The cool surfaces of binary near-Earth asteroids

Here we show results from thermal-infrared observations of km-sized binary Near-Earth Asteroids (NEAs). We combine previously published thermal properties for NEAs with newly derived values for three binary NEAs. The ηvalue derived from the Near-Earth Asteroid Thermal Model (NEATM) for each object is then used to estimate an average thermal inertia for the population of binary NEAs and compared against similar estimates for the population of non-binaries. We find that these objects have, in general, surface temperatures cooler than the average values for non-binary NEAs as suggested by elevated ηvalues. We discuss how this may be evidence of higher-than-average surface thermal inertia. This latter physical parameter is a sensitive indicator of the presence or absence of regolith: bodies covered with fine regolith, such as the Earth's Moon have low thermal inertia,whereas a surface with little or no regolith displays high thermal inertia. Our results are suggestive of a binary formation mechanism capable of altering surface properties, possibly removing regolith:an obvious candidate is the YORP effect.