A study led by Mike Brown (of Killing Pluto fame) detected deposits that hint Earth-like saltwater might be leaking out to Europa’s surface from underneath.
Brown’s team used Keck II’s infrared capabilities to study the composition of brown ice patches on Europa’s surface, something the Galileo orbiter’s early-80s vintage IR spectrometer did not have the resolution to do. One thing that puzzled scientists was the brown patches of ice that appeared on the trailing side of Europa. Some argued that it was magnesium leaking up from the ocean below, while others argued that it was sulfuric acid formed as sulfur spewing out from Io interacted with Jupiter’s intense radiation belts. “There’s been this long-raging debate about whether the dark material on Europa is salts from the salty ocean below, or sulphuric acid from radiation,” said co-author Kevin Hand (JPL).
As it turns out, it’s a little of both. Brown’s team found that the brown spots were magnesium sulfate, formed when magnesium leaking up through cracks in Europa’s icy shell reacted with sulfur molecules raining down on its surface. The end result is magnesium sulfate, commonly known as epsom salt. Magnesium is a common element in seawater here on Earth because it dissolves out of rock, given enough time. Europa has a rocky core and a large subsurface ocean, meaning that there is likely lots of magnesium to go around.
So how do we know that the magnesium is coming from Europa and not from space? The authors made the discovery while studying a different aspect of Europa: looking at the variability in non-water ice on its surface. As it turns out, both the leading and trailing side of the moon have significant amounts of elements like magnesium and potassium incorporated into the ice. This finding led the authors to conclude the magnesium is probably native to Europa.
The sulfur appears only on the trailing side of Europa because of how sulfur gets from place to place in the Jovian system. After it is erupted from Io’s volcanoes, the sulfur becomes charged by electrons caught in Jupiter’s powerful magnetic field. Newly ionized, the sulfur also begins moving along field lines. The magnetic field rotates synchronously with Jupiter, and Jupiter spins faster than Europa’s orbital period. As a result, the field lines always catch up to Europa from behind, depositing the sulfur ions being carried along. When a high-speed sulfur ion collides with a magnesium atom, magnesium sulfide is created.
One interesting implication of the finding is that sulfur probably isn’t present in large amounts in the subsurface ocean – otherwise the magnesium sulfate would be found all over Europa. Instead, the magnesium is likely bound chemically to chlorine, which is also dissociated very easily from rock in water. This has the upshot of making Europa’s oceans look even friendlier to the prospect of life – magnesium chloride is much more tolerable to Earth bacteria than magnesium sulfate. Other chlorides, like sodium chloride and potassium chloride, are probably floating around in Europa’s subsurface ocean. Put it all together and you get a very Earth-like ocean indeed – at least compositionally.
Another conclusion of the study means that we might be able to glean information about Europa’s ocean without drilling to get to it first. Contamination of surface ices suggests that the ocean occasionally bubbles to the surface. If that really is the case, then fresh deposits will likely give us a good estimate on the chemical composition of the ocean. However, up-close investigations of Europa probably won’t happen until 2030, when the ESA’s planned Jupiter Icy Moon Explorer enters Jupiter orbit.
Image: Galileo Project, JPL, NASA; reprocessed by Ted Stryk