I grew up watching Marvin the Martian and Bugs Bunny fight it out over the Illudim Pew-36 Explosive Space Modulator. Along the way, I also had a run-in with Bradbury's The Martian Chronicles, Robinson Crusoe on Mars, E. E. "Doc" Smith's Lensman series, the earlier works of Robert Heinlein and a whole host of works that depicted Mars as a vital, life bearing world. Venus often followed along side, usually as a world of swamps and tropical jungles.
Somewhere in the back of my mind, I knew it wasn't right. I'd been exposed to enough science before I started kindergarten that I knew the other worlds in the solar system were barren, lifeless places. But like any child, I didn't let a little thing like reality distract me from dreams of walking on other worlds rich and flush with life.
I'm a long way from those Marvin the Martian cartoons, but some part of me still longs for a Mars you could run across without a space suit and an oxygen tank. I follow the terraforming discussions the way some people follow their favorite football team.
What I see doesn't make me happy.
These days, most terraforming discussions center around Mars, and the discussions can get pretty heated. People argue about ethics. They scream about protecting any life forms that might exist on Mars. They go on about how easy or how hard it would be to terraform the planet. They put forth ideas and theories, some of which are even intelligent.
What I have yet to see is anyone admit just how massive an undertaking terraforming any of the bodies in our solar system would be. This is disturbing because any attempt to mount a terraforming effort that does not realistically consider all the challenges involved is ultimately doomed to failure.
Since most of the current terraforming discussions focus on Mars, let's start by taking a look at what would be involved in making Mars Earthlike.
One of the most popular suggestions is to start by bombarding the planet with large, fast-moving asteroids. This, proponents suggest, would heat the planet up. They argue that polar strikes could melt the carbon dioxide ice caps, releasing massive amounts of CO2 into the atmosphere which would result in a greenhouse effect which would trap sunlight and heat the planet. Others suggest that a large enough strike could even restart the planet's geological processes.
Using asteroid bombardment would seem to make sense. After all, an asteroid impact can deliver more force than all the nuclear weapons on Earth combined. The problem is, that's still not much energy on a planetary scale. There are two basic scenarios here. One, a single large impact at the poles would result in the entire polar region becoming molten and most of the CO2 would end up trapped in the rock as the lava cools. Alternatively, small impacts could be used to melt both polar caps, releasing millions of tons of CO2 into the atmosphere, which, if the bombardment were performed at midsummer, the CO2 would stay in the atmosphere for about eight months to a year.
The fact is, the Martian atmosphere is already ninety-five percent CO2, and every Martian winter, nearly a quarter of the atmosphere falls on the poles as snow. Even if you managed to get the CO2 in the ice caps into the atmosphere, there wouldn't be enough time before the winter arrived for the greenhouse effect to take hold, and it would all snow back out. Worse, since we're not sure how much of the ice caps are dry ice (frozen CO2) and how much is water ice, it might not even take that long, because the temperature on Mars rarely gets above the freezing point of water.
Surface bombardment to kick start the greenhouse effect would be an ineffective waste of energy at best, and at worst would result in trapping precious CO2 and water in newly formed rock from which it couldn't be released without melting the rock.
As for a heavy bombardment to restart the planet's geological cycle, the idea would have merit on another planet, but on Mars there is a reason to reject the proposal outright. In order to explain why, it's necessary to take a quick look at the history of Mars and understand how it got into the shape it's in today.
Imagine you have a glass coffee cup, a glass coffee pot and a large glass saucepan, all full of boiling water. If you leave them sitting on the counter for about half an hour, when you come back, you'd find a coffee cup full of room temperature water, a coffee pot full of warm water and a saucepan full of hot water. The reason you get this effect is that the water in the coffee cup has a higher ratio of surface area to volume. Since all the water's heat is lost at the surface, a higher percentage of the water in the coffee cup is giving off heat than the water in the coffee pot or the saucepan.
The same principal holds true for planets. They lose all of their internal heat at the surface. Since Mars is a very small planet, roughly fifteen percent of the volume of Earth but only eleven percent as massive because of differences in density, Mars lost the internal heat generated during its formation a long time ago. Hundreds of millions, perhaps even one or two billion years ago, all geological activity on Mars simple stopped. There was no more sea floor spreading, no more subduction, no more marsquakes and most importantly for the future of the planet, no more volcanoes.
Up until that point, Mars was probably incredibly Earthlike. It had vast seas, a thick atmosphere and, quite probably, life. Once that last volcano erupted though, it was all over.
