The “space weathering” of Europa

Abstract:

We explore the processes that fade rays on Europa by combining studies of Europa’s physical and chemical surface evolution and the analog of the space weathering of lunar rays. Silicate regolith on the Moon is bombarded by impacts and solar wind, which change the optical properties of the regolith. State-of-maturity crater rays form when a large impact and the ballistic sedimentation of its ejecta strip the layer of space weathered material from the surface, exposing relatively fresh and optically distinct regolith (e.g. 1, 2). From orbit, crater rays on the Moon appear as albedo bright splash patterns around their parent craters and stretch radially away for hundreds of kilometers. However, not all lunar craters have rays. The combination of space weathering and the mechanical churning of surface regolith by small and frequent impacts, or impact gardening, have been identified as the mechanism by which crater rays fade on the Moon.

Europan rays like those that surround Pwyll may be similar to lunar maturity rays, where fresh material contrasts surrounding material that has been chemically and physically altered by exposure to the surface environment. Like the Earth’s Moon, not all craters on Europa have rays. On Europa, surface material is altered by thermal segregation, radiolyzed, and possibly colored over time by exogenic sulfur from Io and by endogenic surface / subsurface exchange of salts and sulfates (e.g. 3, 4, 5, 6). Like rays on the Earth’s Moon, Europa’s crater rays likely fade from the combination of space weathering and impact gardening. In this work we explore the relative contribution of the agents of space weathering and impact gardening and conclude that sputtering, radiolysis, and thermal processes do not independently dominate ray fading. The combination of a variety of space weathering processes and impact gardening are necessary to explain the fade of Europa’s crater rays.

[1] Hawke, B. R., et al. (2004). Icarus, 170(1), 1-16. [2] Elliott, J. R., et al. (2018). Icarus, 312, 231-246. (3) Eviatar, A., et al. (1981). Icarus, 47(1), 75-83. [4] Trumbo, S. K., et al. (2019). The Astronomical Journal, 158(3), 127. [5] Hand, K. P., et al. (2020). Nature Geoscience, 13(1), 17-19.; [6] Bagenal, F., & Dols, V. (2020). Journal of Geophysical Research: Space Physics, 125(5), e2019JA027485.