8. Mars: the red planet
Telescopic observations , early speculations and missions to Mars
An Earth-like planet
Mars spins on its axis with a rate and tilt that are almost identical to the Earth’s. The day on Mars is only 37 minutes longer than our own, and the polar axis on Mars is tilted at 25 degrees, about the same as Earth with its 23-degree tilt. Both planets therefore have four seasons – autumn, winter, spring and summer – although the Martian seasons last about twice as long since the Martian year is nearly two Earth years.
Mars is a relatively small planet, about half the size and one-tenth the mass of Earth. The total surface area of Mars is about equal to the land area of Earth, owing to the extensive oceans that occupy most of the terrestrial surface. Still, Mars is substantially larger than the Moon or Mercury, so we might expect it to be intermediate in many properties between Earth and the Moon.
It is indeed the Earth-like appearance of Mars in a telescope that has intrigued humanity during the past few centuries. Its orbit is closer to the Earth than any other planet except Venus, enabling us to discern polar caps that change in size with the Martian seasons and dark markings that seasonally distort its ruddy face. A white polar cap grows in the local winter at each pole and recedes with the coming of local spring. Large grayish-green regions flourish in the summer and become dormant in winter, as many plants do on Earth. Their seasonal growth on Mars has been called the “wave of darkening” since a dark band moves from a polar cap toward the equator as the cap shrinks.
White clouds repeatedly form at certain locations on Mars, and clouds are not possible without an atmosphere. Both the Earth and Mars have shallow, relatively clear atmospheres, heated seasonally by varying sunlight. Thus, Mars is in many ways the planet most closely resembling the Earth. Both planets have an atmosphere, clouds, polar caps and seasons.
Early speculations about intelligent life on Mars
Over the past century, our fascination with Mars has been stimulated largely by the prospect that life may exist there, either in the past or the present. Large, seasonally-varying, dark regions seemed to suggest life, since water melting from the polar caps might cause hypothetical vegetation to grow and progress from the poles to the equator.
During a particularly favorable opposition in 1877, when Mars was even closer to the Earth than during most other oppositions, the Italian astronomer Giovanni Schiaparelli (1835-1910), director of the Milan Observatory, reported that a maze of dark, narrow straight lines traverses the planet’s surface. He called them canali, the Italian word for “channels” or “canals”, assuming that they were natural features. Schiaparelli mapped them and gave the broadest ones the names of large terrestrial rivers, such as the Ganges and Indus.
At about the same time, a wealthy Bostonian, named Percival Lowell (1855-1916), convinced much of the American public that there is intelligent life on Mars. Rich enough to do as he pleased, Lowell built an observatory in the clear air of Flagstaff, Arizona with the specific intention of observing and explaining the Martian canals. When Lowell turned his telescope toward Mars in 1894, he found what he expected to see – a vast network of canals bordered by vegetation.
Most astronomers, however, could not see the canals, which had been glimpsed at the limit of telescopic detection, concluding that they were some sort of optical illusion if they existed at all. And no one ever succeeded in photographing the canals using telescopes on Earth.
The space-age odyssey to Mars
Due to distortion caused by the Earth’s atmosphere, the details of Mars remained hidden from view until spacecraft flew past it, and were then sent to orbit the red planet and land on its surface. The search for life on Mars has been one of the main driving forces behind all of these missions, which have successively gathered evidence for and against living things on the planet. We still do not know for sure whether life now exits on Mars, or if it once did, but the possibility of life on Mars remains deeply embedded in human consciousness.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University