Day: April 15, 2010

Titan is cold, so cold that bottled BBQ gas (methane & ethane) are liquid in lakes around its poles and it rains (pours really) the stuff periodically in regions closer to the equator.

Methane-Ethane lakes on Titan (Credit: Copyright 2008 Karl Kofoed)

But chemical disequilibrium, as found on Titan, is one pre-requisite for interesting long-chain chemistry (i.e. LIFE) and so there’s renewed speculation about the possibility… Life on Titan: stand well back and hold your nose! Titanian Life, so the breathless copy tells us, would stink at our ambient temperature, then burst into flames and/or choke us to death.

The idea gains new credence with the discovery of abundant microbial life in the nearest terrestrial analogue to Titan – Trinidad & Tobago’s Pitch Lake – which, while it’s a lot hotter and full of much longer chain hydrocarbons, is pretty impressively nasty. Could Life survive on a moon in lakes of methane/ethane? Could it evolve there? Isaac Asimov was a biochemistry professor in parallel with his most famous authoring period and he has some very interesting speculations on this very question in a web-reprint of his 1962 essay As We Don’t Know It. Here’s a quick summary of possible biochemistries…

There, then, is my list of life chemistries, spanning the temperature range from near red heat down to near absolute zero:

1. fluorosilicone in fluorosilicone
2. fluorocarbon in sulfur
3.*nucleic acid/protein (O) in water
4. nucleic acid/protein (N) in ammonia
5. lipid in methane
6. lipid in hydrogen

Of this half dozen, the third only is life-as-we-know-it. Lest you miss it, I’ve marked it with an asterisk.

…since then we’ve learnt a bit more about the flexible limits of life-as-we-know-it, but lipids in methane/ethane are still more likely than water-based microbes struggling in cryogenic conditions. Yet everything we’ve learnt in the nearly 50 years since his essay tells us “expect surprises”. As much as we think we know our familiar brand of life here on Earth, there’s always something new discovered when we go look.

A recent preprint by Charles Lineweaver & Marc Norman, of ANU, proposes that dwarf planets can be defined physically by the “Potato Radius” – the size at which gravity overcomes the strength of their constituent materials and makes them into a sphere. Why “Potato”? Because below that radius objects are often ellipsoids with three different sizes in the three dimensions i.e. triaxial ellipsoids, much like an average potato. Sure enough amongst the icy moons of the Outer Planets that’s exactly what’s seen – Miranda & Mimas just straddle the radius (~200 km) and Mimas is slightly elliptical. Everything smaller is a ‘potato’.

That’s fine. It means there’s presently about 50 dwarf planets out in the Kuiper Belt, objects about 400 km across or more. School teachers will have nightmares about imagining new memonics to remember them all. But I want something bigger. There’s two good reasons for thinking that something might be Out There, just past the Kuiper Belt. An older reason is the marked truncation of the Kuiper Belt – it just ends at about ~50 AU – and this can be explained by a perturbing body about Mars-to-Earth in size. A newer reason is the oddness that is the Uranian system’s rotational plane versus Uranus’s orbital plane around the Sun. Uranus is knocked over on its side unlike any other planet. More oddly is the fact that its moons share the odd rotational plane too. How did Uranus get knocked over and the moons not get scattered to the six points of the dimensions? Jacques Laskar & Gwenaël Boué have suggested in a preprint late in 2009 that a moon massing ~1% of Uranus (about 1.5 times Mars) could have been perturbed into an odd orbit and the whole system then levered to follow it. Then it was perturbed into the Outer Reaches… perhaps to ‘truncate’ the Kuiper Belt and haunt its Beyond.

So, assuming it exists, what would it be like? If it’s half ice/rock, then it’ll be ~0.72 Earth diameters across – about 9000-9200 km. Massing ~0.16 Earth masses, but saved from the stripping of volatiles experienced by Mars & Earth, it might’ve retained a H/He atmosphere – a so-called primary atmosphere, which the Inner Planets might’ve possessed in the deep past. How much would it be able to capture? Uranus captured maybe ~1.5-3.5 Earth masses of H/He and its moon was ~0.01 times its mass, thus 1/10,000th of its capture area. That would mean ~0.00015-0.00035 Earth masses of primary atmosphere, which spread out over a 9000 km wide object is ~3537-8252 tons/sq.metre of gas. Earth has just ~10.33 tons/sq.metre so that’s a thick atmosphere. Our hypothetical mini-Ice Giant has a surface gravity – at the imaginary 4,500 km radius from its centre – of just 0.32 gee, thus the atmosphere weighs down with 110-256 Earth atmospheres of pressure. Like the bottom of an average stretch of terrestrial ocean.

Here’s where things get interesting. Over a decade ago planetologist David Stevenson pondered planets without a star – he found they could keep a liquid water ocean from freezing due to their primary atmosphere keeping geothermal heat in. Could our mini-Ice Giant have an ocean kept warm beneath a thick H/He blanket? Seems so.