Author Topic: Mega-scale civil engineering on Mars  (Read 11265 times)

Offline Robotbeat

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Mega-scale civil engineering on Mars
« on: 06/18/2010 06:40 PM »
Okay, so I am going to set aside global terraforming for a while and look at something else:

Excavating a portion of Hellas Planitia deep enough that the surface pressure is above the Armstrong Limit.

The Armstrong Limit is about 6260 Pa... where water boils at human body temperature... also, roughly the partial pressure of oxygen at Everest, where some people have climbed without oxygen bottles. At pressures below this limit, a pressure suit is required for survival.

The surface pressure at Hellas Planitia (1155 Pa) is already higher than the average pressure on Mars. This is also higher than the triple point of water, which means that liquid water can exist on the surface at some temperatures just above the freezing point (though it would evaporate, which means that the top part of the Martian soil at this location is still dessicated or frozen). Hellas Planitia is itself a big crater 7 km deeper than the surrounding terrain and thousands of kilometers in diameter.

On Earth, the deeper you are, the greater the air pressure. The same is true of Mars, but the lower gravity means that you have to dig even deeper for the same percentage change in pressure.

We shall use a simple atmospheric model that uses the "scale height" of the planet's atmosphere as one of the parameters to determine how deep or high you need to go in order to achieve a certain pressure.

The pressure (P) at "z" meters altitude difference between a reference altitude is related to the pressure (P0)at your reference altitude in this way:

P = P0 e(-z/H)
where "H" is the atmosphere's scale height ( http://en.wikipedia.org/wiki/Scale_height )

Earth's atmosphere has a scale height of ~7.6 km. Mars's atmosphere has a scale height of ~11km.

If we know "H", "P", and "P0", then we can solve for "z":

z = -H ln(P/P0)
Where H = 11km, P = 6260 Pa, P0 = 1155 Pa
we get:
z=-18.59km (i.e. 18.59 kilometers underground, below the bottom of Hellas Planitia)

That is pretty low! If we excavated a cone (at the angle of repose of <35 degrees) down to that depth, we'd have to excavate over 3000 cubic miles! To get down to ~20km and get a decent sized area down there, it'd take even more... about 7000 cubic miles. Another issue is that if you increase the temperature (like you'd want if you were to grow plants), the scale height is even higher, which means you'd have to dig even deeper.

The only way I could see that happening is if you hit a huge asteroid (~10km in diameter) into Hellas Planitia that would excavate most of that for you (say, a crater depth of ~13km), leaving you with only 7km left... or about 200-300 cubic miles to excavate. That is much more within the realm of reality... but only after Mars had already been industrialized (might as well talk about terraforming, then).

A single asteroid probably couldn't excavate a deeper pit permanently, since larger asteroids leave complex craters, where gravity pushes up the center of the crater. However, perhaps if you had multiple asteroids, a big asteroid ~8km in diameter to dig a little more than half of the 19km and another <1km asteroid to dig the rest.

So, if you somehow excavated a >20km deep hole in Hellas Planitia, you would just need a breathing mask (and warm clothes!!!). You may also be able to heat up your little garden valley with large reflectors placed around your valley.

I actually think if you COULD dig such a deep hole, it would probably fill with water seeping out of the rocks around the hole which would be under great pressure at such depths... I actually think it may not be possible to dig such a hole, since gravity may just force the center upward, like trying to dig a hole on the surface of the water.

Just a thought.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Idol Revolver

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Re: Mega-scale civil engineering on Mars
« Reply #1 on: 06/18/2010 08:48 PM »
The hole could be much shallower if you filled it with SF6. However, you can't grow plants in it.

Offline Robotbeat

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Re: Mega-scale civil engineering on Mars
« Reply #2 on: 06/18/2010 08:50 PM »
The hole could be much shallower if you filled it with SF6. However, you can't grow plants in it.
Besides, Sulfur Hexaflouride would also eventually diffuse into the rest of the atmosphere, whereas the whole would fill in naturally with the regular Martian atmosphere.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Idol Revolver

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Re: Mega-scale civil engineering on Mars
« Reply #3 on: 06/18/2010 09:03 PM »
I would have thought it would be far too dense for that. It should just sit in the hole. The wind may be a problem though. You may have to fill the hole only 80-90% full to stop the wind 'splashing' the SF6 out of the hole.

Offline neilh

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Re: Mega-scale civil engineering on Mars
« Reply #4 on: 06/18/2010 09:19 PM »
Nukes?
Someone is wrong on the Internet.
http://xkcd.com/386/

Offline Idol Revolver

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Re: Mega-scale civil engineering on Mars
« Reply #5 on: 06/18/2010 09:57 PM »
Nukes are always the answer!

