No matter how many Pathfinder photos you've studied, there's still something about the image of green-skinned giants on thoats thundering silently across the mossy plains of Barsoom that whispers, "Mars."

For that matter, a lot of people's mental maps of the Red Planet were shaped by tales of an ancient, golden-eyed race destroyed by chicken pox, looking benevolently on at the new species that replaces them. More recently, the story of a hundred human colonists divided by conflicting ideals of whether Mars should stay red or turn green has sparked our imaginations.

This trio of images spans nearly a full century, bookending science fiction from its beginnings as a genre to today.

The differences in those visions have as much to do with the changing times in which they were written as our changing knowledge of Mars itself.

Peering up at the martian surface through a telescope, Lowell had seen lines he thought were grand canals, and attributed the work of cyclopean "architecture" to an alien civilization struggling to irrigate the planet's dry surface.

At the time, the idea received widespread scientific support, and was even considered "decisively" settled in Lowell's favor as late as 1915.

Burroughs ran with the idea, making his Mars -- called "Barsoom" by the natives -- a dying planet preserved by the efforts of a solar-powered pumping system that kept the atmosphere breathable. Martian cities were scattered along the waterways running toward the equator from the polar caps.

Barsoom was far from a desert world, however. Most of the dried-up seabeds were covered with a moss that fed herds of thoats -- giant beasts of burden -- and other animals. Giant trees lined the waterways, and the vegetation was similar to that found on Earth, only hardier and more garishly colored.

Red and green aliens
The Martians themselves were occasionally technically advanced and always socially primitive. At least in Princess of Mars, Burroughs' first Barsoom book, the dominant civilizations were the red men (near-humans with red skin) and the green men (15 feet tall with four arms and big tusks).

The red men had advanced technology, but both races shared languages, tribal governments, and a violent attitude toward each other. They were honorable, but savage peoples.

This attitude was typical of Burroughs' era. In 1912, the Europeans and the Americans were enjoying the fruits of an age of colonization, having largely subjugated the native populations elsewhere. The natives of Africa and Asia -- to say nothing of the "red" natives of the Western Hemisphere -- were seen as little better than commodities, not humans to be dealt with as equals.

Of course Burroughs' audience would have expected John Carter to be smarter and tougher than the exotic aliens of Barsoom. He was, after all, an American among the natives.

Bradbury's Mars is almost as romantic as Burroughs', but the tone differs substantially. Thanks to the exotic science of the red martians, Barsoom managed to hold its ground against the forces of planetary entropy and extinction. With care and the help of a courageous earthling or two, the planet should sustain life for thousands of years to come.

On the other hand, Bradbury's Mars was truly dying even before the people of Earth inadvertently ended the Martian civilization. It was a desert planet, and the Fourth Expedition discovered that four-fifths of its cities had already been abandoned for thousands of years.

Still, a few Martians remained, to be glimpsed occasionally in the course of the stories -- it's significant that the only Martian to have an open and extended conversation with a human in the book is a ghost.

Bradbury's Martians are ancients passing on, not the friendly and unfriendly natives of Burroughs. The colonial era was ending when Bradbury wrote, and his Martians were vestiges of an archaic era, creatures to be recorded before they died out.

Stop all the clocks
The thread of mass extinction runs strongly through The Martian Chronicles, perhaps as a result of one of the other great changes separating Bradbury from Burroughs' era. Atomic destruction lurks in the background of the stories from the day after the Fourth Expedition sets foot on Mars.

The humans bring universal death -- apocalyptic, atomic -- with them, first to the Martians and later to themselves.

Bradbury's Mars is a world in transition from the romantic fantasy of Burroughs to the more realistic Mars of later science fiction. It's not a transition Bradbury necessarily supports -- he is a poet and a romantic himself, and there is elegiac tone to his stories. The rebellious character Spender claims that "The Martians stopped where we should have, one hundred years ago," and Bradbury seems to agree.

Still, he's too honest a writer to turn the clock back on Mars.

The past is the past -- it sometimes talks to the present, but there is no returning to it. Bradbury looks forward instead, ending The Martian Chronicles with a new kind of Martian, human in form but willing to find a balance between the ways of Earth and Mars. Like Barsoom, the Mars of the Chronicles begins as a place to visit, but unlike Barsoom it ends as a lifetime home for humanity.

The human Mars
Contemporary science fiction doesn't have much room for native Martians. We know too much about Mars to people it with thoats and crystal cities.

Today's SF Mars is a human world, driven by human conflicts, and right now the definitive statement on the subject is Kim Stanley Robinson's award-winning Mars Trilogy.

Red Mars, Green Mars and Blue Mars (along with a companion volume of related short stories called The Martians) trace the adventures of the first hundred humans to colonize Mars. There are about eight major characters, each representative of a central set of ambitions for the planet.

The major division is between the "reds," who wish to preserve Mars as it is, and the "greens," who want to terraform the planet into a new Earth. However, there are also conflicts between those who want or don't want independence from Earth, and those who support several different models of a Martian government.

As a result, Robinson describes the landscape so well that Mars itself becomes a character in the book. By understanding the landscape, you understand what drives each human character, and by understanding both landscape and the people inhabiting it, you can form your own opinions about the future of Mars.

It's too early to tell yet whether Robinson's vision will seep into the public consciousness the way Burroughs' and Bradbury's have. The Mars Trilogy is a dense, difficult work, much less accessible than Burroughs' action-packed adventures or Bradbury's dreams.

The first Martians
Red Mars and its sequels are being closely read by hard SF fans and science fiction writers, but the books may not spread much further -- except to one very important demographic group.

The first settlers of Mars might well be alive today. They're going to be smart, capable people, dreamers with the incredible skills needed to win an alien frontier. Red Mars will be accessible to them because they will be living every day with the survival and colonization issues it raises.

It remains to be seen if Robinson's work will be hard science fiction to them. They may consider the books as fantastic as Barsoom, but there's still a real chance that they will carry the Mars Trilogy in them as their first planetary epic.

Fifty years from now, Red Mars might be considered a prototype of Martian literature.