What Does Europa Taste Like?

Today, I attended a lecture by Mike Brown, a Caltech professor most famous for "killing Pluto" by discovering the Kuiper Belt object Eris. This time, he was talking about his current scientific project: learning about the chemistry (and taste!) of Europa. One of the great things about being a Caltech undergrad is that I get to attend talks like this from world-class scientists without travelling too far from home!

Europa is an icy moon of Jupiter that is about the size of our own moon. Often, we hear that Europa is exciting because it has a subsurface ocean. Brown's rationale for studying Europa is a bit more nuanced than that--Ganymede, another moon of Jupiter, also has a reserve of liquid water underneath its surface but is less interesting to Brown's research because it has more water than Europa. This is because Ganymede has a thick enough liquid water ocean that towards the bottom, the pressure is high enough for the water to freeze out into ice. If you could slice into Ganymede, you would discover an object with an icy outer shell, a liquid water ocean, an inner icy shell and a rocky core, or as Brown described it, "an icy water sandwich." In contrast, the smaller water ocean cannot produce ice at the bottom of the ocean. Water can come in contact with Europa's rocky core, creating a boundary or "interface" where water and rock can interact chemically.

A water-rock interface is important because here on Earth, the interaction of water and rock drives plate tectonics and creates hydrothermal vents inhabited by some of the planet's most exotic creatures.

Water and ice also interact with Europa's surface, producing linear features on its surface and jumbled terrains that resemble icebergs floating on a frozen ocean. These features are produced by upwelling of water from Europa's subsurface ocean. Brown is currently working on identifying the chemical composition of salts present in these features. By doing so, he can sneak a peek at the composition of the ocean beneath. Here on Earth, the main salt in our oceans is sodium chloride. Is this true of Europa, too?

Using spectroscopic measurements from the massive Keck Telescope on the Big Island of Hawai'i, Brown can find out. Europa has a quite varied landscape, with three main compositions roughly corresponding two hemispheres and their border. Europa is tidally locked to Jupiter like our moon is, so one side always faces the giant planet. On the side that leads the planet on its orbit, only pure water ice is detected. On the trailing side, Brown found traces of sulfuric acid, produced by the interaction of sulfur ions from another Jovian moon, Io, carried to Europa by Jupiter's massive magnetic field. Between these two extremes, the surface water is laced with magnesium, potassium, and sodium salts that likely originate from the interaction of the water ocean and rocky core of Europa.

What does this mean for the taste of Europa? Brown suggests a mixture of ice, salt, and grapefruit juice--the sour citric acid in grapefruit replacing the harsher sulfuric acid--for a drink that is out of this world.