Triton Atmospheric Pressure: A New Value Is Obtained
The observation of a stellar occultation by a Solar System object is a very powerful method for measuring its dimensions, as well as possibly detecting rings, revealing an atmosphere and measuring its density.
In particular, this method has been successfully used on several occasions to study the evolution, as a function of time, of the surface pressure of Pluto and that of Triton, Neptune’s largest satellite.
Triton, like Pluto, was probably originally a trans-Neptunian object, and was subsequently captured by Neptune’s gravity field.
Triton’s atmosphere has been known to us since the Voyager 2 probe flew over Neptune in 1989. With a thickness of 100 km, it has since been regularly studied thanks to stellar occultations. [1]
A sixth observing campaign
The increased astrometric precision provided by the Gaia DR3 catalog has improved predictions of this type of phenomenon, and a new occultation by Triton was observed on October 6, 2022; the campaign was coordinated by Observatoire de Paris – PSL as part of the Lucky Star program.
The event was successfully observed from a site in China, another in India, and from the CHEOPS satellite in orbit around the Earth; the campaign included other observation points (eight in total in India, Japan and Russia) but these proved unsuccessful due to the presence of clouds.
A surprising value
From the data collected, the authors of the study were able to measure Triton’s ground pressure, estimated at 14.5 microbars. This value is exactly the same as the one measured in 2017, and also the one measured by Voyager 2 in 1989.
This result is surprising because Triton passed through the summer solstice for the southern hemisphere in 2000 [2] This should correspond to a pressure maximum, and some occultation observations in the 1990s indicated a rise from the Voyager 2 value. We would therefore expect the pressure to decrease since that date, and in particular since 2017, which is not the case.
This new result suggests that current models describing the transfer of volatiles in Triton’s atmosphere as a function of insolation are too simple, and need to be revised to take better account of surface-atmosphere interactions.
Sicardy, B. et al. 2024. Constraints on the evolution of the Triton atmosphere from occultations: 1989 – 2022. Astron. Astrophys. 682, L24. (open access)
Astrobiology
