Central to the Enigma of Enceladus are the fantastic plumes of water vapor and ice that emanate from the "tiger stripe" fractures, or sulci, at the moon's south pole.
Interestingly, the plumes have yet to be seen by Cassini's Imaging Science Subsystem (ISS) cameras under anything but a fairly limited viewing geometry, specifically large angles between the spacecraft and Sun seen from Enceladus.
Observations at multiple viewing geometries allow planetary scientists to determine physical characteristics of plume particles.
The existence of the plumes was confirmed by Cassini's ISS in images obtained at high solar phase angles, or as they were viewed nearly backlit by the Sun. (The solar phase angle is the angle formed between Enceladus, the Sun, and Cassini.) Usually, Cassini cannot aim its cameras within 15 degrees of the Sun, which limits the phase angle at which the plumes (or anything else) can be viewed to 165 degrees or less. If the spacecraft is safely in eclipse behind Saturn, however, then higher phase angles are attainable. Cassini was in fact in eclipse behind Saturn, when it captured the spectacular 'Ghostly Fingers of Enceladus' image below at a phase angle of about 175 degrees.
NASA / JPL / SSI
Ghostly Fingers of Encleadus
Notice how much brighter the plume is, below the black disk that is Enceladus, than the vast E ring, the eventual repository for some fraction of plume particles. Both appear bright because they are efficient forward scatterers. Every eastbound morning (and westbound evening) commuter with a dirty windshield knows all too well how tiny dust particles scatter sunlight in the forward direction... right into their squinting eyes.
But what do the plumes look like at smaller phase angles? The smallest phase angle at which any plume images have been successfully acquired is about 140 degrees, as shown in the figure below.
Plume from Enceladus
ets emanate from Enceladus' south pole as the surface is illuminated by Saturnshine.
Cassini ISS has attempted to obtain plume images at smaller phase angles, but aiming its camera near a bright source (Enceladus is, afterall, the most reflective body in the Solar System!) produces numerous internal reflections rather than useful data. One such image appears below.
nternal reflections hide Enceladus' plumes from view at low phase angles.
So, how to observe Enceladus' plumes at phase angles smaller than 140 degrees? Could the current equinox season at Saturn provide any assistance? With equinox season, comes mutual event season, and the satellites will be eclipsing each other as equinox approaches in August. If the bright surface of Enceladus were to be in eclipse, darkened by the shadow of another saturnian moon, Cassini might have shot at getting a look at the plumes free from those frustrating internal reflections at a phase angle lower than 140 degrees. Of course, viewed from Enceladus, such an event would be a total solar eclipse!
Getting Some Help from Rhea
total solar eclipse by Rhea, viewed from Enceladus.
Such an eclipse doesn't last very long, however, so exposures would have to be well timed to capture the plumes in sunlight while Enceladus is in momentary darkness. With luck, it just might work.
Anne Verbiscer is a Research Associate Professor at the University of Virginia. She studies the surfaces of icy bodies in the outer Solar System and has been involved with the Cassini mission to Saturn since 2007. Currently a visitor at Southwest Research Institute, she has been enjoying the past year living in the mountains above Boulder, Colorado.