Dwarf planet Makemake, which orbits the sun once every 310 Earth-years, has a dark little moon. Just 100 miles across, the moon evaded detection for more than a decade, hiding in the glare of its parent planet.
But it couldn’t escape the stare of the sharpest eye in the sky forever: When scientists aimed the Hubble Space Telescope at Makemake for more than two hours in April 2015, they discovered a faint point of light moving through the sky along with the icy world.
Until now, Makemake was the only officially recognized distant dwarf planet without a moon, a dubious distinction that has now been lost.
Is That … a Moon?
At first, Alex Parker wasn’t sure he’d spotted a new moon in the Hubble observations.
“I was sure someone had seen it already,” says Parker, of the Southwest Research Institute. So, he approached collaborator Marc Buie and asked, “Has anyone seen the moon in the Makemake data?”
Buie’s reply — “There’s a moon in the Makemake data?”— convinced Parker he was onto something.
“It was at that point that everything got exciting and kicked into high gear,” says Parker, who along with Buie reported the discovery of the moon, known as MK2 for now (or S/2015 (136472) 1, more officially) on Tuesday.
By carefully studying the orbit of MK2, scientists will not only be able to determine how the moon formed—whether Makemake’s gravity snatched it or it grew out of a collision—but also learn more about Makemake itself. Specifically, the moon’s orbit will reveal the mass of the small, icy world. From there, they’ll be able to calculate Makemake’s density and determine what it is likely made of, and compare it to other far-flung icy worlds such as Pluto, Eris, and egg-shaped Haumea.
“The wide range of densities of the dwarf planets is one of the most interesting mysteries out there. But we still have so few objects that each one adds a critical part of the story,” says Caltech’s Mike Brown, who, with his colleagues, discovered Makemake in 2005. “I’m accepting bets currently.”
Makemake’s Dark Mystery
Makemake is a strange world. Shaped like a flattened sphere about 870 miles across, it lives in the Kuiper Belt—the icy debris ring beyond the orbit of Neptune—and is reddish in color. Like some other Kuiper Belt objects, Makemake spins very quickly, pirouetting every 7.7 hours. Its slightly oval orbit takes it much farther from the sun than Pluto, which treks around the sun in a comparatively snappy 248 years.
Finding MK2 in orbit around Makemake could solve one of the abiding mysteries about the icy dwarf planet, Parker said.
When scientists first observed the whirling Makemake, they noted that it was continually bright, meaning that its surface is probably uniformly covered in bright, reflective ices. But heat signatures from the faraway planet were slightly varied, suggesting that at least one warm, dark patch might be present on Makemake’s surface. Years of observations failed to reconcile the two data sets, as a dark patch never showed up in observations.
“Well, imagine that the dark material isn’t on Makemake’s surface … it’s in orbit!” Parker said. “If the moon is very dark, it accounts for most previous thermal measurements!”
Indeed, MK2 is much darker than Makemake itself, which is about 1,300 times brighter than its companion.
What else is hiding in our solar system, waiting to be discovered?
THE WOODLANDS, Texas—A system of buried, empty lava tubes hides beneath the moon’s surface, remnants of a bygone age when the volcanically active moon launched fountains of fire into space.
At least, that’s what scientists think.
For years, teams have hunted for these elusive sublunar tunnels, which can be large and sturdy enough to house entire cities. In fact, lunar lava tubes could be ideal locations to establish a moon base, as their thick roofs would shield humans from harmful radiation and small meteorite impacts. But until now, the strongest observational hints of the tubes’ existence came from a smattering of detectable surface features, including skylights and rilles, channel-like depressions thought to form when tubes collapse.
This week, scientists announced that the signatures of at least ten buried lava tubes could be written into a map of the moon’s gravitational field.
Sood and his colleagues began their search for lava tubes in the Marius Hills region, where scientists suspect that a skylight has opened into one of the buried tunnels. That portal, discovered by Japan’s moon-orbiting Kaguya spacecraft and reported in 2009, is approximately 65 meters wide and 80 meters deep. It also sits by two rilles boringly known as A and B. In other words, there are multiple lines of evidence suggesting that lava once oozed and flowed beneath the Marius Hills.
“We see a skylight that is along this rille, but we don’t know if that is an access point into a lava tube or not,” Sood says. “Can we pick that up using gravity data?”
The short answer is yes, most likely.
Sood and his colleagues searched for the Marius Hills tube using data from NASA’s twin GRAIL spacecraft, which flew in tandem above the moon’s surface as they measured and mapped its gravitational field (in 2012, the spacecraft were purposely crashed into a site now named after astronaut Sally Ride). The moon’s gravitational field is affected by masses below the surface, as is Earth’s. Put simply, large chunks of mass will produce an increase in gravity, Sood says, but “if you fly over a lava tube, there’s going to be a dip in gravity.”
The team spotted a gravitational signature that could be a lava tube near the skylight, and then wondered if it would be possible to detect similar signatures in areas with no obvious rilles or skylights. Turns out, the GRAIL data contain at least ten telltale anomalies resembling lava tubes, slithering and twisting beneath the moon’s surface. They’re all located on the moon’s near side, near the dark stains left by ancient volcanic seas, and some of the candidate tubes are more than 100 kilometers long and several kilometers wide—large enough to swallow a small city.
Of course, it’s not certain that the tubes are actually there. The GRAIL data provide the strongest evidence for their presence, but definitive proof would require a moon-orbiting spacecraft that uses ground-penetrating radar to peer beneath the moon’s surface. Sood and his colleagues have proposed just such a space robot, called LAROSS.
“The proposed radar will not only help confirm our findings but will also give us an opportunity to find smaller lava tubes, ones that were beyond the resolution of GRAIL gravity data,” Sood says.
Maybe someday, after looking for lava in all the right places, space-faring humans will not only solve the mysteries of Earth’s closest celestial companion but use it as a giant shield against the dangers of space.
Full citation: Detection of buried empty lunar lava tubes using GRAIL gravity data. R. Sood, L. Chappaz, H. J. Melosh, K. C. Howell, and C. Milbury. LPSC abstract here.
Update, June 29, 2016: The composition of Ceres’ bright spots is no longer a mystery: Wednesday in Nature, scientists revealed those enigmatic splotches are rich in sodium carbonate, a salt that is tightly linked to watery conditions on Earth. In fact, there’s so much of the stuff in Ceres’ Occator crater that the pit holds the record for the largest such deposit in the solar system aside from Earth.
Just how those salts ended up on Ceres’ surface is still a mystery, though. Instead of containing large amounts of ice, as some scientists had expected, Ceres’ interior is considerably drier than suspected, reports a second study published Wednesday in Nature Geoscience. That makes it hard to explain how the dwarf planet’s surface ended up covered in formerly dissolved salts. Now, researchers are trying to sort out how Ceres made those salts and moved liquid brines to the surface; they suspect impacts may be to blame for simultaneously melting buried ice and excavating the planet’s salty sea, leaving bits of it to shimmer in the sunlight.
