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Whilst considering the terraforming Mars in my science fiction project, I realised something rather important: Mars has no plate tectonics.

That means that, assuming the humans in my project manage to terraform Mars without restarting its rather sluggish geological cycle, the planet is inevitably fated to become uninhabitable again: without plate tectonics, the carbon cycle is incomplete, and co2 will slowly build up in the atmosphere. Eventually, won’t it will become like a miniature Venus?.

So, assuming that Mars is given an Earth-like atmosphere, oceans and a magnetic field, (all that jazz), roughly how long before the buildup of co2 causes a runaway greenhouse effect and renders the planet barren again? I’m not looking for a precise date; just knowing the general timescale (thousands, millions, billions of years) will be sufficient.

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    $\begingroup$ The problem of atmosphere being stripped by the "solar wind" without a strong enough magnetosphere to protect it may be a more pressing one. $\endgroup$
    – Pelinore
    Apr 5, 2023 at 16:56
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    $\begingroup$ From where is all that CO2 supposed to come from to turn a terraformed Mars into an imitation Venus? (For example, Earth has an awful lot of carbon than Mars, and not even Earth has anywhere near enough carbon to go the way of Venus.) $\endgroup$
    – AlexP
    Apr 5, 2023 at 17:01
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    $\begingroup$ re: atmospheric stripping, I have read before that it's a "millions of years" process. If you can fix the atmosphere once, then it will be good for quite a long time. Needs occasional top-offs on like the thousand-year scale. $\endgroup$
    – JamieB
    Apr 6, 2023 at 13:30
  • $\begingroup$ @JamieB More often that that. See my answer and my comments to JBH's answer. $\endgroup$ Apr 6, 2023 at 21:23

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This is a Frame Challenge

No group of scientists worth their salt would successfully terraform Mars and not have a way to keep it that way.

Your question only makes sense if there's a story-related incident — like the equipment breaking down. Consequently, the only practical answer to your question as-written is...

  • It will last as long as you need it to in your story because Mars was successfully terraformed in the first place without the benefit of plate tectonics.

Yeah, yeah... but let's pretend that Clarkean Magical equipment broke down? How long could the planet remain habitable?

Even this assertion is worth challenging because you haven't defined what you mean by "terraforming Mars" (which you would need to do in considerable detail). It has a lower gravity and we assume a human-breathable atmosphere, based only on these two variables it's going to have a lower density atmosphere almost no matter what you do,1 which means it will not be a duplicate of Earth. The atmosphere is almost required to be full of greenhouse gasses just to keep the average temperature on Mars equal to the average temperature on Earth. You've obviously imported some water, too. So, the equipment breaks down... how long before...

  • The atmosphere is too thin to breathe or...
  • The atmosphere is too cold to "inhabit," or...
  • The seas sublimate away or...
  • The natural storms return...

Or a thousand other things that will compromise the ability to inhabit the surface without the benefit of greenhouses to grow food, water to drink, and pressurized housing to live in?

Too many variables!

Which makes this version of the question story-based, too. It will become uninhabitable as quickly as you wish using whatever one or more of those variables to rationalize the decision. Keep in mind, we don't have a referent to justify even an educated guess. Many (if not most) of those variables are interdependent. If someone tried to tell you that it would take X number of weeks, months, or years for the atmosphere to thin out, they'd need to prove it by demonstrating they'd dealt with at least a fair number of the other variables (respiration of vegetation, decay in the oceans, compromise of anaerobic bacteria in the soil...).

Yeah, yeah... just give me a number!

Fine... one year.

What? Terran or Martian? I don't know... Aaaaaahhhhhhhhh!


1M.A.Golding correctly points out that there are smaller bodies in our own solar system with higher density atmospheres. If you continue reading my answer, you'll see me conclude that there are "too many variables." If the one and only variable we consider is gravity and all other variables are equal, then the atmospheric density must be lower. Why is Venus' atmosphere almost 100X more dense? Because it's filled with much heavier elements than Earth's atmosphere is. Too many variables.... Keep reading, it'll make sense.

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  • $\begingroup$ @ JBH "It has a lower gravity, so it's going to have a lower density atmosphere almost no matter what you do, which means it will not be a duplicate of Earth." The surface gravity of Venus is 0.904 that of Earth, but it has 93 times the surface pressure of Earth's atmosphere, Titan has 0.138 the surface gravity of Earth and a surface pressure 1.45 That of Earth. Your statement is obviously incorrect. $\endgroup$ Apr 6, 2023 at 11:50
  • $\begingroup$ @HBH "The atmosphere is almost required to be full of greenhouse gasses just to keep the average temperature on Mars equal to the average temperature on Earth." And possibly there are some greenhouse gases which are super efficient and which can keep Mars warm without poisoning or suffocating humans. And possibly the fusion reactors which provide enough energy for the colonists will also produce enough waste heat to keep Mars warm enough. If not extra fusion generators can power heaters to keep Mars warm. $\endgroup$ Apr 6, 2023 at 11:56
  • $\begingroup$ @M.A.Golding "and possibly fusion reactors which provide..." is exactly the point of my challenge. The problem the OP is asking about has already been solved. The only way for the question to make any sense is for your fusion reactors to have stopped working.... $\endgroup$
    – JBH
    Apr 6, 2023 at 14:29
  • $\begingroup$ @M.A.Golding "There is a lot more and heavier gas on Venus than on Earth, and since Venus is smaller than Earth, the pressure that the gas exerts per square inch is much higher." (Source) To quote my answer, "Too many variables!" I regret the simplification (based on the assumption that we're talking about a human breathable atmosphere) as it obviously ignores many variables just as you did with your comparison to (e.g.) Venus. I'll update the answer to reflect this. $\endgroup$
    – JBH
    Apr 6, 2023 at 14:32
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Considering the low escape velocity of Mars, the atmosphere would tend to escape into outer space, Thus it would be necessary to built a roof over the entire surface of Mars to keep the air in, with giant airlocks for spaceships. The roof might be supported by giant pillars and beams, or by air pressure.

