Hydrothermal plumes in a deep, unstratified ocean
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
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.