The Kuiper belt was once a violent and chaotic place; collisions were common and entire worlds were created and destroyed. But over the eons, the solar system's hinterland calmed and found quiescence. However, the scars remain; scars that are just becoming visible as NASA's New Horizons mission races toward dwarf planet Pluto for a close encounter on July 14.
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Planetary astronomer Mike Brown, of Caltech in Pasadena, Calif., refers to these scars as the "blood spatter" of the solar system. In its formative years, the solar system was a hotbed of planetary formation and destruction and, like a forensics team seeking out evidence of a crime, astronomers look toward the Kuiper belt where this evidence has been frozen in time.
Charon: Frozen Witness Astronomers have discovered 4 more moons in orbit around Pluto and Charon, probable signs of a massive impact in the dwarf planet's ancient past. Other tiny worlds in the Kuiper belt also show signs of collisions. Interestingly, in the New Horizons forensics hunt, it's not Pluto that necessarily holds to key to scientists understanding the history of the solar system, it's Charon.
"As the newer images come in (from New Horizons), it's going to be great," Brown told Discovery News. "You can finally see for sure the one big impact crater (on Charon) with rays and a couple of other things that must be impact craters too - that's going to be where the history of the outer solar system is recorded, more on Charon's face than on Pluto's face."
Like our moon, Charon has little to no atmosphere, meaning that impact craters from millions to billions of years ago remain, pristinely etched into the rock with no atmospheric gases to erode the surface. On the other hand, Pluto has a more dominant gravity, which can hold onto a tenuous atmosphere that freezes and sublimes on the surface over the dwarf planet's 248 year orbit around the sun.
"Pluto is going to be interesting as all the frosts have reworked everything on the surface. But Charon is dead enough, a lot like our moon, which just sits there and records whatever hits it. That's where we're going to see (evidence) for all that blood spatter," he added.
On its approach, New Horizons imaged the Pluto-Charon system in color and confirmed a dichotomy; Pluto appears reddish, whereas Charon appears grey and monotone. For two orbiting bodies only 12,000 miles apart, this may seem a bit weird, but it's all down to gravity.
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"Pluto is massive enough that it can hold onto an atmosphere, mostly nitrogen, but a good bit of methane also and when that atmosphere condenses onto the surface, you get all these frosts that methane in particular turns red upon irradiation (by the sun)," said Brown. "We've known this for many decades now that Pluto is this color because of the methane on its surface. Charon is just small enough that it doesn't hold onto an atmosphere, it just escapes off into space."
Regarding the images to come from New Horizons so far, Brown is intrigued.
"Looking at Charon again - it's got the craters, its got other little bright spots that kinda blend into the background. I really want to know what those little bright spots are. I'm holding out for ancient ice volcanoes where water flowed on the surface in the distant past.
"On Pluto, like everybody else, I'm just baffled by what this really bright heart-shaped thing can be and I'm looking forward to having it turned around ... to figure out what the hell is going on. There's no good reason for it. The rest of the equator is dark, as it should be, as predicted 20 years ago. The equator is going to be dark because it's the warmest location and all the ices go away, but then the single brightest spot is almost dead on the equator too! That's just weird."
There's Something About Eris Brown is a prolific discoverer of new worlds in the outermost regions of the solar system. After his discovery of Eris in 2005, which led to the controversial decision by the International Astronomical Union (IAU) to "demote" Pluto from being a planet to a new class of celestial body known as a dwarf planet in 2006, Brown became playfully known as the "Pluto Killer" - a reputation he has embraced with his Twitter handle, @plutokiller.
At the time, it was thought Eris, which orbits the sun well beyond the average orbit of Pluto, was larger than Pluto. On Monday, however, New Horizons scientists announced that they had finally precisely measured Pluto's diameter, crowning it the "king" of the Kuiper belt - Pluto's actual diameter is bigger than Eris', but only by 28 miles.
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"It's a funny thing, as when we first discovered Eris, it was comfortably larger than Pluto because people had the size of Pluto wrong," said Brown. "Pluto has been gradually getting bigger for the past decade, so it's not that Eris has gotten any smaller; Pluto has just gotten bigger. Which is kind of funny."
It's worth noting that Pluto and Eris' masses have been precisely known for some time - Eris is around one-third more massive than Pluto. By measuring the characteristics of moons orbiting both dwarf planets, an accurate mass can be calculated. But Pluto's physical size has been hard to decipher.
From Earth, astronomers use a method known as "stellar occulation" to gauge the diameters of distant planets. As a planet orbits the sun, it occasionally blocks a distant star from view; this can be used as a celestial ruler to measure planetary diameters.
Eris orbits so far away from the sun that, for most of its orbit, any gases in its atmosphere freeze to its surface. But Pluto orbits closer to the sun and atmospheric gases can interfere with occulation measurements, blurring the starlight as the star passes behind. But when measuring Eris' diameter (even though the dwarf planet is further away), its lack of atmosphere makes the measurement more precise.
The fact that New Horizons has discovered that Pluto is larger than thought (but its mass has stayed the same) is critical for understanding its composition.
"The really interesting thing is that Pluto and Eris are almost exactly the same size and yet they are nearly 30 percent different in mass, so they're very different objects," said Brown. "Everyone almost likes to think of them as twins as they are close to being the same size, but I would say that they are very, very different."
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"Pluto is icier than we previously thought and it's got a good bit more ice than Eris does. Eris is almost all rock with an ice layer on the outer edge and Pluto is a lot less rock and a lot more ice."
And that seems to be a recurring theme in the Kuiper belt: from afar, each Kuiper belt object (or KBO) seems to have its very own characteristics.
A Mission for the Generations Although the New Horizons team is hoping to target more KBOs after its Pluto encounter, it's only likely that one, smaller object will be visited, and that will probably be a more distant encounter, said Brown. And as there's no plan to send more missions to the Kuiper belt any time soon, planetary scientists will be poring over the New Horizons flyby data of Pluto and its system of moons for generations to come.
"This is going to stand in for what I imagine Eris to look like, or Makemake to look like," Brown said. "Looking at Charon, it's kind of the size of objects like Quaoar or Orcus, so from having a very big blank palette in my head about what these objects could be before, now having concrete images, it changes how you think about these bodies completely. It's really as exciting as it can be."