Offline Robotbeat

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Re: Mega-scale civil engineering on Mars
« Reply #6 on: 06/18/2010 10:01 PM »
Nukes?
A 1km in diameter asteroid is going to release AT LEAST 10,000 Megatons on impact on Mars (or about ten times the entire arsenal of the United States).
« Last Edit: 06/18/2010 10:04 PM by Robotbeat »
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Offline Robotbeat

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Re: Mega-scale civil engineering on Mars
« Reply #7 on: 06/18/2010 10:11 PM »
BTW, to give a sense of scale, here's one of the largest mining projects on Earth, a lignite-coal mine in Germany:
http://en.wikipedia.org/wiki/File:Panorama_2_Tagebau_Hambach.jpg
The largest moving machine in the world (a bucket wheel excavator) on the other side of the pit can barely be seen in that picture (you have to click on it to see the full size), though you can see the ones which are closer.

This hole I am talking about would be 10,000 times bigger.
« Last Edit: 06/18/2010 10:13 PM by Robotbeat »
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Hop_David

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Re: Mega-scale civil engineering on Mars
« Reply #8 on: 06/18/2010 11:02 PM »
I would have thought it would be far too dense for that. It should just sit in the hole. The wind may be a problem though. You may have to fill the hole only 80-90% full to stop the wind 'splashing' the SF6 out of the hole.

CO2, oxygen and nitrogen gases all have different densities. Yet our atmosphere isn't stratified with layers of these 3 gases.

I suspect convection would tend to mix SF6 with martian atmosphere over time.

Offline Idol Revolver

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Re: Mega-scale civil engineering on Mars
« Reply #9 on: 06/18/2010 11:11 PM »
I would have thought it would be far too dense for that. It should just sit in the hole. The wind may be a problem though. You may have to fill the hole only 80-90% full to stop the wind 'splashing' the SF6 out of the hole.

CO2, oxygen and nitrogen gases all have different densities. Yet our atmosphere isn't stratified with layers of these 3 gases.

I suspect convection would tend to mix SF6 with martian atmosphere over time.
But they are relatively close together when compared to SF6
CO2:1.84212 kg/m3
O2:1.429 kg/m3
N2:1.251 kg/m3
SF6:5.97 kg/m3

Offline Hop_David

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Re: Mega-scale civil engineering on Mars
« Reply #10 on: 06/19/2010 03:02 AM »
CO2, oxygen and nitrogen gases all have different densities. Yet our atmosphere isn't stratified with layers of these 3 gases.

I suspect convection would tend to mix SF6 with martian atmosphere over time.
But they are relatively close together when compared to SF6
CO2:1.84212 kg/m3
O2:1.429 kg/m3
N2:1.251 kg/m3
SF6:5.97 kg/m3

There are other, even lighter, gases in earth's as well as Mars' atmosphere.

H2O: .804 kg/m3
CH4: .668 kg/m3

While SF6 density is a little more than triple that of  CO2, CO2 is nearly triple that of CH4

Gas densities

Hmmm. Wikipedia's atmosphere article says:

Quote
The homosphere and heterosphere are defined by whether the atmospheric gases are well mixed. In the homosphere the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence.[4] The homosphere includes the troposphere, stratosphere, and mesosphere. Above the turbopause at about 100 km (62 mi; 330,000 ft) (essentially corresponding to the mesopause), the composition varies with altitude. This is because the distance that particles can move without colliding with one another is large compared with the size of motions that cause mixing. This allows the gases to stratify by molecular weight, with the heavier ones such as oxygen and nitrogen present only near the bottom of the heterosphere. The upper part of the heterosphere is composed almost completely of hydrogen, the lightest element.

So the Martian atmosphere might be prone to stratification at a lower altitude than earth's atmosphere.
« Last Edit: 06/19/2010 03:03 AM by Hop_David »

Online KelvinZero

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Re: Mega-scale civil engineering on Mars
« Reply #11 on: 06/19/2010 03:21 AM »
Hehe.. I reckon I might have started this idea a few years back with a plan for a 60km deep hole which I somehow worked out would give sealevel pressure.. really dont remember what numbers I put in.

However.. want to know something way way denser than SF6, in fact that you would only need thirty meters of at mars gravity to give you one full atmosphere of pressure? and btw is excellent at blocking cosmic radiation. let me give you a clue.. you can drink it ;)

Offline Idol Revolver

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Re: Mega-scale civil engineering on Mars
« Reply #12 on: 06/19/2010 09:28 AM »
Evaporation would be a problem.

Online KelvinZero

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Re: Mega-scale civil engineering on Mars
« Reply #13 on: 06/19/2010 10:08 AM »
Evaporation would be a problem.

You would think so, but it doesnt need to be at this scale. A large city could support the boiloff of a small open surface. Alternatively of course you can let the surface freeze over.

Also you might think keeping it liquid would require a large waste of energy. In fact that would be getting the problem backwards. Assuming you have sufficient energy for your civilisations day-to-day tasks, the problem will always become how you get rid of the waste heat. Rock and ice are good insulators, and the larger your city, the more heat it produces per external surface area. One of the biggest problems of nuclear power in space is getting rid of the heat. What would be really good to have is a large body of circulating water.

