A few comments about Geospermia (Analog, May 2013)

Yes, I know it isn’t May yet, but the May 2013 issue of Analog with my story in it is out in the wild, and has been sighted by US subscribers (if not yet by me).

Martin Shoemaker alerted me to a discussion on the F&SF forum about the issue in which a few people mentioned my story Geospermia. For those who have followed me on various social networking sites, this is what I loosely termed the “pandas on Mars” story.

It seems that people take away various messages from the story, which is interesting to see.

To me, this story is mostly a biological SF story. Yes, there is terraforming and there are conflicting ideologies in the human population in this habitat, but it is a story about the realities of trying to grow stuff in soil that has never grown anything. I touched on this subject in my posts about farming on Mars or about growing crops in space.

If you try to to replicate some sort of ecosystem under circumstances that are different from the original, it is very likely that something unexpected will happen. Species which should do well don’t, and ones that hadn’t been on the radar become invasive pests. Nature is good at throwing curveballs like that.

In another, much earlier post, I described that I used to work in pasture ecology, where people actively introduce species for the improvement of pasture quality. The process goes like this (simplified): scientists travel overseas to identify species that have desirable characteristics and collect seed. They take the seed home (fumigated through quarantine) and grow plants inside a quarantine glasshouse. Plants that pass inspection will then go into pots to bulk up seed quantity and then into small plots in various locations in the field. People will constantly monitor the plants. It is virtually impossible to predict which plants will do well in the new environment.

Supposing you had a habitat on Mars ready to be populated with living things, how would you go about deciding what to put in? Apart from selecting plants and animals that are adjusted to each other, I suspect that the reality would have a wet-spaghetti element to it (you throw it at the wall to see what sticks). Each of the differences between normal growing conditions and conditions in the new Mars enviroment will influence each species in a different and often unpredictable way. Therefore, you will have a species that may well be timid and unremarkable on Earth run riot on Mars, because it just happens to be less sensitive to the conditions on Mars that are different from Earth. I’m thinking about soil composition (salts and fine particles), light conditions and high carbon dioxide.

Is the story depressing? I don’t think so. What we tend to get from a lot of hard SF is a very big picture, a bird’s-eye camera view of the new society without much detail about what the lives of people inside settled habitats are like on a day-to-day basis. People in these new habitats face the realities and frustrations of trying to grow stuff that should grow but won’t and other stuff that grows but they wish it didn’t. They face the responsibility of churning out food on a regular basis. Their life contracts to their reality, mostly limited to the inside of the habitat, just like many people rarely travel outside the town where they live. This reality is none less interesting than the bigger picture, and is more human.

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

Daylength
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.

How the size of a planet determines its atmosphere

People have asked me to write something about terraforming Mars, a subject that is of great interest to SF buffs. I think Kim Stanley Robinson in his Mars series does a pretty good summary of all the currently-held scientific opinions on techniques of how we could achieve this. Yes, it would involve some form of artificial warming, it would involve melting the polar ice and finding underground water. It would involve long-term (think hundreds of years) and large-scale engineering, but most importantly, two things: it would involve the introduction of large quantities of nitrogen, and it would be a constant and ongoing process, since there is nothing we can do about the main reason why Mars lost nitrogen and oxygen from its atmosphere in the first place: the planet is simply too small.

The mass of a planet defines its gravity. To understand this, we need to go back to physics. According to Newton’s law of universal gravitation, the attractive force between two bodies is proportional to the product of their masses, and inversely proportional to the square of the distance between them:
If one of the masses were really small, say a human body compared to a planet, the force described by this law would be what we experience every day as gravity. The law says: more mass, more gravity. Hence, gravity on the Moon is smaller than that on Earth, which in turn is smaller than that on Jupiter (presuming there was a surface on Jupiter you could stand on).

Another term: escape velocity, which is the speed needed to break free from a gravitational field. Speed itself is attained as function of an object’s mass and the forces holding it in place versus the forces applied to it in the other direction (OK, I’m confuddling speed with acceleration here, but you get the gist). Since the force that holds all objects on a planet is gravity, which is the same for objects of all sizes, it becomes easy to see that if you are planning to propel something off a planet, the best tool available to you to make it easier is to reduce the object’s mass.

In simple terms: your highest jumps on Earth would be mega-leaps on the Moon, while on an asteroid, your best effort at jumping might well hurtle you into space. Try jumping on Jupiter, however, and you might not even get off the ground. Say you wanted to get off the ground on Jupiter, you might train your muscles, but assuming you are reasonably fit, it’s probably more effective to go on a strict diet.

Now imagine you were a particle in the atmosphere. Our atmosphere is made up of gas. The gas state indicates atoms or molecules moving about freely without any attachment to each other. In a gas, particles are constantly bouncing off against each other. In doing so, they acquire speed, helped along by the heat of the Sun’s rays. For a proportion of these particles, that will mean gaining a speed greater than the escape velocity of the planet.

Following the above, the heavier a particle is, the less easy it will gain that speed. Earth’s gravity, for example, is too weak to hold onto its hydrogen or helium. Jupiter’s gravity is not. This is why the large planets are all gas giants. A planet starts retaining helium at about 6 Earth masses, and when it is bigger, about 10 Earth masses, it will retain hydrogen.

Move down the periodic table and see that of the next of the most common elements in the universe that exist in gas form at relatively low temperatures, Earth’s gravity is strong enough to retain atomic nitrogen. Nitrogen (atomic weight 14 grams per mol) is an element that doesn’t tend to form bonds easily. Simply said, it likes being a gas. The next element down, oxygen (atomic weight 16 grams per mol), loves attaching itself to other things, and bonds readily with carbon, another element common in the universe, but not a gas at any temperatures below 4000C. Carbon dioxide, nitrogen dioxide water and methane are all simple combinations of these common elements that we find in Earth’s atmosphere. Add up their atomic weights (44, 46, 18 and 16 grams per mol respectively) and you will see that as planet size becomes smaller, the atmosphere will lose its methane at approximately the same size as it loses its oxygen, then water, then carbon dioxide and then nitrogen dioxide.

Where is Mars on this scale? Well, it’s just big enough to retain carbon dioxide and nitrogen dioxide. There is plenty of the former, but since nitrogen in atomic state is lost from Mars, and since nitrogen doesn’t easily react with anything much, there is not much nitrogen dioxide to be had. Thus the atmosphere of Mars contains mainly carbon dioxide. If we were to artificially establish an atmosphere on Mars, any oxygen wouldn’t immediately evaporate into space, but the atmosphere would require constant and probably expensive maintenance.

This a very simplistic summary of an interesting subject. Yes, things are not quite as straightforward as this (you may stop yelling ‘Titan’ in the background, you over there), but these are the simple, physical processes involved. If you want to read more, I recommend the book Terraforming by Martin Beech.