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European Space Agency

Press Release

Splashing down on Titan's oceans

1 April 2003
 
Titan, Saturn's largest moon, is a mysterious place. Its thick
atmosphere is rich in organic compounds. Some of them would be signs
of life if they were on our planet. How do they form on Titan? Will
they help us to discover how life began on Earth? 
 
ESA's Huygens probe, arriving at Titan in 2005, will help find
answers. Here on Earth, ground-based telescopes are playing their
part also. They will help scientists to decide how and where
precisely Huygens will land. What will it be - on solid ground or in
an ocean of methane? 
 
NASA's Voyager 1 provided the first detailed images of Titan in 1980.
They showed only an opaque, orange atmosphere, apparently homogeneus.
It was so thick that you could not see the surface. However, other
data revealed exciting things. Similarly to Earth, Titan's atmosphere
is mostly nitrogen but there is also methane and many other organic
compounds.

Organic compounds form when sunlight destroys the methane. If
sunlight is continuously destroying methane, how is methane getting
into the atmosphere? On Earth today, it is life itself that refreshes
the methane supply. Methane is a by-product of the metabolism of many
organisms. Could this mean there is life on Titan? 

Titan is not a pleasant place for life. It is far too cold for liquid
water to exist, and all known forms of life need liquid water. Titan'
s surface is -180°C. According to one exotic theory, long ago, the
impact of a meteorite, for example, might have provided enough heat
to liquify water for perhaps a few hundred or thousand years.
However, it is unlikely that Titan is a site for life today.
Jean-Pierre Lebreton, Huygens Project Scientist, is puzzled by the
amount of methane that persists in Titan's atmosphere. Could there be
oceans of methane on or under the surface?

Before Huygens arrival, planned for January 2005, astronomers will
observe Titan using the most powerful ground-based telescopes. "More
and more astronomers are pointing their instruments to this amazing
cold world. And their results are helping us a lot," points out
Lebreton. Images from the W. M. Keck Observatory reveal methane-
containing clouds near Titan's south pole. Could Titan have the
equivalent of a weather cycle? Lebreton says "It is a major
discovery. It means that the atmosphere is much more dynamic than we
used to think." The NASA Cassini orbiter will clearly see these
clouds, carrying out precise observations before, during and after
releasing the Huygens probe.
 
Over the years, scientists have dramatically changed their minds
about Titan's surface. In the mid-nineties, the NASA-ESA Hubble Space
Telescope spied an area on Titan that was brighter than the rest. 

More recent observations show the same feature better. What are these
bright and dark patches? Lebreton wonders if, "the bright area could
be a continent and the rest oceans. We don't know yet. There is no
doubt, though, that the surface appears very diverse, not uniform.
There are a lot of surprises waiting for us there."

Where will Huygens land? On the bright patch or on a dark one? "
Closer to the bright surface, but not on it," answers Lebreton. "Just
imagine! We could be landing in an ocean! It would be really
exciting, the first landing in an ocean outside the Earth!" To land
on an ocean would probably mean better data from Huygens. Even if the
probe lasted only a few minutes before sinking, it would at least
stay in an upright position. Being the right way up is essential for
sending the data back to the Cassini orbiter and to the scientists on
Earth. Moreover, some of Huygens's instruments are better prepared to
analyse liquids. If Huygens lands on a solid surface instead, there
is a higher risk of falling in the wrong direction and not being able
to easily communicate with Cassini. 

Such is the fate of an ESA probe - to travel extremely far to an
object you know comparatively little about to measure extremely
familiar organic compounds extremely quickly. Space is a risky place
and it is all about extremes.

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