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Making it into another earth would be nice if it were possible, but according to our more recent atmospheric models, Venus may simply lie too close to the sun to retain large oceans without having them boil off at the equator and causing a runaway greenhouse. These models could be deficient of course (and I believe they are... pretty sure early earth should've had a runaway greenhouse too under such models since it had a vastly thicker atmosphere with 20% CO2, yet it's supposedly just 5% within the habitable zone... I am a bit skeptical of all of that). Nevertheless, it could be that Venus is too close to be sustainable as a wet planet without some kind of massive solar shield protecting it at all times.
It does fit well within the arid habitable zone under our models though. If H2O in the atmosphere were kept around 1%, and the planet were converted into a dune-like desert planet, it could sustain livable temperatures without any solar shield, and it would be economically useful to us in that form anyway.
If Venus is in the habitable zone, then why isn't it habitable? the calculations for temperature excludes the greenhouse effect. One idea I've got is what happens if we just add water, not necessarily to create oceans, but just enough to saturate its atmosphere? Lets say we crash several comets made mostly of water into Venus, this adds water vapor to the atmosphere, Water vapor is a greenhouse gas, the ground heats up, but what about the upper atmosphere at say 55 km? Air is clear, so the mere fact of light passing through air doesn't heat it up, as air is mostly transparent. Water clouds will reflect light back into space. If we added water to Venus, this would dilute the sulfuric acid. We'd end up with a water cycle where it rains and evaporates without any water ever touching the ground.
Without the greenhouse effect Venus's surface would be 15 degrees C, assuming it maintained an earth-like albedo. Albedo plays a large role in these calculations as well, because with a lower one (darker than earth), you'd be looking at 50 C average temperatures. Earth's would be somewhere around -15.
As for why Venus is a dead place now, the distance to the Sun is an important factor, but also the failure to sequester atmospheric carbon and methane fast enough. Earth's ability to thrive hinged on its ability to remove the ancient greenhouse gases the planet's atmosphere contained in a timely manner. Earth once had much higher CO2 levels in its atmosphere, and higher average surface temperatures (a billion years ago). With those same conditions at Venus's hotter orbit, the greenhouse effect eventually became strong enough to boil the oceans, which then took ~250 million years to lose their hydrogen to space in their atmospheric form. Earth naturally sequesters atmospheric carbon through plate tectonics, in combination with its oceans. Venus doesn't have plate tectonics; its geology is entirely volcanic and the planet takes 200 million more years to renew its surface than Earth.
There wasn't a natural way for Venus to be a habitable place in its orbit due to its geology. A planet with more active plate tectonics could've been just fine there. And if its atmosphere were converted into an earthlike one through artificial means, it could maintain a stable hydrology.
You have to prevent half the light from reaching Venus in the first place. The main problem with Venus and the reason it is such a high pressure oven is because it receives too much energy from the Sun.
It is not necessary to block any light from the planet. It's still in the habitable zone. Blackbody calculations suggest it would have a mean temperature of 30-40 degrees C with an Earthlike atmosphere. (Earth is 16 degrees, for comparison). It would be a warm planet for human standards, but definitely livable.
The issue of rotation is likely a non-issue as well. According to recent models of tidally locked planets orbiting red dwarf stars, which are important places to look in the search for extraterrestrial life, it appears that intense cloud formations dominate the sun-facing side. Similar mechanisms are likely to occur on Venus due to its low spin.
The blunt answer is to focus on Callisto first, which is the only moon outside Jupiter's radiation belts. Until a better solution to the radiation is thought of, the other moons could only be colonized at most.
I used to think Ganymede could be a very interesting terraformed world but that moon receives 0.08 Sv per day. For perspective, Earth receives 0.024 Sv in a year. Ganymede's magnetic field is too weak to stop the incoming radiation. Living on the surface would be like getting two chest x-rays a day. The radiation is so much worse on Europa and Io that any talk of living on those is basically preposterous. 5.4 Sv on the surface of Europa. That's like standing right next to Chernobyl as it was exploding.
I will say I didn't realize just how high it was on Europa until I looked it up for this post, and I'm still trying to wrap my head around that number.
http://www.space.com/22207-jupiter-moon … aphic.html
Check it for yourself. Is that wrong?
Regarding all this talk about changing the planet's spin, I'm not sure this is necessary.
http://arxiv.org/abs/1307.0515
Assuming we could achieve some kind of global ocean on the planet, giving it some fraction of the hydrosphere on Earth, I see no reason why this cloud model for tidally locked planets wouldn't also occur on Venus, allowing it to be habitable despite its slow rotation.
