The Terrestrial Planets (cont.)

Mars

Mars is larger than Mercury, slightly over half the diameter of the Earth. For centuries it has been a mystery planet, a blurred red globe in the telescope, splashed with patches of white and dark. Even before spacecraft vetured there, we knew that Mars had a thin atmosphere, winds, water, clouds, and polar ice caps. There was a continuing debate over whether the so-called "canals" were really there and whether Mars might in fact have life. In science fiction, Mars was the destination for spacefaring humans and also the main source of extraterrestrials, hostile or friendly, who visited Earth.

A new Mars has been revealed by several unmanned spacecraft: flybys, orbiters, and finally landers. Mars is now recognized as a kind of "halfway" world. Part of the Martian surface is ancient like those of the Moon and Mercury, and part is more evolved and Earth-like. The southern Martian hemisphere is heavily cratered, so it too bears the traces of the primordial bombardment that engulfed the Moon and Mercury. But the Martian craters are different: Their rims are low; they are shallow, eroded, and filled with wind-blown dust accumulated over millions of years.

The northern half of Mars is more like the Earth. Huge volcanic mountains climb as much as 25 kilometers (15 miles) into the sky, three times as high as Mount Everest, and the remnants of ancient lava are seen on their slopes. A huge chasm, wider in places than the Grand Canyon is long, runs east-west for about 4500 kilometers (2800 miles). Elsewhere on this younger half of Mars, surprising, twisted channels meander across the surface. These are not the straight "canals" once seen from Earth; those turned out to have been tricks of perception produced by viewing Mars at the limit of Earthbound vision. The real twisted, braided channels seem to have formed catastrophically in huge, sudden floods more than a billion years ago. The water has vanished; no liquid water is now found on Mars. But huge reservoirs of frozen water apparently remain in the Martian polar caps, and in permafrost beneath the Martian soil. The white polar caps are largely made up of water, with a frosting of "dry ice" or frozen carbon dioxide. The caps themselves lie on layered sediments, perhaps the evidence of past ice ages on Mars.

Two places on Mars have been examined on the surface by the two Viking landers (one of which is still operating since its landing in 1976). The robot spacecraft sent back pictures of a pink sky, colored by fine, red, windblown dust. Lander instruments measured mild winds in the thin Martian air and detected a thin frost that formed on rocks and soil during the Martian winter.

The face of Mars.
The different regions of the Red Planet are shown in this single picture taken by Viking I at a distance of 560,000 kilometers (348,000 miles). The dark spots are huge volcanoes, which characterize the younger, more geologically active northern hemisphere. The largest Martian volcano, Olympus Mons, is the isolated dark spot at the upper right. It is more than 600 kilometers (373 miles) across and rises about 25 kilometers (16 miles) above the surface. The more ancient, heavily cratered southern hemisphere is mostly in shadow here, but the large circular feature, Argyre, probably formed by a great meteorite impact, can be seen at the bottom, its shape emphasized by a thin layer of frost or ground fog within.

The atmosphere of Mars is very thin; the barometric pressure is less than one percent that of the Earth at sea level. Unlike our air, Mars' atmosphere is not predominantly composed of nitrogen and oxygen, but consists almost exclusively of carbon dioxide (CO₂),with minor amounts of nitrogen and argon and just a trace of oxygen.

The rocks of Mars are porous and jagged, like the volcanic lavas of the Earth and the Moon. The Martian soil, analyzed by the Viking landers, has a composition very much like weathered lava. The red color of Mars is in fact caused (as generations of science fiction writers had written) by the presence of oxidized iron, a kind of exotic rust.

The greatest mystery of Mars - Is there life? - remains a mystery.

Several experiments on the Viking landers tried to detect life in the soil, and some unusual reactions were encountered. But the reactions probably were due to the unusual chemistry of the soil rather than to the presence of any microscopic life forms. It seems that we will need to carry more so phisticated experiments to Mars - or bring a sample of Mars back to laboratories here on Earth - before we can answer the question with certainty.

Rocks of the Red Planet.
A geological welcoming committee, Martial rocks up to a meter (3.3feet) across surround the Viking 2 lander in the Utopia region of Mars. Dark, sometimes banded, and often bubble-rich, the rocks resemble volcanic lavas (basalt) found on the Earth and Moon. The red soil is colored by oxidized iron; its chemical composition resembles that of weathered basalt lava. The surprising pink sky of Mars draws its hue from fine red dust carried aloft by the winds.

photo from the Viking 2 Lander of the rock strewn martian surface

Mars is a more developed planet than the Moon and Mercury. The evidence of massive volcanism is obvious. In addition, some geologists consider the vast chain of canyons in the northern hemisphere to be the result of rifting, perhaps the beginning of continent-building. Mars has made and retained an atmosphere. But, like the Moon, Mars apparently stopped evolving 1 or 2 billion years ago and has not gone as far geologically as the Earth. On the other hand, like the Earth, it is a world with weather and climate. The records of its past climates, floods, and ice ages remain in the layered sediments of the polar regions, photographed in detail, but as yet unreachable.