The brackish depths of Europa Edge with table salt

Scientists are quite confident that Jupiter's moon, Europa, has an underground ocean, even though they have never seen it.

Hidden under an icy crust, most of the information available to researchers about this ocean is based on the smooth, ridged surface of the moon. Europa lacks mountains or large craters, but scientists believe it is caused by the displacement of ice patches on a liquid ocean, analogous to the tectonics of the Earth's plates.

Using the Hubble Space Telescope to spy on Europa's surface, astronomers have discovered the presence of sodium chloride, better known as table salt, the same compound that makes the Earth's oceans so salty. This could indicate that the Europa Ocean is more like the Earth's or Enceladus's oceans than we thought and reinforces some theories about hydrothermal activity on the Moon.

Salted results

The only spacecraft to have visited Jupiter's moons closely was Galileo in the late 1990s and early 2000s. For an up-to-date view, Samantha Trumbo and her California Institute of Technology colleagues used Hubble to analyze Europa in visible light. This revealed the electromagnetic fingerprints of various elements, also called spectral signatures. One of these fingerprints belonged to sodium chloride and appeared most often near the scars of the chaos ground of the Moon. This is an indication that salt could seep into a submarine ocean, say the researchers in an article published Wednesday in Progress of science.

The chaotic terrain is younger than the rest of Europa's surface and scientists believe it is caused by more distant interactions with liquid or slippery layers, although details remain to be debated. So, if the salt appears near these features, it probably comes from the bottom.

Astronomers have long thought that European Europe was salty, but that salts were based on sulphates instead of chlorides (chemists have a different definition of salt than that of your ordinary chief). Sulphates are common in the European solar system and the ocean could become salty with sulphates just because the water has been interacting with the seabed over the years.

Enceladus, the Earth and Moon of Saturn, possess oceans containing chloride salts and also have known or strongly suspected hydrothermal vents on their seabed. This heating and the resulting circulation are essential for the waters to become rich in chlorides over time. So, if Europa shares the chloride-salt ocean, he could also share the heat source that creates the salty ocean.

And even if Europa does not organize hot vents in deep water, the presence of sodium chloride radically changes the astronomers' assumptions about the chemistry of the invisible ocean.

Despite the frozen nature of moons like Enceladus and Europa, scientists consider them a good choice for extraterrestrial life in the solar system. The gravitational standoff between the moons and their giant planets can keep an oceanic liquid underground, and perhaps even lead to thermal aerators like those that give life to the darkest and deepest oceans on Earth. But it's not because life exists that life exists, and scientists have little data to explore.

This lack of data makes the scientific theories of provisional scientists at the moment. Despite decades of research, scientists still do not know for sure how the chaotic terrain of Europa is formed. And there may be other ways to create an ocean of salty chloride with no vents. But future missions, such as Europa Clipper, NASA, and ESA's Jupiter Icy Moon Explorer, could finally reveal what's happening beneath the icy surface of Europa and find out if this ocean holds any potential for extraterrestrial marine life.

Editor's Note: An earlier version of this article contained errors. Hubble's observations were visible light, and the salts previously suspected on Europa were sulphates. Chloride is the result of hydrothermal circulation.