Constraining the composition of salts exposed on Europa's surface using VIS-NIR spectra derived from Europa Clipper EIS and MISE data

Europa Clipper's goal is to assess the habitability of Europa's ocean [1]. Key is constraining the composition of surface materials derived from the ocean, particularly salts [2-4]. Halides can suggest a chloride-rich ocean [5] that may indicate significant seafloor water-rock interaction [6]; primordial sulfate hydrates can imply a sodium-poor ocean [5]. Halides and sulfates could both exist, as modeling predicts that a Na, Mg, Cl, SO₄ ocean extruded onto Europa's surface would precipitate both sodium sulfates and MgCl₂ [7].

Although many salts are spectrally bland in the NUV-IR, (except minimally diagnostic hydrate features), when irradiated by energetic particles they can develop color-center defects [e.g., 8] that provide diagnostic information [e.g., 4, 9]. E.g., while there are reported detections of NaCl [10] and Mg-chlorinated salts [11, 12], the lack of a strong halide-color-center feature in Europa spectra questions their abundance [13].

Two Clipper instruments can identify these color centers: Europa Imaging System (EIS) [14] and Mapping Imaging Spectrometer for Europa (MISE) [15]. EIS' NAC and WAC both have 6 filters between ~350-1050 nm. MISE provides 10-nm resolution spectra from 0.8-5 µm. EIS WAC and MISE achieve ~10-m/pixel scale from 50km. Although coarse, EIS spectral sampling can discern the spectral shape of many irradiated salts (e.g., distinguishing hydrated MgSO₄ [16] from anhydrous NaCl [10]), and combined EIS and MISE data, afford us the best chance to identify many of these salts. Mapping salts to geologic features will help to determine their origin and relevance to ocean composition. These data will provide a key capability for determining Europa's surface composition and will increase our understanding its ocean composition and potential to harbor life.

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