Bad Astronomy
The entire universe in blog form

July 26 2016 9:00 AM

Asteroid Collisions Create a Forbidden Crystal

Crystals are pretty. They’re also pretty interesting. They’re found in nature in stunning variety, including all kinds of bizarre shapes. I find a lot of these shapes pleasing aesthetically due to their symmetry. Some are box-shaped, some hexagonal… but they’re all fascinating.

Crystals get this symmetry because of the way atoms interact. They’re like puzzle pieces, connecting only in certain ways. For example, carbon atoms can bond to each other to form sheets that are a single atom thick, but contain zillions of carbon atoms interconnected as hexagons. We call this graphene. But they can also connect to form tetrahedrons, four-faced triangular pyramids. The property of that crystal is very different, so we give it a different name: diamond.


Over the years crystallographers have found that there are four kinds of symmetries natural crystals can have: 2-fold, 3-fold, 4-fold, and 6-fold. These are all based on taking a shape and rotating it 360°. For example, take an equilateral triangle. If you spin it 360° it looks the same. But it also looks the same if you spin it 120° and 240°. So after spinning it all the way around, you get the same pattern three times: a 3-fold symmetry.

A regular hexagon has six sides, and looks the same after you spin it 60°, 120°, 180°, 240°, 300°, and finally 360°. So it has 6-fold symmetry.

Now, you could theoretically have a 5-fold symmetry, for an object that goes through multiples of 72° rotations (after five of those you’re back to 360°). But that’s never found in nature. The other symmetries are very strong, and crystals find themselves displaying those instead.

… until recently. In the 1980s scientists were able to create a 5-fold crystal in the lab, which they called a quasicrystal. It’s tough to do, but it can be done. Still, the big question remained: Can that be found in nature?

The answer we now know is: Yes! Scientists found two such crystals in nature, and they came from the same source: a meteorite that fell in far eastern Russia called the Khatyrka meteorite.

Meteorites generally come from asteroids. Collisions break them up into smaller pieces, and sometimes these fall to Earth. Khatyrka has an unusual composition, and when examined microscopically indeed shows signs that it had undergone collisions while it was still part of its parent asteroid body. The scientists wondered if they could replicate this. They created a series of disks made of the same minerals found in the meteorite and stacked them like coins, making something like a hockey puck. They then used a large gas gun to blast it with a projectile moving at about one kilometer per second, which is a typical (or perhaps somewhat slower) collision speed for asteroids in space.

When they examined the result, they found a quasicrystal with 5-fold symmetry, which is now named icosahedrite. The chemical formula for it is Al63Cu24Fe13: 63 atoms of aluminum, 24 of copper, and 13 of iron in each molecule. No wonder it’s so hard to find it in nature!

It’s not entirely clear in detail how it formed, though sudden compression, heating, and then cooling play a role. On Earth those conditions are very rare, but they’re more common in asteroids. The weird composition of the meteorite plays some part too, having the right combination of minerals to start with such that in the end the quasicrystal is created.

five fold symmetry in quasicrystal
When examining the manufactured quasicrystal using a technique called X-Ray Diffraction, the five-fold symmetry becomes apparent. Inset: The fragment examined is attached to a thin length of carbon fiber.

Asimow et al

At this you may be wondering, so what? I have a few whats for you. One is that nature is more clever than we are. These crystals were once thought impossible, but here they are. They’re rare, but not impossible. Just very unlikely and need very special conditions to form.

Second, this gives scientists more insight into the literal structure of nature. Perhaps quasicrystals will never have a practical application, but even if they don’t they still help us understand how the world is put together.

And third, this hints at new branches in the science of crystallography. What other crystals exist, what other strange compounds? What uses will these have? Maybe none, at least not in our ability to exploit them for technological advances, say (the way silicon was used to make computer chips as an example). But again, the more we understand the rules governing the Universe, the better we understand it, and that is a goal unto itself.

And hey, if we can figure out how to make transporters or warp drives or light sabers, then all the better. But in the meantime, just gaining knowledge is pretty cool, too.

P.S. I’ve been meaning to write about this topic for a little while, but tonight I’m giving a talk about science outreach to members of the American Crystallographic Association at their meeting in Denver, so I thought the time was right.

July 25 2016 9:00 AM

More Evidence That Dinosaurs Suffered a One-Two Extinction Punch

What killed the dinosaurs?

