Research ArticlePLANETARY SCIENCE

Sodium chloride on the surface of Europa

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Science Advances  12 Jun 2019:
Vol. 5, no. 6, eaaw7123
DOI: 10.1126/sciadv.aaw7123
  • Fig. 1 HST/STIS spectra showing a distinct 450nm spectral feature, consistent with an NaCl F-center absorption, and a clear lack of a 720nm NaCl M-center absorption.

    (A) Single spectrum from within Tara Regio, which exhibits a particularly strong 450-nm absorption. The dashed line is a third-order polynomial continuum fit. The continuum-removed feature is included underneath the spectrum. Overlain in red is a continuum-removed laboratory spectrum of irradiated NaCl at 100 K, taken from figure 2 of (24). This spectrum corresponds to an NaCl F-center absorption that has evolved in the absence of unrealistic laboratory radiation fluxes. The laboratory F-center absorption has been scaled to match the depth of the observed feature. (B) High signal-to-noise spectrum produced by averaging all spectra from locations exhibiting a 450-nm feature, weighted by the strength of that feature in each location. The weighted average is divided by the average of all spectra from locations in which the 450-nm feature is absent and rescaled to approximate the known Europa continuum. A continuum-removed version is shown underneath the spectrum, where the vertical dashed line indicates the anticipated band center at 720 nm.

  • Fig. 2 A map of the strength of the 450nm absorption.

    The observed feature maps solely to the leading hemisphere. Black outlines correspond to large-scale chaos regions, mapped approximately from (44). The largest absorptions fall within the chaos region Tara Regio (~85°W), with additional concentration in eastern Powys Regio (~125°W). This distribution is separate from the geography of sulfur radiolysis and suggests a subsurface source, consistent with the chloride hypothesis for Europa’s endogenous material. The spatial resolution of the mapped data is ~150 km at the sub-observer point. Background image credit: NASA/JPL/Björn Jónsson/Steve Albers.

  • Table 1 HST STIS G430L/G750L observations of Europa.

    Date
    (universal
    time)
    Time
    (start/end)
    Sub-
    observer
    longitude
    Sub-
    observer
    latitude
    Angular size
    of Europa
    (arcseconds)
    2017 May 2300:20/01:41224−3.060.91
    2017 Jun 2908:19/08:5547−2.910.82
    2017 Aug 104:43/05:20133−2.910.75
    2017 Aug 612:27/13:47314−2.920.74

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Representative HST/STIS spectra of the three chemical terrains on the surface of Europa.
    • Fig. S2. Laboratory spectra of select irradiated salts.

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