Spectacular images of Pluto’s surface have recently been hitting the headlines, made possible by the New Horizon Mission that on 19th January 2016, marked 10 years since the launch of this audacious spacecraft. Now however, insights into Pluto’s atmosphere by scientists studying groundbased stellar occultations of Pluto have revealed that the planet’s atmosphere is expanding, with a significant pressure increase of 5 (±2) % since 2013, ruling out an atmospheric collapse associated with Pluto’s recession from the Sun.
A stellar occultation occurs when the light from a star is blocked by an intervening body (in this case Pluto) from reaching an observer. Consequently, such occultations are very powerful tools in which to study tenuous atmospheres around remote bodies and was the method used to discover Pluto’s atmosphere in the first place, long before probes were sent to image its surface.
Other trans-neptunian objects such as Charon, Eris, Makemake and Quaoar were also analysed using this technique, and so far, none of the objects have exhibited atmospheres that are comparable to Pluto, taking into account the size differences between the bodies. This helps scientists to derive upper limits that constrain the physical conditions necessary for the appearance and maintenance of atmospheres around a body with a given ice composition and heliocentric distance.
Pluto’s atmosphere is in vapour-pressure equilibrium with the N2 ices on its surface. As the subsolar latitude and heliocentric distance of Pluto varies with its distance from the Sun, so too does the temperature of the volatile ices as a result of the amount of solar radiation that reaches Pluto’s surface. This has a knock-on effect on the surface pressure and it is expected to vary by orders of magnitude over the plutonian year (a year on Pluto lasts 248 Earth years!). Rather handily however, these strong seasonal effects can be monitored and analysed through stellar occultations here on Earth.
Scientists studying Pluto’s surface from several sites in New Zealand, have also reported on a central ‘flash’ observation that occurred during a recent occultation two weeks before the NASA New Horizons flyby of the Pluto system. This type of event can help researchers to make predictions about aspects of Pluto’s atmosphere. For example, previous studies of a flash event that occurred in 2007, concluded that the flash is consistent with Pluto having a transparent atmosphere rather than hazy one- a subject that has been hotly debated since it was first discovered that Pluto had an atmosphere at all.
This recent study of an occultation and flash event led by Bruno Sicardy from the Laboratoire d'etudes spatiales et d'instrumentation en astrophysique at the Observatoire de Paris, agrees with this previous hypothesis. The paper states that the flash shape and amplitude are compatible with a spherical and transparent atmospheric layer of roughly 3 km in thickness, whose base lies at about 4 km above Pluto’s surface and where an average thermal gradient of about 5 Kelvin per kilometre prevails. The authors also discuss the possibility that small departures between the observed and modeled flash are caused by local topographic features (mountains) along Pluto’s limb that block the stellar light. The paper concludes that Pluto’s atmospheric pressure has been increasing monotonically since 1988, with an augmentation of 5 (± 2) % between 2013 and 2015. This trend rules out an ongoing atmospheric collapse as Pluto heads away from the Sun and is most likely due to a permanent N2 ice cap at Pluto’s north rotational pole - a result that is seen in models with a high thermal inertia that analyse the energy balance between Pluto’s surface and its atmosphere.
Further information on the discovery can be read here www.arxiv.org