When a volcano erupts, it releases three major gases — carbon dioxide, sulfur dioxide, and water vapor. All three of these are greenhouse gases. At first, water could still enter the atmosphere through evaporation. The problem is, when water condenses and falls as rain or snow, it has a tendency to take CO2 and SO2 out of the atmosphere with it. Slowly, over thousands or even millions of years, the natural cycle of evaporation and rain leeched both gases out of the atmosphere, and as they were leeched away, the temperatures on Mars began to fall. Eventually, the planet got very cold, the rate of precipitation outstripped the rate of evaporation, and the level of water vapor, the last greenhouse gas entering the atmosphere, began to fall. The temperature sank even further. The rain turned to snow and a blizzard started. This blizzard lasted for millions of years and carried the bulk of Mars's atmosphere to the ground with it. In the end, Mars's atmosphere was nothing but a thin envelope of CO2, but the bulk of the original atmosphere and water (not all, but the bulk) is still locked up in ice sheets below the surface of Mars.
Now, if current theory is right, if all of those atmospheric gasses and all that water is trapped in the upper layers of the Martian crust, the last thing you want to do is hit Mars with enough force to restart the planet's geological processes.
To give you an idea of the force involved, let's consider the Earth for a second. At some point in the distant past, and the number varies from around four to around one point five billion years ago, depending on who you ask, an object roughly a tenth the mass of Earth struck the planet at a closing velocity of roughly thirty-five times the speed of sound. Just for reference, Mars has about eleven percent of Earth's mass. In other words, Earth got hit with an object the size of Mars traveling at mach thirty-five. Not only was it not sufficient to turn the entire planet molten, but if you believe the one point five billion year old number, which is supported by some pretty compelling evidence, it wasn't even enough to wipe out all life on Earth. What it did do was cause the eruption of almost every volcano on the planet, boiled away massive amounts of ocean and blew off a divot that we refer to today as "the Moon."
The Moon, coincidentally enough, is about one tenth the mass of Mars. That being the case, if you were to take the moon and toss it at Mars at thirty-five times the speed of sound, it wouldn't turn the planet molten. It would almost certainly turn vast tracts of the surface molten, trapping a large portion of the remnants of the Martian oceans and atmosphere in the rock that would form as the planet re-cooled. It would also blow huge chunks of the Martian surface into space where they would either escape and become a hazard to Earth and anything we put in space, or form a debris field which would make access to Mars that much more difficult. The gravity of Phobos and Deimos would likely prevent them from accreting into a new moon, so, eventually, the debris would fall back to the surface.
There is also something else to consider. When the Earth got hit, the interior was still hot. It could absorb the impact more readily that Mars will be able to, because the interior of Mars is solid. It's entirely possible an explosion large enough to make Mars molten would instead shatter the planet. And that doesn't even consider how long it would take the surface heat to dissipate. It could take thousands of years before the surface was livable again, all for no real gain.
Now, at this point, you might be asking, why bother restarting the geological processes in the first place? Well, restarting the geological processes isn't actually necessary to make the planet habitable. It's only necessary in order to keep the planet habitable. If you can reheat the interior of the planet, you get the CO2 cycle back, which means that the planet can keep itself warm again by putting CO2 back in the atmosphere through volcanism. The other major benefit is the magnetosphere. We'll come back to this in a moment.
I just said it wasn't necessary to restart the geological cycle in order to make Mars habitable, but let's take a quick look at what is necessary. There are two major steps in turning the surface of Mars into an environment that you can survive in without a space suit or some kind of sealed habitat.
The first is to warm the surface of the planet. A lot of methods have been suggested for this, everything from the asteroid bombardment I mentioned earlier, to hanging giant mirrors in orbit. Margarita Miranova, grad student at the California Institute of Technology, recently suggested dumping large quantities of octafluoropropane, a colorless, nontoxic gas with a faint sweet odor into the atmosphere. Octafluoropropane, better known as R218, is used in refrigeration and manufacturing. It's also a persistent, highly effective greenhouse gas. If we pumped enough of it into the Martian atmosphere, it would kick off a greenhouse effect which could, in theory, melt the atmosphere out of the Martian soil. It would take an awful lot of the stuff, it would be incredibly slow, and there is another problem. The latest data from the Mars Express indicates ice as far down as one point eight kilometers in places.
If you want to free all the water and atmosphere you've got to heat the ground down to about two kilometers. The greenhouse effect alone won't accomplish that. Sinking a massive network of underground pipes and running a heat carrying liquid through them could do it. The temperatures in the heat exchange system of a nuclear reactor regularly get up to several hundred degrees. If the coolant exiting the heat exchange was then pumped down into the pipes, you would get an artificial geothermal process which could heat the ground, but again, it would be a slow and expensive process.
