Tiny Atmospheres For Small Worlds

Terraformer · 2021-11-18 11:53:55

Calliban wrote:

Most meteors burn up above a height of 80km, at which point pressure is around 1 microbar. That is a column density of about 0.01kg/m2. For the whole moon, with a diameter of 3480km, an atmosphere of that column density would mass around 380 million tonnes. That is an achievable amount if large scale human presence comes to fruition on the moon. If a lunar magnetic field were put in place, then eventually a combination of human and natural inputs would provide sufficient gas through rocket exhaust and ore processing.

Given that micrometeroids would cause damage to surface equipment, and especially to greenhouses, it may make sense to create very thin atmospheres if possible on the smaller bodies, such as Saturns moons, or Ceres. Even if these worlds cannot hold an atmosphere of any significance, a para-terraforming effort will need to be protected from damage.

On Ceres, that column density would require around 30 million tonnes of gas. By the point it reaches ceres-stationary orbit some 750km up, it should cease to be coupled. In any case, perhaps a magnetic field could grab it back Less of an issue on the outer worlds, given their slow rotation.

Ganymede may already have a suitable atmosphere.

Calliban · 2021-11-19 14:23:07

If the world is icy, then raising temperature will result in evapouration. Waste heat from the power supply will do this naturally.

Vapour pressure of CO2
https://en.m.wikipedia.org/wiki/Carbon_ … data_page)

Vapour pressure of water
https://www.lyotechnology.com/vapor-pressure-of-ice.cfm

Both Callisto and Ganymede contain CO2 ice in their crust. Warming these bodies up would generate thin CO2 atmospheres. In the case of Ganymede, a thicker atmosphere would be useful in shielding out Jupiter radiation. The alternative is either a powerful magnetic field, and electric field or simply to live underground.

The flux of Jupiter radiation is dominated by electrons with energy up to 5MeV. The range of 5MeV beta particles in air is 10m. This indicates a column density of about 12kg/m2 will be needed to shield out the worst effects of Jupiter radiation. On Ganymede, that is equal to a surface pressure of about 0.02mbar, or 20microbar. It wouldn't take a great deal of heating to generate an atmosphere of the pressure from CO2 on Ganymede. Juno used a half inch titanium alloy shield to reduce radiation intensity experienced by its electronics by a factor of several hundred. That is enough column density to reduce surface dose rate on Ganymede to ~0.01rem per day. That equates to a column density of about 50kg/m2. That would equate to a pressure of about 80 microbars.

Both figures would appear to be achievable with modest amounts of heating.

The pressure at the top of the mesosphere on Earth has a pressure of 10 microbar. This is where most micrometeorites burn up. At this height most of the atmosphere is composed of ions. These can be contained by magnetic traps as Terraformer suggested. A column density of 180kg/m2, equivalent to Mars atmosphere, would shield out the worst of cosmic rays, reducing intensity the half. The most damaging heavy ion radiation is completely shielded out at this column density. What remains are protons with an RBE of ~1.

On Ceres, the scale height of a CO2 atmosphere with a temperature of 150K, is almost exactly 100km at the surface. However, Ceres is so small that gravitational field strength declines very rapidly with increasing height. The second scale height (@100km) is 150km. The third scale height at 250km, is 233km. Therefore, at a height of 483km, atmospheric pressure and density have declined by e^3, or a factor of 20. As pressure is assumed to be 1 microbar at the surface, pressure at 483km, will 50nbar. At this altitude, non-ionic gas molecules would escape rapidly. A magnetic field would at least partially protect the atmosphere from complete stripping by the solar wind.

In principle, Ceres could be completely enclosed by a glass world house, by anchoring sections of steel frame to the ground with basalt fibre cables, which are fitted with glass windows. A thin atmosphere above the world house would protect the structure from micro-meteorite impacts. It might be possible to do something similar for even smaller asteroids, by using magnetic fields to contain thin ionic atmospheres, which protect the world house from impacts. The gas used in the ionic portion will probably be quite heavy, to reduce molecular speeds. Sulphur dioxide would be a good choice, as sulphur is relative abundant in volcanic regoliths. The glass world house structure would consist of modular hexagonal domes, with basalt fibre cables transferring load into the ground. For a 50kPa atmosphere, basalt fibres needed to transfer the load would need an area of 1E-4 square metres per pressurised metre.

The further you get from the sun, the less value sunlight provides to large transparent structures. I suspect that habitable volume will be built in underground tunnels and artificial lighting used to grow plants. On Jupiter's moons this will almost certainly make sense, as it conserves heat and shields colonists from dangerous radiation.