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

I presume that a thinner atmosphere would be placed above the roof, to stop most meteoroids from becoming meteorites and striking the roof. The thin present atmosphere of Mars does burn up most meteoroids far above the surface anyway.

With a planetary roof, there would be no need for a planetary magnetic field to stop solar radiation from stripping away the atmosphere.

Because of leakage, more atmosphere would have to be continually imported or manufactured on Mars. So the air factories would be working continually. And no doubt they would break up excessive CO2 molecules into oxygen to be released into the atmosphere and carbon to be taken out of the atmosphere.

Thus there would be no need for Mars to have plate tectonics to remove carbon dioxide from the air.

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  • $\begingroup$ "Considering the low escape velocity of Mars, the atmosphere would tend to escape into outer space..." Venus.... Unless we're talking about a human breathable atmosphere. $\endgroup$
    – JBH
    Apr 6, 2023 at 14:37
  • $\begingroup$ @JBH I'm not sure what you meant with your comment. According to Habitable Planets for Man, Stephen H. Dole, rand.org/content/dam/rand/pubs/commercial_books/2007/… [age 35, table 5, shows the time for a world's supply of a gas in the atmosphere to fall to 0.368 of the original amount depends of the radio of the escape velocity of the world divided by the root-mean-square velocity of that gas in the exosphere of the planet. ON page 54 Dole decides that the minimum escape velocity for a human habitable planet should be 6.25 kilometers per second, Continued. $\endgroup$ Apr 6, 2023 at 21:12
  • $\begingroup$ @JBH Continued Mars has a mass of 0.107 Earth and an escape velocity of 5.027 kilometers per second. If Mars has an exosphere temperature as low as 1,000 degrees Kelvin and thus an oxygen root-mean-square velocity of 1.25 kilometers per second in the exosphere, it should still have 0.368 of the oxygen left after just a few thousand years. Thus the terraformed Martian atmosphere would have to constantly be replenished by the air factories converting various oxygen containing substances to oxygen. Unless they put a roof over Mars and greatly slowed atmospheric loss as I suggested. $\endgroup$ Apr 6, 2023 at 21:17
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Mars will remain marginally habitable for millions to billions of years after being terraformed. Earth like conditions will last for less than 100 million years, being followed by glaciation. Nasa's MAVEN mission found that the current atmospheric loss rate is too small to explain Mars's transition from a warm, wet planet to a frigid desert. Studies rely on the young sun having a stronger solar wind to remove most of Mars's atmosphere. Other papers indicate that asteroid and comet impacts of the late heavy bombardment also cleaved off much of Mars's early atmosphere. Regardless, if Mars was gifted an Earth-like atmosphere it would retain the nitrogen and some of the oxygen over geologic time.

Mars has no plate tectonics and large volcanoes. This combination would not cause Mars to progress towards a Venus-like state because a habitable Mars would have biological carbon sinks. On Mars, volcanoes would emit carbon dioxide to then be cycled into life. Eventually carbon is trapped as peat or ocean sediment, permanently removing it from the atmosphere. Today, Mars's volcanoes rarely erupt. Therefore, photosynthesis would cause Mars to loose its insulating blanket of CO2, causing the planet to freeze over and enter a snowball state.

Mars's habitability could be extended significantly by not bringing any more water to Mars. In this scenario, it would lack oceans, retaining only lakes and seas. The lower precipitation would cause Mars to freeze over more slowly as plant life would struggle to take large amounts of carbon out of the atmosphere. While the low rate of volcanic activity prevents Mars from maintaining a warm, Earth-like climate, a stable ice age might be possible with complex life hanging on in a handful of refuges.

Conditions in the Valles Marenaris would be near-freezing almost year round as its equatorial location prevents temperature variation. The main Hellas Basin refuge would be more seasonal, owing to its higher latitude but much lower elevation. Depending on how warm the Martian summer gets, the lowest parts of Hellas might be able to support taiga. Any large animals would hibernate to survive the long winters.

Every hundred thousand years or so, volcanic blasts from the Cerberus Fossae would produce a slightly warmer period, allowing the barely viable steady state to persist. Much more rarely, activity from Mars's great shield volcanoes would produce a surge in atmospheric CO2 levels, perhaps creating a Earth-like warm period. The imported biosphere would struggle along until the solar wind strips Mars's atmosphere once more or a large impact winter cools the planet to cause CO2 to freeze out of the atmosphere. In either scenario, single cell life is forced into groundwater refuges while multicellular life dies out completely.

IF Mars still has an atmosphere a couple of billion years from now, the increasing luminosity of the Sun would cause the planet to unfreeze itself. However, life would still be crippled by low levels of CO2.

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Mars is "a little bit" further away from the Sun than Venus is. Global warming is not an issue. It hasn't killed Earth's biosphere either.

Insolation on Mars is only 43% of that on Earth. There's less energy going in, and a greenhouse effect would be very helpful to keep it in.

CO2 isn't a magic wand that heats a planet out of nowhere. Terraforming Mars to be even borderline habitable is likely to require much stronger greenhouse gases than CO2. Yes, deliberately produced and introduced into the atmosphere.

The best-case estimates of early Mars climate, with a total CO2 atmosphere at 2 bar, still show temperatures below freezing.

So even with maximum global warming, you're at best looking at a climate like Siberia or Alaska, rather than any concern over excess heat.

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