麻豆传媒

Space particles could trigger chemical explosions on Saturn鈥檚 icy moon Enceladus that could help create its remarkable plumes, a new study suggests.

Scientists were astonished when NASA鈥檚 Cassini spacecraft revealed water vapour spewing from a surprisingly warm region around Enceladus鈥檚 south pole in July 2005 (see Giant water plume spews from Saturn鈥檚 moon).

The prevailing explanation for this unexpected warmth is that Saturn鈥檚 gravity is squeezing and stretching Enceladus, providing energy that warms the 500-kilometre-wide moon in a process called tidal heating.

But now a team of scientists is arguing that at least some 鈥� and possibly all 鈥� of the heat could come from chemical reactions driven by impacts from charged particles.

Called cosmic rays, the particles come from the Sun and from distant supernovae and constantly bombard objects in the solar system, including Enceladus. Like Saturn鈥檚 other icy moons, Enceladus is also struck by particles associated with Saturn鈥檚 magnetic fields.

Seep down

Researchers led by John Cooper of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, US, say they would break down water molecules on the moon鈥檚 icy surface, leaving oxygen as one product. The same process may be partially responsible for the oxygen seen around Saturn鈥檚 icy rings.

The oxygen might work its way down through cracks around the moon鈥檚 south pole. If there is ammonia under Enceladus鈥檚 surface 鈥� Cassini has not yet found any but cannot rule out small amounts of it 鈥� the oxygen and ammonia would react violently. That would release heat that could drive water vapour into space to form the plumes, Cooper says.

鈥淚f those things come into contact, you get 鈥榢aboom!'鈥� Cooper says, pointing out that ammonia and oxygen were used to power the US air force鈥檚 X-15 rocket plane in experiments in the 1960s.

Cassini and previous missions to Saturn have measured the prevalence of energetic particles around the orbit of Enceladus. According to calculations by Cooper鈥檚 team, the particles can more than account for the several gigawatts of power fuelling the plumes.

Surface slush

But Carolyn Porco, who heads Cassini鈥檚 imaging team, says if energetic particles are powering the plumes, then Saturn鈥檚 other icy moons should show them too 鈥� and they do not. 鈥淚f this is going on in Enceladus, why isn鈥檛 it going on in Mimas?鈥� she told 麻豆传媒.

She says she still favours the idea of tidal heating, which would affect Enceladus more than the other moons because it appears to have a rocky core that would have kept the moon relatively warm after it formed.

Cooper says the plumes on Enceladus could be explained if it 鈥� unlike the other moons 鈥� has ammonia-rich slush very close to the surface, making it easy for oxygen to percolate down and react with it.

Cooper presented his group鈥檚 concept on Sunday at a meeting of the Enceladus Focus Group in Pasadena, California, US.