Enshrouded in an atmosphere impenetrable to the visible light, Saturn’s
largest moon has never revealed its surface. No one has been able to see
through the orange-brown atmospheric haze and admire the unknown world
below.
Still, researchers know that Titan is a planet-size organic reactor where
“building blocks” of life are being generated as they might have been
created 4 billion years ago on Earth.
In some ways, Titan resembles early Earth. Its dense atmosphere is mostly
composed of nitrogen and some methane. Scientists once believed that early
Earth’s atmosphere was reducing like Titan’s and that it allowed fast
assembly of long organic molecules. Today many argue that Earth’s primordial
atmosphere contained nitrogen and a lot of carbon dioxide.
“This type of atmosphere is neutral for oxidation and reduction reactions
and does not allow an easy and direct formation of long chains of organic
molecules,” says University of Arizona planetary sciences Professor Jonathan
I. Lunine. “Some particular circumstances may be required to create them.
Although there isn’t much carbon dioxide on Titan, if we see that complex
organic molecules are created on Titan, it would be a very important lesson
about the early Earth and the environment in which life originated.”
Lunine will be talking about Titan at the American Geophysical Union meeting
in San Francisco on Saturday, Dec. 7 at 1:30 p.m.
“Titan has organics, but in what form and how much is not clear. These
molecules are generated in the atmosphere and over time are deposited on the
moon’s surface. Until recently, researchers have been very careful in their
speculations about what might be happening after these molecules get to the
surface of Titan,” Lunine says.
The atmospheric pressure at Titan’s surface is 50 percent higher than on
Earth, which is pressure comparable with pressure at the bottom of a
10-foot-deep swimming pool. Titan’s thick atmosphere protects the surface
and organics from harmful cosmic rays and ultraviolet radiation.
The NASA Cassini spacecraft launched in 1997 with the mission to study
Saturn and its moons will reach its target in 2004. It carries the European
Space Agency’s Huygens probe, which will descend through Titan’s atmosphere
and land on the surface. The Cassini-Huygens mission will conduct a 4-year
survey of Titan’s surface and atmosphere through remote sensing and in-situ
techniques.
“The Cassini mission has the potential to teach us as much about Titan as we
know about Mars today. We will learn about the surface composition, find out
more about the atmosphere, and see what the surface looks like. The Cassini
orbiter will measure the shape of Titan’s gravitational field, which will
help determine the nature of Titan’s interior,” Lunine says.
“Titan will be full of surprises. One of them will be organic chemistry
processes on the surface. It would be interesting to see what their products
might be,” he adds.
“I also hope that Cassini-Huygens will tell us if there are places on Titan
where the organic molecules look different, and therefore, might be modified
over time. Particularly exciting would be finding out if there are any
variations in the apparent organic composition that are correlated with
impact carters or sites of volcanism. If that turns out to be true, these
should be the places to visit in the future,” he says.
Could Titan host primitive life? “It is not the right place, it is too
cold,” Lunine says. “Others have argued that life could exist in the deep
interior of Titan where liquid water may be available all the time. It is
possible, but finding it would be extremely difficult. I do not see Titan as
the place to search for life. But it certainly is the place to explore the
chemistry that may have led to its origin.”
For life to be possible, Titan would need liquid water, which is not stable
for long because Titan is too cold. However, many of the large icy moons in
the outer solar system host active water volcanism. Most of them contain a
lot of liquid water, which flows across their surfaces in the same way lava
does on Earth. Their internal heat initiates a melt that rises to the
surface. These moons also contain various substances that are antifreezes
(e.g. ammonia or formaldehyde). They are mixed into the water which lowers
the density of liquid water and helps the water come up to the surface
through the more dense icy crust. Titan is the second largest moon in the
solar system, and if it hosts such volcanic processes, then water exists
temporarily on the surface.
Titan can also be heated with large impacts. In the early 1990s, Carl Sagan
and W. Reid Thompson of Cornell University suggested that impacts on the
surface of Titan would melt the icy crust and produce liquid water. Lunine
and a colleague from Moscow have been modeling impacts on Titan to see what
fraction of the crater would become liquid due to an impact. They calculate
that an impact of a one-kilometer-diameter comet can turn about 5 percent of
a crater©ˆs interior into liquid. Their simulations also show that the areas
potentially containing organic matter would not be heavily shocked in an
impact. Organic material survives such events and would be tossed in the
crater where the liquid water would exist. When life on Earth originated
about 4 billion years ago, large impacts were frequent.
“An organic soup on Earth did not have much uninterrupted time to form
products relevant to life. Undoubtedly, the environment was changing
dramatically, as young Earth was struck by other impacts or altered by
volcanism,” Lunine says.
Although today the solar system is relatively a quiet place, a
one-kilometer-diameter object could hit Titan once every 10 million – 50
million years.
“There should be areas that haven’t been changed in geologically recent time
and where the products of organic processes that happened after that impact
should be preserved. These may not be possible to investigate with the
Cassini-Huygens probe, but could be done with the following missions. We are
very optimistic that there are places on Titan where organic matter might be
dropped into the liquid water at the bottom of the crater after an impact.
This water can be available for hundreds or even up to a thousand years,”
Lunine says. A thousand years is very short on a geologic time scale, but
it’s a long time for organic chemistry.
“No scientist has a thousand years, so we can’t proceed at this time scale
in the laboratory. Though we don’t have a chance to see organic reaction on
Titan in action, we may find the products of organic chemistry if we go to
the right place,” he adds. “If Cassini finds that organic matter looks the
same everywhere on the surface, then this probably did not happen. But we
need to go and see.”
The chance that the Huygens probe will land in the right place is
infinitesimal, but the Cassini orbiter can map the surface and tell if amino
acids or peptides might be present. “We have been designing miniature
laboratory equipment that may be eventually sent to Titan to analyze the
properties of organic molecules on the surface,” Lunine says. The search
would be for fossil organics, not fossil life, that have been modified at
the bottoms of craters.