I will probably post more on the subject later. Venus is a great deal of interest to me--much moreso than Mars as far as terraforming is concerned. It's a larger planet with a lot more mineral resources to reap, and the gravity would be much more ideal for humans.
However one issue with this is the global resurfacing of Venus every few hundred million years (instead of steady continental drift). Perhaps you would do better to leave a fair bit of the atmosphere there which would also help to insulate your colonies from present day eruptions also as Venus is probably geologically active.
http://www.space.com/9155-hellish-venus … ffect.html
According to newer models of Venus's interior, it appears likely that the global resurfacing phenomenon on Venus is actually a result of its thick atmosphere. This also has the dual effect of explaining Venus's lack of a magnetic field.
As heat released from the interior of the planet builds up in the atmosphere, it eventually reaches a much higher temperature than it currently is at the surface, and the entire crust becomes mobilized. When this happens, the interface between the mantle and space is much more conductive as a result, with heat leaking out of the planet at a far faster rate than it does on Earth. Eventually the mantle cools and the surface solidifies again, and a steady trickle of volcanoes begin the process anew for the next few hundred million years, as the core reheats the mantle.
I find this very interesting. If you boiled off the oceans on Earth, you would literally be looking at the same exact planet in 250 million years. It would take only a couple hundred thousand years for the atmospheric oceans to lose their hydrogen to space. In that short span the planet would dry out like Venus. The surface temperatures would skyrocket, the crust would eventually mobilize, and Earth's interior would lose a large percentage of its thermal energy in a short span of time, losing its geomagnetic dynamo. After the surface cools there is no longer enough energy in the mantle to power plate tectonics, and a steady trickle of volcanoes builds up heat in the atmosphere again.
If you rewind all of that, Venus could've easily looked like Earth as recent as 1 billion years ago, before things went wrong.
So permanent 1.1g would kill somebody?
No, you'd barely notice the difference. Much higher than that probably.
Don't think that higher gravity necessarily means stronger muscles/bones. It could mean exactly the opposite, with the body fatiguing, growing really short and losing a lot of mass to stay comfortable in the strong Gs.
That would be really high though. Anything between 0.75-1.25 should probably feel right at home. That's the difference between walking around with a heavy backpack or with a strong wind.
I think your pretty close to the 35c day temperatures on the day side, maybe a bit hotter in specific locations.
That is an average for the entire planet. It's a hot planet! But you have to think about how that average is distributed. The high temperatures will be close to or in the 40s, and the low temperatures will still be below freezing, causing permanent icecaps at the poles. They will be much smaller icecaps than Earth's, and much warmer. Ours are well into the negative double digits. These would be tenuous, maybe thawing occasionally, in a permanent state of twilight.
As for the speculation about seasons, I'm pretty sure they would be insignificant. Maybe chilly, but never cold enough to freeze at the equator. When you have ocean currents encircling the planet, temperature tends to be regulated pretty well.
Our planet would go from 100 to -100 on the equator every day if we didn't have an atmosphere and oceans to act like a thermal conduit.
I do agree with nickname about the rain though. The weather would probably be ridiculous.
Spatula,
I see an early Venus more in the 50c range with a few bars of co2 and 3 of nitrogen.
I also see a problem for that atmosphere trying to boil off its oceans to get to the 92 bars we see today.
Right, and that's right. My 25 C figure is in reference to Venus without any atmosphere at all. It's an estimate though, for situations where the planet is a greybody with a similar composition to earth on the crust. As a blackbody, reflecting nothing that hits it, the temperature increases quite a bit. Add oceans, and that completely changes too.
A 25c temperature on a Venus with a nitrogen oxygen atmosphere like ours seems pretty straight forward.
Interesting to know that if we did teraform Venus it would tend to stay that way.
Unfortunately, with 1 bar of atmosphere the planet would probably be hotter than 25 C. The oceans would be below 25 C, in no danger of boiling off, but the atmosphere might be in the borderline uncomfortable range, 35 C or higher maybe. Such a planet would only be comfortable in the temperate regions or poles. We could take measures to artificially cool it though. Extra atmospheric oxygen, possibly. Maybe just make the atmosphere thinner, I'm just throwing ideas out!
Not so sure about the day night side staying equal.
A teraformed Venus would have pretty cool nights and pretty hot days.
