Dunes visible from the sky on Pluto
It has been more than 3 years since New Horizons flew over Pluto, on 14 July 2015, 11 000km away. The images it sent revealed a complex surface, more varied and more dynamic than expected. Which is surprising since its atmosphere was measured at 1/100 000 of the Earth’s atmospheric pressure, a priori too little for winds to have an effect on the surface. “In those conditions of very low pressure, we can hardly imagine how these grains can be lifted, François Forget explains. There need to be a dense gas for particles to be suspended in the atmosphere.” So where does all this gas come from, and how does it form winds capable of drawing these dunes?
The New Horizons spacecraft observed the dunes were in the Sputnik Planitia region, at the foot of the al-Idrisi mountains.
First, their formation seems relatively young: probably in the last 500 000 years according to the absence of impacts at the foot the al-Idrisi mountains (5 km altitude), where they were observed. There lies the Sputnik Planitia basin, composed of ice made of molecules that exist in a gas form on Earth. Yet at -230 ° C, it is ice nitrogen N2, carbon monoxide CO, and CH4 methane that covers Pluto. The composition of these dunes is a little different: "We were able to analyse the dunes by spectroscopy and they mainly contain methane, explains François Forget. There is apparently little nitrogen, because it is more volatile: it becomes easily gaseous and leaves behind solid methane in the form of grains, about 200 to 300 micrometres. So they are methane dunes on a nitrogen glacier. "
Where do the winds come from on Pluto?
We must take into account several features of Pluto to explain how methane can produce these dunes. Presumably, it is a combination of its low gravity (15 times lower than on Earth) and the very low temperatures (the cold air is denser): even if the wind is much less powerful, it also takes less force to transport the grains of ice than it would take for sand on Earth. The authors estimate that a moderate speed of less than 10 metres per second (36 km/h) is required. The fact that the dunes are located at the foot of the al-Idrisi mountains is not trivial: at this point, the atmosphere falling by gravity descends the slopes and accelerates by creating horizontal winds that would explain the appearance of these dunes.
However, these winds are very insufficient to lift the particles from the ground and inject them into the atmosphere. For this to happen, there should be another more exotic mechanism at work: with such a low atmospheric pressure, a solid molecule can evaporate into gas without even going through the liquid state. It is sublimation, and that is what nitrogen ice undergoes when it is sufficiently heated by sunlight. The nitrogen composing the basin is sublimated and carries the grains of methane still solid with it. It constitutes a form of erosion that would clearly facilitate the dispersion of ice by Pluto winds. "Since gravity is much lower than on Earth, that's enough to keep methane suspended. We think it is this injection that puts it in the air. It is both very exotic and very fun," François Forget said with enthusiasm.
Comparison between the dunes discovered on Pluto and those known on Earth. On Mars, the interaction between the winds and the sublimation phenomenon is also observed at the poles, with carbon dioxide ice.
To determine the origin of those dunes, the researchers investigated their orientation to check what type of wind it could be. On Earth, it sometimes happens that dunes form in the direction of the wind corresponding to two prevailing winds whose alternation create the dune. On the other hand, with weaker winds or a more tenuous atmosphere, transverse dunes perpendicular to the direction of the wind form: their bifurcations not well defined and their way of eroding against the wind are characteristic. They could thus be observed on Venus and Mars, or even on Triton, the satellite of Neptune. And it is indeed this kind of dunes that correspond to those of Pluto. "One could have imagined that they are only the consequence of condensation and sublimation of ice," François Forget concludes. But in this case their direction would only depend on the Sun, their alignment would be the same everywhere. In fact, their orientation changes slightly depending on the location. This is the sign that there is wind, and that it turns a little bit. That is what we managed to demonstrate with New Horizons."
Telfer et al., Science 01 Jun 2018: Vol. 360, Issue 6392, pp. 992-997
François Forget, CNRS Senior Research Scientist
Laboratoire de Météorologie Dynamique, Institut Pierre Simon Laplace
Phone: +33 1 44 27 47 63 / +33 6 71 20 07 50
Email: forget at lmd.jussieu.fr
Francis Rocard, Solar System Programme Manager
Email: francis.rocard at cnes.fr
Phone: 01 44 76 75 98 / +33 1 44 76 75 98
Fax: 01 44 76 78 59 / +33 1 44 76 78 59
Centre National d'Etudes Spatiales, 2 place Maurice Quentin, 75039 Paris Cedex 1, France