2017AJ....153..250T

We present hemispherically resolved spectra of the surface of Europa from ∼3.1-4.13 µm, 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 µm. In our data, we identify a previously unseen spectral feature at 3.78 µm 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 µm feature of SO₂ frost is absent from our data. This result is contrary to the suggested detection of SO₂ at 4.05 µm 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 µm feature is not easily explained by the presence of SO₂ frost on the surface. We explore alternative explanations and discuss other potential candidate species.