Bad Astronomy

Pluto’s atmosphere is upside-down!

Pluto is a weird place. On average, it’s about 6 billion kilometers from the Sun, so it’s very, very cold there. It’s amazing it has an atmosphere at all… but even that is weird. Pluto’s orbit is an ellipse, and right now – even though it’s slowly pulling away from the Sun – it’s still near the point in its orbit when it’s closest to the Sun. The ever-so-slight increase in heat has warmed the solid surface, turning it into gas, pumping up Pluto’s thin air.

And now astronomers have been able to measure that atmosphere with uncanny accuracy, all the way down to the surface.

Artist impression of the surface of Pluto
Artist impression of the surface of Pluto. Credit: ESO/L. Calçada

Using the ESO’s Very Large Telescope together with a spectrometer that has extraordinary resolution – the ability to distinguish very small differences in the wavelength, or “color”, of infrared light – they’ve been able to make a map of the whole atmosphere from the surface of Pluto all the way up to the top. What they’ve found in general is interesting: the atmosphere is about -170 Celsius, or 50 degrees warmer than the surface! That’s not exactly warm, of course; it’s just barely above the condensation temperature of nitrogen.

What’s really odd, though, is that the atmospheric temperature goes up with height, which is more or less the opposite of the Earth, where the temperature (in the lower atmosphere) decreases with increasing height.

In a sense, Pluto’s atmosphere is upside down. This temperature inversion is due to a process like sweating: as sweat evaporates, it cools our skin. On Pluto, the frozen gas sublimates (turns directly to gas) when warmed, removing heat from the surface. So the surface cools, and the atmosphere warms.

And there’s more: they found that by number, about 0.5% of the atmosphere is methane, a number which was previously not known. In other words, there is one methane molecule for every 199 nitrogen molecules.

This is a pretty astonishing measurement to make. Pluto is a long way off, and its atmosphere is about 1/100,000th as thick as Earth’s; it’s almost, but not quite, a vacuum. Yet there’s enough there, with the right constituents, to profoundly affect how the object behaves.

To be able to make confident measurements of such a thin soup from so far away is another testament to how advanced our science really is. Because we’re curious apes, we have reached out to an iceball 40 times farther from the Sun than we are and taken its measure.

And we’re doing even more. In 2015, the New Horizons probe will fly past Pluto and its small cadre of moons, taking close up pictures and sampling its environment. These new observations using the VLT will help scientists prepare even better for the fleeting encounter, so that we can maximize the brief time the spacecraft will be near this signpost of the outer solar system.

It’s truly incredible to think of what we can do, simply because we’re curious.