Terraforming: Mars or Venus?

In an earlier post, I described the difficulty a small planet, like Mars, faces when people are trying to terraform it: the planet simply doesn’t have enough gravity to hang onto the gases we need to survive.

Some people suggested that maybe instead of Mars, we might look at Venus, since it has the right size.

In this post, I look at the most important deciding factors for the success of a terraforming operation.

Distance from the sun:
Venus is at 0.7 AU from the Sun, which is beyond the inner margin of the habitable zone at roughly 0.9 AU – yeah, we’re very close to that. I hope it doesn’t give you nightmares.
Mars is at 1.5 AU from the Sun, on the very outer edge of the habitable zone. However, the width of the habitable zone depends not just on the sun and its strength, but also on the composition of the planet’s atmosphere and on the presence f clouds. With the addition of clouds, the inner boundary of the habitable zone could be made to stretch inwards. However, water vapour and carbon dioxide (the most likely gases to form clouds) are both greenhouse gases, so the effect likely cuts both ways. There is a lot more leeway on the outer side of the habitable zone. With the addition of carbon dioxide clouds, it can be made to stretch outwards, to as much as 2 AU or even more.
Winner: Mars.

Atmosphere composition and density
The atmosphere of Venus is so dense that we can’t see its surface. It consists of mainly carbon dioxide (96%), some nitrogen and sulphur dioxide. Nasty, nasty stuff
The atmosphere of Mars also consists of mainly carbon dioxide, but it is extremely thin. Both atmospheres lack oxygen and nitrogen, but because of its high temperature, Venus also lacks water, of which Mars has quite a bit, even though it’s frozen. Mars also has frozen carbon dioxide. When thawed, both carbon dioxide and water vapour are greenhouse gases, and both would further increase the rate of warming (this is called positive feedback). At Venus, you’d need to get rid of massive quantities of carbon dioxide. Again, this is possible in theory (see Terraforming by Martin Beech), but even if you got it to work, it would be a process taking millennia, while a warming of Mars could be achieved in a few hundred years.
Winner: Mars

Planet mass
Mars, at only 15% of the Earth’s mass, with 32% of Earth’s gravity, is below the mass required to hold onto a breathable atmosphere.
Venus, at 81% of Earth’s mass, and 90% Earth gravity, is not.
Basically, providing you can create an atmosphere resembling Earth’s, Venus would retain it without the need to replenish it, whereas Mars would not.
Winner: Venus

Mars has a nice 24-and-a-bit hour day. At Venus, however, the planet’s day (243 Earth days) is longer than its year (225 Earth days). This, in combination with stronger sunlight, would make plant growth extremely hard, unless you could somehow speed up the planet’s rotation. I need not say that this would be hard to achieve, although theoretically possible.
Winner: Mars

Required change in temperature
In order to harbour life, the plant’s average temperature must be high enough for liquid water to exist on its surface. Every degree of change a planet requires to bring its temperature within this range is going to cost effort and time, and a lot of resources.
The freezing point of water is 273K; the average surface temperature of the Earth is 288K.
The temperature of Mars is 210K
The average temperature of Venus is 735K, and is the hottest object in the solar system apart from the sun, hotter even than Mercury.
Add to this that an increase in temperature is probably easier to achieve than a decrease, the clear winner is Mars, by many, many miles.

So yes, while people may have concluded from a previous post that Mars is too hard to terraform, and maybe we should look at Venus, maybe these people need to think again. It seems that its size is just about the only thing it has going for it.

The conclusion from this should probably be that terraforming is never going to be easy, and nor should it be in your fiction. Lots of things to go wrong, lots of things taking longer than expected, lots of unexpected stuff happening.


14 comments on “Terraforming: Mars or Venus?

  1. I heard a theory floated that if we could increase Mars’ spin, that might bring its gravity up a little. I’ve never done the homework, so I don’t know how plausible it is or if it would get you a breathable atmosphere for the average person (or even someone from the Himalayas). But it’s a thought.

    • Gravity is related to mass (and indirecty to density), so unless you can increase a planet’s mass significantly (while maintaining or increasing density), you can’t increase the gravity. Rotation has nothing to do with gravity. In fact, it is my half-educated guess that an increased rotation rate will lead to more violent weather, which in turn will lead to a quicker loss of those gases the planet’s gravity is too weak to retain.

  2. Good timing. The other night there was an interesting TV show on just this subject. Apparently Mars used to be quite similar in makeup to Earth, but lost its atmosphere as the millenia progressed. I wish I could remember what it was. I only caught the second half, but it looked like good viewing for SF writers.

  3. There is a way to increase “gravity” by increasing rotation. Unfortunately, it requires you to stand on the INSIDE of the rotating planet/asteroid/space station/Ringworld.


    And if you’re on the inside, you can have a sealed environment. And that defeats the whole terraforming idea anyway. Sigh.

  4. Ah, of course David. The station in the Gaea Trilogy uses that in quite interesting ways (plus it’s one of my fav series):

    I found what that TV show was called. It’s on 9.30 Saturday nights on 7mate – so if you have a life you’re out of luck – it’s called Universe. I haven’t seen most of the shows, but I’m going to try and get hold of it. It has some very good background for SF worldbuilders.

    PS – I should have said it but you’re post was great Patty. I looked, but couldn’t find the other Worldbuilding post you mentioned. Got linky?

