Terraforming Venus

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    In the chronicles of Humanityís settlement of the solar system one planet will stand out for the challenge it offers to those who would make it their home. Almost as big as Earth and close by, in terms of distance and delta-vee, Venus beckons like a rotund houri behind her veil of clouds. But that vaporous veil does not obscure the steaming dinosaur-inhabited jungle of the old science-fiction stories. The clouds themselves consist of sulfuric-acid mist and they cover a landscape that makes Danteís Hell seem benign in comparison.

    If we want to live on Venus as we live on Earth, we must achieve three goals. We must reduce the temperature at the planetís surface from hot enough to melt lead to something just a little under human body temperature. We must transform an atmosphere that consists primarily of carbon dioxide pressing the surface with the pressure of almost nine hundred meters of seawater with an oxygen-rich nitrogen atmosphere with a pressure close to that of Earthís atmosphere at sea level. And we must get the planet to turning on its axis once every twenty-four hours. In short, we must terraform Venus, make it more like Earth.

    The planet needs oceans and a water cycle and producing those will solve other problems as well. The obvious source of the water, which we must import to the planet, is comets. Those lurk in the Oort cloud, 50 AU (7.5 billion kilometers) and more from the sun. If we give the nearest of those a speed of one kilometer per second toward the sun, it will take almost 237 years to reach Venus. Terraforming Venus will require patience, certainly, but that much seems excessive. Perhaps we should get the planetís water from the outer Asteroid Belt?

    Tonne for tonne asteroids contain less water than comets do, but that is actually what this project needs. The process that we use to bring water to Venus will also spin the planet up. We donít want to bring in too much water when we do that.

    If we want to give Venus as much surface water as we find on Earth, we must import 1.664x1021 kilograms of the wet stuff. Earth has even more water dissolved in its mantle, so we may have to bring additional water to Venus over time, but for the nonce letís assume that we need only surface water. Comets coming from the Oort cloud to bring the water will hit the planet at a speed of 50.6 kilometers per second. If they come in on lines that miss the center of the planet, the collisions will change the planetís rotational angular momentum. On the assumption that cometís, on average, consist of 50% water (and we include methane and ammonia, because they are essentially the same thing, needing only oxygen from the Venusian atmosphere or rocks to replace the carbon or nitrogen) and that they strike the planet on a course that skims the surface horizontally (6051.8 kilometers off center), then the amount of angular momentum that we give the planet comes out to 1.02x1027 kilogram-kilometers squared per second.

    That amount of angular momentum will get Venus to rotating once every 4.19 days. To give the planet a 24-hour day we must give it an angular momentum of 4.27x1027 kilogram-kilometers squared per second. To achieve that increase we must bring in 4.19 times as much mass in our comets with none of the extra mass being water; that is, our comets must consist of 11.9% water.

    That latter fact means that we donít have to go to the Oort cloud for our water. Carbonaceous chondrites, meteorites that originate in the Asteroid Belt, contain as much as 10% water by mass. For this project we might use some of the smaller Trojan asteroids, floating on Jupiterís orbit, for our feedstock. We can cut up an asteroid and give each piece a retrograde delta-vee of 6.606 kilometers per second (on average) to put it onto a Hohmann transfer ellipse that will take it to Venus in 2.55 years (931.48 days). The chunk will slide onto the orbit of Venus at 46.4 kilometers per second and slam into the planet at 47.54 kilometers per second. That fact means that, in order to give the planet the right amount of spin, we must increase the mass striking it by a little less than 6.44%. That reduces the necessary water content of the feedstock to 11.18%. Thus we need to send 14.884x1018 tonnes of material to Venus.

    In order to get the asteroidal chunks moving we will mount rocket engines on them. We donít want a lot of thrust: we donít want the chunk to break apart when we turn the engines on, after all. But we do want a large specific impulse, so that the engines will need less propellant. We want that feature because the engines will be using material from the chunk itself as propellant. To calculate how much mass the chunk is going to lose, we simply use the rocket formula, which tells us that the initial mass of the chunk divided by the final mass of the chunk equals the exponential of the ratio we obtain from dividing the desired delta-vee by the exhaust velocity of the engines. If our engines can achieve an exhaust velocity of 100 kilometers per second, the mass ratio for barging a chunk out of Jupiterís orbit and sending it to Venus comes out to 1.006629, which means that for every billion tonnes that we want to send to Venus (and we will want to send the material in billion-tonne chunks) we must run 6.629 million tonnes through the engines.

    The engines themselves will be robots. They will land themselves on an asteroid that has been designated as ready to move to Venus. With feet that look like huge snowshoes, they will press against the asteroidís surface without breaking it up. At the same time they will drill into the asteroid to obtain propellant, both for present thrust and to fill their tanks. They will push hard on the asteroid to put it onto the trajectory that will take it to Venus, then most of them will jump off and reverse their delta-vee to bring themselves back to their Trojan point to push another asteroidal chunk. For humans it would be a traffic-control nightmare, because vast fleets of these robots will be working at any given time.

