Abstract
We simulate the stresses induced by temperature changes in a putative hard layer near the surface of comet 67P/Churyumov-Gerasimenko with a thermo-viscoelastic model. Such a layer could be formed by the recondensation or sintering of water ice (and dust grains), as suggested by laboratory experiments and computer simulations, and would explain the high compressive strength encountered by experiments on board the Philae lander. Changes in temperature from seasonal insolation variation penetrate into the comet’s surface to depths controlled by the thermal inertia, causing the material to expand and contract. Modelling this with a Maxwellian viscoelastic response on a spherical nucleus, we show that a hard, icy layer with similar properties to Martian permafrost will experience high stresses: up to tens of MPa, which exceed its material strength (a few MPa), down to depths of centimetres to a metre. The stress distribution with latitude is confirmed qualitatively when taking into account the comet’s complex shape but neglecting thermal inertia. Stress is found to be comparable to the material strength everywhere for sufficient thermal inertia (≳50 J m−2 K−1 s−1∕2) and ice content (≳45% at the equator). In this case, stresses penetrate to a typical depth of ~0.25 m, consistent with the detection of metre-scale thermal contraction crack polygons all over the comet. Thermal fracturing may be an important erosion process on cometary surfaces which breaks down material and weakens cliffs.
Highlights
Fracturing is prevalent on many scales on Comet 67P/Churyumov-Gerasimenko when observed by Rosetta’s OSIRIS imaging instrument
Stress is found to be comparable to the material strength everywhere for sufficient thermal inertia ( 50 J m−2 K−1 s−1/2) and ice content ( 45% at the equator)
The results of our thermo-viscoelastic model show stresses of up to several tens of MPa induced by seasonal temperature changes on a spherical model of comet 67P down to depths of between a centimetre and a metre. These are seen at most locations on the comet, with the exception of a band of low stress in the middle and low latitudes of the northern hemisphere (Fig. 7), which is pronounced for low values of thermal inertia (I 10 J m−2 K−1 s−1/2)
Summary
Fracturing is prevalent on many scales on Comet 67P/Churyumov-Gerasimenko (hereafter 67P) when observed by Rosetta’s OSIRIS imaging instrument. Thermal stresses may well be the dominant mechanism for the formation of polygonal and irregular fractures (those not associated with the neck) on both large (Thomas et al 2015; El-Maarry et al 2015a) and small scales (Poulet et al 2016) on 67P, and the remainder of this paper will focus on these processes. Kuehrt (1984) modelled Halley’s comet as solid water ice and showed that seasonal thermal stresses at perihelion can reach tens of MPa, exceeding the tensile strength These results were robust when extended from merely elastic stresses to a viscous description (Tauber & Kuehrt 1987), and similar when considering inhomogeneous bodies, showing that comets should be thermally fractured and that this may be important for erosion and outgassing.
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