Pluto’s atmosphere: last models before New Horizons

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The New Horizons spacecraft is now two months away from Pluto. This planet has been in our mental landscape for almost a century, but in July we’ll finally discover what it looks like.

We will also get our first look at its atmosphere. According to a recent set of climate models (Toigo et al. 2015, Icarus 254, 306), New Horizons almost arrived too late. As Pluto gets farther from the Sun, its tiny atmosphere entirely freezes onto the ground. The models indicate that this could have happened before New Horizons reaches the planet, but other measurements (by occultations of background stars) indicate that this is not the case.

Evolution of the size of Pluto's atmosphere with time, showing a "fast freeze" model, the occultation observations, and the date of the New Horizons visit.

Evolution of the size of Pluto’s atmosphere with time, showing a “fast freeze” model, the occultation observations, and the date of the New Horizons visit.

 

Pluto’s peculiar orbital parameters make its climate extremely seasonal. Its orbit is very elliptical (25% eccentricity) and its rotation axis highly inclined (123 degrees). This means that Pluto has two types of seasons, the inclination seasons (analogous to the true cause of Earth’s seasons), and the seasons caused by the variable distance to the Sun (analogous to what the majority of people think causes Earth’s seasons). Moreover, the inclination of Pluto is high enough that its poles receive more sunlight than its equator (see the previous post on the influence of axial tilt on climate).

Along this extreme seasonal cycle, the atmosphere of Pluto “breathes”, between a maximum pressure of around 0.01 millibars, and an almost complete absence of atmosphere, with all the nitrogen, methane and CO2 condensed on the ground.

The breathing of Pluto's atmosphere.

The breathing of Pluto’s atmosphere.

We are now just past the northern summer on Pluto, which is also the “eccentricity summer”, the point of closest approach to the Sun. The atmosphere has started cooling and condensing.

The key factor determining how fast Pluto’s atmosphere collapses is the thermal inertia of its surface. In other words how much heat the ground is able to store. Pluto’s atmosphere is so thin (less than a millimetre of frost on the surface when condensed) that it contains very little heat, and only the heat from the ground can keep it warm after the solstice.

A nice result of the Tiogo et al. climate models is that the global aspect of the planet will be directly affected by the thermal inertia of the surface. If the intertia is low, the atmosphere will freeze in the polar region. However, for higher inertia, a band of ice will form in the tropical regions.

Pluto1

The first image from New Horizons in July might thus immediately tell us how well the ground of Pluto can store heat!

 

Color of Pluto

The color of Pluto according to the internet. This plot comes from a tally by Alex Parker (Planetary Society blogs). About half the internet thinks Pluto is brown, and half thinks it is blue. We know from measurements that Pluto is brown, but the association between cold and blue is so powerful in our mind that many of us think it should be blue.

 

Feature Image: Dailygalaxy.com

 

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About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.