4. A BRIEF HISTORY OF MARS

To the ancient Egyptians, Mars was known as Harmakhis, "the Red One", because of its color. The ancient Greeks named the planet Ares, after their God of War, because its red color reminded them of blood. (In fact, Mars and blood are red for related reasons: the surface of Mars is covered with dust and rock rich in oxidized iron, or rust, while blood carries oxygen bound to iron in hemoglobin, which makes it red.) The Romans, who adopted many Greek ideas, named the planet Mars after their God of War.

In 1604, after years of arduous effort, the German astronomer Johannes Kepler calculated that the orbit of Mars was actually an ellipse, not a circle. He soon recognized that all planetary orbits are elliptical. Kepler's work swept away the ancient prejudice that orbits of heavenly bodies had to be perfect circles. The orbit of Mars provided him with the key that soon unlocked the true laws of planetary motion.

In 1659 the Dutch scientist Christiaan Huyghens (inventor of the pendulum clock) observed Mars through his telescope and determined that the rotation period of the planet was nearly 24 hours, like that of the Earth. In 1666 the Italian astronomer Giovanni Domenico Cassini noticed that Mars had a white cap around one of the rotation poles. Later observers found that Mars had both north and south polar caps.

In 1794 the English astronomer William Herschel (discoverer of the planet Uranus) pointed out the similarities between Mars and the Earth (length of day, tilt of axis, seasonal changes, distance from the Sun) and suggested that the Martian polar caps were made of ice and snow which waxed and waned during the course of the Martian year. (Herschel was right about polar ice on Mars, but it turns out that much of it is frozen carbon dioxide, or dry ice, and not just the water ice he had in mind.)

With better telescopes, astronomers in the 19th century could observe more features on the surface of Mars. In 1840, the German astronomers Wilhelm Beer and Johann Maedler published the first complete (albeit crude) map of Mars.

During the particularly close approach of Mars in 1877, Asaph Hall at the U.S. Naval Observatory in Washington discovered two tiny satellites orbiting close to Mars, which he named Phobos and Deimos (Greek for "fear" and "panic"), after the chariot horses of the God of War. Oddly enough, when Jonathan Swift wrote Gulliver's Travels in 1726, he somehow "predicted" the existence of two tiny moons orbiting close to Mars. In the 18th century there were no telescopes large enough to see the moons of Mars. How did he do it? Swift knew that the Earth had one moon and at that time Jupiter was known to have four. Perhaps he reasoned that if Mars, located between Earth and Jupiter, had two moons, the numbers would make a regular geometrical progression (1, 2, 4). We might then understand why he predicted two moons for Mars, but not why Mars was good enough to oblige. In any case, it was a lucky guess.

Also in 1877, the Italian astronomer Giovanni Schiaparelli first observed straight lines on the surface of Mars, which he named canali or channels. His later maps of Mars showed many of these features . Other astronomers also saw the canali and agreed with Schiaparelli that these features ended in large dark spots which came to be called "oases". Such observations suggested the possibility of life on Mars.

Percival Lowell , a wealthy Bostonian, took this idea much further. In 1894, Lowell made his first observations of the planet from a private observatory that he built in Flagstaff, Arizona. He at once convinced himself that the canali were real, and eventually mapped hundreds of them. Lowell decided that the straight lines were actually artificial canals created by a race of intelligent Martians to carry water from the polar caps to the equatorial regions. Over the next two decades he published Mars the Abode of Life and two other popular books promoting this theory.

Lowell's theories influenced the young English writer H.G. Wells, who in 1898 published The War of the Worlds. With this novel, Wells invented the idea of an invasion of Earth by implacable aliens from Mars, and launched a whole genre of science fiction.

Lowell's popular theory provoked the ire of the biologist Alfred Russel Wallace (co-discoverer, with Charles Darwin, of evolution by natural selection). In 1907, at age 84, Wallace published a remarkable book called Is Mars Habitable? He didn't dispute Lowell's reported observations of straight lines on Mars, but he correctly reasoned that Mars was a frozen desert, and that the polar caps were more likely to be carbon dioxide than water. Any liquid water on Mars would instantly evaporate. Wallace wrote that only "a race of madmen", not intelligent beings, would build open canals to carry water on Mars.

Many experienced observers never saw the Martian "canals" and with good reason. We now know that they never existed. The apparent network of straight lines criss-crossing the surface of Mars was a product of the human tendency to recognize patterns, even where they don't exist. When straining to see a very faint and indistinct group of dark smudges, the eye tends to connect them in a straight line. This has been demonstrated by experiments.

On Halloween eve in 1938, Orson Welles and the Mercury Theater of the Air broadcast a radio adaptation of H. G. Wells' War of the Worlds . The story, presented as a series of "live" news flashes, panicked thousands of listeners across America into believing that the East Coast was actually being invaded by hostile Martians.

In 1950, based on his analysis of asteroid and comet orbits, the astronomer Ernst Öpik correctly calculated and predicted that Mars would be found to have hundreds of thousands of impact craters larger than Meteor Crater in Arizona.

In 1965, the unmanned Mariner 4 spacecraft of the U.S. made the first successful flyby of Mars, followed in 1969 by Mariners 6 and 7. These missions sent back images of the heavily cratered highlands of the southern hemisphere, and thereby created the misleading impression that Mars was like the Moon. That was corrected in 1971-72, when Mariner 9 mapped all of Mars and revealed the astonishing range of geological wonders on the planet -- stupendous volcanoes and rift valleys, and evidence of catastrophic floods.

A major breakthrough came with the two Viking missions of 1976. Viking 1 and 2 each consisted of an orbiter and a lander. The orbiters produced detailed global maps of Mars. Have a look at the so called Schiaparelli Hemisphere of Mars and the Valles Marineris Hemisphere [credit USGS]. The Viking landers provided panoramic images of the surface of the planet. The landers also carried out some inconclusive experiments designed to search for chemical signs of life on the surface of Mars.

In 1996, David McKay and colleagues announced the discovery of what they interpreted to be evidence for fossil microbial life in a meteorite from Mars.

In July 1997, the Mars Pathfinder spacecraft landed on Mars and successfully deployed the first robot vehicle ever placed on the surface of another planet. In September 1997, a second spacecraft, the Mars Global Surveyor, was placed in a large elliptical orbit around the planet. It is scheduled to begin detailed mapping of Mars in March 1999, after completion of "aerobraking" to lower its orbit. For an excellent description, see Roving for Rocks on the Red Planet: Mars Pathfinder Mission by the Mars Pathfinder Project Scientist, Matthew Golombek.

Sources. A fascinating and detailed history of Martian exploration is William Sheehan's The Planet Mars: A History of Observation and Discovery (University of Arizona Press, 1996). See also the introductory chapter by Hugh Kieffer, Bruce Jakosky and Conway Snyder in Mars (University of Arizona Press, 1992), the most comprehensive one-volume scientific survey of the planet.