Hundreds of sinuous channels and valley networks dating back several billion years can be found, mainly in the cratered southern highlands. They suggest a previous epoch of more benign and Earthlike conditions—very different from what we find beneath the tenuous and frigid atmosphere of our time. Some ancient channels seem to have been carved by rainfall, some by underground sapping and collapse, and some by great floods that gushed up out of the ground. Rivers were pouring into and filling great thousand-kilometer-diameter impact basins that today are dry as dust. Waterfalls dwarfing any on Earth today cascaded into the lakes of ancient Mars. Vast oceans, hundreds of meters, perhaps even a kilometer, deep may have gently lapped shorelines barely discernible today. That would have been a world to explore. We are four billion years late.1
On Earth in just the same period, the first microorganisms arose and evolved. Life on Earth is intimately connected, for the most basic chemical reasons, with liquid water. We humans are ourselves made of some three-quarters water. The same sorts of organic molecules that fell out of the sky and were generated in the air and seas of ancient Earth, should also have accumulated on ancient Mars. Is it plausible that life quickly came to be in the waters of early Earth, but was somehow restrained and inhibited in the waters of early Mars? Or might the Martian seas have been filled with life—floating, spawning, evolving? What strange beasts once swum there?
Whatever the dramas of those distant times, it all started to go wrong around 3.8 billion years ago. We can see that the erosion of ancient craters dramatically began to slow about then. As the atmosphere thinned, as the rivers flowed no more, as the oceans began to dry, as the temperatures plummeted, life would have retreated to the few remaining congenial habitats, perhaps huddling at the bottom of ice-covered lakes, until it too vanished and the dead bodies and fossil remains of exotic organisms—built, it might be, on principles very different from life on Earth—were deep-frozen, awaiting the explorers who might in some distant future arrive on Mars.
METEORITES ARE FRAGMENTS OF OTHER WORLDS recovered on Earth. Most originate in collisions among the numerous asteroids that orbit the Sun between the orbits of Mars and Jupiter. But a few are generated when a large meteorite impacts a planet or asteroid at high speed, gouges out a crater, and propels the excavated surface material into space. A very small fraction of the ejected rocks, millions of years later, may intercept another world.
In the wastelands of Antarctica, the ice is here and there dotted with meteorites, preserved by the low temperatures and until recently undisturbed by humans. A few of them, called SNC (pronounced "snick") meteorites2 have an aspect about them that at first seemed almost unbelievable: Deep inside their mineral and glassy structures, locked away from the contaminating influence of the Earth's atmosphere, a little gas is trapped. When the gas is analyzed, it turns out to have exactly the same chemical composition and isotopic ratios as the air on Mars. We know about Martian air not just from spectroscopic inference but from direct measurement on the Martian surface by the Viking landers. To the surprise of nearly everyone, the SNC meteorites come from Mars.
Originally, they were rocks that had melted and refrozen. Radioactive dating of all the SNC meteorites shows their parent rocks condensed out of lava between 180 million and 1.3 billion years ago. Then they were driven off the planet by collisions from space. From how long they've been exposed to cosmic rays on their interplanetary journeys between Mars and Earth, we can tell how old they are—how long ago they were ejected from Mars. In this sense, they are between 10 million and 700,000 years old. They sample the most recent 0.l percent of Martian history.
Some of the minerals they contain show clear evidence of having once been in water, warm liquid water. These hydro-thermal minerals reveal that somehow, probably all over Mars, there was recent liquid water. Perhaps it came about when the interior heat melted underground ice. But however it happened, it's natural to wonder if life is not entirely extinct, if somehow it's managed to hang on into our time in transient underground lakes, or even in thin films of water wetting subsurface grains.
The geochemists Everett Gibson and Hal Karlsson of NASA's Johnson Space Flight Center have extracted a single drop of water from one of the SNC meteorites. The isotopic ratios of the oxygen and hydrogen atoms that it contains are literally unearthly. I look on this water from another world as an encouragement for future explorers and settlers.
