THE 3–4 μm 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 that 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 μm 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 μm in the less-red population. Additional absorption consistent with that 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 μm 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 μm 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 μm for both populations. The continuum of 3.1 μm features and the common absorption beyond 4 μm might suggest that both sub-populations of Jupiter Trojan asteroids formed in the same general region of the early solar system.