Nope, it's just "color centers" in pure NaCl. From the paper: > a strong F-center absorption near 460 nm due to individual electrons trapped within single Cl−

Nope, it's just "color centers" in pure NaCl. From the paper: > a strong F-center absorption near 460 nm due to individual electrons trapped within single Cl− vacancies > a weaker M-center (or F2-center) absorption near 720 nm due to binary aggregates [that is, pairs] of F-centers. I gather that something knocks Cl- ions out of NaCl crystals, leaving electrons stuck in the holes. Relativistic solar wind, maybe? The full quote: > Although spectrally bland in the infrared, alkali chlorides develop distinct spectral features at visible wavelengths under particle irradiation. The bombarding particles lead to the growth of “color centers” by creating anion vacancies in the crystal structures, which trap free electrons and cause compositionally diagnostic absorptions [e.g., (20–22)]. Laboratory experiments have demonstrated that color centers can form in sodium chloride (NaCl) and NaCl brine evaporites under Europa-like surface conditions (23, 24), producing colors in laboratory samples that appear visually similar to those captured in Galileo images of Europa’s surface [e.g., (25)]. Spectrally, these colors largely result from two distinct absorptions caused by two types of color centers—a strong F-center absorption near 460 nm due to individual electrons trapped within single Cl− vacancies, and a weaker M-center (or F2-center) absorption near 720 nm due to binary aggregates of F-centers. To investigate the hypothesis that Europa’s endogenous units contain chloride salts, we used the Hubble Space Telescope (HST) to search for signatures of these color centers on the surface.