Imagine what we might find if a large number of samples, including never melted soil and rocks, were returned to Earth from Martian locales selected for their scientific interest. We are very close to being able to accomplish this with small roving robot vehicles.
The transportation of subsurface material from world to world raises a tantalizing question: Four billion years ago there were two neighboring planets, both warm, both wet. Impacts from space, in the final stages of the accretion of these planets, were occurring at a much higher rate than today. Samples from each world were being flung out into space. We are sure there was life on at least one of them in this period. We know that a fraction of the ejected debris stays cool throughout the processes of impact, ejection, and interception by another world. So could some of the early organisms on Earth have been safely transplanted to Mars four billion years ago, initiating life on that planet' Or, even more speculative, could life on Earth have arisen by such a transfer from Mars? Might the two planets have regularly exchanged life-forms for hundreds of millions of years? The notion might be testable. If we were to discover life on Mars and found it very similar to life on Earth—and if, as well, e were sure it wasn't microbial contamination that we ourselves had introduced in the course of our explorations—the proposition that life was long ago transferred across interplanetary space would have to be taken seriously.
IT WAS ONCE THOUGHT that life is abundant on Mars. Even the dour and skeptical astronomer Simon Newcomb (in his Astronomy for Everybody, which went through many editions in the early decades of this century and was the astronomy text of my childhood) concluded, "There appears to be life on the planet Mars. A few years ago this statement was commonly regarded as fantastic. Now it is commonly accepted." Not "intelligent human life," he was quick to add, but green plants. However, we have now been to Mars and looked for plants—as well as animals, microbes, and intelligent beings. Even if the other forms were absent, we might have imagined, as in Earth's deserts today, and as on Earth for almost all its history, abundant microbial life.
The "life detection" experiments on Viking were designed to detect only a certain subset of conceivable biologies; they were biased to find the kind of life about which we know. It would have been foolish to send instruments that could not even detect life on Earth. They were exquisitely sensitive, able to find microbes in the most unpromising, arid deserts and wastelands on Earth.
One experiment measured the gases exchanged between Martian soil and the Martian atmosphere in the presence of organic matter from Earth. A second brought a wide variety organic foodstuffs marked by a radioactive tracer to see if there were bugs in the Martian soil who ate the food and oxidized it to radioactive carbon dioxide. A third experiment introduced radioactive carbon dioxide (and carbon monoxide) to the Martian soil to see if any of it was taken up by Martian microbes. To the initial astonishment of, I think, all the scientists involved, each of the three experiments gave what at first seemed to be positive results. Gases were exchanged; organic matter was oxidized; carbon dioxide was incorporated into the soil.
But there are reasons for caution. These provocative results are not generally thought to be good evidence for life on Mars: The putative metabolic processes of Martian microbes occurred under a very wide range of conditions inside the Viking landers—wet (with liquid water brought from Earth) and dry, light and dark, cold (only a little above freezing) to hot (almost the normal boiling point of water). Many microbiologists deem it unlikely that Martian microbes would be so capable under such varied conditions. Another strong inducement to skepticism is that a fou
rth experiment, to look for organic chemicals in the Martian soil, gave uniformly negative results despite its sensitivity. We expect life on Mars, like life on Earth, to be organized around carbon-based molecules. To find no such molecules at all was daunting for optimists among the exobiologists.
The apparently positive results of the life detection experiments is now generally attributed to chemicals that oxidize the soil, deriving ultimately from ultraviolet sunlight (as discussed in the previous chapter). There is still a handful of Viking scientists who wonder if there might be extremely tough and competent organisms very thinly spread over the Martian soil—so their organic chemistry could not be detected, but their metabolic processes could. Such scientists do not deny that ultraviolet-generated oxidants are present in the Martian soil, but stress that no thorough explanation of the liking life detection results from oxidants alone has been forthcoming. Tentative claims have been made of organic matter in SNC meteorites, but they seem instead to be contaminants that have entered the meteorite after its arrival on our world. So far, there are no claims of Martian microbes in these rocks from the sky.
