Seems weird to refer to it as table salt. I get that some portion of the audience doesn't otherwise know what sodium chloride is, and just saying "salt" is ambiguous, but still.

Sort of like saying that a lake is filled with "drinking water."

I guess if I was writing the headline for this article I'd go with something like "Europa's ocean may have chemical similarities to Earth's." It gets at the interesting implication of the discovery without being too clickbaity.

The Onion: "Coke-Sponsored Rover Finds Evidence Of Dasani On Mars"

new evidence suggests alien life forms may have high blood pressure

Is water ambiguous? I can name three different compounds I have in my home called salt. Maybe if you're talking about food its clear what you mean, but we're talking about a whole nother planet here.

Yes I realize that, and I don't know what the perfect headline is, but I stand by "it seems weird."

I notice that Wikipedia's entry on salt says "Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts." (it also says "This article is about common table salt. For salts in chemistry, see Salt (chemistry). For table salt used in chemistry, see Sodium chloride. For other uses, see Salt (disambiguation).")

Normally the word "salt" indicates sodium chloride unless it is otherwise clear from context that it might be referring to some other salt.

And there is always the first sentence of the article to clear up what specific chemical you are speaking of.

There are different kinds of water.

https://en.wikipedia.org/wiki/Heavy_water

https://en.wikipedia.org/wiki/Tritiated_water

https://en.wikipedia.org/wiki/Semiheavy_water

Finding any of them on another planet or moon would be interesting.

All of those are H2O, with some contaminants in them. When it comes to water, people mean a material primarily consisting of H2O. There's no other chemical commonly referred to as water.

I had a picture of a wooden table with four legs, on which was a salt cellar containing table salt.

tl;dr: Sodium chloride was found using visible spectroscopy (literally, yellow patches), a technique that has been available for 20 years, but nobody thought to look:

> 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/...

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.