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Sabtu, 17 Agustus 2013

Ballonts in Gas Giants ('Ballonts V')

More ballonts? Well, yes: I had previously explored whether it is possible to produce a fairly small life form floating around using the lighter-than-air mechanism, but there were some loose ends left. As the last one was posted in 2011, it may be wise to recapitulate a bit (or work your way up from here, through here, to this one).

Click to enlarge; copyright Gert van Dijk
The image above show a scene on Earth on sea level at about 20 degrees Centigrade. A default local sophont (let's call him 'Julius') holds a stick indicating two meters. There is also a balloon with a radius of 62.03 cm. Why 62 cm? Because that yields a sphere with a volume of exactly one cubic meter (m^3). The skin is made of a 0.1 mm thick mylar-like material with a mass of 0.5802 kg. The balloon is filled with the lightest possible gas, hydrogen. Hydrogen has a density of about 0.0899 kg/m^3 at 20 degrees, while the air has a density of 1.2019 kg/m^3. So, the 1 m^3 balloon has 0.0899 kg of hydrogen in it, while the corresponding volume of air has a mass of 1.2019 kg. The balloon can therefore lift 1.2019-0.0899 = 1.1120 kg (that is the part needed to understand how balloons work). As the skin masses 0.5802 kg, that leaves 1.1120-0.5802 = 0.5318 kg to build a nice body out of. That is not a nice big body at all; given a body density of 1.1 kg/m^3, which is like our bodies a bit heavier than water, we can hang a spherical body with a radius of just 4.9 cm under our balloon, and the ensemble will then just float. Of course, a real animal would have tentacles and limbs and mouthpieces etc.

As said, I wanted ballonts with a body mass of, say, 10 kg but with only a moderately sized sac. As the example above shows that does not work on Earth. The hydrogen inside the balloon cannot be made lighter, but we can alter the atmosphere outside it; this is speculative biology after all. There are two ways of doing so: the first is to stuff the atmosphere with very heavy gases such as argon, but such elements are quite rare in the universe. The other is to add mass by increasing pressure, as that will squeeze more mass in the same volume. So, let's explore gas giants, where high pressures are easily found.


Click to enlarge; Source: Brian Vanderwende University Colorado
The pictures above show information about 'our' gas giants: the composition of the atmosphere, the temperature and the pressure. Not surprisingly, atmospheric pressure increases the deeper you descend into the atmosphere. For our first try, we should perhaps be a bit conservative and stay with biology in fluid water. A temperature of 20 degree centigrade should not upset Julius; it is the same as 293 degrees Kelvin. For Jupiter, the 293 Kelvin zone results in an atmospheric pressure of some 9-10 times that of Earth, which sounds like a decent start. Instead of jumping in directly, it may be easier to take it in stages, building on the Earth model shown above.

Click to enlarge; copyright Gert van Dijk
The image above shows the first step: Earth's atmosphere is changed to a Jovian one at one atmosphere and 20 degrees centigrade. Internet sources show that the Jovian atmosphere consists of about 86% hydrogen, 14% helium and a smattering of other compounds. Based on the densities of hydrogen (0.0899 kg/m^3) and helium (0.1664 kg/m^3) the density of a 86:14 hydrogen/helium mixture should be 0.1006 kg/m^3. Oops! That is only very slightly denser than pure hydrogen, which we need to fill the ballont with! If you thought Earth air was a bad medium for ballonts, think again. So what are the effects? Well, the liftable mass is 0.1006-0.0899= 0.0107 kg. Remember that the skin had a mass of 0.5802 kg? There's nothing left for a body, so this balloon is not getting off the ground at all.

Click to enlarge; copyright Gert van Dijk
We were aiming for high pressures, so let's increase the pressure to 10 atmospheres. The mass in the balloon will be 10 times higher, and so will the mass of the equivalent volume of air. So the liftable mass also becomes 10 times larger: 10 x 0.0107= 0.107 g. That's still nowhere near the mass of the skin, so this balloon isn't going up either.

Click to enlarge; copyright Gert van Dijk
Let's leave Jupiter and find a ballont-friendlier place. Uranus and Neptune have atmospheric pressures about 50 times Earth's at the 293 Kelvin range. That's better, and apparently the Uranian atmosphere is heavier, with 2.3% methane thrown in. I make the density of its mixture to be 0.1148 kg/m^3 at 1 atmosphere and at 20 degrees C. So, the 1 m^3 balloon can lift 0.1148-0.0899 =0.0249 kg. That is not good enough, but at 50 atmospheres the liftable mass is 50 times that, or 1.2450 kg. Subtracting the skin leaves 0.6648 kg. Finally, a floating balloon! Hurrah!

Or perhaps not 'hurrah', as that is only a tiny bit more than what we had on Earth to start with... Let's go up to 200 atmospheres in Uranus: the liftable mass, skin already subtracted, would be 4.4 kg, and at 500 atmospheres it would be 11.9 kg. Finally we have what we wanted!

Well, not really; these values are not yet adapted for the lower temperature. Julius is left behind, as we need a wholly new biochemistry. The atmosphere is now also so soupy that you would not want to think about the wind or moving in it. Adding even more problems, there is another potential disaster lurking in these gas giants: gravity. The gravity constant for Uranus is nice at 8.85 m.s^-2, a bit less than Earth's at 9.8 m.s^-2. But Jupiter has a value of over 25, so if you thought you could get away with a nice fragile ballont there, waving its slight tendrils through the air and looping in prey with slender tentacles, think again: the animal would need the sturdy limbs befitting a 2.5G environment.

It really does seem as if the universe is trying to sabotage ballonts, doesn't it? Gas giants do have high atmospheric pressures, but their beneficial effects are counteracted by the fact that the atmospheres consist of very light elements. It seems that the only way to get a viable (pun intended) ballont on a Jovian planet is to make the ballont extremely large. But that is where we started... I am beginning to think that there may not be any appreciable advantage in locating ballonts in gas giants, even though science fiction is full of them. They do about as poorly there as they do on terrestrial planets, meaning they can in fact work, but they have to be big, very big. Perhaps gas giants have other advantages for ballonts: there's certainly a lot of atmosphere to play with in them.

Ca I still claim that ballonts are so common in gas giants that they are boring? Yes, but they will be big, as usual; perhaps that's what makes them boring. The best way out for small ballonts seems to be offered by terrrestrial planets with heavy gases and high pressures: Venusian analogues? Perhaps there will be a 'Ballonts VI', one day.   

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