There are numerous difficulties with living on Mars. I've talked about them before (see list of related pieces below). These are the three most serious:
- Radiation, from the sun and from outer space
- Very low temperatures (-60 C on average)
- very low atmospheric pressure (6 mbar compared with roughly 1000 mbar (millibar) at sea level for Earth)
The obvious solution is to give Mars an atmosphere. If the atmosphere is thick enough, temperatures will rise (Earth's temperature is roughly 33 degrees C higher than it would be without an atmosphere). An atmosphere will protect against radiation from the sun and outer space. And a higher atmospheric pressure means that we'd be able to walk outside on Mars's surface wearing only a breathing mask.
So how can we give Mars a thick atmosphere?
Zubrin and McKay in a paper written in the early 90s did some estimates of how we could produce a much denser atmosphere on Mars by melting the southern ice cap. They argued that if we raised Mars's average temperature by 4 degrees C, runaway global heating would occur, because rising temperatures would melt more of the ice cap's carbon dioxide which would in turn raise temperatures, melting more iced CO₂ and so on, until all the polar CO₂ ice was melted. Then it would be the turn of CO₂ adsorbed into the regolith. As temperatures rose, that would also be released into the atmosphere, ultimately raising the air pressure from CO₂ alone to between 500 - 1000 mbar.
However, this calculation is crucially dependent on how much CO₂ there is in the ice caps and the regolith, and 27 years after Zubrin and McKay wrote their article, new estimates suggest there is much less of it than was thought then. In fact, instead of polar CO₂ raising the air pressure by 50 - 100 mbar, it might only raise them by 10-15 mbar.
As the chart below shows, that is not enough to raise temperatures by enough for the CO₂ not to be redeposited at the poles all over again. The freezing point of CO₂ varies with the air pressure; the higher the air pressure, the higher the freezing point. But at 200 mbar (1/5th Earth's air pressure—for comparison air pressure at the summit of Mt Everest is 337 mbar), the freezing point of CO₂, is around -95 Celsius; at 300 mbar around -90 Celsius.
But, if we could get the temperatures up another way, for example by manufacturing sulphur hexafluoride, a greenhouse gas 23,000 times as potent as CO₂, then the CO₂ would remain in the atmosphere and not freeze out. Unlike CFCs, also powerful man-made greenhouse gasses, which are chlorine compounds, sulphur hexafluoride won't destroy any nascent Martian ozone layer, needed to reduce the impact of ultra-violet rays from the sun. By the way, humans would be able to go outside their domes without a pressure suit (but with an oxygen mask) at 200 mbar, though 250 or 300 would be preferable.
As the chart below shows, that is not enough to raise temperatures by enough for the CO₂ not to be redeposited at the poles all over again. The freezing point of CO₂ varies with the air pressure; the higher the air pressure, the higher the freezing point. But at 200 mbar (1/5th Earth's air pressure—for comparison air pressure at the summit of Mt Everest is 337 mbar), the freezing point of CO₂, is around -95 Celsius; at 300 mbar around -90 Celsius.
But, if we could get the temperatures up another way, for example by manufacturing sulphur hexafluoride, a greenhouse gas 23,000 times as potent as CO₂, then the CO₂ would remain in the atmosphere and not freeze out. Unlike CFCs, also powerful man-made greenhouse gasses, which are chlorine compounds, sulphur hexafluoride won't destroy any nascent Martian ozone layer, needed to reduce the impact of ultra-violet rays from the sun. By the way, humans would be able to go outside their domes without a pressure suit (but with an oxygen mask) at 200 mbar, though 250 or 300 would be preferable.
 |
| Data derived from Zubrin and McKay's formulae. |
So do we have to give up hope of terraforming Mars?
No, not at all. Because we can bring other volatiles (gasses, or substances which can be gasses or liquids, like water) from asteroids and Kuiper belt objects. In an article in The Space Review, John Strickland suggested that the volatile of choice should be nitrogen, aiming initially for a 50/50 nitrogen oxygen mix and then, later, a 70/30 mix. The nitrogen would be harvested by robots from nitrogen slush on Kuiper belt objects.
