BACKLIT by a single star, plumes of gas and dust are shooting miles into space from Enceladus, Saturn's sixth-biggest moon.
It is a spectacular sight for stargazers, but it could also bring the possibility of extraterrestrial life-forms closer than before.
After poring over ultraviolet images of Enceladus from Nasa's Cassini probe, astronomers believe the streams are t
ravelling at more than 1,360mph.
Scientists already knew the plumes contained water vapour. But they say reaching supersonic speeds is "hard to do" without liquid water.
Dr Candice Hansen, of Nasa's Jet Propulsion Lab in California, whose research is published in the journal Nature today, describes the phenomenon as Enceladus "helpfully spewing out its innards".
She said: "Using an ultraviolet instrument, we watched a star go behind the plume. We were able to detect individual streams of gas coming from a source on the surface that is probably less than one kilometre square.
"The important thing is that we were able to determine that gas was coming out of Enceladus at supersonic velocities. This agrees with a model that postulates that there is liquid water beneath the surface.
"We may never be able to prove there's liquid water, but what we have done is collected another clue, built up more evidence that points towards it being there."
Enceladus, a 300-mile-wide satellite, acts like a pressure cooker, shooting jets of ice and water vapour from its south pole.
But researchers still do not understand where the energy comes from to heat up the moon's internal "boiler", or why the activity is confined to one area.
Scientists think pressure may build up in an underground chamber before erupting to the surface, in much the same way as a volcano erupting on Earth.
"Working out where the energy comes from is one of the more challenging mysteries in the universe," Dr Hansen said.
"We have some radioactivity there, but not enough. Then you look at tidal energy and try to calculate how much energy could come from that. Then we put the two together, and we're still making observations and testing them."
Each plume is about six miles wide, but because the small moon has such a weak atmosphere and cannot capture the gases, it is hard to calculate how long they are.
Other experts, such as Andrew Ingersoll at the California Institute of Technology, said it was possible to achieve such speeds with ice particles and at cooler temperatures.
John Brown, the Astronomer Royal and regius professor of astronomy at Glasgow University, said: "To me, this is an interesting bit of evidence of terrains that could harbour basic life-forms. It is exciting stuff."
If Enceladus does harbour life, it probably consists of microbes or other primitive organisms capable of living in extreme conditions. Scientists generally agree habitats need several ingredients for life, including water, a stable heat source and the right chemical recipe.
IN NUMBERS60
moons including Enceladus, orbiting Saturn.
99 per cent
of sunlight falling on the surface of Enceladus is reflected, compared with only 7 per cent for our own Moon.
-198 °C
is the warmest mean surface temperature recorded on Enceladus.
800,000
kilometres is the approximate surface area of Enceladus, making it almost the same size as Mozambique.
100 million
years is the youngest age scientists have placed Enceladus' surfaces at.
238,000
kilometres is the distance at which Enceladus orbits the centre of Saturn.
32.9
hours is how long it takes for Enceladus to revolve once.
1,100
feet per second is the technical definition of supersonic speed (or Mach 1) in Earth terms.
Two
Voyager spacecraft passing near Enceladus in the early 1980s sparked astronomers' interest in the satellite.
