Everything You Need to Know About the First Comprehensive Study on Pluto

A tale of craterless plains and unusually shiny moons.

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Oct 15 2015, 6:00pm

Concept drawing of Charon from Pluto. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

Ever since its thrilling flyby of Pluto on July 14, the New Horizons spacecraft has been busy relaying fascinating data about this distant world and its unusual moons back to Earth.

Now, the journal Science has published "the first scientific results and post-flyby interpretations" of the Pluto-Charon system in its October 16 issue, which means we are finally getting the downlow on the real scientific meat of the probe's observations.

"There are many results in this [study] that have not been previously stated at all," Alan Stern, the principal investigator of the New Horizons mission, told me over email. "Equally importantly, this is the first time any results have been peer-reviewed, which is critical in scientific research."

For your reading pleasure, here is a point-by-point breakdown of what the comprehensive new research reveals about Pluto and its moons.

Pluto has been geologically active for a long time, resulting in a wide variety of perplexing surface features such as plains, mountains, and glacial islands. These landscapes may have been caused by erosion or crustal recycling, which occurs when a planet's outer crust is tectonically subducted into its mantle.
As soon as New Horizons started sending back information about the flyby, scientists noted obvious evidence of recent geological activity. Some areas of the dwarf planet, such as the Cthulhu Regio, are heavily cratered, while regions like the Sputnik Planum have no visible craters at all. These smoother patches are likely shaped by erosion and perhaps even plate tectonics, the authors said.

Sputnik Planum. Image: NASA/JHUAPL/SwRI

Many of these planetary processes are also observed on Charon, demonstrating that recent geological activity is not unusual in the Kuiper Belt—a massive circle of planetoids and debris circling the solar system. This bodes well for missions targeting other Kuiper Belt objects, because it shows that far from dead, static worlds, this new frontier of the solar system may be packed with worlds that are as dynamic as any of Earth's closest neighbors.

"[O]ther small planets of the Kuiper Belt, such as Eris, Makemake, and Haumea, could also express similarly complex histories that rival those of terrestrial planets such as Mars, as Pluto does," the team noted.

Pluto has one of the widest ranges of brightness variability in the solar system, and is home to a diverse color palette.
Pluto is so dark at the equator and so bright in the heart-shaped Tombaugh Regio that it ends up second only to the Saturnian moon Iapetus in terms of its range of reflectiveness.

This "tells us that the surface has very different reflectivity in different places and that there is a much higher range of reflectivities on Pluto than essentially any other body in the solar system," Stern told me.

As with so many findings from New Horizons, the team is not yet sure what accounts for this diversity of luminosity on Pluto. The brightest areas suggest the presence of highly reflective methane ice, but the origins of darker regions—such as those at the dwarf planet's northern pole—are still under investigation, as is the puzzling distribution of bright and dark regions.

Pluto has also been revealed to be a very colorful world, thanks to deposits of organic materials known as tholins. "Energetic radiation falling on Pluto's atmosphere and surface, each rich in nitrogen and methane, likely creates tholins that even in small concentrations yield colors ranging from yellow to dark red," the team said. Charon was also found to be rich in tholins, and accordingly, was also rich in color.

Enhanced color portrait of Pluto. Image: NASA/JHUAPL/SwRI

Pluto's surface pressure is lower than expected, and its atmosphere stretches out 150 kilometers into space.
Not surprisingly, Pluto has a very low surface pressure, measuring only 10 microbars during the flyby, compared to Earth's 1,013,250 microbars. As it turns out, this is actually even lower than scientists originally predicted, which is one of many discrepancies the authors hope to figure out with future data. It's possible that Pluto's atmosphere has recently lost some mass, the authors posited, or that perhaps there was an error in the techniques that were used.

The team also found that a rich atmospheric haze extends 150 kilometers above Pluto's surface, and contains two previously undiscovered compounds—the hydrocarbons acetylene (C2H2) and ethylene (C2H4).

Finally, Pluto's moons Nix and Hydra are really, really shiny for no good reason.
The new findings confirm that Pluto does not have any major satellites beyond Charon, and the smaller moons Nix, Hydra, Styx, and Kerberos. The spacecraft is still processing its close-ups of Styx and Kerberos, but fortunately, the study does include more information about Nix and Hydra, which were both revealed to be much brighter than expected.

The high reflectivity of these two small worlds—each only a few dozen kilometers in diameter—suggests that they sport a lot of fairly clean water ice on their surfaces, which is somewhat baffling to the New Horizons team.

Nix (left) and Hydra, as imaged on July 14. Image: NASA/JHUAPL/SwRI

"How such bright surfaces can be maintained on Nix and Hydra over billions of years is puzzling, given that a variety of external processes (e.g., radiation darkening, transfer of darker material from Charon via impacts, impacts with dark Kuiper Belt meteorites, etc.) would each tend to darken and redden the surfaces of these satellites over time," the authors pointed out.

In short, the Pluto-Charon system continues to surprise scientists, presenting them with fresh quagmires to untangle with every new downlink. It's worth reading the new Science paper just to get a sense of how each new datapoint seems to raise ten times more questions about the origins and ongoing evolution of these worlds.

I asked Stern if he has any hunches about which Plutonian mysteries are likely to be solved next. "Yes," he said, "but I can't speak to that because it's embargoed for some releases coming soon, and a new wave of papers to be submitted to Science."

In other words, stay tuned, because more dispatches from the Kuiper Belt are in the works.

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