  5. Cool! Thanks Patty. I was wondering, do you know if creating a thicker ozone layer would compensate for the lesser gravity, or would it still drift into space.

    PS – do you have references for this kind of thing. It’s interesting. I’m (slowly) getting together some non-fiction inspiration (Jared Diamond is a GOD!)

    • I use a wide conglomerate of hard-copy books, plus I follow articles from online magazines, notably Astrobiology magazine. Wikipedia is surprisingly informative and up-to-date with their astronomy references. For some of the books, check the tag ‘references’ at the bottomof the column on the right-hand side.

      I tossed up doing the proper scientific references thing, but I thought it might be intimidating, since I’m trying to appeal to the non-scientifically-trained.

      Then again, it’s probably a good idea to do another references post.

  6. The life zone is extrememly hard to calculate. First, there are so many factors that have not been considered, including planetary mass, composition, and spin. Spin too slow at a marginal orbit, and your planet fries. Have too much CO2 even at Earth’s orbit, and you still fry. Too small, you lose atmosphere. Too many volcanoes, or disturbance of an energy budget of a planet, and you cook. This is why we have so many different estimates of varying widths of lifezones, with almost none of the estimates being correct. The most plausible one I found was Venus and Mars being within the zone, but close to the edges. Of course, this doesn’t help if the other imperfect scenarios previously mentioned are present. Also, there is an equation that calculates the life zone, involving the diameter, and luminosity and spectral type of the star. That formula figures from where Venus is to a little past Mars. Those are the closest correct estimates I could find. Venus really needs a 24 hour long day and not six months, especially due to the closer proximity to the Sun. I would not ever live on or near the Equator, even on a perfectly spinning, and terraformed Venus!!!!

  7. As far as I can see, Venus has some more advantages over Mars in regard to terraforming. First, it is much closer to Earth, so the travel there and back again would be much easier (at least, the convenient “time frames” for the flight come much more often). Second, it is much closer to the Sun – so, the solar panels, which, most probably, will be the main energy sources in the beginning of the colonisation, will work much more efficient there (in the orbit around Venus, at least).

  8. And, considering 243-long day: there are many places on Earth, where there is polar day / polar night. There are even big cities in that zone (Murmansk, Norilsk), people live there, animals live there, plants grow there – so, there’s nothing terrible about 4 months of continous sunlight / darkness. And when the Earth was warmer (oh course, not as hot as Venus now) – there were even tropical forests above the Polar circle.

    Of course, polar day/night may be a bit inconvenient – but, anyway, you can easily manage without speeding up the planet’s rotation.

  9. In a way, I kinda think it may be easier to terraform Venus than mars.

    The real problem with Venus is the extremely dense Carbon Dioxide atmosphere which causes great pressure, as well as global warming effects. However, this means that there is both abundant oxygen and carbon, but it is lacking Hydrogen.
    It has been theorized that since Venus has a weak magnetic field, (probably due to it’s slow rotation) that the Solar Winds were able to carry away much of the hydrogen that was on the planet. So if we can re-introduce Hydrogen to the atmosphere, and separate or extract the carbon from the oxygen, The atmosphere should basically liquify and we would have massive oceans that replace the dense atmosphere, and either massive plant life, giant stores of graphite, or hydrocarbons for storing the Carbon.
    As for the extra solar radiation from being close to the sun, the terraforming process would require massive energy inputs, so if solar shading were installed to capture and harness some of this energy it could reduce the atmospheric temperature while generating this energy at the same time. Also, once the oceans start appearing, the water vapor will create a reflective cloud layer, thus reflecting more light back into space. (I assume more than sulfuric acid clouds do anyway)

    I suppose another challenge would be actually installing the equipment to perform the transforming in the hostile environment, and acquiring the hydrogen but if you have the resources to attempt a transforming project, I’ll assume it could be handled.

    • Yes, your argument sounds good, microbes could do wonders on Venus, by seeding the upper atmosphere at first. Then just gradually adding life that could survive, as conditions change. Just send low cost dispenser spacecraft with the microbes you think would work. Hydrogen if needed may be found in astroids. Mount a rocket engine and send them gradually on there way. Missing Earth I hope.

  10. Another factor in habitability is the carbon cycle. Scientists still say that Venus is too close to the sun to hold water, BUT, I found that water can be lost in other ways too. I have also found out that Venus is too far from the Sun to boil water directly, but an early history of slow rotation, C02 with no oxygen and no living things to release oxygen, would help to break a carbon cycle by causing the water to rise too high in the atmosphere, then having the hydrogen break apart from the water molecule by action of the sunlight, so your water still disappears. Add the rising temperature due to the broken carbon cycle, and then your water boils away, but you will find that it was never due to direct action by the Sun. For an example of where a planet would be Venuslike due to a really too close proximity, you would look at an old Discover magazine. There was an example of a planet around a sunlike star like Venus that was a little smaller, but was actually too close, like only 50% of Earths distance. That would be 4X the solar output. If Venus had Eaths rotation, life, and atmosphere, it is possible that at least some of the water could be retained. The trick is to make it so that the stratosphere never gets wet. Once you have water rise that high, all bets are off. Venus’ orbit is a close case, so it is NOT possible to tell just by atmospheres of planets to know if it is too far or too close to a star. Mercury is obviously too close, of course.

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