Pluto holds more water ice than we thought

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This false-colour image, derived from observations in infrared light, shows where the spectral features of water ice are abundant on Pluto’s surface. – NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Recently released data from NASA’s New Horizons spacecraft suggest there is much more water ice on Pluto’s surface than previously thought.

Scans of infrared light taken at 15-minute intervals by the craft's Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument, during the July fly-by, were stitched into a combined multispectral Pluto “data cube” – a three-dimensional array in which an image of Pluto is formed at each LEISA-sensitive wavelength.

The picture that emerges shows that water ice is Pluto's "bedrock”, with more volatile ices building a changeable pattern over it.

The images on the left, above, is of initial New Horizons maps of Pluto's water ice bedrock comparing LEISA spectra with a pure water ice template spectrum. However, a shortcoming of this technique is that water ice's spectral signature is easily masked by methane ice. That means the map is only sensitive to areas that were especially rich in water ice and/or depleted in methane.

The image on the right uses a much more sensitive technique modelling the contributions of Pluto's various ices all together. NASA warns that this method, too, has limitations in that it can only map ices included in the model, but the team is continually adding more data and improving the model.

The new map shows exposed water ice to be considerably more widespread across Pluto's surface than was previously known — an important discovery. But despite its much greater sensitivity, the map still shows little or no water ice in the informally named places called Sputnik Planum (the left or western region of Pluto’s “heart”) and Lowell Regio (far north on the encounter hemisphere).

This indicates that at least in these regions, Pluto's icy bedrock is well hidden beneath a thick blanket of other ices such as methane, nitrogen and carbon monoxide.

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