Pluto's surface could contain complex hydrocarbon and nitrile molecules which may be responsible for giving the planet its ruddy color, according to the findings of Hubble Space Telescope's Cosmic Origins Spectrograph.

The highly sensitive instrument aboard the NASA telescope discovered the strong ultraviolet-wavelength absorber on Pluto's surface, providing new evidence on the presence of complex hydrocarbon and/or nitrile molecules lying on the planet's surface.

According to a paper recently published in the Astronomical Journal by researchers from Southwest Research Institute and Nebraska Wesleyan University, such chemical species can be produced by the interaction of sunlight or cosmic rays with Pluto's known surface ices, including methane, carbon monoxide and nitrogen.

"This is an exciting finding because complex Plutonian hydrocarbons and other molecules that could be responsible for the ultraviolet spectral features we found with Hubble may, among other things, be responsible for giving Pluto its ruddy color," said SwRI's Dr. Alan Stern.

The project, led by Dr. Stern, also included SwRI researchers Dr. John Spencer and Adam Shinn, and Nebraska Wesleyan University researchers Dr. Nathaniel Cunningham and student Mitch Hain.

Evidence of changes in Pluto's ultraviolet spectrum compared to Hubble measurements from the 1990s was also discovered by the research team. The changes may be related to differing terrains seen now versus in the 1990s, or to other effects, such as changes in the surface related to a steep increase in the pressure of Pluto's atmosphere during that same time span.

"The discovery we made with Hubble reminds us that even more exciting discoveries about Pluto's composition and surface evolution are likely to be in store when NASA's New Horizons spacecraft arrives at Pluto in 2015," Stern added.

The study was supported by a grant from the Space Telescope Science Institute.