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What's in a dwarf planet? First Pluto findings from New Horizons

A massive team of researchers has published the first paper about Pluto based on data received during the New Horizons flyby.

Pluto has been colour enhanced in this image to highlight details of its surface composition and texture. NASA/JHUAPL/SwRI

Up to and during its historic Pluto flyby on July 14 this year, space probe New Horizons collected some 50 gigabits of data. This is probably going to occupy researchers for years to come. Today, the first study has landed, revealing Pluto as an unexpectedly complex and idiosyncratic dwarf planet. Over 150 authors contributed to the paper.

"The Pluto system surprised us in many ways, most notably teaching us that small planets can remain active billions of years after their formation," said first author Alan Stern of the Southwest Research Institute. "We were also taught important lessons by the unexpected degree of geological complexity that both Pluto and its large moon Charon display."

The paper, published in the journal Science, examines in particular Pluto's complex and varied surface geology, and the behaviour of its moons Charon, Nix and Hydra.

Researchers already knew that Pluto's surface was diverse in colour. Flyby images show regions that are dark red, pale blue, golden and white. Now researchers have discovered that this colour diversity is indicative of diversity in Pluto's surface composition. The research also reveals a surprisingly broad variety of landforms and terrain ages on Pluto's surface.

"We knew Pluto's surface was heterogeneous (diverse) based on ground-based data," said astronomer Silvia Protopapa of the University of Maryland, who helped map Pluto's surface composition. "However, I was astonished to see such spectacular surface colour and geological diversity."

Protopapa and her team studied the surface compositions of Pluto and its moons by analysing data from New Horizons' Linear Etalon Imaging Spectral Array, which images the near-infrared signatures reflected by various elements. Certain materials absorb certain wavelengths, which allows the team to determine what an element is by looking at which wavelengths it reflects back.

The team found that there are a number of different volatile ices, associated with different regions of Pluto. The smooth, pale western lobe of the heart-shaped region, nicknamed the Sputnik Planum, is rich in methane ice and carbon monoxide ice. This is in addition to the water ice announced last week (but not included in this paper) discovered in the red areas.

These red areas, the team determined, evidence the presence of organic compounds called tholins, created by the energetic irradiation of methane, nitrogen and carbon monoxide in both gaseous and frozen states. What they are yet to ascertain is if these tholins are related to the presence of water ice, how they are related, and why water ice doesn't appear in other regions.

Nix (left) and Hydra (right), taken by New Horizons' Ralph and LORRI instruments respectively. NASA/JHUAPL/SWRI

The Sputnik Planum, in particular its smoothness, is also revealing. "SP is about the size of Texas, and unlike the rest of Pluto, there's not a single crater on it," Stern told Popular Mechanics. This means the region must be active enough for craters not to form. And evidence of ongoing activity has been found all over the planet.

"Evidence was also found for a water ice-rich crust, geologically young surface units, tectonic extension, surface volatile ice convection, possible wind streaks, volatile transport, and glacial flow," the paper's summary reads.

The research also included analysis of the first resolved images of Nix and Hydra, the smaller two of Pluto's five moons. These are more reflective than the larger moon, Charon. This is partially, researchers have found, due to a coating of surface water ice on the two moons. But even so, external forces should have darkened and dulled their brightness.

According to astronomer Douglas Hamilton of the University of Maryland, who helped analyse the New Horizons data on the moons, and who discovered Pluto's moon Kerberus in 2011 with Mark Showalter of SETI, this is because of their wacky orbits.

"We knew Nix and Hydra were slowly tumbling in unpredictable ways, based on ground-based findings we published earlier this summer in Nature. With New Horizons data, we now believe Nix and Hydra are spinning really fast and rotating in an odd way, and may be the only regular moons, meaning satellites that are near their host planets, which do not always point the same face toward their primary body," he said.

This, he continued, could be because Pluto and Charon dominate the system in a binary orbit. "It's possible that Nix and Hydra can't focus on locking one face toward Pluto because Charon keeps sweeping past and stirring things up," he said.

The team also confirmed that Nix and Hydra are roughly the same size, with Nix measuring 54 kilometres (34 miles) in length to Hydra's 43 kilometres (27 miles). The New Horizons data also shows evidence of no additional moons lurking in orbit around Pluto. Pluto's five moons are its only moons.

Of the 50 gigabits of data collected by New Horizons, only about 15 percent of has been downloaded to date. The New Horizons research team has its work cut out in the months and years ahead.

"The New Horizons mission completes our initial reconnaissance of the solar system, giving humanity our first look at this fascinating world and its system of moons," said Jim Green, director of planetary science at NASA Headquarters in Washington.

"New Horizons is not only writing the textbook on the Pluto system, it's serving to inspire current and future generations to keep exploring -- to keep searching for what's beyond the next hill."