Perhaps because it seems to pander to public interest, NASA and most Viking scientists have been very chary about pursuing the biological hypothesis. Even now, much more could be done in going over the old data, in looking with Viking-type instruments at Antarctic and other soils that have few microbes in them, in laboratory simulation of the role of oxidants in the Martian soil, and in designing experiments to elucidate these matters—not excluding further searches for life—with future Mars landers.
If indeed no unambiguous signatures of life were determined by a variety of sensitive experiments at two sites 5,000 kilometers apart on a planet marked by global wind transport of fine particles, this is at least suggestive that Mars may be, today at least, a lifeless planet. But if Mars is lifeless, we have two planets, of virtually identical age and early conditions, evolving next door to one another in the same polar system: Life evolves and proliferates on one, but not the other. Why?
Perhaps the chemical or fossil remains of early Martian life can still be found—subsurface, safely protected from the ultraviolet radiation and its oxidation products that today fry the surface. Perhaps in a rock face exposed by a landslide, or in the banks of an ancient river valley or dry lake bed, or in the polar, laminated terrain, key evidence for life on another planet is waiting.
Despite its absence on the surface of Mars, the planet's two moons, Phobos and Deimos, seem to be rich in complex organic matter dating back to the early history of the Solar System. The Soviet Phobos 2 spacecraft found evidence of water vapor being out-gassed from Phobos, as if it has an icy interior heated by radioactivity. The moons of Mars may have long ago been captured from somewhere in the outer Solar System; conceivably, they are among the nearest available examples of unaltered stuff from the earliest days of the Solar System. Phobos and Deimos are very small, each roughly 10 kilometers across; the gravity they exert is nearly negligible. So it's comparatively easy to rendezvous with them, land on them, examine them, use them as a base of operations to study Mars, and then go home.
Mars calls, a storehouse of scientific information—important in its own right but also for the light it casts on the environment of our own planet. There are mysteries waiting to be resolved about the interior of Mars and its mode of origin, the nature of volcanos on a world without plate tectonics, the sculpting of landforms on a planet with sandstorms undreamt of on Earth, glaciers and polar landforms, the escape of planetary atmospheres, and the capture of moons—to mention a more or less random sampling of scientific puzzles. If Mars once had abundant liquid water and a clement climate, what went wrong? How did an Earthlike world become so parched, frigid, and comparatively airless? Is there something here we should know about our own planet?
We humans have been this way before. The ancient explorers would have understood the call of Mars. But mere scientific exploration does not require a human presence. We tan always send smart robots. They are far cheaper, they don't talk back, you can send them to much more dangerous locales, arid, with some chance of mission failure always before us, no lives are put at risk.
"HAVE YOU SEEN ME?" the back of the milk carton read. "Mars Observer, 6' x 4.5' x 3', 2500 kg. Last heard from on 8/21/93, 627,000 km from Mars."
"M. O. call home" was the plaintive message on a banner hung outside the jet Propulsion Laboratory's Mission Operations Facility in late August 1993. The failure of the United States' Mars Observer spacecraft just before it was to insert itself into orbit around Mars was a great disappointment. It was the first post-launch mission failure of an American lunar or planetary spacecraft in 26 years. Many scientists and engineers had devoted a decade of their professional lives to M. O. It was the first U.S. mission to Mars in 17 years—since Viking's two orbiters and two landers in 1976. It was also the first real post-Cold War spacecraft: Russian scientists were on several of the investigator teams, and Mars Observer was to act as an essential radio relay link for larders from what was then scheduled to be the Russian Mars '94 mission, as well as for a daring rover and balloon mission slated for Mars '96.
