  We present hemispherically resolved spectra of the surface of Europa from
~3.1--4.13 microns, which we obtained using the near infrared spectrometer
NIRSPEC on the Keck II telescope. These include the first high-quality L-band
spectra of the surface to extend beyond 4 microns. In our data we identify a
previously unseen spectral feature at 3.78 microns on the trailing hemisphere.
The longitudinal distribution of the feature is consistent with that of a
radiolytic product created by electron or Iogenic ion bombardment. This feature
is coincident with an absorption feature of SO2 frost seen in both laboratory
spectra and spectra of Io. However, the corresponding, typically stronger 4.07
micron feature of SO2 frost is absent from our data. This result is contrary to
the suggested detection of SO2 at 4.05 microns in Galileo NIMS data of the
trailing hemisphere, which was severely affected by radiation noise. We use
simple spectral modeling to argue that the 3.78 micron feature is not easily
explained by the presence of SO2 frost on the surface. We explore alternative
explanations and discuss other potential candidate species.
