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Old 01-22-2020, 11:40 PM   #21
Michael Thayne
 
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Default Re: Sci-Fi World-building: Mars the farm planet?

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Your main issues are nitrogen and water. Everything living requires nitrogen, and Mars practically has no nitrogen and very little water. In order to create 1 atmospheric pressure, you would require the equivalent of two Earth atmospheres worth of nitrogen (around 10 quadrillion metric tons). The nearest sources of massive quantities of free nitrogen are Earth (~0.8 atmospheric masses), Venus (~4 atmospheric masses), Titan (~4 atmospheric masses).

As for water, you would need a minimum of 10 quadrillion tons of water to make Mars as habitable as the Atacama Desert, and a minimum of 100 quadrillion tons of water to attempt any massive agriculture for local support. Anything less, and the atmosphere will just suck up the water.

So, we are talking about a minimum of 110 quadrillion metric tons of nitrogen and water. Now, you can produce oxygen locally, so you could reduce that to around 21 quadrillion metric tons, but you would need a way to transport the hydrogen (ammonia would be a partial solution, but 10 quadrillion tons of hydrogen can only transport ~2 quadrillion tons of hydrogen).

But wait, there is more. Every kilogram of matter traveling from LMO to Mars surface possesses ~10 MJ of energy, meaning that 21 quadrillion metric tons generates 210 YJ of energy. Mars only receives around 25 PJ/s, so the energy from transporting the necessary materials to Mars would be equal to 8.4 billion seconds of sunlight, around 250 Earth-years. To avoid boiling off everything, you would probably need to spend 1000 Earth-years just dropping stuff onto Mars.
I think some of your math might be wrong. According to this NASA fact sheet, the current mass of Mars' atmosphere is only 25 trillion metric tons. I might be wrong about this, but I thought the relationship between atmosphere mass and atmospheric pressure was linear?
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Old 01-23-2020, 12:00 AM   #22
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Default Re: Sci-Fi World-building: Mars the farm planet?

Also, could some plants—perhaps genetically engineered ones—survive in a mostly-CO2 atmosphere derived from melting Mars' (dry) ice caps?
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Old 01-23-2020, 01:07 AM   #23
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Default Re: Sci-Fi World-building: Mars the farm planet?

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Also, could some plants—perhaps genetically engineered ones—survive in a mostly-CO2 atmosphere derived from melting Mars' (dry) ice caps?
GEd single cell organisms is more plausible. We already have some living in pretty weird places to work with.
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Old 01-23-2020, 08:41 AM   #24
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Default Re: Sci-Fi World-building: Mars the farm planet?

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Your main issues are nitrogen and water.
On the amount of nitrogen, you "only" need about 60% of an earth atmosphere. You've got to stack twice and a half as much on any given spot, but mars has about a quarter of the surface area as earth.

But that's not a farming problem, that's a terraforming problem, and some neo-emperor or the other already spent the stupid amount of money to get you there. You've on a planet mostly terraformed but with too much CO2 where mars is, how do you profit off the former crazy whale's extravagance?
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Old 01-24-2020, 01:52 AM   #25
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Default Re: Sci-Fi World-building: Mars the farm planet?

If you have liquid surface water in a mostly CO2 atmosphere it will have significantly lower pH than on earth as the CO2 dissolves and form carbonic acid. Don't know what pH will be when the oceans are saturated as what I've read is about earth with 0.04% CO2.
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Old 01-24-2020, 05:30 AM   #26
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Default Re: Sci-Fi World-building: Mars the farm planet?

It depends on the partial pressure of CO2 and the temperature.

There's a table here:

https://en.wikipedia.org/wiki/Carbonic_acid

But genetic engineering can probably overcome the pH problem.
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Old 01-24-2020, 11:35 AM   #27
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Default Re: Sci-Fi World-building: Mars the farm planet?

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Still, you have to reckon that it will take on the order of centuries to create topsoil.
I'm not sure it needs that long. Volcanic lava flows can become vegetated in a matter of years or decades. They do have the advantage of having biotic material carried in from neighbouring regions, but they still develop a sustaining soil fairly quickly.

Source: My visit to Sakurajima volcano, and a tourist sign showing a diagram of revegetation. 30 years for grasses to colonise bare rock, 80 years for low shrubs, 100 years for tall trees, 200+ years for so-called "climax forest" (mature, steady state growth).
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Old 01-24-2020, 03:17 PM   #28
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Default Re: Sci-Fi World-building: Mars the farm planet?

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I'm not sure it needs that long. Volcanic lava flows can become vegetated in a matter of years or decades. They do have the advantage of having biotic material carried in from neighbouring regions, but they still develop a sustaining soil fairly quickly.

Source: My visit to Sakurajima volcano, and a tourist sign showing a diagram of revegetation. 30 years for grasses to colonise bare rock, 80 years for low shrubs, 100 years for tall trees, 200+ years for so-called "climax forest" (mature, steady state growth).
Around here a lot of spotted-gum forest grows on poor shale country where there isn't enough topsoil to grow crops. (Where the soil is eighteen metres deep the land has been cleared for farming.) Trees will grow to considerable heights in ground you couldn't possibly draw a plough through or pick a potato from if one grew, and put down deep roots in freely-draining material that won't retain moisture in the root zone of grasses and herbaceous crops.

You can grow plants in sand or vermiculite if you pour on enough water and fertilisers, but you have to keep them out of the rain. That's essentially hydroponics, not agriculture.

Of course futuristic designer crops could well be deep-rooted perennials that you run a header over but don't plough in and re-plant over.
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Old 01-26-2020, 06:13 PM   #29
Michael Thayne
 
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Default Re: Sci-Fi World-building: Mars the farm planet?

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As for water, you would need a minimum of 10 quadrillion tons of water to make Mars as habitable as the Atacama Desert, and a minimum of 100 quadrillion tons of water to attempt any massive agriculture for local support. Anything less, and the atmosphere will just suck up the water.
Thinking about this more, I wonder if it's more complicated than that. Sure you might need 100 quadrillion tons to get significant ocean coverage on Mars, but less isn't going to produce a uniform desert—I suspect 10s or 100s of trillions of tons of water would be enough for small seas / lakes here and there? Might be wrong about the dynamics of it tough.
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Old 02-12-2020, 09:57 AM   #30
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Default Re: Sci-Fi World-building: Mars the farm planet?

The why is simple. Somebody nukes Morocco and Earth no longer grows food.

https://en.wikipedia.org/wiki/Peak_phosphorus
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