The scientific instruments aboard Mars Observer would have napped the geochemistry of the planet and prepared the way for future missions, guiding landing site decisions. It might have cast a new light on the massive climate change that seems to have occurred in early Martian history. It would have photographed some of the surface of Mars with detail better than two meters across. Of course, we do not know what wonders Mars Observer would have uncovered. But every time we examine a world with new instruments and in vastly improved detail, a dazzling array of discoveries emerges just as it did when Galileo turned the first telescope toward the heavens and opened the era of modern astronomy.
According to the Commission of Inquiry, the cause of the failure was probably a rupture of the fuel tank during pressurization, gases and liquids sputtering out, and the wounded spacecraft spinning wildly out of control. Perhaps it was avoidable. Perhaps it was an unlucky accident. But to keep this matter in perspective, let's consider the full range of missions to the Moon and the planets attempted by the United States and the former Soviet Union:
In the beginning, our track records were poor. Space vehicles blew up at launch, missed their targets, or failed to function when they got there. As time went on, we humans got ;)otter at interplanetary flight. There was a learning curve. The ,adjacent figures show these curves (based on NASA data with NASA definitions of mission success). We learned very well. Our present ability to fix spacecraft in flight is best illustrated by the Voyager missions described earlier.
We see that it wasn't until about its thirty-fifth launch to the Moon or the planets that the cumulative U.S. mission success rate got as high as 50 percent. The Russians took about 50 launches to get there. Averaging the shaky start and the better recent performance, we find that both the United States and Russia have a cumulative launch success rate of about 80 percent. But the cumulative mission success rate is still under 70 percent for the U.S. and under 60 percent for the U.S.S.R./Russia. Equivalently, lunar and planetary missions have failed on average 30 or 40 percent of the time.
Missions to other worlds were from the beginning at the cutting edge of technology. They continue to be so today. They .ire designed with redundant subsystems, and operated by dedicated and experienced engineers, but they are not perfect. The amazing thing is not that we have done so poorly, but that we leave done so well.
We don't know whether the Mars Observer failure was due to incompetence or just statistics. But we must expect a steady background of mission failures when we explore other worlds. No human lives are risked when a robot spacecraft is lost. Even if we were able to improve this success rate significantly, it would be far too costly. It is much better to take more risks and fly more spacecraft:.
Knowing about irreducible risks, why do we these days fly only one spacecraft per mission? In 1962 Mariner 1, intended for V
enus, fell into the Atlantic; the nearly identical Mariner 2 became the human species' first successful planetary mission. Mariner 3 failed, arid its twin Mariner 4 became, in 1964, the first spacecraft to take close-up pictures of Mars. Or consider the 1971 Mariner 8/Mariner 9 dual launch mission to Mars. Mariner 8 Was to map the planet. Mariner 9 was to study the enigmatic seasonal and secular changes of surface markings. The spacecraft were otherwise identical. Mariner 8 fell into the ocean. Mariner 9 flew on to Mars arid became the first spacecraft in human history to orbit another planet. It discovered the volcanos, the laminated terrain in the polar caps, the ancient river valleys, and the aeolian nature of the surface changes. It disproved the "canals." It mapped the planet pole to pole and revealed all the major geological features of Mars known to us today. It provided the first close-up observations of members of a whole class of small worlds (by targeting the Martian moons, Phobos and Deimos). If we had launched only Mariner 8, the endeavor would have been an unmitigated failure. With a dual launch it became a brilliant and historic success.
There were also two Vikings, two Voyagers, two Vegas, many pairs of Veneras. Why was only one Mars Observer flown? The standard answer is cost. Part of the reason it was so costly, though, is that it was planned to be launched by shuttle, which is an almost absurdly expensive booster for planetary missions—in this case too expensive for two M. O. launches. After many shuttle-connected delays and cost increases, NASA changed its mind and decided to launch Mars Observer on a Titan booster. This required an additional two-year delay and an adapter to mate the spacecraft to the new launch vehicle. If NASA had not been so intent on providing business for the increasingly uneconomic shuttle, we could have launched a couple of years earlier and maybe with two spacecraft instead of one.