High School Problems With a Terraformed Moon -- Maybe?

[Oomuu]

While watching a Neil DeGrasse Tyson documentary they mentioned something about terraforming the moon. What I have never seen anyone mention is even if it was possible to terraform the moon, give it an atmosphere, and plant life, and water, wouldn't the extra mass from the water and atmosphere essentially make the moon have a stronger gravitational pull?

 

[Karen Anne]

How are you going to get a large amount of water up there? I would think any water would have to come from sources already on the moon, therefore the mass wouldn't change.

 

[Bystander]

wouldn't the extra mass from the water and atmosphere essentially make the moon have a stronger gravitational pull?

What percentage of Earth's mass is "the water and atmosphere?"

 

[Karen Anne]

A very fast scan through the web (no time to be neater or include refs):

"But in terms of mass, scientists calculate that the oceans on Earth weigh about 1.35 x 1018 metric tonnes (1.488 x 1018 US tons), which is the equivalent of 1.35 billion trillion kg, or 2976 trillion trillion pounds. This is just 1/4400 the total mass of the Earth, which means that while the oceans cover 71% of the Earth’s surface, they only account for 0.02% of our planet’s total mass."

"According to the American National Center for Atmospheric Research, "The total mean mass of the atmosphere is 5.1480×1018 kg."

I'm surprised the % mass of water is so low. Must be swamped by the iron core, etc. or depth of the oceans vs. radius of the earth.

 

[Orodruin]

I'm surprised the % mass of water is so low. Must be swamped by the iron core, etc. or depth of the oceans vs. radius of the earth.

The depth of the oceans is at most of order 10 km. The Earth's radius is roughly 6300 km. Even if the Earth was covered by an even 10 km deep layer of water, this would only be equal to a fraction $3 d/R = 30/6300 = 1/210$ of the Earth's total volume. Add to this that water's density is generally much lower than the density of the rest of the Earth and that the Earth is (luckily) not covered by an even 10 km deep ocean.

 

[mfb]

The average depth is about 4 km, spreading the oceans over the whole surface (instead of 3/4) would lead to 3 km. Water has ~1/5.5 the average density of the Earth. 3*3km/(6370km) * 1/5.5 = 1/3900, a pretty good estimate.

The moon has a mass of 7.3*10²² kg or 7.3*10¹⁹ tons, but giving it an ocean would require less mass as the surface area is smaller by a factor of 13.

Actually, giving the moon (approximated as sphere) a global ocean would reduce the surface gravity, as the increased radius has a larger impact (~factor 2) than the increased mass for realistic ocean layers.

 

[Karen Anne]

How are you going to get a large amount of water up there? I would think any water would have to come from sources already on the moon, therefore the mass wouldn't change.

It occurs to me that an asteroid with a lot of water (are there such things?) could be dragged to the moon.

 

[mfb]

Comets have a lot of water. A large impact on the Moon would lead to many smaller secondary impacts on Earth.

 

[Karen Anne]

I was thinking more of putting an asteroid in a stable orbit and mining it for water than crashing it into the surface.

 

[nikkkom]

You would need to consume something like the Ceres to have enough water for Moon oceans.

While in principle that probably can be done (it is FAR beyond our current space capabilities, but laws of physics allow it and we can imagine necessary technologies), the efforts would be much better spent otherwise. Building artificial habitats, even enormous ones, gives better bang for the buck.

 

[Karen Anne]

Where are you going to get the water for these artificial habitats?

 

[mfb]

Water for city-sized habitats is orders of magnitude easier than water for oceans. From asteroids, from Earth, from hydrogen in the solar wind and oxygen from the surface, ...

 

[nikkkom]

Water for city-sized habitats is orders of magnitude easier than water for oceans.

Exactly. Oceans are inefficient. Sure, when they are already there free of charge, they are very convenient. When you need to create them from scrathc first, it does not make any economic sense.

For much less money, you can build a completely artificial habitat with Moon (~= Africa) worth of living space. (For one, such habitat is a fantastic spaceship, by virtue of necessarily having fully closed life support systems it's basically a generation ship, usable for interstellar journeys)

 

[mfb]

The moon will have the sun as convenient power source for a few billions of years, interstellar journeys would need an internal power source.

 

[Mikey16]

I guess we can infer it based on the case of the earth. The total mass of water on the Earth is about 1/4400 of the mass of the earth. Therefore, I guess if we do terraform it, there must be some influences on the Earth (such as tides etc). But I don't think it will be much.
But I don't think you can get so much water there. Even though you do transport it on the moon, it will not exist in the way we like. We see water like that on the Earth just because they Earth has a proper gravity which can make it in this form. The gravity on the moon is only 1/6 that on the earth.

 

[nikkkom]

Water does not need gravity to be liquid. It needs pressure.
It is technically possible to give the Moon Titan-like atmosphere, then it can have liquid water on its surface.

 

[DrStupid]

It is technically possible to give the Moon Titan-like atmosphere

Theoretically? Yes. But technically? That would require 90 t/m² of nitrogen. Where should that come from?

 

[mfb]

Moon is not heavy enough to keep an atmosphere at the temperatures required for liquid water over geological timescales. Hydrogen atoms (from water vapor) escape freely, helium won't stay long, and even nitrogen and oxygen have some chance to escape.

 

[Mikey16]

Water does not need gravity to be liquid. It needs pressure.
It is technically possible to give the Moon Titan-like atmosphere, then it can have liquid water on its surface.

