The Case for Mars, page 5
Observations of Mars continued through the decades, especially around “oppositions,” those times when Mars (technically, any planet outside Earth’s orbit) lies on the opposite side of the Earth from the Sun. At these times, Mars is at its closest to Earth and thus shines most brightly in the sky. By the early nineteenth century, astronomers had collected a basketful of basic Mars statistics: its orbital period; the length of its day; the planet’s mass and density; distance from the Sun, and surface gravity. But what truly intrigued observers was the changing face of Mars. Through the years the telescope’s eyepiece revealed that Mars’ face was mottled with ar darkpatches that came and went with time. Likewise, the bright white spots observers noted at the poles appeared to vary with the Martian seasons, expanding and contracting over the course of a Martian year. And Mars apparently hosted an atmosphere, as some observers spied vague indications of clouds above the Martian surface.
The opposition of 1877 proved especially fruitful for observers and for Martian studies. Asaph Hall of the U.S. Naval Observatory discovered two small moons of Mars and promptly named them Phobos and Deimos—fear and terror, an appropriate entourage for the planet of war. But in hindsight, 1877 is perhaps best remembered for a series of observations that launched a turbulent episode in the history of Mars observations and one of the strangest chapters in the history of astronomy.
Among those who turned a telescopic eye toward Mars in 1877 was the Italian astronomer Giovanni Schiaparelli, director of the Brera Observatory in Milan. Schiaparelli’s reports of his observations noted the location of more than sixty features on the Martian surface. But, along with many standard features, he reported sighting linear markings crisscrossing the face of Mars. He named these features after terrestrial rivers—Indus, Ganges—but referred to them in his writings as “canali,” the Italian plural for channels or grooves. While not the first to note these strange markings, he was the first to identify an extensive system of “canali.” More than a decade later, the enthusiasms of Percival Lowell would catapult Mars and its “canali” to headline status throughout the world.
Born into an illustrious New England family of poets, educators, statesmen, and industrialists (the great poet Amy Lowell was his sister, his brother Abott was president of Harvard), Lowell while in his late thirties became intrigued with Mars, especially with Schiaparelli’s observations. For Lowell there could be only one interpretation—for “canali” Lowell read not channels, but canals. Canals reflect the work of minds in collaboration, of life. For reasons that remain unclear, Lowell decided that Mars demanded his attention and devote his attention to Mars he did, with a passion and pocketbook few could match.
The tool Lowell built for his investigations—the Lowell Observatory in Flagstaff, Arizona—saw first light in April 1894, just a few weeks before Mars reached its biennial opposition with Earth. Lowell and his staff atop Mars Hill spent more than a decade studying and mapping the face of Mars. Lowell and his assistants mapped hundreds of canals. In their number and organization, Percival Lowell saw the history of an alien race trying to survive on an arid, dying world plainly writ.
Lowell captured the popular imagination with his sympathetic picture of an intelligent race of Martians trying to forestall its inevitable doom. The effect of his writings was amplified further by adventure writers such as Edgar Rice Burroughs, who used the Lowellian Mars as the setting for an extraordinary romantic Martian civilization that called its home planet “Barsoom.” Burroughs’s Mars novels featured swashbuckling heroes rescuing daring and beautiful princesses endangered by monsters, savages, and power-mad Martian tyrants, all set against a rich tapestry of life on Barsoom. In its Barsoomian incarnation, Lowell’s Mars enchanted millions of readers.
Over the years though, neither Lowell’s eloquence as a writer and speaker nor his energy and enthusiasm could defend his theories against the barbs of the astronomical community. The tide of opinion slowly turned against Lowell as other observers that remag more powerful telescopes found no evidence whatsoever of canals. We now know that Lowell was absolutely wrong in his investigations of Mars, but he did leave an important legacy behind: he fired the imaginations of people to make them see a world on Mars. True, that world turned out to be wildly inaccurate, but its envisionment led to a massive uplifting of at least a segment of the popular mind, which three centuries after Kepler was and still is largely addicted to the ancient geocentric view of the Earth as the only world, orbited by tiny lights in the sky. Lowell made Mars habitable in the imagination only, but it is from imagination that reality is created. It was Lowell’s works that inspired the pioneers of rocketry, including Robert Goddard and Herman Oberth, to begin their quest to develop the tools that would soon make the solar system accessible, not only to the eye, but to the hand of man. It was the spirit of Lowell that touched the rocky surface of Mars as Viking landed.
