The Mid-UV Spectrum of Irradiated NaCl at Europa-like Conditions

Abstract

Recent observations from the Hubble Space Telescope show a mid-UV absorption feature localized to leading-hemisphere chaos regions on Europa. The same regions were previously found to have a visible absorption at 450 nm that was attributed to the presence of irradiated NaCl. The lack of any additional diagnostic absorptions for NaCl in the visible spectrum of these terrains made confirmation of this identification difficult. Here we use laboratory experiments to show that NaCl irradiated at Europa’s surface temperatures develops an absorption at ∼220 nm, consistent with the new detection in Europa’s mid-UV spectrum, strongly supporting the NaCl identification. Irradiated NaCl in leading-hemisphere chaos terrain would suggest that sodium and chlorine are important components of Europa’s subsurface ocean.

A New UV Spectral Feature on Europa: Confirmation of NaCl in Leading-hemisphere Chaos Terrain

“…The new detection of an NaCl V 3 -center absorption, in combination with the parallel laboratory work explaining the HST data in the context of color-center formation and decay at Europa-like conditions (Brown et al 2022;Denman et al 2022), effectively confirms the presence of NaCl on the surface of Europa. In addition, the clear association of both the V 3 -and F-center features with geologically disrupted chaos terrain implies that this NaCl originated in Europa's internal ocean, which has important implications for the nature of Europa's endogenous salts.…”

“…As the laboratory data do not extend below 200 nm and the HST spectra are of very poor quality at those wavelengths, it appears likely that these bounds do not entirely capture the extent of the feature on the shortwavelength end. The result of the laboratory comparison is included in Figure 1, which demonstrates that the band minimum, shape, and apparent width of the observed 230 nm feature are well matched by the V 3 center of laboratoryirradiated NaCl at 120 K. Indeed, we obtain equivalent, marginally different fits using any of the spectra of Brown et al (2022), which show slight variations in the V 3 band with different laboratory conditions.…”

“…The HST spectra of some locations on Europa reveal a subtle absorption near 230 nm (Figure 1), which appears discrete against the sloping continuum in the mid-UV and at roughly the wavelength of the NaCl V 3 band at 120 K (Brown et al 2022). To facilitate comparison with the laboratory data, we remove linear continua from both an average HST spectrum containing the feature (Figure 1) and a 120 K laboratory NaCl spectrum from Brown et al (2022) using identical bounds.…”

“…Here, we use mid-UV HST spectra of Europa's surface to detect a previously unseen absorption near 230 nm, which geographically correlates with the recently detected F-center feature. The parallel laboratory work of Brown et al (2022) shows that NaCl irradiated under Europa-like conditions forms an additional feature, known as the V 3 center, at similar wavelengths. We find that the V 3 band matches our observed feature well and argue that this detection confirms the presence of NaCl within geologically young chaos regions on Europa.…”

“…The dashed line is a linear continuum fit, and the continuum-removed feature is included above the spectrum. The continuum-removed NaCl V 3 band from the 120 K laboratory spectrum ofBrown et al (2022) is overlain in red and scaled to match the depth of the observed feature in the HST data.…”