A SALTY ocean containing billions of cubic kilometres of water may lie buried
deep beneath the icy crust of Ganymede, Jupiter鈥檚 largest moon, say planetary
scientists. And where there is water there might just be life. Magnetic
measurements from NASA鈥檚 Galileo probe, which is orbiting Jupiter, suggests that
an ocean girdles the moon 170 kilometres beneath the surface at a chilly
鈥�13 掳C.
鈥淲e have now reached the point of saying there is very strong evidence of
water beneath the surface, after wrestling with the data for months,鈥� Margaret
Kivelson of the University of California at Los Angeles said at the American
Geophysical Union meeting.
The discovery promotes Ganymede into the first division of possible homes for
extraterrestrial life. Ganymede joins Mars and the Jovian moons Callisto and
Europa as the only bodies apart from Earth to show signs of liquid water.
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Galileo flew within 900 kilometres of Ganymede in May and detected a magnetic
field from the moon which changed as Ganymede passed through Jupiter鈥檚 much
stronger magnetic field. For Jupiter鈥檚 field to affect Ganymede鈥檚, it must set
up an electric current within the moon itself. And this current itself would
generate a magnetic field. But the field requires an electrically conductive
layer within the moon. While ice is a poor conductor, salty water is very
efficient.
The subsurface oceans on Callisto and Europa were detected using similar
reasoning, but Ganymede鈥檚 own permanent magnetic field meant Kivelson and her
colleagues to had to show that the field varied in sync with Jupiter鈥檚 field as
it passed through it.
Dave Stevenson of the California Institute of Technology in Pasadena says
natural radioactivity in Ganymede鈥檚 rocky interior should generate enough heat
to maintain a layer of liquid water about 150 to 200 kilometres below the
surface.
