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#1 2007-11-25 23:07:05

samy
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From: Turku, Finland
Registered: 2006-01-25
Posts: 180
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Re: One crater at a time

There's a lot of talk about what a massive undertaking it would be to build up an atmosphere for the entire planetary mass of Mars.

There's less talk about building local small-scale atmospheres. And I'm not talking about domed cities and habitats, but open-sky.

If you flooded, say, Hellas Planitia with SF6, the gas would stay at the bottom of the crater, would it not? By virtue of being heavier than the CO2 atmosphere above it?

Mars is pretty rich in craters, why not make that work for us? Basically, there are a lot of ready-made "cups" that you could pour "liquid" into. The crater rims should serve as natural containers to prevent the (heavy) gases from spilling out significantly. (Light gases like O2, N2, would rise above the CO2 and mix into the planetary atmosphere.)

At 1150 km radius and 9 km depth from rim to bottom, Hellas Planitia for example would fit approximately 3.7E16 cubic meters of gas. Which is still a hell of a lot (and you'd want to practice on smaller craters first) but it is also a hell of a lot less than doing the entire planet.

If you had a 9000m x 1m x 1m pillar of SF6 on top of a square meter of Hellas soil, it would be 9000m3 x 6.164kg/m3 = 55 476kg per square meter. Given martian gravity 3.69m/s2 * 55476kg ~= 205 000 N/m2 = 200 kPa = 2 bar.

(Martian gravity would be trivially more at the bottom of Hellas Planitia but not enough difference to worry about.)

One thing where my calculation is probably wrong is that the density of SF6 should be lower at Martian lower pressures. Calculation help with that?

If you didn't fill it to the rim, you'd reduce the amount of gas loss to the outside by staying away from the border zone, at the price of providing less pressure to the bottom of the crater.

One of those "black holes" we've seen on the surface could also be an interesting target to pour heavy gases into.

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#2 2007-11-25 23:37:27

RickSmith
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From: Vancouver B.C.
Registered: 2007-02-17
Posts: 244

Re: One crater at a time

There's a lot of talk about what a massive undertaking it would be to build up an atmosphere for the entire planetary mass of Mars.

There's less talk about building local small-scale atmospheres. ...

If you flooded, say, Hellas Planitia with SF6, the gas would stay at the bottom of the crater, would it not? By virtue of being heavier than the CO2 atmosphere above it? ...

Hi Samy, everyone.
  What a neat idea.  But I am worried that it won't work.  At this link here it says that the bottom of the crater is ~9 km below the surrounding crater rim.

Wiki Hellas Planitia

But Mars' scale height is 11 km.   So if you had (say) 0.1 mBars of greenhouse gases at the bottom of Hellas Planitia, you would have 0.037 mBars above the crater being blown in the local winds.  Now the density might be a little less that high up (since as you say CO2 is less dense than SF6) but I don't think it will be enough.  There is plenty of CO2 in Earth's stratosphere even tho it is significantly more dense than air.

So I think it very likely that given a few years, most of the greenhouse gases would leak out. 

Now if you put some sort of strong plastic dome over it, a slightly higher pressure inside could keep it up.  You wouldn't much care about small leaks as they would be many orders of magnitude slower than if you left it open.  Such a system would allow a vast area to warm up.  Sadly being in the southern hemisphere, you would have the short summers / long winters problem.

Warm regards, Rick.

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#3 2007-11-26 03:57:27

samy
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From: Turku, Finland
Registered: 2006-01-25
Posts: 180
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Re: One crater at a time

CO2 has a molar mass of 44, versus O2 of 32 and N2 of 28. So it's not so much heavier that some of it couldn't be thrown higher up in the air by currents.

SF6, conversely, has a molar mass of 146...which is pretty drastically different from all of the above.

I'm not saying it wouldn't leak at all -- in an open crater, everything would eventually leak -- but I think it would be a really slow process, especially if you only, say, half filled the crater to 4.5km and let the top half be CO2. The SF6 would have to work its way 4.5km up through the much lighter CO2.

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#4 2007-11-26 04:33:49

Terraformer
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From: Lancashire
Registered: 2007-08-27
Posts: 3,062
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Re: One crater at a time

Put some sort of a plastic bag over it? The pressure would keep the bag up.

So you'd end up with an artic coat, oxygen mask enviroment but wouldn't need to worry about the pressure. If it warmed the area enough you can do without the artic coats.

