ATOM & COSMOS
Scientists begin to analyze data
from Cassini’s last hurrah
BY LISA GROSSMAN
It’s been more than six months since
NASA’s Cassini spacecraft plunged to
its doom in the atmosphere of Saturn,
but scientists didn’t spend much time
mourning. They got busy analyzing the
The Cassini mission ended September 15, after more than 13 years orbiting
Saturn (SN Online: 9/15/17). The probe’s
final 22 orbits, dubbed the Grand Finale,
sent Cassini into the potentially dangerous region between the gas giant and its
rings; the final orbit sent the spacecraft
directly into Saturn’s atmosphere.
That mission-ending ride helped
solve mysteries about the planet’s atmosphere, rings and moons that could not
be tackled any other way, scientists
reported March 19.
“The Grand Finale orbits provided
information that was totally unexpected,” said Cassini project scientist
Linda Spilker of NASA’s Jet Propulsion
Laboratory in Pasadena, Calif.
One surprise came from new mea-
surements of Saturn’s gravity. Cassini’s
final daredevil orbits allowed the space-
craft to measure the gravity of Saturn
and its rings independent of one another.
Looking at the planet’s gravity field alone
revealed that Saturn’s swirling bands of
clouds penetrate much deeper into the
planet than expected.
Last month, astronomers announced
a similar discovery for an even larger gas
giant, reporting that Jupiter’s cloud belts
reach about 3,000 kilometers below the
top of the atmosphere (SN: 3/31/18, p. 10).
Saturn’s clouds reach a few times
deeper than that. “People used to think
that maybe Saturn was just a slightly
smaller version of Jupiter, but it’s evident
that that’s not the case,” says planetary
scientist Paul Schenk of the Lunar and
Planetary Institute in Houston, who was
not involved in the gravity measurements.
The difference speaks to how diverse
planets are, he says. “Every place you look,
everywhere we’ve been to, it’s just been so
dramatically different and unique.”
Cassini also confirmed that bits of ice
from Saturn’s rings rain into the planet’s
atmosphere, an idea first suggested in the
1980s. In its last five full orbits before
diving into Saturn, Cassini found a zoo
of organic molecules in and just above
the planet’s atmosphere, said planetary
scientist Kelly Miller of the Southwest
Research Institute in San Antonio.
Cassini found a lot of water, as expected.
But there were also lots of hydrocarbons
similar to propane, plus some methane
and sulfur-bearing molecules.
The types of molecules became less
well-mixed as the spacecraft looked
deeper into Saturn’s atmosphere, which
is what would happen if the particles
came from the rings and sank at different
speeds. The researchers think this mate-
rial is raining especially from Saturn’s
thin innermost ring. Other Cassini data
suggest this ring is losing mass.
The organics in the ring rain seem
to resolve a debate about why Saturn’s
rings appear reddish in some spots.
“We’ve had this debate going on for
a couple of years now—are they red
because of good old-fashioned rust like
Mars, or because of the same kinds of
organic materials … that make carrots and
tomatoes and watermelon red?” asked
planetary scientist Jeff Cuzzi of NASA’s
Ames Research Center at Moffett Field,
Calif. “To me, this answers the question
of what makes the rings red: It’s organics.”
With the last of Cassini’s data in hand,
researchers also have a new understand-
ing of one of the forces nudging plumes
on Saturn’s tiny moon Enceladus.
The spurts of liquid water were discovered in 2006. Over the next six years, scientists noticed that the plumes varied in
brightness (a proxy for how much material is gushing from the moon) on a daily
cycle, probably driven by changes in gravitational strength as Saturn and Enceladus
moved with respect to each other.
Then, in 2015, some researchers noted
that the plumes’ overall brightness had
The final data
taken by the
before it plunged into
Saturn reveal new details
about the planet’s clouds,
rings and moons.
Disappearing islands on Saturn’s moon
Titan may actually be tall waves reflecting sunlight. Such a reflection (yellow
spot at upper left) is shown in the sea
Kraken Mare in this infrared image.