Pluto's Water Ice And Its Blue Atmosphere in the Infrared

This image from NASA’s New Horizons spacecraft is the first look at Pluto’s atmosphere in infrared wavelengths, and the first image of the atmosphere made with data from the New Horizons Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument. In this image, sunlight is coming from above and behind Pluto. The image was captured on July 14, 2015, while New Horizons was about 112,000 miles (180,000 kilometers) away. The image covers LEISA’s full spectral range (1.25 to 2.5 microns), which is divided into thirds, with the shortest third being put into the blue channel, middle third into the green channel, and longest into the red channel. North in this image is around the 10 o’clock position. The blue ring around Pluto is caused by sunlight scattering from haze particles common in the atmosphere of Pluto - scientists believe the haze is a photochemical smog resulting from the action of sunlight on methane and other molecules, producing a complex mixture of hydrocarbons such as acetylene and ethylene. These hydrocarbons accumulate into small particles – a fraction of a micrometer in size – which scatter sunlight to make the blue haze. The new infrared image, when combined with earlier images made at shorter, visible wavelengths, gives scientists new clues into the size distribution of the particles. The whitish patches around the limb of Pluto in this image are sunlight bouncing off more reflective or smoother areas on the surface of Pluto – with the largest patch being the western section of the informally named Cthulhu Regio. Future LEISA observations returned to Earth should capture the remainder of the haze, missing from the lower section of the image. Credits: NASA/JHUAPL/SWRI

New data from New Horizons spacecraft point to more prevalent water ice on the surface of Plutothan previously thought. This false-color image, derived from observations in infrared light by the Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument, shows where the spectral features of water ice are abundant on the surface of Pluto. It is based on two LEISA scans of Pluto obtained on July 14, 2015, from a range of about 67,000 miles (108,000 kilometers). The scans, taken about 15 minutes apart, were stitched into a combined multispectral Pluto “data cube” covering the full hemisphere visible to New Horizons as it flew past Pluto. A data cube like this is a three-dimensional array in which an image of Pluto is formed at each LEISA-sensitive wavelength. Water ice is crustal "bedrock” for Pluto, the canvas on which its more volatile ices paint their seasonally changing patterns. Initial New Horizons maps of water ice on the bedrock of Pluto compared LEISA spectra with a pure water ice template spectrum, resulting in the map at left. A disadvantage of that technique is that the spectral signature of water ice is easily masked by methane ice, so that map was only sensitive to areas that were especially rich in water ice and/or depleted in methane. The much more sensitive method used on the right involves modeling the contributions of various ices of Pluto all together. 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 the surface of Pluto 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 the “heart” of Pluto) and Lowell Regio (far north on the encounter hemisphere). This indicates that at least in these regions, the icy bedrock on Pluto is well hidden beneath a thick blanket of other ices such as methane, nitrogen and carbon monoxide. Credits: NASA/JHUAPL/SWRI

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