Subsurface ocean on Europa could be habitable for life

Laurel Kornfeld

Jupiter’s moon Europa has long been thought to harbor a subsurface ocean of liquid water. Now, a study conducted by a group of scientists at NASA’s Jet Propulsion Laboratory (JPL) suggests that ocean may have a balance of hydrogen and oxygen similar to that found in oceans on Earth.

The scientists, who published their findings in this week’s issue of the journal Geophysical Research Letters, compared the processes on Europa that could produce the proportion of hydrogen and oxygen necessary for life with those occurring in Earth’s oceans.

They found Europa has the potential to produce a balance of hydrogen and oxygen similar to that found on Earth, with both worlds producing approximately ten times more oxygen than hydrogen.

Volcanic activity has long been thought necessary to make the Jovian moon’s underground ocean habitable. Without such activity, oxidants from Europa’s surface, created when radiation from Jupiter breaks up molecules of water ice, would acidify the ocean to the point that it would be toxic to any life.

The icy world’s neighboring moon Io is the most volcanically active object in the Solar System due to it being squeezed and stretched by Jupiter’s powerful gravitational pull, which generates heat. Scientists are still uncertain as to whether volcanic activity also occurs on Europa.

However, they now think the correct balance of elements needed for life may not require volcanic activity at all. This is because Europa’s rocky interior is now believed to be far more complex than previously thought as well as surprisingly Earth-like.

Hydrogen could be produced by the interaction of Europa’s salty ocean water with rock. Through a process known as serpentinization, water flows into spaces between mineral grains and interacts with the rock, resulting in the creation of different minerals and a release of hydrogen.

As Europa’s interior cools, more cracks open up in its ocean floor, exposing more rock to ocean water, leading to increased hydrogen production.

While fractures in the crust of Earth’s oceans extend downward between three and four miles, those in Europa may extend as much as 15 miles (25 kilometers), meaning hydrogen-producing water-rock interactions might be occurring deep into its ocean floor.

Oxidants and other compounds capable of reacting with Europa’s hydrogen are produced when radiation from Jupiter breaks up water ice molecules on the moon’s icy surface. They are then cycled into the moon’s interior and subsequently brought to its ocean.

“The oxidants from the ice are like the positive terminal of a battery, and the chemicals from the seafloor, called reductants, are like the negative terminal,” said study co-author Kevin Hand, a planetary scientist at JPL. “Whether or not life and biological processes complete the circuit is part of what motivates our exploration of Europa.”

Planetary scientist Steve Vance, lead author of the study, said cold rock easily fractures, meaning serpentinization could be releasing large amounts of hydrogen that would balance out the oxidants in a rate comparable to Earth’s oceans.

Europa may have volcanic activity and hydrothermal vents from which hot water filled with minerals erupts onto the surface.

The researchers are also studying the cycling of other elements in the large moon’s underground ocean, such as carbon, nitrogen, phosphorus, and sulfur.

“We’re studying an alien ocean using methods developed to understand the movement of energy and nutrients in Earth’s own systems,” Vance said. “The cycling of oxygen and hydrogen in Europa’s ocean will be a major driver for Europa’s ocean chemistry and any life there, just as it is on Earth.”

NASA is planning a detailed mission to Europa during the 2020s involving a spacecraft that will orbit Jupiter and conduct numerous flybys of the large moon.

The proposal calls for cameras and spectrometers to take high-resolution images of Europa that will be used to determine its composition and map its surface.

An ice-penetrating radar will be used to search for subsurface water and measure the thickness of Europa’s icy shell while a thermal instrument will look for evidence of warm water having recently erupted on the surface. Gravity instruments will also be used to confirm if an underground ocean is present.

Video courtesy of JPL