Scanning for "Fingerprints" in the UVSo how do we detect different components of atmospheres using occultations? Depending on composition, atmospheres will absorb sunlight more strongly or weakly at different wavelengths. Each individual component of the atmosphere has its own 'fingerprint' that varies with wavelength and one of the most revealing parts of the solar spectrum is in ultraviolet (UV) wavelengths, where even very thin atmospheres absorb strongly. Mission scientists designed the Alice UV spectrograph onboard New Horizons to detect the 'fingerprints' of a wide range of possible molecules that we expected to find on Pluto, including nitrogen (the most prevalent molecule in our own atmosphere), as well as hydrocarbons like methane. This picture is an example spectrum showing the absorption features of several atmospheric molecules in the far-ultraviolet. For a given line of sight abundance, each component contributes differently to the overall transmission of sunlight in the UV.
Image credit: Credit: SwRI/Josh Kammer