Thermal stresses as a possible mechanism for initiating the destruction of comet Shoemaker-Levy 9

Abstract

It is now widely accepted that the breakup of comet Shoemaker-Levy 9 (SL9) was caused by tidal forces. This explanation applies to bodies that enter a planet's Roche zone and have a loose 'rubble pile' structure with no internal strength. Observations from the Rosetta mission to comet 67P/Churyumov-Gerasimenko initially supported this hypothesis, as the comet's surface strength was exceptionally low. However, beneath this loosely bound surface layer, a dense mixture of ice and rock was detected, as evidenced by deep cracks. Similar fractures on Mars yielded strength estimates of around 2 MPa. In this paper, we hypothesize the observed fragmentation pattern of comet SL9 caused by preliminary fracturing due to internal stresses. We calculate the internal thermal stresses using the analytical solution of the heat diffusion equation for bodies approaching the Sun along parabolic trajectories. The results indicate that upon reaching Jupiter's orbit, thermal stresses develop within bodies composed of crystalline ice with radii ranging from 60 m to 5 km, exceeding the material's tensile strength. When the tidal forces subsequently separated the fragments, they formed a chain of large bodies, with the central body corresponding to the inner core of the comet. The fragments closest to the surface were observed before and behind the central core. Therefore, the preliminary fracturing of the comet by thermal stresses, followed by the separation of resulting fragments due to tidal forces, appears quite plausible.