The gravity will be marginally higher? Then dig dig dig! Build your habitat near the core where the gravity can hold on to an atmosphere. Probably a crazy idea but who cares.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#5 2007-11-26 04:58:32

RickSmith
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From: Vancouver B.C.
Registered: 2007-02-17
Posts: 244

Re: One crater at a time

Hi Samy, everyone.
  Good point.  However, I am not sure that how to calculate this.  Let us say that there was no CO2 and we had nothing but the SF6.  The scale height (which is based on the gravity and temperature of the planet) would still be 11 km would it not?  (Doing some research here...)

OK, the formula for the scale height is:

H = kT / Mg

where:
k= gas constant = 8.314 j/ (mol K)
T = mean planetary surface temperature in kelvins.
M = mean molecular mass in dry air (kg/ mol)
g = acceleration due to gravity on surface in m/s^2.

(Note at heights above 100 km (basically a vacuum), molecular diffusion means that each gas species has its own scale height.)

All of these are constants (for Mars that is) except the molecular mass.  SF6 is about 3.3 times larger than for the CO2 atmosphere that Mars has.  So the scale height for SF6 would be 11km / 3.3 = 3.33 km.

The ~9 km height of the crater walls is: 9/3.33 or 2.73 times the scale height.  So at the top of the Hellas Crater, we would have 0.065 times the atmospheric pressure as on the ground.  (Assuming that the scale heights can be treated as independent partial pressures which is not true.)

(So if the pressure is 100% at the bottom it will be 6.5% at the top.)

Now on Mars, would things be better or worse than this?  Well the winds by the CO2 atmosphere can only make things worse.  Also, if you use SF6 to increase the temperature in Hellas Basin by 50 degrees C then that would increase the SF6's scale height by 1.23 times or to 4.1 km.  (165K / 115K = 1.23)  Finally the scale height is based on the average mass of the gases.  Basically the CO2 bouys up the SF6 more than if the SF6 was on its own, which makes sense.

So I don't think that your idea will work for the Hellas basin.


However, I don't think that we have explored all the possibilities here.  Let us say we drop several large asteroids in the Hellas Basin itself.  An asteroid with a velocity of 10 km/s and a 50 km diameter will make a crater ~500 km in size.  Figuring out the depth is complex.  I found a study that suggest that the depth to diameter ratio is a 0.3 exponent (but they also say that huge craters are shallower than you would expect).

Crater Depth to Diameter Ratios.

500 km to the 0.3 power is 6.45 km deep.  If we conservatively say that an impacter this big only makes a hole 3 km deep and then we drop 5 asteroids into the same hole it will be 15 km below the floor of Hellas Basin.

This will be so deep that much of the atmosphere of Mars will flow into it meaning that you would have no need of a pressure suit even with the current amount of atmosphere.  Your SF6 would have   (15 + 8) / 4.1 = 5.6 times the scale height. 

So if the SF6 is at 100% at the bottom of this hole, then the pressure at the top would be 0.36% at the top of the crater walls.  (Which is low enough that colonists might be able to replace losses.  A hole this deep will likely outgas a fair amount of volatiles.   if the local temperature (hot rocks) is above zero, you may end up with a sea down there and have to build house boats.

Since the CO2 buoys the SF6 up, you likely would want to make the hole deeper.  If it was 20 or 30 km deep, it would be better.  However a hole that deep would not need worry too much about winds.  The winds blowing around the outside of Hellas likely would not have any effect down inside the hole.

Our hole might be 500 km across at the top and say 100 km at the bottom.  This would greatly reduce the amount of greenhouse gases needed both speeding up the filling of the hole and reducing the cost to keep it topped up.


Interesting idea!  I had never considered using gravity to keep the greenhouse gases in a small area before.  I think that digging a deep hole may be a practical way to maximize any changes we wish to make to a volume of atmosphere and instantly get a local partial pressure where you would not need space suits.  In addition, digging that deep into the crust may expose any number of useful ores.

Very warm regards, Rick.

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#6 2008-02-04 01:24:59

RickSmith
Member
From: Vancouver B.C.
Registered: 2007-02-17
Posts: 244

Re: One crater at a time

Hi all,
  I bought a book, "The Geology of Mars: Evidence from Earth - Based Analogs".  I'll do a review of it later but I found in chapter 2 formulas for craters on Mars.  They are:

d= depth (km)
D= diameter (km)

For simple craters less than 7 km:
d = 0.26 D^0.67

For complex craters between 7 and 110 km:
d = 0.36 D^0.49

This is very different than the formula I used last post.  This book says that the formula for complex craters on Luna is:
d = 1.044 D^0.301
  ... which is basically the formula I found.

Using the Martian formula, we get a deeper hole (~7.6 km rather than 6.45 km).  My estimates for the amount of digging were very conservative so this strategy looks even more viable with better data.  (Note that this formula is only good up to 110 km diameters and we are talking 500 km for the first impacts, so there are still no guarantees.)