There are other tricks that can be used as well. You can pump liquid water out of the heated ground and spray it into the atmosphere where it can be heated by microwave towers. The microwave towers can be powered by building solar towers which generate power by heating the air inside a large greenhouse and letting the resulting convection currents drive turbines. Not only does this power the microwave towers, but the heat generated by the process is dumped straight into the atmosphere. You can even build fusion reactors in orbit to serve as miniature suns. Whatever methods you employ, the process will be slow and expensive. It will also be highly labor intensive.
Once you're done cooking the atmosphere out of the ground, you get to step two, making the atmosphere breathable. There are three ways to go here, and two require the large scale introduction of anaerobic bacteria into the Martian ecosphere. The first method is to introduce a bacteria that takes in CO2 and releases O2. The problem is, unless you've restarted the geological processes, you'll be stripping your atmosphere of C02, which it needs. The second option is to introduce an organism that eats iron oxide (rust) and excretes O2 and iron. Since the entire surface of the planet is covered with iron oxide, you would get a lot of oxygen this way. You'd also get a surface covered with conductive iron dust, which is going to play all kinds of hell with your electronics. The third is to split your water into oxygen and hydrogen via electrolysis (the same method used to produce oxygen on nuclear subs). This method leaves you with a lot of surplus hydrogen that can be used for fuel, but unless you've restarted the planet's geological processes, it's not that great an idea.
Yes, we're back to restarting the geological processes, because in addition to releasing greenhouse gasses, which can be done artificially, heating the planet's core also would allow the planet to develop a magnetic field, which in turn gives the planet a magnetosphere.
The magnetosphere is an important feature of any planet you want to make habitable. The magnetosphere largely prevents the direct interaction of the planetary atmosphere and the solar wind. Without it, the solar wind would strip away the upper layers of the atmosphere. Among other nasty effects, this can lead to the breakdown of water vapor into oxygen and hydrogen and the stripping of the hydrogen from the atmosphere, preventing the reformation of the water, which can lead to some nasty chemical reactions in the atmosphere that can destroy your ecosystem. The magnetosphere also helps to protect the planet from various radiation hazards including cosmic rays. It helps shield communications hardware from solar flares and coronal mass ejections (although not perfectly, and under the right circumstances, it can actually worsen the effects). Like the volcanic out gassing of CO2 and SO2 mentioned earlier, a magnetosphere is not necessary to make a planet habitable, but it is necessary to keep it that way for an extended period. It is also extremely beneficial in reducing radiation hazards from solar phenomena.
So, if you want to terraform Mars, and you want it to stay terraformed, you have to reheat the interior of the planet. If we eliminate the idea of tossing a largish moon at Mars, or a smaller body accelerated up to a much higher velocity, there is only one thing we know of that could reheat the interior of a planet.
Gravity.
Or more specifically, differential gravitational stressing of the planet. To better explain this, let's take a look at two of the moons of Jupiter, specifically, Io and Europa. Both formed about the same time as Mars, give or take a few hundred million years. Io is about twenty percent more massive than Earth's moon, while Europa is about thirty-five percent less massive. Remember that the Moon is about a tenth the mass of Mars. It is also considerably younger than Mars, Io, or Europa and like Mars, geologically dead. If we follow the logic that smaller bodies cool more quickly than larger bodies, Io and Europa should be frozen solid.
Io is the single most geologically active body in the solar system, and Europa, where the temperatures found on the surface are more commonly associated with liquid oxygen than liquid water has a massive, moon-wide subsurface ocean and exhibits signs of cryovolcanism (volcanic activity in which liquid water and ice take the place of molten lava and rock). They are kept warm through a mechanism called tidal heating.
Every astronomical body, be it moon, planet, star, or black hole has an invisible line around it called a Roche limit. Inside a body's Roche limit, no object that is held together only by the force of its own gravity can survive. It's torn apart by the object whose Roche limit it has fallen below. It is important to note that the object torn apart is always the less massive of the two, because the more massive object has a larger Roche limit, so it destroys the less massive object before entering the Roche limit of the less massive object.
Io and Europa don't actually pass below the Roche limit of Jupiter, but both of them get close to it at perigee (the closest point to Jupiter in their orbits), then move back away from it until they reach apogee (the furthest point in their orbits). The difference in the stress Jupiter's gravity puts on these two bodies between perigee and apogee is so great that it keeps both bodies warm inside. This process is called tidal heating, and while there are other factors that contribute to the heating of both worlds, they are far less significant than their simple proximity to Jupiter's Roche limit.