THE WOODLANDS, Texas –
Today, Ceres is a salt-covered dwarf planet whose main claim to fame is that it’s the largest body in the main asteroid belt. But back when it was younger and hotter, scientists have found, Ceres was an ocean world—much like the watery moons of Jupiter and Saturn.
“Ceres appears to have been one of these in the past,” Carol Raymond, deputy principal investigator for NASA’s Dawn mission, said Tuesday at the Lunar and Planetary Science Conference. The Dawn mission put a spacecraft in orbit around the tiny planet in March 2015. “What we’re looking at now, we believe, is the remnants of a frozen ocean.”
Scientists hoped that when Dawn arrived at Ceres, it would help solve the mysteries of this strange little world. But instead, Ceres is throwing puzzle after puzzle at the team and proving to be a much tougher space-nut to crack than anticipated.
Among Ceres’ enigmas are those perplexing bright spots, which scientists first thought could be extremely reflective water ice. But closer inspection reveals the spots are likely to be salts – perhaps leftover from a briny, frozen ocean that’s exposed when impacts gouge craters into Ceres’ crust. “We’re interrogating the chemistry, essentially, of that ocean-rock interface,” Raymond said.
The brightest and best-known spots are in Occator crater, a 92-kilometer-wide hole in the ground that is roughly 80 million years old. Occator is covered in vaguely blue, young terrain at its bottom, and shaded somewhat reddish and older around its edges. But it’s the formation on the crater’s floor that has most intrigued scientists: There, among the brightest of the bright spots, is a pit. And rising out of that pit is a fractured, reddish dome.
It’s possible that when Occator formed, the impact not only excavated the crater but heated a portion of the subsurface material enough for ice and other volatiles to waft into space, said Tim Bowling of the University of Chicago. “If you remove all the ice from a region, then you’re able to form a pit,” he said, during a presentation at the meeting. And when that happened, the materials left in the resulting hollow should be the reflective salts.
But how about that dome? Stay tuned. “I just found out yesterday there’s a mound inside the pit,” Bowling said.
It’s no secret that craters are sprinkled across Ceres’ surface. Take a spin around the dwarf planet and multiple pockmarks will fill each frame. But what’s noticeably absent are extremely large craters, the ones more than several hundred kilometers across. These, based on the collisional history of the solar system, should also be carved into Ceres – and yet, they’re missing.
“We must think that those craters formed, and then they got erased,” said Simone Marchi of the Southwest Research Institute. “The question is, How can you erase all those large craters?”
And then there’s that mountain, a pyramidal oddity called Ahuna Mons. About 5 kilometers tall with steep, bluish slopes, Ahuna Mons looks as though it has been thrust straight out of Ceres. “The mountain is really coming out from the subsurface and taking the surface features, which are a little bit older, up the mountain,” said Ralf Jaumann of the German Aerospace Center. It’s not the only mountain on Ceres, he said, but the others are older and less developed.
When it’s millions of miles away from Earth, the new rover will not only need to search for signs of ancient life, it will need to be prepared for all the alien rocks it meets. Especially because, unlike its cousin Curiosity, this next-gen robot won’t just be drilling holes in those rocks: It will also be collecting rock cores and stashing them in various spots on the planet’s surface. If all goes well, the rock samples will be picked up and returned to Earth by a future rover, kind of like an interplanetary golden retriever. It’s a complicated strategy that forms the next step in NASA’s long-term plan to explore our small reddish neighbor, a plan that could eventually add human footprints to the rover tracks already pressed into the dusty, formerly wet Martian surface.
But first things first. The Mars 2020 rover needs to be able to drill, extract, store, and deposit all those cores.
“We don’t quite know what we’ll encounter when we get to Mars,” says JPL’s Matt Robinson, deputy product delivery manager for the rover’s sampling and caching subsystem. “We need to have a range of rocks we’re going to test.”
Robinson and I are standing in a cavernous lab on the JPL campus, in a place appropriately called the Rock Room. Somewhat ominously, there’s a sign on the door warning against the dangers of inhaling pure nitrogen gas. “Two breaths of 100% nitrogen can cause immediate unconsciousness with no warning,” it says in all-caps, which seems a bit out of place, considering that Mars wears a thin, gassy veneer of carbon dioxide. But no matter.
As its name suggests, the rock room is filled with baskets and baskets of rocks, mostly collected from sites in southern California. There are hard rocks and soft rocks and in-between rocks, and they’re all the geological equivalent of guinea pigs for the drill prototypes Robinson and his colleagues are developing. Like these rocks, some of the Martian targets will be hard and less likely to yield to the machinations of a nosy robot, while others will easily crumble under pressure. As evidence of the team’s ongoing experiments, there are piles of rocks bearing numerous cylindrical gouges.
“It’s somewhat of a design-by-test,” Robinson says. “You do the mechanical design, you build a prototype, then you come into the test bed and you test it.”
Opposite the rock piles are illuminated racks of clear plastic tubes, each holding a 4-inch-long cylindrical rock core. These dozens of tubes are the handiwork of the 2020 drill prototypes, and are almost exactly the type of thing the rover will be stashing on the Martian surface. The various cores are different colors and consistencies, and there’s more material in some tubes than others.
It occurs to me that it would be bad for the rover to accidentally cache an empty tube for future retrieval.
“Will the tubes on the rover be clear? Could you see into them?” I ask.
“It’s going to be a metal tube so you can’t really see into it, but there’s a camera that will look in and see how much sample there is,” Robinson assures me. He pauses, thinking. “But it would be nice to – like in Star Trek IV — have transparent aluminum…that would be really cool.”
Whales on Mars would also be nice.
Alien humpbacks aside, the plan right now is for the 2020 rover to retrieve as many as 42 rock cores and distribute them in strategic sites that are still TBD. That way, a future rover will only need to visit several locations to retrieve the cached samples instead of inefficiently retracing 2020’s path.
“This is our ambient robotic coring station,” Robinson continues, showing me one of the three drill prototypes the team is currently working with. As imagined, the 2020 drill would have a variety of bits available for tackling rocky obstacles; when the rover encounters an enticing rock, all it needs to do is shove its arm into a tool carousel and select the bit that’s best for the task. When it’s done with one tool, the rover can easily swap it out for another.
Right now, the retinue of options being tested includes a brush that will help smooth rock surfaces pre-drilling, as well as a dozen different bit prototypes. The team is also experimenting with stabilizing legs so the rover can achieve maximum rock-punch, as well as having the rover collect loose material from the surface, called regolith.
“The great thing about this test bed is it’s got a drill press that’s easy to set up and operate,” Robinson says, before the clamor of an ongoing test drowns out the rest of his sentence.
