Scientists working with data from the New Horizons spacecraft have published a series of papers revealing for the first time detailed information and analysis of the geology, atmosphere and behaviour of Pluto and its moons.

New Horizons has been sending back data and images of the distant dwarf planet and its satellites since the spacecraft carried out a successful fly-by in July 2015, collecting 50GB of measurements in the process. About half of that data has now been transmitted back to Earth, and all the remaining readings are expected to arrive by the end of 2016.

Pluto's surface



NASA/JHUAPL/SwRI


The team was able to date the age of Pluto's surface by counting how many craters were visible. They found that the dwarf planet has been geologically active throughout the past four billion years. There are signs of relatively recent geological formations, too. Nasa said that "the surface of Pluto’s informally-named Sputnik Planum, a massive ice plain larger than Texas, is devoid of any detectable craters and estimated to be geologically young - no more than ten million years old."

The dwarf planet's surface proved to be far more diverse and active than anyone had anticipated. Jeff Moore of Nasa's Ames Research Center said that "observing Pluto and Charonup close has caused us to completely reassess thinking on what sort of geological activity can be sustained on isolated planetary bodies in this distant region of the solar system, worlds that formerly had been thought to be relics little changed since the Kuiper Belt's formation."

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Its icy landscape is primarily made up of a combination of highly volatile and mobile methane, nitrogen and carbon monoxide ices, alongside inert and sturdy water ice. This leads to what Will Grundy of the Lowell Observatory describes as "fascinating cycles of evaporation and condensation" that are "a lot richer than those on Earth, where there's really only one material that condenses and evaporates - water. On Pluto, there are at least three materials, and while they interact in ways we don't yet fully understand, we definitely see their effects all across Pluto's surface."

Grundy and his team's paper concluded that "although Pluto's durable [water] ice is probably not active on its own, it appears to be sculpted in a variety of ways through the action of volatile ices of [nitrogen] and [carbon monoxide]. [Methane] ice plays a distinct role of its own, enabled by its intermediate volatility. [Methane] ice condenses at high altitudes and on the winter hemisphere, contributing to the construction of some of Pluto’s more unusual and distinctive landforms."


Pluto's atmosphere

Taken 15 minutes after New Horizons' closest approach to Pluto, this panorama captures a view of the Sun setting over the distant, icy world NASA/JHUAPL/SwRI

New Horizons revealed that Pluto's atmosphere is about 21 degrees colder than anticipated by previous Earth-based studies, as well as being more compact, although the reason for its frigidity is not yet known. These characteristics mean that less of Pluto's atmosphere is being lost to solar winds -- streams of charged particles from the Sun --than previously thought. Pluto is also less exposed to solar winds than previously thought. "We've discovered that pre-New Horizons estimates wildly overestimated the loss of material from Pluto's atmosphere," said Fran Bagenal of the University of Colorado, Boulder. "The thought was that Pluto's atmosphere was escaping like a comet, but it is actually escaping at a rate much more like Earth’s atmosphere."

Researchers also found that methane, rather than nitrogen, was the primary gas that escaped Pluto's atmosphere, even thought the atmosphere near the dwarf planet's surface is 99 percent nitrogen.

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New Horizons observed distinct, bluish layered hazes in the atmosphere, thought to be produced by methane, acetylene, ethylene and ethane gases that make up abundant minor elements of the dwarf planet's atmosphere. Scientists have concluded that the haze is most likely caused by "buoyancy waves" that are created by winds blowing across Pluto's mountainous surface, which in turn compress and concentrate haze particles into distinct layers.

Pluto's moons



This photo of Charon was taken late on July 13, 2015 from a distance of 289,000 miles (466,000km) NASA-JHUAPL-SwRI

Pluto is orbited by one large moon, Charon, which has a diameter of 1,172km, and four small, irregularly-shaped moons: Styx, Nix, Kerberos and Hydra. These range from around 40km in diameter in the case of Nix and Hydra, to tiny Styx and Kerberos, which come in at around 10km across.


The moons' reflectivity when compared to small bodies common to the nearby Kuiper Belt indicates they are unlikely to have been captured from the Belt, and instead formed when even smaller bodies merged. Their surfaces date from at least four billion years ago.

"These latter two results reinforce the hypothesis that the small moons formed in the aftermath of a collision that produced the Pluto-Charon binary system," said Hal Weaver, New Horizons project scientist from the Johns Hopkins University Applied Physics Laboratory.

Charon itself has a similarly ancient surface. The smooth planes at its equator, informally named Vulcan Planum, is thought to have comes from cryovolcanoes that spewed icy material onto the moon's surface four billion years ago. It's thought that such eruptions were caused by an internal ocean that froze and ruptured Charon's crust.