Cosmic Connection by Carl Sagan Read Free Book Online Page B

There are at least two objections to this argument. First, oxides of nitrogen are poisonous gases only to some organisms on Earth. Second, what quantity of oxides of nitrogen were thought to be discovered on Mars? When I calculated the amount, it turned out to be less than the average abundance above Los Angeles. The oxides of nitrogen are an important constituent of smog. Life in Los Angeles may be difficult, but it is not yet impossible. The same conclusion applies to Mars. The final problem with these particular observations is that they are very likely mistaken; later studies–for example, observations Tobias Owen and I made with the Orbiting Astronomical Observatory–have shown no oxides of nitrogen in the atmosphere of Mars.
    Oxygen chauvinism is common. If a planet has no oxygen, it is alleged to be uninhabitable. This view ignores the fact that life arose on Earth in the absence of oxygen. In fact, oxygen chauvinism, if accepted, logically demonstrates that life anywhere is impossible. Fundamentally, oxygen is a poisonous gas. It chemically combines with and destroys the organic molecules of which terrestrial life is composed. There are many organisms on Earth that do without oxygen and many organisms that are poisoned by it.
    All of the earliest organisms on Earth did not use molecular oxygen, O 2 . In a brilliant set of evolutionary adaptations, organisms like insects and frogs and fish and people learned not only to survive in the presence of this poisonous gas but actually to use it to increase the efficiency with which we metabolize foodstuffs. But that should not blind us to the fundamentally poisonous character of this gas. The absence of oxygen on a place such as Jupiter is, therefore, hardly an argument against life on such planets.
    There are ultraviolet light chauvinists. Because of the oxygen in the Earth’s atmosphere, a variety of oxygen molecule called ozone (O 3 ) is produced high in the atmosphere, about twenty-five miles above the surface. This ozone layer absorbs the middle-wavelength ultraviolet rays from the Sun, preventing them from reaching the surface of our planet. These rays are germicidal. They are emitted by ultraviolet lamps commonly used to sterilize surgical instruments. Strong ultraviolet rays from the Sun are an extremely serious hazard to most forms of life on Earth. But this is because most forms of life on Earth evolved in the absence of a high ultraviolet flux.
    It is easy to imagine adaptations to protect organisms against ultraviolet light. In fact, sunburn and high melanin pigmentation in the skin are adaptations in this direction. They have not been carried very far in most terrestrial organisms because the present ultraviolet flux is not very high. In a place like Mars, where there is little ozone, the ultraviolet light at the surface is extremely intense. But the Martian surface material is a strong absorber of ultraviolet light–as most soil and rocks are–and we can easily imagine organisms walking around with small ultraviolet-opaque shields on their backs: Martian turtles. Or perhaps Martian organisms carry about ultraviolet parasols. Many organic molecules also could be used in the exterior layers of extraterrestrial organisms to protect them against ultraviolet light.
    There are temperature chauvinists. It is said that the freezing temperatures on planets like Jupiter or Saturn, in the outer Solar System, make all life there impossible. But these low temperatures do not apply to all portions of the planet. They refer only to the outermost cloud layers–the layers that are accessible to infrared telescopes that can measure temperatures. Indeed, if we had such a telescope in the vicinity of Jupiter and pointed it at Earth, we would deduce very low temperatures on Earth: We would be measuring the temperatures in the upper clouds and not on the much warmer surface of Earth.
    It is now quite firmly established, both from theory and from radio observations of these