We move into the space where the third prototype lives, and it looks like I’ve stepped into a concert hall. On a small stage, the proto-arm is surrounded by multiple rocks, each with multiple lights and cameras aimed at it. The stage, it turns out, is hydraulic powered – and when the team wants to test the drill under Mars-like conditions, the whole platform can be launched upward until it docks with a pressure chamber hovering overhead.
“We actually raise it up and there are clamps up there which it clamps onto,” Robinson says. “You can pump down to Martian pressure and Martian temperature.”
On Mars, the surface pressure is about 60 0.6 percent that of Earth’s, and temperatures range from a wintry day in Ithaca, NY to somewhere you absolutely wouldn’t want to be. Robinson says the team runs roughly half its tests in the Mars-chamber, under the watchful eye of all those cameras.
But there are more differences between Mars and Earth than simply temperature and pressure.
“Does the different Martian gravity have any effect on the drill?” I ask.
“Gravity actually does affect the robotic arm quite a bit,” Robinson says. He explains that the team has to factor in weaker Martian gravity when teaching the rover where to retrieve bits and stash tubes. “If you were to go to those same joint angles on Mars, it would not work,” he says. “So we have to compensate for that in software. On the flipside, we’re now more capable because we carry less weight. So it’s both tricky and it’s good.”
All of this means that when NASA’s next rover arrives on Mars, expect the red planet to be poked, prodded, zapped and drilled into with all the precision that a wheeled, nuclear-powered robotic surgeon can offer – courtesy of these piles of geologic guinea pigs collected from spots on Earth.
In other words, as Albert Markovski astutely notes in the movie I Heart Huckabees: “You rock, rock.”
A large, undiscovered planet lurking on the fringe of the solar system is a compelling idea—and it’s nothing new. It’s been proposed many times. And those predictions have always been wrong.
So what about this latest prediction, the one that might actually be right?
(In case you missed it, a new study from Caltech suggests there might be a large world way out there—a planet whose gravitational hand is sculpting the orbits of distant, icy worlds and forcing them to take weird paths around the sun.)
How can we figure out if the planet is actually there? And will it even be bright enough to see? Here are some answers to these and other pressing questions.
Are we sure it’s there?
No. The evidence is tantalizing, but it’s circumstantial. UCSC astronomer Greg Laughlin gives the planet a 68.3 percent chance of actually existing (“That’s odds-on, but it’s not huge odds-on. It’s also not a coin flip.”) Konstantin Batygin, who’s half of the Caltech team, says he’d put the planet’s chances at 83 percent (“I made that up right now…I’m just being a little bit more realistic than Greg.”)
Others aren’t quite so sure. “I’m very skeptical of this turning up because I’ve seen so many predictions like this—and so far they’ve never turned out,” says Alan Stern, principal investigator of the New Horizons mission that sent a spacecraft zooming by Pluto this summer. “But I’m sure that they ultimately will. I have no doubt that there are lots of planets out there.”
This planet would obviously orbit the sun, and something with 10 Earth masses is more than massive enough to be round. Regarding the last point—the clearing of small bodies and other junk from its orbit—“Planet Nine is forcing any objects that cross its orbit to push into these misaligned positions. It fits that concept perfectly,” Caltech’s Mike Brown, the other half of the Caltech team, said to the Washington Post.
What would this planet be like?
Cold, for sure. At its closest, the planet is still 200 times farther from the sun than Earth is, and at its most distant is a whopping 600 to 1,200 times farther away than Earth.
At somewhere around 10 Earth-masses, the frigid world will be more like a gassy, mini-Neptune than a rocky planet. This inference is based on information from exoplanets, which tend to show up most frequently in this mass range. So far, though, this type of extremely common planet has been notably absent from the solar system.
Could it have moons?
Possibly. “It would be very interesting to know if it has satellites,” Laughlin says. If those moons are big enough to see, then we could determine the mass of the planet and get a better idea of what it’s made of.
What should we call this planet?
For now, whatever you want, really (but not Nibiru. This is not Nibiru). The world hasn’t been detected yet, and if it ever is, naming the thing will be a formal and lengthy process. The Caltech team is calling it Planet Nine, which is a pointed reference to the controversial reclassification and removal of Pluto from the planetary lineup in 2006, a decision was motivated in part by the work of Mike Brown, who could now be on the ironic cusp of resurrecting a ninth planet.
But…this could also be Planet Ten, based on how you define planet (hi, Pluto!). Or Planet Fourteen (hello, Ceres, Haumea, Makemake, and Eris). Or Planet One Hundred and Something (hey, every round thing). “Apparently, Caltech can’t count,” Stern says.
How could scientists find it?
“Go to a big telescope with a wide field of view, and look at as much sky as possible,” says the Gemini Observatory’s Chad Trujillo, who has spotted a number of very distant solar system objects. “Take three images of the sky, with maybe 1.5-2 hours between each image, and look for things that move. Things that move fast are asteroids and are nearby, and things that are slower are farther out.”
Yet Planet Nine, if it’s there, will be a bit harder to find than Pluto. It will be dimmer and farther away, and scientists estimate that it takes between 10,000 and 20,000 years to orbit the sun once—so it will move very slowly across the sky.
We don’t know where the planet is in its orbit, but chances are it’s not nearby (because of the way orbits work, this planet spends a lot of time hanging out very far away). Plus, the patch of sky its orbital path covers is huge and crosses the plane of the Milky Way twice. Pulling a planetary needle from a cosmic haystack is difficult under the best of circumstances, but pulling that needle out from a star-studded galactic streamer is even harder.
But it’s not impossible. Scientists could manage to catch a glimpse of something on the fringe using several telescopes on Earth, Trujillo says. He and others say the Subaru telescope on Mauna Kea, as well as a smaller telescope in Chile are capable of finding something that faint and far away. If teams do see something moving, they’ll try to get at least three observations of the object so they can plot a preliminary distance and brightness. And then, over many months of observations, they’ll work to pin down the exact orbit. Brown and Batygin are already looking; Trujillo may start searching next month. “We’re racing, but it’s a friendly race,” he says. “We don’t try to trip each other or anything.”
Wait…how bright is it?
The real answer is no one knows for sure. A planet’s brightness depends on its size, distance, and composition—and we don’t really know any of those things right now. But there are ways to estimate how bright the world could be, which is something astronomers refer to as magnitude. Because astronomers sometimes like to do things backwards, the magnitude scale is a bit counterintuitive: Objects with higher magnitudes are actually dimmer. For example, Pluto is currently around magnitude 14. The sun? That’s –26.74. If you’re in a big city, you probably can’t see anything dimmer than magnitude 3, maybe 4 at best.
Batygin and Brown have calculated that when it is closest to the sun, this planet is around 18th magnitude, which is bright enough to be picked up with high-end backyard telescopes. That’s why it’s unlikely to be close to the sun now—we would have already seen it.
At its farthest, the planet would be around 24th magnitude, or about 10,000 times dimmer than Pluto. That’s not too dim for a telescope to see, but it doesn’t make spotting the planet easy. Remember, this faint speck of light is going to be moving very slowly through a dense field of stars.
