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Exploring Mars is a bit like doing brain surgery through a mile-long soda straw. At an average distance of fifty million miles from Earth, with a one-way radio message time of twelve to twenty minutes, roving the dry, treacherous surface requires the utmost in planning and careful execution. One false move can end a mission in seconds, and there are rarely many options to correct a mistake. That is why the people who dare seek the truth about Mars are so remarkable. This is the story of human striving, from early times through tomorrow, to discover what makes Mars tick.

Orbiting in the dark cold of space at an average of about 140 million miles from the sun, or about half again as far as Earth, Mars is the fourth planet in the solar system. It is also the last stop for the rocky, or terrestrial, worlds before the gas giants Jupiter and Saturn and the icy balls of Uranus and Neptune. It is separated from these giant worlds by the asteroid belt, a planet which failed to form from the large disk of material that still orbits the sun beyond Mars.

The air on Mars is thin and cold; the highest temperatures hover at about 60°F, and can plummet to -180°F at the poles. Its day lasts about 24 hours, 37 minutes, and its axial tilt matches Earth's at 24 degrees. Its year lasts 686 Earth days. Mars is about half the diameter of Earth, less than a quarter its size, and has only 11 percent of Earth's mass and 38 percent of its gravity. Despite this, it has almost the same dry surface area, due to a lack of seas and oceans; in fact, bodies of liquid water do not exist on its surface. It is a bone-dry, ultracold place with only about 1 percent of Earth's atmosphere.

Why then, one might ponder, are we so fascinated with this seemingly inconsequential world? Because Mars is a planet of dreams. Always, it has inspired feelings in humanity as no other planet in our solar system. Early on, it was the reddish hue, resembling the color of aging blood, that attracted the naked eye. Later, in wavering telescopic images transmitted across the tens of millions of miles from its surface, it was the odd markings and, still later, imagined lines crossing its surface that inspired. One could imagine life there. One could imagine…empires.

And why not? The planet is not that far from Earth, and must then be not so unlike our own world, or so the thinking went. If Venus, one step closer to the molten sun, might be covered with tropical oceans and riotous growths of green, steaming jungles under its impenetrable cloud cover, why couldn't Mars, still within the so-called Goldilocks Zone, harbor an older, wiser, more advanced civilization?1

But of course, those dreams vanished along the way. Venus turned out to be an unbelievably hot hellhole with over nine hundred pounds per square inch of pressure crushing its deadly surface. But the real Mars, as we know it today, is not so much less interesting than the one of previous generations. The empires of Edgar Rice Burroughs's green men and eight-legged Thoats (dinosaurlike steeds) might be gone, but in its place is an old, highly weathered, and geologically fascinating place with signs of vast and ancient floods of liquid water, and more recent indications of smaller flows.

The planet seems in many ways to be an older version of our own. But it is an Earth with planetary evolution gone awry. Once rich with oceans and cloud cover, it is too small to any longer harbor liquid water on its surface for more than a few moments. Its thin atmosphere is almost entirely carbon dioxide, with bits of nitrogen, argon, and oxygen existing in wisps. Most of the life-sustaining oxygen that we prize so highly has been long spent, slowly turning the iron in the soil a ruddy, oxidized red. And that thin atmosphere has also allowed for billions of rocks, most of which would burn up or explode in Earth's denser atmosphere, to slam into Mars's surface with impunity. Many millions of these were large enough to leave wounds on the planet, and some created vast new surface features.

Another seeming indicator of a dead world is Mars's lack of a meaningful magnetic field. This is probably due to a largely inert core, or a cooling one. Whatever the case, there is not the same molten, metallic dynamo that creates Earth's robust lines of magnetic force, and what magnetism Mars does possess is lumpy and erratic. The gravitational field is also unlike Earth's, with mascons (mass concentrations), not unlike those within Earth's moon.2 Whatever the case, Mars has, proportionally, a much thicker crust and a smaller, less active molten core than our planet.

Still, Mars has possessed life, though not the kind we seem to wish upon it. It was geologically alive, with vast lava flows and wind and water erosion working their healing magic upon its tortured and pockmarked surface. While much of the southern half of the planet still bears the scars of bombardment, the northern half is largely covered with younger lava flows that filled in the offending craters. And the source of much of this once-molten rock can be clearly seen with even low-resolution cameras from the myriad probes that have flown past the planet, in the form of the Tharsis Bulge and its huge volcanoes. This region is so remarkably swollen that it noticeably deforms the otherwise spherical planet. And it is home to some of the most impressive mountainous real estate in the solar system.

What follows is a brief primer of Martian geography. Entire sets of texts are available on the subject; what is presented here is merely the briefest of samplings. The intention is to present a general idea of the important regions of the planet in both historical and scientific terms.

First among the huge volcanoes identified within the Tharsis region is Olympus Mons, the largest known mountain anywhere, which flanks the bulge. Three times the height of Mount Everest, the now-extinct volcano soars fourteen miles into the thin Martian air. It is a shield volcano, resembling those that comprise the islands of Hawaii, with a base width of almost four hundred miles. The area it rests upon is roughly the size of Arizona. It is also the youngest of the major volcanic structures on Mars.

