Cassini, a Nasa spacecraft the size of a truck, carrying Huygens, a European robot not much bigger than a commercial washing machine, will complete one more preparatory loop around Titan, the largest moon of Saturn. Then, on Christmas Day, Huygens will be released, spinning five times a minute, on a collision course with the only moon in the solar system that has an atmosphere.
Cassini will fly past Titan, its receiver turned towards Titan’s dense cloud tops to receive the last messages from Huygens as it disappears into the freezing hydrocarbons.
European scientists admit they still have no idea of what Huygens will see on the way down, whether it will land with a splash, a thud or a squelch, nor how it will perish.
Cassini’s instruments have picked up what might be evidence of topography: seas of frozen methane, huge rafts or low hills of solid chemical ice, perhaps even volcanic flows of ammonia. At -177C (-287F), Titan is cold enough for methane to exist simultaneously as gas, solid and liquid.
Christophe Sotin, a planetary scientist based at Nantes in France, believes that Titan could have an ice sheet over an ocean of liquid water, washing in turn over a second layer of thick ice that sheathes a core of rock and iron.
If Titan has liquid water, it has one of the ingredients for life. The highly reactive mix of hydrocarbons could be another: methane, ammonia and other gases could be the raw material of amino acids, the building blocks of proteins.
On January 14, Huygens will hit the upper atmosphere of Titan at about 4 miles a second. Its heat shield will absorb enough of the energy of impact to slow the probe. Then the first of three para chutes will help it brake to a relatively leisurely descent through the atmosphere. A microphone will tune in to the sounds of Titan, a camera will collect snapshots of the journey to oblivion through the orange-coloured clouds, and sensors will start making chemical analyses of the atmospheric density and chemistry.
Instruments will take the temperature, measure the howling winds, and record the moon’s thunder and lightning. It will pump its discoveries across 37,000 miles of space to Cassini, which will relay the precious data back to Earth. Anxious scientists will have no way of changing the experiment. Even at the speed of light it will take more than an hour to transmit the first bleep to Earth and Huygens will be dead by that time.
The impact may not be immediately fatal: Huygens will hit the deck at five metres a second, about the speed of a human jumping off a chair.
“We do not know what we are going to land in,” says John Zarnecki, of the Open University. “We might squelch down. Or land on icy surface, with mountains in the background.”
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