The plumes of Enceladus
The plumes of Enceladus are jets of water vapor and ice grains that erupt from four long fractures, nicknamed "tiger stripes", near the south pole of this small Saturnian moon. Discovered by Cassini in 2005, they draw their material from a global ocean of salty liquid water beneath the ice. The plumes feed Saturn's broad E ring, and their chemistry, including salts, organic molecules, molecular hydrogen and phosphates, makes Enceladus one of the most promising places to look for life beyond Earth.
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Discovery
Enceladus had long been known as the most reflective body in the solar system and was suspected of being linked to the E ring, which is densest near its orbit. Cassini's first flybys in early 2005 gave the decisive clues. In February and March 2005, the magnetometer detected a bending of Saturn's magnetic field around Enceladus, as if the moon had a thin, localized atmosphere. In July 2005, during a close flyby, several instruments detected water vapor and ice grains over the south pole, along with unexpectedly warm temperatures there. In November 2005, images taken with the Sun behind the moon showed the plumes directly: bright fountains rising above the south polar limb.
The tiger stripes
The plumes originate from four nearly parallel fractures in the south polar terrain, named Alexandria, Cairo, Baghdad and Damascus Sulci, after cities in The Thousand and One Nights. Each is about 130 km long and about 2 km wide, flanked by ridges, and they are spaced about 35 km apart. The surrounding terrain is almost free of craters, which indicates it is geologically young, perhaps only a few million years old or less.
Cassini's infrared instrument found that the stripes are much warmer than their surroundings, with the hottest spots exceeding -100 °C in places, against an ambient temperature closer to -200 °C. The total heat flow from the south polar terrain is several gigawatts, with estimates ranging up to about 15 GW, much more than expected from radioactive decay in a body of this size.
A 2014 study by Carolyn Porco and colleagues identified about 100 individual jets along the tiger stripes and found that they correlate with the hot spots, supporting the idea that the jets are fed by liquid water from below rather than by ice sublimating at the surface.
What drives the plumes
Enceladus is heated by tides. Its orbit is slightly eccentric, maintained by a 2:1 resonance with Dione, so the moon is squeezed and stretched by Saturn's gravity during each 33-hour orbit. This flexing generates heat in the interior and opens and closes the tiger stripes.
Cassini confirmed that tides control the plumes: they are brightest when Enceladus is farthest from Saturn, when tidal stresses pull the fractures open, and fainter when it is closest. In 2015, measurements of a slight wobble, or libration, showed that the ice shell is detached from the rocky core, which requires a global ocean rather than a regional sea. The ice shell is thought to be about 20 to 25 km thick on average, but only a few kilometers thick at the south pole.
The chemistry of the plumes
Cassini flew through the plumes many times, sometimes very close: on 28 October 2015 it passed only about 49 km above the surface. Its mass spectrometer and dust analyzer sampled the material directly.
| Finding | Year reported | Significance |
|---|---|---|
| Water vapor, CO₂, methane, ammonia, simple organics | 2006–2008 | Volatile chemistry similar to comets |
| Sodium chloride and other salts in ice grains | 2009 | Grains come from liquid water in contact with rock |
| Silica nanoparticles | 2015 | Hydrothermal activity, likely above 90 °C |
| Molecular hydrogen (H₂) | 2017 | Chemical energy source available to microbes |
| Large, complex organic molecules | 2018 | Rich organic chemistry in the ocean |
| Phosphates | 2023 | Phosphorus, essential for life, is available in the ocean |
The phosphate discovery, published by Frank Postberg and colleagues in Nature in 2023, was especially significant. Phosphorus is a key element of DNA, RNA and cell membranes, and it had been considered potentially scarce in icy moon oceans. The concentrations inferred for Enceladus's ocean appear to be relatively high. These results are discussed in the context of habitability on life on Titan and Enceladus.
Feeding the E ring
Most of the plume material falls back onto Enceladus, coating it with fresh frost and helping explain its extreme brightness. A fraction escapes the moon's weak gravity and spreads along its orbit to form the E ring, a broad, tenuous ring of tiny ice grains that extends to about 480,000 km from Saturn. E ring particles coat nearby moons such as Tethys and the Trojan moons. Water vapor from the plumes also forms a torus of gas around Saturn, and once ionized it becomes the main source of plasma in Saturn's magnetosphere.
In 2023 the James Webb Space Telescope observed a plume of water vapor extending more than 9,000 km from Enceladus, nearly 20 times the moon's diameter, confirming that the activity continued after Cassini ended its mission in 2017.
FAQ
What are the plumes of Enceladus?
They are jets of water vapor and ice grains erupting from fractures near the south pole of Enceladus. The material comes from a global subsurface ocean of salty liquid water.
What are the tiger stripes on Enceladus?
They are four parallel fractures, each about 130 km long, in the south polar region of Enceladus. They are warmer than the surrounding ice and are the source of the plumes.
What is in the Enceladus plumes?
Mostly water, with carbon dioxide, methane, ammonia, molecular hydrogen, salts, silica nanoparticles, complex organic molecules and phosphates.
Do the Enceladus geysers feed Saturn's rings?
Yes. Part of the material escapes Enceladus and forms Saturn's E ring, a broad, tenuous ring of tiny ice grains along the moon's orbit.
Are the plumes still active?
Yes. In 2023 the James Webb Space Telescope observed a large plume of water vapor from Enceladus, years after Cassini's mission ended.
Sources
- NASA Science: Enceladus
- Porco, C. C. et al., "Cassini observes the active south pole of Enceladus", Science 311 (2006)
- Waite, J. H. et al., "Cassini finds molecular hydrogen in the Enceladus plume: evidence for hydrothermal processes", Science 356 (2017)
- Postberg, F. et al., "Detection of phosphates originating from Enceladus's ocean", Nature 618 (2023)
- Thomas, P. C. et al., "Enceladus's measured physical libration requires a global subsurface ocean", Icarus 264 (2016)