How did they do that brightness calculation, given we don’t know how big it is, where it is, or what it’s made of?
Batygin and Brown assumed the planet is between two and four times larger than Earth (more on that in a minute) and has a Neptune-like reflectivity, which is a property that matters when you’re thinking about how much light bounces off an object. Reflectivity is known as albedo, and is reported on a scale from 0 (very dark) to 1 (very bright).
The reason for the Neptune-like assumption is that, at roughly 10 Earth-masses, this planet is likely more of a gas-shrouded mini-Neptune than something with a hard, rocky surface. Not surprisingly, different atmospheres have different reflectivities (and what if it’s covered in bright white clouds?), so there is some uncertainty in this portion of the calculation.
But the biggest uncertainty in the brightness calculation (aside from distance) comes from the planet’s size, which could be between two and four times bigger than Earth, and the area of a reflecting surface scales as the square of the radius. “The size uncertainty of a factor of 2 leads to an area uncertainty of a factor of 4,” says Andy Rivkin of The Johns Hopkins University Applied Physics Lab. “That corresponds to 1.5 magnitudes all by itself.”
Overall, that kind of uncertainty isn’t terrible, and most estimates of this planet’s brightness at its farthest point tend to hover between 23rd and 25th magnitude. “I would say that at a given distance there’s probably 1-2 magnitudes of uncertainty in the prediction,” says Laughlin, who independently reached a conclusion very similar to Batygin’s and Brown’s, assuming a world that is ¾ the size of Neptune and has the same albedo.
Could we use a similar technique to find planets around other stars?
Yes, if we could make such detailed observations of small bodies around other stars (which we can’t). The question would then become, is it easier or harder to prove the existence of an exoplanet based on how it perturbs exo-smallthings?
Laughlin argues that it would be easier. Discoveries in the solar system, he says, are held to a higher standard of proof than discoveries around other stars. “If there was some mechanism for returning very accurate observations of comet orbits in an exoplanet system, and you saw this amount of evidence, I think it would just be a no-brainer that there’s a 10-Earth-mass planet perturbing that stuff.”
Stern agrees. “I’m trying to think about whether that’s sociological or actually scientific,” he says.
NASA’s Natalie Batalha, a member of the planet-hunting Kepler team, notes that planets around other stars have already been discovered based on how they perturb other objects—notably, other planets. In 2011, scientists described a planet called Kepler 19c. The planet hadn’t been directly detected, but its presence was inferred from the way it jiggled its planetary sibling, called Kepler 19b. As the pair orbits its star, 19c’s gravitational hand pulls on 19b, changing the frequency with which 19b periodically blocks out its star’s light. Scientists can use those slight variations, called transit timing variations, to constrain the orbital period and mass of the invisible, perturbing planet, and the method is thought to be sensitive enough to detect low-mass planets.
It’s not clear whether transit timing variations offer more precise constraints than the whacked-out orbits of icy worlds in the faraway solar system, but it’s certainly interesting to think about.
Is this planet the fifth giant that scientists think Jupiter kicked out way back when?
No. First of all, the idea that our solar system once had a fifth giant planet comes from simulations of our neighborhood’s early days. Plunk a fifth world in the realm of the giants (Jupiter, Saturn, Uranus and Neptune), and you end up with a solar system that looks much more like the one we live in today. If that fifth large sibling was there, it would have been punted into space by that jerk Jupiter’s gravity hundreds of millions of years after being born.
Batygin and Brown think their planet would have been thrown outward within several million years after the sun formed. Back then, our star was still in its native birth cluster and the surrounding stars would have helped keep the flying planet from hurtling forever into space (it’s worth mentioning that some scientists, such as Hal Levison of the Southwest Research Institute, don’t consider this scenario to be incredibly likely).
What’s more, this planet would have been less of a full-fledged giant and more of a planetary core—something like a seed from which a larger planet could have grown, had it been given the chance.
“These two events are both ejections, but they are well separated in epoch,” Batygin says. “The planet that was ejected during the giant instability of the solar system—which, by the way, coincided with the formation of the Kuiper Belt—that planet, if it was there, it was just ejected. It doesn’t get to stick around.”
Curving across the surface of Saturn’s moon Tethys are crimson streaks – and scientists have no idea what the material is or how it got there.
“It’s clearly painted on the surface in some way that we do not as yet understand,” says Paul Schenk of the Lunar and Planetary Institute, who presented the observations Tuesday at the American Geophysical Union’s annual meeting. “We basically have a little mystery.”
At just a bit more than 1,000 kilometers across, Tethys is a medium-sized moon and is made almost entirely of water ice. Aside from the bloody arcs, its surface is pretty normal as far as outer solar system moons go: There are a bunch of craters, including a 450-kilometer-wide behemoth called Odysseus, and a lot of fractures. And then there are the streaks, which are a few kilometers wide and hundreds of kilometers long.
“We have these bloody stains on Tethys,” Schenk says.
The red streaks were faintly visible in early images from NASA’s Cassini spacecraft, which swooped into the Saturn system in 2004. But it wasn’t until April that Cassini got a close look at the extraterrestrial artwork. Now, after a close flyby in November, scientists can peer even more closely at the smudges. And what they’re finding doesn’t make a lot of sense.
“You don’t see any trace of scarps or ridges or depressions of any kind,” Schenk says, meaning there are no obvious landforms associated with the smears – or at least nothing that’s big enough to see at Cassini’s current resolution. Some of the nearby craters have odd, dark material inside them, but it’s not clear what that material is, how it got there, or if it’s associated with the streaks at all.
Instead, it appears as though someone simply painted the moon red.
“If you didn’t have the color, you wouldn’t know they were there,” Schenk says.
Perhaps the best clue about where the streaks are coming from can be found by plotting their locations on the moon. When Schenk mapped the lines onto the moon’s surface, he saw a pattern suggesting the moon is being squeezed or deformed by some kind of global stress – such as irregular rotation, a shifting orbit, or the migration of its poles. But simulations of those processes don’t produce landforms that quite line up with where the streaks are.
One thing is clear, though: The streaks are relatively young. Normally, dust from Saturn’s E ring and charged particles from space would erase the smudges. But they’re still there. And, they’re drawn on top of the Odysseus basin, meaning that the crater came first. Scientists aren’t sure precisely how old Odysseus is, but Schenk suggests it couldn’t have been made more than 2 billion years ago.
Schenk’s best guess now is that the streaks are associated with fractures that Cassini can’t quite see, and that those fractures are currently forming or have been reactivated recently, exposing material that might not be water ice like the rest of the surface.
As Cassini’s days of exploring the Saturn system wind to a close, scientists are hoping to solve this little mystery – and spy on a host of otherworldly enigmas associated with the giant, ringed planet and its clutch of moons.
