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Data from NASA’s Cassini spacecraft have revealed Saturn’s moon
Titan likely harbors a layer of liquid water under its ice shell.
Researchers saw a large amount of squeezing and stretching as
the moon orbited Saturn. They deduced that if Titan were composed entirely of
stiff rock, the gravitational attraction of Saturn would cause bulges, or solid
“tides,” on the moon only 3 ft (1 m) in height. Spacecraft data show
Saturn creates solid tides approximately 30 ft (10 m) in height, which suggests
Titan is not made entirely of solid rocky material. The finding appears in Science.
“Cassini’s detection of large tides on Titan leads to the
almost inescapable conclusion that there is a hidden ocean at depth,” said
Luciano Iess, the paper’s lead author and a Cassini team member at the Sapienza
University of Rome, Italy. “The search for water is an important goal in
solar system exploration, and now we’ve spotted another place where it is
abundant.”
Titan takes only 16 days to orbit Saturn, and scientists were
able to study the moon’s shape at different parts of its orbit. Because Titan
is not spherical, but slightly elongated like a football, its long axis grew
when it was closer to Saturn. Eight days later, when Titan was farther from
Saturn, it became less elongated and more nearly round. Cassini measured the
gravitational effect of that squeeze and pull.
Scientists were not sure Cassini would be able to detect the
bulges caused by Saturn’s pull on Titan. By studying six close flybys of Titan
from Feb. 27, 2006, to Feb. 18, 2011, researchers were able to determine the
moon’s internal structure by measuring variations in the gravitational pull of
Titan using data returned to NASA’s Deep Space Network (DSN).
“We were making ultrasensitive measurements, and thankfully
Cassini and the DSN were able to maintain a very stable link,” said Sami
Asmar, a Cassini team member at NASA’s Jet Propulsion Laboratory in Pasadena,
Calif. “The tides on Titan pulled up by Saturn aren’t huge compared to the
pull the biggest planet, Jupiter, has on some of its moons. But, short of being
able to drill on Titan’s surface, the gravity measurements provide the best
data we have of Titan’s internal structure.”
An ocean layer does not have to be huge or deep to create these
tides. A liquid layer between the external, deformable shell and a solid mantle
would enable Titan to bulge and compress as it orbits Saturn. Because Titan’s
surface is mostly made of water ice, which is abundant in moons of the outer
solar system, scientists infer Titan’s ocean is likely mostly liquid water.
On Earth, tides result from the gravitational attraction of the
moon and sun pulling on our surface oceans. In the open oceans, those can be as
high as 2 ft (60 cm). While water is easier to move, the gravitational pulling
by the sun and moon also causes Earth’s crust to bulge in solid tides of about
20 in (50 cm).
The presence of a subsurface layer of liquid water at Titan is
not itself an indicator for life. Scientists think life is more likely to arise
when liquid water is in contact with rock, and these measurements cannot tell
whether the ocean bottom is made up of rock or ice. The results have a bigger
implication for the mystery of methane replenishment on Titan.
“The presence of a liquid water layer in Titan is important
because we want to understand how methane is stored in Titan’s interior and how
it may outgas to the surface,” said Jonathan Lunine, a Cassini team member
at Cornell University, Ithaca, N.Y. “This is important because everything
that is unique about Titan derives from the presence of abundant methane, yet
the methane in the atmosphere is unstable and will be destroyed on geologically
short timescales.”
A liquid water ocean, “salted” with ammonia, could
produce buoyant ammonia-water liquids that bubble up through the crust and
liberate methane from the ice. Such an ocean could serve also as a deep reservoir
for storing methane.
Source: NASA Jet Propulsion Laboratory
