Europa's Hidden Ocean: New Data Suggests 29km Thick Ice Shell
NASA's Juno mission has provided new measurements suggesting Jupiter's moon Europa may have a solid ice shell approximately 29 kilometers thick, potentially impacting the exchange of materials between its subsurface ocean and the surface.
New data from NASA’s Juno spacecraft indicates that Jupiter's icy moon Europa may possess a solid ice shell roughly 29 kilometers (18 miles) thick. This measurement, derived from the spacecraft's Microwave Radiometer (MWR) instrument during a close flyby on September 29, 2022, offers critical insights into the moon's potential habitability by constraining the depth of its vast subsurface saltwater ocean.
Europa, a moon smaller than Earth's own, is believed to harbor a global ocean containing more than twice the water of all Earth's oceans combined. This immense volume of water, hidden beneath a shell of ice, has long fascinated scientists. The Juno mission's findings, published in Nature Astronomy, used 129 measurements across six microwave frequencies to estimate temperatures at various depths within Europa's ice. This allowed researchers, led by Steve Levin, to model the conductive ice shell's thickness.
The estimated 29-kilometer thickness is a central figure in a range of 29 plus or minus 10 kilometers. This means the shell could be as thin as 19 kilometers or as thick as 39 kilometers, depending on various factors and the specific region observed. For perspective, the official height of Mount Everest is approximately 8.85 kilometers; thus, three Everests stacked would fall short of the lower bound of the estimated ice thickness. This measurement is significant because it provides the first data capable of distinguishing between models proposing thin shells (less than a kilometer) and those suggesting much thicker shells extending for tens of kilometers.
Challenges in Measuring Europa's Icy Crust
While Juno was primarily designed to study Jupiter, its MWR instrument proved capable of gathering data on Europa's surface during a close encounter, passing within approximately 360 kilometers of the moon. The instrument detects microwave radiation emitted by ice, with lower frequencies penetrating deeper than higher ones. The researchers analyzed the brightness temperature across different frequencies and the impact of radio emissions from Jupiter's radiation belts and the galaxy reflecting off Europa's surface. A key finding came from the difference between 0.6-gigahertz and 1.2-gigahertz measurements, which helped constrain the vertical temperature gradient, a key indicator of shell thickness.
The model assumes a pure water ice composition and a rigid, conductive outer layer. However, the researchers acknowledge limitations. Dissolved salts, which are expected in Europa's ocean, could make the ice more opaque to microwaves, potentially reducing the estimated thickness by about 5 kilometers. Conversely, a warmer, convective layer beneath the rigid ice could increase the total thickness of the solid shell. Furthermore, the analysis used a laterally uniform model, despite visible geological differences across Europa's surface, such as ridged plains and chaotic terrain. The 29-kilometer figure represents the best fit for the observed region under idealized conditions, not a definitive global measurement.
The existence of Europa's subsurface ocean is supported by multiple lines of evidence, primarily from NASA's Galileo spacecraft. Galileo detected an induced magnetic field around Europa, strongly suggesting the presence of a deep, electrically conductive liquid layer, with salty water being the most likely candidate. Europa's surface, characterized by few impact craters and numerous ridges and bands, also points to a geologically active interior. Tidal forces, generated by Jupiter's gravitational pull and Europa's slightly eccentric orbit influenced by other Galilean moons, are thought to provide internal heat, preventing the moon from being frozen solid and potentially driving geological activity.
The potential habitability of Europa's ocean hinges not only on the presence of liquid water but also on the exchange of essential chemical elements and energy between the ocean, the moon's rocky mantle, and the irradiated surface. A thicker ice shell, like the one now suggested, presents a greater barrier to this exchange. Scientists are keen to understand if chemical compounds from the rocky seafloor, potentially driven by hydrothermal activity, can reach the ocean, and if oxidants produced by Jupiter's radiation breaking down surface ice can be transported to the ocean depths. The Juno data, while indicating a substantial ice shell, also detected shallow structures that could represent pores or cracks, offering potential, albeit challenging, pathways for this vital chemical transport.