These preliminary conclusions are based on data from NASA’s Dawn spacecraft, which has been orbiting the dwarf planet Ceres since March. Large, round and watery, Ceres isn’t quite like the rest of the space rocks that live between Mars and Jupiter. And the more scientists learn about the place, the stranger it gets.
“Ceres has been a mystery,” says UCLA’s Christopher Russell, principal investigator for the Dawn mission, noting that there are no pieces of Ceres that have fallen to Earth. “We had to go out there and see what it was because we didn’t have the clues.”
Dried Up Salty Spots?
Before Dawn even spiraled into orbit around 950-kilometer-wide Ceres, scientists were transfixed by what appeared to be extremely bright splotches on the world’s surface. For months, the Dawn team guessed the splotches might be made of highly reflective water ice –- and even suggested that one of them could be spitting water into space. Now, as often happens in science, it looks like those early guesses weren’t quite right.
As Dawn neared Ceres, the team realized the spots were dimmer than they’d expected – and not nearly bright enough to be water ice. (In fact, Ceres itself is more or less as reflective as freshly laid asphalt – which is to say, not very bright at all.)
“The next brightest thing is salt,” Russell says, referring to the spots. “There’s a number of different salts that could have been made in the interior by the chemistry that goes on between rocks and water.”
The team doesn’t know precisely what the spots are made of, but scientists suggest they could be reflective, sulfate-containing salts – the kind of stuff that might be left behind as ice tucked into salty crystals warms and turns into water vapor, says Andreas Nathues of the Max Planck Institute for Solar System Research.
“The bright spots are remnants of a water ice sublimation process,” he says.
So far, Dawn has taken its closest look at the cluster of spots in Occator crater, which are the biggest and brightest, and also among the youngest. Mostly found inside craters, more than 130 of the enigmatic splotches sprinkle the world’s surface. Nathues notes that the spots’ occurrence within craters is no accident, and that the team suspects impacts trigger their generation by digging into a buried, frozen layer of water. So it’s likely that while water ice still exists in Occator, it has disappeared from the older spots.
“We think these bright spots have the same origin,” Nathues says. “Most of them are today dehydrated.”
But there’s more: Observations suggest that each morning, as the sun rises, a morning haze or mist fills two of Ceres’ craters, Occator and Oxo. The haze isn’t very thick and is likely less than a few hundred meters high – and it disappears by dusk, only to reappear the next morning. Nathues suspects the morning mist could be produced as buried water ice is warmed by the rising sun. If it’s there, that hovering water vapor could explain a 2014 observation made by the Herschel space observatory, which showed that Ceres had thin tufts of water vapor surrounding it.
“The longitudinal position at which Herschel found the strongest water vapor absorption line fits with the longitude of Occator,” Nathues says.
But it’s still too soon to say if the haze is really there, says William McKinnon of the Washington University in St. Louis. He’s unconvinced that what appears to be haze isn’t just an artifact of the angle at which Dawn is peering into these craters, and says he’d like to reserve judgment until the spacecraft can take an even closer look at what’s going on.
“It’s not a slam dunk,” McKinnon says. “Dawn is going into its lowest orbit, so presumably will be getting even more and better pictures of this puppy in the coming months.”
At a recent conference, scientists shared some observations that might support this idea, and the same observations were published in Nature today: On Ceres’ surface, Dawn spotted what appear to be clays containing ammonia, which isn’t a compound that’s normally found in the warm inner solar system. Here, ammonia-containing ice would just evaporate. That’s why Maria Cristina De Sanctis, an astronomer at Rome’s National Institute of Astrophysics, says she wasn’t expecting to see this particular type of clay, known as ammoniated phyllosilicates.
“So the question boils down to, where does the ammonia come from?” McKinnon asks. “Maybe stuff from the outer solar system got mixed into the asteroid belt and got widely distributed. Or maybe Ceres as a whole got implanted in the asteroid belt.”
The only solid surfaces where significant amounts of ammonia have been found are in the outer solar system: Charon, Pluto’s largest moon; Orcus, another body in the Kuiper Belt; and Miranda, the innermost moon of Uranus.
But both McKinnon and Andy Rivkin, of the Johns Hopkins University Applied Physics Laboratory, note that recent work has uncovered tiny amounts of ammonia in meteorites that have been chipped off of main belt asteroids. “If it turns out to be ammoniated phyllosilicates, which is a decent if not slam dunk bet, that doesn’t mean it has to be an escaped Kuiper Belt Object or necessarily have these outer solar system pebbles,” Rivkin says.
It’s also possible, they both say, that Ceres was once much more active, and that water percolating through its interior may have helped form and push the the ammonia-containing clays to the surface, concentrating whatever small amounts of ammonia were naturally inside Ceres into a region where Dawn could see it.
Determining which of these scenarios actually took place will be complicated, especially from orbit – but as Dawn snuggles in close to Ceres this week, scientists will give it their best shot.
It’s not every day you hear about a troubled spacecraft making a desperate attempt to cling to a planet — for the second time.
After missing its first chance to orbit Venus, Japan’s Akatsuki spacecraft circled the sun for five long years, waiting for the right time to try again. That moment came on Dec. 7, a half-decade to the day after a broken nozzle sent Akatsuki hurtling toward the sun instead of falling into the gravitational clutches of Earth’s sister planet. But with its large, main engine crippled, the spacecraft needed another way to slip into orbit. That responsibility went to four smaller thrusters that are normally used to adjust where the spacecraft is pointing; for 20 minutes they fired, nudging Akatsuki onto a course for capture as it skimmed the Venusian cloud tops.
And then the team commanding the spacecraft, at the Japan Aerospace Exploration Agency, waited. Shifts in the radio waves Akatsuki uses to communicate with Earth would indicate whether the spacecraft had changed course. If it hadn’t, there was a small window in which the team could try again, using an alternate set of thrusters. And if that didn’t work? Well, no one wanted to think about that. The third time might be the charm on Earth, but in space, getting even a second chance is exceedingly rare.
An hour later, scientists shared the exciting news: “It is in orbit!!” reported Sanjay Limaye, of the University of Wisconsin-Madison, who was with the team in Sagamihara, Japan. The question was, which orbit? Was it stable? Could the team talk with the spacecraft? Is Akatsuki healthy?
It would take two days of analysis to work out the precise path Akatsuki is taking around the cloud-shrouded world. On Dec. 9, scientists announced that the orbit is a bit more stretched out than anticipated, though it’ll do. Now, Akatsuki takes a little more than 13 days to orbit Venus, swinging from 400 kilometers above the planet’s surface out to 400,000 kilometers away. Over the next four months, engineers will reshape the spacecraft’s path so it will complete one trip around Venus in fewer days — and then in April, the real science observations will begin.
For now, the team will be making sure that all the instruments on board the spacecraft are working. After all, Akatsuki was originally designed for a two-year mission in space — an amount of time it has already surpassed while circling the sun. The spacecraft’s extended trip around the solar system brought it closer to the sun than scientists had intended, meaning Akatsuki’s instruments have encountered temperatures a bit warmer than they were designed for.
