Titan – Archvillain’s Lair

Still continuing my fascination for Saturn’s moon Titan this week. This time adding local power production to my sketch for the habitat.

But first a few other fun facts.

Using a variety of instruments aboard the Huygens probe scientists have been able to recount a by-by-blow of all 10 second of its bounce, wobble and slide across the surface of Titan.

At first contact the probe made a dent 12cm deep before bouncing onto Titan’s flat surface. They estimate the 200 kg probe hit the surface with the impact speed of a ball dropped from around waist height on Earth. Just think about that for moment – the effect of Titan’s reduced gravity. I find that awesome.

The analysis is so precise, scientists then report the probe tilted by 10 degrees, slid about a foot across the surface and came to its final resting place – where it wobbled back and forward four to five times. (Of course what they don’t realise is that a local Methanomorph – who had the sh*t scared out of him by the appearance of this strange object – kicked the Probe five times in sheer frustration.)

You can see an animation of the landing here.

Recent radar images from Cassini – orbiting Titan – reveal a circular feature described as a ‘hot cross bun’. This tasty region, which is 70km long, is characterised by surface fractures, with steam – possibly from rising magma below the surface, driving the uplift.

Having a think about where you would want to set up a habitat on Titan, I realised heat would be a premium. So you would set up either in a geologically active area like the ‘hot cross bun’ or perhaps near the mouth of an active volcano – just like the old Archvillain’s Lair!

OK, before you tell me I’m crazy, remember that things are very cold on Titan, around -179 C (-290F). The volcanic ‘lava’ is actually a liquid ammonia and water mixture. At the surface pressure of 1.45 atmospheres or 147 kPa (~21 psi) that means water is boiling at a slight elevation of 106C (~224F). [If you want to check out this boiling point elevation effect, find yourself a set of steam tables and look at the Saturated Water section.]

Perched up on the rim of the volcano, you would have a nice source of heat, and also a source of liquid water. Of course you would probably need to drive piles down to a nice solid foundation of ice or rock to keep your hab steady.

What you also have is a power Engineer’s dream – a temperature difference! Forget the portable nuclear reactor you brought to power things while you set up, now you can generate electricity directly from the ‘lava’.

You could operate a modified Rankine cycle using local materials for the working fluids. You would need to bring the key components with you (turbines etc), but you could use ammonia (R717) as the working fluid. Ammonia has a critical point at 132C. So you could fit a conventional cycle say between the temperatures of 106C and 10C – which would give you a theoretical cycle efficiency of around 25%. Not that great, but straightforward.

But what about using liquid methane as well? If you mixed liquid methane from the surface with the water ‘lava’ (50:50), you would end up with a methane-water-ammonia mixture with a temperature of around -79C. Now, because the ambient temperature is so low (-179C), you can theoretically operate a power cycle to recover heat right down to this temperature. The theoretical efficiency of recovering heat between 100C and -70C is 46%, much better. You could improve this by playing the proportions of the mix.

Of course you would have to do something tricky with the working fluid to allow this, perhaps by using a binary mixture of methane and ammonia.

You could pump the hot water from the volcano mouth, then generate power while you cooled the water using the ammonia cycle, then get even more power by mixing some of this water with methane. Rejecting heat from the working fluid would be a snap, given the low ambient temperature.

If you are interested,  here are the temperature Vs entropy diagrams for methane and ammonia. A typical steam-based Rankine cycle on a T-s diagram looks like this:

You would probably need to set up your Lair on the northern hemisphere of Titan, the only place where vast hydrocarbon seas have been observed.

Next week – sailing ice boats on methane seas. . .

Titan

I’ve always been fascinated by Titan. The idea of a moon in our solar system that has an atmosphere has always intrigued me – and it’s the only one in the whole solar system with more than a whiff.

Bigger than Mercury, if it was orbiting the sun Titan would make an impressive planet in its own right. It is the second-largest moon in the solar system, coming in just slightly smaller than Ganymede, and is twice the size and ten times the mass of poor demoted Pluto. Discovered in 1655 by Christiaan Huygens, Titan is an old friend.

Like Venus, until recently its surface has been obscured by cloud. However unlike the hothouse second planet, Titan has an anti-greenhouse effect.

Its distinctive orange colour – pretty familiar now from photographs – is due to a thick organonitrogen haze. In face Titan’s atmosphere is remarkably dense, holding an astounding 7.3 times more per square metre of surface than Earth (1.19 times overall mass). Only its reduced gravity (around 1/7th of earth – 6/7ths of the Moon’s) brings its atmospheric pressure into the highly reasonable range, being roughly one and a half times our own (1.45 atmospheres).

This comparable atmospheric pressure immediately gets me thinking about habitats. You could have fairly lightweight , non-pressurised, structures that maintained a comparable internal pressure to the external environment. Perhaps with an adjusted nitrogen-oxygen mix to bring the oxygen back to 0.21 atmospheres of pressure i.e. 0.21 atm of oxygen pressure with the remainder 1.45-0.21 = 1.24 atm nitrogen, so the actual percent oxygen by volume in the mix is 0.21/1.45 => 15%, yet equivalent to our home-grown partial pressure. Why nitrogen as the balancing gas? There’s heaps of it on Titan!

Titan’s atmosphere comprises about 98.4% nitrogen, with 1.4% methane and the remainder hydrogen. Its thickness also removes the threat of radiation, which is a particular problem for a human presence on the Moon, Mars or asteroids.

Another intriguing reason why Titan is an interesting target for colonisation is the amount of water there. OK. It’s pretty cold: around -179C (-290F). The water is locked up in the planet as solid ice. In fact, it pretty much is Titan ‘rock’. It is estimated that up to half of Titan’s bulk composition is water ice, with the other half rocky material. Volcanoes there spew ‘lava’ composed of water and ammonia.

So the bad news is that you would need some sort of energy source to melt the ice and harvest the oxygen through electrolysis. A nifty little nuclear power plant of the sort proposed for Martian missions would do the trick.

Due to these temperatures, and the light composition of the moon, the ‘methane cycle’ takes the place of the water cycle on Earth. By now descriptions of the vast methane lakes on Titan would be familiar following the descent of the Huygens probe to the surface in January 2005 (wow! Has it really been that long). Based on Cassini data from 2008, Titan bas hundreds of times more liquid hydrocarbons than all known oil and gas reserves on Earth. Now that’s something to consider.

The surface also has many features similar to Earth, despite the difference in composition, such as ‘sand dunes’. Space exploration and settlement advocate Robert Zubrin is keen on Titan, stating “In certain ways, Titan is the most hospitable extraterrestrial world within our solar system for human colonisation”. The nitrogen, methane and ammonia can all be used to produce fertiliser for the biodomes.

Probably the most significant problem (apart from getting there) would be managing the health effects of the low gravity.

So when’s the next bus?