Mars has been born afresh in the human mind with each exploratory thrust. Now it was happening again under the astonished gaze of Ken Edgett as he sat at his computer, tucked away in a leafy industrial park in the Pacific coast hills outside San Diego. One of Edgett's tasks, unique on Earth at the moment, was to inspect virtually every one of the thousands of images raining in at the rate of 5,000 pixels per second from the only operational spacecraft in the vicinity of the red planet—the Mars Global Surveyor (MGS). For a little while each day, in some sense, the young geologist had this hatchling Mars all to himself.
The world unfolding in front of him was not, as he put it, "your mother's Mars." It was not the Mars of the Viking missions of the 1970s or even of the last successful landing mission a few years ago. And although he had spent many of his 34 years honing his own sense of the place—as if he had scaled its Everest-dwarfing heights, slogged across its frosty dunes, felt the sting of its dust storms, and shaken its fine sand out of his boots—he confessed this Mars shocked even him. "The biggest thing to come out of the MGS images is bafflement," Edgett said the first morning of my visit. "Much of it doesn't add up. It's spine-chilling. It's…mind-boggling!"
In November 1996 when Surveyor was launched, there had been no completely successful mission to Mars in 20 years. Its goal was to assemble a global portrait of the planet over a full Martian year (about two Earth years, or 687 Earth days). A boxy, one-ton (0.9-metric-ton) craft, it carried, in addition to the camera, a suite of remote-sensing instruments that bounce laser beams off the surface to measure topography, scan for heat emissions to study atmosphere and mineral composition, and probe the planet's interior through its gravity and magnetism.
Surveyor never attracted the concentrated burst of global acclaim bestowed in 1997 on its charismatic (but scientifically much less ambitious) sister mission, NASA's Mars Pathfinder with its lander and rock-sniffing sidekick, the rover Sojourner. Slowly and steadily, however, Surveyor fomented its own quiet revolution in scientific understanding of the seemingly most Earthlike planet known.
By late 2000 the workhorse camera had churned out some 80,000 images, on average 50 times as detailed as any previously taken from orbit. Other onboard instruments had mapped the topography and gravity of the entire globe, provided a new understanding of the plant's overall shape, and discovered a puzzling pattern of magnetic strips in the planet's surface—apparent remnants of an ancient global magnetic field. The craft had operated a continuous Martian weather service, recording huge dust storms, the shadows of tornado-like dust devils, and wildly rapid swings from dusty and warm to cold and clear. (At the summit of Olympus Mons, for instance, the typical daily high is 40 degrees Fahrenheit (4.4 degrees Celsius), and the low is minus 16 degrees Fahrenheit (minus 8.9 degrees Celsius).
Some of the images were, aside from any scientific revelations, simply dazzling displays of nature's art: Byzantine dune-field sculptures, black scrawls etched by Martian dust devils. "When I first gazed at some of the images from Surveyor's camera," said Jim Garvin, NASA's chief scientist for Mars exploration, "I was moved to tears."
The quest to understand the history of water on Mars has been an overarching theme in the study of the planet. Right now that quest was perhaps nowhere more concentrated than here at Malin Space Science Systems (MSSS, or sometimes "MS Cubed"), where the Surveyor camera was designed, constructed, and tested under the guidance of Edgett's boss and mentor, geomorphologist Michael Malin.
On this particular spring morning Edgett was escorting me on a virtual flying tour over Mars's surface. Exhibit A was a disheveled-looking region known as Gorgonum Chaos. Here, captured in the MGS images, we saw part of a rugged crater wall that had collapsed into a gully with a number of deep, sinuous channels fanning out, ending abruptly in an apron of deposited material.
In shape, the features resembled gully washes in the American West. The flows appeared to come to a sudden halt, suggesting the material was thin—perhaps liquid filled with dirt and debris. Mud on Mars? But what really brought up the goose bumps was the panoramic repetition of the startling features. As the flight continued along a strip of the planet's surface, the flow patterns showed up on the cliff walls and escarpments of other craters, mesas, and troughs, always erupting near their tops, always apparently from the same geologic layer 100 to 500 meters (328 to 1,640 feet) down.
The evidence disturbed the scientist in more than one respect. First, conditions on Mars are such that any water reaching the surface supposedly would not remain liquid for very long but would boil, freeze, or poof into vapor. Second, from the absence of craters, sand dunes, or anything else on top of the gullies, they appeared to have formed very recently, possibly as recently as yesterday.
By this time the signature of weeping or seeping liquid had shown up in some 200 Surveyor images. Most of the evidence was found, strikingly, in some of the coldest places on the surface—on shadowed slopes facing the poles, in clusters scattered around latitudes higher than 30 degrees Fahrenheit (minus 1.1 degrees Celsius)—rather than at the warmer equatorial latitudes. This suggested that the flows contained frozen volatiles, substances that would vaporize if exposed to the warmth of sunlight.