So far, though, the team says the spacecraft is healthy — and as proof that at least some of the onboard cameras are undamaged by Akatsuki’s sojourn in space, has released three images taken as the spacecraft slipped into orbit (see gallery, above). These are the first closeup images of Venus we’ve seen in a while, and with no new missions to the planet on the schedule, they could be among the first of the last for the foreseeable future.
If all goes well, Akatsuki will hover above the planet for the next two years, peering into the churning, super-rotating atmosphere that whips around Venus faster than the world itself rotates. Once thought to be very much like the Earth, the Venus of today is a hellish wasteland, transformed into a planetary inferno by runaway greenhouse gases. It has the hottest recorded temperature in the solar system (aside from the sun), a young surface that shows signs of recent volcanism, and sulfuric acid clouds that flash with lightning.
But from afar, Venus shines more brightly than just about anything else in the sky. It’s a shimmering, tranquil pinprick of light that has bewitched astronomers, poets and entire cultures for millennia — and now, after a drama of interplanetary proportions, it has a plucky little robot friend once again.
Two weeks ago, I wrote about how difficult it can be to focus on science when the world is tearing itself apart. As this latest terrible chapter is being written into the already overwhelming history of violence on Earth, I worry I might find myself seeking solace from that post’s optimism more often than I want to.
In it, a friend reminded me that tales of science and adventure can be powerful antidotes to stories of suffering and destruction. Exploring new worlds and uncovering new knowledge? “That’s what we should be doing,” Kareem Shaheen, who covers the Middle East for the Guardian, told me.
And so today, when NASA revealed the newest images of Pluto taken by the New Horizons spacecraft, I switched off everything and dove into the intricate, exotic landscapes of an alien world. These are the highest resolution images of Pluto we might see in our lifetimes — images where features smaller than a football stadium are visible. In them, we see jumbled blocks of ice-mountains that look as though they’re being pushed to the shoreline of a frozen sea that’s bubbling in slow motion. Impacts that excavated chunks of Pluto’s surface reveal curiously colored layers beneath its crust. There are pits and ridges that look as though they’ve been stretched and bent as Pluto’s ices move across its surface, areas where erosion has sculpted some intricate landscapes, and things I’m having a hard time even describing.
There’s no question that exploring this distant, icy world is a story about the human mind and spirit at its best; so give yourself a break and zoom over these Plutoscapes.
Billions of years ago, a piece of interplanetary debris smashed into Pluto and left an 825-kilometer-wide crater. But instead of turning into an ugly pockmark, that mighty scar may be responsible for one of Pluto’s most charismatic features: Its icy heart.
Recently, observations revealed a circular feature surrounding the western ventricle of the striking, heart-shaped region known as Tombaugh Regio.
“The question is, could this be a relic giant impact basin?” asked Paul Schenk, at the 47th meeting of the American Astronomical Society’s Division for Planetary Sciences. “The key to that question is, is it deep? And the answer is yes.”
The basin is about 4 kilometers deep and stretches roughly one-third of the way across the icy world, said Schenk, of the Lunar and Planetary Science Institute. Finding a similarly sized gouge means going all the way to the other end of the solar system – to Mercury, where the Caloris impact basin stretches roughly one-third of the way across that roasted, dense world. (To put this in Earthly perspective, it would be as if a crater obliterated everything between Mexico and Canada, from California to North Carolina — aka, the majority of the United States.)
On Pluto, the smooth icefield known as Sputnik Planum sits within the basin. It’s a region that — unlike the possibly 4-billion-year old scar — is relatively young, at about 10 million years old. The edges of the basin are steep in the north but degraded in the south, where a large portion of the rim is missing. But the general shape is still obvious.
“It is indeed circular, except for the southern extension,” Schenk said. “The floor is basically flat.”
The impact created a depression that may have been perfect for accumulating flowing ices, especially given its location on the side of Pluto that never sees its large moon, Charon, said Douglas Hamilton of the University of Maryland, College Park. Pluto and Charon are locked in a whirling dance in which they keep the same face pointed at one another all the time. So Charon never fills the skies over Sputnik Planum – and none of the meager sunlight it reflects as Charonshine ever warms the cockles of Pluto’s heart.
That geometry helps make the region an efficient cold trap, or an area where ices can congregate. And that does seem to be what’s going on here. Will Grundy, from the Lowell Observatory, reported at the meeting that Sputnik Planum is full of basically every type of ice that has been spotted on Pluto, including carbon monoxide, nitrogen, and methane. The only species that’s conspicuously absent is water ice, which makes up the planet’s soaring mountains.
In fact, Sputnik Planum is more like an ice cap than anything else.
“We see ice caps throughout the solar system — ice caps on Earth, ice caps on Mars,” Hamilton said. “What we have to do is explain why this ice cap is at 30 degrees north on Pluto.”
It will take a bit of work to sort out exactly what happened, but scientists are well on their way to solving the mystery of Pluto’s heart.
The strange, duck-shaped comet that ESA’s Rosetta spacecraft has been orbiting for more than a year just got a bit stranger: Like plants on Earth, the comet is blowing molecular oxygen, O2, into the space around it. Molecular oxygen is thought to be rare in the cosmos – or at least exceptionally tricky to detect.
“It is the most surprising discovery we have made so far,” says Rosetta team member Kathrin Altwegg of the University of Bern. The team first spotted the oxygen about a year ago and took its time ruling out sources other than the comet itself. “The first time we saw it,” Altwegg says, “I think we all went a little bit into denial because it is not expected to be found in a comet.”
Of course, molecular oxygen is common on Earth, having first been pumped out in enormous quantities by photosynthetic blue-green algae about 2.5 billion years ago. Until now, though, astronomers have only spotted gaseous O2 in a handful of other places, including two distant molecular clouds. The new observations, reported today in Nature, not only force a reconsideration of the very early solar system, they also throw a bit of a curveball at scientists hoping to identify the signatures of life on other worlds.
“The finding is definitely a wake up call for exoplanets and the search for life,” says Sara Seager of MIT. “O2 is the most prominent gas on our biosignature gas list.”
Back to the Beginning
Comets are icy, space-traveling time capsules. Unlike planets, where internal ovens have more or less cooked and rearranged the planet’s ingredients, a comet’s original building blocks are preserved. So, scientists can use the icy dirtballs to peer back in time, all the way to the beginning of the solar system when small bits of frozen debris were colliding and forming comets. As the thinking goes, the molecules trapped in a comet reflect the composition of the dusty primordial nebula that swirled around the very young sun.
Out there, far beyond the orbit of Neptune, temperatures were obviously quite cold. But until now, no one thought it was cold enough or placid enough for two oxygen atoms to meet, link up, and stay together.
“All the models say it shouldn’t be there,” says study author Andre Bieler of the University of Michigan in Ann Arbor.
