Cassini Prepares to Say Goodbye to a True Titan

Mere weeks away from its dramatic, mission-ending plunge into
Saturn, NASA’s Cassini spacecraft has a hectic schedule,
orbiting the planet every week in its Grand Finale. On
a few orbits, Saturn’s largest moon, Titan, has been near
enough to tweak Cassini’s orbit, causing the spacecraft to
approach Saturn a bit closer or a bit farther away. A couple of
those distant passes even pushed Cassini into the inner fringes
of Saturn’s rings.

Titan will be waiting once again when the road runs out in
September. A last, distant encounter with the moon on Sept. 11
will usher Cassini to its fate, with the spacecraft sending
back precious science data until it loses contact with Earth.

But this gravitational pushing and shoving isn’t a new behavior
for Titan. It’s been doing that all along, by design.

The True Engine of the Mission

Repeated flybys of Titan were envisioned, from the mission’s
beginning, as a way to explore the mysterious planet-size moon
and to fling Cassini toward its adventures in the Saturn
system. Scientists had been eager for a return to Titan since
NASA’s Voyager 1 spacecraft flew past in 1980 and was unable to
see through the dense, golden haze that shrouds its surface.

Titan is just a bit larger than the planet Mercury. Given its
size, the moon has significant gravity, which is used for
bending Cassini’s course as it orbits Saturn. A single close
flyby of Titan could provide more of a change in velocity than
the entire 90-minute engine burn the spacecraft needed to slow
down and be captured by Saturn’s gravity upon its arrival in
2004.

The mission’s tour designers — engineers tasked with plotting
the spacecraft’s course, years in advance — used Titan as
their linchpin. Frequent passes by the moon provided the
equivalent of huge amounts of rocket propellant. Using Titan,
Cassini’s orbit could be stretched out, farther from Saturn —
for example, to send the spacecraft toward the distant moon
Iapetus. With this technique, engineers used Titan flybys to
change the orientation of Cassini’s orbit many times during the
mission; for example, lifting the spacecraft out of the plane
of the rings to view them from high above, along with high
northern and southern latitudes on Saturn and its moons.

What We’ve Learned

Over the course of its 13-year mission at Saturn, Cassini has
made 127 close flybys of Titan, with many more-distant
observations. Cassini also dropped off the European Space
Agency’s Huygens probe, which descended through Titan’s
atmosphere to land on the surface in January 2005.

Successes for Cassini during its mission include the revelation
that, as researchers had theorized, there were indeed bodies of
open liquid hydrocarbons on Titan’s surface. Surprisingly, it
turned out Titan’s lakes and seas are confined to the poles,
with almost all of the liquid being at northern latitudes in
the present epoch. Cassini found that most of Titan has no
lakes, with vast stretches of linear dunes closer to the
equator similar to those in places like Namibia on Earth. The
spacecraft observed giant hydrocarbon clouds hovering over
Titan’s poles and bright, feathery ones that drifted across the
landscape, dropping methane rain that darkened the surface.
There were also indications of an ocean of water beneath the
moon’s icy surface.

Early on, Cassini’s picture of Titan was spotty, but every
encounter built upon the previous one. Over the course of the
entire mission, Cassini’s radar investigation imaged
approximately 67 percent of Titan’s surface, using the
spacecraft’s large, saucer-shaped antenna to bounce signals off
the moon’s surface. Views from Cassini’s imaging cameras,
infrared spectrometer, and radar slowly and methodically added
details, building up a more complete, high-resolution picture
of Titan.

“Now that we’ve completed Cassini’s investigation of Titan, we
have enough detail to really see what Titan is like as a world,
globally,” said Steve Wall, deputy lead of Cassini’s radar team
at NASA’s Jet Propulsion Laboratory in Pasadena, California.

Scientists now have enough data to understand the distribution
of Titan’s surface features (like mountains, dunes and seas)
and the behavior of its atmosphere over time, and they have
been able to begin piecing together how surface liquids might
migrate from pole to pole.

Among the things that remain uncertain is exactly how the
methane in Titan’s atmosphere is being replenished, since it’s
broken down over time by sunlight. Scientists see some evidence
of volcanism, with methane-laden water as the “lava,” but a
definitive detection remains elusive.

Cassini’s long-term observations could still provide clues.
Researchers have been watching for summer rain clouds to appear
at the north pole, as their models predicted. Cassini observed
rain clouds at the south pole in southern summer in 2004. But
so far, clouds at high northern latitudes have been sparse.

“The atmosphere seems to have more inertia than most models
have assumed. Basically, it takes longer than we thought for
the weather to change with the seasons,” said Elizabeth Turtle,
a Cassini imaging team associate at Johns Hopkins Applied
Physics Laboratory, Laurel, Maryland.

The sluggish arrival of northern summer clouds may match better
with models that predict a global reservoir of methane, Turtle
said. “There isn’t a global reservoir at the surface, so if one
exists in the subsurface that would be a major revelation about
Titan.” This points to the value of Cassini’s long-term
monitoring of Titan’s atmosphere, she said, as the monitoring
provides data that can be used to test models and ideas.

Results from the Last Close Pass

Cassini made its last close flyby of Titan on April 22. That
flyby gave the spacecraft the push it needed to leap over
Saturn’s rings and begin its final series of orbits, which pass
between the rings and the planet.

During that flyby, Cassini’s radar was in the driver’s seat —
its observation requirements determining how the spacecraft
would be oriented as it passed low over the surface one last
time at an altitude of 608 miles (979 kilometers). One of the
priorities was to have one last look for the mysterious
features the team dubbed “magic islands,” which had appeared
and then vanished in separate observations taken years apart.
On the final pass there were no magic islands to be seen. The
radar team is still working to understand what the features
might have been, with leading candidates being bubbles or
waves.

Most interesting to the radar team was a set of observations
that was both the first and last of its kind, in which the
instrument was used to sound the depths of several of the small
lakes that dot Titan’s north polar region. Going forward, the
researchers will be working to tease out information from these
data about the lakes’ composition, in terms of methane versus
ethane.

As Cassini zoomed past on its last close brush with Titan,
headed toward its Grand Finale, the radar imaged a long swath
of the surface that included terrain seen on the very first
Titan flyby in 2004. “It’s pretty remarkable that we ended up
close to where we started,” said Wall. “The difference is how
richly our understanding has grown, and how the questions we’re
asking about Titan have evolved.”

The Cassini-Huygens mission is a cooperative project of NASA,
ESA (European Space Agency) and the Italian Space Agency.
NASA’s Jet Propulsion Laboratory, a division of Caltech in
Pasadena, manages the mission for NASA’s Science Mission
Directorate, Washington. JPL designed, developed and assembled
the Cassini orbiter.

More information about Cassini:

https://www.nasa.gov/cassini

https://saturn.jpl.nasa.gov/

News Media Contact

Preston Dyches
Jet Propulsion Laboratory, Pasadena, Calif.
818-394-7013
preston.dyches@jpl.nasa.gov

2017-215

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