In the face of all this uncertainty, it would be difficult to say anything more unequivocal than that a liquid ocean at Europa is possible. However, a new line of evidence from an unexpected source, results from Galileo’s study of magnetic fields, has suggested that the "possible" should be replaced with "probable" – we now believe that it is likely that Europa has a liquid ocean beneath its icy surface. How is this possible?

Europa has no magnetic field of its own, but Jupiter has a very strong magnetic field. The Jovian magnetosphere extends out to about 10 Jupiter radii (R_J), between the orbits of Europa (9.4 R_J) and Ganymede (15 R_J). So the satellites Io and Europa actually orbit Jupiter within its magnetic field. However, Jupiter’s magnetic field is not symmetric around the center of Jupiter–it is tilted by almost ten degrees with respect to Jupiter’s spin axis, and is also offset by about 1 R J from the center of Jupiter. Because of this offset as Jupiter rotates, Europa experiences a time-varying magnetic field over a period of 11.23 hours. The field also varies as Europa orbits around Jupiter, but Jupiter’s rotational period (and thus the rotation of its magnetic field) is a much stronger effect.

We know from the laws of electromagnetism that a time-varying magnetic field will induce an electric field. This electric field then causes a current to flow within the interior of Europa, with a direction that changes on a timescale of half the rotational period. This current loop then creates a secondary magnetic field with a direction that’s approximately opposite to the primary magnetic field from Jupiter. This is called an induced magnetic field.

Data from Galileo's magnetometer (an instrument which measures the strength and direction of magnetic fields) showed that Europa has an induced magnetic field that varies in direction and strength in response to Europa's position within Jupiter's strong magnetic field. The periodic variation in direction shows that the field is not due to a permanent internal dipole, meaning that the field is not created in the interior of Europa (unlike the Earth's magnetic field).

The strength and response of the induced field at Europa can tell us about subsurface structure. The results measured by the Galileo magnetometer require a near-surface, global conducting layer. The most likely layer that meets these requirements is a global layer of salty water, with a salt content of no less than ~0.02 times the salinity of Earth’s oceans. The magnetometer results allow a range of solutions with different values for the conductivity of the ocean, the depth below the surface at which it is located, and the ocean layer’s thickness. For example, if we assume a Europan ocean with a conductivity equal to that of the terrestrial oceans, then such a layer would have to be at least several kilometers thick and located no farther than 200 km below the surface of Europa.

The magnetic field data from the Galileo spacecraft put a substantial set of constraints on Europa’s subsurface structure. For example, the data for Europa cannot be explained by localized pockets of salty water, and instead require a nearly complete spherical shell of liquid water. A frozen ice layer, even if it had pockets of briny water, could not produce the observed response because ions in solid ice would be insufficiently mobile.

It is possible that a type of conducting layer other than a global salty ocean could account for the induced magnetic field, but the salty ocean explanation appears the most plausible. In particular, the strength of the observed induced field is not consistent with currents induced in a metallic core; the induced dipole field strength falls off with the cube of the distance and the core is simply too far away to provide the observed field. The data are also not consistent with a field induced in Europa's ionosphere; the ionosphere is too tenuous to support the electrical currents needed to explain the strength of the field. A subsurface layer of a different conducting material, instead of salty water, is possible, but such layers (such as graphite) would be implausible given what we know about Europa’s composition and formation.

Intriguingly, the Galileo magnetometer data suggest that Callisto and Ganymede may also harbor subsurface oceans. These latter oceans likely exist in a layer sandwiched between two phases of water ice, so they would not provide the astrobiologically more interesting rock/water interface (with possibilities for hydrothermal vents) that may be present at the bottom of Europa’s ocean.

The magnetic field results for Europa, therefore, currently provide our best indirect evidence for the presence of liquid water beneath Europa’s icy surface. These results, while intriguing, will not be confirmed until a direct detection of Europa’s ocean is made. Such measurements will require dedicated instruments on a spacecraft that is orbiting Europa. A future article will discuss these measurements (including altimetry, high-resolution gravity, and radar sounding) and the current plans to fly a spacecraft to Europa with instruments to make these and other measurements.