VIKING’S SEARCH FOR LIFE
Life had brought Viking to Mars. Though Lowell’s visions had long since died, the idea that Mars might harbor some form of life had itself never died. Streaking by the planet in July 1965, the first spacecraft to visit Mars, the American Mariner 4, certainly quashed once and for all the Lowellian vision of the Red Planet, revealing a barren, cratered surface, more Moon-like than Barsoom-like. Those who hoped for postcards from life’s far edge got, instead, funereal images of an aged, dead planet, a “cosmic fossil” in science fiction author Arthur C. Clarke’s words. During the summer of 1969, Mariners 6 and 7 confirmed their predecessors’ findings. Science experiments confirmed Mariner 4’s atmospheric findings—the atmospheric pressure of the carbon dioxide-rich atmosphere was low, just 6-8 millibars. (A millibar is 1/1,000th of Earth’s sea-level atmospheric pressure, so at 7 millibars, Mars’ atmosphere was a bit less than 1 percent as thick as Earth’s.) Temperatures measured near the south pole supported the notion that frozen carbon dioxide—dry ice—formed the polar cap. Mars, according to the Mariner flybys, was a cold, dead, cratered planet—not a place you want to linger. Then came Mariner 9.
Unlike the previous American spacecraft, Mariner 9 would go into orbit around Mars. Where the early Mariners shot by the Red Planet and captured what information they could, Mariner 9 and a companion spacecraft would map the planet’s surface and observe planetary dynamics over a sixty-day period. Unfortunately, that companion spacecraft, Mariner 8, ended up in the waters of the Atlantic shortly after launch in the spring of 1971. Mariner 9, though, lifted off flawlessly on May 30th, bound for Mars. Just days earlier the Soviet Union had launched Mars 2 and Mars 3 combination orbiter/lander spacecraft. No great surprises arose on board the spacecraft as they sped toward their destination. The same couldn’t be said for Mars.
On September 22, about two months before the Mars probes and Mariner were due to arrive, astronomers noticed a bright, white cloud begin to develop over the Noachis region of Mars. The cloud grew quickly, by the hour. Within days the cloud, now recognized as a dust storm, had enveloped the planet. As robotic eyes sped toward Mars, the planet pulled a shroud around itself. Far-encounter photographs of the planet captured by Mariner 9 on November 12th and 13th showed a blank disk, save for a slight brightening near the south pole, and a few small, dark smudges above the equator. On the 14th, the spacecraft slipped into Mars orbitMariner gazed down on an essentially featureless planet. The probe’s controllers rewrote the mission plans, allowing for some science experiments and photography to be undertaken, but, in essence, told the spacecraft to kick back and ride out the storm.
Mars 2 and 3 didn’t have that option. Unlike Mariner, the Soviet program did not have “adaptive operational capability.” On arrival at Mars, the orbiters duly released their landers into the maw of the largest Martian dust storm ever recorded. Parachuting blindly through an atmosphere whipped by 160 km/hr winds, both probes hit the ground too hard for their airbag deceleration systems to save them. Mars 2 was destroyed on impact; Mars 3 managed to transmit 20 seconds of data after crashing, and then died.
The Soviet orbiters hardly fared any better than the descent probes. Nearly all data from Mars 2 was lost because of poor telemetry, and Mars 3 pulled into a wildly elliptical orbit about Mars, producing only one released photograph.
While the dust storm raged, and the Soviet probes met their respective fates, Mariner 9 serenely orbited the planet, waiting for the dust to clear, both literally and figuratively. Toward the end of December and into early January 1971, the Martian skies started clearing, and Mariner began to return staggeringly vivid images of an unimagined world.
The small smudges Mariner imaged during far encounter could now be seen for what they were: enormous mountains whose tops Mariner had spied through the dust storm. A century earlier, optical astronomers had noted a bright region in the area of the largest of these massifs, and dubbed the region “Nix Olympica,” the Snows of Olympus. It was an apt name, as Nix Olympica proved to be the largest mountain in the solar system—Olympus Mons—looming some 24 kilometers above the Martian surface and covering an area about the size of the state of Missouri. Another region of Mars well-known to astronomers, the Coprates region, yielded surprises as well. Through the telescope, Coprates appeared as a dark, stubby, bright, cloud-like band. As skies cleared, Mariner’s audience of scientists realized they were looking at a dust cloud slowly settling into the bottom of a valley of, again, Olympian proportions. Now known as Valles Marineris (in honor of Mariner 9), this ragged scar stretches nearly 4,000 kilometers across the planet. Up to 200 kilometers wide and 6 kilometers deep, Valles dwarfs any similar feature on Earth (if need be, you could tuck the Rocky Mountains in one of Valles’s side valleys; nobody would see them).
With each orbit of the planet, Mariner returned ever more astonishing information. The greatest surprise, though, proved to be images of sinuous channels (yes, canali!) that appeared to have been carved by running water—there were riverbeds on Mars.