Last edited by Calliban (2021-11-19 17:46:01)

Calliban · 2022-03-25 18:31:45

I just ran a few calculations that suggest we may be able to confine thin atmospheres on small bodies using magnetic fields. Taking the example of Eros, which has an average diameter of 20km. A conducting ring around Eros, with a circumference of 60km, carrying a current of 40A, would generate a magnetic pressure of 1mbar. A 2cm thick aluminium cable would mass some 50 tonnes, would have resistance 5.06 ohm. A potential difference of 202 Volts would be needed, requiring a power of 8kW.

The 1mbar magnetic pressure would inflate a magnetic bubble of solar wind gases hundreds of km in diameter, with ion density increasing closer to the cable and average temperature declining closer to the surface. The plasma pressure may allow a heavier gas to be introduced at the surface of the asteroid, with buoyancy keeping the lighter solar wind gases above the denser gas. I have no idea how much pressure and column density we could build up this way. Could it be sufficient to keep a body of water stable on the surface? Somehow I doubt it. But a microbar atmosphere, sufficient to take out micrometeorites and the worst heavy ion cosmic rays, would be valuable.

Last edited by Calliban (2022-03-25 18:34:28)

Void · 2022-03-26 10:31:30

It is good to put another potential tool in "The toolbox".

Done.

Calliban · 2022-09-20 10:49:57

An interesting option occured to me today when thinking about one of Voids posts. Traditional paraterraforming involves putting a roof over a world, effectively wrapping it in a huge pressure vessel or anchoring sections to the ground. Shell worlds use the planet's gravity to ballast the weight of a thick shell against internal pressure.

However, a pressure vessel of sufficient strength to enclose an entire planet with breathable air would be immensely thick. One option for reducing the required thickness is to take advantage of scale height. This is the height above the surface at which atmospheric pressure declines by e (2.718). Even Pluto with a diameter of ~2400km, has a scale height of 50km. A pressure shell with diameter 2500km, would have 8.5% more surface area than Pluto, but would only need to contain 37% of the pressure, because atmospheric pressure at that height will be lower than ground by a factor of e. A shell would also need to be less massive if anchored at 50km, rather than closer to ground level. For worlds with surface gravity and radii above a certain threshold, we can use gravity to reduce shell or pressure vessel thickness.

For progressively smaller bodies, the advantages diminish and ultimately disappear, because gravitational field strength diminishes faster than scale height at constant g is able to reduce pressure. Beyond this point, increasing shell radius will increase it's total mass. For Pluto, a shell with diameter 2500km, wouod only weigh 43% as much as a 2400km diameter shell. So this probably won't be much use for worlds much smaller than Pluto. The scale height advantage, appears to disappear for bodies smaller than 1600km is diameter, for nitrogen atmospheres at Kuiper belt temperatures. For a body like Ceres, the most economic shell would be close to the ground. Triton, is 65% more massive than Pluto and has 42% greater volume. So scale height can be used here also. The Uranian satellites and all of Saturns bar Titan, are too small for such an effect to be significant.

Last edited by Calliban (2022-09-20 11:14:44)

tahanson43206 · 2022-09-20 12:01:19

It is fun to see this topic back in view after too long an absence ....

It didn't take long to read from the top ...

A question I have, after trying to understand the posts, is whether any of them hypothesize simply enclosing candidate bodies entirely.

In that case, the mass of the enclosing "roof"/"bag"/"enclosure" could be quite small indeed, compared to some of the examples that appear to be included in posts earlier in the topic.

The "roof"/"bag"/"enclosure need only have the property of keeping atmosphere molecules inside.

As a detail for planning ... the sectors of an enclosure can be fabricated separately and then joined to make a sphere. Since gravity around most objects that might be considered for this treatment is small, moving the panels into place and securing them to each other should not be difficult, compared to trying to accomplish that in a gravitational field of any magnitude.

If the body is located from from the Sun, then the "roof"/"bag"/"enclosure" might be fitted with solar panels which would collect starlight as well as Sunlight, for whatever that might be worth.

If the owner/claim-holder of this object is able to build such an enclosure, then the atmosphere can be pumped up to whatever pressure might seem agreeable, including Earth sealevel standard, although Mars Habitat Standard (per RobertDyck) might seem more reasonable, to facilitate EVA sessions to perform maintenance inspections and the occasional repair.

