Pluto is a weird little world with a familiar planetary recipe.
If you combine many of the materials found in the solar system -- rocks, water, methane, nitrogen, carbon-monoxide -- and assemble them into an object far from the sun, pummel it with impacts from other objects in the solar system and then wait billions of years, you might produce something like Pluto.
Scientists have some sense of this recipe because they got their first close-up look at the underdog dwarf planet when NASA's New Horizons spacecraft flew past Pluto in July, beaming back breathtaking images of the complex geology and atmosphere of a world never studied from close range.
The probe is continuing to send back the data it collected during the flyby, giving researchers drips of tantalizing information about the world.
Now, months after New Horizons' close approach, scientists are slowly starting to piece together exactly where Pluto fits within the context of the solar system.
"I can tell you, no one expected what we found, which is this stunningly complex little object that is alive on a massive scale today," New Horizons principal investigator Alan Stern said in an interview.
He and his colleagues published five new Pluto studies in the journal Science this week.
In many ways, pieces of the small world do look somewhat familiar.
Pluto's layers of atmospheric haze look similar to those on Saturn's moon Titan, and it may even have a liquid subsurface ocean like those found on Jupiter's Europa or Saturn's Enceladus.
Triton, Neptune's largest moon, also appears to be compositionally close to Pluto, with volatile, exotic ices like methane on its surface.
But even that comparison isn't exactly fair.
Triton on steroids
"Our model going in was Triton... or maybe Triton on steroids," Stern said.
"But it's not Triton, and it's not Triton on steroids. It's really its own thing, and we've never been to a small planet that's out there on its own and formed in the solar nebula."
Pluto's main distinctions from other objects in the solar system come from the fact that it is so far from the sun and not orbiting another body.
Because of Pluto's extreme distance, the molecules that compose it behave differently than they would on other objects nearer to the star.
For instance, molecules of methane, carbon monoxide and nitrogen appear as ice on the world's surface, whereas those molecules on Earth are mostly in a gaseous state.
These ideas are straightforward enough in theory, but seeing them in practice is another story.
"The expectation all along was that these things [molecules] would be there, but I think people didn't really put one and one together and realize how these two different classes of materials could work together to create active geology even at really, really low temperatures," co-author of one of the studies, Will Grundy, told Mashable.
Nitrogen glaciers flow in a part of the world called Sputnik Planum, which makes up part of Pluto's distinctive heart.
Huge blocks of water-ice appear to float on the nitrogen as it moves on the surface.
"It's a world that has undergone many of the processes that we've seen on other worlds," said planetary scientist Sarah Hörst, adding that even though we see similarities, the physics and the chemistry of these glaciers and other objects on Pluto are still completely new.
Sputnik Planum itself is also incredibly young in geological terms, at less than 10 million years old, but other parts of Pluto are ancient, having formed not too long after the solar system did more than 4 billion years ago, according to one of the new studies.
A new testbed in the solar system
Pluto also plays host to huge mountains the size of the Rocky Mountains that are made of water-ice. On Earth, this kind of ice is too brittle to support large mountain ranges, but on Pluto, these mountains can grow high thanks to extremely cold surface temperatures.
Other molecules like methane also appear to seasonally condense from the atmosphere onto the high peaks of the ice mountains, covering them in frost, the studies suggest.
"It's a really important way for us to test our understanding of how these things work," Hörst, who is unaffiliated with the New Horizons team, said. "Okay, great, we think we understand what determines the height of mountains and how mountains form, now make them out of water-ice."
Titan may also have mountains made of water-ice, according to Hörst, but that moon is much warmer than Pluto, plus its gravity is different. So now scientists have two testbeds with which to figure out how planets form ice mountains in different parts of the solar system.
The same goes for Pluto's nitrogen, carbon-monoxide and methane-rich atmosphere. Scientists can now compare the way Pluto's atmosphere looks to that of Triton or Titan, which have similar compositions.
This may seem like a small question, but it gets to a fundamental desire to understand exactly how our cosmic neighborhood functions and how it came to be.
Catastrophic birth and subsurface oceans
These discoveries are also leading to a fuller understanding of Pluto's evolution over the course of more than four billion years.
The Pluto that New Horizons saw during its flyby of the dwarf planet is the product of a violent, catastrophic event that formed the small world and its system of moons.
Scientists still aren't exactly sure of the details of the cosmic crash, but it is clear that some kind of impactor slammed into an early version of the dwarf planet.
That impact likely formed Charon and left a mess of material circling the world which eventually condensed into Pluto's other, smaller moons Kerberos, Nix, Hydra and Styx.
After their formation, Pluto and Charon may have been left with subsurface oceans that drove a fair bit of the geology seen by New Horizons last year, one of the new studies says.
Scientists now think that Charon's subsurface ocean -- if there was one -- likely froze, creating the incredible, complex fissures and cracks seen on the surface of the moon, one new study suggests.
When the ocean froze, it would have expanded, stretching Charon from the inside out, New Horizons scientist William McKinnon said, breaking up the crust.
The New Horizons team has also found some evidence for a subsurface ocean that may still exist within Pluto.
Scientists think that Pluto is about two-thirds rock, and within that rock are elements that are slowly degrading -- like uranium -- possibly keeping the world warm enough to sustain an ocean beneath its icy surface.
"It's a largely theoretical argument," McKinnon told Mashable.
"When you put the numbers together, it really could have a sustained ocean. There's really no hard argument you could make that it has to have frozen."
Kuiper belt context
Pluto is likely a great example of bodies in its part of the Kuiper belt -- the mass of cold objects in the dwarf planet's part of space -- but scientists aren't done exploring that large swath of the solar system.
The Kuiper belt tends to be divided into different parts.
On the one hand, Pluto's neighborhood is perhaps dominated by other, similar objects, which may have started closer to Neptune and then migrated out to where they are now.
This massive solar system restructuring would have occurred about four billion years ago, when the orbits of the giant planets shifted somehow, McKinnon added.
Those potentially movable bodies, like Pluto, may have gone through some serious changes after the solar system formed, leaving them with complex geology, though no one is exactly sure.
In fact, seeing Pluto close-up may have produced more questions than answers.
"I think everyone kind of assumed that Pluto would probably be relatively heavily cratered, not really have a whole lot of interesting things going on geologically, and that's obviously not the case," Hörst said.
"So, what does that mean about the other large objects [in the Kuiper belt], are they also that way, or is Pluto a special case potentially because of the catastrophic things that obviously happened to it? Maybe they all look like that. We don't know."
Other objects farther out in the Kuiper belt, known as classical Kuiper belt objects, are likely pristine examples of the original material that existed as the solar system first formed.
And soon -- well, soon by scientific standards -- New Horizons will get a chance to explore one of those pristine objects when it flies by 2014 MU69 in the Kuiper belt in 2019.
2014 MU69 was discovered by the Hubble Space Telescope and likely belongs to the second, non-Pluto-like category of Kuiper belt objects that may represent the solar system's earliest material.
Scientists working with New Horizons can't wait to check it out up-close, assuming NASA funds an extended mission for the spacecraft.
"We basically know nothing about it [2014 MU69]," McKinnon said. "For all we know, it's a binary world itself."
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