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Boulders on Bennu: Subsurface fractures can explain low thermal inertia

Presentation #102.09 in the session Asteroids: Objects of Interest (Oral Presentation)

Published onOct 23, 2023
Boulders on Bennu: Subsurface fractures can explain low thermal inertia

Pre-encounter telescopic observations of Bennu in the infrared suggested that it has a thermal inertia lower than CM chondrites [1]. Through these observations and thermophysical analysis, [1] inferred that Bennu’s surface was covered in regolith grains ranging in size from several millimeters to about a centimeter, because regolith has a lower thermal inertia than coherent rock. OSIRIS-REx confirmed the telescopic infrared observations, but found a surface covered predominantly by meter to 10’s of meter scale boulders implying that the rock itself has a thermal inertia significantly lower than that of CM chondritic meteorites [2]. Even though spectral features indicate that Bennu is similar to CM chondrites [3], the low thermal inertia of boulders on Bennu led to the prediction that the returned samples would have thermophysical properties unique from any of those found in the existing meteorite collection [e.g. 4].

Here I show that a subsurface fracture parallel to the surface in CM chondritic material can explain the inferred low thermal inertia on Bennu, and exotic material properties need not be invoked. I used commercial finite element software, COMSOL Multiphysics, to model the surface temperature of CM chondritic material above a range of subsurface void space configurations. A fracture parallel to the to the surface prohibits heat conduction through contact points between grains and instead results in heat transport only through the less efficient radiation of heat across the fracture. This impeded heat transport results in a higher daytime and lower nighttime surface temperature above the fracture, which could be confused for lower thermal inertia material. Widespread surface-parallel fractures are expected on Bennu as a result of thermal fatigue [5] and observed boulder morphologies are consistent with this type of fatigue-driven exfoliation [6]. Therefore, I suggest that boulders on Bennu do not have a surprisingly low thermal inertia but instead have bulk properties similar to CM chondrites and widespread subsurface fractures resulting from thermal fatigue. If correct, the thermophysical properties of Bennu’s returned sample will be more similar to those of CM chondrites than those inferred from remote sensing observations of Bennu.

[1] Emery J. P. et al. (2014) Icarus 234, 17-35. [2] DellaGiustina D. N., Emery J. P. et al. (2019) Nat. Ast. 3, 341-351. [3] Hamilton V. E. et al. (2019) Nat Ast. 3, 332-340. [4] Rozitis B. et al. (2022) JGR: Planets 127 e2021JE007153. [5] Molaro J. L. et al. (2020) JGR: Planets 125, e2019JE006325. [6] Molaro J. L. et al. (2020) Nat Com 11: 2913.

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