How Pluto's atmosphere is spared by a moon shield
For a demoted dwarf planet hanging about at the edge of the Solar System, Pluto has been presenting scientists with many surprises. Not the least of these is that it not only has an atmosphere, but it's losing it a lot slower than previously thought. According to a team of researchers at Georgia Tech, this is due to Pluto's largest moon, Charon, which acts as a sort of intermittent shield protecting Pluto from the solar wind.
When NASA's New Horizons probe made its historic flyby of Pluto, it showed conclusively the dwarf planet has a tenuous atmosphere. With a pressure 100,000 times less than the Earth's atmosphere, it's consists of nitrogen with traces of methane and carbon monoxide, all of which have sublimated from Plutonian ice.
But what really surprised the scientists back on Earth is that Pluto isn't losing its atmosphere very fast. In fact, the loss rate is a hundred times slower than previously estimated.
One of the reasons why smaller planets and moons like Mars don't have much or any of an atmosphere is because the Sun is constantly blasting out a stream of charged particles called the solar wind. When these winds strike the outer atmosphere of a planet, they relentlessly strip it away.
The Earth manages to maintain its atmosphere because its powerful magnetic field acts as a shield that deflects most of the winds away, but Pluto doesn't have any magnetic field to speak of, so what's keeping its tenuous atmosphere in place?
According to the Georgia Tech team, the answer is Charon, which acts like a barrier when it is between the Sun and Pluto.
The largest of Pluto's three moons, Charon is over half the width of Pluto and is only about 12,000 mi (19,300 km) away from the planet. This, the team says, would be like our Moon being three times closer and the size of Mars.
"Charon doesn't always have its own atmosphere," says Carol Paty, a Georgia Tech associate professor in the School of Earth and Atmospheric Sciences who co-authored a paper on the study with John Hale, a Georgia Tech student. "But when it does, it creates a shield for Pluto and redirects much of the solar wind around and away."
The scientists say that Charon generates its own atmosphere periodically in one of three ways. It can develop a "parasitic" atmosphere by grabbing some of Pluto's as it escapes; cryovolcanism can cause eruptions that create one; or giant meteor impacts can create one. In fact, says Hale, a one kilometer (.6 mi) impactor would cause Charon to have an atmosphere for hundreds to thousands of years, although one wasn't present when New Horizons made its flyby.
While the moon deflects the solar wind from Pluto whether or not it has an atmosphere, it works best when it has one.
"As for the effect on the solar wind interaction, when Charon has an atmosphere, some of that material is ionized, creating an ionosphere," Hale told New Atlas via email. "This creates much more effective obstacle to the solar wind because of 1) its larger size compared to the surface of Charon and 2) the fact that it is highly conductive electrically, which diverts the magnetized plasma of the solar wind more efficiently.
"Because Charon is a more effective obstacle to the solar wind when it has an atmosphere, it decreases the ability of the solar wind to strip atmosphere from Pluto. You can think of it as being somewhat similar to how a bicyclist has an easier time when he or she is drafting behind another cyclist."
Hale says that their model of Charon/Pluto interaction is valuable because it helps us look back in time at the chemicals present when our Solar System was formed, as Pluto hasn't had its atmosphere blasted away by either the extreme heat of the Sun or its solar wind.
"As a result, Pluto still has more of its volatile elements, which have long since been blown off the inner planets by solar wind," he 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."
The research was published in a special "Pluto" edition of the journal Icarus.
Source: Georgia Tech