Our heroine parachutes down out of a dusky sky toward the desert planet. Her journey to this place has taken months, and yet she cannot rest now. The ancient world's surface is harsh and forbidding: howling winds whip through rocky terrain, past cliffs and craters. Water is nowhere in sight. Can our heroine locate that precious liquid before her time runs out? Can she find shelter from the bitter cold, as winter turns the air itself to snow? She is so alone, far from home. Perhaps her sister is still alive, on the other side of the world—she does not know. Wait, look above. Is that a reconnaissance satellite wheeling overhead, its telescopic cameras searching for her tracks?

Is that the back cover blurb for a new novel? Or perhaps the teaser for the latest Holly-wood SF action flick? No, this is not fiction. The story I've just told you—albeit with a bit of dramatic license, I'll admit—took place in recent months, not too far from here, and its latest chapters are still unfolding right now. Our heroine, you see, is none other than the famed interplanetary robot adventurer, Spirit, and that ancient world beneath her wheels is our crimson neighbor, the fourth rock out from the sun.

Mars! The true protagonist of our story (despite the protests of our six-wheeled heroine), Mars scarcely needs an introduction. From ancient days the namesake of the God of War, the Fire Star, Mars has captured the human imagination like no other celestial body. Schiaparelli and Lowell reported faint linear surface features, and thousands dreamed of canals wrapped around a dying desert planet. Edgar Rice Burroughs entranced the masses with tales of an exotic and vibrant world, home to princesses and sword-swinging heroes. Orson Welles infamously broadcast a radio play about a Martian invasion of Earth, and millions believed—and panicked. Today, we know there are no canals, no Martians, and yet the Red Planet's pull continues. From Philip K. Dick to Larry Niven, Kim Stanley Robinson to Greg Bear, we continue to dream about how we will someday get to Mars, what we might find there, how mankind will shape that world, and perhaps be shaped by it in return. A manned mission to Mars is at last officially one of NASA's goals, but it remains a distant one, with no target dates for a landing yet announced. The first footprints on Mars will come no earlier than 2025, or more likely 2035. By that time, though, will there be many Martian mysteries left?

Mars' secrets are today being rapidly unravelled by an astonishingly extensive fleet of robotic explorers. Not just the impressively long-lived rovers Spirit and Opportunity, but also the orbiting Mars Global Surveyor, Mars Odyssey, Mars Express, and now Mars Reconnaisance Orbiter are currently sending back reams of data on our next-door neighbor. The Mariner and Viking missions of the 1970s first revealed the truth about Mars' present state: cold, dry, dusty, dead. But today's missions are revealing its past, opening the door to interplanetary paleogeology. The hardy Mars Exploration Rovers have now lasted more than ten times longer than their originally warranteed lifetimes, returning a treasure trove of information that has revolutionized our understanding of the red planet. The full saga of the rovers and their orbiting counterparts can (and does!) fill books. In this article, we'll touch on a few of the highlights of our newly enriched understanding of Mars—what we've learned from Spirit and other recent missions, how the present conditions of Mars tell us about its ancient past, and what the prospects are for future exploration, both unmanned and manned.

The most profound difference between Earth and Mars is size. At 3396 km, Mars has barely half Earth's diameter—and is only 10% as massive. It cooled correspondingly faster after its formation, and today has much less internal heat than the Earth. Its surface is ancient, generally between 1.7 and 3.5 billion years old, and heavily cratered like our Moon. But unlike the Moon, Mars does retains a thin atmosphere: very tenuous, but still enough so that wind is a major erosive force, still enough for morning clouds, winter frost, and seasonal dust storms. Mars conspicuously lacks plate tectonics, and thus is without the mountain ranges, subduction canyons, and rings of volcanoes that characterize our own world. Vulcanism on Mars instead has taken the form of repeated eruptions in the same location over billions of years, creating a handful of supervolcanoes far larger than any terrestrial counterpart. The entire island of Hawaii, one of the largest shield volcanoes on Earth, could comfortably nestle inside the crater of mighty Olympus Mons. The four largest of these volcanoes, Olympus and its neighbors Arsia, Pavonis, and Ascraeus Mons, cluster together in the Tharsis bulge, a raised highlands on the equator some 4000 km across and 10 km high. Tharsis has clearly played a major role in shaping the entire global structure of Mars. The sheer mass of the Tharsis region cracked the crust of Mars, producing a vast network of fissures and canyons stretching over nearly a third of the planet. What caused the original uplift of Tharsis remains unknown, but cratering and lava flow patterns indicate that it happened during the very earliest periods of Martian history.