Those temperature difference are sure to cause some pretty intense weather.
An atmosphere of 0.1 bars is enough to stabilize the temperature on both sides to an earthy range, but there is some variation in the models. Some show ice caps forming on on the night side. It's difficult to be certain. Part of it is that the planet is simply hotter. It will take longer to lose heat on the night side through blackbody radiation, because there's more of it.
Part of it is that the planet has no axial tilt, so the yearly temperature variation would be the same as Earth's equator, everywhere. No seasons. There are other small details.
I think the main problem with these Venus threads is that people see how close the planet is to the Sun, and they immediately assume it would be hot without the thick atmosphere.
They also see the slow rotation and think 'ah, much hotter days.'
Without any atmosphere, the predicted temperature for the planet is around 25 degrees C.
The disparity in temperature between days and nights would not be significantly greater than Earth's.
When the concept of exoplanets that were tidally locked to their stars came up, it generated a lot of interest, and the scientific community spent quite a bit of time modeling these to see if life would be possible on them. It was thought for a long time that earthy atmospheres would simply get blown off from the turbulence, but apparently the presence of a water cycle, and greenhouse gases, is sufficient at distributing heat and keeping the prevailing wind speeds down.
You are wrong.
Oxygen oxidised the rocks and it never remains in the atmosphere for more than million years without replenishing and there is no reason why Venus was formed with little water.
Venus does not have the big layer of rust Mars has, though even if it did that wouldn't be nearly enough oxygen for a significant ocean.
Oxygen has nothing to react with on Venus... all of the surface rocks and atmospheric compounds are oxygen-rich igneous formations, just as they are on Earth. Earth and Venus started out with a very large percent of oxygen in their mass. The planets are ready-made with every oxidation reaction completed, and atmospheric oxygen is only derived from water or organic processes, after which it has nowhere to go.
Maybe I'm wrong and we've been breathing leprechauns.... maybe.
If we were to assume that all of the oxygen in the atmosphere used to be from water, at best we'd get 10% the hydrosphere of Earth, but this is unlikely, because it would only result from a very oxygen-poor planet. Venus has about as much CO2 on its surface as Earth, just in a different chemical form. If we say it started with none... that's getting into dangerously ridiculous territory.
For us oxygen is an essential gas but it a strong oxidant that never remains in the atmosphere of a planet without plantlife.
And there are forms of life on Earth for what it is extremely toxic and oxidise them in a while;
This I just...aah... it's not even true in sort of a 'well I can kind of see what you mean' sort of way. Ganymede and Europa have tenuous oxygen atmospheres. These formed from the same processes you're claiming removed Earthloads of water from Venus. This is not a fast thing. It happens over billions of years, and that's how long much of the oxygen in these atmospheres has been floating around for.
And WHY BOTHER GATHERING ICE WHEN YOU COULD CONVERT ACID TO WATER!?H2SO4>REMOVE SULFUR>H2O+S8.USE HEAD NOT "ASSUMING".
And that thing that you said about stellar evolution is a nonsense.
If we converted all of the H2SO4 to water, we'd have a few lakes worth. If we want a sizable fraction of Earth's hydrosphere, it would have to be several times the volume of Venus's current atmosphere. That's the main reason.
Spatula, the temperature on Venus' surface is ~465C which will melt lead. This is above the critical temperature of water (374C), where no matter what the pressure it is at it does not form a liquid. So there will be no water to dissolve CO2 and do the other things you mention. The water in the atmosphere, will photo-disassociate and the hydrogen will be lost. I believe I've read that NASA space probes have detected hydrogen being lost from Venus. Anyone have a link?
Something to consider is that when most of the atmosphere becomes water by composition, how heat will be distributed in such an atmosphere.
Antius, when the sun was only 70% as hot as it is now, Venus was thought to have a hot wet greenhouse. The temperature then was enough to evaporate the seas. The carbonates were baked out of rocks releasing CO2 and Venus has turned into the dry greenhouse it now is as the hydrogen was lost.
I think I made a good case for the probability that Venus never had appreciable amounts of water. Stellar evolution models and the lack of atmospheric oxygen would preclude significant water in Venus's past. It was always a furnace.
Now if we add water (which is a VERY powerful greenhouse gas) we make the temperature problems worse. If we could somehow turn the CO2 atmosphere into oxygen then Venus will then radiate a lot of its heat and we might be able to get it to a stable state. But for that to happen, you have to have liquid water at the same place as the phosphorus, iodine, iron & other trace elements. I don't see this happening unless the planet is made much cooler, presumably with some sort of space mirror or the like.
