Glaciers on Pluto might 'grow' with the seasons

Methane and nitrogen glaciers on Pluto expand and "grow" in response to temperature changes on the dwarf planet, new research suggests.

When NASA's New Horizons probe flew by Pluto five years ago, it spotted glaciers made of solid methane and nitrogen on the icy dwarf planet's surface. In a new study, inspired by the stunning images the spacecraft snapped of Pluto's icy mountain peaks, scientists provide evidence that the grains of methane and nitrogen in the dwarf planet's glaciers move as temperatures rise, indicating that the glaciers might expand and grow as things warm up. 

While Pluto lies billions of miles from Earth out in the far, icy reaches of the solar system, taking a whopping 248 Earth years to complete each orbit around the sun, temperatures on the dwarf planet do vary seasonally, and there are warmer periods on its surface. In fact, Pluto's seasonal temperature range from minus 418 to minus 364 degrees Fahrenheit (minus 250 to minus 220 Celsius). So, while it's never warm on Pluto, it has warmer seasons. 

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"In the warmer seasons of Pluto, still about -220 C, both the methane and nitrogen molecules are freely rotating in the solids — the molecules are not bound together very well," planetary scientist Helen Maynard-Casely of at the Bragg Institute, ANSTO, the lead author on the new study, said in a statement. "Studies of the mechanical properties of these materials at very low temperatures are really challenging, so we are missing useful information for the unusual conditions on the outer planetary bodies."

In performing the first thermal expansion study of Pluto's methane and nitrogen, the team found that these molecules change orientation with warming temperatures. The researchers also learned more about how the dwarf planet's methane and nitrogen behave in general. 

"The fact that methane and nitrogen molecules can flow at such extremely low temperatures," Maynard-Casely said, "has to do with how the methane and nitrogen molecules are arranged in their crystal structures."

The team recreated conditions on Pluto using Wombat, a high-intensity neutron diffractometer located in the OPAL Neutron Guide Hall at the Australian Center for Neutron Scattering. The researchers observed changing densities in the molecules in response to temperature, but, while they expected a straightforward study, they came away with some surprising outcomes. 

"Nitrogen actually has two crystal structures in the range of temperatures seen on Pluto," they unexpectedly found, Maynard-Casely said. "The nitrogen story is really interesting because the molecules have the ability to cool into an ordered structure, which is the alpha nitrogen phase and at this point there is a big volume drop … Whereas at a slightly higher temperature, around 44 Kelvin [minus 380.47 degrees Fahrenheit, or minus 229.15 Celsius], the nitrogen molecules are freely rotating in a plastic state."

This work was published July 29 in the IUCrJ journal. 

Email Chelsea Gohd at cgohd@space.com or follow her on Twitter @chelsea_gohd. Follow us on Twitter @Spacedotcom and on Facebook.