by Rhodri Evans, School of Physics and Astronomy, Cardiff UniversityIn the film Silent Running, the Earth has become uninhabitable to all but humans, and a fleet of space ships out in the vicinity of Saturn each have 6 large geodesic domes. Some plant and animal life is being sustained in these domes, in the hope that planet Earth will be able to support them once more in the future. In this article I will discuss the evolution of Earth's current human-friendly environment, and also the idea of terra-forming, creating an Earth-like environment elsewhere.
The Earth is very old by human timescales, 4,600 million (4.6 billion) years old to be precise. This age was accurately determined in the 1950s by Clair Patterson of the University of Chicago, using the radioactive decay of Uranium in the Earth's interior. The Earth, and the other planets in the Solar system, formed out of a cloud of gas and dust which surrounded our Sun as it formed to become a star, this is known as the Solar nebula. We see similar nebulae around other young stars in our neighbourhood of the Milky Way galaxy.
Most of the material in the Universe, and in our Solar system, is hydrogen and helium. These are not only the two lightest elements, but also the only two which were created in any quantity in the early Universe. All heavier elements (with the exception of a small amount of lithium, barium and boron) are created inside stars; either in their nuclear furnaces, or when massive stars explode as supernovae.
Thus, because we find elements heavier than the lightest ones in our Solar System, we know that our Sun is not a first generation star. Our Sun and the planets in our Solar System have all formed from material which had already been enriched with heavier elements by a previous generation of stars. The rocky material of our Earth is made primarily of compounds of iron, silicon and magnesium, but many other elements, all the way up to uranium, exist in the material which makes up our planet.
When these heavier elements came together to form a nascent Earth, there was enough hydrogen and helium left in the Solar nebula after the formation of the Sun to give our Earth its first atmosphere. This primary atmosphere of hydrogen and helium was quickly lost. The Earth is too close to the Sun and does not have a strong enough gravitational field to be able to hold on to light molecules and atoms like hydrogen and helium, so these were lost into space.
The early Earth had a great deal of intense volcanic activity, and this volcanism lead to the out-gassing and creation of a second atmosphere, composed mainly of water, carbon dioxide and nitrogen. This is the atmosphere the Earth had when life first appeared.
The first signs of life on Earth go as far back as about 3.5 billion years, which is pretty early in Earth's history. In fact, it seems that as soon as Earth had survived a period of heavy bombardment by debris left over from the formation of the Solar System, life on Earth first appeared. The earliest life forms were not only simple single cell bacteria, but in fact were anaerobic respirators, meaning they didn't use oxygen to respire. Instead, they would have used carbon dioxide; and some, possibly, nitrates (compounds of nitrogen and oxygen).
From 3.8 billion years ago through to 1 billion years ago, all life on Earth was single celled. Green plant life first appeared about 3 billion years ago, and with the appearance of green plant life came the oxygenation of our atmosphere through photosynthesis. The amount of oxygen in our atmosphere started increasing from essentially 0% to the present value of 20% due to the photosynthesis of green plant life. Thus, our current atmosphere is actually the third one the Earth has had. The first appearance of life which respired aerobically (using oxygen) is believed to have happened about 2 billion years ago. This too would have been single celled life, the first multi-celled life appeared only about 1 billion years ago.
Thus, when we talk about our Earth being habitable to life, we need to bear in mind that for most of its history it would have been uninhabitable to humans, and most other multi-celled life.
We, as humans, have evolved to live in the conditions we find on Earth today. These conditions have been fairly constant for the last several thousands of years. The last ice-age peaked 20 thousand years ago, but humans survived this climatic swing, presumably by migrating to warmer climes. The causes of ice ages are not well understood, but clearly humans were able to adapt to them.
The Sun is about half-way through its lifetime, in another 4.5 billion years it will begin to undergo dramatic changes. This is because it will have used up the Hydrogen in its core, and so its source of current fuel will be gone. Without going into the details, the Sun will swell up into a red giant as it begins to burn Helium in its core. When the Sun does this, its size will increase many thousands of time, and it will probably engulf the Earth. Thus, eventually, the Earth will become uninhabitable due to natural processes over which we have no control.
