NASA has published a comparative analysis of the spectral measurements of three moons of the dwarf planet Pluto which prove that in particular Nyx but also Hydra have a surface covered in water ice. The New Horizons space probe’s LEISA instrument carried out the measurements during the July 14, 2015 flyby. Unfortunately the location of the other two small moons, Styx and Kerberos in those hours didn’t allow spectral measurements.
In early May, NASA published some data about Pluto’s moons including spectral data of the small moon Hydra, the outermost one, detected from a distance of about 240,000 kilometers (150,000 miles). Those are in particular infrared measurements, which tell the amount from their absorption the amount of water ice present. The spectrum of Hydra turned out to be very similar to that of pure ice, even more than that of Charon, on whose surface there’s mostly water ice in crystalline form.
The spectral characteristics of Hydra suggest that the ice grains on its surface are larger and reflect more light at certain angles than on Charon. Now the spectral data of Nyx, detected from a distance of approximately 60,000 kilometers (37,000 miles), are even more similar to those of pure ice, so much that the relative spectral lines nearly overlap.
Concerning Styx and Kerberos, lacking spectral data the New Horizons mission scientists could make only approximate estimates. They took into account the reflectivity of these two moons’ surface that suggests that they also have a surface covered with ice water.
The spectral measurements made by the LEISA (Ralph/Linear Etalon Imaging Spectral Array) instrument confirms the hypothesis that Pluto’s small moons formed from the debris left after a planetoid struck the dwarf planet a few billion years ago. However, most of the debris formed Charon, in a event similar to the Moon’s formation.
Nyx and Hydra’s reflectivity are similar but not the same the reason for the difference isn’t clear why. Scientists keep on analyzing the data collected by the New Horizons space probe and more will keep on being sent for a few more months. They might get other useful information to better understand the composition of Pluto’s moons and have some more clues about their history.