Seen from orbit, Mars displays many tantalizing clues that it may once have been much warmer and wetter than it is today. Eroded canyons appear to have been carved by water, just as on our own planet. In some places, irregular jumbles of gigantic blocks and collapsed cliffsides, called chaotic terrain, suggest catastrophic collapse of the land after vast amounts of subsurface water flowed away. Channels extending from the chaotic terrain often look very much like former riverbeds, complete with smooth and meandering banks and teardrop-shaped islands. But evidence from orbit, no matter how suggestive, can never be as definitive as that from the ground, and thus we return to our roving heroine Spirit and her ground-floor view of Gusev Crater.

Though Mars' volcanoes are now dormant, their handiwork dominates the Martian land-scape. The boulders and rock fragments that litter the planet's surface are primarily basalt, a type of rock produced when lava cools. Geologists believe that most of these rocks were scattered across the surface as debris ejected from craters. The other main surface feature is of course the omnipresent red sand. Landers on opposite sides of the planet have measured nearly identical sand compositions, suggesting that the annual dust storms have long since mixed and homogenized the sand. It is in this setting that our intrepid electronic emissaries must search for evidence of a possible watery past—amid volcanic rubble that tells no tales of water, only of fire and violence, and surrounded by sand that could have blown in from anywhere.

The twin rovers brought an impressive array of scientific tools to bear on this problem, far more sophisticated than those of the previous landers Viking and Pathfinder. In addition to sophisticated stereo cameras and a microscopic imager, each rover carries two different spectrographs for determining the composition of rocks. The Alpha-Particle X-Ray Spectrometer (APXS) bombards samples with alpha particles and X-rays (naturally!) produced from radioactive decay of a small piece of curium 244. The echo of radiation that returns indicates the elemental composition of the target, with the alpha particles providing information on light elements and the X-rays identifying heavier elements. The second spectrometer—and the most important in the search for water—is a Mossbauer spectrometer, contributed by the University of Mainz in Germany. Like the APXS, the Mossbauer spectrometer sends radiation at its target, in this case gamma rays from energized iron 57 atoms, produced by decay of radioactive cobalt 57. Unlike the APXS, the Mossbauer spectrometer literally waves its radiation source back and forth at the target on a small motorized stage. The resulting Doppler shift of the gamma rays changes the way they scatter from iron in the target. The shift is slight, but nonetheless renders the instrument exquisitely sensitive to the electronic state of iron atoms, indicating precisely how those atoms are bound up into crystalline minerals. This ability to carefully distinguish between different iron-bearing minerals has proven key to unraveling the history of water on Mars.

Gusev Crater, the landing site for Spirit, was chosen because it lies along the apparent path of an ancient flood channel. Scientists hoped to find on the surface confirmation that water had indeed once flowed across the crater floor. But the triumph of a successful landing soon gave way to scientific disappointment. As Spirit rolled across the landscape, measuring rocks with its spectrometers, scanning its cameras, peering into craters, a consensus picture slowly emerged: Gusev Crater was a rock-strewn desert, with no signs of water ever having been present. For five long months, every single rock the rover met was plain old volcanic basalt, the product of lava flows that had poured across the crater and buried any evidence of water forever out of sight beneath tons of solid stone. On the far side of Mars, its sister rover Opportunity stole the spotlight, with reports of water-formed sandstone, acid-etched boulders, and strange round pebbles that could only have formed in a wet environment. Compared with those successes, for a hundred and sixty days, Spirit's mission looked like a failure.