Water is a very strong greenhouse gas, but unlike CO2 it undergoes phases. Hot water vapor is not as dense as CO2, and rises quickly to the top of the atmosphere, taking heat away from the surface. Upon radiating this heat into space, it precipitates to the surface as liquid or solid depending on the atmospheric temperatures.
The water itself traps a lot of infrared radiation due to its polar hydrogen bonds, but because of its phases it tends to be more of an anti-greenhouse gas in planetary settings.
For more anti-greenhouse effects, see Mars. Mars is so cold that CO2 experiences phases on its surface, and because CO2 can freeze and sublimate, it decreases the planet's temperature further. Or perhaps Titan, where Methane, another greenhouse gas, experiences phases and cools the moon off. Or perhaps Pluto, where the cryogenic temperatures are low enough for Nitrogen to have phases and act in a similar fashion.
Venus is a lot hotter than Earth (1.913 times the Earth's insolation), so its oceans will always be in danger of ending up in the run away wet greenhouse that it suffered when Venus was getting 1.33 times Earth's current insolation. This suggests to me that we will ALWAYS want to have a mirror reducing the amount of heat it gets from the sun, or be adding more floating mirrors, manually sequestering CO2, moving Venus farther from the sun, etc. It just sounds like more work than for Mars.
A good concern. With increasing levels of light comes increasing reflectivity. As a material gains heat energy, its specific heat increases, causing it to have trouble gaining further energy. Instead it becomes reflective. A lot of this ties into black-body radiation - as it gets brighter, an object will act more and more like a white body. Venus's natural temperature without the greenhouse effect should not be twice as high as Earth's. Closer to the edge of livability, but well within it.
Floating mirrors are the objects of sci-fi, and will probably remain that way, concerning terraforming. If we screw up, and the planet is still a bit too hot, supplementing the atmosphere with reflective particles and anti-greenhouse gases would push it into safe territory.
RD, I'm always glad to see your posts on this topic. I think there is some support here for terraforming the second planet, but a lot of misinformation generally abounds when people talk about it.
The lack of water is simple enough to fix. Redirect comets from the outer solar system to Venus.
Adding a significant proportion of earth's ocean water to Venus would have a huge impact on the planet without life. Think about it for a bit.
-The atmosphere becomes hundreds of times thicker.
-Sunlight never reaches the surface. It is completely dark. The slow process of cooling can begin.
-Water can condense at higher pressures and temperatures.
-CO2 dissolves readily in water, because it is polar.
As for seeding the atmosphere with algea, there are many problems with this.
-- First, we know of no cyanobacteria that can live in sulfuric acid.
-- Second we know of no cyanobacteria that can live in the upper atmosphere (or indeed have positive boyancy).
-- Third, the upper atmosphere is totally lacking in Sodium, Magnesium, Potassium, Calcium, Phosphorus, Selenium, Iron, Copper, Zinc, Iodine and many other trace elements needed to form cells. (This is the same reason why it is unlikely we will find life floating in Jupiter's atmosphere.)
-- Fourth, all known life requires liquid water. None (I know of) grows in micrometer (which are smaller than bacteria) sized droplets of water (let alone concentrated sulfuric acid).
Not problems.
-Acidophiles, read up on them.
-Venus's atmosphere is a lot thicker than 1g/mL, and quite a few things can live above that on Earth.
-With an active water-cycle, materials on the surface would dissolve and get picked up in trace amounts in the atmosphere. Not much, but enough.
-Microorganisms, especially spores, migrate around the planet in airborne water droplets. Helps them propagate.
Carbon is not soil. Soil is made up of fungus, bacteria, decomposing life, grains of minerals ions, microscopic mites, etc.
Topsoil is, but below that is a much larger layer of subsoil, which is a combination of rock and organic particles chemically derived from it. The subsoil is what really matters, as it contains most of the organic material above the planet's surface, and it is necessary for forming topsoil. It's much harder to assemble from the stratum underneath it though, that's why we have so many threads about it.
Wrong.Read something about runaway greenhouse.
And Sun was cooler when it was young.
When the Sun formed, over the first few million years, it was substantially hotter than it is now. When it reached the main sequence it cooled off, but by then we had the planets close to their current configuration.
Most of the mass of the atmosphere could come from the moon, as could 90% of the mass of the water needed to produce an active hydrosphere.