With current technology this is impossible, but (a) who knows what will be available in 20 years?, and (b) we are talking about colonising Mars, and we haven't even sent humans there yet. SpaceX will have gone from a Mariachi band and a carpet to uncrewed Starships on Mars in 20 years. My guess is that 20 years after we put the first humans on Mars, we will be exploring the asteroids and the Kuiper belt. Using Strickland's calcs, I calculate that every 36 loads of 100 million tonnes each of nitrogen would raise air pressure by roughly 1 mbar. If we wanted to raise pressures by 10 mb a year, we'd need 360 loads annually, and we'd reach 200 mb in 20 years, ignoring any desorption of CO₂ from the regolith. A target of 3 mb air pressure increase a year would mean 120 loads, or 1.2 billion tonnes, annually. A massive undertaking, but by then it will be technologically feasible. After all, we transport 2 billion tonnes of oil across the globe each year, while all we have to do once we have mined the nitrogen is to give it a push. In space, there is no air friction to slow vessels—once started, the load will keep going, though it will make it quicker if we can accelerate it. Once we have fully re-usable spacecraft, space will become really, really cheap. Shipping stuff from the asteroid or Kuiper belts to Mars and Earth will be easily affordable.
Without using lots of fuel, the journey by asteroids from the asteroid belt could take a decade, and from the Kuiper belt several decades. So this kind of rapid terraforming won't start until we've been on Mars for 30+ years, assuming we take 20 years from the first colonisation to being able to mine the asteroid and Kuiper belts. (Asteroid belt mining will likely happen long before we start crewed explorations of the Kuiper belt)
Another way to get volatiles like water and nitrogen is to bombard Mars with comets or asteroids. We're talking hundreds of comets, all aimed with exquisite precision. You don't want to hit a domed city, but even the shockwave from an impact with the surface would be very destructive. To prevent this, the comets will have to be disintegrated before they hit the atmosphere. It will likely be doable by 2040. We'll have the technology. It's be costly, but as the population of Mars rises, the political will and financial ability to terraform Mars will also rise. Because the asteroids and Kuiper belt objects are not in deep gravity wells, the fuel needed to nudge their orbits so they intersect with Mars's orbit, or to send nitrogen from them to Mars, is low. Even Pluto (now a planet again, I see) only has an escape velocity of 0.62 m/sec² compared with Mars's 3.711 m/sec² and Earth's 9.807 m/sec².
But all these terraforming techniques will take many many decades to take full effect. A century, prolly. Or more. For the first couple of hundred years we will be living on Mars in domes or underground. And perhaps only when air pressure reaches 300 mbar will we be able to start greening the surface of Mars, and then only with genetically modified plants. Although there is some evidence that lichens might grow in deep valleys or craters on Mars at current pressures, and oxygen-making cyanobacteria might survive when there is liquid water for much of the year at the equator.
A final point: Mars lost its atmosphere because it had no magnetic field to deflect the solar wind (a stream of charged particles from the sun, essentially the nuclei of hydrogen atoms). To stop this happening again, we'd have to give Mars an artificial magnetosphere, perhaps by situating a giant magnetic dipole (electric magnet) at the L1 Lagrange point. The authors of the NASA study estimate that because there is still outgassing of volatiles from Mars's interior, an artificial magnetosphere alone would be enough to eventually raise atmospheric pressure on Mars to allow for liquid water on the surface. Other analysts disagree, saying that volcanic outgassing is no longer enough to build an atmosphere. Either way, an artificial magnetosphere will stop Mars losing its new atmosphere like it lost its old one. If there is outgassing, then that will be a bonsella.
Our society will be wealthy enough one day to terraform Mars using nitrogen and water from asteroid or Kuiper belt objects, with an artificial magnetosphere to protect it. Just as the Dutch tax themselves to maintain the dikes which protect them, so will Martians pay for the terraforming of their planet. It just won't be as easy as Zubrin and McKay hoped. Not their fault: we have much better information about Mars now than we did 30 years ago. But terraforming Mars will happen, and prolly much sooner than we think.
These are the other pieces I've written about living on Mars:
These are some of the reference works I consulted:
Temperature of Mars
Carbon Dioxide and water phase diagrams
Terraforming Mars, Wikipedia
Could we terraform Mars? -- PBS Space Time
Nuking Mars's Icecaps won't geoengineer it
Rethinking the Mars terraforming debate (Strickland)
The biology of low atmospheric pressure
Technological requirements for terraforming Mars (Zubrin and McKay)
Inventory of CO2 available for terraforming Mars
NASA proposes building an artificial magnetic field to restore Mars's atmosphere
No, not at all. Because we can bring other volatiles (gasses, or substances which can be gasses or liquids, like water) from asteroids and Kuiper belt objects. In an article in The Space Review, John Strickland suggested that the volatile of choice should be nitrogen, aiming initially for a 50/50 nitrogen oxygen mix and then, later, a 70/30 mix. The nitrogen would be harvested by robots from nitrogen slush on Kuiper belt objects.
