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6 mins read 12 Aug 2020

How Does Ice Flow on Pluto?

Australian scientists from ANSTO and the Australian Synchrotron have published new research about the unusual structure of the glaciers on the surface of Pluto. 

Enhanced-colour view from NASA’s New Horizons spacecraft zooms in on the southeastern portion of Pluto’s great ice plains, where at lower right the plains border rugged, dark highlands informally named Krun Macula. Credit: NASA/JHU-APL/SwRI

Given its distance from us, Pluto is a difficult celestial body to study. We know some basic information, but there is still much mystery surrounding the dwarf planet. However, in a newly published report, researchers have explored the temperature-based structure of the solid methane and nitrogen glaciers on Pluto. 

This study was inspired by the New Horizons flyby mission to Pluto five years ago. The results of the study conducted by Dr. Maynard-Casely and her associates was unexpected, showing that the orientation of the methane and nitrogen grains within the glacier changed with temperature. Dr. Maynard-Casely, a Planetary scientist who specialises in off-Earth mineralogy, commented on the inspiration for this study - NASA's ground breaking New Horizons mission. 

“There’s not going to be many times in your life that a whole new landscape is revealed, that’s the awe inspiring thing about a mission like New Horizons.  In the space of a few months in 2015 Pluto went from being a fuzzy ice blob on the outer fringe of the solar system, to being a diverse and exciting world to explore.”

“Rather than being a cratered and old surface, the way those glaciers smoothed out the terrain on Sputnik Planitia (Pluto’s heart) straight away highlighted to me that there’s so much more to discover.” 

“I take things from the material point of view - if I can re-create the conditions on the surface of Pluto, and the chemistry that we know is there, then we can discover the material properties of what is shaping the landscape,” she said.

Pluto: the Dwarf Planet

The dwarf planet Pluto. Credit: NASA

Situated approximately 5.9 billion kilometres from the sun, Pluto is perhaps most well known for being reclassified as a dwarf planet in 2006. It is tilted 120 degrees on its axis, and orbits the sun once every 248 years. 

Pluto also experiences seasonal variations in temperature, ranging from 24 to 54 Kelvin (-250 to -220 Celsius). This change in temperature has proven to be a driver of geological change on Pluto, a producer of its unique landscape. 

This dwarf-planet exhibits towering icy mountains surrounded by lower terrain with apparent flowing features. Due to the extreme cold of this environment, the apparent ‘flowing’ had been difficult to explain up until now.  

“Gone is the thought that Pluto is a dead world. New Horizons has picked up evidence that the dwarf planet has been geologically active throughout its 4 billion year life,” said Dr. Maynard-Casely.

Studying Other Worlds

New Horizons close-up of the surface of Pluto. Credit: NASA

In research published in the IUCr Journal, Dr. Maynard-Casely and her associates have provided evidence that could explain the mechanisms that have contributed to the landscape on Pluto. This was achieved by performing a crystallography study on thermal expansion at a range of temperatures using the Wombat high intensity diffractometer and cryogenic furnaces at Australian Nuclear Science and Technology Organisation (ANSTO). 

Wombat can be used to study icy planets and their moons by recreating their conditions. In this novel study, Dr. Maynard-Casely examined the thermal expansion of methane and nitrogen, similar to what would occur in the glaciers of Pluto throughout the seasonal temperature variation. 

Dr. Maynard-Casely commented, “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.”

Shaping the Landscape

The surface of Pluto revealed by New Horizons has come to be known to have vast glacial flows. Credit: NASA

In this latest report, Dr. Maynard-Casely documents that the research found results that her and her associates did not expect; the nitrogen model found at lower temperatures did not match the accepted model that had been cited since the 1970’s. Nitrogen is found in two different crystalline structures in the range of temperatures seen on Pluto: one form at higher temperatures, one at lower. Similar effects are seen in the methane of these glaciers. According to this new data from Wombat, this may explain the shape of the landscape on Pluto. 

“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,” explained Dr. Maynard-Casely.  “Whereas at a slightly higher temperature, around 44 Kelvin, the nitrogen molecules are freely rotating in a plastic state.” 

This means that as the temperature cools and warms, the nitrogen in glaciers on Pluto contracts and expands respectively. Simultaneously, due to the expansion of nitrogen at these warmer - albeit still freezing temperatures - the weaker bonds and increased freedom of orientation is thought to impart a mechanical softness, and hence the ability to ‘flow’.

 “The fact that methane and nitrogen molecules can flow at such extremely low temperatures has to do with how the methane and nitrogen molecules are arranged in their crystal structures, which is why crystallography might be able to answer questions about these unusual landscapes,” explained Dr. Maynard-Casely.  

“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,” she said.

Dr. Maynard-Casely went on further to explain the importance of this data and its potential application elsewhere. “The big potential for applying these results elsewhere is Neptune’s moon Triton...it has a big variety of terrain and some potentially active spots.  Nitrogen is definitely going to be an important material on the surface of this moon, and could be what is driving the cryo-volcanism there.  To understand how that can work, the more we know about the solid material properties of these materials, that we normally think of as gases, the better,” she said.

With another piece in the puzzle put in place for Pluto, this research paves the way for further exploration of the icy planets and moons of our outer solar system and beyond. 

Read the full research paper to learn more.