And then Spirit reached the Columbia Hills, and instantly everything changed forever. These hills, named for the seven astronauts who lost their lives in 2003 shortly before the rover's landing, rise hundreds of feet above the surrounding plains. Scientists had long recognized their potential: high on the hills, unburied by any lava, was the last remaining chance to find signs of water in Gusev Crater. Controllers raced furiously to get Spirit to the hills, well aware that she was aging, already nearly twice her design lifetime (little did they know how durable the rovers would prove to be, or how lucky gusts of wind would periodically clean off the solar panels and prolong the mission!) Once there, though, it was almost too easy: the very first rock measured on the very first hillside showed strong signs of being altered by water, and nearly every rock examined since has done so as well.

Over the next two years, scientists identified several distinct classes of rocks in Gusev Crater, with each class named after a particularly exemplary rock. Adirondack-class rocks are the pure basalts, the rocks of the plains. The iron in these rocks is primarily in the form of the minerals olivine and pyroxene, some 85% by weight. Magnetite is also present, indicating that these rocks crystallized from lava under slightly oxidizing conditions. The Clovis class rocks, in contrast, first encountered on the Columbia Hills, are primarily composed of goethite and hematite, two minerals which indicate the former presence of water. Goethite in fact contains water, in the form of embedded hydroxyl groups, and can only be created in an aqueous environment. Clovis rocks are also notably enriched in chlorine and bromine, the signature of evaporated salt water. Overall, the Clovis rocks appear to have once been regular basaltic lava, but were long ago heavily altered by the pervasive presence of water. The Watchtower and Wishstone classes show an intermediate level of modification by water, along with strangely high levels of titanium and phosphorous. They tell a different story: of rocks partially altered in a damp environment, then violently melted and resolidified during tremendous impacts. Similar rocks have been found on Earth near Manicouagan Crater in Canada. Lastly, and most rarely, two sandstone outcrops have been found, comprising the Peace class. Sand of volcanic origin was cemented together by sulfate salts. The level of mineralogical alteration in the sandstone is low, however, suggesting that these rocks were only briefly exposed to water.

Together, the Clovis, Wishstone and Peace rocks paint a picture of a young Mars very different from the geologically quiescent, cold and dry world we see today. The young Mars was a violent place, as volcanic eruptions and meteor impacts churned up the landscape. Water was commonly present, sometimes for brief periods, sometimes longer. The salty and acidic water left a handiwork of pervasive mineralogical changes in the volcanic rocks it encountered. Later volcanic eruptions buried most of the altered rocks under new basaltic lava plains, but the Columbia Hills patiently preserved a small portion of the geologic record for three billion years, waiting for our intrepid robot emissary.

Today, nine hundred and seventy days into a ninety day mission, both rovers are showing signs of old age. The right front wheel on both rovers is failing: Opportunity's can't steer, while Spirit's can no longer turn at all. The rovers can still move using their five good wheels, though controllers say they're now a bit lame and have to avoid certain kinds of terrain. The shoulder joint of Opportunity's robot arm is wearing out as wires start to fray, and the grinding teeth on both rovers' rock abrasion tools are long since worn down to uselessness. For months, Spirit has been parked in one spot, carefully chosen to maximize solar energy through the harsh Martian winter. But seen in another light, both rovers are going stronger than ever: recent software upgrades have given them greater autonomy and new capabilities. The rover team leader, Cornell professor Steve Squyres, recently joked that the rovers may be getting older and creakier, but they're also getting wiser as they age, just like people. Martian spring is just beginning, and solar energy is on the rise. Soon Spirit's wheels will begin to turn once again. Soon Opportunity will peer into a crater the size of a football stadium, its destination for more than a year of driving. Stay tuned.

