Rotationally resolved midultraviolet studies of Triton and the Pluto/Charon system I: IUE results

We present a uniform analysis of the full set of usable International Ultraviolet Explorer (IUE) long wavelength spectra of Pluto+Charon and Triton. Our objective is to study the variation in UV photometric properties of these objects with rotational phase. The data presented here contain useful information in the range 2550-3200 Å. In the case of the Pluto+Charon system, there are eight spectra spanning about 30% of the rotational lightcurve. Six of these spectra were selected for analysis on the basis of S/N; each good quality spectrum required an integration of 6-13 hr to obtain. Owing to IUE's limited angular resolution, the data represent the combined spectra of Pluto and Charon. Based on analysis of these data, we report the first evidence of UV lightcurve variation in the Pluto system, finding that the ultraviolet albedo decreases substantially between 0.45 and 0.7 rotational phase; this is in contrast to the visible lightcurve, which reaches a maximum near the 0.65 rotational phase. This implies the visible-UV color difference increases as Pluto reaches its maximum bolometric brightness. Based on the IUE data, we suggest that such behavior could be caused by a UV surface absorption feature on Pluto or Charon. In the case of Triton, seven spectra were obtained, and three 12- to 13- hr integrations were selected for analysis. All seven measurements were made near greatest E or W elongation from Neptune. We find typical UV albedos on Triton in accord with Voyager photopolarimeter results, and a hemispheric lightcurve asymmetry in accord with the sense of Voyager visible imaging measurements. The UV hemispheric asymmetry is ∼3× higher than the Voyager-derived visible-light hemisphere asymmetry. Over the 1-year time-base 1988-1989, we found no significant change in Triton's UV reflectance properties. Comparing Pluto to Triton, we found that Pluto's UV albedo is much lower than Triton's, but that the amplitude of Pluto's UV lightcurve is greater than Triton's. The lower UV albedo of Pluto's surface indicates that fresh frost may have been more recently deposited on Triton than on Pluto.