That was a mystery for decades; when I was a kid, there were tons of ideas but precious little evidence for any of them, making them little more than speculation. In the late 1970s and early ’80s, though, the hypothesis was put forward that a giant asteroid or comet impact did the deed, and over the years evidence mounted.


The impact idea gained wide acceptance, but some details remained stubbornly difficult to explain with a single catastrophic event. Another idea that started gaining traction was that a series of huge and sustained volcanic eruptions occurred for a couple of hundred thousand years before the impact. These were no ordinary eruptions; they formed the Deccan Traps, a soul-crushingly huge region in India consisting of igneous rock layers more than two kilometers deep and covering an area of 500,000 square kilometers.

Half a million square kilometers. Yeah: huge.

This long-lasting eruption did ecological damage across the planet, weakening life and killing species. The clock was ticking on the dinosaurs and so many other species. When the impact came, their time was up.

There’s pretty good evidence that it took both catastrophes to do in the (non-avian) dinosaurs and 75 percent of species on Earth, but a new study provides more support: Scientists found two warming pulses in Earth’s ocean temperatures corresponding to the times of both the volcanic eruptions and the giant impact. This suggests that large-scale global climate change effects were behind the mass extinction.

It's hard to imagine, but the creatures inhabiting these shells may have witnessed the demise of 75 percent of species on Earth.

Sierra V. Petersen

The scientists examined the shells of bivalves that lived around Seymour Island, near the tip of the peninsula sticking out from Antarctica. This is a good location to do this; the island dates back to before the time of the impact (called the Cretaceous-Paleogene boundary, or K-Pg boundary—C was already used for something else, so geologists went with K) and fossils from that time are abundant on the island. The deposits layered there provide a continuous sampling from before to after the boundary with little or no jumps in time, so the record is relatively clean.

They looked at bivalves because the critters absorb minerals from the water to make their shells, and the ratios of the amounts of some minerals in the water is temperature dependent. By carefully measuring the minerals in the shells (specifically, the ratio of oxygen-18 to oxygen-16), the scientists can then use them as a proxy for temperature.

temperature spikes
The plot of temperature versus time (measured in millions of years ago, or Ma) shows a long rise and fall, then a sharp uptick during the Deccan Traps eruption, another fall, then another smaller spike at the time of the impact.

Petersen et al.

What they found was a huge ~8° C (± 3°) leap in water temperature starting around 66.25 million years ago (roughly 150,000 years before the K-Pg boundary), which is when the Deccan Traps eruption started. The water started cooling again, but then there was another sharp rise of a little over a degree right at the K-Pg boundary, corresponding to the impact. That’s on average; some of the bivalve samples showed much larger spikes in temperature.

I’ll admit, the data look a little rough to me. The second pulse has pretty big uncertainty bars, so it’s hard to tell exactly how big it was. The first pulse looks quite real, though, and does support the idea that the Deccan Traps contributed to geologically sudden and biologically deadly worldwide warming.

Interestingly, there was a long, slow warming and cooling period for a couple of million years before all this. While it was a large rise (about 8°) it happened over a long period of time, so the ecological impact wasn’t as bad. A smaller rise can do far more damage if it happens quickly, a point many climate change deniers ignore.

We use the dinosaurs as a metaphor; something slow, plodding, not terribly intelligent, and in imminent danger of going extinct. This new study shows that to still be apt; they were ill-prepared for both sudden climate change and a giant asteroid impact.

Humans are smarter than that. We have a space program, which is useful in both aspects; it can help prevent an impact, and our Earth-observing satellites are telling us very plainly that the climate of our planet is rapidly changing.

We’re not dinosaurs … if we choose not to be.

July 24 2016 9:00 AM

Time Lapse: The Skies Over Dry Tortugas

How have I never heard of Dry Tortugas National Park until now? It’s a little more than 100 kilometers west of Key West, Florida, in the Gulf of Mexico. It’s pretty remote, accessible only by boat or seaplane, and has an interesting history.

It’s the location of Fort Jefferson, a huge brick fortress that was constructed over a period of 30 years in the 19th century but never completed. It was built to protect the busy waters at the entrance to the Gulf and was also a prison—Samuel Mudd was sent there, convicted for conspiring with John Wilkes Booth in the assassination of Lincoln.


It was a miserable place to be incarcerated, with terrible conditions. When I read that I was astounded; that’s bizarre given how gorgeous the waters are there: crystal clear, with coral and abundant fish. Ironically, it’s a tourist destination now known for its beauty.

