The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 456 2017 November 5
FRESH FINDINGS FROM CASSINI
NASA
The Cassini spacecraft ended its journey on Sept. 15 with an intentional
plunge into the atmosphere of Saturn, but analysis continues on the
mountain of data the spacecraft sent during its long 'life'. The
spacecraft's Ion and Neutral Mass Spectrometer (INMS) returned a lot
of direct measurements of the components in Saturn's upper atmosphere,
which stretches almost to the rings. From those observations, the team
sees evidence that molecules from the rings are raining down onto the
atmosphere. That influx of material from the rings was expected, but
INMS data show hints of ingredients more complex than just water, which
makes up the bulk of the rings' composition. In particular, the
instrument detected methane, a volatile molecule that scientists would
not expect to be abundant in the rings or found so high in Saturn's
atmosphere.
Chief among the questions that scientists hope to answer by using data
from Cassini is the age and origin of the rings. Theoretical modelling
has shown that, without forces to confine them, the rings would spread
out over hundreds of millions of years -- much younger than Saturn
itself. Such spreading happens because faster-moving particles that
orbit closer to Saturn occasionally collide with slower particles on
slightly farther-out orbits. When that happens, some momentum from the
faster particles is transferred to the slower particles, speeding the
latter up in their orbit and causing them to move farther out. The
inverse happens to the faster, inner particles. Previous research had
shown that gravitational tugs from the moon Mimas are solely responsible
for halting the outward spread of Saturn's B ring -- that ring's outer
edge is defined by the dark region known as the Cassini Division. Ring
scientists had thought that the small moon Janus was responsible for
confining the outer edge of the A ring, but a new modelling study shows
that the A ring's outward creep is kept in check by a confederation of
moons, including Pan, Atlas, Prometheus, Pandora, Janus, Epimetheus and
Mimas.