits to the resolution due to telescope size, too. And you need a very sophisticated technique to combine the resulting frames to tease out all the information in them. But speckle interferometry has been around a long time, and its methods are solid (here’s a detailed and technical description).

The astronomers observed a bunch of stars known to have planets (found by the Kepler mission), and then used speckling to see how many had detectable stars very close by. They then used some pretty nifty simulations to find out how many binary companions they might miss—some might be too close to the star to separate even using these techniques. By comparing the two results, they were able to conclude that 40–50 percent of all exoplanets orbit a star in a binary system.

That’s pretty amazing. There are billions of planets orbiting stars in binary system in our galaxy!

What I find most interesting about all this, oddly, is the mundanity of it. Back in the day, we didn’t even know if other planets existed. Was there something special about the Sun that allowed it to host a solar system? Size, chemical content, position in the galaxy? But now we see that a wide variety of stars have planets. Even having another star orbiting the host star doesn’t seem to be a problem!

... well, as long as the other star is far enough out. Closer in, and its gravity could disrupt the planet orbit. But if the stars are close enough together, a planet could orbit both stars. Quite a few such circumbinary exoplanets (I love sciencey words) have been found. I’ve written about a few of them, like Kepler 16b and PH-1b.

Forty years ago this picture was science fiction. Now … well, now not so much.

Tatooine double sunset
Cue the french horn and the wavy hair.

Photo from LucasFilms

Sept. 15 2014 11:00 AM

The Comet and the Cosmic Beehive

In mid-October, the comet C/2013 A1 (Siding Spring) will have a very close encounter with Mars. It will pass just over 130,000 km from the Red Planet; while engineers have been working to make sure our spacecraft are safe from debris, scientists are eager to gather data about the comet using those same spacecraft.

In the meantime, the comet has become a favorite for astrophotographers, because it’s relatively bright and has been passing by one astronomical gem after another. Marco Lorenzi is one such astrophotographer. He has a remotely-operated observatory outside of Coonabarabran, Australia, and on Aug. 30, 2014, took this stunning shot of the comet as it passed by one of the most beautiful objects in the sky: the globular cluster 47 Tuc.

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