And it’s even more bizarre when you think about how beautiful the skies are there. With no land except a few small sandy islands all the way out to the horizon, the skies there are fiercely dark at night, and the stars shine with brilliant intensity.

I learned all this when photographer Harun Mehmedinovic sent me a note about his newest time-lapse video, “Tortugas Rock,” which is a stunning display of just how spectacular this site is:

Whoa. Mehmedinovic employed a variety of effects to enhance the scenes, including stacking frames to create star trails that show the motion of the stars in the sky, reflecting the Earth’s rotation on its axis. Because it’s so far south, from this location the Milky Way gets higher overhead than it does from most of the U.S. and really dominates the video when it’s visible.

I also really like the music. It's called "Escape Velocity," and was written by Terry Devine-King. You can find more of his work online.

Mehmedinovic made this as part of the Skyglow Project, an effort he’s undertaking with fellow photographer Gavin Heffernan to visit remote locations and document the skies there. Their goal is to underscore the issue of light pollution and the damage it causes. I support this effort.

Watching this video and reading about the park, I have to add this place to my ever-growing list of spots on Earth I wish to see. It’s no coincidence that many of these are remote, difficult to access, and not well known. All of these are ingredients that add up to a dark sky and spectacular viewing at night. I sometimes wonder if there are enough nights to visit them all.

I’ve written about some of Mehmedinovic’s work before; check these out:

July 23 2016 9:00 AM

Rosetta’s Final Resting Place Has Been Chosen

On Sept. 30 at approximately 10:30 UTC (06:30 EDT), the Rosetta mission will come to an end.

After many days of slowly approaching the comet 67P/Churyumov-Gerasimenko—sending images and data back to Earth the whole way—it will settle down onto the surface of the bizarre little worldlet, what the European Space Agency is calling a “controlled impact.” And at that moment, the spacecraft is expected to stop transmitting.


It’ll have done what it came to do: observe a comet up close, investigate the environment around it, and send a lander down to the surface of a comet for the first time in human history.

That’s quite a docket. And it performed these tasks amazingly.

Sure, the situation with the Philae lander could’ve gone a lot better. But it did send back a passel of data and a handful of amazing images, and even in failure it succeeded in teaching us more about the surface of a comet.

The final resting place of the Rosetta spacecraft itself has been chosen as well: Ma’at, an area that has some “active regions” sending out plumes of gas. It’s located on the smaller of the comet’s two lobes (the head of the rubber ducky, about halfway from the neck up to the top). It’s good choice; if active regions are still doing their thing, we’ll get some truly amazing close-ups of cometary outgassing, the phenomenon that creates the fuzzy head and long, long tail of a comet.

The ESA hasn’t released too many details just yet, but they expect to have more soon. I’ll let you know when I hear.

Follow Rosetta on Twitter for current info, too.

July 22 2016 9:00 AM

EPIC Earth: A Year of Days From Space

NASA has released a pretty amazing video: It consists of more than 3,000 images of Earth taken by the Earth Polychromatic Imaging Camera (EPIC) on the DSCOVR spacecraft.

DSCOVR is in a special orbit, 1.5 million kilometers closer to the Sun than Earth, that keeps it more or less between the Earth and Sun. It faces us, so it always sees the daylit side of Earth, and takes an image every two hours, watching the Earth rotate.


The video shows the Earth over the course of an entire year, and it’s mesmerizing. I suggest you listen to the narration while you watch, done by EPIC lead scientist Jay Herman.

I love the bit about the solar eclipse (I wrote about that when it happened). Sadly, the video ends just before the Moon photobombed the Earth.

There’s one thing not mentioned in the time lapse that I think is really important. Take a look at these two frames from the video:

Earth over the seasons
The view of DSCOVR in August (left) and December (right). Note the change in the position of Australia (arrowed) and Antarctica.

NASA's Goddard Space Flight Center/Kayvon Sharghi

The one on the left is from August 2015, and the one on the right from December 2015. See the difference? I put arrows pointing to Australia, and you can see that in December Australia is up “higher.” Note too you can see Antarctica clearly in December, but barely at all in August.

What you’re seeing is literally a graphic demonstration of Earth’s seasons due to its tilt!

If you watch the video again, you’ll see this. It starts in late July, during the Northern Hemisphere summer. At that time, the north pole is tipped toward the Sun (and therefore toward DSCOVR). You can see Greenland go by, and other northern features.

But over the course of weeks and months, the North Pole tips away from the Sun and the South Pole tips toward it. Antarctica becomes visible. Then near the end of the video the North Pole comes into view again.

