The text below is the caption to the graphics shown here, taken from a new Phys.org report. It gives some interesting insights into the physics of moons and sets some new puzzles for theorists.
This set of images from NASA’s Cassini mission shows how the gravitational pull of Saturn affects the amount of spray coming from jets at the active moon Enceladus. Enceladus has the most spray when it is farthest away from Saturn in its orbit (inset image on the left) and the least spray when it is closest to Saturn (inset image on the right).
Water ice and organic particles gush out of fissures known as “tiger stripes” at Enceladus’ south pole. Scientists think the fissures are squeezed shut when the moon is feeling the greatest force of Saturn’s gravity. They theorize the reduction of that gravity allows the fissures to open and release the spray. Enceladus’ orbit is slightly closer to Saturn on one side than the other.
A simplified version of that orbit is shown as a white oval. Scientists correlate the brightness of the Enceladus plume to the amount of solid material being ejected because the fine grains of water ice in the plume are very bright when lit from behind. Between the dimmest and brightest images, they detected a change of about three to four times in brightness, approximately the same as moving from a dim hallway to a brightly lit office.
This analysis is the first clear finding that shows the jets at Enceladus vary in a predictable manner. The background image is a mosaic made from data obtained by Cassini’s imaging science subsystem in 2006. The inset image on the left was obtained on Oct. 1, 2011. The inset image on the right was obtained on Jan. 30, 2011. Credit: NASA/JPL-Caltech/University of Arizona/Cornell/SSI
Phys.org report: Enceladus jets: Surprises in starlight
Just to repeat: the above is only the caption to the graphics, none of the actual linked report has been quoted.