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Extraterrestrial life a Titanic question

On the moon Titan you can see some extraordinary things. Dr Karl gets suited-up for a trip to one of our galaxy's planetary wonders.

Way back in 1996, I visited the Jet Propulsion Laboratory in California.

I got dressed up in the super-clean bunny suit so I wouldn't contaminate anything, and I was actually allowed to touch the Cassini–Hyugens space probe. It was one of the magical moments of my life.

Cassini–Huygens still holds the record for the largest such spacecraft ever built — 5.6 tonnes, 6.8 metres high, over 4 metres wide, and with over 14 kilometres of cabling.

And this was the spacecraft that discovered sand dunes in space! Or, at least sand dunes on the surface of Titan, the giant moon that orbits the ringed planet, Saturn.

When the spacecraft got to Saturn in 2004, it split into two parts. The Cassini part of the spacecraft is still orbiting Saturn doing great science.

But, on January 14, 2005, the Huygens part parachuted down to land on Titan.

On the way down, its photos showed branching river beds just like the ones on Earth. And it actually landed on the wet, pebble-littered location of a flash flood.

We now know that Titan has vast deserts with kilometre-high hills circling the equator, giant lakes near the poles, and in between, eroded landscapes with flowing liquids.

Titan is a bit special. It's the largest moon in the entire solar system. At 5000 kilometres in diameter, it's bigger than the planet Mercury, a bit smaller than Mars, and about 40 per cent of the diameter of Earth.

Titan is the moon that would be a planet.

Most of the planets and moons in our solar system have an atmosphere that is either enormous or microscopic, either millions of times thicker than our earthly atmosphere, or millions of times thinner.

But the atmosphere of Titan is surprisingly close to Earth's atmosphere — only about 1.5 times thicker.

And the composition of Titan's atmosphere is amazingly close to ours as well. Our earthly atmosphere is about 80 per cent nitrogen and 20 per cent oxygen. Titan's is about 95 per cent nitrogen and five per cent methane.

The sunlight that falls on Titan is about one per cent of the strength of our sunlight, because Titan is so far from the sun.

So the temperature is very cold, around -180°C. At this temperature, methane, the stuff you burn in your gas stove at home, can exist as a gas, a liquid and a solid.

It sounds just like the situation here on Earth where, thanks to the 'lucky' temperature, water can exist as a gas, a liquid and as the solid that we call ice.

On Titan, methane does the job that water does on Earth. Methane evaporates from the methane lakes near the poles and floats as methane clouds and then drops as methane rain.

This methane rain carves out river beds and valleys in the surface of Titan, and then flows back into the methane lakes. The shorelines on Titan look like those we see here on Earth.

The climate sounds similar, but there's a major difference between Earth and Titan.

On Earth, the energy from the nearby Sun will evaporate about one metre of water from the ocean each year.

But the Earth's atmosphere will hold only a few centimetre's worth of water before the water turns into clouds and rain.

So on Earth, we have a couple of centimetres of water every week or so, dropping as rain out of the atmosphere.

On Titan, the much weaker energy from the distant Sun will evaporate only about a single centimetre of methane each year.

But the atmosphere is a giant sponge — it can hold about 10 metres of methane. So on Titan, it's a long time between rains, but when it comes, it's huge.

On Titan, water does the job that rocks do here on Earth.

Frozen water-ice under the surface behaves like rock, and brings the overall density of Titan down to about 1.9-times the density of water.

We think that Titan has a core of rock and iron, which is surrounded by water-ice compressed to very high pressure.

This ice is, in turn, surrounded by an ocean of water and ammonia, which is then covered with water-ice.

And finally we get to the surface, and above that, the atmosphere that is hauntingly similar to that on Earth.

But does that mean there is life on Titan? Well, that is a very big question, and I'll talk about that next time.

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