Offline JohnFornaro

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Re: Mega-scale civil engineering on Mars
« Reply #14 on: 06/19/2010 01:49 PM »
This is interesting:

Quote
we get:
z=-18.59km (i.e. 18.59 kilometers underground, below the bottom of Hellas Planitia)

That's pretty deep.  McKay speculated about possibly life "deep" in martian crevices, but he didn't specify how deep.

Quote
if you hit a huge asteroid (~10km in diameter) into Hellas Planitia that would excavate most of that for you (say, a crater depth of ~13km), leaving you with only 7km left...

Hit it again, what the heck!

Quote
Nukes are always the answer!

Uhhh... What was the question again?
Sometimes I just flat out don't get it.

Offline khallow

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Re: Mega-scale civil engineering on Mars
« Reply #15 on: 06/19/2010 02:00 PM »

The only way I could see that happening is if you hit a huge asteroid (~10km in diameter) into Hellas Planitia that would excavate most of that for you (say, a crater depth of ~13km), leaving you with only 7km left... or about 200-300 cubic miles to excavate. That is much more within the realm of reality... but only after Mars had already been industrialized (might as well talk about terraforming, then).

Two problems here. First, a huge asteroid strike is going to be hard on the local industry. Second, what happens to the rest of Mars? After all, you're effectively increasing the atmosphere present in Hellas Planitia by a factor of six. In turn, Hellas Planitia seems to be roughly 2-3% of the surface area of Mars. My guess is that would reduce the atmospheric pressure over the rest of Mars by 10-15%. Any industry or organisms outside of this basin that extract resources from Mars atmosphere would take more work to do so than before.
Karl Hallowell

Offline Robotbeat

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Re: Mega-scale civil engineering on Mars
« Reply #16 on: 06/19/2010 05:15 PM »

The only way I could see that happening is if you hit a huge asteroid (~10km in diameter) into Hellas Planitia that would excavate most of that for you (say, a crater depth of ~13km), leaving you with only 7km left... or about 200-300 cubic miles to excavate. That is much more within the realm of reality... but only after Mars had already been industrialized (might as well talk about terraforming, then).

Two problems here. First, a huge asteroid strike is going to be hard on the local industry. Second, what happens to the rest of Mars? After all, you're effectively increasing the atmosphere present in Hellas Planitia by a factor of six. In turn, Hellas Planitia seems to be roughly 2-3% of the surface area of Mars. My guess is that would reduce the atmospheric pressure over the rest of Mars by 10-15%. Any industry or organisms outside of this basin that extract resources from Mars atmosphere would take more work to do so than before.

I'm actually talking only about a very small part of Hellas Planitia, less than 1% of Hellas Planitia for even the first asteroid crater. The second crater would be about 1% of that. In fact, it may be possible that out-gassing from the heat caused by the asteroid strikes could lead to a net gain in the total Martian atmosphere.

Excavating something as large as Hellas Planitia is not going to happen.
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Offline Hop_David

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Re: Mega-scale civil engineering on Mars
« Reply #17 on: 06/19/2010 06:17 PM »
I'm actually talking only about a very small part of Hellas Planitia, less than 1% of Hellas Planitia for even the first asteroid crater. The second crater would be about 1% of that. In fact, it may be possible that out-gassing from the heat caused by the asteroid strikes could lead to a net gain in the total Martian atmosphere.

Excavating something as large as Hellas Planitia is not going to happen.

Well, this mitigates one of my reservations.

If you hit Mars with a big object at sufficient velocity, you get atmospheric erosion.

All the atmosphere above the tangent plane at point of impact is blasted away.

However, you're talking about a tiny crater within a small crater within an existing big crater. So the impacts may not be big eough to blast away atmosphere.

Using Kelvin Zero's idea, the craters could be even smaller. I like the idea of a frozen ice cap. Maybe cover it with clays to prevent sublimation. Then the lower liquid water could carry away the city's heat. Since cold water sinks and hot water rises, it's possible you wouldn't even need pumps for this mammoth radiator.

Offline kkattula

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Re: Mega-scale civil engineering on Mars
« Reply #18 on: 06/19/2010 06:31 PM »
...

However.. want to know something way way denser than SF6, in fact that you would only need thirty meters of at mars gravity to give you one full atmosphere of pressure? and btw is excellent at blocking cosmic radiation. let me give you a clue.. you can drink it ;)

What a great idea! 

Instead of worrying about how to keep a 1 atm dome from blowing out or tearing loose, it's under a compressive load. Micro-meteors aren't a problem either.

There are probably simple methods to prevent most evaporation or simply capture most of the water vapour.

Any leak would simply be a water drip, not an emergency.

Make the supporting structure a glass dome, and you still get a, somewhat wavey, view.

Offline kkattula

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Re: Mega-scale civil engineering on Mars
« Reply #19 on: 06/19/2010 06:41 PM »
Of course you need 30,000 litres of water for every square metre of dome. That's 235 million litres for a 100 m diameter dome. Although it would only weigh 90,000 tonnes on Mars.

Would mining that much water on Mars, and transporting it to the base, count as mega-scale civil engineering?
« Last Edit: 06/19/2010 06:47 PM by kkattula »

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