Recent findings from the New Horizons team reveal that Pluto’s third-largest satellite Nix is covered in the purest water ice yet observed in the dwarf planet system, even purer in spectra than what was seen on its slightly larger sibling Hydra. This analysis further supports the hypothesis that Pluto’s moons were created in an impact event that formed the Pluto-Charon system, over 4 billion years ago.
“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 New Horizons Project Scientist Hal Weaver, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “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,” Weaver said.
The deeper spectral features on Nix seen in the graph above 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. Scattering from smaller, or less pure, icy grains tends to wash out spectral absorption features, making them shallower. (Source)
The Nix observation was captured on July 14, 2015, by New Horizons’ LEISA – the compositional spectral imager aboard the spacecraft – from a range of 37,000 miles (60,000 km).
If Pluto’s moons are indeed all bits of debris left over from an impact, that may partially explain their observed haphazard rotations and off-kilter alignments.
Nearly a year after its close pass New Horizons is now 2.7 AU from Pluto and 34.75 AU from Earth, a one-way light-travel time of over 9 hours, 38 minutes. Data from the pass continues to arrive from the outbound spacecraft, bit by bit, gradually filling in our decades-old knowledge gap about these distant members of the Solar System.