    Every 237 days the launch window opens for putting asteroidal chunks onto Hohmann trajectories that will hit Venus. The window opens first on the Trojan asteroids following Jupiter and then, 79 days later, opens on the leading Trojans. Thousands of chunks, each roughly one kilometer wide, will be sent gliding into the inner solar system. The robot rockets that stay on the chunks once they get moving will carry out the necessary midcourse corrections to ensure that the chunks hit Venus and do so in the right way. We want the chunks to hit Venus as far off-center as possible without skipping off the atmosphere and heading back out into space.

    Once the robots have determined that the chunk is on the correct course and is making its final approach to the planet, they will jump off and put themselves onto a trajectory that misses Venus and takes them back to Jupiterís orbit. When they return to Jupiterís orbit their starting point wonít yet have made half a revolution around the sun, so the robots will add a little delta-vee to their motions in order to raise their perihelia and take longer on their second revolution around the sun. Then, 11.861 years after setting out on their voyage, the robots will return to the Trojan asteroid group whence they set out.

    Every 237-day cycle thousands of asteroidal chunks will hit Venus. If we send one thousand billion-tonne chunks every cycle, then this part of the project will take 9.66 million years. Of course we expect propulsion technology to improve and perhaps reduce that time to several millenia. The project will still be a major exercise in human patience.

    In addition to spinning up the planet and bringing water to it, the initial stage of the terraforming project will also involve cooling the planetís surface and atmosphere to temperatures suitable for life. At first the project wonít be doing that: kilometer-sized rocks hitting the planet at nearly 50 kilometers per second will generate a lot of heat. In fact, we will want to keep the planetís thick carbon dioxide atmosphere to act as an absorbent cushion that prevents material from being spattered or evaporated into space. The planet will not cool down with that heat-trapping blanket in place. The initial cooldown, when it comes, will require something else, a feature that will remain in place when the project is complete.

    On the sunward side of Venus will float the Parasol. Our asteroidal chunks will all approach Venus from the starward side of the planetís orbit, so we can build the Parasol immediately without anything interfering with it or it interfering with the rain of asteroids coming down onto the planet. The Parasol will float at the point where its orbital centrifugal force plus the gravitational attraction of Venus balance the gravitational attraction of the sun. In essence it floats in a 224.701-day orbit about Venus, coming just a little closer to the planet than the calculated 1.46 million kilometers from the planetís center, doing so in order to compensate the sunís gravity. In that position it must cover the image of the sun as seen from anywhere on the planet.

    Simple geometry tells us that the Parasol must span 31,000 kilometers and that the structure must prevent 47.7% of the sunlight reaching it from reaching the planet. We conceive the Parasol, then, as a gridwork of solar cells attached to a frame with anchorages for the electric rockets that the structure needs for holding its position. The balance point where we have put the Parasol is unstable and the structure will tend to drift away from it if we do not actively provide a little thrust now and then. One source of that problem also provides, in part, the solution.

    In addition to the electromagnetic radiation, a wind of plasma blows from the sun. Gusting at speeds between 330 and 700 kilometers per second, the wind consists of protons, electrons, and some alpha particles (helium nuclei) in densities that range from 7500 particles per cubic meter in the faster wind and 22,800 particles per cubic meter in the slower gusts. That corresponds to a particle flux that varies from 5.2x1012 particles per square meter per second to 7.52x1012 slower particles per square meter per second, with the particles carrying average energies that range from 7 electron volts to 14 electron volts. That flux of hot particles will erode the Parasol and will also exert a starward pressure upon it.

    Glown steadily into space, sunlight will exert a constant pressure on the Parasol. By moving the Parasol ever so slightly closer to the sun, the builders can use the minuscule increase in the solar gravity to compensate the photonic force. The fluctuating force of the solar wind will require more active compensation.

    With an average mass of about 470 Mev (8.36x10-28 kilogram), the particles will strike the Parasol with en masse momentum fluxes of about 2x10-9 newton per square meter to about 3x10-9 newton per square meter. A complex array of magnetic mirrors will cover the Parasol and intercept the solar wind, drawing in the particles and sending them back toward whence they came. Vibrating electromagnetic fields, driven by the solar cells that cover the opaque parts of the Parasol, will accelerate or decelerate the plasma as needed. The magnetic mirrors will enable the Parasol to achieve three goals:

    1) floating a little too close to the sun, the Parasol will use the thrust from the magnetic mirrors to maintain its position relative to Venus, always shading the planet;

    2) the magnetic fields deflect the particles away from the solar panels and structural members that constitute the Parasol, thereby protecting the material parts of the Parasol from erosion by spallation; and

    3) it protects the atmosphere of Venus, which doesnít have its own magnetic field, from erosion; in particular, it protects the atmosphere from losing the hydrogen that the project is bringing from the outer solar system.

    By cutting the isolation from 2.614 kilowatts per square meter to 1.368 kilowatts per square meter, the Parasol will enable Venus to cool down to Earth-like temperatures. The theoretical blackbody temperature of the planet will go from 328 Kelvin (55 Celsius, 131 Fahrenheit) to 279 Kelvin (6 Celsius, 42.8 Fahrenheit). These are average figures, of course: the polar regions will be colder than the tropics. But the planet wonít get there without a lot of help.

