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Charon protects Pluto’s atmosphere from solar wind

Charon in natural color

Charon, a moon of Pluto. When Charon is positioned between the Sun and Pluto, Georgia Tech research indicates that the moon can significantly reduce atmospheric loss. Image Credit: NASA/JHUAPL/SwRI

Pluto’s largest moon, Charon, acts as a barrier between the solar wind and Pluto’s atmosphere, preventing that atmosphere from being stripped away when the large moon is positioned between the Sun and Pluto, according to a new study published in the journal Icarus.

John Hale and Carol Paty of the Georgia Institute of Technology predicted Charon’s effect on Pluto’s atmosphere before New Horizons returned data from its flyby of the Pluto system. Their findings were confirmed by the spacecraft’s data.

Prior to the flyby, Pluto’s atmospheric escape rate was thought to be at least 100 times greater than it actually is.

Their study also confirmed another New Horizons finding, specifically that the dark red areas on Charon’s poles are composed of magnetized particles that originated in Pluto’s atmosphere.

The large moon is about half Pluto’s diameter, and the two objects are separated by just 12,000 miles (∼19,310 kilometers). This makes them a binary system because their center of gravity is located between the two objects rather than inside the larger one.

According to Hale and Paty, Charon’s position between Pluto and the Sun gives Pluto’s bow shock a more acute angle, slowing the rate at which the solar wind strips its atmosphere and redirecting the solar wind away from Pluto.

However, when Charon is positioned behind or next to Pluto, the large moon has a much smaller influence on the interaction between the dwarf planet and the solar wind.

Pluto has managed to retain its volatile elements – something the inner planets have not been able to do – because the solar wind is weaker at Pluto’s distance from the Sun. Also, Pluto has not experienced the extreme temperatures that have affected planets in closer orbits.

“Knowing the rate at which Pluto’s atmosphere is being lost can tell us how much atmosphere it had to begin with, and therefore what it looked like originally. From there, we can get an idea of what the Solar System was made of during its formation,” Hale said.



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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