This also assumes that impacts this size would not cause massive volcanic eruptions.

Anyway, it's fun to think about.

Warm regards, Rick.

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#7 2019-05-15 15:22:37

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,513
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Re: One crater at a time

We were talking about digging 1000 km holes to mine the lunar core over in the Lunar Nickel Mining thread, and while I came out against it there I think there's an interesting use case for the technology on an unterraformed Mars.

What I'm thinking about is sort of like what was discussed in this thread, using a large deep hole to get a higher atmospheric pressure.  Unlike the discussions in the thread I was thinking not to use SF6 but instead to use Mars's native CO2 atmosphere.  The ten trillion dollar question is how deep of a hole you'd need to dig before the pressure becomes tenable for humans.

Luckily, we have the barometric formula, reproduced below:

4fba1b5af4f6ebe93e2c4e2e78cca45efb6f69d4

Where

P is the pressure at height h
P_b is the pressure at h=0 (0.6 kPa at the datum)
M is the mean molar mass of the gas (0.044 kg/mol for Mars's predominantly CO2 atmosphere)
g_0 is the gravity (3.7 m/s^2)
R is the universal gas constant, 8.314 J/molK
Tb is the atmospheric temperature (I'll assume 250 K)
(h-hb) is the height

To get a 40 kPa atmosphere (40% of Earth mean sea level) according to this equation you would need to dig a hole to about 54 km below the datum.  Mars's extreme topographical variation helps a bit: Valles Marineris is 4 km below the datum, and Hellas 9.  You might dig a hole with angled sides at 45 degrees having funiculars running up and down the sides (angled so that sunlight reaches the bottom and for material stability) with a town at the bottom where a gas exchange seal (but not a pressure seal) keeps in an oxygenated atmosphere.  A Trench of Babel, if you would.  The much thicker atmosphere at the bottom would likely result in fairly warm air there compared to the surface.  Plants could certainly survive, and even begin the process of oxygen generation.  You might even fill some of the bottom with water to create a nice lake or sea (but on the other hand, perhaps not: Any humidity would certainly condense into clouds or fog near the top and block the sunlight).

It's a megaproject for sure.  My question is, once you get past the massive scale of earthmoving that would be required, just how hard it would be.  Could it be done?


-Josh

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#8 2019-05-15 17:49:03

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 15,831

Re: One crater at a time

Thanks for finding this older topic where teraforming down a bore hole diameter to its bottom to get atmospheric pressure that would be useable on mars.
So roughly a 6psi earth content breathable....

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#9 2019-05-15 17:59:53

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,513
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Re: One crater at a time

You wouldn't want to use Earth's mix of gas at 40 kPa because it probably wouldn't be breathable (not enough oxygen) so you might instead go for something like:

18 kPa O2
22 kPa N2/Ar mix (at whatever the ambient Martian ratio is, so probably about 13 kPa N2 and 9 kPa Ar)

This gives more Oxygen than Denver with a total pressure just a bit higher than you'll find at 8000 m on Earth, what is considered the death zone.


-Josh

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#10 2019-05-15 18:58:25

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 15,831

Re: One crater at a time

Sounds like the trace levels of mars normal atmopsphere with that would make living there quite nice and natural.
Glass or other transparent means to cap the opening and to let natural light in makes it near complete under reduced gravity.

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#11 2019-05-16 05:39:11

Terraformer
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From: Lancashire
Registered: 2007-08-27
Posts: 3,062
Website

Re: One crater at a time

It would, however, not be compatible with terraforming Mars. Any terraforming would put the town under a crushing depth of water, and probably drain the rest of the planet in doing so.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#12 2019-05-16 11:28:05

Void
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Registered: 2011-12-29
Posts: 3,011

Re: One crater at a time

I appreciate how your mind works JoshNH4H.

But lets consider how to make reality serve what we want.

I think that Terraformers  words are probably ~Accurate.

This exists as a possibility.
https://www.space.com/mars-deep-groundw … ineae.html
Others claim that they can drill down 1-10 KM and find liquid water and life perhaps (Watch out for the Mars Roach Motel on that).  You pay for the science, and they don't let you go to Mars.  I actually am sympathetic to protection of presumed Martian life, but to limits.

Ignoring the life issue, I am tempted to think that the only things that prevent the discovery of ponds and lakes on Mars, is the thickness of the permafrost, which will inhibit artesian springs, the evaporative potential of the surface, and the low rate of underground melt in water sources of Mars.

Permafrost might allow you to have a deeper hole than expected by blocking water flows.

