The first color images returned of Pluto by NASA's New Horizons spacecraft reveals blue skies and surface water-ice deposits. Previous non-color images of the dwarf planet's tenuous atmosphere have displayed a surprisingly complex multilayer structure, and the new color images have allowed scientists to deduce the composition of the haze.
An analysis of the new image snapped by the spacecraft's Ralph/Multispectral Visible Imaging Camera (MVIC) has led scientists to theorize that the observed composition of Pluto's atmosphere arises from sunlight acting as the catalyst for a chemical reaction between nitrogen (N) and methane (CH4).
In basic terms, the sunlight breaks down and ionizes the CH4 and N molecules, freeing them to react and form complex negative and positively charged ions, which recombine to create very large molecules known as macromolecules. The macromolecules grow to form particles known as tholins.
This process, which is very similar to the one taking place in the atmosphere present around the Saturnian moon Titan, results in Pluto's blue haze having a soot-like consistency. NASA scientists note that the individual particles are most likely grey or red, but the manner in which sunlight is scattered by the haze throws off a blueish hue.
"That striking blue tint tells us about the size and composition of the haze particles," explains New Horizons science team researcher Carly Howett, of the Southwest Research Institute (SwRI), Boulder, Colorado. "A blue sky often results from scattering of sunlight by very small particles. On Earth, those particles are very tiny nitrogen molecules. On Pluto they appear to be larger — but still relatively small — soot-like particles we call tholins."
It is believed that volatile gasses condense on the surface of the tholin particles, coating them with ice. When these icy particles reach the dwarf planet's surface, they contribute to the reddish color that has been observed across Pluto's surface, and on the north pole of her large moon Charon.
Not content to leave us with the knowledge that Pluto hosts blue skies, the New Horizons team had one more surprise in store. The newest release of images from the New Horizons probe highlights significant deposits of water ice on the surface of the dwarf planet.
Data harvested by the spacecraft's Ralph spectral composition mapper has revealed numerous small deposits spread across the dwarf planet's surface. The data appears to show concentrations of water ice in areas synonymous with the reddish regions discolored by the aforementioned tholin particles, though the relationship between the two phenomena has yet to be explained.
"Large expanses of Pluto don’t show exposed water ice," states science team member Jason Cook, of SwRI, "because it’s apparently masked by other, more volatile ices across most of the planet. Understanding why water appears exactly where it does, and not in other places, is a challenge that we are digging into."