    The problem originates in 4.8x1020 kilograms of carbon dioxide covering the planet like a thick wool blanket. At the surface the atmosphere presses down 92 times as hard as Earthís atmosphere does at sea level and raises the temperature, through the greenhouse effect, to 737 Kelvin (464 Celsius, hot enough to melt lead (melting point - 327.502 Celsius)). To cool the planet to livable temperatures we need to rid Venus of that atmosphere and replace it with an oxygen-rich nitrogen atmosphere like the one we have on Earth.

    We wonít be shipping carbon dioxide off-planet. Instead, the project will sequester the gas in the planetís structure, in essence burying it. The project will simply create large quantities of carbonate rock, such as limestone. To achieve that goal we must ensure that the new rock stays cooler than the temperature at which it outgasses its carbon dioxide. And therein lies a challenge.

    We must cool the planet down before we can produce carbonate rock. The first step in that direction comes when the builders so construct the Parasol that it eclipses the sun completely, leaving the planet in total darkness. With no more heat coming in from the sun, the planet can cool down without being constantly reheated. Active processes will speed up the cooldown, driving it faster than a purely passive radiation of heat into space from an unmoving atmosphere.

    Kilometer-wide chunks of matter blasting into Venusí atmosphere will not make a significant contribution to the cooldown directly; indeed, they will bring more heat to the planet in the form of their kinetic energy. But indirectly they will have a significant impact on the planetís temperature.

    Water brought to Venus will vaporize in the impacts. Lighter than carbon dioxide, it will rise to the top of the sensible atmosphere and radiate its heat into space. The water will condense and fall back into the atmosphere until it evaporates again. As more water comes in, the process will speed up and the rain will fall progressively deeper into the atmosphere before evaporating.

    Eventually the rain will hit the ground and boil away almost immediately. In the darkness the rocky surface will cool and water will be able to pool into growing ponds that will become oceans. But it wonít actually be water in those pools: it will be a mixture of carbonic acid and sulfuric acid and it will corrode the rocks, transforming oxides into sulfates and carbonates. Dolostone and limestone dust will muddy the creeks and rivers flowing into the spreading oceans.

    As the asteroidal impacts increase the planetís rotation, the atmosphere will turn over. Shaped by the growing Coriolis force, the Venusian air will form the great roiling cells, like the Hadley cells on Earth, that move the weather. The weather will cause more corrosion, which will draw more carbon dioxide out of the atmosphere.

    When Venus begins cooling too much, robots will begin opening up the gaps between the solar panels on the Parasol, allowing some sunlight to reach the planet. The surface will still be as dark as a cloudy day, but there will be enough light to keep the planet from freezing and to drive the next stage in the evolution of the atmosphere. That next stage will be carried out by a special form of self-replicating robot life. Specifically, the robots carrying out the project will introduce algae to the planet, because only plant life can survive in a carbon-dioxide atmosphere.

    The algae will also have to be tough enough to survive and thrive in highly acidic water. The Venusian oceans will still contain a large proportion of carbonic acid. Absorbing the carbon dioxide, the algae will reduce the acidity of the oceans as the atmospheric concentration of carbon dioxide continues to decline and it will release free oxygen into the atmosphere. With the formation of an ozone layer in its atmosphere Venus will become hospitable to more delicate forms of life, including animal life.

    Little by little, century by century, the robots will open gaps in the Parasol until Venus has become essentially Earth-like, with an oxygen-nitrogen atmosphere containing only traces of carbon dioxide and water. With a 24-hour day, the planet will redistribute the sunís heat more efficiently than it would otherwise do. If the planetís axis of rotation tilts relative to the axis of the planetís orbit, Venus will have seasons like those on Earth, though each season will last only 3/5 as long as the analogous season on Earth. Or the axes may be made parallel to each other, locking Venus into an endless spring.

    We can expect that, at minimum, this project will take longer than human civilization has taken to rise from the first farming villages to its present manifestation. Venus will not change noticeably in a human lifespan. The people who initiate this project have no hope of seeing it come to fruition. So why would any human society undertake such an endeavor?

    It would certainly test human patience. True, the actual work will be performed by self-maintaining robots. But the project wonít yield any material benefit in historical time, so why would Humanity bother with it?

    It certainly wonít be for living space. Most, if not all, of Humanity will be living in space, residing in artificial habitats. Few, if any, will want to live on a planetary surface. In any case, with trillions of humans living throughout the solar system, there would not be enough surface area to give people a fair share of the land.

    No, most likely the motive for terraforming Venus will come from the urge that drove the founding of our national parks. Freed from the inhibiting force of human settlement and activity, the grand waltz of mutation and selection will weave the tapestry of natural evolution across the surface of another world. Seeded with simple forms of terrestrial life, Venus will evolve its own native species. And perhaps, someday millions of years hence, the planet will fulfill the old science-fiction dream of being covered with steaming jungles full of dinosaurs.

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