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A quick aside about Titan: Back in the mid 1970s there were temperature measurements, via radio scans, of Titan which indicated temperatures of up to ~200 K. Like the argument over Venus, which inspired the Mariner 2 science experiments, there were two possible locations for the observed 'warmth' - the surface and the upper atmosphere - which were 'explained' by 3 scenarios: a warm surface, warmed by a massive greenhouse effect (up to 20 bars of N2 was computed); a warm ionosphere over a cold surface at 80 K, with a low methane surface pressure (~80 mb); and something in between the two.
Just before "Voyager 1" firmed up the data, Arecibo data supported the third option - the observed surface temperature was ~100 K, thus a 2 bar N2 atmosphere, with a bit of methane, was indicated. The top of the stratosphere was measured to be ~170 K.
Another unrealised 'fact' about magnetospheres is they can't stop the high energy neutrons, x-rays and gamma-rays that are produced by Solar Wind particle events. Yet we survive them just fine here on Earth as we would on any planet with sufficient atmosphere. What matters is the mass-loading - on Earth, the 10 tons per square metre.
On smaller objects, the same mass-loading means a lower surface pressure. Atmospheric column mass scales with the inverse of the surface gravity. Thus the same surface pressure would mean greater radiation protection - 2.5 times more on Mars, 6 times more on the Moon, and 7 times more on Titan. Additionally, as the scale height also inversely scales to the gravity, the atmospheres on lower gee worlds are much deeper than on Earth. This means more time for, for example, muons generated by cosmic-rays, to decay. On Earth, the muons penetrate a couple of kilometres underground before they decay. On a terraformed Mars, the atmosphere would be 2.5 times deeper and the muon exposure at the surface near negligible.
Focusing solettas can provide Earth-like insolation levels, as desired. They would be 'pseudo-lenses' with angled rings of mirrors to focus the sunlight onto the target object. Their radius would scale with the distance of the target world from the Sun in AU. Thus a Titan soletta at 10 AU would be 10 times the size of Titan. But, as Titan as a lot of methane and Callisto has a lot of dry ice, Earth-like insolation levels probably aren't needed to make significant changes to the surface temperature.
Earth's magnetic moment is 8E+22 A.m^2. The current needed, if the wire is around the Equator, is 626 MA. Not easy with copper, but if it's superconducting, then no problem. Of course we want room temperature superconductors and there's a possible candidate - compressed magnesium hydride [MgH6 specifically] might be superconducting at ~400 K so we might have a ready supply of materials if we can make it and quench it to STP.
On Mars, if we're scaling the field to produce a Solar Wind stand-off distance of 8 planetary radii, the required super-current is about 144 MA. Powering up could take a while - the total magnetic energy will be ~6.8E+17 J, a couple of decades output from a 1 GWe reactor.
A 3/2 Day/Year cycle, like Mercury's, is another possible end state of tidal despinning. If so, then the Sol (sun-rise to sun-rise period) will be twice the Year - thus a 3 week diurnal cycle is a real possibility. Will make for very interesting effects on any life that might have evolved there, let alone what we might introduce (some day).
Of course there's also the additional radial velocity trend that might be a super-Earth further out. Long term orbital evolution of the system can be computed once we have harder figures.
As I've discussed in this essay, "Interstellar Comparisons", it's something of a false dichotomy. Chiefly because the power levels required are of the same magnitude - if we can do one, then we can do both.
As for the Jeans escape rate for hydrogen on the Moon, it probably escapes directly into space from the exosphere. A background gas is needed to dilute the water and allow a stratospheric cold trap to form, else the Moon would lose hydrogen as soon as it was dissociated.
On the water photolysis issue, as the atmosphere would be exposed directly to the Sun's UV, without the benefit of an ozone shield or stratospheric cold-trap, then it'd only be energy limited. Direct photolysis, which knocks a hydrogen loose and forms a hydroxyl and hydrogen atom, requires UV shorter than ~150 nanometres or so. I did look up the correct energy some time ago, so don't quote me. As UV and shorter wavelengths are ~6% of the Sun's flux and the efficiency is probably not 100%, especially once a bit of ozone is formed, then the actual rate would be harder to compute. There are complex atmospheric codes for this sort of thing, though they usually assume an N2/CO2 background mix. Andy Ingersoll's old paper on Venus's runaway greenhouse notes that the UV flux at the frequency which water absorbs highly is enough to dissociate about ~40 times Earth's ocean mass in 4.5 gigayears. That's about ~4,000 times the column mass of water we're discussing on the Moon, so the whole lot might dissociate in a megayear, roughly.