And while molecular oxygen only accounts for a small percentage of the total amount of stuff escaping from the comet – about 3.8 percent, relative to water – finding it at 67P is still enough to make scientists reconsider the composition and temperature of that primordial dust cloud. “This ice hasn’t been heated up enough to be reprocessed,” Bieler says.
Gassy molecular oxygen has only been observed around two other stars, suggesting that it’s a rare component of the interstellar medium. Perhaps, scientists now say, that result reflects the difficulty of detecting O2 remotely.
“When we find new molecules in comets, they’ve nearly always been found in the interstellar medium,”says Mike A’hearn of the University of Maryland, College Park. But, he adds, “the abundance [in 67P] is low enough that it’s unlikely we would have ever seen it in remote sensing.”
O2 and the Search for Little Green Microbes
Here’s what the problem could be for scientists hunting for the signatures of life on exoplanets: No one thinks there are exhaling microbes on 67P. Yet molecular oxygen, as Seager says, is at the top of the list of gases that could indicate the presence of extraterrestrial life. And if it is naturally common in the cosmos, then O2 might need to be reconsidered as a potential biosignature.
On the other hand, high levels of O2 in a planet’s atmosphere could still reflect the presence of life.
“The O2 lifetime is so short in atmospheres it won’t be present for long unless it is continually produced,” Seager says, noting that there are many ways to produce molecular oxygen that don’t necessarily involve life. “The comet shows us there are situations we hadn’t considered, and this will happen over and over again.”
Something is rotten in the state of Charon, Pluto’s giant moon. OK maybe not rotten, exactly, but it’s just not…right.
Before the New Horizons spacecraft took a tour of the Pluto system in July, scientists guessed they might see a few interesting features on Charon — things like craters, and more craters, and maybe some notable landforms because Charon’s water ice surface is sturdy and capable of supporting dramatic landscapes. Several team members even dared to suggest that Charon would be surprisingly interesting — perhaps even more interesting than Pluto.
But overall, guesses seemed to converge on Charon being a dead, cratered husk.
“Craters were the number one thing people said we’d see on Charon,” said Amanda Zangari of the Southwest Research Institute, who conducted a survey of New Horizons team members before the flyby. But, she noted back in June, “I think we’re going to find that Charon is not just a dull, gray featureless thing.”
Turns out, Charon is totally not a dull, gray featureless thing.
It has curiously few craters, especially in its southern hemisphere, and a surface riven with massive canyons (some of which have cool informal names, like “Serenity Chasma”). It even has a mountain in a moat. Charon’s north pole is reddish — the same color red as the band near Pluto’s equator — and there’s a dark blotch on the hemisphere that faces away from Pluto.
In fact, the whole moon is dark, way darker than Pluto. Charon might not look especially shadowy in the image above, but that’s because of how those pixels were processed. If you look at the giant moon in relation to Pluto, its companion in binary planethood, you’ll see just how spooky it is. While Pluto is splattered with bright ices and colorful smears, Charon looms in the background like a scarred, charred world emerging from the darkness.
To me, it looks like half of Charon’s crust is missing, as if someone started peeling it like an orange and gave up after the southern hemisphere. The remaining northern rind is cratered, stained and old, while the southern pith is younger and smoother. The real story is probably much more interesting (yes, even more interesting than a giant peeling a moon). One early idea explaining the startling differences in terrain between Charon’s hemispheres invokes cryovolcanism, or the eruption of icy, frozen, maybe oozy materials. Icy satellites in the outer solar system appear to be particularly good at generating flavors of frozen “lavas”: Think geysers on Enceladus, plumes on Triton, and (possible) ice volcanoes on Titan.
Once upon a time, as the story might go here, a buried ocean sloshed inside Charon’s icy shell. At some point, as the moon froze, pressure started to build in the liquid water locked inside. As that pressure forced the water upward, it found a weak spot in Charon’s shell — and then all hell broke loose. Like water bursting through a crack in a dam, the ocean spilled onto Charon’s surface, coating part of it in smooth, young plains and erasing all the stuff that was there before. Younger crust, explains The Planetary Society’s Emily Lakdawalla, would be denser and thus sink below the older crust, explaining the elevation differences between Charon’s halves.
If that did happen, it’s not the only violent event in Charon’s history. After all, the Pluto system was born from a giant impact — a collision that knocked proto-Charon into proto-Pluto and chucked out all kinds of icy shards that ended up forming the smaller moons. How the system evolved into the spectacularly diverse cast of characters we see today — paintballed Pluto, fractured Charon, super-dark Kerberos, odd-looking Nix, and so on — is still a mystery.
*Thank you to Emily Lakdawalla and Sarah Hörst for helpful discussions.*
Well, it depends on who’s asking. To a virus, we’re colossal, even vast. To a giraffe, we’re small. If it’s me asking, a virus looks microscopic (minuscule?), while the solar system—ah, the solar system—has gotta be in the colossal-to-vast range, but I really have no idea. I can look up at what might be Mars (the rosy-looking one) in the night sky, but I haven’t the imagination, the metaphor, the math to make sense of that distance. All I know is what Doug Adams says in The Hitchhiker’s Guide to the Galaxy: “Space is big. Really big. You just won’t believe how vastly hugely mindbogglingly big it is.” Yup. That’s how I measure deep space: I don’t. My mind just boggles.
But this week, along with a million or so other folks, I saw the light. Or rather, I saw space. I saw, maybe for the first time, how hugely mindbogglingly big space is. Two wonderful filmmakers, Wylie Overstreet and Alex Gorosh, figured out what’s wrong with every image of the solar system we’ve ever seen. In every one, they say, space gets cheated. Planets get exaggerated. And in their short film To Scale: The Solar System, they fix that.
What they do is build our solar system with the heavenly bodies true to scale, which means the sun, Mercury, Venus, and, all the way out, Neptune (sorry, Pluto) are crazily small. Space, meantime, gets back its vastness. As you see here, their Earth (this is Overstreet demonstrating) is a little marble.
Using a ten-foot chain-link fence hooked to the back of their car, they created the orbits of all eight planets on a dried lake bed in Nevada (Black Rock Desert, home to Burning Man), carving ellipses into the sand. Then, when night fell, they drove the orbits, Gorosh holding a large lamp out the car window. The resulting time-lapsed film was composed into a carnival-looking, swooshing solar system, with teeny planets poised on poles, each a pinpoint of light.
The most wonderful moment comes at the very end, when we stand nose to nose with the marble that is Earth and look back at the actual sun coming up in the east and, astonishingly, their model sun and the real sun … match! They’re the same size. So the model suddenly feels real, and that’s when Overstreet takes Earth and tosses it along the desert floor so it rolls into orbit, and you see, really think you see, how small (minuscule? tiny? Lilliputian?) our little planet—home to all of us—actually, really is.
It’s lip-bitingly beautiful.
