> Meanwhile, JPL scientist Kevin Hand had used sample ocean salts, bombarded by radiation in a laboratory under Europa-like conditions, and found that several new and distinct features arose in sodium chloride after irradiation. He discovered that they changed colors to the point that they could be identified with an analysis of the visible spectrum. Sodium chloride, for example, turned a shade of yellow similar to that visible in a geologically young area of Europa known as "Tara Regio.
> ...
> "We've had the capacity to do this analysis with the Hubble Space Telescope for the past 20 years," Brown said. "It's just that nobody thought to look."
Sodium chloride is the major dissolved component of the Earth's ocean. The significance is that Europa's ocean may be much more similar to the Earth's ocean than previously thought.
Where there's similarity in environment, there's a chance for similarity in life that may be found there.
The article doesn't note the exact component(s) giving rise to the yellow color. It may be important that ocean salts, not pure sodium chloride itself, were irradiated by JPL. Ocean water contains many other dissolved minerals. Knowing the minimum component mixture necessary to produce the yellow color could say much more about the similarity between the oceans.
It's also not clear from the article what other materials might account for the 450 nanometer absorption.
Link to research paper:
https://advances.sciencemag.org/content/5/6/eaaw7123
The supplementary material Figure S2 contains some comparisons in the spectra of various samples:
> Here, we reproduce a selection of irradiated salt spectra that can be examined for the presence of a 450 nm absorption. With the exception of the NaCl brine taken from (23), all of the spectra shown were taken at room temperature. The bloedite spectrum (Na2Mg(SO4)2 4H2O) is from (29), the MgCl2 spectrum is from (35), and the remaining salt spectra are from (34). Of all of the spectra, only NaCl can explain the observed 450 nm absorption on Europa, and most have strong absorptions elsewhere that we do not observe in our HST data. All spectra are normalized to unity at 750 nm, and each spectrum is offset vertically by 0.4 units from the one below it.
https://advances.sciencemag.org/content/advances/suppl/2019/...