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Thermal Inertia of Asteroids -- Regolith Coverage and Yarkovsky Effect

Wednesday, January 16, 2008
Michael
Müller (Steward Obs, U of AZ)

Thermal inertia is a measure of the resistance to changes in surface temperature. It triggers the Yarkovsky effect and is a sensitive indicator for surface regolith (the latter fact is widely used in Mars geology).

The largest few main-belt asteroids (MBAs) are well known to have a very low thermal inertia, indicating a thermally insulating regolith cover. However, very little has been known about the thermal inertia of small asteroids, including near-Earth asteroids (NEAs), which are most strongly affected by Yarkovsky forces and may be expected to be regolith free due to their low gravity.

Thermal-inertia measurements of five NEAs are reported, increasing the total number of NEAs with measured thermal inertia to six. To this end, extensive mid-IR observations were performed (using IRTF and Spitzer) and a detailed thermophysical model was developed and tested; this is the first such model shown to be applicable to NEA data. The size and albedo of all targets was determined, too.

Our results allow the first determination of the typical thermal inertia of NEAs, which is around 300 J s-1/2K-1m-2 (corresponding to a thermal conductivity of ~0.08 W K-1m-1), larger than the typical thermal inertia of large MBAs by an order of magnitude and indicating the presence of coarse and/or thin regolith on most of their surfaces.

In particular, thermal inertia appears to increase with decreasing asteroid diameter. This is qualitatively consistent with present regolith formation theories and has important repercussions on the size-dependent strength of the Yarkovsky effect and the drift of small MBAs into near-Earth space (see Delbo' et al., 2007).

Observations of MBAs and a Trojan are also presented, including the first thermal-IR observations of an eclipsing binary asteroid system, allowing a uniquely direct determination of the system's thermal inertia.

Most of the reported work was part of the author's thesis project, performed at DLR Berlin, Germany under the supervision of Alan Harris (the younger).

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