Supply chain from Enceladus to Saturn's E ring observed
Enceladus is the sixth-largest of Saturn's moons and orbits along the E ring, the broadest and outermost of the planet's main rings. The tiny moon is geologically active, and over a hundred geysers have been identified on Enceladus bursting out of surface fractures known as "tiger stripes" in the south polar region. These jets of water ice and vapour particles, which originate from a sub-surface ocean are a major supplier of the E ring.
Now Cassini has been able to image the supply chain, observing the streams of these icy particles stretching tens of thousands of kilometers from the moon's geysers all the way to the E ring.
A new study led by Colin Mitchell, a Cassini imaging team associate at the Space Science Institute, describes these connecting streams as "tendrils" and has used computer simulations to trace the trajectories of the ice grains ejected from individual geysers.
This collage of Cassini spacecraft images and computer simulations shows how long, sinuous features from Enceladus can be modelled by tracing the trajectories of tiny, icy grains ejected from the moon's south polar geysers. Image credit: NASA/JPL-Caltech/SSI
"We've been able to show that each unique tendril structure can be reproduced by particular sets of geysers on the moon's surface," stated Mitchell.
The team demonstrated that different tendril structures correspond to different sizes of geyser particles. Cassini images taken at different times and from different positions around Saturn, also show that the detailed appearance of the tendrils changes over time.
"It became clear to us that some features disappeared from one image to the next," stated John Weiss, an imaging team associate at Saint Martin's University, and another author of a paper reporting the study.
These changes in the tendrils' appearance likely result from the cycle of tidal stresses on Enceladus as it orbits Saturn. The small moon is squeezed and stretched by gravitational forces, varying the widths of the fractures from which the geysers erupt. The wider the fracture, the greater the eruption of material.
"As the supply lanes for Saturn's E ring, the tendrils give us a way to ascertain how much mass is leaving Enceladus and making its way into Saturn orbit," said Professor Carolyn Porco, Cassini imaging team leader and a co-author on the paper. "So, another important step is to determine how much mass is involved, and thus estimate how much longer the moon's sub-surface ocean may last."
Professor Porco told Sen, "The oceans are not expected to dry up. But even if they did, we believe that Enceladus is in a cycle such that, far in the future, another large ocean could be regenerated. That's what we think, anyway. Hopefully, future study of the moon's tendrils will help us figure this out."
The Cassini probe launched in October 1997. Arriving at Saturn in 2004, the spacecraft completed its initial four-year mission in June 2008. The mission has subsequently been extended and has funding through to September 2017.
The mission has provided strong evidence that Enceladus, with a subsurface ocean, an internal energy source and simple organic compounds, is potentially one of the most habitable environments in our Solar System for microbial life, making it a major target of investigation. Many observations, including imaging of the plume and tendril features, are planned during the final couple of years of the Cassini mission.
"There will be three more very close flybys of Enceladus before Cassini goes off the air forever. They happen at the end of this year. Cassini will fly through the plume during those flybys and the instruments that can capture geyser material and analyze it will do just that. Other instruments will determine temperature of the surface in between the tiger stripes, so we'll finally be able to estimate how much energy is being generated by tidal flexing." Professor Porco told Sen.
"Enceladus' activity has been such a mind-blowing find, that there is a small group of us designing a mission to go back and do a proper job of checking for biological activity. Fingers crossed that we soon get the chance to do that," Professor Porco concluded.