However there are at least three obvious low areas on Mars where Artesian springs should have the potential to exist, if there are slowing flowing aquifers.

The most likely is Hellas.  Then the Mariner rift valley, and then the Northern plains, where variations in elevation might allow it.

I think that Hellas is the most likely.  But very cold.  Deep permafrost I expect.  High evaporation rates if water does emerge to the surface, and really I suspect that the South Pole may melt some water, but it is not that much.

You could warm the permafrost and drill it to get a flow.  You could try cactus methods with tech, devices, to reduce evaporation.  You could increase melting on the south pole, using orbital solar or fusion.

These people will or will not create fusion power devices by 2024+ or not.  If they do, then reality changes vastly.  Options also change for the better.

https://www.thedrive.com/the-war-zone/1 … on-reactor

So, without plate tectonics, I suspect that Mars, has vast underground capacity for flows.  But not that much melted water.  If any emerges to the surface, it likely evaporates very quickly.

But the flow systems may be there.  If humans explore it, the time will tell.  The Dead Sea is below sea level.  So on the Earth this is one example where a hole can be kept somewhat reduced in water by evaporation rates exceeding liquid water inputs.  So your plan is not absolutely out of the question.

The surface of Mars is a very evaporative environment for liquid water.  Of course if you have a hole with a "Dead Sea" in it (Which is not absolutely dead), you might hope that evaporation will reduce the sea level, and that water will go elsewhere to the polar caps, to snow, and to even rain if you have a terraformed planet.

I meander, and that is alright I think.

There are so many variables which are not properly defined, that it is too soon in my opinion to state as orthodoxy, what we may or may not try to do on Mars and elsewhere.

As you know from pervious I am quite fond of using hydrostatic pressure to exceed the Armstrong limit and more.  I will set that aside for now, and try to remain more true to your post's intent.

There is a group member here that is apparently a collective of individuals that proposes to drill a hole on Mars using asteroid crashes.  I recall that idea as having emerged in the 70's or 80's not sure.  In that idea, a asteroid would be cut into pieces, and would be made to crash sequentially in a serial manner into a particular spot on Mars.  I would think if that was attempted, Hellas would be the place to try.

I don't think that the idea conceived of rubble pile asteroids.  Rubble pile asteroids may be mining opportunities, but may contain fine materials, and big chunks.  I you could mine the finer materials, construct orbital habs from the smaller (Radiation shielding for metal machines within), then you might have a series of big chunks which are hard to process.  Could you string them together with tethers, and move them through a solar orbit to an impact of Mars?  In order to have a serial impact, you would need to actively give propulsion to each of the chunks just prior to impact.  Maybe that way you could drill a deep hole.  It would indeed be partially filled with water, but evaporation may, limit the level of fill.

As you know I am rather fond of ice covered reservoirs, but I will just concentrate on the hope to have a pressurized hole not filled entirely with water.

So, I have tried to help your case.


Done.

Last edited by Void (2019-05-16 16:22:36)


I like people who criticize angels dancing on a pinhead.  I also like it when angels dance on my pinhead.

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#13 2019-05-16 14:01:47

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,513
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Re: One crater at a time

Hey SpaceNut, Terraformer, and Void,

It's true that the political economy of digging this sort of trench-town is bad if you want to promote terraforming.  Even if it's not flooded, the pressure will always be 50+ times the pressure at the datum down there meaning that if the datum is habitable the trench will not be.

From a political standpoint, if you choose to terraform you will more or less have to tell people that you're filling in their city and they will have to move for the greater good.  Seems tough, but so does terraforming.

Subterranean water would be a big challenge to any project like this.  I don't know how best to deal with it.


-Josh

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#14 2019-05-16 17:13:03

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 15,831

Re: One crater at a time

Teraforming in the open entirety of mars but localized not really that much of an issue.

It will depend on the water flow rate as to if you can pump it away to tanks or even to another underground hole for storage.

Tunnels even under the channel do not leak and thats covered with plenty of water that could fill it.

Converting the water to oxygen and methane sounds like a plan for some of it.

We drain mining operations even here on earth so it should not be an issue for mars as long as you got power.

We can also design the first submarines for mars if there is that much water to live in....

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#15 2019-05-16 17:51:57

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,513
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Re: One crater at a time

I'm not concerned about what to do with the water so much as its effect on the structure of the sides of the trench.

As a point of comparison, at Mars g 54 km of rock will exert a vertical pressure of about 6 GPa.  This is way more force than rock alone can withstand, but the structure of the rock makes a substantial difference.  If it's rendered into a sort of slurry or fractured and shot through with ice that will make a big difference to whether or not something like this can be done at all.


-Josh

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