If you could lick Jupiter鈥檚 moon Europa, it would taste like earthly seawater. A fresh look at the moon鈥檚 icy surface has revealed magnesium salts 鈥� the best evidence yet that material can seep from Europa鈥檚 buried ocean to its frozen surface, providing a glimpse of the moon鈥檚 liquid innards.
鈥淲e think the ice shell of Europa is serving as a window into the ocean below,鈥� says of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. 鈥淓uropa likely has a sea salt composition similar to our own ocean.鈥�
Glimpses of Europa from the passing Voyager spacecraft in the 1980s first hinted that it might harbour a liquid ocean. Later investigations by the Galileo spacecraft, which orbited Jupiter from 1995 to 2003, showed a world covered in cracks and littered with dark debris, strengthening the case that Europa has a briny ocean under a relatively thin icy crust.
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But some researchers pointed out that Europa is constantly bathed in sulphur erupted by its volcanic neighbour, the moon Io. Europa is also swathed in Jupiter鈥檚 magnetic field, so is constantly bombarded with charged particles that could turn that sulphur into sulphuric acid.
鈥淭here鈥檚 been this long-raging debate about whether the dark material on Europa is salts from the salty ocean below, or sulphuric acid from radiation,鈥� says Hand. Galileo鈥檚 instruments were not sensitive enough to tell the two scenarios apart.
Sea salt ahoy?
In September 2011, Hand and colleague Mike Brown of the California Institute of Technology used the Keck Observatory in Hawaii to take another look at the light reflected off Europa and tease out the chemical signatures in its spectrum. The resolution was good enough for them to see a feature that Galileo had missed: a hint of magnesium sulphate.
Europa is tidally locked, which means that one side always faces Jupiter. The magnesium sulphate only showed up on the side that gets the brunt of the material ejected from Io and the radiation from Jupiter. Hand and Brown think some other magnesium compound must be welling up from the ocean, to be broken down by radiation on the surface. The magnesium then reacts with sulphur blasted over from Io, forming the sulphate.
A strong candidate for the upwelling substance is magnesium chloride, which doesn鈥檛 have a definitive spectral signature and so would be invisible on the surface. Sodium, potassium and chlorine have all been seen before on or around Europa, so if the ocean holds magnesium chloride, the pair say, there鈥檚 a strong chance it is dominated by other chlorine salts, like sodium chloride and potassium chloride, making the waters distinctly Earth-like.
Bodes well for life
It鈥檚 a reasonable argument, says of the SETI Institute in Mountain View, California, and if it鈥檚 right, it bodes well for the chances of life on Europa. 鈥淐hlorine salts are better for life as we know it.鈥�
Phillips is also impressed at the quality of data Hand and Brown were able to get from a ground-based telescope. 鈥淚f you can do something this good on Europa from the ground, I鈥檓 really looking forward to seeing this technique applied to other targets.鈥�
But Hand isn鈥檛 satisfied with watching from afar: 鈥淚f we could just send a spacecraft out there to inspect the surface in much greater detail, we might be able to detect salts and maybe even organic compounds or possible indicators of life in Europa鈥檚 liquid water ocean.鈥�
Although NASA has no firm plans for a Europa explorer, the European Space Agency is due to launch its Jupiter Icy Moon Explorer, or Juice, in 2022. It will visit the icy moon and two of its neighbours, Callisto and Ganymede.
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