Planetary Oceanography: Europa
|Europa, the second major moon of Jupiter. The surface of the moon is composed of water ice, but Galileo spacecraft data indicate the presence of a liquid water "ocean" beneath the ice crust. (NASA/JPL, Galileo Imaging Team)|
|The Conamara Chaos region of Europa. What has caused the pits, domes, spots, cracks, and shattered blocks and plates in this picture?] (NASA/JPL, Galileo Imaging Team)|
|Many scientists believe that upwelling plumes of warm water beneath the ice partially or completely melted it, causing the chaotic terrain seen in the photo above.
|Numerical simulation of a buoyant hydrothermal plume within Europa's ocean. Purple cones indicate direction of fluid flow. Note how Coriolis forces constrain the plume into a thin column, and lead to intense counter-rotating vortices at the top and bottom of the plume.
Geoffrey Collins (Wheaton College)
One of the most exciting discoveries of the Galileo spacecraft mission to Jupiter is
that some of Jupiter?s large ice-covered moons, most notably Europa, appear to
possess thick liquid water layers beneath the icy crust (Showman and Malhotra,
1999). Thus, the Earth may not be the only solar system body which possesses
oceans, and we may contemplate studies in ?planetary oceanography?.
One must be careful in approaching this topic, due to the extreme lack of hard data on these oceans. My approach has been to look at the governing physical processes and most basic sources of energy to drive flow in an ice-covered ocean, and generate rough but robust predictions for how that ocean ought to interact with the ice layer above it.
I have been working with planetary geologist Geoffrey Collins (Goodman et al, 2003; Goodman and Collins, 2006a) to test geologists? proposed explanations for geological features seen on the ice crust (Greenberg et al, 1999), to see whether the required flows of mass and heat in the liquid ocean are consistent with basic geophysical fluid dynamics.
For instance, a substantial group of planetary geologists, including Greenberg et al (1999), argue that the small pits, domes and spots seen in detailed views of Europa?s surface result from intense melting by warm, buoyant hydrothermal plumes upwelling from localized vents at the sea floor. If this is so, the fluid dynamics of the liquid layer strongly affects the shape and size of these features. Using a combination of simple laboratory tank experiments, large-scale numerical modeling, and simple thermodynamic models of the ice crust, we have compared fundamental fluid dynamical limits on the scale of the buoyant plume (such as the Rossby radius of deformation) to the observed pits, domes, and spots.
We are only beginning to ask the most interesting questions about these exotic
oceans, trying to probe their composition, equations of state, and general circulation. Despite the frustrations caused by a lack of good data, I?m excited to pursue pioneering research investigating a newly-discovered ocean, a ?Mare Incognitum?.
The following videos show the ascent of warm, buoyant water from a heat source at the base of Europa's ocean, as modeled by the MITGCM ocean model software on a 16-node Linux computer cluster. The plumes mix turbulently with their surroundings, but Coriolis forces constrain the plumes, forming a narrow cylinder which eventually breaks up into swirling eddies.
Note that the simulation is flipped upside-down, so the plume appears to be descending rather than ascending.
Hydrothermal Plume Simulation, Part 1
Hydrothermal Plume Simulation, Part 2
The videos are in Quicktime format.
Showman, A. P. and R. Malhotra, 1999. The Galileian Satellites. Science 284, 77-84.
Goodman, J. C., G. C. Collins, J. Marshall, and R. T. Pierrehumbert, 2004.
Hydrothermal plume dynamics on Europa: Implications for Chaos Formation.
Journal of Geophysical Research ? Planets 109:E03008, doi:
Goodman, J. C. and G. C. Collins, 2006a. Flow of Warm Ice in a Melt-Through Model of Europa?s Ice Shell. In preparation.
Greenberg, R., G. V. Hoppa, B. R. Tufts, P. Geissler, and J. Riley 1999. Chaos on
Europa. Icarus 141, 263?286.