Friday, 19 April 2013

I'm In A Different World

Today's video:

Click to tweet:  .  This is another in my series on aliens, quite closely linked to the one on silicon-based life in fact.

Assuming life to be based on chemical reactions, which is not inevitable, it's sometimes claimed that water is unnecessary.  Water is so familiar to us that we tend to assume it's a normal substance.  In fact, although it's very common and one of the most widespread chemicals in the Universe, it is in a sense very exotic and weird, so the fact that we need it to live might be connected to its weirdness.

First of all, let's consider water.  It has a very high melting and boiling point compared to other compounds with the same molecular weight, expands as it freezes, has very strong surface tension, is a good solvent and is amphoteric - both acidic and basic.  The consequences for life are that there is less risk of bodies of water freezing solid from the bottom up - they're more likely to freeze over and this will also insulate the water underneath - membranes form more easily within them leading in the case of Earth to the appearance of cells with controllable internal environments, many substances and reactions can take place in solution and the temperature means the reactions will take place relatively fast compared to substances which are gaseous at the temperature at which life here is possible.

However, ammonia is a viable alternative.  Ammonia is also polar, a good solvent, in some ways better than water in fact, and can accept or donate hydrogen like water.  There are many organic groups which would function similarly in ammonia to other groups in water.  Two of the drawbacks of ammonia, however, are that it has low surface tension and contracts when it freezes, like most other liquids.  Therefore, on a moon or planet with liquid ammonia and an atmosphere about as dense as ours, the chances are that the ammonia would freeze solid permanently.  However, if the world either had a much denser and therefore more thermally conductive atmosphere or a low axial tilt, temperatures could be stable enough for the ammonia to stay liquid.  Higher atmospheric pressure would also give ammonia a much higher boiling point, higher in fact than the boiling point of water at sea level on this planet.  The low surface tension may mean that membranes are unlikely to form, so it might be that life on an ammonia world would have to work without a protected internal environment, or that isolation would have to come about by other means.  There are no planets or moons with liquid ammonia in our solar system.  If there were, they would be orbiting at about the distance of Mars or inside the asteroid belt, which might explain their absence.  The fact that ammonia could still be liquid at a higher temperature under pressure means that chemical reactions could also be as fast as they are in life here.

An interesting third option, not often explored, is sulfuric acid (or sulphuric acid).  There are at least three bodies in our solar system where sulphur compounds are important:  Venus, Mars and Io.  Volcanic springs on Earth are also sometimes high in sulphuric acid.  Sulphuric acid is less likely to suffer from the "freezing solid" problem due to the fact that its melting and boiling points are so far apart that a world with sulphuric acid oceans would be much more likely never to have frozen sulphuric acid on its surface.  Even high mountains are unlikely to be "snow" capped since the strongly erosive acid rain would quickly level them.  Under pressure, the boiling point of sulphuric acid is extremely high, so a high-pressure planet with the liquid form on its surface could orbit considerably closer to the Sun than Earth, and in fact even inside the orbit of Venus.  It's also clearly a very good solvent.

To an alien living on an ammonia planet, Earth looks extremely hostile.  It has oceans of boiling acid and an atmosphere high in the corrosive gas oxygen, capable of reactions so violent that organic matter can heat up to over 1000 degrees Celsius and be completely destroyed.  To one from a sulphuric acid world, Earth looks a little less hostile but still quite inhospitable, since it would be cold and covered in alkaline seas - rather like an outside toilet in a way.

Presumably this means that to us, an ammonia planet would also seem like an extreme version of an outside khazi too.  Anyway...

I'd really like to get away from the slideshow format but don't know how exactly.  Clearly the mpreg vlog is very different but is still visually appealing, so something along the lines of actually using scenery or myself, or perhaps practical demonstrations, would work well too.

In this case there's quite a lot in the doobly-do which is missing from the video because i did it unscripted.  One of these bits of information is the highly specific nature of ammonia worlds.  They would have to have dense atmospheres and/or low axial tilt to avoid runaway snowball effects.  I was also surprised how narrow the habitable zones were, not to mention how hard it was to work out where their borders were.  The same applies to sulphuric acid worlds, which as it happens feel like the opposite end of a range with Earth in the middle.  Here's what seems to be roughly the water-based ecosphere:

 This assumes pressure roughly equivalent to sea level on this planet and is about some part of the solid surface of the body involved.  Superimposing ammonia on it gets you this:

This is somewhat unfair as the inner edge of the ecosphere for ammonia assumes the critical temperature of the compound.  It also extends into the inner part of the asteroid belt, which is probably one reason there are no ammonia planets in this solar system.  By contrast, the one for sulphuric acid looks like this:

This is actually well inside the orbit of Venus.  In a sense, Venus is a sulphuric acid planet, or would be were it not for the runaway greenhouse effect.

To me, these diagrams reveal the possibility of marginal environments of this nature such as sulphuric acid planets at about the level of insolation this planet has, which would probably be snowball planets covered in, of all things, frozen sulphuric acid, another type of planet absent from this solar system.  The inner ring might include planets with acid lakes at the poles, but not mountainous glaciers due to their low topography.  There is also a Venus-like ammonia planet possibility where there are droplets of ammonia in the upper atmosphere but none at the surface.

I must admit to being rather surprised about the sulphuric acid possibilities as to me it seems quite likely that they could be quite common.  Orion's Arm argue that they require a combination of several unlikely circumstances.  They need to start off high in both water and surface sulphur, then photolysis needs to split the water, creating an early atmosphere high in oxygen.  The sulphur would then oxidise and dissolve in the water, forming sulphuric acid itself.  Interestingly, sulphuric acid worlds, referred to on Orion's Arm as "Vitriolic", benefit from the wide temperature range under which concentrated sulphuric acid is liquid and are unlikely to experience ice ages, partly because of the lack of mountain ranges and partly due to the fact that the greater density of the substance never comes into play except at the outer edge of the range.  Even thinking about them brings stinging tears to my eyes.

Meanwhile on the pregnancy vlog, i think my plans for next week are going to have to exclude leaving the house.  I've been quite physically harsh on myself and it hampers the development.  I'm also still a bit out of it when i do it, probably an inevitable physical effect of the process, so probably intense scripting and rehearsal would be a good idea.  Massage also looks like it'll help.  I've also relocated the old "equipment", so now i have two!