(th)

Calliban · 2022-09-20 14:19:32

Spherical pressure vessel equation:

t = Pr/(2 x stress)

Tensile strength of carbon steel is about 600MPa. Lets say we go for an operating stress 1/3rd of tensile strength. So that's 200MPa. How thick would a steel pressure vessel need to be to enclose a 10km diameter asteroid, if internal pressure is 50KPa?

t = (50,000 x 5000)/(2 x 200MPa) = 250,000,000/400,000,000 = 0.625m

How much would it weigh?

Steel volume = 4pi x 5000^2 x 0.625 = 196 million cubic metres.

Density of steel is 7.9 tonnes per cubic metre. Mass = volume x 7.9 = 1.55 billion tonnes.

How much would asteroid weigh? Let us take asteroid density to be 2.7 tonnes per cubic metre (Eros). Mass = volume x density.

Mass = 4/3 x pi x 5000^3 x 2.7 = 1414 billion tonnes.

Maybe we could use these pressure vessels to assist in mining small bodies.

Last edited by Calliban (2022-09-20 14:39:17)

Terraformer · 2022-09-20 14:33:28

How does the pressure vessel thickness change if it is anchored to the world? A Cererean bubble would be say 1000km across, but presumably wouldn't need the thickness of an empty pressure vessel 1000km across, since the tension is being transferred into the planet itself...

tahanson43206 · 2022-09-20 14:44:11

For Terraformer ... re #8

Neat question .... I'm looking forward to how Calliban will reply...

For Calliban ... thank you for the helpful numbers in #7

I went to Google (as usual) to try to refresh on pascals ...

Atmospheric pressure
Atmospheric pressure, also known as barometric pressure, is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 101,325 Pa, which is equivalent to 1013.25 millibars, 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. Wikipedia
Psi: about 14.7 pounds per square inch nationalgeographic.org
P_h= P_0 e^{\frac {-mgh}{kT}}

It would appear that 100 pascals is sea level equivalent, so your estimate of 50 kpa is a match with the Mars habitat pressure proposed by RobertDyck.

I conclude from your presentation that a spherical shell supported entirely by air pressure at 50 kpa would have the dimensions (thickness) you've proposed.

I'm surprised by the result you presented, but only because I had no prior numbers to work with.

I do have a quibble ... there is no weight under consideration in an asteroid 10 km in diameter.

So (I would assume) the correct term might be mass ...

The mass of the asteroid should be knowable, if assumptions are made about the content.

If an asteroid does not contain the quantity of iron required for your design, is there an alternative material that might be drawn from the asteroid?

Basalt may have excellent compression strength, but it may prove lacking under tension.

Never-the-less, this topic of Terraformer's seems to have a way to go on this round.

(th)

Calliban · 2022-09-20 15:19:56

The shell itself could be much thinner if anchored to the ground, as radius of curvature woukd be much smaller. The problem is that gravity of Ceres is 2.9% of Earth. So the dome would need to be anchored a least a few hundred metres underground. For a body the size of Ceres, the mass of cabling would be high. Maybe someone can work it out?

Mars_B4_Moon · 2023-12-03 21:03:38

NASA has an ambitious blueprint for building homes on the moon by 2040
https://www.foxnews.com/lifestyle/nasas … homes-2040

The "world house" concept or Paraterraforming, Mars tech could be then exported offworld for partial or paraterraforming include large moons such as Titan, Callisto, Ganymede, Europa, and the dwarf planet Ceres, it could be similar to the concept of collections of domed habitats but one which covers a larger area.

Making a Greenhouse on Another World: Where Can We Paraterraform in Our Solar System?
https://interestingengineering.com/scie … lar-system

How Do We Terraform Ceres?
https://www.universetoday.com/128711/ho … orm-ceres/

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#1 2021-11-18 11:53:55

Tiny Atmospheres For Small Worlds

#2 2021-11-19 14:23:07

Re: Tiny Atmospheres For Small Worlds

#3 2022-03-25 18:31:45

Re: Tiny Atmospheres For Small Worlds

#4 2022-03-26 10:31:30

Re: Tiny Atmospheres For Small Worlds

#5 2022-09-20 10:49:57

Re: Tiny Atmospheres For Small Worlds

#6 2022-09-20 12:01:19

Re: Tiny Atmospheres For Small Worlds

#7 2022-09-20 14:19:32

Re: Tiny Atmospheres For Small Worlds

#8 2022-09-20 14:33:28

Re: Tiny Atmospheres For Small Worlds

#9 2022-09-20 14:44:11

Re: Tiny Atmospheres For Small Worlds

#10 2022-09-20 15:19:56

Re: Tiny Atmospheres For Small Worlds

#11 2023-12-03 21:03:38

Re: Tiny Atmospheres For Small Worlds

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