WATCH: A group of friends build a scale, illuminated model of the solar system stretching seven miles across a dry lakebed in Nevada. Video courtesy Wylie Overstreet and Alex Gorosh
The more we see of Pluto, the more stunning it gets – and that’s not because we understand what’s going on. It’s the opposite. Rather than neatly fitting pieces into a big, Pluto-shaped puzzle, NASA’s New Horizons spacecraft is returning data that make it clear just how unbelievably enigmatic this little world is.
How that all works, we don’t know – but if space exploration and planetary science were easy, it would be nowhere near as rewarding.
Understanding Pluto is a lot like climbing a mountain: The satisfaction comes from the struggle it takes to reach the peak. After all, no one writes home about simply driving to a summit — the views might be the same, but the sense of achievement? Not even remotely similar.
Earlier this year, I climbed Kilimanjaro for the first time, and last night, I got back from a few too-short days in the Canadian Rockies. There, bare slabs of rock erupt from tree-covered valleys, their ancient layers outlined by a fresh frosting of snow. Lakes tucked into canyons slowly collect the silty slough of glaciers and snowpack, which turn frigid water into brilliant, unearthly shades of green and blue.
Landscapes like this exert an almost irresistible pull on me. I want to know what it’s like to charge up those slopes and slip through those trees, to meet the boundary where that evergreen carpet lost its battle with gravity, and then to continue going, to tiptoe up to alpine shorelines, trek through alien landscapes and ogle the myriad ways nature sculpts Earth’s stones into craggy gargoyles.
For me, reaching a summit isn’t the sole motivation for a climb; in fact, on the highest mountains, lingering at the top is downright dangerous. On lower peaks, summits can be nice places to rest and enjoy the view, clouds permitting, but they’re merely markers signifying the end of a particular route (and descents can be more punishing, in many ways).
The tug, for me, comes from the struggle of the journey, in the breathless effort it takes to continue climbing higher and higher, step after burning step. It’s in the challenge of billy-goating through tricky passes and surviving slippery trails, in finding the path through the scree and in putting training and skills to the test.
It’s the same with figuring out Pluto. You have to love the uncertainty and the challenge, accept the questions and mysteries. As images of Pluto come back to Earth, the fun is not in foolproof explanations for how a particular feature came to be, but in the thrill of seeing something for the first time and knowing that right now, we don’t know what it’s doing there. It’s in the need to tinker with theories and revise what we think we know. It’s in allowing science to lead us to answers, as surely as relying on all that training will get us to a summit.
“The short version is, I don’t really know the answer,” team member Will Grundy said to me last week, when images of Pluto’s haze prompted me to ask what a weather forecast would look like on Pluto. He then proceeded to beautifully lay out the logic and the questions we’d need to consider while tackling the problem.
Right now, we’re still on the tree-covered foothills of Pluto, staring at its peak through needled branches and trying to gain the next ridge.
Someday, I would love to be able to tell the story of Pluto — how it took root in the swirling disk of dust and gas that ringed our infant sun and, over millions of years, grew into a mottled, complex world with a giant moon and four smaller, icy companions. When, in Pluto’s history, did its mountains first punch through the exotic ices frosting its surface? How did that even happen? Did Pluto get pummeled by space rocks early in life, and if so, what happened to all the craters from that era? When did Pluto start getting its wrinkles, which look so much like the lines carved into wizened human faces?
What will its next 4.6 billion years be like?
I’m glad I can’t tell that story now, because it means our journey isn’t over. Once we know everything about Pluto (if we ever do), there is no more up. No more adventures, no more unexpected obstacles, no more surprises…until we descend, spend a minute recuperating, and get to work finding another mountain, or another route to Pluto’s peak.
There’s a world near the solar system’s fringe that, in many ways, is more Earthlike than anyone could have imagined. Here, frosted glaciers slowly drain moisture from icy mountains that tower over smooth, lowland basins. Overhead, the sky is filled with haze – lots and lots of it, carved into multiple layers.
That world is Pluto.
In July, the New Horizons spacecraft flew within 8,000 miles of the dwarf planet’s surface, furiously snapping photos and gathering data about the composition of the world’s frosty veneer. Then, 15 minutes after its closest approach, New Horizons flipped around and caught a Pluto, just before sunset. Today, NASA released the spacecraft’s dusky postcard.
If you randomly stumbled across the image, you’d be forgiven for thinking you’d found a black-and-white image of the Earth.
The image, “reminds me of the Transantarctic Mountains along the Ross Ice Sheet, because of the tall mountains looming over a flat open expanse of ever-changing ice,” says New Horizons team member Simon Porter of the Southwest Research Institute.
On Earth, we have a pretty good idea of how mountains form. On Pluto, it’s not clear yet. There are no obvious volcanoes, eroding plateaus, or colliding tectonic plates. “The formation of blocky mountains on Pluto may be more analogous to the breakup of sea ice on Earth, but on a much grander scale,” says team member Alan Howard of the University of Virginia.
And then there are those glaciers, which scientists assume flow as glaciers do on Earth, advancing inch by inch and draining moisture from the mountains. Except Pluto’s glaciers are made from nitrogen, not water ice. Scientists think it’s likely Pluto has something analogous to Earth’s hydrological cycle, where water evaporates from the oceans, rains or snows back down, and returns to the seas by rivers and glaciers. But Pluto’s chemistry and temperature are different, perhaps dominated by soft nitrogen ice, and also featuring methane and carbon monoxide ices.
“What happens is that these ices evaporate from the icy plains informally called Sputnik Planum [the heart], are transported through the atmosphere tens to hundreds of kilometers, and then are deposited on the surface, probably as frosts,” Howard says. “Where enough frost has accumulated, it flows like glaciers back to the source, which we think is Sputnik Planum.”
So, a nitrological cycle, perhaps fueled by Pluto’s beating heart?
Among the features in the image that have scientists most excited is the haze shrouding Pluto’s surface – haze that is more complex than anticipated, and more voluminous than on Earth. “I still haven’t managed to wrap my head around the way that Pluto’s haze looks,” says Sarah Hörst of the The Johns Hopkins University. “Everything I know and believe about haze formation told me Pluto would have haze, but I really wasn’t sure we would be able to see it in the images.”
On Earth, hazes tend to form near the ground, when the gases in car exhaust (for example) reacts with sunlight to produce particles. On Pluto — and around Saturn’s moon Titan — haze forms all over, including high in the atmosphere, and is compositionally different than on Earth. For Hörst, the most remarkable thing about the haze in this new image are its spectacular layers, the origin of which is still a mystery. “The incredibly detailed structure will be able to tell us a lot about the atmospheric processes that are occurring here,” she says.
On Pluto, not everything hazy is alien, though. “If you were standing on the top of one of the mountains rising maybe 1.5 miles above the surrounding terrain, and looked towards the setting (or rising) Sun, you could see ground-hugging hazes like you often see on Earth,” Howard says.