  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.
