A direct translation of the word terraforming is ''Earth shaping,'' and this is further taken to mean the process by which a planet is made Earth-like, and by implication a world capable of supporting human life. Depending upon how literal one wants to be, there is really only one planet within our Solar System that might be made Earth-like, and that's the planet Venus.
The second planet out from the Sun, the mass, radius, and surface gravity of the Venus in Earth-units are 0.815, 0.949, and 0.90, respectively. In other words, it is already an Earth-like planet. The problem for humanity, however, is that Venus has a surrounding atmosphere that currently makes surface life impossible. In the case of terraforming Venus, therefore, it is essentially atmospheric alteration that must be performed in order that life might eventually exist upon its surface.
This may seem like a tall order, but if we think about it, in a timeframe of less than 200 years, human industry has changed
(though in the wrong way for our survival) the atmosphere of the Earth. This observation alone provides us with the very real sense that atmospheric manipulation on a planetary scale is entirely possible, and that it is possible on a timescale of centuries rather than millennia. Indeed, the term geoengineering has recently been introduced to the scientific lexicon to describe the manner in which the harmful effects of global warming might be ameliorated.2
Although Venus and Earth can be thought of as planetary doppelgangers, it is the planet Mars that is most often called the Earth's twin. At first glance this seems a rather odd statement. In Earth-units, Mars has a mass, radius, and surface gravity of 0.107, 0.532, and 0.38, respectively. Indeed, Mars is nothing like the Earth in physical terms. It is in this (admittedly semantic) respect that Mars cannot be terraformed (that is, made into something like the Earth), but it can be made habitable, at least in a dynamical sense, as will be discussed in Chapter 6. In addition, it is now clear that Mars was a very different world in the past, and in some sense terraform-ing it in the future will be a partial process of reinstituting what was once there, when the Solar System was much, much younger.
The term planetary ecosynthesis has also been used to describe the manner in which Mars might be transformed into a life-supporting domain, and this expression gives us some sense of the great complexity of the problem at hand. An ecosystem is typically described as a natural setting that consists of a multitude of species of plants, animals, and microbacteria that function and interact within the same environment. To make Mars habitable, therefore, very specific ecosystems will have to be nurtured and sustained. Canadian biophysicist Robert Hall Haynes (1931-1998) further coined the expression ecopoiesis (from the Greek words for house and making) to describe the deliberate production of new ecosystems on other planets. In addition, inherent to the meaning of the word ''ecosystem,'' the process of ecopoiesis entails the generation of a self-supporting system hosting many hundreds, if not many thousands, of subsystems that are all interacting with one another, but all of which are stable over long periods of time. But this will be a topic for further discussion in Chapter 4.
If at the heart of the terraforming (or ecopoiesis) process is the goal of making another planet habitable, the question that can reasonably be raised is, ''What kind of life is the world being made habitable for?'' Clearly, microbial life forms have very different requirements to, say, plants or humans. Extremophile microbes, for example, can thrive in rock pools where the temperature is 100°C, or where there is no light at all—regions in which no human being could live. Likewise, the typical winter temperature in the central Antarctic continent is about -80°C, and as far as is known, no plant, microbe, human, or other animal can survive for extended periods of time under these conditions, and yet Antarctica is very much part of the Earth, a planet that is otherwise teeming with life. As shall be seen in Chapter 4 the range of conditions necessary for life, specifically human life, to thrive are quite narrowly defined, but for the process of terraforming this is actually helpful, since it makes clear exactly what conditions must eventually be brought into existence.
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