Solid nitrogen

Solid nitrogen is the solid form of the element nitrogen. It is an important component of the surfaces of Pluto and outer moons of the Solar System such as Neptune's Triton. Under low or moderate pressure solid nitrogen contains dinitrogen molecules held together by London dispersion forces. Non-molecular forms of solid nitrogen produced by extreme pressures, have a higher energy density than any other non-nuclear material.

Solid nitrogen was first made in 1884, by first liquefying hydrogen with evaporating liquid nitrogen, and then allowing the liquid hydrogen to freeze the nitrogen.Karol Olszewski achieved a world record lowest temperature by evaporating vapour from solid nitrogen getting down to 48 K. Solid nitrogen is normally made in a laboratory by evaporating liquid nitrogen in a vacuum. The solid produced is porous.

Solid nitrogen has several properties relevant to its formation of rocks in the outer Solar System. Even at the low temperatures of solid nitrogen it is fairly volatile and can sublime to form an atmosphere, or condense back into nitrogen frost. At 58 K the ultimate compressive strength is 0.24 MPa. Strength increases as temperature lowers becoming 0.54 MPa at 40.6 K. Elastic modulus varies from 161 to 225 MPa over the same range. Compared to other materials, solid nitrogen loses cohesion at low pressures and flows in the form of glaciers when amassed. Yet its density is higher than that of water ice, so the forces of buoyancy will naturally transport blocks of water ice towards the surface. This effect has been most clearly observed on Pluto (by the New Horizons space probe in 2015), where water ice makes up a major part of the surface layers as icebergs on top of nitrogen ice.

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