Previous flybys detected water vapor , sodium and organic molecules , but scientists need to know more aboutthe plume 's composition and density to characterize the source , possibly a liquid ocean under the moon 's icy surface .
Over the past months , nasa 's cassini spacecraft has made several close flybys of saturn 's moons , caught the sun 's reflection glinting off a lake on titan , and has brought us even more tantalizing images of ongoing cryovolcanism on enceladus .
But not all of the salt found in the flybys was created equal .
但并不是在飞越所有被发现的盐一样的。
Messenger 's earlier flybys didn 't get a good look , but the spacecraft 's neutron detectors will seek signs of hydrogen - the h in h2o - which would suggest it might be ice .
During venus flybys in 1998 and 1999 , the cassini spacecraft failed to detect the high-frequency radio noise that lightning would be expected to generate - like am radio static during terrestrial thunderstorms .