Geysers from the Tiger Stripes of Enceladus

Enceladus, the sixth-largest moon of Saturn, is a cold, icy world — but it’s also remarkably active. Recent studies have charted over a hundred geysers venting gas and dust into space from Enceladus’ south polar region. New research addresses the question of how the moon’s extreme surface terrain influences the locations and behavior of these geysers.

Active Plumes

Saturn's E ring

Enceladus orbiting within Saturn’s E ring. Enceladus’ plumes probably created this ring. [NASA/JPL/Space Science Institute]

A decade ago, scientists discovered that Enceladus’ south polar region is home to a prominent set of four fractures known as the “tiger stripes”. This region was found to contain roughly 100 geyser jets, which form plumes of gas and dust venting into space at a combined rate of ~200 kilograms per second! These plumes are probably the source of the material in Saturn’s E ring, in which Enceladus orbits.

Recently, Carolyn Porco (UC Berkeley and CICLOPS Space Science Institute) led a study that analyzed 6.5 years of Cassini data, surveying the locations and orientations of 101 geysers. The outcome was peculiar: the geysers are distributed along the tiger stripes, but their directions are not all pointing vertically from the surface (see the video below!).

Now, Paul Helfenstein (Cornell University) has teamed up with Porco to examine whether the surface terrain surrounding the geysers affects where the jets erupt, what direction they point, and even when they’re active.

Surface Influence

Helfenstein and Porco demonstrate that the locations and behavior of the geysers are very likely influenced by Enceladus’ surface features in this region. In particular, they find:

  1. The spacing of the geyser jets on Enceladus is not random.
    The jets are roughly uniformly distributed along the three most active tiger stripes, spaced about 5 kilometers apart. This fixed spacing might be due to shear fractures — produced by fault motion along the tiger stripes — cutting across the stripes at regular intervals and providing convenient outlets for the geysers.
  2. The orientation of the geysers also isn’t random.
    Instead, the directions of jets are correlated with directions of the local terrain — be it the tiger stripes, the cross-cutting fractures, or the fine-scale tectonic fabric.

The authors further theorize that the timing of the plume activity may also be influenced by the terrain. Plume activity is thought to result from tidal flexing of Enceladus in its struggle against the gravitational forces of Saturn. The authors propose that under these stresses, the tiger stripes and fractures cutting across them might open and close at different times. The combinations of these motions may play a significant role in determining when the plumes are most active.

Bonus

Check out this 3D model, based on Cassini observations, of the locations and directions of the ~100 geysers coming from the tiger stripes in Enceladus’s south polar terrain. [NASA/JPL-Caltech/Space Science Institute, Porco et al. 2014]

Citation

Paul Helfenstein and Carolyn C. Porco 2015 AJ 150 96. doi:10.1088/0004-6256/150/3/96

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