The search for evidence of life or of life-related substances is of the greatest general interest, but the planet Mars presents a number of problems which have intrigued scientists for many years. The answers would not only clarify the situation concerning Mars, but they would also help in the understanding of many matters related to Earth. In order to explain some of the problems of Mars, a brief outline will be given here of certain aspects of the planet. The various subjects mentioned are treated more fully in later chapters of this book.
Observations of Mars through the telescope have revealed three distinct types of surface: namely, polar caps, bright areas, and dark areas. The brilliant white polar caps are formed in the region of each Martian pole during the local autumn and winter. The caps gradually diminish in size in the spring and summer and start to form again in the autumn in each hemisphere in turn. Some scientists argue that the polar caps consist of solidified water, similar to hoarfrost, whereas others think they are mainly solid carbon dioxide. It is true that water frost is more readily deposited but, on the other hand, the Martian atmosphere contains much more carbon dioxide than it does water vapor. The first problem is then: What is the composition of the polar caps?
The bright areas, covering much of the surface, are generally reddish or orange in color. It is to these areas that the planet owes its characteristic appearance as seen by the naked eye. The great majority of the so-called canals, which are usually darker in color, cross the bright areas. The large dark regions occupy about one-fourth of the Martian surface and are mainly in the northern hemisphere. The dark areas and the canals are much lighter in color during the local winter; they become darker in the course of the spring and summer in each hemisphere. Some dark areas appear to remain constant in size, whereas others are quite variable. Occasionally a new dark area seems to develop and it may last from a few months to many years. Conversely, an existing dark area may fade out and perhaps disappear.
Most students of Mars would probably agree that the bright areas have a covering, at least, of a reddish-colored mineral containing iron. But what is the composition of the material on the dark areas? Why do these areas show seasonal changes? Are these changes caused by vegetation, as some have suggested, or is there another explanation? Why are some dark areas permanent whereas others are variable? What are the so-called canals?
It has long been assumed that, as on the Moon, the dark areas on Mars are lowlands, whereas the bright areas are uplands. But is this necessarily true? There are arguments which suggest that the reverse may be the case. It would be of great interest to know more about the relief of the Martian surface. For example, are there really elevation differences of as much as 10 to 20 kilometers (32 800 to 65 600 feet), as has been suggested? If such elevations occur in the equatorial region, one of the strange problems concerning the flattening of Mars in the polar direction would be solved.
Although Mars has an atmosphere, it is now known to be a very tenuous one. The atmospheric pressure at the surface of the planet is roughly one-fiftieth to one-hundredth part, that is, about 1 or 2 percent, of the atmospheric pressure on Earth. The Martian atmosphere apparently consists mainly (or contains a considerable proportion) of carbon dioxide. It is expected that nitrogen and argon will also be present, but are the amounts as small as some have suggested? An answer to this question might help to explain the origin of the terrestrial atmosphere.
The temperature variations with increasing altitude in Earth's atmosphere are quite complex and not completely understood. The situation is complicated by the presence of large bodies of water as well as by the constituents of the atmosphere. Mars has no seas or lakes and its atmosphere is much less dense than that of Earth. A knowledge of the temperatures in the Martian atmosphere would undoubtedly clarify some of the problems on Earth. Terrestrial meteorologists would also benefit from information on the related problem of the wind structure on Mars and its variation with the seasons.
Clouds and haze of various types frequently appear in the atmosphere of Mars. White (or blue-white) clouds of different types are fairly common, but they usually cover only relatively small areas. Are these clouds droplets of water or small ice crystals? Or are they small particles of solid carbon dioxide? Occasionally yellow clouds form and cover very large areas of the planet. Presumably these clouds consist of dust particles of the same material as covers the bright areas on the surface. But other ideas have been suggested, and they need to be studied further.
A baffling Martian phenomenon is the so-called "blue haze" or "violet layer." If Mars is viewed (or photographed) through a telescope with a red or orange filter, or without a filter, the details of the surface are generally clearly visible. But with a blue filter, which passes only blue light, the details disappear, although the polar caps are seen to remain bright. Occasionally, there is a local or planetwide clearing for periods of hours to a few days, and then a photograph in blue light will show the surface details.
What is responsible for these strange effects? Most explanations, none of which is really satisfactory, attribute the phenomena to something in the atmosphere. They may, however, be caused largely by certain properties of the Martian surface. More must be known about the light and dark areas, as well as about the atmosphere, before the mystery of the blue haze and its occasional clearing can be solved.
Finally, what about the interior of Mars? It can be surmised, from the low density of the planet, compared with that of Earth, and the essential absence of a magnetic field, that the interiors of Mars and Earth differ in some respects. Earth probably has a core consisting mainly of the heavy metals, iron and nickel, partly molten and partly solid. If Mars has such a core at all, it is expected to be small. It would be of considerable interest to know if Mars has a heavy-metal core. This is related to the question of whether or not there has been melting in the interior of the planet, such as is believed to have occurred in Earth. Information about the interior of Mars would undoubtedly throw light on the origin of the solar system and on the early history of Earth.
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