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Planetary News: Cassini-Huygens (2005)

Cassini Photographs the Fountains of Enceladus

By Emily Lakdawalla
November 30, 2005

Cassini now has photographic proof of the geologic activity of Saturn's little moon Enceladus. Enceladus' south polar eruptions were discovered earlier this year on the basis of observations made with several of Cassini's instruments, including the magnetometer, UVIS and CIRS spectrometers, and dust detector. On November 27, the camera instrument participated in a campaign to photograph the tenuous plumes that should be erupting from such geysers, and succeeded in catching them in the act.

The camera took advantage of an alignment between the Sun, Enceladus, and the spacecraft such that the three were nearly in a straight line with the spacecraft behind Enceladus with respect to the Sun. From this point of view, tiny particles scatter sunlight forward to Cassini's cameras, revealing what appear to be multiple plumes erupting from Enceladus' south pole.

Fountains of Enceladus
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Fountains of Enceladus
On November 27, 2005, Cassini captured a series of images of Enceladus from its night side. The back-lit view lights up fine particles streaming from Enceladus' south-polar geysers. Credit: NASA / JPL / Space Science Institute

A possible plume had actually first been spotted from a similar point of view during an encounter on January 16, 2005. However, while the image was compelling, it alone was not conclusive proof of the existence of the plume. The apparent plume could have been an image artifact. In order to rule out that possibility, the camera team conducted two follow-up campaigns:

  • First, they captured many images from similar points of view of other icy moons that were not suspected of geologic activity, such as Tethys and Mimas. The absence of plume-like features in those images supported the interpretation of the Enceladus image as a plume.
  • Second, the team commanded the Cassini spacecraft to rotate as it gathered the data during the November 27 plume search. If the plumes appeared in all images, regardless of Cassini's viewing geometry, it would tend to rule out the artifact theory. In fact, seven sets of images were captured at different orientations, and they all show the multiple plumes visible in the image above:
Enceladus
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144,063 km
Enceladus
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145,652 km
Enceladus
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147,488 km
Enceladus
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149,146 km
Enceladus
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165,845 km
Enceladus
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169,835 km
Enceladus
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174,176 km
Raw images credit NASA / JPL / Space Science Institute

The geologic activity of Enceladus had been confirmed even without these pictures, but seeing them is a boost to the Cassini science team. The only other places in the solar system where such plumes have been photographed are Jupiter's moon Io, Neptune's moon Triton, and, of course, Earth.

Triton's South Polar Geysers
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Triton's South Polar Geysers
Triton's south pole is dotted with about 50 dark plumes. The plumes are interpreted to be actively erupting geysers. Source Credit: NASA/JPL
Loki erupts on Io's limb
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Loki erupts on Io's limb
This Voyager 1 image of Io shows the active volcanic plume of Loki on the limb. Credit: NASA / JPL