A team of scientists from the Southwest Research Institute (SwRI) have combined data from NASA’s New Horizons mission with state-of-the-art laboratory experiments and exospheric models to reveal the likely composition of the red cap on Pluto’s moon Charon and how it might have formed. have formed. Red might not be an unusual color on iron-rich worlds like our own, or even Mars, but in the outer reaches of the solar system, red is much more likely to indicate something different.
Where does this strange red spot on Charon come from?
The new studies (two), published in the journals Geophysical Research Letters and Science Advances , provide fascinating insight into the origins of the red spot on this large moon of Pluto.
That kind of rust-tinted ‘hat’ captured by New Horizons in its 2015 flyby has had many hypotheses. The heaviest weight suggested that the iron-colored spot (nicknamed Mordor Macula) was methane captured from Pluto’s surface, and that color came from slow cooking in ultraviolet light from the Sun.
“Before New Horizons, the best Hubble images of Pluto revealed only a blurry patch of reflected light,” said New Horizons team member Randy Gladstone, a researcher at the Southwest Research Institute and the Department of Physics and Astronomy at the University of Pennsylvania. Texas in San Antonio and co-author of the work. “In addition to all the fascinating features discovered on Pluto’s surface, the flyby revealed an unusual feature on Charon, a striking red cap centered on its north pole.”
Charon south pole view
Now, we can certainly say that they weren’t far off in their theory. The researchers attribute the color to seasonal changes in Charon’s thin atmosphere. The process would involve several stages beginning in the Charon winter , when methane freezes in the presence of Lyman-alpha radiation , a form of ultraviolet light scattered by interplanetary hydrogen. Scientists carried out an experiment in the laboratory to prove it:
“Our experiment condensed methane in an ultrahigh vacuum chamber under exposure to Lyman-alpha photons to replicate conditions at the poles of Charon with high fidelity,” said Ujjwal Raut. “We believe that ionizing radiation from the solar wind breaks down Lyman-alpha baked polar frost to synthesize increasingly complex and redder materials responsible for the unique albedo on this enigmatic moon.”
“Ethane is less volatile than methane and remains frozen on Charon’s surface long after spring sunrise,” says Raut, who modeled changes in the densities of evaporating and freezing methane.
According to the studies, seasonal spikes in Charon’s thin atmosphere, as well as light breaking down condensing methane frost, are key to understanding the origins of Charon’s red polar zone.
Around the equinox, the frozen methane turns to gas, increasing atmospheric pressure almost a thousand times, before refreezing in the other hemisphere. The authors believe that about 10 percent of the methane Charon captures eventually turns into reddish hydrocarbons, while the other 90 percent eventually escapes into light gravity, only to be replaced by more material from Pluto.
Referencia: Ben Teolis, Ujjwal Raut, Joshua A. Kammer, Caleb J. Gimar, Carly J. A. Howett, G. Randall Gladstone, Kurt D. Retherford. Extreme Exospheric Dynamics at Charon: Implications for the Red Spot. Geophysical Research Letters, 2022; 49 (8) DOI: 10.1029/2021GL097580
Ujjwal Raut et al. 2022. Charon’s refractory factory. Science Advances 8 (24); doi: 10.1126/sciadv.abq5701
