February 17, 2016 | David F. Coppedge

Tiny Enceladus Beats Saturn in Plasma Output

An astonishing case of the tail wagging the dog: tiny moon is major source of plasma in Saturn’s magnetosphere.

Take all the plasma sources at Saturn: the planet, the rings, and the icy moons as far out as Titan. None of them holds a candle to the plasma output of Enceladus, a little moon no wider than the state of Iowa.

Tiny Enceladus orbits outside Saturn's rings

Enceladus orbiting outside Saturn and its rings

The south pole geysers of Enceladus pump out 93 kilograms of ice per second. The water molecules become ionized and are swept into Saturn’s magnetosphere. A press release from University College London (UCL) says this is the main source of plasma in the magnetosphere, an astonishing output for a tiny moon, only matched by the plasma being ejected from Saturn’s ionosphere.

Saturn's moon Enceladus with "Tiger Stripes" fissures where geysers erupt

Saturn’s moon Enceladus with “Tiger Stripes” fissures where geysers erupt

A journal paper in the Journal of Geophysical Research confirms the winner based on observations taken by the Cassini spacecraft when it passed through the magnetotail in 2006.

Saturn’s magnetosphere is a complex multicomponent plasma system with several internal plasma sources in addition to the solar wind. The largest internal plasma source is from photoionization and electron impact ionization of neutral water and nitrogen molecules from the icy moon Enceladus. These ions are subsequently processed by photolytic and radiolytic processes to produce H+ and a variety of water group ions such as OH+ and O+ that are collectively referred to as W+. The other natural satellites, the rings, and Saturn’s atmosphere are minor internal sources. The solar wind also plays a role as an external plasma source. A number of studies have focused on the moons, rings, and solar wind as plasma sources, to constrain the extent to which they drive the system. In this paper we provide the first in situ constraints on the role that the ionosphere plays as a mass source for Saturn’s magnetotail, via the first observation of ionospheric outflow at a giant planet.

Such ionospheric outflow occurs on Earth, but this is the first observation at another planet. Jupiter, too, gets plasma from a moon that is tiny compared to its planet. The press release states:

In the case of Saturn, its moon Enceladus ejects water from its icy plumes which is ionised into H2O+, O+, OH+ and then transported throughout the magnetosphere. For Jupiter, its moon Io provides plasma from its sulphurous volcanoes whereas Earth’s magnetosphere is strongly driven by the solar wind but fed by a polar wind from the ionosphere – the atmospheric layer ionised by solar and cosmic radiation.

The contribution by Enceladus to Saturn’s environment has been known since Cassini arrived at Saturn, but this is the first time Saturn’s ionospheric contribution has been published. One might expect that the planet’s contribution of plasma to its magnetosphere would swamp the input from a tiny moon with one 1/50,000th its mass, but Saturn barely keeps up with its tiny satellite:

The Cassini mission previously established the importance of Enceladus as the dominant mass source for Saturn’s magnetosphere but this is the first time that Saturn’s ionosphere has been seen providing, at times, a similar plasma production rate.

Enceladus outputs so much mass, it creates a vast ring of fine particles from Mimas to Titan.

Diagram of Saturn's E-ring created by Enceladus

Diagram of Saturn’s E-ring created by Enceladus

Expressions of amazement in the paper and the press release are mild for such an unexpected discovery:

Professor Andrew Coates, a co-author on the paper and Cassini co-investigator, said: “Cassini never ceases to amaze us. First, it found that the plume of Enceladus is the main source of the water-rich magnetosphere which ultimately escapes from the planet. Now, we find that solar wind compression allows much lighter hydrogen ions to escape from Saturn’s upper atmosphere at times.”

Which is more amazing, though? Who would have expected a moon 300 miles in diameter to beat a planet 75,000 miles in diameter? Saturn with its rings and all its other moons combined can hardly keep up with the ionized mass output of this little moon.

There is more than just plasma coming out of Enceladus. There is so much ice and dust coming out of its insides, it creates a whole ring around Saturn (the E-ring). The paper and press release, of course, fail to ask whether this rate of mass loss could keep going for 4.5 billion years. The burden is on the moyboys to explain this.


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