Charon Helps Pluto Keep Its Atmosphere

Charon from New Horizons. NASA/JHUAPL-SwR

One of the many amazing finds of New Horizons was Pluto’s thin atmosphere, and it was even more remarkable when astronomers realized that the dwarf planet was losing less of it than expected.

Researchers from Georgia Tech have proposed an unusual mechanism to explain the reduced rate of loss. Planets can lose their atmosphere due to the solar wind blowing it away, so the team simulated the effect of Charon, Pluto’s largest and closest moon, on the stream of particles arriving at the dwarf planet.

The findings, published in Icarus, indicate that when Charon is upstream, it is capable of deflecting the solar wind, shielding Pluto. The simulations looked at scenarios in which Charon either has or hasn’t got a very thin atmosphere, and the results are consistent with the data from New Horizons.

“As a result, Pluto still has more of its volatile elements, which have long since been blown off the inner planets by solar wind,” lead author John Hale said in a statement. “Even at its great distance from the sun, Pluto is slowly losing its atmosphere. 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.”

The complex interaction between the solar wind and the system does not exclusively affect Pluto. One of Charon’s characteristics, its dark-colored poles, has actually been formed by this. As the moon moves around the dwarf planet, molecules from Pluto’s atmosphere fly onto Charon and begin to accumulate at the poles over billions of years. The researchers show that this would be particularly significant when Charon is downstream.

The Pluto-Charon system is often considered a double dwarf planet, as the moon is so close and so large (compared to Pluto) that it doesn’t orbit Pluto, both orbit a point between each other. They are also tidally locked and always showing each other the same face. The complexity of the interaction has been seen as evidence for a potential ocean underneath the heart of Pluto.

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