Directly atop the bulge and spanning its crest diagonally are three older volcanoes, all shield volcanoes, Arsia Mons, Pavonis Mons, and Ascraeus Mons. While subordinate in size to their larger sibling, these lava factories contributed greatly to the basaltic flows that inundated much of the surrounding area. Overall, the Tharsis region is the size of a terrestrial continent.

The formation of this region was not without its side effects, and a gigantic wound in the planet can be found nearby, stretching east from the Tharsis area and continuing along the Martian equator for about a quarter of the planet's circumference. This gigantic gash in Mars's hide is called Valles Marineris. In keeping with the Texas-style “bigger is better” nature of Martian topography, it is the largest valley in the solar system. Our own Grand Canyon would be scarcely noticeable alongside it. Almost 2,500 miles long, it was formed when the Tharsis region rose out of the planet, and the nearby crust could not take the stresses of this enormous violation. So it cracked and slumped, resulting in the huge channel. It averages 125 miles in width and is as much as 4.5 miles deep. It is outclassed only by the underwater Mid-Atlantic Ridge on Earth.

To the north rests Alba Mons, also known as Alba Patera, the oddest of the volcanoes and unlike anything else on Mars (or Earth, for that matter). It has the gentlest slope of any Martian volcano, with an inclination of just one-half a degree, or about a tenth of that of Olympus Mons. Its volcanic outflow forms an ellipse almost 2,000 miles across and 1,200 miles north to south, making it (here, again, the Texas-style attributes) among the largest known magma generators in the solar system. There are many theories seeking to explain its productivity, including the existence of highly fluid magma that flowed freely and fluidly out of the caldera and across the surface of Mars. On Earth, the total outflow of the volcano would have covered most of twelve states if centered in Colorado. It's a big beast.

Other volcanic regions include an area thousands of miles west of Tharsis called Elysium. Here we find three main volcanoes: Elysium Mons, Hecates Tholus, and Albor Tholis. Finally, down toward the equator is the region of Syrtis Major Planum with its own volcano of the same name. About 750 miles wide, but only one mile in elevation, this vast, low-lying monster produced lavas that seem to be different than that from the Tharsis volcanoes; it is more complex and differentiated in geological terms and is thought to have formed in the vast three-mile-deep magma chamber below when heavier elements settled out, leaving the lighter lavas to spew forth.

In any case, by the time Mariner 9 had begun sending back images of the Tharsis volcanic complex, any thoughts that Mars had been dull or uninteresting in its youth were banished. While the planet may have slowed down in its old age, in earlier eras it was a geologically active toddler, with regular volcanic tantrums to match.

Pulling farther back, we can see that almost half the northern hemisphere is covered by the Borealis Basin. Its origins are uncertain, but it was likely the result of a huge, planet-shifting impact. What is apparent is that there are far fewer craters in this area, and the Tharsis Bulge was formed subsequent to the events that spawned Borealis. If it is an impact feature, it would again be a record setter as the largest in the solar system. And the object that impacted Mars would have been about the size of Pluto, and probably arrived during the Late Heavy Bombardment period of about four billion years ago.

In the southern hemisphere can be found Hellas Planitia, another huge impact basin, about 1,430 miles wide with a depth of about 30,000 feet. It is so deep that the atmospheric pressure at the bottom is about 90 percent more than at the surface, enough to allow liquid water to exist for brief periods. While much smaller than the planet-girdling Borealis Basin, it is the largest obvious impact feature clearly visible on Mars. Like its northern cousin, it is thought to be about four billion years old.

Drawing a line from Hellas Planitia through Mars's interior to the other side of the planet, we return to Alba Patera, home of the Tharsis volcanoes. It is hypothesized that the impact at Hellas was sufficient to cause at least part of the formations on the antipodal, or opposite, side of the planet, as the seismic shock rattled through, slamming into the far side.

Much of the southern hemisphere, ranging a bit into the north, is extensively cratered, another result of the Late Heavy Bombardment, when copious amounts of interplanetary junk smashed into the rocky, or terrestrial, planets.3 Overall, the southern regions sit much higher than the northern hemisphere, and the crust of the planet is over twice as thick in the south.

The geological record of Mars can be summarized in three eras:

Maps of Mars list two general sets of features. The first are those differentiated by apparent brightness, called albedo features. Albedo is the amount of sunlight reflected back from another world. On maps, these have Latin names. One such example is the enormous Sinus Meridiani, or Meridian Bay, one of the few major features visible through a telescope from Earth. It is noteworthy that the darker of these features were originally thought to be seas or other bodies of water, and were named accordingly. Hence, some of the major features in this group are Mare Erythraeum (Erythraean Sea), Mare Sirenum (Sea of Sirens), and Aurorae Sinus (Bay of the Dawn). The largest dark feature seen from Earth is Syrtis Major Planum, a classical Latin name for a region near present-day Libya. Areas thought at the time to be dry land include Arabia Terra (Land of Arabia) and Amazonis Planatia (Amazonian Plain). The north polar cap is Planum Boreum (Nothern Plain), and the southern cap is Planum Australe (Southern Plain).