This same mechanism could be applied to Mars through two methods. The first is to move Mars close enough to a more massive body that the tidal heating effect kicks in.
There are problems with this approach. The first and most obvious being, how do you move a planet? Well, as it turns out, we happen to know the answer to that. In fact, if you want to get really, really technical about it, we've even done it.
All those probes we've sent to the outer solar system, like Voyager 1 and 2 and Cassini-Huygens have made close flybys of Jupiter. In the process, a tiny amount of the kinetic energy locked up in Jupiter's orbit of the sun was transferred into the spacecraft. The amount of energy involved is so small that our sun will go cold long before the shift in Jupiter's orbit is measurable, but the process scales well. If you were to use a much larger relative mass, and to fly that same mass by over and over again, the gravitational interactions would begin to move Jupiter closer to the sun. Fly the mass by on the other side of the planet, and you transfer the energy into Jupiter, moving it further away from the sun. We know the process worked for two reasons. One, the math involved has been tested repeatedly. Two, the current location of the Jovian planets and the existence of the asteroid belt instead of a fifth terrestrial planet in the inner solar system can only be explained by this process occurring in the early solar system.
Unfortunately, moving Mars would be the simple part, and there are other obstacles that couldn't be overcome. First, there are only two real candidates for where to put Mars, either in Jupiter's or Earth's orbit.
With Jupiter's orbit, in order to get Mars into a position where the tidal heating can occur, you would have to drop it right smack in the middle of Jupiter's radiation belt. Sure, you'd heat the planet up, but you'd also render it completely sterile and incapable of bearing life.
With the Earth's orbit, you'd have to get rid of the Moon and the process would disrupt the oceanic tides and cause all kinds of trouble with earthquakes and disruption of satellite orbits, and just make space near Earth a nightmare to deal with. Not to mention that moving planets in and out of Earth orbit is insanely dangerous.
Then there is the really big problem. In order to put Mars into orbit around another body, you would have to accept a retrograde orbit (Mars would go around the planet in the direction opposite the spin of the planet). Retrograde orbits invariably decay, and since you would have to put Mars close to the Roche limit to begin with, that decay would quickly drag Mars in below its Roche limit which would tear it to pieces, every one of which would eventually fall onto the planet.
In short, moving Mars is out.
That being the case, the only real option for restarting the core of Mars is to build a gravity source to cause the tidal heating. In other words, to build a planet of equal mass, or slightly larger and place it in such a way that Mars regularly makes close approaches to the new planet's Roche limit.
This has several things to recommend it. Because you would be dealing with masses small enough to manipulate directly, you wouldn't be forced to accept a retrograde orbit, so the new body would never fall on Mars. It would also be much faster and safer than trying to move a planet. It would also take considerably less energy.
The process itself is relatively straightforward. There is enough mass in various belts out beyond Neptune to build several planets the size of Mars. In fact, it's highly likely that there are several bodies out there the size of Mars or larger. But it's the smaller bodies that are of interest. They're easier to move, which means they would be easier to drag back to Martian orbit. Mars' existing moons, Deimos and Phobos are both small asteroidal moons that would make the perfect starting place. Phobos needs to be boosted to a higher orbit anyway because it is slowly falling towards the surface of Mars, and it's going to hit eventually.
Again, the process is going to be long, expensive and labor intensive, but it could be done in a few centuries, which is about the same about of time it would take you to cook Mars's atmosphere out of the ground. It also has the added advantage that the same tidal forces which would reheat the interior of Mars would also help build the dynamo which would create a strong Martian magnetic field by stirring the liquid iron core of the planet. In the end, what you would get is a binary planetary system where one of the worlds looked a lot like Earth.
If Mars is that much of a challenge, what about other planets or moons in the solar system?
Mars has one major problem: the lack of internal heat. Venus doesn't have that problem. Venus is extremely geologically active. Venus, however, has three major problems, and in order to explain them, we need to take another brief trip into the distant past.
At some point in the past, and unlike the event which formed the Moon (we have no way to date it) Venus was struck by an object that was close to, if not greater than, Martian mass. Unlike the Earth, however, where the collision was to spinward (i.e. the collision wasn't dead center but off to the side in the direction the planet was spinning), the collision with Venus occurred antispinward (the collision was offset in the direction opposite of the planet's rotation). The collision was so hard, the direction in which Venus spins was reversed. Venus not only spins in the wrong direction, but it takes longer to complete one rotation than it does to complete an orbit around the sun. Because of the incredibly low rate of spin, Venus has no measurable magnetic field. If it has one at all, it is too weak to extend from the core of the planet all the way to the surface.