30 tons per square metre on the Moon would provide ~half a bar of pressure. At average lunar temperatures the ice would sublimate until the pressure reached some equilibrium point at maybe ~0.25 bar or so. The rest of the (now liquid) water would soak into the regolith. It's porosity is hard to guesstimate, but it's something like ~5% or so. Thus 15 metres of liquid water, would soak into ~300 metres of regolith, but very slowly. There might be significant amounts of frozen ground water a few metres down, soaked up as vapour over the aeons from the solar-wind and cometary inputs that seem likely. If I was a betting man I'd say about 10 metres worth would remain as water loose on the surface, quickly pooling into all the low lands. Some areas would be dessicated during the day, but it depends on what clouds form. The Moon spins a bit too fast for a sub-solar cloud bank to form, but having the atmosphere evaporating or condensing on a regular basis en masse, it's very hard to predict what the atmosphere would be like on average.
Void, we already have ion drives. This system is an inside-out ion drive, using the atmosphere as working fluid. A space version is the enclosed grid system that is used on a variety of satellites for station-keeping and on a few space-probes, especially "Dawn". There's nothing new or novel in the concept - Robert Goddard experimented with an ion wind system back in 1916.
I have wondered if this could be converted into a space propulsion. Of course you would need to bring the gasses with you where there is no atmosphere, but also I am thinking that those can be more normal gasses.
qrall01 I appreciate that you provided that article also. A great addition, more dimensions. For instance spin.
So, if it is not feasible to increase the spin of Venus, could the humidity of Venus be throttled down further, containing water in habitats only. I presume the spin is needed so that the polar water has time to cool down so that it does not actually boil humidity into the atmosphere. Covering it with a shading? I believe plants can grow successfully with 10% Earth light so on Venus you would let 5% through the shaded moist locations?
The spin is needed to sustain a temperature gradient so the pole remains cool. Else a slow atmosphere would mix too well and heat everything evenly - ala current Venus.
I had heard of these desert planets, but only the most vague reference to this point.
Is it reasonable to say that Sulfuric Acid along with such water vapor as is present, is one of the keys to the current high temperatures of Venus?
No. Clouds are a double edged sword in a sense, but the H2SO4 layers reflect heat away on current Venus more than they trap. The real problem is all that CO2 which enhances its own heat-trapping by pressure-broadening the spectral windows it absorbs best at.
CO2, at lower pressures, helps cool the very high atmosphere and trap in the water, keeping the planet wet. We want much lower CO2, but not too low. More importantly we don't want too much water, else a runaway Greenhouse will result from self-accelerating evaporation of the oceans.
It looks to me that a pathway if found between the current stable condition, and a preferred new stable desert planet condition, might involve a sequence of manipulations. I can only offer that removal of Hydrogen from the atmosphere, and therefore H20 vapors and Sulfuric acid vapors might help to lead to an initial cooling, along with that dust could improve that situation, and I might hope that a cooling surface might absorb some atmospheric gasses. (The reverse has been proposed for a Mars where the surface is warming, that gasses would be released).
Dust absorbs as much as it reflects, unless engineered right. Really what Venus needs is for about 99% of the CO2 to become a convenient solid and fall out of the sky. The sulfuric acid will react with the regolith and basically neutralise itself, once it can condense on the ground at about ~500 K. What remains will be the equivalent of about 10 centimetres of water over the whole surface - enough for a smallish lake. Not enough to make trouble.
Qraal, that is very interesting. Do you have any sense of how much land area one would get from Venus if it were transformed into that kind of desert planet?
Welcome back, by the way.
Thanks Josh. As for land area, most of it. The humidity has to remain low, so we're talking <10% water coverage of the planet. Roughly. So such a Venus would be very dry compared to Earth, but as we use only a tiny fraction of the water we have, it's more a matter of water useage efficiency. As the equatorial and tropical regions would be very hot, all the water bodies will be clustered around the poles. A scenario might result in which canals linking the two polar regions are dug, probably requiring coverage to reduce evaporation.
This paper has significance for terraforming Venus: http://arxiv.org/abs/1304.3714
Towards the Minimum Inner Edge Distance of the Habitable Zone
Andras Zsom, Sara Seager, Julien de Wit
...the punch-line being that a desert planet can remain habitable - i.e. able to sustain some liquid water on the surface - up to an insolation ~4 times Earth. The trick is the conditions required to do so at high insolation aren't met on present day Venus, not just because of the thick atmosphere, but also the slow rotation. If Venus rotated quicker then quite high temperature gradients could be sustained and it could, for example, have cold polar caps with above boiling point equatorial deserts. The thermal inertia of the current atmosphere is probably too high presently, but removing the CO2 would leave about ~2 bars N2 on the surface, which would support a habitable climate if the planet rotated quicker. Question is: how fast is fast enough?
Intriguingly it also means that burying the Earth's oceans will allow it to remain habitable well into the deep future. The Main Sequence, by current models, will end with the Sun only about ~84% brighter (x1.84 present day) and even the billion year long Redwards Traverse will only see the insolation climb to ~x2.7 present day. The luminosity will rise a bit more rapidly after that, then do a mad scramble towards the Red Giant Tip - when the Sun bloats to ~256 times bigger and 2730 times brighter than today. All crammed into a few million years! A Desert Earth might remain habitable to about 7 billion AD or so.
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