Screaming in at high speed from darkest interplanetary space, the surveillance satellite rushes toward the red world. As it plunges into the tenuous outer reaches of the atmosphere, it spreads its wings, dissipating speed against the furious plasma wind. Soon it will unfurl its radar booms, soon it will train its telescopes on the ground below. The Red World's secrets will not remain hidden for long.

The Rovers are of course not our only eyes on Mars. In fact, despite their relatively media-friendly good looks and the excitement of first-hand science on the ground, the rovers are actually fairly limited. Even with their unexpected endurance, each has traveled only a few kilometers across the surface. For the big picture of overall global structure, we have to turn to the orbiters. The sketched Martian maps of Schiaparelli have long since given way to detailed topographic and mineralogical studies spanning the entire planet.

Mars' rotation defines an equator, just as on Earth. Without any convenient Greenwich Observatory on Mars, 19th-century astronomers picked an arbitrary prime meridian (though later astronomers noticed a tiny crater in the right spot, and declared that crater to be the official zero point for longitude). The zero point for altitude is even harder, since there is no sea level to use as a reference. Martian zero altitude was defined to be that at which atmospheric pressure is 6.1 millibar (about 0.6% that of sea-level pressure on Earth). Mars Global Surveyor, the first of the modern generation of Mars explorers, has been orbiting the red planet for more than a decade now. One of its instruments, a laser altimeter, has provided a detailed topographic map of the planet, relative to the above-defined zero altitude. By bouncing a laser off the planet's surface and measuring the delay of the return pulse, this instrument can measure the altitude of any given point with a vertical precision of ten to twenty feet, comparable to the accuracy of GPS on Earth.

One of the most striking features about Mars is the dichotomy between its two hemispheres: the southern hemisphere is high, rocky, and ancient, pitted by craters, while the northern hemisphere is relatively smooth, less cratered, and much lower than the south. Even a small backyard telescope can show the sharp difference between the relatively dark southern highlands and bright northern plains, and orbital mapping by Mars Global Surveyor and others shows the division in stark contrast. The entire southern hemisphere, plus a small adjacent portion of the northern hemisphere, lies several kilometers higher than the northern plains. The boundary between the uplands and lowlands is frequently terraced, much like dry fossil shorelines seen in the American southwest. In some places, heavily eroded canyons suggest tremendous floods once poured northward onto the plains. With all the clues for a wet young Mars, it is very tempting to imagine the northern lowlands are the now-dry floor of a vanished ocean.

But there's just one problem with that scenario: if you trace the putative ancient coastline around the whole northern hemisphere, it's nowhere near flat. The altitude of the fossil coastline varies by hundreds of meters, far larger than the measurement accuracy of our maps. In other words, Martian sea level doesn't look even remotely level. Water always flows downhill to a level surface (an equipotential) on Mars just as on Earth, so how can we explain the distortion in the supposed coastline?

One promising answer is that the coastline used to be level, even though it isn't now. Without plate tectonics to raise and lower continents, though, some other force must have caused the distortion after the water went away. Recently, a team of researchers from Berkeley, Toronto, and Washington has proposed an elegant solution in the form of polar wander. The rotation axis of a planet is not fixed; the Earth's pole wanders a few meters each year, which isn't a very big deal, except for the staffers at the South Pole's Amundsen-Scott research station who have to hammer in a new marker every January 1 (true!). But over time, a similar effect may have moved the Martian pole many thousands of kilometers.

A spinning planet is a carefully balanced thing. Centrifugal force causes the equator to bulge outwards very slightly (for the Earth, the difference between polar and equatorial radii is about 20 km; for Mars, it's even less). Any major distortion to a planet, such as an unbalanced mass in one region, will cause the rotation axis of the planet to drift in such a way that the extra mass moves toward the equator. (The details of this process unfortunately require more tensor algebra than the editorial staff is likely to let me get away with in this column, but see the references in the sidebar if you want the mathematical whole nine yards!) As the planet tilts to bring the extra mass toward the equator, it changes its orientation with respect to the centrifugal force. The new equator bulges out and the former equator relaxes inward, warping the entire planet slightly in the process. Mars does indeed have an unbalanced mass located on the equator, in the form of the tremendous Tharsis rise—but Tharsis is old, far older than the ancient shoreline, so it cannot be the cause of the polar wander. However, missing mass can unbalance a planet just as easily as extra mass can:

