Albedos of Small Jovian Trojans

TL;DR

This study uses thermal infrared observations to determine that small Jovian Trojans have higher and more varied albedos than larger ones, likely due to their younger, collision-generated surfaces.

Contribution

It provides the first comprehensive albedo measurements of small Jovian Trojans, revealing a correlation between size and surface reflectivity not observed in larger Trojans.

Findings

Small Trojans have median albedo of about 0.12.

Albedo range among small Trojans is wider than in larger ones.

Higher albedo correlates with smaller diameter, indicating younger surfaces.

Abstract

We present thermal observations of 44 Jovian Trojan asteroids with diameters (D) ranging from 5 to 24 km. All objects were observed at a wavelength of 24 microns with the Spitzer Space Telescope. Measurements of the thermal emission and of scattered optical light, mostly from the University of Hawaii 2.2-meter telescope, together allow us to constrain the diameter and geometric albedo of each body. We find that the median R-band albedo of these small Jovian Trojans is about 0.12, much higher than that of "large" Trojans with D > 57 km (0.04). Also the range of albedos among the small Trojans is wider. We attribute the Trojan albedos to an evolutionary effect: the small Trojans are more likely to be collisional fragments and so their surfaces would be younger. A younger surface means less cumulative exposure to the space environment, which suggests that their surfaces would not be as dark as those of the large, primordial Trojans. In support of this hypothesis is a statistically significant correlation of higher albedo with smaller diameter in our sample alone and in a sample that includes the larger Trojans.