A lunar atmosphere consisting of 90% pure oxygen and perhaps 10% water vapour at 1/3rd bar, would produce the same O2 concentration in human blood as Earth's atmosphere. Only trace amounts of nitrogen would actually be needed.
Don't forget that under 1/6th earth gravity, it would take an atmosphere twice as dense as Earth's to create 1/3rd pressure at the Moon's surface.
If, at any point, we have to rely on industrial refining to get something, such as chemically extracting components from the moon's crust, we might as well give up. This would be an impossibly expensive step to conduct on a planetary scale. Getting stuff like Oxygen without refining would be insurmountable.
The best thing I can think of is genetically engineering a very sturdy type of bacteria to pull it out of the minerals in the crust.
I'm not saying it's impossible to terraform the Moon. I'm saying that for the cost, it's not worth it.
Forget it. The Moon's too small to be properly terraformed at the surface.
Feel free to intellectually cripple yourself with premature, ill-informed judgements - god knows you won't be in the minority - the rest of us will continue to examine the possibility until actual evidence rules it out, as if scientists.
Have fun figuring out how to import water, nitrogen, oxygen, carbon, phosphorus, sulfur, and lots of other fun ingredients to this dead rock then. I'll stick with Mars and Venus for now, which have at least some of those in appreciable quantities to make it seem economically worthwhile. Jupiter's moons are a long-shot, but I see the possibility for them.
To terraform the Moon, the planetoid itself would contribute nothing to the effort except gravity, and not particularly good gravity in of itself. We'd have to build a colossal magnetic field to supplement it. Everything else we're just adding from other things. Tens of thousands of TNOs would have to contribute materials. This is quite an operation.
I'm going to agree with RobertDyck on this one. We have four other better candidates for terraforming. Earth's Moon has no distinguishing characteristic that gives it an advantage over devoting resources to terraforming these other worlds. But that's not to say it won't be colonized heavily and be a very important center for growth. Without an atmosphere we'll be able to build up many industries that require high thermal efficiency. It's close to a habitable world, it has useful mineral resources as-is, and it happens to be a good place to set up telescopes and communications systems to talk to all of the other worlds being colonized without atmospheric interference.
With the low gravity birds will have much greater advantages than they would on Earth. Birds and fish would be the way to go.
Forget it. The Moon's too small to be properly terraformed at the surface. It also seems to be valuable simply as a vacuum staging area for lots of industries. Subsurface colonies that utilize the deep interior heat might work though.
Speaking of Nitrogen, I actually did think of a way to get large amounts of Nitrogen to Mars finally.
There are a lot of TNOs that should have a high percentage of Nitrogen frozen throughout them. That part of the solar system is cold enough to keep the room temperature gas locked in solid form. Redirect the most Nitrogen rich comets on a trajectory that crashes them into Mars. This has to be done quickly, of course, since they'll leak gas all the way over.
Interesting. Just for the fun of it, I did some calculations to figure out the mean density of the Earth-Luna binary and got 5.472 g/cm3. The combined mass of Earth and Luna is 6.0494e24kg. The density of Earth is 5.515g/cm3 which is 5515kg/m3 for a total volume therefore of 1.0835e21 cubic metres. Doing the same math for Luna gives it a total volume of 2.2002e19m3. You add the two volumes together and divide into the total mass and get an average density. I wondered if this might drop the denisty below the figure for Venus (5250kg/m3) but no such luck.
Mercury is only more dense than Earth if you account for gravitational compression. Earth has much more mass, so its gravity increases its density, despite being made of lighter materials. The same applies to Venus, so you would actually get gradually decreasing density in order from Mercury to Mars.
Note that the outer planets are much further apart than their inner counterparts. These giants probably experienced rapid inward migration during their formations, so predicting their densities by their positions shouldn't be very possible. What we have is a situation where more material is located further out, but denser material is located further in. Planets close to the sun don't have enough material in their orbits to form giants, and planets far away accrete rapidly, and move inward.
Mars, for instance, probably would be more massive if Jupiter hadn't cleared a lot of material from its own orbit.
Hot Jupiters are just an extreme effect of this process. A planet forms fast enough to move right up next to its star.
Pretty reasonable. I would put 10-15% Earth's water as the potential upper limit Venus could've had, with something around 1-2% as the most likely amount. Now it wouldn't even be comparable. 1.86 mBar, pretty pitiful.
