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Ever since it was known that the planets were bodies more or less like the Earth,Cartier rotonde, people have asked: "Are there living things on them too?" This is a hard question, but by pooling the resources of most of the sciences, we can make a pretty good answer.
First, let us consider life on our Earth for a bit. Our world would not be habitable if it did not have water on it. All living things are absolutely dependent on water. We digest our food, for example, only when it is dissolved in water in our stomachs. Moreover, this water must be liquid—neither ice nor gaseous water vapor, but ordinary liquid water. Many Breitling Replica living things—such as the inner bark of trees—can survive being frozen and thawed out again; but nothing grows while it is frozen. And no living thing can stand having the water boiled out of it. Hence the temperature .on a habitable planet must be above freezing part of the time, and below the boiling point all the time.
Light is necessary too. Practically all the food in the world is produced by green plants. These are the most marvelous laboratories in the world. Plants take in simple raw materials, carbon dioxide gas from the air and water with some things dissolved in it from the ground,Cartier declaration, and build up out of these, as they grow, all the complex substances of which they themselves are made, and some of which serve as food for animals. The plants get the energy required for this process from sunlight.
The waste product of the green-leaf laboratories is oxygen gas, which is turned back into the atmosphere. There is good reason to believe that the vast store of oxygen in the air has all been put in by plant life during the long course of geological time. Without plants, there would be no food for us animals to eat, nor oxygen-containing air for us to breathe.
There is, however, another geological process which takes oxygen out of the air. Most of the igneous rocks, which, like lava, have come up melted from below, contain a good deal of incompletely oxidized iron. As these rocks are "weathered", that is, broken down and carried off by rain and streams, the iron in them combines with more oxygen, taken from the air, and the originally black or gray rocks give rise to red or yellow sand, mud or clay. If we inquire whether other planets are habitable, we must then try to find out whether their temperatures are suitable for life, whether they have water and atmospheres on them, and, if so, whether oxygen and carbon dioxide are present in these.
As for the composition of the atmosphere, certain gases, if present in them, absorb Cartier Roadster Replica light of particular wavelengths. We therefore study the planet's light with a spectroscope—an instrument which can tune out and separate different wave lengths of light many hundreds of times more powerfully than your radio can separate two stations of nearly the same wave length. And with this sensitive device we can find whether or not oxygen, carbon dioxide, and water vapor are present in any considerable quantity, and estimate their amounts.
Electrical heat-measuring device can be made so sensitive that, attached to a great telescope; they could measure the heat from a single candle 100 miles away. With such a device it is possible to get a pretty close estimate of the temperature of a plant's visible surface.
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