The Thickness of Europa's Ice Shell from JEO

Okay, after I saw Emily practically dare me to talk about something other than Io with respect to these flagship missions, I thought fine, I will break my one rule here and talk about...Europa. Cue dramatic prairie dog! In particular, I want to look at perhaps one of the most important science goals for the Europa/Jupiter System Mission: determining the thickness of Europa's water-ice shell.

Following the Galileo mission, the Europa scientists were split into two armed camps: those that felt that the evidence found by Galileo indicated that Europa had a thick ice shell (read: the ocean did not "communicate" with the surface) and those that thought Europa had a much thinner ice shell (read: the ocean did "communicate" with the surface). Okay, they weren't literally armed, though I hear they had to put metal detectors at the entrances to the Europa sessions at LPSC. In the thin ice case, the ice shell would be on the order of a few kilometers thick (Greenberg et al. 2000), while in the thick ice case, the ice shell would be 10 to 30 kilometers thick (Pappalardo et al. 1999). Knowing the thickness of the ice shell is important for understanding Europa's habitability as well as designing a future submarine that might explore the ocean beneath. Obviously digging through two km of ice is quite a bit different from digging through 30. Knowing the thickness of the ice shell is also important for understanding Europa's surface geology. With a thin shell, chaotic terrain and double ridges can be explained by break-throughs of the crust by the underlying water ocean. With a thick shell, these features are better explained by a convecting ice layer producing diapirs, which imping on the surface.

While in orbit around Europa, the Jupiter Europa Orbiter would use four primary measurements to constrain the thickness of the ice and water shells: gravity and topography measurements, radar sounding, and magnetometer data. In a previous post, I reported on the types of science the Ice-Penetrating Radar can obtain at Io, but the instrument's primary purpose is identifying shallow water pockets and detecting the ice-water interface at Europa. The ability of the IPR to detect the ice-water interface can vary depending on a number of factors. For example, in the thick ice case, the ice shell is expected to be split into an upper, brittle cold ice layer and a lower, ductile warm ice layer. The warm ice layer has a much higher dielectric constant, and this would reduce the penetration depth of the radio signal IPR transmits. Heavy fracturing of the ice layer can also reduce the penetration depth. Using a tectonic models, the team reports that penetration depths of 15 km are expected. The study team thinks that even a non-detection of the ice-water interface with IPR can be useful as a lower bound. Magnetometer measurements can be used to constrain the thickness of the water ocean by measuring the strength of the induced magnetic field at Europa.

Another pair of measurements of the ice shell thickness are gravity and topography. This requires the use of the antenna for Doppler tracking and the Laser altimeter for altitude measurements. These would be used to derive Europa's Love numbers, h₂ and k₂. The Love number h₂ is dependent on the tidal deformation of Europa's surface and can be measured by calculating the difference between laser altimetry of the same point on the surface at different times of day. Combining the two Love numbers can constrain the thickness of the ice shell as these numbers are a function of the rigidity of the shell.

Based on the measurements acquired by the Jupiter Europa Orbiter, the thickness of Europa's water ice shell can be constrained and the thick and thin water ice shell debate should be settled. However, depending on the thickness of that shell, it maybe difficult to derive a specific value for its thickness. The Ice-Penetrating Radar may not be able to see the ice-water interface if the shell is thicker than 15-30 km, which would be expected from the thick ice shell interpretation. If the shell is more on the order of less than 10 km, the ice thickness should be pretty well determined.