Having already established that the surface of Pluto’s outermost small moon, Hydra, is comprised predominantly of water ice, recently received observations from the New Horizons spacecraft has revealed that Nix, the moon it was discovered alongside in 2005, does as well.
In May, the probe sent back compositional data confirming that Hydra’s surface was dominated by nearly pristine water ice, confirming hints detected by scientists in photographs showing that the satellite had a highly reflective surface. On Thursday, NASA announced that newly obtained spectral observations of Nix showed that its surface has a similar ice-rich composition.
The new data was obtained by New Horizons’ Linear Etalon Imaging Spectral Array (LEISA) instrument, a near-infrared imaging spectrometer, and could provide new insight into how the distant dwarf planet’s satellite system originally formed. Furthermore, the agency added that it will help mission scientists piece together details of Pluto’s quartet of smaller outer moons (a group which also includes the moons Styx and Kerberos).
“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,” explained New Horizons Project Scientist Hal Weaver with the Johns Hopkins University Applied Physics Laboratory (APL) in Maryland. “So we would expect them all to be made of similar material.”
Observations shed new light on similarities, differences of Pluto’s moons
Weaver and his colleagues have collected spectra data from three of Pluto’s moons thus far (Nix, Hydra and its largest moon, Charon) and compared the results to pure water ice. They found that the surface of Nix displayed the deepest water-ice features amongst the three satellites.
The deeper features discovered on Nix are indicative of water that is relatively pure and coarse-grained, because the shape and depth of water-ice absorption is dependent upon the size and the purity of the icy grains on the surface, NASA explained. Scattering from smaller, less pure, icy grain is likely to cause spectral absorption features to become shallower and washed out.
The strong signature of water-ice absorption on the surfaces of all three satellites adds weight to this scenario,” Weaver said, adding that while he and his colleagues did not collect spectra of Styx or Kerebos – Pluto’s two smallest moons – “their high reflectivity argues that they are also likely to have water-ice surfaces.”
Captured on July 14, 2015, the Nix observations may have answered some questions, but raised others, such as why it and Hydra appear to have different surface ice textures, despite the fact that they are similar in size. In addition, researchers are puzzled as to why Hydra’s surface reflectivity is visible at wavelengths higher that of Nix’s, even though the latter moon’s surface appears to be icier and should theoretically be more reflective at visible wavelengths.
Image credit: NASA