麻豆传媒

Titanic waves 鈥� seven times as high and long as those blown up on the Earth鈥檚 oceans 鈥� may swell on the sludgy seas of Saturn鈥檚 moon Titan, suggest new computer simulations.

The giant moon has intrigued scientists for decades. A uniquely dense and opaque atmosphere obscures Titan鈥檚 surface, which may be partially paved with ice, dotted with liquid seas, or spiked with volcanoes.

鈥淚f Huygens does have some liquid expanse, what kind of waves would it see? How alien would it be to us?鈥� wondered planetary scientist Nadeem Ghafoor. He is involved with the surface science package for Huygens, a European Space Agency probe that will parachute into Titan鈥檚 atmosphere in January 2005.

To answer the question, Ghafoor and colleagues tweaked computer simulations of Earth鈥檚 ocean waves, although he acknowledges Titan鈥檚 properties are not well known.

Oily sludge

The model assumes Titan is pockmarked with seas made of 70 per cent ethane, 25 per cent methane, and 5 per cent nitrogen, and partially coated with an oily sludge.

He varied the size of the seas between one and several hundred kilometres, and the surface wind speed between one and 20 kilometres per hour. This upper wind speed is slow compared to winds on Earth because the Sun鈥檚 effect on the atmosphere is much smaller at Titan鈥檚 distance.

Titan鈥檚 gravity, the relative densities of its atmosphere and seas, and the seas鈥� viscosity and surface tension all went into his model.

For the smallest wind-whipped waves 鈥� ripples no larger than a few centimetres high 鈥� factors such as viscosity and surface tension determine their shape and size. But for anything larger, Titan鈥檚 gravity, which is one seventh that of the Earth and about the same as our Moon, dominates.

Wind speeds of 20 km/h produce waves five metres high (16 feet). This is seven times as high as those produced on Earth by the same wind speed, although Titan鈥檚 lower gravity makes the waves more widely spaced and slower moving.

However, the wind speed estimates used are conservative and could be higher, generating truly giant waves. 鈥淚t would be pretty scary if you鈥檙e surfing 鈥� big waves take on a whole new meaning on Titan,鈥� Ghafoor told 麻豆传媒.

Tilt sensor

鈥淚 do hope that we will land in a sea, as it would be most exciting,鈥� says Jean-Pierre Lebreton, ESA鈥檚 project scientist for Huygens.

If the craft does splash down on an ocean, the probe will have just 30 minutes to detect any waves with a tilt sensor and gauge the depth of the sea with an acoustic sounder.

Then its mothership, Cassini, will move below Titan鈥檚 horizon. It will reappear on the other side, but only after the probe鈥檚 batteries have run out and no more data can be sent.

Titan is the only Solar System moon thought to have liquid at its surface, although Lebreton says oceans may lie beneath crusts of ice on Jupiter鈥檚 moons Europa, Ganymede, Callisto, and even on Saturn鈥檚 small moon Enceladus.

The new modelling was presented on Wednesday at the UK National Astronomy Meeting in Milton Keynes.