Studying small-body atmospheres through stellar occultations

Pluto and Triton are strikingly similar bodies in the outer Solar System, with comparable sizes, densities, and compositions. They both can be considered links to early conditions in the outer Solar System, with Triton likely to have originated in the Kuiper Belt and been captured by Neptune. Pluto and Triton each host a thin, global atmosphere, which is intimately connected with their surface ices through vapor-pressure equilibrium. Pluto’s atmosphere has been studied since 1988 through stellar occultations and ground-based observations, as well as intensively in 2015 during the New Horizons flyby. The atmosphere has changed during this time, notably increasing in size and doubling in pressure from 1988 to 2002 and having variable extinction or temperature gradients in the lower atmosphere on yearly timescales. Triton’s atmosphere was measured by Voyager in 1989, and it grew as a result of global warming in the late 1990s. Since then, it has been difficult to monitor due to the low number of available stellar occultation candidates, with only one observation in 2017. The primitive nature of both objects, combined with the complicated relationship between the atmosphere, surface, and environmental conditions, makes Pluto and Triton compelling and important targets for study.

Here, we propose to gain insight into thin atmospheres around small, icy bodies by studying Pluto and Triton in the 2021-2023 epoch. We will observe stellar occultations by both bodies to determine atmospheric size, pressure, distortion, and extinction. Occultations are the most effective method (short of sending spacecraft) to obtain this vital information on Pluto’s and Triton’s climates, the temporal evolution of which is particularly interesting given seasonal variations that result from energy-balance-driven changes in insolation due to their unique orbits. Our new atmospheric measurements will be combined with previous results to gain a more complete picture of atmospheric changes over time for each object. Atmospheric evolution will then be compared to outputs from existing thermophysical models to tie the results into seasonal variations and connections between the surfaces and atmospheres. We will also carry out comparative planetology to determine commonalities and differences in characteristics and evolution between the Pluto and Triton systems, particularly over seasonal timescales. Since the surfaces of some large trans-Neptunian objects are rich in volatiles and could support thin global, or even localized, atmospheres, this work is applicable to our overall understanding of how primitive bodies form and evolve.