New insights into Pluto’s outer moons

A clearer picture of the composition of Pluto’s smaller moons is emerging as NASA releases new spectral data from the New Horizon’s mission. It now seems almost certain that the surface of the moon Nix is covered in water ice, as is Pluto’s small satellites, Hydra.

NASA says the new result provides further clues about the formation of Pluto’s satellite system.

The data was collected by New Horizons’ LEISA – the compositional spectral imager aboard the spacecraft – of four small, outer moons of Pluto – Styx, Nix, Kerberos and Hydra.

The deeper spectral features on Nix seen in the graph below are a signature of water ice that is relatively coarse-grained and pure, because the shape and depth of water-ice absorption depends on the size and purity of the icy grains on the surface, NASA says.

Scattering from smaller, or less pure, icy grains tends to wash out spectral absorption features, making them shallower.

“Pluto’s small satellites probably all formed out of the cloud of debris created by the impact of a small planet onto a young Pluto,” said Hal Weaver, of Johns Hopkins University. “So we would expect them all to be made of similar material. The strong signature of water-ice absorption on the surfaces of all three satellites adds weight to this scenario. Although we didn’t collect spectra of Pluto’s two tiniest satellites, Styx and Kerberos, their high reflectivity argues that they are also likely to have water-ice surfaces.” 

While the new information answers some questions, it also raises new ones, for example, why do Nix and Hydra apparently have different ice textures on their surfaces, despite their similar sizes.

The LEISA Nix observation was captured on 14 July 2015, from a range of 60,000 kilometres.