[spdustin]

The moon's core isn't made of the swirling molten iron dynamo like Earth's is. No swirling molten iron, no magnetic field. No magnetic field, nothing deflects the solar wind that then, literally, blows away any fledgling atmosphere.

It's believed that Mars' atmosphere was considerably more dense than it is today, but along the way, its core cooled, and it lost its protective magnetic shield, allowing the solar wind to strip away much of its atmosphere.

[drdrey]

Even with a magnetic field, would the moon's gravity be sufficient to retain an atmosphere?

[simulate]

Yes, it is possible that with a magnetic field the moon's gravity would be sufficient to retain an atmosphere. Titan has an atmosphere denser than earth's atmosphere despite having a lower gravity than the moon. There are a few hypotheses why and one of them is that, although Titan has no magnetic field of its own, it orbits inside Saturn's magnetosphere. https://astronomy.stackexchange.com/questions/8345/how-does-...

[SimbaOnSteroids]

No, for that matter Mars doesn't have the gravity to retain an atmosphere either. I can't cite a source on this because I don't remember where I read it but I believe having a magnetic field surrounding a celestial body is a relatively minor factor in atmosphere retention compared to mass. If someone who specializes in planetary atmosphere retention knows better feel free to eviscerate me.

[pavel_lishin]

Someone replied mentioning that Titan has a denser atmosphere than Earth, despite being lighter than the Moon: https://news.ycombinator.com/item?id=15531798

[logfromblammo]

Temperature is also a factor.

PV=nRT. T is temperature.

Titan is not only protected by Saturn's magnetic field, but it is also very cold. Some of the molecules that are gaseous on Earth are liquids on Titan. Cold gases have lower pressure, and less likelihood of bouncing a light molecule high enough up in the atmosphere that the solar wind can grab it and blow it away.

Earth has a hard time holding on to light molecules like H2 and He, but its He is replenished somewhat by alpha decay, and it takes a long time to get from the inside of a rock to the upper reaches of the atmosphere. Most of the hydrogen is attached to heavier molecules. But it happens eventually, and even the Earth's magnetic field and gravity can't keep them. Venus is almost as massive as Earth, but it is hotter than Mercury and has no core-generated magnetic field. So most of its water has already thermally dissociated (which happens slowly starting at around 800 degC) and the H, H2, He, and monoatomic O bounces high up into the atmosphere, ionizes, and blows away. So now Venus has about 90 bar of CO2 and barely any water left.

One of the terraforming proposals for Venus is to transport a large quantity of hydrogen from Jupiter to Venus, and use Fe catalyst to react it with the CO2, to get graphite C, H2O, and O2. That would strip off much of the greenhouse blanket, but the planet would still have to be cooled off and protected from the solar wind to keep all that hydrogen around on a geologic time scale.

[eradicatethots]

>feel free to eviscerate me

Off topic but I’d like it quite a lot if people could be informative without being eviscerating lol. I suppose this doesn’t usually happen, but it should happen more. That’d be so much better, so much more tolerable and happier. It’d be good for all of us probably lol.

[averagewall]

I think eviscerating comes when people present false statements as facts. Qualifying it with uncertainty as the OP did probably won't anger anybody. But telling lies because you don't know what you're talking about justifiably does.

[brw12]

You're pointing to an important difference: between being uninformed but trying to get towards being informed, and being uninformed but fiercely protective of your certainty.

What Would Feynman Do?

In this case, I'm imagining he'd say these are some of the things we think might explain bodies' keeping or losing atmosphere, but we have to remember that almost everything we think we know about planets and moons comes from looking at the light that bounces off of them and making guesses about the underlying rules, but we still don't know do much about the underlying rules of physics, so there's probably more that we don't know than what we do know about atmosphere retention.

[u801e]

I've read that it's the same case with Venus' core, but its atmosphere is far more dense than either Earth's or Mars'.

[Scaevolus]

Venus has an induced magnetic field that may protect its atmosphere. https://www.astrobio.net/also-in-news/a-magnetic-surprise-fr...

[nonbel]

Apparently so does Mars though? http://www.sciencedirect.com/science/article/pii/S0019103509...

[kirillkh]

Then we have no hope of ever engineering Mars to have enough air for us to breath?

[LeifCarrotson]

A Martian atmosphere (not sure about a lunar atmosphere) would be lost on a timescale of tens of millions of years. That's very quick in cosmological timescales, but slow enough that any human effort to create or replenish it could be very successful.

