London, Feb 23 : Scientists have developed a new theory to explain the formation of the enormous plume of dust and water vapour that spurts violently into space from the south pole of Enceladus, Saturn's sixth-largest moon.
According to a report from the University of Leicester, UK, physicist Nikolai Brilliantov and his colleagues in Germany, have revealed why the dust particles in the plume emerge more slowly than the water vapour escaping from the moon's icy crust.
Enceladus orbits in Saturn's outermost "E" ring. It is one of only three outer solar system bodies that produce active eruptions of dust and water vapour.
Moreover, aside from the Earth, Mars, and Jupiter's moon Europa, it is one of the only places in the solar system for which astronomers have direct evidence of the presence of water.
The erupting plume on Enceladus is ejected by geyser-like volcanic eruptions from deep, "tiger-stripe" cracks within the moon's south pole. Some astronomers have suggested that the myriad tiny grains of dust from these eruptions could be the actual source of Saturn's E-ring.
However, the dynamics and the origin of the plume itself have remained a mystery.
Now, Brilliantov, along with his team, has developed a new theory to explain the formation of these dust particles and to explain why they are ejected into space.
The researchers point out that once ejected the dust particles, which are in fact icy grains, and water vapour are too dilute to interact with each other and so the water vapour cannot be the cause of the dusty slowdown.
Instead, the team suggests that the shift in speed must occur below the moon's surface before ejection.
The numerous cracks through which the plume material escapes from the moon's icy surface, and which can be hundreds of metres deep, are narrower at some points along their length. At these points temperature and pressure of vapour drop drastically down, causing condensation of vapour into icy grains and hence to formation of the dust-vapour mixture.
These peculiar conditions allow the water vapour to erupt rapidly carrying with it the dust particles.
However, these particles undergo countless frequent collisions with the inside of the channel walls which causes friction that slows them down before final ejection. In fact, the larger the particle, the slower the ejection speed.
This effect, quantified by the new theory, explains the structure of the plume and eventually the particle size distribution of the E-ring of Saturn.