Pluto's 'Thick' Air Isn't Going Anywhere

When astronomers were selling the idea of a NASA mission to Pluto, they added a sense of urgency.  No, they didn’t want to scout out Pluto as an outpost for setting up an early warning system in case of an alien invasion; planetary scientists warned that if we don’t visit the far-flung planet soon, the Pluonian atmosphere will freeze out and collapse onto the surface as fresh nitrogen-methane snow.

This was predicted because the distant world was passing its summer season and heading out billions of miles farther from the sun along a highly elliptical orbit. The warning worked. Launched in 2006, NASA’s New Horizons probe is now sprinting toward a 2015 rendezvous with Pluto as the fastest manmade object ever built.

But don’t worry if the long distance runner doesn’t get there soon enough.  New observations by Catherine Olkin of the Southwest Research Institute in Boulder, Colo., predict that the tenuous nitrogen-methane-carbon monoxide atmosphere will remain buoyant year ’round. In fact, as Pluto enters late summer, the atmosphere is now three times denser than when first measured in 1988. That means it isn’t going anywhere.

These results provide insights into the nature of Pluto’s surface crust, say the researchers. To explain the atmospheric changes there must be a rock-hard water-ice surface that apparently soaks up feeble sunlight and holds onto the heat it for a long time.

Some astronomers have previously dismissed Pluto as an oversized comet nucleus. A comet forms a temporary atmosphere, called a coma, when it is close enough to the sun for surface ices to warm and sublimate. Some scientists called Pluto’s atmosphere just a giant and temporary coma. But comets have frothy surfaces that lose heat quickly and Pluto doesn’t according to the researchers.

We’ve never had an opportunity to observe what really happens during winter on Pluto because it was springtime on the icy world when it was discovered in 1930. Pluto’s summer happened in the late 1980s when it swung inside Neptune’s orbit.

Because of its sluggish 248-year orbit, Pluto won’t be deep into winter until the year 2130. (Astronomers won’t be able to celebrate Pluto’s completion on one orbit since its discovery until the year 2178! Imagine what Earth history will have transpired by then.)

The tilt of Earth’s axis drives seasonal changes on our surface. But in Pluto’s case, seasons are driven by its highly elliptical orbit. Pluto shuttles between distances of 2.7 and 4.6 billion miles from the sun. In winter it is at the outer rim of the Kupier belt’s icy debris that encircles the sun. The consequences are that Pluto receives roughly 30 percent as much sunlight during the long winter than during the comparatively brief summer.

The team looked at Pluto observations taken from 1988 to 2003. These were occultation events where Pluto passes in front of a background star and astronomers can measure the light filtered through Pluto’s atmosphere. This allows for precise measurement of atmospheric pressure.  The team made observations this year to show the atmosphere is thicker than ever before measured.  Still, the anemic atmosphere is just one one-hundred-thousandth the surface pressure of Earth’s atmosphere.

Read more at Discovery News