One of the most compelling pieces of evidence for Europa having an ocean was obtained by NASA’s Galileo spacecraft (Figure 1), which explored the Jupiter system from 1995 to 2003.

Figure 1 Photograph of the Galileo spacecraft during construction at NASA's Jet Propulsion Laboratory, taken 5 years before launch.Show descriptionA photograph of the Galileo spacecraft on a raised platform inside a large room. The main body of the spacecraft is to the left of the image, and the main antenna (which is shaped like an upside-down umbrella) is found at the top of the spacecraft. A long rod extends from the body of the spacecraft. There are six people dressed in white coveralls and white hats working on the spacecraft.

Galileo’s evidence

A scientific instrument onboard Galileo – the magnetometer – recorded the magnetic field in the vicinity of Europa as it performed its multiple flybys of the ice-covered moon. By reconstructing this data, scientists found that Jupiter’s magnetic field lines were disturbed around Europa, suggesting Europa itself was producing a magnetic field. However, unlike Earth’s magnetic field, which is generated by a molten iron core, Europa’s magnetic field is likely to be generated in an electrically conductive layer 200 km below the moon’s surface. Using computer modelling, this layer has been proposed to be salty liquid water ocean underneath the outer ice shell.

Further, Galileo also observed a relatively young and crater-free surface, possibly reflecting the exchange of material between the ice shell and an ocean below.  It also detected salts on the surface, which could arise from an ocean interacting with rocks at a seafloor, and arc-like surface fractures that appear to require a subsurface ocean for their formation.  Scientists using the Hubble Space Telescope have also reported cryovolcanic plumes – jets of material being shot out of Europa’s surface and into space.  The material in these plumes may have come from an ocean. 

It is important to remember, however, that these observations are all only indirect evidence for a subsurface ocean – there is not yet any direct proof that one exists.

What might the ocean be like?

Data collected by the Galileo and Juno missions have helped determine the internal structure of Europa (Figure 2). Models have suggested that Europa has a metallic core surrounded by a rocky layer, like the Earth.  This, plus an understanding of the moon’s physical properties, such as density and radius, have allowed scientists to constrain the depth of Europa’s ocean. Current estimates suggest the water-ice layer is ~100 km deep, which is nine times deeper than the deepest part of the Earth’s ocean, the Challenger Deep in the Mariana Trench.

Figure 2 Artist’s drawing of the internal structure of Europa.Show descriptionTwo illustrations of Europa’s interior, side by side. On the left-hand side is Europa drawn as a globe. A wedge has been removed from the globe to show the moon’s core at the centre, labelled ‘metallic core’. The outer surface of the globe is labelled ‘H₂O layer’. The space between the core and the water layer is labelled ‘rocky interior’. Two lines are drawn from the intersection of the rock interior and H₂O layer directing us to the right-hand side drawing. This shows a 3D wedge shape that widens towards the top of the image. At the bottom of the wedge is a brown coloured layer. In the middle of the wedge is a blue-coloured layer labelled ‘liquid ocean under ice’. At the top of the wedge is a white-grey layer with an irregular bottom border and a smooth top. There are cracks across the smooth top, and some of these cut down into this layer.

Observations of Europa’s icy crust have also provided clues as to what the subsurface ocean is made of. The icy crust is made of water ice with some carbon dioxide ice, but salts such as magnesium sulfate, magnesium chloride, sodium carbonate and sodium chloride (common salt) have also been detected. Assuming these salts and the ice crust formed by freezing of the underlying ocean, this would suggest that magnesium, sulfur, sodium, carbon and chlorine are all present in Europa’s ocean. However, it is unclear from the ice crust alone how much of these elements there might be, as they could be influenced by processes occurring in the icy crust, or if there are any other elements, ions or molecules present in the ocean.

The presence of these salts also indicates that the ocean is interacting with the rocky seafloor, like on Earth. This causes the rocks and minerals to break down (a process known as dissolution), liberating ions into the ocean (Figure 3) where they can form new minerals and become incorporated into the ice. Knowing this process has occurred (and is possibly still occurring) on Europa means scientists can find out more about the ocean’s likely composition by mimicking these water-rock interactions using computer modelling or laboratory simulation experiments. For example, researchers in AstrobiologyOU have used geochemical modelling to investigate what might happen when rocks of a similar composition to those on the europan seafloor react with fluids from different sources, such as cometary ice and water extracted from hydrated minerals, to assess what elements could be released into the ocean.

Figure 3 An illustration of water-rock interactions occurring at the ocean-seafloor interface of Europa. Credit: Lewis Sym.Show descriptionThere are two illustrations side by side. The left image is a wedge shape with four layers, the first grey layer at the bottom of the image is labelled ‘metal core’. This is overlain by a brown layer labelled ‘rocky mantle’, which is overlain by a blue layer labelled ‘subsurface ocean’. The final top light grey layer is labelled ‘ice shell’. There is a small box that covers the region where the rocky mantle and subsurface ocean layers join. This box expands to the square image on the right. At the bottom of this expanded square there is a brown layer with an uneven surface labelled ‘seafloor rocks’. Directly above this there is a light yellow-brown layer with an uneven surface, and the remainder of the square is blue and labelled ‘ocean’. Dark blue arrows extend from the ocean into the brown layer labelled seafloor rocks and the yellow-brown layer above. Yellow arrows extend from the yellow-brown layer into the ocean. The arrows are labelled magnesium, sodium, or chlorine. In the top left of the square is a key, which shows the yellow-brown colour corresponds with areas of dissolution on the image, the blue arrows signifies interactions between the ocean and the seafloor, and the yellow arrow signifies release of elements.

Why is the ocean important?

Europa’s subsurface ocean may be significantly deeper than the Earth’s oceans, but it may harbour environments that are common in terrestrial oceans. The Earth’s seafloor is rich with life that ranges from microscopic organisms to larger animals, such as fish and coral. Although we are not anticipating fish and coral living in Europa’s ocean, it is possible that it could sustain microbial life. In AstrobiologyOU we combine multiple scientific disciplines, including geochemistry and microbiology, to help determine whether microbial life can be supported in the europan ocean and what signatures it might leave behind that could be detected by future missions.