A knowledge of the characteristic properties of the Martian atmosphere is, of course, of scientific interest, both for its own sake and for the light it may throw on the origin of Earth's atmosphere. It has also, however, a practical significance. Within the foreseeable future, a vehicle containing scientific instruments, to be released from a spacecraft, will be landed on the surface of the planet. To slow down the lander, by the use of retrorockets or a parachute, or both, so that it does not crash on the surface, the composition, temperature, and pressure of the atmosphere of Mars, and their variation with altitude, should be known. Recent studies have shown that the atmospheric pressure at the surface of the planet is much lower than had been accepted for some 20 years. As a result, preliminary plans for the design of a landing vehicle have undergone extensive revision.

In the year 1784, William Herschel reported his opinion (p. 17) that Mars has a

"considerable" atmosphere, although this view was challenged a few years later. There is no doubt that Mars has an atmosphere, but it is very much less dense than Earth's atmosphere. Moreover, its composition appears to be quite different. Until suitable instruments can be carried by spacecraft into the Martian atmosphere (ch. XIII), definite information concerning the identity of the gases present can be obtained only from spectroscopic studies of sunlight reflected from Mars as observed from Earth.

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