Abstract
In the light of NASA's New Horizons mission, the solid-phase behaviour of methane and nitrogen has been re-examined and the thermal expansion coefficients of both materials have been determined over their whole solid temperature range for the first time. Neutron diffraction results indicate that the symmetric Pa 3 space group is the best description for the α-nitrogen structure, rather than the long-accepted P213. Furthermore, it is also observed that β-nitrogen and methane phase I show changes in texture on warming, indicating grain growth.
Highlights
This study is motivated by the striking images returned from the New Horizons fly-by of Pluto (Stern, 2009), including images of towering mountains surrounded by seemingly flowing terrain (Moore et al, 2016)
The explanation for this terrain has its base in crystallography, where at 44 K the strength of the hydrogen bond endows water–ice with the resilience to build such mountains, while the rotational disorder in methane (Press, 1972) and nitrogen (Press et al, 1982) allows these materials to flow at cryogenic temperatures
Rather than studying deuterated methane, which could exhibit isotopic effects and be less relevant to the Plutonian surface, we have taken advantage of the high flux of neutrons offered by the WOMBAT instrument to study hydrogenous methane
Summary
This study is motivated by the striking images returned from the New Horizons fly-by of Pluto (Stern, 2009), including images of towering mountains surrounded by seemingly flowing terrain (Moore et al, 2016). The explanation for this terrain has its base in crystallography, where at 44 K the strength of the hydrogen bond endows water–ice with the resilience to build such mountains, while the rotational disorder in methane (Press, 1972) and nitrogen (Press et al, 1982) allows these materials to flow at cryogenic temperatures. We determined the volume change and derived the linear expansion from neutron diffraction data, encompassing the whole range of Pluto temperatures (24 to 54 K) while allowing insights into their crystal structures
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