Now it is time to discuss dirt. Earth has dirt, also called earth. But on Mars, and other solid bodies like the moon, one cannot properly refer to the soil as dirt. The proper term is regolith, from the Greek rhegos or blanket, and lith or rock. It denotes a layer of loose material over bedrock, essentially ground-up rocks. Spacecraft have studied or observed the regolith of our own moon, Titan (moon of Saturn), Venus, and Mars. In this book, however, we will generally refer to regolith as soil for the sake of expediency.

Martian soil is highly alkaline, and apparently filled with perchlorate. It is highly toxic stuff, at least so far as lower forms of life are concerned. But the areas of Mars sampled as yet are small, and orbital data inconclusive, so the true nature of planetwide soil is yet to be decisively determined.

Most of the planet is also overlain by a thin layer of oxidized dust, which gives Mars its red color. This dust is very finely grained and, when winds whip up, can stay suspended in the atmosphere for weeks or even months.

The planet has two icy polar caps, some of the first features to be observed through early telescopes. With a tilt of about 25 degrees, these polar areas experience growth in the local winter and shrinkage in the local summer. This led many eighteenth-century astronomers, most notably Percival Lowell, to conclude that poles were water ice and melted in the summers, sending life-giving water cascading to the equatorial regions of the planet and contributing to the “wave of darkening” that seemed to occur every Martian year, thought then to be plant life gone wild as the moisture from the poles nourished it. We now know that the poles are a mix of a thin layer carbon dioxide ice, or dry ice, and water ice. The CO2 freezes groundward in the local summer and then is re-released into Martian skies in the summer. This seasonal exchange can account for over a quarter of the atmosphere freezing out, then being released back into the air. The water ice below also melts in the summer; evidence of flowing meltwater, once thought to be very unlikely, is seen in orbital images of the planet.

In fact, the observation of subterranean and frozen water is becoming relatively common on Mars. At one time thought to be dry, arid, and dead, Mars is turning out to be quite active in terms of weather and erosional processes. Wind is of course the primary agent of erosion today, but water is slowly, stubbornly revealing itself more and more.4 The southern ice cap alone, the smaller of the two, is thought to have enough water tied up in its frozen reservoir to cover the entire planet to a depth of over thirty feet—but not for long. The atmosphere is so thin, at less than 1 percent of Earth's, that liquid water quickly boils away in the tenuous air. But scratch the surface, literally, and there is plenty of water ice all over the planet.

That water was far more evident in the past than it is today, slumbering beneath the dust, dirt, and sand of the present Mars. Huge features called outflow channels, about twenty-five of them, litter the surface of Mars, indicating a truly massive and disastrous outpouring of water at some time within the last few million years—and smaller ones are still being formed to this day, via liquid water. There are also areas strongly resembling river deltas, alluvial fans, and channels that speak to a once-watery Mars.

So what's the big deal about water? Simply this: life as we understand it needs water to exist. Increasingly, extreme forms of life on Earth are discovered that can survive on trace amounts and in hideously difficult environments, but all forms need some water. So when we find water in any form on Mars it is exciting, for it allows us to think, once again, of Martians…microscopic though they might be.

And why are microbes on Mars so important? It's not like they will be the next explosive market for Big Macs® or directly impacting our daily lives if discovered. But the discovery of some form of life on Mars would be a game changer in other ways. Philosophy, religion, and of course science would all be rocked by such a discovery. The existence of life on Mars would, for some, seem to diminish our special place in the universe as humans. Of course, if this life were something more akin to streptococci than your next door neighbor, it's hard to get too intimidated. But for some, any discovery of life elsewhere would be a threat.

For others, it would be a delight. The idea that life, that poorly understood miracle of amino acids and organic compounds, could have sprung up independently on another world is a big one. Some scientists have hypothesized that life may not have even begun here on Earth, that is may have started first on Mars and then hitchhiked a ride to Earth via a meteoric fragment. This idea has gained credibility in the last few decades, when meteors found on Earth (in Antarctica) were definitively traced back to Martian origin. And, since Mars calmed down from its evolutionary throes much sooner than Earth, this idea makes some sense. Life could have slowly evolved there, then come to Earth and survived once our own planet was less volatile. It would certainly give the organisms more time to mature in evolutionary terms.5

Of course, there are others who consider this to be hogwash, and still others who consider these ideas blasphemous. The former group will likely come around given sufficient evidence. The latter will never be convinced, regardless of the science; the idea is simply too threatening.

The important thing is this: until we go to Mars, sift through the soils, test its properties, and search cracks, crevices, caverns, glaciers, the polar caps, the equatorial regions, and everything in between for microbial (or other) life, we will probably not know the answer. And it may well take the hand of humanity present on the surface of that cold, ruddy world to make this work—robots are simply too limited and too inflexible.

It's almost time for us to visit the red planet. But first, let's look back a few millennia, to a time when Mars was not a destination, not yet even a plane, but a harbinger of death and destruction in the night sky.