Geophysical Fluid Dynamics Laboratory

The surface of Europa, Jupiter's second major moon, consists of a thick shell of water ice. But several lines of evidence suggest that a 100-km-thick layer of liquid water exists beneath the ice crust. This layer is maintained by tidal and radiogenic heating generated within the moon's rocky interior. Planetary geologists are interested in the ocean's role in reshaping Europa's ice crust, and planetary biologists speculate that it may be the most likely place to find extraterrestrial life. Both groups focus their attention on hydrothermal plumes which should rise into the ocean from hot spots on the sea floor, but very little is known about the behavior of plumes in such a deep, unstratified ocean.

We are developing scaling analyses to describe the shape and structure of hydrothermal plumes in a deep, rotating, unstratified medium. Apart from its relevance to Europa, this is an interesting geophysical fluid dynamics problem which has not yet been tackled. We are testing our theoretical predictions using a series of laboratory tank experiments. Figure 1 shows the experimental setup. A tank mounted on a rotating turntable is filled with fresh water. An injector at the top of the tank releases a steady flow of denser, salty water, which also contains a fluorescent dye. The salty fluid descends into the tank, mixing turbulently with its surroundings. The descending salty plume behaves identically to a warm rising plume, just flipped upside-down.

The plume is visualized using planar laser-induced fluorometry (PLIF). A green diode laser is reflected off a pair of rapidly-rotating mirrors, an arrangement similar to a supermarket checkout scanner. The rotating mirrors create two perpendicular sheets of laser light, which illuminate the interior of the tank. The fluorescent dye glows yellow-green where it is illuminated by the laser, so that two cross-sections through the plume are visualized: a plan view near the bottom of the tank and an elevation section through the plume's center. Video cameras record the cross-sections.

PLIF allows visualization of a single thin section through the fluid, uncluttered by dye in the foreground or background. When the recorded images are properly calibrated, dye concentration can be estimated, allowing us to measure the dilution of the source fluid as it is mixed in the plume.

Figures 2 and 3 show snapshots from the video cameras during a typical experiment. Figure 2 is a side view early in plume development: the plume is cone-shaped near its source at the top, but as the fluid descends its outward expansion ceases once Coriolis forces come into play. Figure 3 shows an overhead view much later in time. Here, the plume has become unstable and begun to shed baroclinic eddies which travel away from the plume source. The white lines mark the location of a metal bar which supports the injector and obstructs the view.

Analysis is currently underway to compare the behavior of laboratory plumes to the theoretical predictions.