Saturday, February 1, 2014

Terraforming Mars vs Orbital Habs

Those who advocate Mars settlement like to say Mars can be terraformed. First I will take a look at what it would take to terraform Mars.

How much air do we need to add to Mars?

From NASA's Mars Fact Sheet, surface density of the Martian atmosphere is about .02 kg/m3. That is about 1.5% of Earth's surface air pressure of 1.27 kg/m3. Mars' atmosphere is virtually a vacuum.

Mars surface gravity is about 38% earth gravity. That means given an atmosphere of comparable temperature and composition, Mars atmosphere scale height is 264% earth atmosphere scale height. But Mars surface area is about about 28% that of earth's. 2.64 * .28 is about .75. To get comparable air density, we would need Mars' atmospheric mass to be about three quarters that of earth's atmosphere.

The total mass of the Martian atmosphere is about 2.5 x 1016 kg. Earth's atmosphere is about 5 x 1018 kg. So to make Mars surface air density earth like, we'd need 3.6 x 1018 kg of air added to Mars.

But do we need sea level air density? No, there are people who survive at higher elevations. This list of the world's highest cities show several places at around 5000 meter elevation. Granted the dwellers of the highest city La Rinconada, Peru don't live comfortably. But they demonstrate humans can endure air density half that of sea level. If half is sufficient, Mars only needs 1.8 x 1018 additional kilograms of air.

Would be Mars terraformers like to point at the frozen CO2 at the Martian poles. If Mars temperature is raised just a little, they hope the vaporized carbon dioxide would create a greenhouse effect that would cause more carbon dioxide to be vaporized. Their hope is that a runaway greenhouse effect could substantially boost Mars' atmosphere from frozen volatiles already in place.

According to Wikipedia, there is thought to be a 1 meter thick layer of CO2 at Mars north pole, a cap about 1,000,000 meters in diameter. At the south pole there is an 8 meter thick layer of CO2 over a cap having a 350,000 meter diameter. That's about 1.6 x 1012 cubic meters of CO2. Dry ice has a density of 1.6 thousand kg/m3. If all of that CO2 is vaporized (an optimistic assumption) that totals about 2.5 x 1015 kg of atmosphere. Short by almost 3 orders of magnitude, a miniscule contribution toward the needed 1.8 x 1018 needed kilograms.

Zubrin and McKay believe runaway greenhouse could boost Mars atmosphere to 300 to 600 millibars. Besides the polar dry ice, they also mention CO2 in Martian regolith. I believe most of Zubrin's optmistic estimates are influenced more by wishful thinking than hard data. But for the sake of argument I'll grant 300 millibars of CO2. 300 millibars of CO2 is not breathable. But let's say green plants combine Martian water and CO2 to make sugars and starches plus oxygen. Taking the carbon out of 300 millibars of CO2 leaves about 220 millibars of oxygen. Earth's 1000 millibar atmosphere is 1/5 oxygen, so perhaps a 220 millibar oxygen atmosphere would be breathable. But it would also be an extreme fire hazard. Apollo 1 taught us a pure oxygen atmosphere isn't a good idea.

Even with Zubrin's very optimistic scenario, it seems we'd still need to import 1.5 1x 1018 kilograms of nitrogen.

Can we add to Mars' air with comets?

Zubrin and McKay suggest  it'd take .3 km/s to nudge an ammonia asteroid in the outer solar system towards Saturn and then Saturn's gravity could throw the ammonia snowball Marsward.

"Consider an asteroid made of frozen ammonia with a mass of 10 billion tonnes orbiting the sun at a distance of 12 AU. Such an object, if spherical, would have a diameter of about 2.6 km, and changing its orbit to intersect Saturn's (where it could get a trans-Mars gravity assist) would require a DV of 0.3 km/s. If a quartet of 5000 MW nuclear thermal rocket engines powered by either fission or fusion were used to heat some of its ammonia up to 2200 K (5000 MW fission NTRs operating at 2500 K were tested in the 1960s), they would produce an exhaust velocity of 4 km/s, which would allow them to move the asteroid onto its required course using only 8% of its material as propellant. Ten years of steady thrusting would be required, followed by a about a 20 year coast to impact. When the object hit Mars, the energy released would be about 10 TW-years, enough to melt 1 trillion tonnes of water (a lake 140 km on a side and 50 meters deep). In addition, the ammonia released by a single such object would raise the planet's temperature by about 3 degrees centigrade and form a shield that would effectively mask the planet's surface from ultraviolet radiation. As further missions proceeded, the planet's temperature could be increased globally in accord with the data shown in Fig. 12. Forty such missions would double the nitrogen content of Mars' atmosphere by direct importation, and could produce much more if some of the asteroids were targeted to hit beds of nitrates, which they would volatilize into nitrogen and oxygen upon impact. If one such mission were launched per year, within half a century or so most of Mars would have a temperate climate, and enough water would have been melted to cover a quarter of the planet with a layer of water 1 m deep."

