Ice Shell Structure of Ganymede and Callisto Based on Impact Crater Morphology. Issue 4 (4th April 2022)

• Record Type: Journal Article
• Authors: Bjonnes, E.; Johnson, B. C.; Silber, E. A.; Singer, K. N.; Evans, A. J.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 4(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-04-04
• Subjects: impact craters -- Galilean satellites -- ice deformation; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2021JE007028
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: Understanding the thermal structure of the ice shells around Ganymede and Callisto remains a critical step in unraveling the geologic histories of the moons. The depth‐diameter ( d‐D ) trends of pristine craters on each surface have an inflection point in crater morphology at approximately 26 km diameter, at which point observed crater depths transition from craters deepening with increasing crater diameter to craters shallowing with increasing crater diameter. In this work, we use iSALE‐2D to simulate impact crater formation in ice shells. We test conductive thermal gradients between 5 and 15 K/km and convective ice temperatures between 240 and 260 K in a modeled ice shell to determine if these observed d‐D trends correlate to specific thermal parameters. We find that the conductive thermal gradient has a more pronounced effect than the temperature of the underlying convective ice on reproducing the d‐D trends of craters up to 100 km in diameter, and that the inflection point in the observed crater depth trend on both Ganymede and Callisto is replicated in ice shells with a conductive thermal gradient of ∼10 K/km. With this conductive thermal gradient, the conductive ice was approximately 12–14 km thick on each body at the time they accrued these unmodified craters. The similar thermal constraints for both moons suggest that there are other differences between Ganymede and Callisto responsible for their divergent evolutionary paths, such as an increased proportion of strong impurities in Callisto's ice shell. Plain Language Summary: Properties within the ice surrounding Jupiter's moons Ganymede and Callisto are not well understood. However, the depths and diameters of pristine craters on these moons are affected by the temperatures within their ice shells, making impact crater studies a possible avenue for learning about these ice shells. Observations of fresh‐looking craters on these moons shows that wider craters are deeper than smaller craters up to a diameter of approximately 26 km, at which point craters show a maximum depth of 1.2 km, and craters larger than 26 km across on each moon get progressively shallower or, in some cases, develop the same depth. We simulate the formation of craters on Ganymede and Callisto using a computational model to better understand the relationship between the temperatures within the ice shells and the dimensions of the craters on their surfaces. We find that the upper portion of the ice shell, where heat transfer occurs through conduction, is 12–14 km thick for both Ganymede and Callisto, despite their very different surface appearances and geologic histories. The differences between the moons may be due to different levels of strong material in Callisto's ice shell compared to Ganymede's ice shell. Key Points: We model crater formation on Ganymede and Callisto to test the thermal structure of their ice shells We reproduced the observed crater depth‐diameter trends in ice shells on Ganymede and Callisto with a 10 K/km conductive thermal gradient Results show craters on Ganymede and Callisto formed under similar thermal conditions despite their apparently divergent geologic histories