Abstract DGP2026-102

The asteroids (162173) Ryugu and (101955) Bennu are rubble piles with surfaces dominated by dm-sized pebbles and larger boulders. Ryugu was the target of JAXA sample-return mission Hayabusa2 which included the DLR/CNES MASCOT lander. Bennu was the target of NASA’s sample return mission OSIRIS-REx. The boulders of both asteroids showed a surprisingly low thermal inertia that was attributed to a high porosity of the asteroid material. However, analysis of returned samples challenged that picture as their thermal inertia was, in the case of Bennu, 2x higher than that of the boulders, and 3.5x higher for the case of Ryugu. The samples of both asteroids show fractures at various scales. Recent works show that a single fracture parallel to the surface can constitute an obstacle to heat flow, and effectively lower the thermal inertia of a boulder. While these studies can partially reproduce the in-situ infrared observations of OSIRIS-REX and MASCOT, they do not match as well as earlier studies that assumed homogenous subsurface for their boulders. Nevertheless, given the complex geometry of fracture networks, a better match might be obtained by reproducing fracture networks in a 2-dimensional thermophysical model.

In this work, we present a 2-dimensional extension of a thermophysical model capable of simulating heat flows in structured materials.  In this model, the 2D heat conduction equation is solved by setting up a grid of points in x -and y-direction respectively. This grid defines M × N cells that can be defined to be either material or voids. Each cell is assumed to be isothermal. The temperature change of a cell is proportional to the net heat flow from neighboring cells. For surfaces, that is the boundary between a material and a void cell, the heat flow into the cell is defined by the balance of net radiative heat exchange to visible surfaces and illumination, if the surface is not shadowed. For the boundary between two cells filled with material, the flow is given by the net conductive heat transfer between these cells. With this 2D TPM, we investigate fractures at various depths, as well as the heat flow in surface structures. The latter is accounting for surface roughness and we investigate how lateral heat flow through such structures influence kinetic and brightness temperatures. With our work we aim to better link the inner structure of asteroid materials to the thermal infrared emission observable through remote sensing and telescopic observations.