HISTORICAL PHOTOMETRIC EVIDENCE FOR VOLATILE MIGRATION ON TRITON

Analysis of CCD images of Triton obtained with the 1.5-m telescope on Palomar Mountain shows that in the time period surrounding the Voyager 2 encounter with the satellite (1985-1990), no changes in the satelli te's visual albedo or color occurred. The published observations of Tr iton in the 0.35- to 0.60-mum spectral region obtained between 1950 an d 1990 were reanalyzed to detect historical variability in both its al bedo and visual color. Analysis of the photometry indicates that there is little, if any, change in Triton's visual geometric albedo. This r esult is consistent with the albedo pattern observed by Voyager and th e change in sub-Earth latitude. Two distinct types of color changes ar e evident: a significant secular increase in the blue region of the vi sual spectrum since at least the 1950s, and the reported dramatic redd ening of Triton's spectrum in the late 1970s. The latter change can be explained only by a short-lived geological phenomenon. Triton's chang ing pole orientation with respect to a terrestrial observer cannot exp lain the secular color changes. These changes imply volatile transport on a global scale on Triton's surface during the past 4 decades. We p resent two models which show that either removal of a red volatile fro m Triton's polar cap or deposition of a blue volatile in the equatoria l regions can explain the secular color changes. A third possibility i s that the changes are the result of the alpha-beta phase transition o f nitrogen and subsequent fracturing of the polar cap region (N. S. Du xburv and R. H. Brown, 1993, Science 261, 748). (C) 1994 Academic Pres s, Inc.