# B Problems With a Terraformed Moon -- Maybe?

1. Dec 29, 2016

### 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?

2. Dec 30, 2016

### 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.

3. Dec 30, 2016

### Bystander

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

4. Dec 30, 2016

### 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.

5. Dec 30, 2016

### Orodruin

Staff Emeritus
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.

6. Dec 30, 2016

### Staff: Mentor

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.

7. Dec 30, 2016

### Karen Anne

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

8. Dec 30, 2016

### Staff: Mentor

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

9. Dec 30, 2016

### 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.

10. Dec 31, 2016

### 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.

11. Dec 31, 2016

### Karen Anne

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

12. Dec 31, 2016

### Staff: Mentor

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, ...

13. Jan 1, 2017

### nikkkom

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)

14. Jan 1, 2017

### Staff: Mentor

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

15. Jan 11, 2017

### 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.

16. Jan 11, 2017

### 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.

17. Jan 11, 2017

### DrStupid

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

18. Jan 11, 2017

### Staff: Mentor

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.

19. Jan 11, 2017

### Mikey16

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.

20. Jan 11, 2017

### Staff: Mentor

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.