The column density of Venus' atmosphere is 1000 tonnes CO2 per m2. If it were liquified, it would form a global CO2 ocean with a depth averaging 500 metres. That is a lot more carbon than we have here on Earth, even taking into account things like deep limestone deposits. The biosphere is a relatively thin layer, confined to the top few inches of the Earth's surface, on average. And it would be impossible to start a biosphere on Venus of any meaningful scale on Venus.
I really do believe that the idea of terraforming Venus is a non-starter.
That's the same amount of CO2 we have on Earth, all over the crust. Otherwise known as dirt. Big ingredient in carbonates. CO2 is the basic building unit in organic chemistry. It's essentially a dehydrated carboxylic acid. Chain them together and you've got a sugar.
The abundance of CO2 will make things a lot easier for us, as it would on Mars. Instead of building massive factories to convert things on a planetary scale, we can use bacteria to do all the heavy work.
Using the 60 bars of leftover oxygen to "fix carbon into better solid forms".
That leftover oxygen won't exist unless you import water. And it would not be easier to import hydrogen, since you'd have to construct ships to hold it. Water is already locked into solid forms in comets. Just give it a little push so that their orbits intersect Venus.
Look, we probably will have to use hydrogen imports at some point to sequester extra oxygen as it piles up, but if we converted all of the oxygen in the atmosphere into water, we'd get a hydrosphere that was maybe 10% the size of Earth's if we're lucky. We're going to need a bit more water after that. And by a bit, I mean a lot. No matter how you cut it, most of the imported material to Venus is going to be water.
Math... hmm. The best I can do is show that you're not going to get a 1:1 ratio of CO2 to O2 conversion in the atmosphere, from organic processes.
Phosphates, carbonates, silicates, sugars, all have more oxygen than carbon, overall.
It actually does need the oxygen for fixing the carbon into better solid forms.
Look at it this way, if we import hydrogen, using the bosch reaction to form water and carbon from the carbon dioxide atmosphere, we'll be left with a pretty thick carbon layer on the crust that's essentially dead. It's also black, and it would really bring up the surface and atmospheric temperatures. How do you propose to get rid of it?
And just where are you going to get hydrogen from? Electrolysis? Don't be silly.
Import water and aerobic microbes, and they'll eat the atmosphere up, fix all the carbon and some of the oxygen into their organic chemistry. Extra oxygen will have somewhat of an anti-greenhouse effect; it will form an ozone layer, and it will dissolve in the planetary ocean when it forms. But most importantly, we're not just importing pure water. It's actually dirty water, from comets. The metals from these comets, along with the planet's surface, will help to eat up extra oxygen, forming minerals.
When we think about terraforming, we often only think about the first half of it - 'giving it an Earth-like atmosphere'. Part of terraforming is also getting enough life onto a planet to keep it in ecopoiesis. A lot of methods people propose for terraforming Mars and Venus result in a dead world. Part of the trick is giving life a jump-start while the process is still occurring.
Spatula,
Could be just the sulphur has gobbled up all the free oxygen.
Sulphur dioxide.If Venus has had volcanic activity for most of its life then the quantity of sulphur should be a good candidate to gobble it up.
It sounds like a decent creation theory for Venus, so its probably wrong LOL
Still not nearly enough SO2 to account for oceans worth of water. Which is good, because if the Sulfur Iodine cycle really were responsible for eating Venus's oceans, Earth would look like Venus too. We have exactly the same process here, due to Earth's volcanic history. Alternatively, oxygen could be on the surface as FeO2, as it is on Earth and Mars, or in other mineral forms. It probably is, but still can't store nearly enough oxygen to make a dent in Earth's oceans.
We're talking about enough water to make a very respectable sized moon.
I agree that there is no reason Venus could not hold an impact formed moon, but such a moon is born out of very special circumstances in an impact. What I am saying is that there appears to be some conditions in the stellar disk which work against formation of planetary satellites in the inner system. It may be just "material deprivation" in the womb.
What special circumstances? 11% of TNOs are in multiple-body systems. That ratio goes up a lot the larger they get. This region of the solar system is a lot like the early solar system, but frozen in time. These objects were far enough away from each other and the Sun to remain in the same region without coalescing. In the inner solar system, large collisions and moon formations should be similarly common, but most of them wouldn't be stable over geological periods.
Perfectly fitting for this case. Though, as Nickname said, the destroyed moon isn't the only explanation for Venus's rotation or surface composition.