Mars perhaps first caught public fancy in the late 1870s, when Italian astronomer Giovanni reported using a telescope to observe "canali," or channels, on Mars. A pos-sible 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 that he energetically promoted for many years.

By the turn of the last century, popular songs envisioned sending messages between worlds 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, pocked with craters similar to those seen on 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 a suite of experiments that con-ducted chemical tests in search of life. Most scientists interpreted the results of these tests as negative, deflating hopes of identifying another world on where life might be or have been 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 concentrated on a study of the planet's geology and climate. Over the next 20 years, however, new findings in laboratories 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 and NASA's Johnson Space Center that a meteorite believed to have originated on Mars contained what might be the fossils of ancient bacteria. 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 comet impacts. They are thought to come from Mars because of gases trapped in the rocks that match the composition of Mars' atmosphere. Not all scientists agreed with the conclusions of the team announcing the discovery, but it reopened the issue of life on Mars.

Another development that shaped scientists' thinking was 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 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 other hospitable environment. Or it might still exist in some form in niches below the frigid, dry, windswept surface.

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.

Mars and Water

Mars today is far too cold with an atmosphere that is much too thin to support liquid water on its surface. Yet scientists studying images acquired by the Viking orbiters consistently uncovered landscape features that appeared to have been formed by the action of flowing water. Among those features were deep channels and curving canyons, and even landforms that resemble ancient lake shorelines. Added to this foundation is more recent evidence, especially from observations made by Mars Global Surveyor, that suggested widespread flowing water on the Martian surface in the planet's past. On the basis of analysis of some of the features observed by both the Mars Pathfinder and Mars Global Surveyor spacecraft, some scientists likened the action of ancient flowing water on Mars to floods with the force of thousands of Mississippi Rivers.

Continuing the saga of water in the history of Mars, in June 2000 geologists on the Mars Global Surveyor imaging team presented startling evidence of landscape fea-tures that dramatically resemble gullies formed by the rapid discharge of liquid water, and deposits of rocks and soils related to them. The features appear to be so young that they might be forming today. Scientists believe they are seeing evidence of a groundwater supply, similar to an aquifer. Ever since the time of Mariner 9 in the early 1970s, a large part of the focus of Mars science has been questions related to water: how much was there and where did it go (and ultimately, how much is accessible today). The spectacular images from Mars Global Surveyor reveal part of the answer -- some of the water within the Mars "system" is stored underground, perhaps as close as hundreds of meters (or yards), and at least some of it might still be there today. Still, there is no general agreement 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.

Even among those who subscribe to the warmer-and-wetter theory, the question of what happened to the water is still a mystery. Most scientists do not feel that the scenario responsible for Mars' climate change was necessarily a cataclysmic event such as an asteroid impact that, say, disturbed the planet's polar orientation or orbit. Many believe that the demise of flowing water on the surface could have resulted from a gradual process of climate change taking place over many millennia.

Under either the warmer-and-wetter or the always-cold scenario, Mars must have had a thicker atmosphere to support water that flowed on the surface even only occasionally. If the planet's atmosphere became thinner, then liquid water could not flow without evaporating. Mars' atmosphere today is overwhelmingly composed of carbon dioxide. Over time, carbon dioxide gas reacts with elements in rocks and becomes locked up as a kind of compound called a carbonate.

On Earth, the horizontal and vertical motions of the shifting tectonic plates that define the crust of our planet are continually plowing carbonates and other widespread miner-als beneath the surface to depths at which the internal heat within Earth releases carbon dioxide, which later spews forth in volcanic eruptions. This terrestrial cycle replenishes the carbon dioxide in Earth's atmosphere. Although we are not sure Mars today harbors any active volcanoes, it clearly had abundant and widespread volcanic activity in its past. The apparent absence of a long-lasting system of jostling tectonic plates on Mars, however, suggests that a critical link in the process that leads to carbon dioxide recyling in Earth's atmosphere is missing on Mars.

These scenarios, however, are just theories. 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 perma-nent cap at Mars' north pole. Water ice may also 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.

NASA's next mission to the red planet, 2001 Mars Odyssey, will provide another vital piece of information to the ³water puzzle" by mapping the basic chemistry and minerals that are present in the upper centimeters (or inches) of the planet's surface. Odyssey and within the surface of Mars, and hydrogen may provide the strongest evidence of water on or just under the Martian surface since it is one of the key elements within the water molecule. The high-resolution imaging system on Odyssey will be able to identify regions such as hot springs, if any exist, which could serve as prime sites in which to refine our surface search for signs of simple biological processes.

Even if we ultimately learn that Mars never harbored life as we know it, scientific explo-ration of the red planet can assist in understanding life on our own home planet. Much of the evidence for the origin of life here on Earth has been obliterated by the incredi-ble dynamics of geological processes which have operated over the past 4 billion years, such as plate tectonics and rapid weathering. Today we believe that there are vast areas of the Martian surface that date back a primordial period of planetary evolu-tion -- a time more than about 4 billion years ago that overlaps the period on Earth when pre-biotic chemical evolution first gave rise to self-replicating systems that we know of as "life."

Thus, even if life never developed on Mars -- something that we cannot answer today --scientific exploration of the planet may yield absolutely critical information unobtain-able by any other means about the pre-biotic chemistry that led to life on Earth. Furthermore, given the complexity we recognize in Earthıs record of climate change, some scientists believe that by studying the somewhat simpler (but no less bizarre ) Martian climate system, we can learn more about Earth. As such, Mars could serve as Mother Nature's great "control experiment" providing us with additional perspectives from which to understand the workings of our own home planet. The 2001 Mars Odyssey mission continues us on the path of understanding the red planet as a "system" by probing what it is made of, and where the elusive signs of surface water may have left their indelible marks.