Earth orbital motion
The Earth's axis stays fixed in space as it orbits the Sun.

Wikipedia user Tfr000

Mind you, the poles of the Earth aren’t really tipping at all; the axis of the Earth is pretty well fixed in space, and always points in the same direction. But over the course of the year the Earth goes around the Sun, and in the northern summer the pole is aimed “over” the Sun, dipped toward it, and six months later the North Pole is tipped away from the Sun. From DSCOVR’s (and the Sun’s) point of view, it looks like the Earth’s axis is moving, but it’s a matter of perspective.

I suspect this video, combined with a good lesson plan, would work really well in the classroom to show how seasons work. If a student represents the Sun and another student, holding a globe, pretends to be the Earth in orbit, they could physically see how this works, too. A lot of people don’t really understand that it’s the Earth’s tilt that causes the seasons—I’ve explained it many times in talks, and it’s fun to see the look on someone’s face when they really figure it out.

The beauty of this, too, is that this is one of the purposes of the DSCOVR mission: to help people better understand the Earth and our place in the cosmos. The fact that we are on a tilted, spinning, revolving planet affects our lives every single day. If more people understood that, I wonder how that too would affect our lives every day?

July 21 2016 9:15 AM

These Are the Antiheroes We Don’t Deserve

So you’re anti-reality and anti-human-driven climate change, but you can’t find any way to get your kids to listen to you about it? I have just what you need: antiheroes for the age of anti-science.

Presenting Climate Inaction Figures! Seven of the most oil-fueled deniers of science, ready to take up arms and fight against the experts, bad mouth good data, and confuse the public with verbal legerdemain!


Each comes with his or her own super-powered weapon against reality. There’s Mitch McConnell with his Coal Gauntlet, Ted Cruz and his Anti-Science Shield, Sarah Palin and her Drill Sword, and my personal favorite, James Inhofe and his Spiked Snowball:

James Inhofe
James Inhofe's logic is a little slushy.

Climate Inaction Figures

I wonder if he’d use that on the floor of the Senate.

There’s also Donald Trump, Chris Christie, and my other favorite, the two-headed Koch brothers (superpower: money. Lots and lots of money).

I love this idea so much. They even have a commercial:

Sadly, you can’t buy them. Unless you’re a fossil fuel company!

These figures are brought to you by admen Cabot Norton and Arturo Aranda, who partnered with the Years Project—the group that made the documentary Years of Living Dangerously—to create these figures.

The purpose is to raise awareness about the GOP head-in-the-sand syndrome currently scuttling any attempt to do anything about global warming. But it’s more than that: It’s an attempt to get people to understand that there is something we can do. They started the website the Climate Solution to promote the idea of putting a price on carbon, getting companies that pollute the air with carbon dioxide to pay for that pollution, and creating an incentive to reduce emissions.

As former U.S. Labor Secretary Robert Reich explains in a short video, this is the same idea that was behind reducing acid rain due to pollution; companies were forced to pay for that pollution and in a few years the problem was greatly reduced. Fossil fuel companies complain about a “carbon tax” now as they did for the acid rain tax then, but in the end it worked out pretty well. A carbon tax will force these companies to look into more efficient renewable energy production, as well they should.

It’s not that easy, of course, but the problem right now is really the Climate Inaction Team. They won’t even allow it to be discussed, and if we can’t get our politicians to at least talk about it, we can be sure it’ll never happen.

So remember that come November. While the world gets ever-hotter, and the U.S. bakes under a massive heat wave, it’s way past time that the folks who get burned are the ones denying it’s even happening. Vote ’em out of office.

You can follow the Climate Inaction Figures on Facebook, Twitter, Instagram, and Tumblr.

July 20 2016 11:30 AM

March … I Mean April … I Mean May … I Mean June 2016 Is the 6th … I Mean 7th… I Mean 8th … I Mean 9th Temperature Record-Breaking Month in a Row

N.B. If this article sounds familiar, it should. This has been happening so frequently I just copied the post for March April May and updated it.

October. November. December. January. February. March. April. May And now June.


For the sixth seventh eighth ninth month in a row, we’ve had a month that has broken the global high temperature record.

