Hypotheses for Triton's Plumes: New Analyses and Future Remote Sensing Tests

At least two active plumes were observed on Neptune's moon Triton during the Voyager 2 flyby in 1989. An eruption model based on a solar-powered, solid-state greenhouse effect was previously considered the leading hypothesis for Triton's plumes, in part due to the plumes' proximity to the subsolar latitude during the Voyager 2 flyby and the distribution of Triton's fans, which are putatively deposits from former plumes. Two other eruption hypotheses, cryovolcanism and a model based on basal heating of an insulating volatile-ice sheet, are powered by internal heat, not solar insolation. Based on new analyses of the ostensible relation between the latitude of the subsolar point on Triton and the geographic locations of the plumes and fans, we argue that neither the locations of the plumes nor fans are strong evidence in favor of the solar-powered hypothesis. We conclude that all three eruption hypotheses should be considered further, and that new hypotheses should be pursued. Five tests are presented that could be implemented with remote sensing observations from future spacecraft to confidently distinguish among the eruption hypotheses for Triton's plumes. The five tests are based on the: (1) composition and thickness of Triton's southern hemisphere terrains, (2) composition of fan deposits, (3) distribution of active plumes, (4) distribution of fans, and (5) surface temperature at the locations of plumes and/or fans. The tests are independent, but complementary, and could be implemented with a single flyby spacecraft mission.