CHARON, MY CRYOVOLCANIC PROBLEM CHILD: A WINDOW INTO THE GEOLOGICAL WORLDVIEW OF J.M. MOORE (Invited Presentation)

For nearly forty years, a persistent theme in the research of this year’s Gilbert award winner, J.M. Moore, has been the question of volcanism on icy bodies. Whether arguing (e.g.) for aqueous cryovolcanism on Ganymede, against its facile acceptance on Titan, or marshaling marginal evidence from Voyager images for viscous cryovolcanic eruptions on Dione or Enceladus, this most vexsome topic has engaged the good “doktor.” A hallmark of Jeff’s approach has always been the rigorous and dispassionate evaluation of working hypotheses, in the manner of G.K. Gilbert. A clear barrier to understanding cryovolcanism has been the lack of observations of extant and active examples. Although this barrier fell with the discovery and characterization of the plumes on Enceladus, geysering of water into a vacuum is only one possible manifestation of cryovolcanism. It is remarkable that it has taken the robotic exploration of the Pluto system to reveal reasonably clear evidence for constructive, ammonia-bearing cryovolcanism in the outer Solar System. Evidence for this classic, cryovolcanic mode has long been hampered on multiple fronts: 1) low NH₃ abundances in solar system ices (~1%); 2) subtle near-IR band depths compared with those of H₂O ice; and 3) instability long-term under UV and charged particle irradiation. On Charon, the surface abundance of NH₃ appears sufficient, and the radiation environment sufficiently benign, that ammonia is manifest, and globally distributed. Most notable is Vulcan Planitia* (VP), a broad (at least 400,000 km²), lower-lying (by ~ 1 kilometer), cryovolcanic plain displaying morphologies consistent with massive, viscous flow (embayments, lobate margins, sinuous rilles). Crater frequencies resemble those of the lunar maria, and models of Kuiper belt bombardment support a similarly old age (several Ga) for VP, but younger, less cratered patches (~50–100 km wide) also exist. Models of Charon’s evolution are consistent with tectonic evidence for global expansion due to ocean freezing, followed by late-stage flooding of low-lying areas by ammonia-rich eutectic cryomagmas. Most intriguing is the pitted or textured terrain at the VP terminator, seen in the highest resolution images. We will discuss multiple explanations/analogs for this texture, both endogenic and exogenic. *Informal name.