Pluto Shows Its True Colors

“This is definitely an extraordinary system,” Cathy Olkin, New Horizons Deputy Project Scientist, told NOVA Next on Tuesday.

Pluto is a celestial oddball in more ways than one. It’s the only (former) planet that boasts a binary system status; Charon, its largest moon, is massive enough that the two bodies rotate around a common center of gravity. Some of Pluto’s moons wobble unpredictably as they orbit. The dark spot atop Charon—a feature that was, according to geologist Bill McKinnon, “a complete surprise” to the team—may have something to do with the way Pluto and Charon have historically interacted. The leading theory is that this material was originally found on Pluto but got absorbed onto Charon’s surface. Finally, the New Horizons team is identifying extreme bright and dark patterns that indicate a kind of diversity unmatched in the solar system by anything other than Earth itself.

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Pluto, as seen from New Horizons on the anti-Charon side of the dwarf planet

Light and Dark

“Brightness is very good because it says that the planet is active,” project co-investigator Bonnie Buratti told NOVA Next. Bright spots on Enceladus, Saturn’s highly reflective moon, once foreshadowed tales of plumes and geysers. But until the New Horizons team acquires more data about the specific geologic features and composition of the surface, most anything an expert can say about the latest image (above) is speculation.

Undulating terrain, multi-hued tendrils, and other linear features populate the snapshot, and to the left of the prominent heart feature (which scientists presume is some form of ice) lies a large crater and a veneer of dark material—probably made from complex hydrocarbons called tholin. Tholin forms when ultraviolet rays and galactic cosmic rays disturb methane, breaking down the molecules and generating a reddish gunk that gives Pluto its color. This area left of the heart is darker, redder, and possibly warmer. The heart itself may be eroding slightly over time, but we’ll get a closer look at its composition and landscape soon.

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Pluto rendered in false color—a technique that allows scientists to get a more precise reading on the distinctions between geographic areas.

Exaggerated color images that NASA released this afternoon show that the heart is actually two separate colors, one “ventricle” more blueish than the other. Pluto’s northern polar cap is a yellowish-orange. New Horizons scientists will be analyzing this photo and others to help determine what aspects of the dwarf planet they want to hone in on further. For now, the color photos suggest that some craters on Pluto could be two or three billion years old. When it comes to the system’s ancient past—including these craters’ formation—Charon may actually be “a better witness plate,” Olkin told NOVA Next. That means that in some cases, Charon might be better equipped to tell us Pluto’s secrets than Pluto itself.

“There’s nowhere on Triton that’s as old as this,” said science team co-investigator John Spencer in a group press interview. He’s referring to the now-outdated idea that Triton, Neptune’s largest moon, is a good analogue for Pluto. Triton is thought to have been captured from the Kuiper belt, and it’s similar to Pluto—they both have a dark equatorial region and bright poles. But as it becomes more apparent that Pluto is staggeringly varied in its chemistry and geology, the NASA team is re-considering that comparison. Spencer postulated that Iapetus and Callisto might be acceptable correlates for Pluto, since they also have sharp boundaries between light and dark terrain (also known as “albedo contrasts,” the difference in reflecting power between surfaces).

“The complexity of the albedo patterns is surprising,” Spencer said. “There’s an enormous variety of colors and they do seem to correlate with the geology.”

The Atmosphere’s Hazy Edge

NASA hopes to better understand the boundary between Pluto and interstellar space by analyzing its atmosphere. One reporter at Johns Hopkins Applied Physics Laboratory posed the idea that Pluto’s atmosphere may bleed into Charon’s, or vice versa; the New Horizons team member being interviewed said this could very well be a possibility. In a group press interview, Fran Bagenal of the plasma and particles team summed up what we do know so far: “This seems to be the purest nitrogen atmosphere that we’ve got.” The only other bodies in our solar system with comparable amounts of nitrogen are Earth and Saturn’s moon, Titan.

As the data starts coming in, scientists will calculate the escape rate of molecular nitrogen. “The Earth has lost some of its atmosphere over time,” Banegal said. “Mars has lost its water, too, and a lot of that has to do with atmospheric escape.” On Pluto, gravity is lower and the atmosphere is more tenuous—Banegal explains that gaseous escape from Pluto occurs at a distance seven times the radius of the dwarf planet.

Michael Summers, Deputy Lead of the Atmosphere Theme Team, thinks that the early Earth could be a good analogue for Pluto. “Understanding [Pluto’s] escape process, called hydrodynamic outflow, could be key to applying those ideas to the early Earth,” Michael Summers, Deputy Lead of the Atmosphere Theme Team, told NOVA Next.

“Every planet has its own personality, and each atmosphere does, too,” Summers said. “We’re not even close to understanding the diversity of atmospheres out there.”

Moreover, if Pluto has a troposphere—the lowest layer of Earth’s atmosphere and the site of all weather—it would be very small and therefore extremely complex. Summers says that our terminology for atmospheric conditions may change as we discover new phenomena. Pluto also experiences intense seasonal changes due to its tilt and eccentric orbit. Collectively, these factors could help us re-evaluate atmospheric physics, making for a more complete knowledge base of atmospheric physics and climatology throughout our many worlds.

Watch “Chasing Pluto” Wednesday 9/8c on PBS.