Jupiter's large moon Europa has been drawing attention for decades, ever since the first space missions to the giant planet sent back images hinting that an ocean of liquid water may lie concealed beneath its icy surface. So crucial is liquid water to life on Earth that detecting it anywhere else sets off alarm bells — or rather, excitement bells — in scientists' heads.

Interest in the Jovian moon ratcheted up further in 2019 when scientists found evidence of sodium chloride — better known as ordinary table salt — on its frozen surface. If the underground ocean believed to exist on Europa is salty, like Earth's oceans, it must be exposed to rocky minerals that could provide the building blocks of living organisms.

Now, new images indicate that Europa's salty ice surface may literally glow in the dark, potentially giving researchers a road map of its surface composition and further guiding the search for any Europan life.

In an unusual twist, as Space.com reports, the images came not from Europa but from a lab in Maryland. What's more, they led to a completely unexpected discovery.

The Heart of an Icy Moon of Jupiter

According to NASA's Jet Propulsion Laboratory (JPL), a research team built a special instrument, intending to study the effects that Jupiter's powerful radiation field of charged particles might have on any organic compounds on the surface of Europa.

The instrument is officially called Ice Chamber for Europa's High-Energy Electron and Radiation Environment Testing — a jawbreaker name that is much easier to remember as an acronym: ICE-HEART.

To carry out the intended research, ICE-HEART was set up at a high-energy lab in Maryland. There, as Science News notes, the device cooled salty ice samples to around minus 173 degrees C (the temperature of Europa's surface), then bombarded them with radiation.

When ICE-HEART was switched on, the ice samples started to glow. This in itself was not a huge surprise. While we usually associate glowing astronomical bodies with very high temperatures (think stars), a variety of radiation-related effects can make even cool objects, including living organisms, glow.

When Serendipity Strikes

But when the scientists varied their salty ice samples, they got a surprise. Salts, to a chemist, are a family of compounds with similar properties. Table salt, or sodium chloride, is one of these compounds. Epsom salt, or magnesium sulfate, is another. And it turns out that each of these salts produces a different glow when run through ICE-HEART. Ice containing sodium chloride glows less than pure ice with no salt, while ice with magnesium sulfate glows brighter than pure ice.

In all the history of science, no one had ever previously tried the specific experiment of cooling salty ice to outer-solar-system temperatures, then bombarding it with electrons. So — until now — no one knew that each variety of salt produces its own distinct signature glow.

NASA research team member Bryana Henderson detailed the moment of discovery: "When we tried new ice compositions, the glow looked different. And we all just stared at it for a while and then said, 'This is new, right?'" The researchers described this finding as a serendipity, a sheer good-luck discovery made by explorers who were looking for something else entirely.

What Lies Beneath

So, does the icy moon of Jupiter actually glow in the dark? We don't yet know. Earth-based telescopes can't get a decent view of the night side of Europa, and no space-based telescopes are currently in position for a good view.

As Science News notes, the ICE-HEART samples were only exposed to a Jupiter-like radiation field for 20 seconds, not millions of years. Astronomer Jennifer Hanley told Science News that she wonders, "Is there ever a point where you might break down the salts, and this glow stops happening?"

The only sure way to find out is to go out to Europa and look. Space.com reports that NASA's Europa Clipper probe, scheduled for launch about mid-decade, may give us our first chance to do just that. Scientists are currently examining whether its instruments should be able to detect the glow.

If the glow of salty ice is detectable on Europa, it could provide a powerful tool for mapping the chemical composition of the surface and help to determine where a future Europa landing mission (now in preliminary planning) should go.

If we do find life in the underground ocean of Europa, we may have the glow of salty ice to thank for the discovery.

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Jupiter’s Moon Europa May Glow in the Dark

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