The global understanding we may thus gain is fine-tuned by our exploration of still more planets. Let us consider Mars now. Since Mars was smaller and less dense than the Earth, it did not have available as much internal energy as the Earth. And since Mars is further from the sun, it receives less sunlight. To compensate, if Mars were to have as comfortable a climate for life, it would have to have a much greater greenhouse effect than the Earth. This would require truly large amounts of CO2 in the atmosphere. At the present time the CO2 in Mars is 50 times per unit volume that of the Earth's atmosphere. That sounds like much, but it produces only a puny greenhouse effect. The reason is that since the atmosphere is already so cold and thin, the Martian water is frozen at the poles or spread as permafrost under large areas of the surface. This means that the initial boost that CO2 gives to the greenhouse effect is not multiplied by the action of water vapor in trapping even larger amounts of infrared radiation.
It seems, however, that earlier in its history, when Mars' internal energy was much higher, Martian volcanoes might have filled the atmosphere with as much as 100 times as much CO2 as today. This factor would have raised the density and temperature enough to permit liquid water and large amounts of water vapor, and hence much more of a greenhouse effect. As we look at Mars now, that appears to have been the case. For years, spacecraft photographs showed what seemed to be riverbeds and suggested other indications of significant amounts of liquid water in the past, perhaps even an ocean. After the recent exploration of the Martian surface by robots, the case for water in Mars is very strong.
This view of Mars is strengthened by the recent discovery that Mars at one time did have plate tectonics as well. It seems, then, that life could have existed on Mars. If so, why did not Martian life control the climate the way Earth's presumably did? Mars apparently did not have enough energy to run the cycles that have made a sustainable biosphere on Earth possible, just as it did not have enough heat to support the motion of tectonic plates for billion of years. Life, if it ever existed on Mars, was thus powerless to stop the ultimate collapse of its global environment.
The history of Mars should prove most instructive. If Martian life did exist, its perils might tell a tale as dramatic as it would be fascinating. For in the old layers of Martian rock, human geologists may one day find the mark of life just as they do in very old rocks in our home planet. And those rocks would provide a record of a series of interactions between life and the environment in which the mechanisms of the "thermostat" finally collapsed. Just as we learn much about the human brain by studying those brains that break down through injury or disease, we can learn about a terrestrial planet's global environment by studying terrestrial planets in which the global environment broke down.
As we will see in Chapter 6, the consequences of Martian life for our understanding of Earth’s biology would be truly extraordinary, even if we can find only fossils. The geological exploration of Mars is made even more tantalizing by the suggestion that at least one Martian meteorite, ALH84001, contains evidence of past life on our sister planet.
. Some researchers have suggested that the Earth's early atmosphere, following the initial heavy bombardment by asteroids, also had a very high percentage of CO2. The ensuing greenhouse would then compensate for the dimmer sun. Life eventually removed much of the CO2, thus preventing a runaway temperature when the sun's luminosity increased. This hypothesis runs contrary to other ideas on the composition of the early atmosphere, according to which a primitive atmosphere would exhibit either a highly reducing mixture of methane, ammonia, water and molecular hydrogen (similar to that of Jupiter, Saturn and the other planetary gas giants) or else a mildly reducing mixture of carbon monoxide, carbon dioxide, nitrogen, and water, with not much molecular hydrogen. (In this context a mixture is reducing to the extent that it contains hydrogen). To decide between these and perhaps other alternatives it will be helpful to study not only the histories of Mars and Venus, but the largely methane atmospheres of Titan and Triton, the large moons of Saturn and Neptune respectively. The reason these matters are so worth looking into is that knowing more about the composition of the primitive atmosphere can tell us much about the origin and evolution of the global environment of a planet; in this case, of our planet.
Consider also one of the most interesting aspects of Jupiter's atmosphere: the famous Red Spot. In a dense atmosphere with winds of hundreds of miles per hour, how could a storm, which is what the Red Spot is, remain stable for centuries, perhaps for many thousands of years? The answer seems to be that the fast spin of Jupiter (once every ten hours) produces very strong Coriolis forces, which in turn produce the turbulent winds that drive the gigantic eddy of gas otherwise known as the Red Spot. As the planet spins, many smaller eddies develop, but these eddies eventually feed the Red Spot. In the midst of turbulence the Red Spot has achieved stability within the Jovian atmosphere. Thus stability arises from chaos. But interesting as this may be, what significance does it have for people on Earth? The significance is that space scientists see many parallels between the dynamics of the Red Spot and some weather patterns in the atmosphere of the Earth. In particular, these scientists see parallels to systems of high pressure that sit still for weeks or even months. Understanding this phenomenon, known as "blocking", would be a great help in forecasting the weather here on Earth. Of course, it may still turn out that what we learn about the stability of the Red Spot does not apply to stationary high-pressure systems on the Earth.