Malin and Edgett had been puzzling over these images for more than a year, trying to come up with an explanation that would point to something other than liquid water before publishing their discovery. "We were dragged kicking and screaming to this conclusion," Edgett said. But they could find no plausible "dry" explanation. And proposals for other substances that might behave as liquids on the Martian surface raised so many other questions that they failed to solve the problem.
As if this mystery weren't enough, Malin and Edgett were pondering other images that told a separate and possibly more fundamental tale of ancient water on Mars. Soon after Surveyor's arrival there, the researchers noticed images of Valles Marineris (a great canyon system that would span the continental United States) which show that the planet's upper crust is dramatically layered: light, dark, light, dark. Something like what you see in Arizona and Utah at Zion, Bryce, or the Grand Canyon.
Bruce Murray of Caltech, an old friend and adviser of Malin who stopped by MSSS around Christmas 1997, happened to see the first images of the layering. "I can't believe it!" he said.
"Everything we knew from Mariner 9 and Viking led us to expect that the uppermost subsurface of Mars would resemble that of the moon," Malin said. "To see hundreds of individual layers defied the common wisdom."
As Surveyor's orbiting camera zoomed in, there were more surprises in the details. The presence of the layers and the way they are interbedded with ancient craters suggest that here is a priceless record of a Martian history previously unknown. The layers speak of the Mars that existed before the one that humans have observed for hundreds of years.
"The surface of today represents the preserved end of time," Malin said. The layering "certainly preserves a story about Mars few had ever thought of before."
Actually Malin, 50, had been fretting about this issue for decades and even predicted elements of the hypothesis in his thesis written in 1975, based on Mariner 9 data, with its thousand times poorer resolution. "Nobody bought my story," he said, "so I stopped harping on it."
Until now. "If I had to bet, I probably would say these layers represent lakes that occurred on Mars very, very early in its history. I probably would say that Mars had lakes all over its surface at one time, and that materials were being transported into these lakes." Just as with the eerie signs of seepage elsewhere, the layering evidence introduces more riddles to be solved. But, says Malin, "it all points to a Mars that was substantially more dynamic in terms of its environment, weathering, erosion, and transport than anything we see on the surface today."
Perhaps the layers in the Surveyor images represent the only record of the erosion of landscapes long gone because the processes that created them no longer operate on Mars. "Craters the size of Washington, D.C., were completely filled and then exhumed," says Edgett. "Unbelievable amounts of material were moved around in ways that just don't add up."
The human perspective on Mars has flickered over the centuries, but in some sense it has always centered on water. Percival Lowell's famous notebooks—drawn from telescope observations enhanced by a hopeful imagination—proposed in the late 1800's that vast, engineered canals, or irrigation ditches, carried water from the Martian poles to a mighty civilization concentrated nearer the equator, where the Martian climate was no less comfortable than, say, "the South of England."
A more skeptical assessment by Alfred Russel Wallace in 1907 described a much colder, perpetually frozen place as dead and forbidding as Earth's moon. Still, life-giving waters flowed compellingly through the Mars novels of Edgar Rice Burroughs, while H. G. Wells conjured a Martian race of "intellects vast and cool and unsympathetic."
Our first space-age close-up—a glimpse from Mariner 4 as it swept over a slice of ancient cratered terrain—revealed a staggeringly inhospitable world. The view brightened again with Mariner 9, the first Mars orbiter. Though it could see no features smaller than a few football fields, its images suggested a more interesting, changeable world: one where water had indeed once flowed, where the polar caps had ice in them and expanded and retreated as the seasons changed, where there was water in clouds that drifted through Martian skies.
With the ambitious Viking missions of the 1970s, scientists were emboldened to ask the big question: Is there life? Were there organics in the surface soil? At least for that place and time, the answer came back No.
Meanwhile, scientists studying life on their home planet were gaining a new perspective on the topic. Evidence was turning up all over the globe that microbial life thrived under extremes of pressure, temperature, and other conditions previously considered lethal. Certain organisms could use chemical energy in place of sunlight.
In 1996 the startling claim that signs of possible microbial life had been discovered in a Martian meteorite helped complete the link between the Earth microbes known as extremophiles (lovers of extremes) and the hunt for life on other worlds. Although most of the scientific community rejected the more dramatic interpretations by the NASA-Stanford team, some features of the rock remain mysterious, and there is little dispute that its evidence confirms the presence of certain kinds of organic molecules on or within Mars. At the same time the intriguing complexity found within the Martian meteorite has made scientists painfully aware that they might not be able to recognize extraterrestrial life, or agree about it, if they found it.
