Cryovolcanism on Charon: How and When?

Charon (left; NASA/JHUAPL/SwRI/Alex Parker) and Uranus' moon Ariel (right; NASA/JPL) are both thought to have been cryovolcanically resurfaced.
Many icy moons in the solar system—Charon, Enceladus, Ariel, etc.—show evidence of at least partial resurfacing possibly due to cryovolcanism—the eruption of water, ice, and dissolved volatiles onto a body’s surface. The role of cryovolcanism on these bodies has not been fully assessed, mainly because many details of this process are still unknown. Here, we ask: How does cryovolcanism work, and how might it be expressed on different icy bodies? To make predictions, constraints must be placed on how much cryomagma these bodies might have had, how long they had it for, what was its composition, how much could have erupted onto the surface, and when in a body’s history it could have happened.

Charon is an understudied but particularly well-suited world at which to examine these questions. A wealth of data from the New Horizons mission indicate strong surface evidence for extensive cryovolcanism. One region, Vulcan Planitia, covered with relatively smooth material made of ice and ammonia, has been interpreted as a massive cryovolcanic flow that occurred as a subsurface ocean froze and caused global extension, creating tectonic features that may have served as cryovolcanic sources (Beyer et al., 2017). This interpretation is complicated by a lack of large, embayed craters and a paucity of small craters (although this may reflect a dearth of small impactors in the Kuiper Belt; see Singer et al., 2019). More seriously, crater counts put the likeliest age of the smooth terrain at ~4 Gyr (Singer et al. 2019), whereas thermal models put the likely timing of a large freezing event, and therefore the extensional tectonism, between 1.7 and 2.5 Gyr ago (Desch and Neveu 2017). The uncertainties surrounding the origin of Vulcan Planitia’s smooth terrain hinder efforts to infer Charon’s evolution: the explanation of extension-driven cryovolcanism fails if the time of ocean freezing cannot be reconciled with the craterderived age of the smooth material. By using complex geophysical and geochemical modeling, spatial statistical analyses, and the rich dataset at Charon to resolve this discrepancy, we can finally time the evolution of Charon’s surface and interior, constrain the role of cryovolcanism in resurfacing Charon, and extend our findings to other icy moons.

This project was funded in summer 2021 through the NASA ROSES Solar System Workings program.

Objectives of our research:
  1. Test the hypothesis that Charon's widespread resurfacing resulted from the freezing of a subsurface ocean: could a large subsurface ocean have frozen ~4 billion years ago?
  2. Assess the relative importance of localized vs. widespread cryovolcanism
  3. Constrain the physical parameters that could enable both types of cryovolcanism on Charon consistent with the timeline suggested by crater densities and age estimates
  4. Explore cryovolcanism's role in forming or altering some of Charon's specific geological features
A part of Charon's surface noting areas of low (purple) and high (orange) crater densities. Places with low crater densities may have been resurfaced by cryovolcanism. Evaluating how clustered these areas are is a part of our investigation.