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On Charon’s Unusual North Pole

Deep space exploration brings a surprise with each new destination. New Horizons made the point over and over again, and today we get word of new work on one of the mission’s discoveries, that dark red polar cap at the north of Pluto’s large moon Charon. Will Grundy (Lowell Observatory) and colleagues are behind the study, which digs into the theory that methane from Pluto’s atmosphere is trapped at Charon’s north pole.

In a new paper in Nature, Grundy and team have used New Horizons mission data in conjunction with their own modeling of the evolution of Charon’s ice cap over the course of a Plutonian year to demonstrate that the model of trapped methane works. It involves the processing of methane into complex organic molecules called tholins. From the paper:

The distribution of dark, reddish material around Charon’s northern pole is notable for its generally symmetric distribution across longitudes and its gradual increase with latitude, although there are local irregularities associated with craters, topographic features and perhaps subsurface variations in thermal properties. These characteristics, and the existence of an albedo feature around the southern pole with a similar latitude dependence, are consistent with our hypothesis that the combination of Pluto’s escaping atmosphere and Charon’s long, cold winters enables CH4 to be seasonally cold-trapped at high latitudes, where some is photolytically processed into heavier molecules that are subsequently converted to reddish tholin-like materials.

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Image: Mosiac of New Horizons MVIC color observations of Charon obtained during the final 6.4 day rotation on approach to the system during July 7-14, shown in polar orthographic projection. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Tholins are complex organic molecules that can be formed when ultraviolet light strikes simple organic molecules like methane. We find the dark red of Charon’s polar cap duplicated on the surface of Pluto in areas where these interactions occur. And as the paper notes, the nearby atmosphere of Pluto provides ‘potential feedstock’ for this complex chemistry at Charon’s pole. Grundy’s team was able to model the surface thermal environment on Charon, capturing the cold-trapping of material from Pluto and its processing into more complex molecules, all consistent with earlier theories but modeled for the first time quantitatively.

New Horizons provided data on the escape rates of methane and nitrogen. The former exists in a frozen state on Charon’s surface only during the polar winter night, which means that it is being processed quickly, although it need not be fully converted into tholins right away, but only into molecules that are non-volatile enough to remain on the surface when the pole is again exposed to sunlight.

As to radiation sources with an impact on Charon’s surface, the paper notes ultraviolet photons, solar wind charged particles, galactic cosmic rays and interstellar pickup ions. The latter result from solar wind protons interacting with neutral hydrogen atoms from the interstellar medium. The paper pegs the most important night-side radiation source as solar ultraviolet photons that have been scattered by the interplanetary medium.

Once the pole is back in sunlight, ultraviolet radiation directly from the Sun can drive further chemistry. Interestingly, we find a reddish spot on the small Plutonian satellite Nix, but the authors argue that it and the other small moons orbit farther from Pluto and have lower masses, which makes the process less efficient. Charon is a different story. With sufficient mass and a winter lasting for over a century, its trapping of Pluto’s methane produces a highly visible result.

The paper is Grundy et al., “The formation of Charon’s red poles from seasonally cold-trapped volatiles,” Nature 15 September 2016 (abstract).

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  • ljk September 15, 2016, 18:30

    https://arxiv.org/abs/1608.06955

    New Horizons Constraints on Charon’s Present Day Atmosphere

    S. A. Stern, J. A. Kammer, G. R. Gladstone, A. J. Steffl, A. F. Cheng, L. A. Young, H. A. Weaver, C. B. Olkin, K. Ennico, J. W. Parker, A. H. Parker, T. R. Lauer, A. Zangari, M. Summers, the New Horizons Atmospheres Team

    (Submitted on 24 Aug 2016)

    We report on a variety of standard techniques used by New Horizons including a solar ultraviolet occultation, ultraviolet airglow observations, and high-phase look-back particulate search imaging to search for an atmosphere around Pluto’s large moon Charon during its flyby in July 2015.

    Analyzing these datasets, no evidence for a present day atmosphere has been found for 14 potential atomic and molecular species, all of which are now constrained to have pressures below 0.3 nanobar, as we describe below, these are much more stringent upper limits than the previously available 15-110 nanobar constraints (e.g., Sicardy et al. 2006); for example, we find a 3σ upper limit for an N2 atmosphere on Charon is 4.2 picobars and a 3σ upper limit for the brightness of any atmospheric haze on Charon of I/F=2.6×10−5.

    A radio occultation search for an atmosphere around Charon was also conducted by New Horizons but will be published separately by other authors.

    Comments: 17 pages, 7 figures. Submitted to Icarus, in revision

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:1608.06955 [astro-ph.EP]
    (or arXiv:1608.06955v1 [astro-ph.EP] for this version)

    Submission history

    From: Joshua Kammer [view email]

    [v1] Wed, 24 Aug 2016 20:14:26 GMT (5116kb)

    https://arxiv.org/ftp/arxiv/papers/1608/1608.06955.pdf

  • ljk September 15, 2016, 18:31

    X-rays detected from Pluto – and not by New Horizons:

    http://chandra.si.edu/press/16_releases/press_091416.html

    To quote:

    The team recently published its findings online in the journal Icarus. The report details what Lisse says was a somewhat surprising detection given that Pluto – being cold, rocky and without a magnetic field – has no natural mechanism for emitting X-rays. But Lisse, having also led the team that made the first X-ray detections from a comet two decades ago, knew the interaction between the gases surrounding such planetary bodies and the solar wind – the constant streams of charged particles from the sun that speed throughout the solar system — can create X-rays.

  • Supernaut September 20, 2016, 19:13

    Thanks. I heard a talk by L. Young today, where she was asked about the most surprising things discovered by New Horizons. One thing she mentioned was the reddish polar cap of Charon. Previous ground-based observations did not detect prominent light-curve variations on Charon therefore, she explained, pre-New Horizons most people assumed Charon was ‘bland’. But, since the polar axis pierces almost though the middle of the cap, it’d have been very difficult to detect. Interesting moon.