The quest to understand the history of water on Mars not only guides scientists but also serves as an organizing principle for NASA's long-range Mars exploration plan. Liquid water is essential for all known biology, and scientists note that everywhere life is found on Earth, it exists in combination with organic material, an energy source, and liquid water. If the Malin-Edgett seepages do represent water, says Edward Weiler, NASA's associate administrator for space science, "It could have profound implications for the ultimate question: Are we alone?"
But biology takes time. For three decades planetary scientists have debated a fundamental question about Mars: Did it rain? The answer is crucial, many believe, to the ease with which life could arise there, at least on the surface. Rain implies a warmer climate, where water could flow or pool for long periods, a water cycle persistent enough for life to have developed. The geologic record has suggested that Mars was warmer, wetter, and more Earth-like billions of years ago—possibly back to the very time that life was emerging on Earth. There is abundant evidence that water in huge quantities once flowed there, but then something changed on Mars, leaving its surface barren, cold, dry, and desolate.
For some 30 years the primary evidence for a wetter, warmer early Mars has been the many branching channels, known as valley networks, that some believe were carved by cascading rainwater. Because they occur on heavily cratered terrain, the assumption was that they are very ancient and the valleys carved among them are very ancient.
Other Surveyor data have been more consistent with a "no rain" scenario, suggesting negative implications for life on the surface. What if the streambeds and channels seen on Mars were created in brief, spasmodic events—such as molten volcanic flows or explosive asteroid and comet impacts—that cooked the surface and melted the ice? If so, some argue, it is unlikely that there was a climate temperate enough for rain to fall and unlikely that water was present on the surface long enough for life to have begun here. Also, Surveyor images show branching channels ending abruptly in box canyons, which indicated flooding rather than rain. "The evidence against rainfall is much greater than it was," says Arden Albee, Surveyor project scientist at the Jet Propulsion Laboratory (JPL) in Pasadena, which manages the Mars program for NASA.
One of the consistently puzzling aspects of the data so far is in the evidence of huge basins within the structure of the Martian crust. Findings from Surveyor's laser altimeter and gravity sensors, published in early 2000, revealed signs of buried channels that could been carved by floods of water flowing from the southern highlands into the northern lowlands billions of years ago. Once on the surface but since buried in sediment, these channels are as much as 125 miles (201 kilometers) wide and more than a thousand miles (1,600 kilometers) long. They hint at the existence of oceans. A depression near the north polar cap looks the way the site of an ancient ocean could look on earth.
But intensive searches for confirming details, such as shoreline, have turned up nothing conclusive. (Although geomorphologist Tim Parker of JPL has argued persistently that such details are there.) And scientists can't tell yet whether the basin's flat bottom represents sediment that settled out, which would also be a key indicator of oceans. One of the more intriguing ideas under debate is the possibility that the water flows were ice covered.
The data also have some scientists rethinking their techniques for determining the age of features on Mars (counting craters and the like), where the uncertainties of chronology are already vast. They also see problems in some cases with their practice of basing assessments of Mars geology on features similar to those they've studied on Earth.
"Now that we have the data from MGS, we know much more about Mars's surface features, and we find that few field sites on Earth are good analogues. In fact, many Mars features don't look like anything on Earth," said Dan McCleese, chief scientist of the Mars program at JPL. Even in the case of Malin and Edgett's gully-like seepages, he notes, the Mars versions are sometimes way off scale, much larger than their best counterparts on Earth. "The absence of an Earth analogue for these at the right scale," he adds, "has been a problem."
Another jarring turn in the data comes from the Thermal Emission Spectrometer, which has been searching for certain minerals that are formed over long periods in the presence of water, usually warm water. Contrary to expectations, only one such mineral, gray crystalline hematite, has been detected, and in only two places. And even this case is inconclusive, since hematite can be formed without water. Scientists plan to look again with higher resolution instruments.
But for now, it seems, the conviction that there was a wet, warm Mars a long time ago has been shaken. Where there is evidence that water once flowed, scientists don't understand what processes drove it.
Veterans of Mars exploration have felt their enthusiasm increasingly tempered of late by a renewed sense of respect, even humility, regarding the planet. There is of course the pain and shock of losing an orbiter, a lander, and two microprobes in rapid succession in late 1999 and the awareness that of the 30 Mars missions launched by the United States or the former Soviet Union to date, 21 have failed completely or partly. And of 12 attempts at landings, only four have made it.
Even more daunting is the new appreciation arising from the Surveyor data of how poorly the planet is understood and how ill-equipped scientists and engineers are to confront it.
All these factors informed NASA's drastic alteration last year in its strategic approach to Mars, which had been driving toward the goal of bringing back samples gathered from the Martian surface for study in sophisticated Earth labs, in hopes of finding evidence of past or present biology. "With the evidence we have now," JPL's McCleese said, "our problem is that we don't know where to land."