Imagine a young Mars tilted 50 degrees on its side from what we see today. Its north pole is located in what will someday be known as Acidalia Planitia, 2900 km from the modern pole's location. A shallow ocean covers, not its northern hemisphere, but its eastern. Over time—hundreds of millions of years, perhaps— the ocean slowly disappears. Without the weight of those several quadrillion tons of water, Mars becomes unbalanced. Ever so slowly, the whole planet pivots. The pole shifts. The equatorial bulge shifts along with it, and parts of the now-dry shoreline of the receding ocean are lifted or lowered hundreds of meters. A once-level shoreline, level no longer: mystery solved.

A giant robot moves silently through the rugged canyonlands. It is nuclear-powered, invulnerable to the planet's harshness. The robot slowly rotates its head, scanning the landscape for targets. There! Its laser flashes once, twice, and the target is vaporized. With a parting glance at the dissipating debris, the robot trundles on its way.

I can hear my readers complaining now: "Oh, come on! A nuclear-powered robot armed with lasers? Surely that must be fiction." But no, we really are sending such a beast to Mars in the very near future. The 2009 Mars Science Laboratory will be far larger and more capable than our present rovers, and yes, it will feature a laser capable of blasting bits off of rocks and reading their composition from the resulting vapor cloud. In addition to the laser, it will carry within itself the most advanced chemistry set ever sent to another world, in order to detect and study any organic chemicals present in the martian environment. (Note that the word "organic" here is used in its chemical sense, meaning carbon-based molecules, and not necessarily to mean chemicals of biological origin—though the quest to find evidence of life is of course one of the major goals of the mission! Also notably, that list of goals no longer includes "observe the seasonally changing conditions on the Martian surface for at least one complete Martian year," since that particular goal has already been achieved far ahead of schedule, by those upstart rovers Spirit and Opportunity.)

It's a cliched truth about science that every question answered opens up at least two new ones. We now know without a doubt that there was once water on Mars. It seems increasingly probable that oceans once filled the lowlands. But where did all that water go? Was it all lost to space as hydrogen escaped from Mars' feeble gravity? Much of it seems buried underground in the form of permafrost, as indicated by hydrogen traces picked up by orbiting ground-penetrating radars. The Mars Reconnaisance Orbiter has just arrived at Mars with a newly improved radar system, scheduled to start operating in November, and the most advanced telescopic camera ever sent to another world. The hunt for the missing water continues.

We know for a fact that some water ice is present today at Mars' poles. Not just in the polar caps themselves, but also in permafrost extending far from the polar regions into temperate latitudes, and elsewhere. Last year, the European Mars Express craft returned spectacular images of a northern crater partially filled with an ice lake some 10 km across. Anyone up for ice skating in 1/3 gravity? The poles remain one of the most interesting, yet least explored, parts of Mars, particularly given the failure of 2001's Mars Polar Lander. The upcoming 2007 Mars Phoenix mission has recycled extra components from a previous cancelled mission into a low-cost lander targeted at Mars's northern pole. With a powerful robot arm, Phoenix will be able to dig down into the permafrost layer and directly study its nature.

Further out, the path becomes murkier. The first European rover is planned for 2011; another NASA one may follow in 2014. Other, wilder proposed missions include robot gliders that would wing their way down the spectacular Valles Marineris canyon system and a torpedo-shaped robot that would melt its way deep into the polar cap. Sometime around 2020, a rocket might lift off from the surface of Mars, carrying back to earth a precious collection of rocks gathered by rovers. And perhaps a decade after that, maybe, just maybe, the rover tracks will be joined by the first of many footprints.