Venus's second problem is its lack of a moon. If Venus had a moon, than the slow rotation wouldn't be as much of a problem, because the orbit of the moon would perturb the liquid core enough to generate a magnetic field. Venus's third problem is its location. If Venus were where Mars is, it might very well be a garden, but its proximity to the sun killed it over the last billion years or so.
The thing is, a billion years, a billion and a half years ago, Venus probably didn't look that different from Earth. The atmosphere was probably about four times thicker, but very Earthlike. There were oceans, Earth type plate tectonics, and much like Mars, there is a high probability that there was life. Unlike Mars, we'll never know one way or the other because any traces would have been destroyed as Venus has resurfaced itself over the last 50 million years.
But our sun is a main sequence star, and as main sequence stars burn, they swell. Because their energy output is a function of their surface area, over the course of the last billion years, the total solar output has increased by about thirty percent. As that happened, the temperature on Venus began to rise as the greenhouse gasses captured more and more energy. The natural precipitation cycle of water slowed the process down until the planet reached the point where temperatures stopped getting low enough for it to rain. Once that happened, there was no way to remove any of the greenhouse gasses from the atmosphere, and the planet was caught up in the runaway greenhouse effect. The oceans slowly boiled off over several million years and by the time they were all gone, the temperature on the planet began to reach levels where CO2 and water were literally being baked out of the rocks. With the change in the chemical composition of the rocks, the increase in pressure due to the fact that the entire mass of the oceans and a good deal of the mass of the crust had entered the atmosphere, and the incredibly high temperatures, the surface of the planet became soft and somewhat flexible.
To make matters worse, the pressure pushed the water vapor, the lightest of the gases making up the soupy new atmosphere, to the top where it was unprotected by the thick clouds. In the upper reaches of the atmosphere, UV radiation broke the water apart, and without a magnetosphere to shield the planet, the solar wind stripped most of the hydrogen away. The oxygen and what hydrogen was left reacted with the sulfur dioxide to form sulfuric acid, and the planet was slowly transformed into the hell it is today.
Terraforming Venus where it is would require a way to shield it from the sun and the solar wind. If you could accomplish that feat, you would just need to wait until the planet had cooled off significantly, then dump a volume of water approximately equivalent to the entire hydrosphere of Earth into the atmosphere, then wait about three hundred million years while the planet completely resurfaced itself.
Moving Venus to a different location presents the same difficulties as moving Mars would. It can be done. The process is relatively straightforward. The tricky part is that if you did attempt it, you would very likely knock Earth out of its orbit. It's far too dangerous and would result in no tangible benefits. I don't want to say terraforming Venus can't be done, but I will say that we know of no way, based on current science, to terraform Venus in a realistic timeframe.
As to other candidates that have been put forward from time to time, Io sits in the heart of Jupiter's radiation belt and anyone on the surface would receive a lethal dose within minutes. Europa is, despite the tidal heating, insanely cold, so cold that at certain times, liquid oxygen could exist on its surface. It also sits on the edge of Jupiter's radiation belt. The lethal dose would take longer, but the radiation levels are just too high. Titan has the same temperature problems as Europa, without the tidal effects that keep the oceans of Europa liquid.
Which brings us back to Mars.
If we want another planet, Mars is still our best bet. The key is to take a different approach. You won't have any open oceans or wilderness, but what you could have, in fifty to a hundred years from now, are thriving cities, productive farms, parks, pastures, factories and anything else you need. All you have to do is give up the notion of making the entire world habitable at once.
Instead, you build domes and arcologies (cities enclosed in a single building). Build domes twenty miles across and you can make the planet habitable a hundred square miles at a time. If built right, the whole thing would act as a shield against radiation and electromagnetic interference from the sun. The iron and carbon needed to make the domes is readily available on the surface and in the atmosphere. If you don't want to thin out Mars's atmosphere, you can just dig it out of the ground and scrape it off the poles.
It's not as sexy as terraforming. It doesn't capture the imagination the same way the idea of transforming an entire planet does. What it does do is present us with projects that can be completed in a decade or two. It lets the people paying for it, be they investors or taxpayers, see the results of their spending.
Ultimately, I'm in favor of terraforming. I like planets. But realistically, it makes more sense to crawl before we try to run a marathon, and even a relatively simple terraforming project is a marathon. Building domes on Mars, on the other hand, is a simple, practical solution that can be built with today's technology and science. Call it a starter project.
