The Greening of Mars
Sheffield, Charles, The World and I
The barren red planet could become a blossoming garden if strategies for changing it to the point where organisms from Earth, including humans, can live and thrive there prove viable and gain adequate support.
The phrase "terraforming a planet" is somewhat misleading. Interpreted literally, it ought to mean that another world is to be made similar to Earth.
In practice, that is impossible. No matter what we do we cannot make Mars, Venus, or any other planet of the solar system the same size as Earth, or give it an identical orbit. And if similarities in size and position were the criteria for planetary terraforming, Venus would win hands down. Venus is often spoken of as a "sister world" to Earth. It is only 5 percent smaller in radius and closer to us than any other world except our own Moon. A man or woman who weighs 140 pounds here would weigh 127 pounds on Venus--hardly enough change to notice. But that same person would weigh 370 pounds on Jupiter, 53 pounds on Mars, and a mere 4 pounds on the largest asteroid, Ceres.
Why, then, do we not think of terraforming Venus before we consider Mars? It is not enough to say that humans have been fascinated by the red planet, in fact and fiction, for over 100 years, since Percival Lowell's "discovery" of an extensive pattern of Martian canals (canals that later observations of higher quality failed to find).
In practice, "terraforming" does not imply the creation of an Earth look-alike; it describes, rather, the changing of another world to the point where organisms of our own planet, including humans, can live and thrive there.
When we examine Venus with this idea in mind, we see huge problems. Yes, Venus is almost the same size and has close to the same surface gravity as Earth. But Venus is blazing hot, with a surface temperature of more than 450 [degrees] C--easily enough to melt lead. The Sun delivers twice as much radiation to the planet, and it proves easier to warm a world that is too cold than to cool a too-hot one.
Also, the air pressure at the surface of Venus is 90 Earth atmospheres. That air is mostly carbon dioxide, while the pale yellow clouds that shroud the planet are not water vapor but poisonous sulfuric acid. Finally, the "day" on Venus, the time from noon to noon, is 117 of our days.
Venus, then, is not an attractive candidate. What about Mars?
At first glance, we see major problems. Mars has an atmosphere, but it is as thin as Venus' is dense. The pressure at the surface is only I percent of an Earth atmosphere, and the Martian air is mostly carbon dioxide and nitrogen. The polar ice caps of Mars are mainly solid carbon dioxide rather than water ice, good evidence that Mars is cold (solid carbon dioxide turns to gas at minus 79 [degrees] C). The planet is half as far again from the Sun as Earth and receives less than half as much solar radiation. The Martian lander, Pathfinder, and its mobile robot, Sojourner, reached Mars on July 4, 1997, and recorded a comfortable Sunwarmed temperature of 21 [degrees]C (70 [degrees] F) in a thin layer of dust and gravel right at the surface. Nighttime and polar temperatures dip to minus 100 [degrees] C or colder, however. By comparison, the lowest natural temperature ever recorded on Earth is minus 89 [degrees] C. At such temperatures there is naturally no sign of liquid water on the Martian surface, though photographs reveal convincing evidence of ancient water runoff.
On the other hand, Mars provides some positive features. The available area is roughly equal to the land area of Earth. The atmosphere, though thin, is dense enough to be useful for aerobraking a spacecraft, or for flying an aircraft (the low gravity, only two-fifths of Earth gravity, helps). We know of terrestrial organisms that can stand the temperature extremes of the Martian surface, provided they have access to liquid water. And there is water, or rather, water ice, on Mars. …