Cassini Data Says It's Separate From Saturn's Own
Data from the NASA/ESA/ASI Cassini spacecraft indicate that
Saturn's majestic ring system has its own atmosphere -- separate
from that of the planet itself. During its close fly-bys of the
ring system, instruments on Cassini have been able to determine
that the environment around the rings is like an atmosphere,
composed principally of molecular oxygen. This atmosphere is
very similar to that of Jupiter's moons Europa and Ganymede,
according to scientists.
The finding was made by two instruments on Cassini: the Ion and
Neutral Mass Spectrometer (INMS) has co-investigators from USA and
Germany, and the Cassini Plasma Spectrometer (CAPS) instrument has
co-investigators from US, Finland, Hungary, France, Norway and
Saturn's rings consist largely of water ice mixed with smaller
amounts of dust and rocky matter. They are extraordinarily thin:
though they are 250,000 kilometres or more in diameter they are no
more than 1.5 kilometres thick.
Despite their impressive appearance, there is very little
material in the rings - if the rings were compressed into a single
body it would be no more than 100 kilometres across.
The origin of the rings is unknown. Scientists once thought that
the rings were formed at the same time as the planets, coalescing
out of swirling clouds of interstellar gas 4000 million years ago.
However, the rings now appear to be young, perhaps only hundreds of
millions of years old.
Another theory suggests that a comet flew too close to Saturn
and was broken up by tidal forces. Possibly one of Saturn's moons
was struck by an asteroid smashing it to pieces that now form the
Though Saturn may have had rings since it formed, the ring
system is not stable and must be regenerated by ongoing processes,
probably the break-up of larger satellites.
Water molecules are first driven off the ring particles by solar
ultraviolet light. They are then split into hydrogen, and molecular
and atomic oxygen, by photodissocation. The hydrogen gas is lost to
space, the atomic oxygen and any remaining water are frozen back
into the ring material due to the low temperatures, and this leaves
behind a concentration of oxygen molecules.
Dr Andrew Coates, co-investigator for CAPS, from the Mullard
Space Science Laboratory (MSSL) at University College London, said:
"As water comes off the rings, it is split by sunlight; the
resulting hydrogen and atomic oxygen are then lost, leaving
"The INMS sees the neutral oxygen gas, CAPS sees molecular
oxygen ions and an ‘electron view’ of the rings. These
represent the ionised products of that oxygen and some additional
electrons driven off the rings by sunlight."
Dr Coates said the ring atmosphere was probably kept in check by
gravitational forces and a balance between loss of material from
the ring system and a re-supply of material from the ring
Last month, Cassini-Huygens mission scientists celebrated the
spacecraft's first year in orbit around Saturn. Cassini performed
its Saturn Orbit Insertion (SOI) on July 1, 2004 after its six-year
journey to the ringed planet, travelling over three thousand