Ultraviolet observations of Triton in 1999 with the Space Telescope Imaging Spectrograph: 2150-3180 angstrom spectroscopy and disk-integrated photometry

We observed Triton using the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) on 1999 August 28, September 11, and Sept ember 13 UT. On all three occasions, we obtained images at both 271 and 374 nm, and spectra from 215 to 318 nm. We find that Triton's geometric albedo is high through the near- and mid-UV, decreasing from 0.64 +/- 0.02 at 374 nm to 0.53 +/- 0.05 at 223 nm. We compare these data with a previous UV da ta set obtained by the HST Faint Object Spectrograph (FOS), taken primarily during 1993 September 1-4. Between 250 and 320 nm, Triton was roughly 20% brighter in the STIS (1999) data set than in the FOS (1993) data set, but b etween 220 and 250 nm the albedos are essentially unchanged. As expected fr om the HST FOS and other data sets, we find that Triton had higher spatial contrast at shorter wavelengths in 1999, but it exhibited a light-curve amp litude that is as much as 33% lower than derived from Voyager 2 photometry taken in 1989. We discuss some possible causes for these observed changes.