On Pluto the Winter is approaching, and the atmosphere is vanishing into frost
2019 May 10

With less than a fifth of the Moon's mass, Pluto can still retain an atmosphere, though a tenuous envelope of gas produced by the periodical sublimation of nitrogen ices. A study that followed the evolution of Pluto's atmosphere for fourteen years shows its seasonal nature, and predicts that it will now start to condensate as frost. This study¹ was published today in the journal Astronomy and Astrophysics and had the participation of Pedro Machado, of Instituto de Astrofísica e Ciências do Espaço (IA²) and Faculdade de Ciências da Universidade de Lisboa (FCUL).

The authors analysed data from this dwarf planet's atmosphere in the altitude range of 5 to 380 kilometres, collected between 2002 and 2016. This period overlapped with the Summer in Pluto's northern hemisphere³, where are mostly concentrated the reservoirs of nitrogen ice, which sublimate under the exposure and the proximity to the Sun. Data indicate that the atmospheric pressure at the surface has risen by about twofold and a half since 1988 until its maximum in 2015, yet still one hundred thousand times thinner than the average atmospheric pressure on Earth at sea-level.

“More and more we look at Pluto's seasonal atmosphere as a cometary activity,” says Pedro Machado. “Since it is a body of small mass, nitrogen molecules gain the escape velocity very easily, and Pluto looses atmosphere, like the comets. What will happen now is that temperatures are dropping and the nitrogen molecules are again forming ice crystals near the surface,” Machado adds, “in a process similar to water frost here on Earth.”

Data came from observations of eleven times Pluto has passed in front of stars in the sky. On this occasions, the light from the star, though hidden from the Earth by the solid body, is bent by the atmosphere to our line of sight. This technique, know as stellar occultation, enables the use of the light of the star, which passed through the atmosphere, to infer its properties. For instance, light is deflected to a greater or lesser degree depending on the density at different altitudes, allowing to determine the variation of pressure and temperature with the distance from the ground.

Pedro Machado's team provided two observations conducted from the observatory at Constância Ciência Viva Science Centre, and also with their experience in the processing and analysis of the data. “Our group of observing of stellar occultations has nearly six years of activity. We belong to an international network and receive international alerts whenever there are occultations visible from Portugal.”

The second author of the paper published today, Bruno Sicardy, is supervisor, at the Paris Observatory, of PhD student Joana Oliveira, member of IA's Solar System research group. Joana Oliveira is applying the method of stellar occultations to the study of Triton, one of Neptune's moons. Another member of the group, João Ferreira, at the Nice Observatory and co-supervised by Pedro Machado, is using data of stellar positions published by the GAIA⁴ mission, of ESA, to increase the accuracy in the prediction of future stellar occultations.

Pedro Machado underlines the link between this area and the study of exoplanets. “We are actually learning hands-on a technique similar to the one used to detect and characterise the atmosphere of exoplanets. There's a direct synergy between the research we are doing in the Solar System and the work that IA is doing, or will do, for instance, with the future Ariel⁵ mission, of ESA, a mission in which we lead one of the goals, one that stands precisely on that synergy.”


Notes

1. The article “Pluto's lower atmosphere and pressure evolution from ground-based stellar occultations, 1988-2016”, by E. Meza, B. Sicardy, et al., was published online in the journal Astronomy and Astrophysics (DOI: 10.1051/0004-6361/201834281).
2. The Instituto de Astrofísica e Ciências do Espaço (Institute of Astrophysics and Space Sciences – IA) is the reference Portuguese research unit in space sciences and integrates researchers from the University of Porto and the University of Lisbon. The institute encompasses most of the field’s national scientific output and it was evaluated as Excellent in the last evaluation Fundação para a Ciência e Tecnologia (FCT) commissioned from the European Science Foundation (ESF). IA's activity is funded by national and international funds, including Fundação para a Ciência e a Tecnologia (UID/FIS/04434/2019).
3. Due to its strongly tilted rotation axis, Pluto spins almost laying on its orbit. This causes it to expose permanently to the Sun the northern latitudes during a fraction of the more than two centuries that it takes to complete a full turn around the Sun. This period overlaps with the crossing of the point in its orbit closest to the Sun (perihelion), which happened in 1989. Pluto has a very eccentric orbit, varying its distance from the Sun between about 30 and 49 times the average distance of the Earth from the Sun.
4. The Gaia satellite, launched on the 19th November, 2013, has the goal of producing the largest and most accurate catalogue of positions, distances, movements and other properties of about one percent of the total of stars in the Milky Way.
5. See IA's press-release (in Portuguese) of 23rd March, 2018, “Portugal no grande salto para os exoplanetas”

Contacts 
Pedro Machado

Science Communication Group
Sérgio Pereira
Ricardo Cardoso Reis
Daniel Folha (Coordenação, Porto)
João Retrê (Coordenação, Lisboa)