According to NASA’s Goddard Institute for Space Studies, March April May June 2016 was the hottest March April May June on record, going back 136 years. It was a staggering 1.28°C 1.11°C 0.93°C 0.79°C above average across the planet.* The previous March April May June record, from 2010 2014 2015, was 0.92° 0.87° 0.86° 0.78° above average. The good news, such as it is, is that unlike previous months, June 2016 didn’t shatter the previous record; it just edged it out in a statistical photo finish. But don’t let that distract you from the important issue: The Earth is way above average in temperature, and overall that temperature is increasing all the time.

Welcome to the new normal, and our new world.

New for July 2016: NASA has created a short video describing its efforts to measure global warming, specifically pointing out that the first six months of 2016 have all been the hottest months on record of their kind:

As you can see from the map above, much of this incredible heat spike is located in the extreme northern latitudes. That is not good; it’s this region that’s most fragile to heating. Temperatures soaring to 7° or more above normal means more ice melting, a longer melting season, loss of thinner ice, loss of longer-term ice, and most alarmingly the dumping of billions of tons of fresh water into the saltier ocean which can and will disrupt the Earth’s ability to move that heat around.

What’s going on? El Niño might be the obvious culprit, but in fact it’s only contributing a small amount of overall warming to the globe, probably around 0.1° C or so. That’s not nearly enough to account for this. It’s almost certain that even without El Niño we’d be experiencing record heat.

Most likely there is a confluence of events going on to produce this huge spike in temperature—latent heat in the Pacific waters, wind patterns distributing it, and more.

JAXA June temperature anomalies
The Japanese Meteorological Agency measured similar temperatures as GISS (though it uses a different baseline for the average), and has June 2016 in a virtual tie for hottest June on record with 2015. Note the trend. See a "pause"? I don't.

Japanese Meteorological Society

And underlying it all, stoking the fire, is us. Humans. Climate scientists—experts who have devoted their lives to studying and understanding how this all works—agree to an extraordinary degree that humans are responsible for the heating of our planet.

That’s why we’re seeing so many records lately; El Niño might produce a spike, but that spike is sitting on top of an upward trend, the physical manifestation of human induced global warming, driven mostly by our dumping 40 billion tons of carbon dioxide into the air every year.

Until our politicians recognize that this is a threat, and a very serious one, things are unlikely to change much. And the way I see it, the only way to get our politicians to recognize that is to change the politicians we have in office.

That’s a new world we need, and one I sincerely hope we make happen.

*GISS uses the temperatures from 1951–1980 to calculate the average. The Japanese Meteorological Agency uses 1981–2010, which gives different anomaly numbers, but the trend remains the same. Realistically, the range GISS uses is better; by 1981 global warming was already causing average temperatures to rise.

You may have noticed that the actual temperature anomaly for each month over March through June appears to be dropping; 1.28 to 1.11 to 0.93 to 0.79. That may be due to El Niño weakening, but it’s hard to know over such a short time period. Even if the trend continues, I’d bet 2016 will be the hottest year on record.

July 20 2016 9:00 AM

The GOP Fiddles While America Burns

You want to know the very definition of irony? While the Republican National Convention is going on in Ohio—loaded to the hilt with people who deny the reality of global warming—the country itself is baking under a heat wave that is likely amplified by global warming.

The National Oceanic and Atmospheric Administration just released this model temperature map of the United States:

heat dome
A map of the temperatures across the U.S. on July 18, using sophisticated computer models.



That is a model of the air temperatures in the U.S. at 5 p.m. ET on July 18, showing what’s colloquially called a “heat dome” over about two-thirds of the country. It’s a high-pressure system that squats over an area and can lead to high temperatures. High-pressure systems have sinking air, and when the air drops down, it compresses and heats up. This is causing elevated temperatures from the Southwest to the East Coast, and elevated humidity in much of that area as well.

Worse, the system is moving only slowly … which is very likely to be due to global warming. Usually, such weather patterns don’t hang out very long. But the planet is warming, and this has consequences. Warming affects the Arctic more than lower latitudes, and the strength of the jet stream depends in part on the difference in temperatures at lower latitudes to those in the Arctic. With the North Pole warming, the jet stream weakens, and “blocking patterns” can result, where weather systems move more slowly or not at all for some length of time.

While it’s difficult to pin down any specific event to global warming, overall the effects of warming will make patterns like this more common, and we are seeing more of them.

And at the very same time as this stubborn heat wave with temperatures 5–10°C higher than normal, the Republicans in Cleveland deny that any of it is happening. Donald Trump called global warming a hoax. His vice presidential pick, Mike Pence, also denies it’s real. In a recent interview, Pence said:

I don't know that [global warming] is a resolved issue in science today ... just a few years ago we were talking about global warming. We haven't seen a lot of warming lately. I remember back in the 70's we were talking about the coming ice age.

