
Table Salt Compound Spotted on Europa - el_duderino
https://www.jpl.nasa.gov/news/news.php?feature=7423
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robbrown451
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."

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apatters
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.

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

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apo
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](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/...](https://advances.sciencemag.org/content/advances/suppl/2019/06/10/5.6.eaaw7123.DC1/aaw7123_SM.pdf)

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mirimir
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.

