Jupiter's moon Europe is an intriguing world. It's the most smooth body in the solar system, and the sixth-largest moon in the solar system, though it's the smallest of the four Galilean moons. Most intriguing of all is Europe's subsurface ocean and the potential for habitability.
The scientific consensus is that Europe has a subsurface ocean underneath its exceptionally smooth, icy surface. The crust is 10-30 km (6-19 mi) thick, and the ocean below it could be about 100 km (60 mi) deep. If true, then the volume of Europe's ocean is about two or three times the volume of Earth's oceans.
The interior of Europe is kept warm by tidal heating, and possibly by radioactive decay of elements in its rocky mantle. But studies do show that radioactive decay alone is not enough to produce the heat in Europe. Whatever the exact source of heat is, it's enough to create the subsurface ocean.
It's likely to be a salt-water ocean, which is important for habitability. Initially, scientists thought that the saltiness came from magnesium chloride, which is basically Epsom salts. But it does not matter magnesium chloride, but rather sodium chloride, the same type of salt that makes Earth's oceans salty.
The new study is called "Sodium Chloride on the Surface of Europe" and is published in the June 12th issue of Science Advances. The authors are Samantha Trumbo, Michael Brown, and Kevin Hand. Trumbo is the lead author of the paper.
Hubble observations of the surface of Europe. There are yellowish areas on the surface of the moon.
Europa's surface is a geologically young icy shell. So anything that's on the surface is likely from the ocean below. That, and the Cracks and Fractures in the Icy Shell, is what scientists are looking for at Ocean Under There. An ocean rich in sulfate salts.
But new spectral data from the Keck Observatory suggested that the salts on the surface were not magnesium sulphates. Absorption lines indicating the presence of magnesium sulphates were absent in the Keck data. Those types of salts have very distinctive absorption lines and they were just not there. Scientists thought that they could not be made known on the surface, but the problem is that it does not make its presence known in infrared.
"We thought that we might smell chlorine, but they are essentially featureless in an anomalous spectrum," says Mike Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy at Caltech and co-author of the Science Advances paper.
But a colleague of Brown's, and eventual co-author of the new paper, had an insight into the problem.
His name is Kevin Hand, of JPL. He had irradiated ocean salts in a laboratory, under Europe-like conditions. He found that after irradiation. The color it changed into? You guessed it: yellow. Just like in the yellow region on Europe's surface, named Tara Regio.
"Sodium chloride is a bit like invisible ink on Europa's surface. "Before irradiation, you can not tell it's there, but after irradiation, the color jumps right out at you," says Hand, scientist at JPL and co-author of the Science Advances paper.
No one has taken this view of the spectrum before. The Galileo spacecraft did not have a visible spectrometer. It just had a near-infrared spectrometer, "says Caltech graduate student Samantha Trumbo, the lead author of the paper.
The Hubble Space Telescope to advance the idea. They pointed the Hubble at Europe and found an absorption line in the visible spectrum that perfectly matched the irradiated salt. This confirmed the presence of irradiated sodium chloride on Europe. And the probable source for that is the subsurface ocean.
"We've had the capacity to do this analysis with the Hubble Space Telescope for the past 20 years," Brown says.
This is evidence in support of a subsurface ocean with sodium chloride like Earth's oceans. But it's not a slam dunk. It might be evidence of different materials in the icy crust.
In any case, the study presents more intrigue-surrounding Europe.
As the authors say at the end of their paper, "Regardless of whether or not the observed NaCl
If the salt in the ocean is magnesium sulphate, it could have leached into the ocean from the rocks on the ocean floor. But if it's sodium chloride, that's a different story.
"Magnesium sulfate would simply have leached into the ocean from rocks on the ocean floor, but sodium chloride may indicate that the ocean floor is hydrothermally active," Trumbo says. "That would mean Europe is a more geologically interesting planetary body than previously believed."
Fire up the rockets. Let's go and find out!