2024-03-22 07:00:13
Triton, Neptune’s main satellite, passed in front of a star on October 6, 2022. This phenomenon was the occasion for a vast observation campaign in India, China and from space, carried out under the direction of a scientist of the’Paris Observatory (The Paris Observatory was born from the project, in 1667, of creating an astronomical observatory…) – PSL, making it possible to measure the atmospheric pressure (Atmospheric pressure is the pressure exerted by the gas mixture constituting…) reigning on its surface. Published in the journal Astronomy & Astrophysicson February 27, 2024, the value obtained raises questions.
© NASA
The observation of a occultation (An occultation is a phenomenon of apparent covering of an element by an…) stellar (Stellaria is a genus of annual or perennial herbaceous plants, starworts,…) by an object of Solar system (The solar system is a planetary system composed of a star, the…) is a very powerful method for measuring dimensions (In common sense, the notion of dimension refers to size; the dimensions of a room…)but also possibly detect rings, highlight a atmosphere (The word atmosphere can have several meanings 🙂 and measure its density (The density or relative density of a body is the ratio of its density to the…).
This method has in particular been used successfully on several occasions to study, as a function of time, the evolution of the surface pressure of Pluto and that of Triton, Neptune’s largest satellite.
The two objects, of comparable size, are also of a similar nature with a very thin atmosphere of molecular nitrogen; Triton, like Pluto, is undoubtedly originally a trans-Neptunian object, which was subsequently captured by the field of gravity (Gravitation is one of the four fundamental interactions of physics.) by Neptune.
Triton’s atmosphere has been known to us since the Voyager 2 probe flew past Neptune in 1989. With a thickness of 100 km, it has since been regularly studied thanks to stellar occultations. (1)
A sixth observation campaign
The increased astrometric precision provided by the catalog DR3 by Gaia made it possible to improve the predictions of this type of phenomenon, and a new occultation by Triton was observed on October 6, 2022; the campaign was coordinated by the Paris Observatory – PSL as part of the programme Lucky Star.
The event was successfully observed from a site in China, another in India, and from the CHEOPS satellite orbiting the Earth; the campaign included other observation points (eight in total in India, Japan and Russia) but which, due to the presence of clouds, proved unsuccessful.
Left: Observation geometry, shown on Earth (top) and in the sky plane (bottom). In the top image, the two lines frame the region of the Earth from where the occultation was visible. At the bottom, the three straight lines represent, seen from the three observation sites, the path (The trajectory is the line described by any point of a moving object, and…) apparent of the star behind Triton’s disk; this one presents its South Pole (The South Pole is the southernmost point on the Earth’s surface, diametrically…) towards Earth.
On the right, the occultation curves obtained at the different sites. The observed curve is represented in black, the blue curve is the model corresponding to a ground pressure of 14.5 microbars.
© DR
Surprising value
From data (In information technology (IT), data is an elementary description, often…) collected, the authors of the study were able to measure Triton’s ground pressure, estimated at 14.5 microbars. However, this value is exactly the same as that measured in 2017, and also that measured by Voyager 2 in 1989. This result is surprising because Triton passed the summer solstice for the southern hemisphere in 2000 (2), which should correspond to maximum pressure; certain occultation observations, in the 1990s, had also indicated an increase compared to the value of Voyager 2. We would therefore expect the pressure to decrease since that date, and in particular since 2017, which it’s not the case.
This new result therefore suggests that the current models describing the transfer of volatiles in Triton’s atmosphere as a function of insolation are too simple and must be revised to better take into account the interactions between surface and atmosphere.
Reference:
Sicardy, B. et al. 2024. Constraints on the evolution of the Triton atmosphere from occultations: 1989 – 2022. Astron. Astrophys. 682, L24. https://arxiv.org/abs/2402.02476
Notes:
(1) Previous occultations have made it possible to probe Triton’s atmosphere, in 1989, 1995, 1997, 2008 and 2017.
(2) In 2000, Triton experienced an extreme solstice that occurs every 650 years.
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