The 3-4 $\mu$m Spectra of Jupiter Trojan Asteroids

To date, reflectance spectra of Jupiter Trojan asteroids have revealed no distinctive absorption features. For this reason, the surface composition of these objects remains a subject of speculation. Spectra have revealed, however, that the Jupiter Trojan asteroids consist of two distinct sub-populations which differ in the optical to near-infrared colors. The origins and compositional differences between the two sub-populations remain unclear. Here we report the results from a 2.2-3.8 $\mu$m spectral survey of a collection of 16 Jupiter Trojan asteroids, divided equally between the two sub-populations. We find clear spectral absorption features centered around 3.1 $\mu$m in the less red population. Additional absorption consistent with expected from organic materials might also be present. No such features are see in the red population. A strong correlation exists between the strength of the 3.1 $\mu$m absorption feature and the optical to near-infrared color of the objects. While traditionally absorptions such as these in dark asteroids are modeled as being due to fine-grain water frost, we find it physically implausible that the special circumstances required to create such fine-grained frost would exist on a substantial fraction of the Jupiter Trojan asteroids. We suggest, instead, that the 3.1 $\mu$m absorption on Trojans and other dark asteroids could be due to N-H stretch features. Additionally, we point out that reflectivities derived from WISE observations show a strong absorption beyond 4$\mu$m for both populations. The continuum of 3.1 $\mu$m features and the common absorption beyond 4 $\mu$m might suggest that both sub-populations of Jupiter Trojan asteroids formed in the same general region of the early solar system.