You can see that in space station with little gravity, water looks like a ball floating in the air. Without enough gravity, water can not be steady as needed to generate life. Moon doesn't have water on its surface just because it doesn't have enough gravity to "grasp" water. Same reason as Mars. A theory believes that there used to be liquid water on Mars but then disappeared. Because Mars had not enough gravity to prevent water molecule from escaping.

You mentioned Titan and its atmosphere. Titan is heavier than the moon therefore has more powerful gravity. There is methane rain in the atmosphere of Titan because methane is much "lighter" than vapour (water) and easier to be pulled down.

 

[mfb]

Liquid surface water needs a minimum pressure to exist - about 0.5% of the atmospheric pressure on Earth (sea level). Otherwise all water freezes/evaporates. Even a super-Earth could not have liquid surface water if the atmospheric pressure does not exceed this minimal pressure.

Liquid surface water will always lead to some water vapor as part of the atmosphere, and solar wind and UV radiation will split some hydrogen atoms off. Here the combination of temperature and escape velocity determines if the hydrogen atoms will escape. The surface gravity (the local gravitational acceleration) is irrelevant, only the escape velocity matters here.

 

[Mikey16]

Liquid surface water needs a minimum pressure to exist - about 0.5% of the atmospheric pressure on Earth (sea level). Otherwise all water freezes/evaporates. Even a super-Earth could not have liquid surface water if the atmospheric pressure does not exceed this minimal pressure.

Liquid surface water will always lead to some water vapor as part of the atmosphere, and solar wind and UV radiation will split some hydrogen atoms off. Here the combination of temperature and escape velocity determines if the hydrogen atoms will escape. The surface gravity (the local gravitational acceleration) is irrelevant, only the escape velocity matters here.

I got a question for this topic: Should I say a planet with strong surface gravity must have large atmosphere pressure?

 

[mfb]

It does not have to have that. It is likely, but not guaranteed.

PSR J1719-1438 b for example is probably the core of a gas giant that survived the death of its host star, but got stripped of all its atmosphere. It has about the mass of Jupiter, but is much smaller, with an average density of at least 23 g/cm³ its density exceeds the density of gold. It orbits the host star remnant (now a pulsar) in a very close orbit.

 

[nikkkom]

You can see that in space station with little gravity, water looks like a ball floating in the air. Without enough gravity, water can not be steady as needed to generate life. Moon doesn't have water on its surface just because it doesn't have enough gravity to "grasp" water.

This is nonsense.
A cup of water in Moon gravity in a pressurized habitat (say, 1 atm 25% O2 75% N2 atmosphere, 25 C) stays in the lower part of the cup, as it should.

 

[Mikey16]

This is nonsense.
A cup of water in Moon gravity in a pressurized habitat (say, 1 atm 25% O2 75% N2 atmosphere, 25 C) stays in the lower part of the cup, as it should.

But you can only do this in a closed venue on the surface of moon. If you want to make the whole moon like the Earth ("terraform" it), the water molecules will escape to the space soon. What you said should be in a "space base", the environment in which is totally isolated to the outside. This is not "terraform".

 

[nikkkom]

Theoretically? Yes. But technically? That would require 90 t/m² of nitrogen. Where should that come from?

There are no laws of physics which prevent a very advanced spacefaring civilization from transporting N2 from e.g. Titan to the Moon.
As I said before, I do not think it makes any sort of economic sense to do it, but it can be done.

 

[nikkkom]

But you can only do this in a closed venue on the surface of moon.

And this proves that it's not weak gravity which makes water unstable on the Moon, since gravity is the same in a closed habitat.

the water molecules will escape to the space soon.

Now try to use your brain and figure out WHY water molecules would escape to the space soon, and why this process depends on not being in the closed habitat.

 

[Mikey16]

And this proves that it's not weak gravity which makes water unstable on the Moon, since gravity is the same in a closed habitat.
Now try to use your brain and figure out WHY water molecules would escape to the space soon, and why this process depends on not being in the closed habitat.

Just because your habitat on the moon is totally isolated (sealed), the water molecules can not escape outside and you still have them in your habitat.
We don't need to use our brain too much to imagine why a planet without enough gravity will have their water molecules escaped into space. Because if the surface gravity on the planet, or to say, moon, is not strong enough, the speed of the molecules can reach escape velocity easily. Don't forget that once there're radiation from the universe, the molecules get more energy and then accelerate their speed.

 

[mfb]

There are no laws of physics which prevent a very advanced spacefaring civilization from transporting N2 from e.g. Titan to the Moon.
As I said before, I do not think it makes any sort of economic sense to do it, but it can be done.

The water would still get lost to space over time. The nitrogen wouldn't matter much, ignoring temperature effects. First water evaporates until its partial pressure is in equilibrium with the liquid water - same process as without nitrogen. Then the hydrogen slowly escapes.

 

[nikkkom]

The water would still get lost to space over time.

The "over time" part has rather different magnitude for two different scenarios.

On the airless Moon, water would boil and thus escape right away (a meter-thick global layer would be gone in a few years).

But if there is a ~1bar atmosphere with twice molecular mass than H20, water won't boil (or rather, would boil _much_ less), and escape of the atmosphere and water would take much longer (I think thousands of years at least).

 

[nikkkom]

Just because your habitat on the moon is totally isolated (sealed), the water molecules can not escape outside and you still have them in your habitat.

You missed the point. The water molecules are not just in the habitat. They _stay in the cup_. Outside, they will not.
I probably need to stop responding.