Mars perhaps first caught public fancy in the late 1870s, when Italian astronomer Giovanni Schiapparelli reported using a telescope to observe canali, or channels, on Mars. A possible mistranslation of this word as canals may have fired the imagination of Percival Lowell, an American businessman with an interest in astronomy. Lowell founded an observatory in Arizona, where his observations of the red planet convinced him that the canals were dug by intelligent beings - a view which he energetically promoted for many years.
By the turn of the century, popular songs told of sending messages between Earth and Mars by way of huge signal mirrors. On the dark side, H.G. Wells' 1898 novel The War of the Worlds portrayed an invasion of Earth by technologically superior Martians desperate for water. In the early 1900s novelist Edgar Rice Burroughs, known for the Tarzan series, also entertained young readers with tales of adventures among the exotic inhabitants of Mars, which he called Barsoom.
Fact began to turn against such imaginings when the first robotic spacecraft were sent to Mars in the 1960s. Pictures from the first flyby and orbiter missions showed a desolate world, pockmarked with craters like Earth's Moon. The first wave of Mars exploration culminated in the Viking mission, which sent two orbiters and two landers to the planet in 1975. The landers included experiments that conducted chemical tests in search of life. Most scientists interpreted the results of these tests as negative, deflating hopes of a world where life is widespread.
The science community had many other reasons for being interested in Mars apart from searching for life; the next mission on the drawing boards, Mars Observer, concentrated on a study of the planet's geology and climate. Over the next 20 years, however, new developments in studies on Earth came to change the way that scientists thought about life and Mars.
One was the 1996 announcement by a team from Stanford University, NASA's Johnson Space Center and Quebec's McGill University that a meteorite believed to have originated on Mars contained what might be the fossils of ancient microbes. This rock and other so-called Mars meteorites discovered on several continents on Earth are believed to have been blasted away from the red planet by asteroid or meteor impacts. They are thought to come from Mars because gases trapped in some of the rocks match the composition of Mars' atmosphere. Not all scientists agreed with the conclusions of the team announcing the discovery of fossils, but it reopened the issue of life on Mars.
Other developments that shaped scientists' thinking included new research on how and where life thrives on Earth. The fundamental requirements for life as we know it are liquid water, organic compounds and an energy source for synthesizing complex organic molecules. Beyond these basics, we do not yet understand the environmental and chemical evolution that leads to the origin of life. But in recent years it has become increasingly clear that life can thrive in settings much different from the longheld notion of a tropical soup rich in organic nutrients.
In the 1980s and 1990s, biologists found that microbial life has an amazing flexibility for surviving in extreme environments - niches that by turn are extraordinarily hot, or cold, or dry, or under immense pressures - that would be completely inhospitable to humans or complex animals. Some scientists even concluded that life may have begun on Earth in heat vents far under the ocean's surface.
This in turn had its effect on how scientists thought about Mars. Life might not be so widespread that it would be found at the foot of a lander spacecraft, but it may have thrived billions of years ago in an underground thermal spring. Or it might still exist in some form in niches below the frigid, dry, windswept surface wherever there might be liquid water.
NASA scientists also began to rethink how to look for signs of past or current life on Mars. In this new view, the markers of life may well be so subtle that the range of test equipment required to detect it would be far too complicated to package onto a spacecraft. It made more sense to collect samples of Martian rock, soil and air to bring back to Earth, where they could be subjected to much more extensive laboratory testing with state-of-the-art equipment.
Mars and Water Mars today is too cold, with an atmosphere that is too thin, to support liquid water on its surface. Yet scientists who studied images from the Viking orbiters kept encountering features that appeared to be formed by flowing water - among them deep channels and canyons, and even features that appeared to be ancient lake shorelines. Added to this were more recent observations by Mars Pathfinder and Mars Global Surveyor which suggested widespread flowing water in the planet's past. Some scientists identified features which they believe appear to be carved by torrents of water with the force of 10,000 Mississippi Rivers.
There is no general agreement, however, on what form water took on the early Mars. Two competing views are currently popular in the science community. According to one theory, Mars was once much warmer and wetter, with a thicker atmosphere; it may well have boasted lakes or oceans, rivers and rain. According to the other theory, Mars was always cold, but water trapped as underground ice was periodically released when heating caused ice to melt and gush forth onto the surface.
In either case, the question of what happened to the water remains a mystery. Most scentists do not feel that Mars' climate change was necessarily caused by a cataclysmic event such as an asteroid impact that, perhaps, disturbed the planet's polar orientation or orbit. Many believe that the demise of flowing water on the surface could have resulted from gradual climate change over many millennia as the planet lost its atmosphere.
Under either the warmer-and-wetter or the always-cold scenario, Mars must have had a thicker atmosphere in order to support water that flowed on the surface even only occasionally. If the planet's atmosphere became thinner, liquid water would rapidly evaporate. Over time, carbon dioxide gas reacts with elements in rocks and becomes locked up as a kind of compound called a carbonate. What's left of Mars' atmosphere today is overwhelmingly carbon dioxide.
On Earth, shifting tectonic plates are continually plowing carbonates and other minerals under the surface; heated by magmas, carbon dioxide is released and spews forth in volcanic eruptions, replenishing the carbon dioxide in the atmosphere. Although Mars has no known active volcanoes and there are no signs of fresh lava flows, it had abundant volcanic activity in its past. However, Mars appears to have no tectonic plates, so a critical link in the process that leads to carbon dioxide replenishment in Earth's atmosphere is missing. In short, Mars' atmosphere could have been thinned out over many eons by entrapment of carbon dioxide in rocks across its surface.
That scenario, however, is just a theory. Regardless of the history and fate of the atmosphere, scientists also do not understand what happened to Mars' water. Some undoubtedly must have been lost to space. Water ice has been detected in the permanent cap at Mars' north pole, and may exist in the cap at the south pole. But much water is probably trapped under the surface - either as ice or, if near a heat source, possibly in liquid form well below the surface.
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