Take some water, add table salt and simmer for millions of years.
It's almost as if some divine hand were starting a nice soup.
But the broth on Europa — Jupiter's fourth largest moon — may be cooking up something scientists have overlooked for decades: Life.
According to a study published this week in Science Advances, Europa's brine is covered with sodium chloride. That's table salt, or the main component of sea salt.
And it suggests the vast ocean beneath Europa's icy enamel could be a lot more like Earth's oceans than anyone previously thought.
For the study, researchers at Caltech and NASA's Jet Propulsion Laboratory focused on the swathes of yellow coloring in the Tara Regio region captured by NASA's Voyager and Galileo spacecrafts, as well as the Hubble Space Telescope. A closer look at those patches, thanks to data from Galileo's built-in infrared spectrometer, revealed the presence of sodium chloride.
"Sodium chloride is a bit like invisible ink on Europa's surface," NASA's Kevin Hand noted in a press release. "Before irradiation, you can't tell it's there, but after irradiation, the color jumps right out at you."
Surprisingly, this discovery has been sitting under our noses for decades.
"We've had the capacity to do this analysis with the Hubble Space Telescope for the past 20 years," Mike Brown, who co-authored the paper, explained in the release. "It's just that nobody thought to look."
We may consider ourselves a primarily blue planet, thanks to the salty oceans that cover 71 percent of the Earth's surface and represent 97 percent of its water, but Europa is far more flush with water.
Much of it may be like sea ice in the Antarctic.
"It indicates that the ice is geologically pretty young and it could be proof of its interaction with a reservoir of liquid water," François Poulet from the Institute of Space Astrophysics at the Université Paris-Sud, told Chemistry World last year.
This week's discovery that Europa's ocean is a lot like our own may also broaden our horizons in the search for life in the cosmos. For the most part, scientists assume life is most likely to form on planets within a certain range of the star it orbits. A planet too close to its sun will be a smoldering husk; too far and it's an ice cube. The perfect real estate for a planet capable of supporting life would be a region in between, called the "Goldilocks zone."
But Europa doesn't get its energy from our sun. As a moon, it relies on its host planet — in this case, Jupiter — for that. In effect, the giant gas planet is its sun, using its gravitational pull to keep the moon in orbit. Gravity's stretching and flexing effect on the Europa provides the energy it needs to simmer. No Goldilocks zone required.
But what exactly is cooking on Europa? Jupiter and several of its moons will be so close to Earth this month, we only need binoculars to spot them, but Europa keeps its secrets beneath its unassuming exterior.
It's the puzzle on the inside that scientists are looking to crack. If Europa's sodium chloride does indeed spring from the planet's core — rather than having been leached into the ocean from rocks on its sea floor — than those very Earth-like oceans could host some very Earth-like life.
At the very least, Europa offers a vital lesson to scientists as they cast their gaze farther and farther into space.
"That would mean Europa is a more geologically interesting planetary body than previously believed," Brown added.
Another reason to never judge a world by its cover.