This scheme presupposes we could land a 20 gigawatt power source on a rock in the outer solar solar system. For comparison the Palo Verde Nuclear Power Plant, the largest nuclear power plant in the United States, produces about 3.3 gigawatts. So we're sending 6 Palo Verde Nuclear Power Plants out past Saturn. McCay's scheme stipulates using the comet's mass as reaction mass. So now we have a mining and transportation infra structure on the comet that digs up the ice and places this reaction mass in the nuclear rocket engine.

If we have the wherewithal to establish such infrastructure, we certainly have the ability to build habs on these rocks.

Asteroidal Real Estate

How much asteroidal real estate could 1.5 1x 1018 kilograms of air give us? An O'Neill cylinder 8 kilometers in diameter and 32 kilometers long would give us 804 square kilometers of real estate. Such a cylinder would have a volume of 1.6e12 cubic meters. On earth's surface, our air has a density of about 1.27 kg per cubic meter. So that volume at 1 bar density would be 2e12 kilograms of air.

1.5e18/2e12 = 750,000. Three quarters of a million O'Neill habitats. Recall each cylinder has 804 square kilometers of real estate. 750,000 * 804 km2 = 603 million km2. Mars' surface area is 145 million km2. So if we put the asteroidal resources to use where they're at, we get 4 times as much real estate.

Some would point out that O'Neill cylinders are very extravagant pieces of mega-engineering. I completely agree! It's my belief that humans don't need a full g to be healthy, I believe .4 g (a little more than Mars' gravity) would suffice. In which case the hab radius could be 1.6 km. Such a hab would have only  321 km2 of real estate but a volume only 2.6e11 cubic meters. 2.6e11 m3 * 1.27 kg/m3 = 3.3e11 kilograms. 1.5e18/3.3e11 = ~4.5 million. 4.5 million of the smaller O'Neill habitats. 4.5 million * 321 = 1460 million square kilometers. Or about as much real estate as 10 Mars planets.

If the goal is to provide more real estate and resources for humanity, terraforming Mars is an extravagant waste. We should ditch planetary chauvinism and go for the small bodies.

Robert Walker also takes a look at terraforming Mars.


Doug Plata said...

I would encourage a consideration of a third approach namely paraterraforming. This is where "green houses" are extended as the population grows. When the population covers Mars then you would have a so-called "world house". Pump the air into the greenhouses until it reaches the needed pressure.

However, my concern is that these mega projects can mislead us when determining which is the next right step to take. Rather we should give more consideration as to lower cost more near-term solutions.

The Moon is a safe distance away, there are more than enough volatiles at the poles to support an initial permanent base, shielding is readily available, the volatiles have a near-term market, operations could be expanded inexpensively using telerobotics.

AlanSE said...

I like the idea of getting the Nitrogen gas from Titan. It already has more Nitrogen than Earth. I bet the delta V is lower too, although I have not looked into it.

In its present state, it's only a few 10s of degrees from freezing anyway. So freeze it the rest of the way and truck liquid nitrogen from Titan to your destination. The legs of the journey can be broken up into ~2-3 km/s legs so that lots of reusable and reaction-less infrastructure can be employed.

But that's for the long future. Several manned spacecraft have had low Nitrogen content. A full atmosphere might be a luxury for space travelers. The NSS recommends a 50:50 mix of Oxygen and Nitrogen, which is a compromise. Nonetheless, they would have a different relationship with wind.

GabeT said...

Are you seriously suggesting that building 450,000 of your 321 km^2 O'Neil cylinder type habitats would be cheaper than fully terraforming Mars? If so, I would very much like to see your math. If not, then what's your point?

In any case, no one is suggesting terraforming Mars is easy. Rather, it is desirable. A new world rather than a steel container for storing humans.

Serious question. If you had to choose, would you personally spend a your life in a 321 km^2 space hab, or on a fully terraformed Mars?

Hollister David said...

Are you seriously suggesting that building 450,000 of your 321 km^2 O'Neil cylinder type habitats would be cheaper than fully terraforming Mars?

Ummm.... No.

Where did you get that? In no place have I suggested any price tag. Price tags for either is highly speculative.

If not, then what's your point?

Reread my post, this time for comprehension.

Using asteroidal volatiles where they sit would give a lot more real estate than crashng asteroids on Mars.

The small bodies offer many orders of magnitude more real estate and resources than planetary surfaces can give.

Serious question. If you had to choose, would you personally spend a your life in a 321 km^2 space hab, or on a fully terraformed Mars?

I believe attmpting to terraform Mars would make it an even more unpleasant hell hole than it already is. On the other hand I believe it's possible to make very pleasant spin habs. Definitely the 321 km^2 space hab.

Also asteroidal habs would be shallow gravity wells easy to enter and exit. Visiting an asteroid hab would not be the life time commitment of descending a deep gravity well.

Anonymous said...

