Lesson 2: Earth Planet of Life 2.3 The Water Planet Figure 2.3.1 The Pacific Ocean There are (at least) two things we would like to know concerning the fact that the home planet has an ocean and its siblings in space do not: What is so special about Earth that allows it to have an ocean? And how does having an ocean make the Earth a good place for Life? We have already identified two vital ingredients of the answer to the first question: size and distance to the home star (or better: energy received). Are there others? Is it important what stuff the Earth is made of? Or did Earth, as a planet, have a history of evolution that is fundamentally different from the histories of its siblings? To put the question within the framework of the thought experiments of the previous section: If we put Venus in the position of Earth, will it acquire an ocean? Let's look at these various questions one at a time. First, we note that the mass of the film of water on Earth, the "hydrosphere", is very small when compared to the mass of the planet. The mass of Earth is about 6 x 1027 grams (1027 stands for "10 multiplied by itself 27 times", that is, a 1 with 27 zeroes). The mass of the hydrosphere is about 1.5 x 1024 grams or 1:4000 (0.025%) of Earth's total mass. This value of 0.025% is significant. It means that there is no problem in deriving the water from the kind of planetary matter that is abundant in the solar system. We know what this material looks like from studying the composition of metallic and stony meteorites. On average the water content of meteorites is well above 1:1000 (0.1%). Scientists generally agree these meteorites are a good sample of the kind of stuff that formed the Earth, by gravitational accretion. Thus, Earth's ocean evolved from the water expelled from this material, as it heated up during the early history of our planet. Whatever the exact course of events might have been during the formation of Earth, we would have to admit that there was enough water in the primordial meteoric matter to make the ocean. Figure 2.3.2 Present day outgassing of water by Old Faithful Geyser Yellowstone National Park This line of reasoning pretty much eliminates the likelihood that our neighbors in space, the Moon, Venus and Mars, don't have enough water within their bodies to make oceans. Perhaps, then, it is a question of expelling the water from the original rock? To check this idea, we must look at the structure of Earth, which we shall turn to presently. For now, we note that Earth and all its neighbors in space show the effects of gravitational segregation, with heavy materials concentrated toward the center of a planetary body, and light matter farthest away from the center of gravity. For Earth, this means a heavy metallic core, a thick mantle of basaltic rock surrounding the core, a relatively thin crust of rocks to walk on, a layer of water, and an envelope of gas. What is especially interesting is that the mantle of Earth (which makes up two thirds of its volume) shows "convection", like soup being heated on a stove. Through this process, through geologic time, different parts of the mantle come to the surface of the planet and then disappear again deep into the interior. The convection is driven by the heat generated from decay of radioactive elements inside the Earth. Again, size is crucial for translating this type of heat into convection. A small planet (such as the Moon) also has radioactive elements releasing energy, but the heat generated can be dissipated by diffusion and does not build up sufficiently to soften up the rocks and make things move. Figure 2.3.3 Internal structure of the Earth Now let us turn to the question of what the presence of water does for the standard of living on Earth. The fact that water has existed in its three phases (solid, liquid, gas) for billions of years, is of crucial importance for the climatic conditions on Earth, and hence for the evolution of life. Great amounts of energy can be taken up (or released) by water when it changes phase. This property of water, and others, makes the hydrosphere a planetary air conditioner of great efficiency. How does this work? First, let us take a look at the properties of water as they affect the heat budget of Earth. We already met with one important property in connection with the "greenhouse effect": absorption of heat radiation by water vapor. Without it, our home planet would be inhospitable at an average temperature of near zero degrees Fahrenheit. But there is a much more elementary question: How does water react to heating or cooling? How much energy can it store? Figure 2.3.4 Phase diagram for water. The horizontal axis is temperature, the vertical axis is pressure. Freezing and boiling temperatures are indicated for 760 millimeters of mercury which is normal atmospheric pressure at sea level. It turns out that water is much more efficient in storing heat than any other common substance on Earth. To increase the temperature of fluid water by one degree Centigrade takes one calorie of heat energy for each gram of water. To do the same for a rock, for comparison, takes 0.2 calories per gram, and for petroleum, 0.5 calories per gram. To change a gram of ice into water (without increasing the temperature) takes 80 calories. Conversely, when water freezes, it releases this amount of heat into the environment. To change a gram of water into vapor takes 580 calories! This means that evaporation is the best cooling mechanism around. Our body takes advantage of this fact when cooling itself by sweating: The evaporation of the sweat extracts heat from the surroundings, lowering the temperature of the skin. The heat expended in the phase change of water to vapor is not lost, but is contained in the vapor in latent form. For this reason, this energy is called "latent heat". When the vapor condenses, the heat is freed for warming the surrounding air. This process can be observed when a thunder storm develops: the newly forming clouds release heat to the air and the heated air rises to make more clouds. The phase changes of water, combined with the unique heat-related properties of water, are intimately involved in all aspects of climate and weather. Water transfers and stores heat on an immense scale, and thereby evens out the temperature differences between day and night, summer and winter, tropics and polar areas. It is in this fashion that water makes Earth a nice place to live, even for those creatures (like us) who do not have the benefit of being immersed in water. Table 2.3.1 Comparison of surface temperatures for the terrestrial planets. Consider the temperature differences between day and night. They are largest in the water-starved desert. In areas where water is abundant, evaporation during the day tends to lower the temperature, and condensation during the night (the familiar dew) tends to raise it. During the day, clouds protect the ground from the heat of the Sun, and at night, from the black coldness of space. Seasonal differences are similarly tempered by the great heat reserves of water, hence the mildness of coastal climates. Likewise, geographic differences are moderated by the transfer of heat through water motion and water phase changes. Most of this transfer is by water vapor in the air. Warm winds blowing over warm ocean regions pick up the water, cooling the sea surface. In cold regions, the air becomes unable to hold the vapor. The vapor condenses and gives off its heat of evaporation. If it freezes to snow, it also gives off its heat of fusion, to the surrounding air. Thus, a blizzard brings warmth -- a somewhat unexpected conclusion, perhaps, to those who have been surprised by a snowstorm during a hike in the mountains. Ocean currents also play an important role in transferring heat from the warm tropics to the cold polar regions. A familiar example is the relatively mild climate of Norway, which depends upon the warm waters brought north along the Norwegian coast through an extension of the Gulf Stream System. Excepting the poles, the most severe climates are in the interior of continents, far from the sea. Central Siberia is the prime example of a "continental" climate with great seasonal temperature contrasts.