Pluto's moon is saving its atmosphere from decay

Pluto's moon Charon may be significantly reducing the loss of atmosphere of the icy dwarf planet by creating a shield and redirecting much of the solar wind around and away, according to a new study.

Pluto's relationship with its moon Charon is one of the more unusual interactions in the solar system due to Charon's size and proximity. It is more than half of Pluto's diameter and orbits only 19,312 kilometres away. To put that into perspective, picture our moon three times closer to Earth, and as large as Mars, researchers said.

The study by researchers from the Georgia Institute of Technology in the US provides additional insight into this relationship and how it affects the continuous stripping of Pluto's atmosphere by solar wind. When Charon is positioned between the Sun and Pluto, the research indicates that the moon can significantly reduce atmospheric loss. "Charon does not always have its own atmosphere. But when it does, it creates a shield for Pluto and redirects much of the solar wind around and away," said Carol Paty, a Georgia Tech associate professor.

This barrier creates a more acute angle of Pluto's bow shock, slowing down the deterioration of the atmosphere. When Charon does not have an atmosphere, or when it is behind or next to Pluto, then Charon has only a minor effect on the interaction of the solar wind with Pluto.

The study's predictions, performed before the New Horizons probe collected and returned data to Earth, is consistent with the measurements made by the spacecraft about Pluto's atmospheric loss rate. Previous estimates at the time of the study were at least 100 times higher than the actual rate.  According to John Hale, the Georgia Tech student who co-led the study with Paty, the Pluto system is a window into our origins because Pluto has not been subjected to the same extreme temperatures as objects in closer orbits to the Sun. "As a result, Pluto still has more of its volatile elements, which have long since been blown off the inner planets by solar wind," Hale said.

"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," he said.  The study affirms a popular hypothesis of Charon. The areas of discolouration near its lunar poles are likely caused by magnetised particles that have been shorn from Pluto's atmosphere, researchers said. These particles have accumulated and settled on Charon over billions of years, particularly when it is downstream of Pluto.
The research was published in the journal Icarus.