Ocean--the Case Strengthens
When a conductor is placed in a time-varying magnetic field, electrical currents are induced. These currents in turn create magnetic fields that can be detected. It is these physical principles that underlie the operation of metal detectors at airports and that give the magnetometer on the Galileo spacecraft the ability to "see" electrical conductors inside the moons of Jupiter. On page 1340 of this issue, Kivelson et al. present overwhelming evidence for a conducting layer near the surface of one of these moons, Europa. The most likely explanation is that Europa has a salty, global water ocean beneath its ice shell.
Europa is similar in size to Earth's moon and is thought to be mostly Earthlike in composition, but with a layer of some 100 km or so of water on top, which may be either liquid or solid. The very cold surface ice is extensively cracked and deformed, a testament to the flexing by tides as Europa follows its forced eccentric orbit about Jupiter. The possibility of water beneath this ice, perhaps as little as 10 km below the surface, has excited those interested in extraterrestrial environments for life and established a major role for Europa in NASA's plans for outer solar system missions.
Theoretical, geological, and spectroscopic arguments have all been used to support the presence of an ocean beneath Europa's icy shell, but none of these arguments are compelling (see the table). In contrast, the magnetic field evidence is remarkably strong. To appreciate this, consider the behavior expected for a sufficiently conducting shell close to Europa's surface. In this "high conductivity" limit (reached for a conductivity many orders of magnitude lower than those for a metal), a simple induction model predicts that the time-dependent field created by the induced currents almost exactly cancels the component of the time-dependent external field perpendicular to Europa's surface. The largest contribution to the latter comes from Jupiter's dipole tilt, which causes the field direction at Europa's location to oscillate with an amplitude of roughly 20° and a period of about 10 hours. The predicted strength and direction of Europa's induced magnetic dipole depend on the instantaneous position of Europa in the reference frame defined by Jupiter's rotating magnetic field .
The observational data match the model well. This is all the more remarkable considering that the model has no adjustable parameters. Moreover, these most recent data show that the field arising from currents or magnetization within Europa changes direction by 180° for spacecraft encounters 180° apart in longitude, as the induction model requires. This behavior is very different from that expected for a fixed field [such as the core dynamo that appears to dominate at Ganymede]. The model does not represent the data perfectly: Europa is deeply immersed in the jovian magnetosphere and in the plasma contained within that field, and understandable (though quite complex) field disturbances arise from this interaction of Europa with this environment.
EVIDENCE FOR EUROPA'S OCEAN
1.Theoretical study of tidal deformation and heating
2.Observations of surface deformation, especially "chaotic" regions, rafting, cycloidal ridges,possible low-viscosity surface flows
3.Near-infrared spectroscopy suggesting salt deposits on surface
4.Magnetic field evidence for an induction response
5.Altimetry and gravity field with sufficient resolution to determine tidal variation
1.Predicts that an ocean will persist once formed
2.Suggests thin ice and highly mobilized ice, consistent with an underlying ocean
3.Salt could arise from sublimation of a salty water "eruption"
4.Requires a near surface,global conducting layer,most readily explained by a salty ocean
5.Clear determination of whether there is an ocean; information on ice thickness
1.Rheology of ice is poorly known, especially at tidal frequencies, so predictions are uncertain
2.Might be explained by thin,cold, brittle ice "floating" on thick,warm, soft, easily deformed ice
3.Even if water is implicated,it need not come from an ocean--there may be melting within the ice
4.Is there any other possible conducting layer?
5.Requires Europa orbiter
To a good approximation, the induction response depends on the product of conducting layer thickness and conductivity. However, the layer must be a nearly complete spherical shell; the data cannot be explained by a patchwork of highly conducting regions with much less than global coverage. A global layer of water with a composition similar to Earth seawater and a thickness greater than about 10 km could explain the data. The dominant source of ions in Europa's ocean may be different from those in Earth's oceans , but they should satisfy the conductivity requirement. A much thicker layer of water ice, even if it is heavily contaminated with frozen brine, cannot explain the data because the ions are relatively immobile compared with those in liquid water. Any plausible ocean flow (fluid currents relative to the ice shell) is unimportant because it will have a much lower velocity than the rotational motion of Europa's surface. A partially melted ice layer could match the required conductivity but is physically implausible because the melt would have to be interconnected over large distances, which would result in the melt percolating through and separating from the ice driven by the density difference of ice and water. Kivelson et al. argue that Europa's tenous, external ionosphere also cannot provide the required conducting layer. The induced field declines as the inverse cube of the radius from the surface of the conducting layer, and any deep-seated conducting layer (such as a metallic core or a magma ocean in the rocky core) would therefore lead to a much lower field than is observed.
Some more exotic possibilities cannot be excluded (such as graphite or some other relatively high conductivity material, plausibly carbon-rich, intermingled within the ice but interconnected at the grain size scale), but a water layer is the most plausible explanation. A compelling demonstration of its existence or absence may be reached from gravity and altimetry data in the proposed Europa orbiter. The predicted diurnal tidal amplitude is over an order of magnitude larger for a Europa with a global ocean than for a Europa without one. More complex, intermediate scenarios can be envisaged (such as ice "grounding" on the underlying rocky topography in some places and not others). But the orbiter results will likely settle the fascinating question of whether Europa has an ocean. Defined broadly enough, oceans may not be that rare, but Europa's case may be special because the tidal heating may allow liquid water to get closer to the surface, possibly including occasional eruptions or flows. After Mars, it remains the most attractive extraterrestrial environment within our solar system in which to seek evidence of past or present life.
*The author is in the Division of Geology and Planetary Science, California Institute of Technology
Copied From[Science Aug. 25th 2000]