Dione. Image: Los Alamos National Laboratory |
Los
Alamos National Laboratory scientists and an international research team have
announced discovery of molecular oxygen ions in the upper-most atmosphere of
Dione, one of the 62 known moons orbiting the ringed planet. The research
appeared in Geophysical Research Letters
and was made possible by instruments aboard NASA’s Cassini spacecraft, which
was launched in 1997.
Dione—discovered
in 1684 by astronomer Giovanni Cassini (after whom the spacecraft was
named)—orbits Saturn at roughly the same distance as our own moon orbits Earth.
The tiny moon is 700 miles wide and appears to be a thick, pockmarked layer of
water ice surrounding a smaller rock core. As it orbits Saturn every 2.7 days,
Dione is bombarded by charged particles (ions) emanating from Saturn’s very
strong magnetosphere. These ions slam into the surface of Dione, displacing
molecular oxygen ions into Dione’s thin atmosphere through a process called
sputtering.
Molecular
oxygen ions are then stripped from Dione’s exosphere by Saturn’s strong
magnetosphere.
A
sensor aboard the Cassini spacecraft called the Cassini Plasma Spectrometer
(CAPS) detected the oxygen ions in Dione’s wake during a flyby of the moon in
2010. Los Alamos researchers Robert Tokar and
Michelle Thomsen noted the presence of the oxygen ions.
“The
concentration of oxygen in Dione’s atmosphere is roughly similar to what you
would find in Earth’s atmosphere at an altitude of about 300 miles,” Tokar
said. “It’s not enough to sustain life, but—together with similar observations
of other moons around Saturn and Jupiter—these are definitive examples of a
process by which a lot of oxygen can be produced in icy celestial bodies that
are bombarded by charged particles or photons from the sun or whatever light
source happens to be nearby.”
Perhaps
even more exciting is the possibility that on a moon with subsurface water,
such as Jupiter’s moon Europa, molecular oxygen could combine with carbon in
subsurface lakes to form the building blocks of life. Future missions to Europa
could help unravel questions about that moon’s habitability.
Two
sensors aboard Cassini built by Los Alamos National Laboratory are expected to
come into play beginning later this month (March, 2012), and again in April and
May (2012), when the Cassini spacecraft flies by the moon Enceladus. The moon
is one of the brightest objects in our solar system, reflecting back nearly all
of the sunlight that strikes it, thanks to a shimmering surface of snowy ice
crystals. The moon also unleashes plumes of material from its south polar
region. Los Alamos’ ion-beam spectrometer and
ion-mass spectrometer may help answer key questions about the composition of
these plumes.
