Enceladus' Geysers in Relation to Surface Geological Features

September 28th, 2015

Saturn's active moon Enceladus is famous for its icy geysers that erupt across the satellite's south polar region. The geysers spew jets of water vapor and ice crystals upward through four parallel rifts, called “tiger stripes” and into a giant plume that feeds Saturn's diffuse E-Ring. In a paper published in the September 2015 issue of The Astronomical Journal, Cornell senior research associate Paul Helfenstein and Cassini Imaging Team lead, Carolyn Porco at the Space Science Institute in Boulder, Colorado, examine the surface locations of 100 of these geysers and the three-dimensional orientations of their associated jets in relation to geological details visible in Cassini high-resolution images.   

While many of the geysers erupt in near-vertical orientations, a significant fraction of them are tilted away from the zenith direction. The study tested if the azimuth orientations of tilted jets are influenced by their placement relative to surface geology and tectonic structures. To study the jets in geological context, the investigators created a gallery of high-resolution image mosaics that show the placement of all the jets in their source region. They found that the azimuth orientations of tilted jets are not randomly distributed; rather their azimuths correlate with the directions either of tiger stripes, fractures that cross-cut the tiger-stripes, or else fine-scale local tectonic fabrics.  One possible implication is that the presence of cross-cutting fractures on tiger stripes could affect the timing and duration of geyser eruptions, which are largely controlled by daily tidal interactions with Saturn. In addition, the highest-fidelity Cassini mosaics (resolutions of a few meters/pixel) reveal evidence that surface geological features surrounding geysers have been modified by sublimation-aided erosion, and ablation, and scouring by the jets.

 Among the most important findings is that discrete geysers emplaced along the three most active tiger stripe fractures (Damascus Sulcus , Baghdad Sulcus, and Cairo Sulcus) occur in local groupings with relatively uniform nearest-neighbor separation distances ( about 5 km). Their placement may be controlled by small, uniformly spaced diagonal cracks that are expected to cut across each tiger stripe from shearing motion along the faults. The observed spacing would imply that the icy shell in the vicinity of the tiger stripes has a thickness of only about 5 km over the water source.

Reference:  Helfenstein, P. & Porco, C.C. 2015. Enceladus’ geysers:  Relation to geological features. The Astronomical Journal, 150:96 (33pp),

Figure: The locations and azimuth angles of geyser jets along the Baghdad Sulcus tiger stripe fracture are shown superposed on a high-resolution Cassini image mosaic map. Each circle is centered on a geyser location, and the circle diameter shows the estimated uncertainty in measured position. Fully open circles identify geysers for which estimated azimuth uncertainties were too large to be used for detailed analysis. The nominal jet azimuth direction is identified by a white radial line extending outward from the circle. For geysers with accurately measured jet azimuths, a gray radial sector is marked inside of the circle along with two extended lines outside of it to show the azimuth angle measurement uncertainty.