Those are very typical and very wrong climate change denier talking points. Despite Pence’s claims, we have indeed seen a lot of warming lately, and that bit about a future ice age is just baloney. But this load of fertilizer isn’t too surprising coming from a guy who wrote that smoking cigarettes isn’t so bad for your health.

I’ll note that some Republicans do think the Earth is warming, but very few of them are attending the convention. That doesn’t surprise me; if they actually have a grip on reality, they would want to put as much distance between themselves and the train wreck in Cleveland as they possibly can.

But they should remember that it’s the GOP that created Trump, and in November we should all remember the GOP candidates who deny reality.

Tip o’ the gavel to Gizmodo.

July 19 2016 9:00 AM

Hubble Takes a Long Look Into the Heart of the Crab

7,500 years ago, a massive star exploded.

It had lived a short life, comparatively, running through its nuclear fuel of hydrogen, helium, carbon, neon, oxygen, silicon; each element fusing faster in turn in a sprint to the finish.

Silicon fuses to iron. When that began, everything for the star ended. Iron built up in the core, and several processes conspired to cause the core’s demise. It was already incredibly dense, but fierce and inexorable quantum mechanical laws took hold, squeezing it further. It collapsed down further, becoming a ball of tightly squeezed neutrons just a few kilometers across.

When it did, it released a flash of energy so colossal it defies our puny human brains. To wit: When the core collapsed, the outer layers of the star started to fall in as well. Moving at high speed, this material was a tsunami with a mass of several octillion tons crashing down… yet the flash of energy from the core was so huge it was able to halt the infall, turn it around, and have enough power left over to fling that same material — several times the mass of our own Sun — outward at a significant fraction of the speed of light.

That is the immensity of a supernova. The star had finished, but a new phase had begun.

July 18 2016 12:00 PM

Throwing Shade on a Comet

On Valentine’s Day last year—Feb. 14, 2015—the Rosetta space probe passed an astonishing six kilometers above the surface of the comet 67P/Churyumov-Gerasimenko. Mind you, the comet is only about four km end to end, so this was a very, very close shave.

The images it took at the time are amazing, but not for the obvious reason. Cooler to me was that the flyby was done in such a way that the Sun, spacecraft, and comet were in a perfect line.


So what? Well, that means the spacecraft could see its own shadow on the comet’s surface! I wrote about this event at the time; what’s new is that now all the high-resolution images taken during that pass are online… and that the folks at the European Space Agency put the shadow pass images together into this very cool animation:

Rosetta’s shadow moves from right to left along the bottom of the images. To give you a sense of scale, the shadow is about 20 x 50 meters across! The resolution of the full-scale images is about 11 centimeters per pixel. Wow.

This trajectory was chosen on purpose to measure just how the surface reflects light. With the Sun directly behind Rosetta (called “zero phase angle”) shadows disappear, and there’s a surge in light reflected back. Different surface materials and different surface textures affect that surge, so this is a way to observe that.

The pass took about three minutes, during which time the spacecraft took a dozen shots of ½ a second exposure each.

The high-res images from that pass that are online are fun to look at; here’s one I grabbed nearly at random:

comet surface
The surface of the comet in very high res; objects smaller than a half-meter in size can be seen.


“Nearly at random” just means I scrolled around until I found something neat. This shows a couple of boulders and assorted smaller rocks sitting in a grainy but smooth surface, probably from fine-grain material that’s flowed down from higher elevations into a pit. On the comet, some pits are where ice sublimates, turning directly into a gas, creating the cloud of material surrounding the comet itself, and which get blown back by the solar wind and pressure from sunlight to form the comet’s tail.

Note the parts of the surface that look slumped. That may be due to this very thing; the support under the material goes away as the ice sublimates, so you get a partial collapse. I’m guessing, but I’d bet that’s what this is. There are faint sweeping features around the rocks that look like regions where this happened before, and are being filled in by more material. There might even be some subtle wind action going on here as gas escapes; such things have been seen before on the comet.

The mission received an extension last year, and the end is planned for September 2016, when Rosetta will be gently guided down to the surface. Once down it’s not expected to function any more, but I hope we get images as it descends. What will the comet look like not from six km, but three? One? A hundred meters?

It wouldn’t be true to say I’m looking forward to the end of this wonderful and spectacular mission. But what an end it will be.