I think one reason some people prefer terraforming is that they do not want to feel confined inside a structure and prefer to be under an open sky.
Please correct me if I'm wrong but I seem to remember a proposal for very large O'Neil type habitats that would meet that requirement. IIRC a torus spinning to create 1 g of say 100 miles diameter and open to space in the center would be able to hold onto its atmosphere without needing physical containment. If the center hole in the donut is say 30 miles in diameter then the interior of the torus should be able to hold one atmosphere shouldn't it?
Occupants would be living in a circular valley that would be open to space and have a blue sky. If 20 miles tall or so maybe several thousand square miles of living space on an inside out world.

Steve Mickler said...

Mistakenly submitted the previous comment as "anonymous". Just want to add if the torus is oriented outer edge toward the Sun that mirrors above and below the central torus opening could even give a 24 hour day and climate control. The outer perimeter could give radiation shielding in this orientation just like an O'Neil colony.

Steve Mickler said...

Another option is to partially terraform Mars by carving a deep crater or craters near the equator as suggested years ago by James Oberg in "New Earths" by directing an asteroid to impact. If combined with similar impacts on the poles to thicken the atmosphere the bottom of such a crater might have a pretty nice temperature. You'd still need to have an oxygen supply but the CO2 would help with the temperature so it might not be desirable to convert much of it to oxygen using plants, but if you did it might still be pretty pleasant and one could breathe the air and not need a pressure suit.
The mass of mirrors needed to redirect the asteroids using the "mirror bees" idea would be relatively modest and the mirrors could be retained in Mars orbit to increase insolation in the crater.
Might give a better bang for the buck than habitat construction. Far less infrastructure is needed and so it might be possible sooner and for much less initial investment.

Hollister David said...

Steve, I sympathize with the "Open Sky" argument. I live in Arizona and I love walking underneath the beautiful, wide open desert skies. That said, I think we grow accustomed to the environments we live in. I suspect there are people in our planet's more densely populated cities who dwell contendedly enclosed in mostly artificial environments.

With conventional terraforming, a column of air would extend tens of kilometers high and only a small part of the lower volume would be usable volume for humans, animals and plants.

Paraterraforming is more reasonable. With ceilings, the thick atmosphere isn't 10s of kilometers tall and you can get quite a lot of usable volume with a smaller quantity of atmosphere.

The lower the ceiling, the more square feet of real estate you get from a given volume of atmosphere. This geometry might influence hab designers towards small radius cylinders. Althouth I hope there will be some structures like the Island One O'Neill envisioned.

Anonymous said...

This post is compelling in its arguments, and I am convinced (as I had previously suspected) that orbital habs are more effective than the far less plausible mega-project of terraforming. However, a thought came to me on the subject of its long term sustainability - growth of human civilization is not invariably forward, there are flukes, regimes, dark ages. And habs have to be sustained by a capable, knowledgeable crew, and (I assume) must rely on regular foreign supplies. Any lapse in authority, it seems, and everybody living there dies. A habitable planet doesn't need station-keeping, imports of raw materials, or EVA capability if a bit of space debris hits. And if authority fails, and the people living there resort to barbarism, it will be devastating as it is when it happens on Earth, but it won't be certain death. Perhaps humanity's permanent future is confined to planets, while habs come and go. But maybe not.

jesrad said...

"That is about 1.5% of Earth's surface air pressure of 1.27 kg/m3. Mars' atmosphere is virtually a vacuum."

Err, no, it's nowhere near a vacuum, that is the same density our airplanes are designed to cruise through.

I'm intrigued, what do your calculations look like if you aim for the compromise of "merely" increasing Mars' atmospheric density to the point where humans can stay there by breathing pure oxygen without pressure suits (that is, 160 mBar) ?

Hollister David said...

Jesrad, scale height of earth's atmosphere is about 8.5 kilometers. That means density falls by a factor of e (about 2.72) each 8.5 km. .015 = ~e^-4.2. 4.2 * 8.5 = ~36. So at 36 kilometers altitude our atmosphere's density is about the same as Mars surface. 747s etc. normally cruise at about 11 km altitude.

Apollo 1 is a good argument against pure O2 atmosphere. In any case an atmosphere from Mars volatiles and imported asteroids and comets would be a mixture of CO2, NH3, H2O and other CHON compounds. Making a pure O2 atmosphere would be a lot harder, even if it weren't an extreme fire hazard.

jesrad said...

Silly me confusing feet and meters for altitude :)

I like the idea of carving an habitat into a reasonably big asteroid. The dug material may feed a mass driver for rotating the whole thing and for course corrections...

glenn robinson said...

I once caught a mouse on a glue trap. Its cries were pitiful until I dispatched it with a shovel. Earth's gravity well is also a glue trap. I fail to see why, when, multo labore, we exit earth's trap, we would voluntarily settle another. The point is to exponentially expand geographically to avoid the otherwise inevitable planet-killer object. To do this, we must create a viable non-planetary barter economy on the inter-planetary equipotential plane. Tall order, but do-able. Once more, it's a time for visionaries and heroes. Accountants and other apparatchiks can have their day later.

Regarding the "open sky" issue, SF made the point long ago that a moderately large office tower is, in a way, a space ship.