Shortly after New Horizons’ closest approach to Pluto the craft’s on-board Solar Wind Around Pluto (SWAP) instrument found a region of cold, dense ionized gas tens of thousands of miles distant.
SWAP discovered a ‘hole’ in the solar wind — the outflow of electrically charged particles from the Sun — at a distance of between 48,000 miles (77,000 km) and 68,000 miles (109,000 km) from Pluto.
Within this “hole,” SWAP data registered nitrogen ions that made up a “plasma tail” of undetermined structure and length extending behind the dwarf planet.
The discovery that the solar wind is acting like a hair-drier, blowing away Pluto’s atmosphere may be one explanation for a mysterious reddish brown area near the north polar region on the surface of Pluto’s companion Charon.
Earlier images returned by New Horizons showed Charon to have a rusty-hued blotch similar in coloration to darker areas seen on Pluto’s surface.
Unlike Mars where the Red Planet’s reddish-rusty colour is just that, iron oxide a.k.a. rust, Pluto’s “rust” is most likely caused by an entirely different chemical process.
As Universe Today reports, the reason for Pluto’s coloration may be due to hydrocarbon molecules, known as tholins, which form when cosmic rays and solar ultraviolet light interact with methane in Pluto’s atmosphere and on its surface. Airborne tholins precipitate out of the atmosphere coating Pluto’s surface with a reddish substance.
Charon also has a patch of its surface similarly coloured. Charon and Pluto circle each other at a mere 20,000 kilometers. That may put Charon comfortably within the “area of influence” of the newly found cavity in the solar wind, so much so that even if only a few molecules of Pluto’s rusty tholins were solar-blasted from their parent planet each year, over billions of years Charon may have become the receptacle from some of these tholins lost to Pluto.
Pluto isn’t the only object in the solar system where plasma tails have been observed. Both Venus and Mars have similar phenomena.
With Pluto’s predominantly nitrogen atmosphere, solar ultraviolet light ionizes escaping molecules which the solar wind then collects. The molecules are then conveyed past Pluto, in the process forming the newly discovered plasma tail.
Another instrument on-board New Horizons — the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) — detected nitrogen ions far upstream of the dwarf planet, suggesting Pluto’s atmosphere was leeching away, prior to New Horizons making its closest approach.
In a mission that’s not been short of surprises, Pluto’s plasma tail formation, so far little understood, and just how fast Pluto is losing its atmosphere, are aspects of the dwarf planet that astrophysicists will be investigating for years.
Fran Bagenal of the University of Colorado, Boulder, leader of the New Horizons Particles and Plasma team, commented, “This is just a first tantalizing look at Pluto’s plasma environment,” adding, “We’ll be getting more data in August, which we can combine with the Alice and Rex atmospheric measurements to pin down the rate at which Pluto is losing its atmosphere. Once we know that, we’ll be able to answer outstanding questions about the evolution of Pluto’s atmosphere and surface and determine to what extent Pluto’s solar wind interaction is like that of Mars.”