With current technology this is impossible, but (a) who knows what will be available in 20 years?, and (b) we are talking about colonising Mars, and we haven't even sent humans there yet. SpaceX will have gone from a Mariachi band and a carpet to uncrewed Starships on Mars in 20 years. My guess is that 20 years after we put the first humans on Mars, we will be exploring the asteroids and the Kuiper belt. Using Strickland's calcs, I calculate that every 36 loads of 100 million tonnes each of nitrogen would raise air pressure by roughly 1 mbar. If we wanted to raise pressures by 10 mb a year, we'd need 360 loads annually, and we'd reach 200 mb in 20 years, ignoring any desorption of CO₂ from the regolith. A target of 3 mb air pressure increase a year would mean 120 loads, or 1.2 billion tonnes, annually. A massive undertaking, but by then it will be technologically feasible. After all, we transport 2 billion tonnes of oil across the globe each year, while all we have to do once we have mined the nitrogen is to give it a push. In space, there is no air friction to slow vessels—once started, the load will keep going, though it will make it quicker if we can accelerate it. Once we have fully re-usable spacecraft, space will become really, really cheap. Shipping stuff from the asteroid or Kuiper belts to Mars and Earth will be easily affordable.
Without using lots of fuel, the journey by asteroids from the asteroid belt could take a decade, and from the Kuiper belt several decades. So this kind of rapid terraforming won't start until we've been on Mars for 30+ years, assuming we take 20 years from the first colonisation to being able to mine the asteroid and Kuiper belts. (Asteroid belt mining will likely happen long before we start crewed explorations of the Kuiper belt)
Another way to get volatiles like water and nitrogen is to bombard Mars with comets or asteroids. We're talking hundreds of comets, all aimed with exquisite precision. You don't want to hit a domed city, but even the shockwave from an impact with the surface would be very destructive. To prevent this, the comets will have to be disintegrated before they hit the atmosphere. It will likely be doable by 2040. We'll have the technology. It's be costly, but as the population of Mars rises, the political will and financial ability to terraform Mars will also rise. Because the asteroids and Kuiper belt objects are not in deep gravity wells, the fuel needed to nudge their orbits so they intersect with Mars's orbit, or to send nitrogen from them to Mars, is low. Even Pluto (now a planet again, I see) only has an escape velocity of 0.62 m/sec² compared with Mars's 3.711 m/sec² and Earth's 9.807 m/sec².
But all these terraforming techniques will take many many decades to take full effect. A century, prolly. Or more. For the first couple of hundred years we will be living on Mars in domes or underground. And perhaps only when air pressure reaches 300 mbar will we be able to start greening the surface of Mars, and then only with genetically modified plants. Although there is some evidence that lichens might grow in deep valleys or craters on Mars at current pressures, and oxygen-making cyanobacteria might survive when there is liquid water for much of the year at the equator.
A final point: Mars lost its atmosphere because it had no magnetic field to deflect the solar wind (a stream of charged particles from the sun, essentially the nuclei of hydrogen atoms). To stop this happening again, we'd have to give Mars an artificial magnetosphere, perhaps by situating a giant magnetic dipole (electric magnet) at the L1 Lagrange point. The authors of the NASA study estimate that because there is still outgassing of volatiles from Mars's interior, an artificial magnetosphere alone would be enough to eventually raise atmospheric pressure on Mars to allow for liquid water on the surface. Other analysts disagree, saying that volcanic outgassing is no longer enough to build an atmosphere. Either way, an artificial magnetosphere will stop Mars losing its new atmosphere like it lost its old one. If there is outgassing, then that will be a bonsella.
Our society will be wealthy enough one day to terraform Mars using nitrogen and water from asteroid or Kuiper belt objects, with an artificial magnetosphere to protect it. Just as the Dutch tax themselves to maintain the dikes which protect them, so will Martians pay for the terraforming of their planet. It just won't be as easy as Zubrin and McKay hoped. Not their fault: we have much better information about Mars now than we did 30 years ago. But terraforming Mars will happen, and prolly much sooner than we think.
o—o—o—o—o
These are the other pieces I've written about living on Mars:
These are some of the reference works I consulted:
Temperature of Mars
Carbon Dioxide and water phase diagrams
Terraforming Mars, Wikipedia
Could we terraform Mars? -- PBS Space Time
Nuking Mars's Icecaps won't geoengineer it
Rethinking the Mars terraforming debate (Strickland)
The biology of low atmospheric pressure
Technological requirements for terraforming Mars (Zubrin and McKay)
Inventory of CO2 available for terraforming Mars
NASA proposes building an artificial magnetic field to restore Mars's atmosphere
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