Impacts on Icy Worlds

Introduction

When objects collide with icy bodies in space, they can create chambers of melted ice and cause the surrounding area to heat up. Recent research shows how non-penetrating impacts affect Europa's ice shell over time. Simulations of how the ice deforms under stress showed that when dense impact melts are formed, they quickly cool and solidify again. However, if the impact creates a cavity that is more than half as deep as the ice shell's thickness, over 40% of the melt can drain into the underlying ocean. Interestingly, this drainage of melts into the ocean happens no matter how thick or viscous the ice shell is, indicating that it's a natural result of icy impacts. This post-impact deformation is important for understanding how cryovolcanism and crater formation occur on icy worlds. Additionally, it could create pathways through the ice for material to move from the surface down to subsurface oceans, potentially contributing to the development of life in these hidden watery worlds.

Europa, along with other icy bodies in the outer reaches of our solar system, are prime candidates for hosting subsurface oceans that could potentially support life. To explore the possibility of habitable environments within these oceans, the JUICE and Europa Clipper missions have been planned to investigate the habitability of Europa, one of Jupiter's largest moons. One of the key unanswered questions is where the oxidants come from that are essential for creating and maintaining redox gradients within Europa's ocean. It's known that oxidants are generated by radiation at the moon's surface, but how they're transported through the icy shell to the ocean remains a mystery. Understanding this process is critical to determining whether Europa's subsurface ocean is truly habitable.

Crater Morphology

A new potential transport mechanism focuses specifically on the smaller non-penetrating impacts that have left a record of craters on Europa’s surface. These impacts are believed to generate large melt chambers that can sustain cryovolcanism over long periods of time. Models for the evolution of these melt chambers have typically assumed that the ice around them remains rigid, but this may not be the case. Non-penetrating impacts can actually heat and soften the surrounding ice, making it more susceptible to viscous deformation. This in turn may facilitate the transport of melts short distances away from the crater melt pond. The observed features of icy moon craters, including collapsed pits, domes, and central massifs, suggest that post-impact evolution is significant.

Deformation causes downward vertical transport, which impacts the local composition and thermal structure of the ice shell. Mathematical models that support this theory tended towards making the results ultra conservative. The simulations assumed a conductive ice shell, neglecting convective heat transfer in the melt chamber and melt transport through the ice by porous or fracture flow. Furthermore, the simulations neglect weakening due to impact damage and still produced results that supported vertical transport. One model explored the uncertainty in the rheology of the ice, varying the viscosity at the melting temperature and the viscosity activation energy. Despite the wide range of viscosity variations, the cut-off between impacts that drain into the ocean and those that do not is largely consistent. Even for the thickest and most viscous ice shells, the foundering of impact melt chambers and drainage into the ocean is possible. Therefore, this previously ignored process is highly likely to occur, despite the present uncertainties in ice shell thickness and rheology.

The melt formed from the ice is negatively buoyant, yet another factor that could lead to post-impact viscous deformation and foundering of the melt. This process could explain complex crater expression on Europa and possibly on other ocean worlds with similar characteristics, such as Ganymede and Titan. We must acknowledge that ocean worlds exhibit a wide range of characteristics that could influence the prevalence of viscous foundering, such as ice shell thicknesses, surface temperatures, gravities, material compositions, and ice grain sizes, and further research is needed to fully understand the impact behavior on these diverse worlds.

Cryovolcanism

Europa has well documented active water vapor plumes erupting from its surface. The impact melt chambers located beneath the Manannán Crater are being considered as a potential source for this activity. The drainage of impact melts in less than 10,000 years limits the duration of cryovolcanism, reducing the amount of near-surface melt available for eruption. Although some near-surface melt may remain, the remnant volume is a small fraction of the original and, in the case of a Manannán-sized crater, would freeze within 3,000 years of impact. Availability of near-surface melt is also affected by downward melt percolation and drainage into fractures, which may form central pits.

Melt drainage generates a continuous high porosity channel through the ice shell. This channel can reach close to the surface and remain open for over a thousand years, providing a potential ocean-to-surface pathway if the ocean is pressurized or contains exsolvable gasses. This connection of impact melts to deep fluids may extend cryovolcanism and initiate the venting of ocean materials, as seen on Enceladus, with impacts potentially serving as suitable sites to sample deep overpressured fluids and search for signs of life. Gaining insight on the formation of porous columns through Europa's crust due to impact melt drainage could influence the way we view impacts on other icy bodies and resulting cryovolcanism. A notable exception to previously discussed melt drainage mechanisms may be Occator Crater on Ceres, where the ice-rich crust has a higher density that could potentially reverse the buoyancy of the brine and prolong cryovolcanism. The mere existence of alternatives such as this gives further credence to the importance of cryovolcanism research.

Conclusion

Astrobiologists should maintain great interest in the study of post-impact viscous deformation resulting from non-penetrating impacts on Europa. The processes involved could hold the answers to many or our questions regarding oxidant transport on icy worlds. According to the simulations, impacts that generate significant melt chambers undergo substantial post-impact viscous deformation due to the foundering of the impact melts. Foundering of large volumes of melt below craters likely alters crater morphology, affects cryovolcanism, and may contribute to the habitability of oceans within icy worlds. Future work could explore the effects of ice shell thermal structure, salts, impurities, and melt drainage by porous and fracture flow on this surface-to-ocean exchange process, as well as evaluate the detailed geomorphological changes to craters that this process is likely to cause. While in this article, we focused on Europa, the viscous foundering of impact melts to the ocean occurs for all ice shell thickness and ice viscosities explored in the study, and is therefore likely to occur on other icy worlds similar to Europa.

References:

Howell SM, Pappalardo RT. NASA's Europa Clipper-a mission to a potentially habitable ocean world. Nat Commun. 2020 Mar 11;11(1):1311. doi: 10.1038/s41467-020-15160-9. PMID: 32161262; PMCID: PMC7066167.

Carnahan, E., Vance, S. D., Cox, R., & Hesse, M. A. (2022). Surface‐to‐ocean exchange by the sinking of impact generated melt chambers on Europa. Geophysical Research Letters, 49(24). https://doi.org/10.1029/2022gl100287

Travis, B. J., Palguta, J., & Schubert, G. (2012). A whole-moon thermal history model of Europa: Impact of hydrothermal circulation and Salt Transport. Icarus, 218(2), 1006–1019. https://doi.org/10.1016/j.icarus.2012.02.008