[waegawegawe]

That said, the earth's atmosphere has a mass of about five billion billion tons. For reference, as a species, we produce about ten billion tons of concrete each year. This is just to give a sense to the scale of effort involved in replacing a planetary atmosphere.

[larkeith]

Depending on the volume of surface ice (especially at the poles), might it be possible to produce atmosphere on a massive scale via orbital lenses or mirrors? With recent advances in solar sail technology, I can't imagine the implementation would be too far removed from current capabilities.

[waegawegawe]

Think about how large of a lens you're talking about. Even if it were one hundred meters across and was able to collect 100% of the sun's energy passing through it, the amount of energy produced would be utterly insignificant compared to the problem we're discussing. And how would you get a one hundred meter lens to mars orbit? Even after that, you have to consider that we want an oxygen atmosphere, not one made of water vapors.

I don't know what you mean by "too far removed from current capabilities", but I doubt we'll even start working on the problem for two or three centuries.

No, it seems more likely to me that we'd use our growing knowledge of genetics and psychology to hack out the part of ourselves that needs to be outside, and opt for a purely enclosed existence on Mars.

[larkeith]

The IKAROS sail (launched 2010) is 196 m^2, using aluminum as a reflector (about 90% efficiency). With regards to transferring a lens to orbit, that is a self-solving problem - a large lens or mirror can both be used as a solar sail, potentially even hauling additional mass to Mars orbit.

Mars receives 593 W/m^2 flux, so each IKAROS-sized reflector could produce about 100 KW of energy. Given the expected difficulty of large scale terraforming and colonization efforts, it seems the cost of, say, the equivalent of 10,000 IKAROS-sized mirrors (~1 GW, comparable to a large nuclear plant) would be relatively minor.

Whether that would be more cost effective than shipping an equivalently powerful reactor or other generator is questionable - it will presumably depend on our lifting capacities.

[kobeya]

That's about ~250x the JWST. Given that we're talking about a society that has developed far enough to be sending people to Mars and terraforming the landscape, I don't think that 8 doublings would be an unreasonable multiplier of current capability. Still a flagship, many-decade mission though.

[hutzlibu]

With lenses you could also burn the soil to produce gases that add up to he atmosphere ... but we want breathable atmosphere. With lot's of oxygen and very little co2. And that is a bit harder ...

So you also couldn't just vaporize the ice, you need to split it up. Solar heat could be sufficient, but the water will then be missed everywhere else on mars where life wants to grow.

And Mars is dry.

So I would first use the water in enclosed habitats. And then after, if there is plenty of water left, one could start to think about smoking that up ..

But there might be other options, once you have lot's and lot's of autonomous machines and rockets available and allmost unlimited fuel (sun?). But without that? Not a chance ...

[larkeith]

I wonder if it would be a reasonable colonization process to establish a non-breathable atmosphere, allowing postprocessing facilities to later separate oxygen from airborne water vapour.

The notable benefits from such an approach would be the ability to easily deliver asteroid-based water deposits (aim it at Mars, let reentry do the rest), as well as the significant simplification of ground based colonization technology - it's far easier to build resilient habitats for a non-breathable atmosphere than it is for a vacuum, and the risk of accidents and difficulty of venturing outside is much, much lessened (a face mask or filter and oxygen tank instead of a bulky spacesuit), not to mention the radiation protection afforded by a thick atmosphere.

[kobeya]

Also many thermoforming efforts consider adding an artificial dynamo (using, say, superconducting rings) to protect both the atmosphere and life on the surface. The energy cost of maintaining an artificial dynamo would be less than the incremental maintenance cost of maintaining atmosphere and dealing with health risks from radiation.

[jlebrech]

we could also use the resources on mars or the moon more sparingly, making dome bases would provide that.

we could also bore into those tunnels and put caps on the hole afterward, have some of those channel light into those tunnels too.

[icanhackit]

NASA proposes building artificial magnetic field to restore Mars’ atmosphere: https://www.universetoday.com/134052/nasa-proposes-magnetic-...

[extrapickles]

Interesting, it only needs a 1-2 Tesla magnetic field. MRI machines go up to 3T, which means this isn't too outlandish with known technology. The main issue is that they probably want a Mars sized field at 1-2T.

[andai]

Does that mean it needs a bigger coil?

[larkeith]

I don't know the rate of atmospheric loss, but it might be possible to introduce air at a high enough rate to outstrip the losses, making for a sustainable atmosphere. It would presumably depend on the availability of ice and other volatiles.

[abandonliberty]

Unlikely because you can't use a noun as a verb :)

Someday though we may be able to breathe on Mars