And you see they're shown at the same scale and the Earth is in fact no bigger than the great red spot of Jupiter, which is a giant cyclone. And all of the planets that have been detected around other stars - a very exciting research frontier in astronomy - are of the class of planets like Jupiter. Some day perhaps we shall learn whether there are other Earths. But these planets are certainly very different from Earth, and don't tell us very much about the Earth. The planets that are more similar in size to the Earth are Venus and Mars, shown here you along with a collection of other objects： Mercury and some moons (fig.11). These two planets share many processes with the Earth, but not enough that they are similar in all ways. And you will see that what little we know about Venus and Mars say that the Earth is indeed special.
Venus (fig.12) has a massive atmosphere of carbon dioxide, 100 times more massive than that of the Earth. The atmosphere has created a greenhouse that had left the surface very hot. The surface of Venus is something like 500 degrees Centigrade. And even though there are land forms of the sort familia on Earths such as this giant volcano that has been imaged by radar (fig.13) because of this greenhouse the planet is almost completely dry. And so water is unimportant on this planet. There are important climatic variations, and the study of Venus can tell us something about the general topic of climate variation, but this is not an Earth. We wouldn't want to live there.
The planet Mars (fig.14) is interesting. It has a thin atmosphere. It is currently the subject of study by a number of spacecraft, including the Nozomi spacecraft which is headed to Mars to study the atmosphere and its interaction with the solar wind. We in fact believe we have samples of Mars - twelve of the more than 30,000 meteorites in the worlds collections, including many collected by the Japanese and the United States in Antarctica, including this Antarctic meteorite (fig.15), are believed to come from Mars. So we know quite a bit about this planet, including the fascinating fact that in the geological past, water running on or near the surface and carving out these land forms (fig.16) was evidently quite common. So there was an era in the history of Mars involving processes similar to those that operate on Earth.
But today Mars is a frozen place. The only water is at the polar caps (the north pole is shown in fig.17) and probably in permafrost. These bands, however, are interesting for future exploration. They are layered deposits that are thought to record variations in climate on Mars driven by variations in orbital parameters. So if we were thinking very broadly in terms of objectives for the whole 21st century, perhaps some day we might think about drilling on Mars to understand climate variations there. But that is not the topic that we're talking about here today.
Given that the Earth is a special place (fig.1), because it has oceans, because it has life, because it has a range of temperatures that is permissive of both liquid water and the evolution of life including ourselves, it is obviously very important for us to study.
The second message that I wanted to convey in this talk was the message that we are learning that the Earth is quite complicated, that it can be thought of as 2 set of complex interacting systems. And I want to give you four examples today of how these systems interact.
The first is the plate tectonic system, which mostly involves the solid Earth, but I will argue that the interaction with the oceans is a critical part of the plate tectonic cycle. Here you see on your left a map of the Earth's plates (fig.18), which are moving relative to one another, and in the middle a cartoon cross-section of the plate tectonic cycle (fig.19).
Most of the Earth, of course, is under oceans, and the oceanic plates are the simplest to understand. They are born at the mid-ocean ridge, where hot material partially melts to give rise to the oceanic crust and where vigorous interaction with the sea water cools the lithosphere and changes the chemistry of the oceanic crust. The plate rides across until it encounters another plate, where it subducts, deep into the mantle.