However, the timescales involved are huge. They are so large that it is useful to compress the Earth's 4.6 billion year history down to 24 hours. On this timescale, human beings don't make an appearance until the last 20 seconds! 20 seconds out of 24 hours, that is all our presence on Earth amounts to. It is therefore very naive, in my opinion, to think that our species will be around in another 24 hours (4.5 billion years), when the Sun swells up to become a red giant star. I think we will be lucky to survive another few minutes on this timescale, let alone a whole 24 hours.
Of course, if climate change is as dramatic as some people fear, we may have to look to create a present-Earth like environment elsewhere in the not too distant future. How feasible is this? Certainly it is something NASA and the European Space Agency ESA have looked at. But where would we go?
The two obvious places are the Moon and Mars. The Moon is the only place beyond the Earth that humans have actually visited. Although it is our closest neighbour, it is very different from the Earth in many ways. The gravity is only about one sixth of the Earth's gravity, and as a consequence it has no atmosphere. No atmosphere means no atmospheric pressure. If a human were to step out of a space ship which had landed on the surface of the Moon without wearing a space suit, before he or she died from lack of oxygen the astronaut would explode due to the lack of atmospheric pressure.
Therefore, on the Moon, the only way we could create a habitable environment would be inside of giant domes or greenhouses. This would enable us to create an oxygenated atmosphere (using green plants to do the work for us), and also to create an atmospheric pressure so that we didn't explode! There isn't much we could do about the lower gravity, but one way to quickly adapt to this would be to wear weighted shoes to artificially increase our mass and thus our weight. There is increasing evidence that the Moon has frozen water in the pulverised rock on its surface, and there may also be frozen water in the depths of craters near the Moon's poles. Along with many other minerals in its surface, the presence of water on the Moon makes it feasible to, maybe one day, establish a colony there.
In many ways Mars would be a more attractive destination. Sadly, we appear to be many many years away from sending humans to Mars. In a book written in the same year as the first Moon landing, 1969, I recently read confident predictions that we would send humans to Mars by the year 2000. Apart from a simulated mission to the red planet conducted in a facility just outside of Moscow, we are probably further away from sending humans to Mars than we were in 1969. In my opinion, it is unlikely to happen before 2030.
When we eventually do get to Mars, what sort of environment awaits us? The short answer is one which, in many ways, is similar to Earth. Mars has about half the Earth's radius, and a similar density. Therefore its surface gravity is 0.4g, where 1g is the acceleration due to gravity at the Earth's surface. There is also increasing evidence that water exists on Mars. We believe it actually flowed on the surface in the past, but now it seems to be in the form of frozen water on the surface. But, there is some evidence that liquid water may presently exist below the surface.
Mars has an atmosphere, but it is much thinner than Earth's. It is 95% carbon dioxide, 3% nitrogen, with the remainder being mainly Argon. At the surface the atmospheric pressure is only about 600 Pascals, compared to 100 kPascals here on Earth, so about 160 times less. This means that, even on Mars, we would initially have to create an artificial environment to enable humans to survive, again presumably in large geodesic domes where green plant life could create a pressurised, oxygenated atmosphere. It is possible that, over time, green plant life could create a thicker, oxygenated atmosphere allowing humans to survive outside of these geodesic domes, but this is very speculative.
It is my belief that terra-forming, although it presents large technological challenges, is not something which is beyond our current capabilities. The main hurdles are ones of resources, both financial and material. But, to think that terra-forming could provide us with a way to survive the destruction of the habitability of our planet is, I think, very naive. It could only ever serve to provide for a small fraction of humanity and other life on Earth.
We need to realise that we have evolved to live in quite a narrow range of conditions, and if the conditions on Earth change too much it may well become uninhabitable to us. Most natural changes which we know have happened in the past appear to have happened over many thousands, if not millions, of years. What is alarming about the current increases in e.g. carbon dioxide levels is how quickly they are happening.
However long our remaining time is in the 4.6 billion years our Earth has left to its lifetime, let us hope we take better care of our planet than we have done in the last few hundred years.
