When the European Huygens probe on the Cassini space mission parachutes down
through the opaque smoggy atmosphere of Saturn’s moon Titan early next year,
it may find itself splashing into a sea of liquid hydrocarbons. In what is
probably the first piece of "extraterrestrial oceanography" ever carried
out, Dr Nadeem Ghafoor of Surrey Satellite Technology and Professor John
Zarnecki of the Open University, with Drs Meric Srokecz and Peter Challenor
of the Southampton Oceanography Centre, calculated how any seas on Titan
would compare with Earth’s oceans. Their results predict that waves driven
by the wind would be up to 7 times higher but would move more slowly and be
much farther apart. Dr Ghafoor will present their findings at the RAS
National Astronomy Meeting at the Open University on Wednesday 31 March.
The team worked with a computer simulation, or ‘model’, that predicts how
wind-driven waves on the surface of the sea are generated on Earth, but they
changed all the basic inputs, such as the local gravity, and the properties
of the liquid, to values they might expect on Titan.
Arguments about the nature of Titan’s surface have raged for a number of
years. Following the flyby of the Voyager 1 spacecraft in 1980, some
researchers suggested that Titan’s concealed surface might be at least
partly covered by a sea of liquid methane and ethane. But there are several
other theories, ranging from a hard icy surface at one extreme to a
near-global hydrocarbon ocean at the other. Other variants include the
notion of hydrocarbon ‘sludge’ overlying an icy surface. Planetary
scientists hope that the Cassini/Huygens mission will provide an answer to
this question, with observations from Cassini during several flybys of Titan
and from Huygens, which will land (or ‘splash’) on 14 January 2005.
The idea that Titan has significant bodies of surface liquid has recently
been reinforced by the announcement that radar reflections from Titan have
been detected using the giant Arecibo radio dish in Puerto Rico.
Importantly, the returned signals in 12 out the 16 attempts made contained
reflections of the kind expected from a polished surface, like a mirror.
(This is similar to seeing a blinding patch of light on the surface of the
sea where the Sun is being reflected.) The radar researchers concluded that
75% of Titan’s surface may be covered by ‘open bodies of liquid
hydrocarbons’ – in other words, seas.
The exact nature of the reflected radar signal can be used to determine how
smooth or choppy the liquid surface is. This interpretation says that the
slope of the waves is typically less than 4 degrees, which is consistent
with the predictions of the British scientists, who showed that the maximum
possible slope of waves generated by wind speeds up to 7 mph would be 11
degrees.
"Hopefully ESA’s Huygens probe will end the speculation" says Dr Ghafoor.
"Not only will this be by far the most remote soft landing of a spacecraft
ever attempted but Huygens might become the first extraterrestrial boat if
it does indeed land on a hydrocarbon lake or sea." Although not designed
specifically to survive landing or to float, the chances it will do so are
reasonable. However, the link back to Earth from Huygens via Cassini, which
will be flying past Titan and acting as a relay, will only last for a
maximum of 2 hours. During this time, if the probe is floating on a sea, one
of the 6 instruments Huygens is carrying, the Surface Science Package
experiment, which is led by John Zarnecki, will be making oceanography
measurements. Among the 9 sensors that it carries are ones that will measure
the height and frequency of the waves and also the depth of the sea using
sonar. It will also attempt to determine the composition of the sea.
What would the sea look like? "Huygens does carry a camera so it is possible
we shall have some direct images," says Professor Zarnecki, "but let’s try
to imagine that we are sitting onboard the probe after it has landed in a
Titan ocean. What would we see? Well, the waves would be more widely
dispersed than on Earth but they will be very much higher – mostly as a
result of the fact that Titan gravity is only about 15% of that on Earth. So
the surface around us would probably appear flat and deceptively calm, but
in the distance we might see a rather tall, slow-moving wave advancing
towards us – a wave that could overwhelm or sink us."
CONTACTS
Dr Nadeem Ghafoor
Surrey Satellite Technology Ltd, Surrey Space Centre,
University of Surrey,
Guildford. Surrey. GU2 7XH
Tel (work): +44 (0) 1483 873346
Tel (home): +44 (0) 1483 567779
Mobile +44 (0) 7976 704931
E-mail: n.ghafoor@sstl.co.uk
Professor John Zarnecki
Planetary and Space Science Research Institute, The Open University
Walton Hall, Milton Keynes MK7 6AA
Tel. (work): +44 (0)1908 659599
Tel. (home): +44 (0)1908 608125
Mobile: +44 (0) 778 9900099
E-mail: J.C.Zarnecki@open.ac.uk
NOTE
Professor Zarnecki has a quarter-scale model of the Huygens probe at the
Open University and on request can provide a variety of relevant video clips
showing Huygens descending and landing on a sea.
Videos, images and more information available at
