A New Spectral Feature on the Trailing Hemisphere of Europa at 3.78 μm
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 SO_2 frost seen in both laboratory spectra and spectra of Io. However, the corresponding, typically stronger 4.07 µm feature of SO_2 frost is absent from our data. This result is contrary to the suggested detection of SO_2 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 SO_2 frost on the surface. We explore alternative explanations and discuss other potential candidate species.
Additional Information© 2017. The American Astronomical Society. Received 2017 February 1. Accepted 2017 April 8. Published 2017 May 12. This research was supported by Grant 1313461 from the National Science Foundation. K.P.H. acknowledges support from the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration and funded in part through the internal Research and Technology Development program. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. The authors thank Bethany L. Ehlmann, George R. Rossman, and Robert P. Hodyss for helpful discussions. Software: Astropy (Astropy Collaboration et al. 2013), skimage.transform (van der Walt et al. 2014).
Published - Trumbo_2017_AJ_153_250.pdf
Submitted - 1706.03295.pdf