After a period of lively debate, NASA last October announced the outlines of a long-term strategy designed to be less vulnerable to failures and better able to respond to new discoveries and evolving scientific understanding of the planet. "You have to assume Mars will continue to surprise us," Edward Weiler said.
Two spacecrafts equipped to pursue signs of water on or below the Martian surface—the U.S 2001 Mars Odyssey and the European Mars Express—were already on track for launch this year and in 2003 respectively. In addition, Weiler has decided to send two robotic rovers to the planet's surface in 2003, each a much more capable "geologist" than the experimental Pathfinder model.
Beyond that, missions later in the decade will depend on what the 2001 and 2003 explorations reveal about Martian climate and geology and how the technologies advance. Tentative plans call for the 2005 launch of the Mars Reconnaissance Orbiter, a craft that could photograph surface details as small as beach balls and follow up on the mystifying hints of water seen in the Surveyor images. NASA also envisions a new generation of smart robots that could land with greater precision and explore as much as a hundred miles (160 kilometers) of terrain, avoiding hazards while doing so. While past landings have been aimed at relatively bland, safe spots, the evidence is urging the explorers toward some of the most treacherous, cliff-covered badlands on the planet.
Bringing home samples of Martian rocks and soil is still a goal, Weiler emphasized, but not until at least 2011. To make sure such a costly mission (an estimated one to two billion dollars) goes to the right place on Mars, the exploration leading up to it will focus on determining whether the ingredients required for life—liquid water, complex organic molecules, and a source of energy—ever existed together anywhere on Mars and, if so, where.
The space agency has already begun developing better technologies for the eventual return of samples: improved methods for preserving them on the trip home, for detecting life, for preventing contamination of Earth's biosphere should there be any captured microbes in the trove, and for analyzing the rocks once they are in hand.
Planetary exploration is inherently risky, but the avoidable 1999 failures convinced managers that, among other problems, the philosophy known as "faster, cheaper, better" had been pushed too far toward faster and cheaper. With little additional money in their exploration budget, program leaders plan to reduce the chance of failure in part by stretching the mission schedule over longer periods.
Through their camera, the Malin team will tell you, they live the Surveyor mission with dailiness hard to explain to the outside world. They experience the Martian seasons, watch the polar caps shrink and expand. They take their vacations once each Martian year (every two Earth years) when Mars is on the opposite side of the sun from the Earth.
And they have learned that when you have the only camera on Mars, the pressure can get intense. Malin and his beleaguered staff of ten struggle constantly to cope with the sheer volume of incoming data. When I visited MSSS, machines from the entertainment industry used to mass-produce CDs were cranking out discs like pancakes, leaving them stacked up everywhere in the lab. It took 44 CDs to contain just the first six months of Surveyor data, and scientists around the world had been clamoring impatiently for copies.
I arrived one morning to find Edgett at a computer, where he'd been since about 5:30 a.m. camera-targeting—planning out the sequence of images to be taken three days hence. Using a mouse, he clicked on some instructions on his screen and shifted the position of a small white box superimposed on the Martian surface to a spot along the spacecraft's predicted ground track for that day.
At peak data-transmission rates, the camera takes 300 pictures a day, and a mere 60 during slower periods when Mars is farthest from Earth. At full throttle, when the two planets are closest together, Edgett can spend 12 hours planning 24 hours of observations.
This is an obscure field of expertise with just the one leading practitioner, who has much of the "mission memory" stored in his brain. Despite a long list of previously identified targets, Edgett notes that many of what have turned out to be the most surprising and informative sites—such as those showing seepage—were products of serendipity. "You don't have a roomful of engineers and scientists making decisions," said Edgett. "The target list is due in six hours, and I've got to get it done. The ability to learn as we go, to select what we shoot, that's what makes for a powerful mission."
But as Malin and Edgett pondered those elusive landscapes, they admitted to a certain melancholy. "We are constantly aggravated by the fact that all the questions we have about Mars could now be answered by Ken and me if we could just walk around on the planet for a few days," Malin said.
Edgett agreed: "It's unusual to hear people like us argue for manned space exploration. But for about two years now Malin and I have been absolutely convinced that we're going to have to send people there."
Short of digging with his own trowel into the Martian sand, Malin has his heart set on dispatching a sophisticated Mars airplane, built by a consortium of Earth's best high altitude aircraft experts. He can almost see it—soaring on its ten-meter (thirty-eight-foot) wingspan on a three hour tour over the eastern region of Valles Marineris to study the layering or to Gorgonum Chaos and one other seepage site. Its camera and remote-sensing instruments would probe the planet's interior, and "if there's liquid water there now, it will glint like gangbusters at radio frequencies.
"I would give them a Mars they'd never dreamed of."