Saturn's Moon Titan May Have Been Planetary Punching Bag

Charles Q. Choi, SPACE.com Contributor
Date: 08 May 2011 Time: 01:00 PM ET
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This false-color image from NASA
CREDIT: NASA/JPL/Space Science Institute
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An untold number of cosmic impacts could have created the
mysteriously thick atmosphere of Saturn's largest moon Titan, suggest
experiments with laser guns.

Titan has always stood out as the only moon in the solar system with
a substantial atmosphere. In fact, the surface pressure on Titan is
50 percent greater than the pressure on Earth.
[Photos:
The Rings and Moons of Saturn]

The main ingredient of
Titan's
atmosphere is nitrogen, just as it is on Earth. Where this nitrogen
came from has long been debated. For instance, it could be
primordial, accumulating as Titan formed, or it could have originated later.


Weighing the options

In 2005, the Huygens probe carried by
NASA's
Cassini spacecraft to Saturn ruled out a primordial origin for this
nitrogen. Titan's atmosphere apparently has extremely low levels of
the isotope argon-36, while high amounts are expected in an
atmosphere rich in primordial nitrogen.

There are a number of other explanations for how this atmospheric
nitrogen might have formed after Titan's birth. For instance,
sunlight in Titan's atmosphere might have broken apart ammonia, a
molecule made of nitrogen and hydrogen.

However, nearly all these suggestions require that Titan formed at
relatively high temperatures, which would have led the moon to
differentiate into a rocky core and an icy mantle layer, and
Cassini's radar scans suggested that Titan is not fully
differentiated. Comets loaded with nitrogen might have delivered it
to Titan, but that would have also led to higher levels of argon-36
than currently seen.

Now scientists in Japan suggest that countless numbers of
asteroids
and comets slamming into ammonia ice on Titan could have converted it
to nitrogen gas several hundred million years after the moon's formation.

"Our results suggest that hypervelocity impacts have played a key
role," researcher Yasuhito Sekine, a planetary scientist at the
University of Tokyo, told SPACE.com.

Solar system dodgeball

During an era known as the Late Heavy Bombardment about four billion
years ago, the solar system was very much like a shooting gallery,
with cosmic impacts regularly blasting planets and moons. To see if
such impacts would deliver enough energy to convert ammonia ice to
nitrogen, researchers used laser guns and "bullets" made of gold,
platinum or copper foil. The beams vaporized the back of these
bullets, propelling them at high speeds at targets made of ammonia
and water ice.

The researchers found "ammonia is very easily converted to nitrogen
molecule by impacts," Sekine said.

They calculated that 330 million billion tons (300 million billion
metric tons) worth of impactors could have produced the current
amount of nitrogen seen on Titan, "a plausible mass of impactors
during the Late Heavy Bombardment," noted planetary scientist
Catherine Neish at Johns Hopkins University, who did not take part in
this research.

"It's an interesting new hypothesis," Neish told SPACE.com.
"Differentiating between the different hypotheses will require a more
detailed understanding of Titan's internal structure, and the
composition of comets and-or other Saturnian satellites." She
suggested that a future mission to a comet would very likely provide
key evidence to help confirm or refute the idea.

One question would be where all the craters from such impacts might
be. Titan has only about 50 recognized craters, Neish said. "Does
this imply that Titan's surface is very young?" she asked, suggesting
a young surface could have covered up most of the
craters
on Titan.

The scientists detailed their findings online May 8 in the journal
Nature Geoscience.

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