**Snowstorm on Pluto**

by Dr. Tony Phillips

There's a nip in the air. Outside it's beginning to snow, the first fall
of winter. A few delicate flakes tumble from the sky, innocently enough,
but this is no mere flurry.

Soon the air is choked with snow, falling so fast and hard it seems to
pull the sky down with it. Indeed, that's what happens. Weeks later when
the storm finally ends the entire atmosphere is gone. Every molecule of
air on your planet has frozen and fallen to the ground.

*That* was a snowstorm---on Pluto.

Once every year on Pluto (1 Pluto-year = 248 Earth-years), around the
beginning of winter, it gets so cold that the atmosphere freezes. Air on
Pluto is made mainly of nitrogen with a smattering of methane and other
compounds. When the temperature dips to about 32 K (-240 C), these
molecules crystallize and the atmosphere comes down.

"The collapse can happen quite suddenly," says Alan Stern of the
Southwest Research Institute. "Snow begins to fall, the surface reflects
more sunlight, forcing quicker cooling, accelerating the snowfall. It
can all be over in a few weeks or months."

Researchers believe this will happen sometime during the next 10 to 20
years. Pluto is receding from the warmth of the Sun, carried outward by
its 25% elliptical orbit. Winter is coming.

So is New Horizons. Stern is lead scientist for the robotic probe, which
left Earth in January bound for Pluto. In 2015 New Horizons will become
the first spacecraft to visit that distant planet. The question is, will
it arrive before the snowstorm?

"We hope so," says Stern. The spacecraft is bristling with instruments
designed to study Pluto's atmosphere and surface. "But we can't study
the atmosphere if it's not there." Furthermore, a layer of snow on the
ground ("probably a few centimeters deep," estimates Stern) could hide
the underlying surface from New Horizon's remote sensors.

Stern isn't too concerned: "Pluto's atmosphere was discovered in 1988
when astronomers watched the planet pass in front of a distant star---a
stellar occultation." The star, instead of vanishing abruptly at Pluto's
solid edge, faded slowly. Pluto was "fuzzy;" it had air. "Similar
occultations observed since then (most recently in 2002) reveal no sign
of \[impending\] collapse," says Stern. On the contrary, the atmosphere
appears to be expanding, puffed up by lingering heat from Pluto's waning
summer.

Nevertheless, it's a good thing New Horizons is fast, hurtling toward
Pluto at 30,000 mph. Winter. New Horizons. Only one can be first. The
race is on....

Find out more about the New Horizons mission at
[http://pluto.jhuapl.edu](http://pluto.jhuapl.edu/) . Kids can learn
amazing facts about Pluto at spaceplace.nasa.gov/en/kids/pluto.

This article was provided by the Jet Propulsion Laboratory, California
Institute of Technology, under a contract with the National Aeronautics
and Space Administration.

![](media/image1.jpeg){width="6.0in" height="4.503472222222222in"}

*This artist's rendering shows how Pluto and two of its possible three
moons might look from the surface of the third moon. Credit: NASA/ESA
and G. Bacon (STSci)*

*Note to editor:*

*This image may be found at
spaceplace.nasa.gov/astro_clubs/pluto_3moons_art.jpg.*