Bonnie J. Buratti
Phew! We made it through the deepest swoop yet down into the plume of Enceladus, the encounter we call “E7″ because it’s the seventh targeted flyby of Enceladus.
But now we have our work cut out for the next few weeks as we pore over the data, painstakingly analyzing every signal to understand the composition of the plume and its structure.
So far, we know the Visual and Infrared Mapping Spectrometer (VIMS) was able to get images and data in a variety of wavelengths of light and saw that the plume extends out to at least 1,000 kilometers (600 miles).
We also have striking images of the moon crowned by its glorious plume, which Cassini captured right before its plunge. The images illustrate well that the spectacular plume spewing from the south polar region is composed of many much smaller jets.
The images and VIMS data both show that as the moon becomes less and less illuminated by the sun (similar to when our moon approaches the phase known as “new moon”), the plume gets much brighter. These data will be valuable for understanding the detailed structure of the plume and where it connects to the surface.
We have also learned that the density of the plume appears to be less than half of that predicted. Still, the heart of the plume measured on this flyby was about three times denser than the sparser parts of the plume we flew through previously.
There is more good news. We will be able to do the Enceladus flyby on April 28, 2010, on the spacecraft’s reaction wheels. This means we will be able to perform the Radio Science Subsystem experiment with Cassini’s main antenna to understand the interior of Enceladus under the hot south polar region.
During this experiment, antennas from the Deep Space Network (DSN) on Earth will be tracking the spacecraft to see how much Enceladus tugs on it. By measuring this tug, scientists will be able to answer such questions as: How much is the shape of the moon deformed by tidal forces from Saturn? Is there an unusually dense mass under the south pole? (The higher the mass, the larger the tug?)
We know that heating by tidal forces is what drives the plumes, but we’re not sure exactly how. In addition to a possible liquid subsurface ocean, Enceladus may be harboring a dense mass underneath its surface that helped to start and maintain the moon’s current activity.
Just wanted to share our excitement about the reams of data we’re combing through. Now, back to work!