Scientists combined data from NASA's New Horizons mission with novel laboratory experiments and exospheric modelling to reveal the likely composition of the red cap on Pluto's moon Charon and how it may have formed. This first-ever description of Charon's dynamic methane atmosphere using new experimental data provides a fascinating glimpse into the origins of this moon's red spot as described in two recent articles. "Prior to New Horizons, the best Hubble images of Pluto revealed only a fuzzy blob of reflected light," said SwRI's Randy Gladstone, a member of the New Horizons science team. "In addition to all the fascinating features discovered on Pluto's surface, the flyby revealed an unusual feature on Charon, a surprising red cap centred on its north pole."
Soon after the 2015 encounter, New Horizons scientists
proposed that a reddish "tholin-like" material at Charon's pole could
be synthesized by ultraviolet light breaking down methane molecules. These are
captured after escaping from Pluto and then frozen onto the moon's polar
regions during their long winter nights. Tholins are sticky organic residues
formed by chemical reactions powered by light, in this case, the Lyman-alpha
ultraviolet glow is scattered by interplanetary hydrogen molecules.
"Our findings indicate that drastic seasonal surges in
Charon's thin atmosphere, as well as light breaking down the condensing methane
frost, are key to understanding the origins of Charon's red polar zone,"
said SwRI's Dr Ujjwal Raut, lead author of a paper titled "Charon's
Refractory Factory" in the journal Science Advances. "This is one of
the most illustrative and stark examples of surface-atmospheric interactions so
far observed at a planetary body."
The team realistically replicated Charon surface conditions
at SwRI's new Center for Laboratory Astrophysics and Space Science Experiments
(CLASSE) to measure the composition and colour of hydrocarbons produced on
Charon's winter hemisphere as methane freezes beneath the Lyman-alpha glow. The
team fed the measurements into a new atmospheric model of Charon to show
methane breaking down into residue on Charon's north polar spot.
"Our team's novel 'dynamic photolysis' experiments
provided new limits on the contribution of interplanetary Lyman-alpha to the
synthesis of Charon's red material," Raut said. "Our experiment
condensed methane in an ultra-high vacuum chamber under exposure to Lyman-alpha
photons to replicate with high fidelity the conditions at Charon's poles."
SwRI scientists also developed a new computer simulation to
model Charon's thin methane atmosphere.
"The model points to 'explosive' seasonal pulsations in
Charon's atmosphere due to extreme shifts in conditions over Pluto's long
journey around the Sun," said Dr Ben Teolis, lead author of a related
paper titled "Extreme Exospheric Dynamics at Charon: Implications for the
Red Spot" in Geophysical Research Letters.
The team input the results from SwRI's ultra-realistic
experiments into the atmospheric model to estimate the distribution of complex
hydrocarbons emerging from methane decomposition under the influence of
ultraviolet light. The model has polar zones primarily generating ethane, a
colourless material that does not contribute to a reddish colour.
"We think ionizing radiation from the solar wind
decomposes the Lyman-alpha-cooked polar frost to synthesize increasingly
complex, redder materials responsible for the unique albedo on this enigmatic
moon," Raut said. "Ethane is less volatile than methane and stays
frozen to Charon's surface long after spring sunrise. Exposure to the solar
wind may convert ethane into persistent reddish surface deposits contributing
to Charon's red cap."
"The team is set to investigate the role of solar wind
in the formation of the red pole," said SwRI's Dr Josh Kammer, who secured
continued support from NASA's New Frontier Data Analysis Program.
0 comments:
Post a Comment