Whatever romance the earlier Mariners had killed, Mariner 9 renewed. The probe reinforced many of the earlier Mariner findings, but overturned others, including the notion that Mars was simply a knock-off of the Moon. Imagine the Martian globe bisected by a line running at roughly a 50° angle to the planet’s equator. Below that line to the south lies the heavily cratered, ancient terrain Mariners 4, 6, and 7 discovered and recorded. North of the line, craters are few while evidence of more recent geological activity is plentiful. It just happened that the first three Mariners visited the south, offering no clues as to what other regions of the planet mit reveal. Mariner 9’s images (more than 7,000 of them) and data swept away the notion of the Red Planet as a “cosmic fossil.” Instead, Mariner 9’s findings told the tale of a planet of fire and ice. In the distant past, Mars’ surface had been geologically alive. Volcanoes had roared and resurfaced vast areas of terrain; internal mechanisms of some sort had fractured and split the landscape, lifting the Tharsis region (on which Olympus Mons stood) kilometers above the landscape; and water had flowed across the planet’s surface in volumes large enough and for periods long enough to carve the face of the planet. Mars was once warm, wet, and alive with geologic activity. And that begged the question once again: Was Mars now, or perhaps in the past, bustling with biologic activity, with life?
To answer that question, astronomers and biologists found themselves stepping back from the concept of “life on Mars” to the simpler but still complex concept of, simply, life. What is it? If you can’t define what life is, if you can’t distinguish between life and nonlife here on Earth, you’ll have a devilish time looking for it on a red dot 400 million kilometers distant. So the search for life on Mars began with a review of the only known sample of life in the universe, terrestrial life. While terrestrial life comes in all forms, shapes, and sizes, its presence invariably causes changes to its local environment. These changes can be small, tiny even, especially if you’re dealing with tiny life forms. But, no matter the size, life will still alter its environment simply by the fact of metabolism and respiration, the complex physical and chemical business of keeping something, anything, alive. Seal up an airtight box and the mix of gases (assuming there’s no outgassing from the walls) will remain stable. Stick a cat in the same box and the mix will change pretty quickly (as will the state of the cat). So, if you’re casting about for signs of life, establish a controlled environment, insert whatever sample you have, and then observe the changes, chemical or physical, inside the box. Chances are, any large changes will be attributable to biological processes. This, in essence, is what the scientists of the Viking project chose to do.
The Viking program was fairly straightforward in description—two orbiters, two landers, all to head for Mars in 1973 to search for life—but proved staggeringly difficult in execution. A budget squeeze delayed launch until 1975, which, in retrospect, was a hidden blessing, as the spacecraft simply would not have been ready by 1973 without, in the words of a Viking team member, “compromising both capability and reliability.”
The four Viking spacecraft bristled with instruments for imaging, water-vapor mapping, thermal mapping, seismology, meteorology, and more, but the heart of the mission lay with the landers’ biology packages. Viking engineers had packaged three biology labs weighing about 9 kilograms total into something that could sit quite comfortably in your bookcase.
The three experiments in the biology package operated on the same basic principle: seal some Martian dirt in a container with a culture medium, incubate it under different conditions, and then measure the gases emitted or absorbed. The experiments differed in the specific approaches they took to incubate samples and in what they sought to detect and measure as evidence for life. The Viking landers also carried an X-ray florescence instrument capable of assessing the elemental composition of the soil, and a gas chromatograph mass spectrometer (GCMS) capable of detecting and identifying organic compounds in the soil.
The search for life began on Viking 1’s eighth Martian day—“sol” 8 in the local time zone, July 28, 1976, here on Earth—as the lander extended its sampler arm, dragged it across the Martian surface, and delivered soil to the biology package. The three experiments received their small allotments of soil and set to work. Over the course of the next three days, incredibly, all three biology experiments reported powerful gas releases, positive signals for life, in some cases virtually immediately after exposure of the culture media to Martian soil.
The Viking biology team was, to say the least, stunned. Three experiments, three positive responses, three indications of life . . . maybe. The gas release signals were definite, but their suddenness of both onset and cessation had more of a ring of chemical reaction than biological growth. So caution was called for. The discovery of life anywhere in the solar system would have profound ramifications not just for the world of science but for the entire world community. Once again, as in Kepler’s time, humanity would come to know its place in the universe more fully, more truthfully. We would know that while we are not the center of the universe, we are part of a phenomenon that is general throughout the universe. We would know that life owns the universe. This was, most definitely, no small announcement.
No one on the biology team was eager to rush out such an announcement, only to discover that he had jumped the gun. So conservatism prevailed, especially since many on the biology team had strong suspicions that the reactions witnessed were nonbiological in origin. One of the biology team’s principal investigators, Norman Horowitz, stated his position quite clearly during a press conference announcing his own experiment’s first positive readings. “I want to emphasize,” he told an eager group of journalists, “we have not discovered life on Mars—not.”
On sol 23, the gas chromatograph mass spectrometer analyzed a sample of Martian soil and found not a trace of organic carbon in the sample. After the reactions recorded by the three biology packages, this came as an enormous surprise and heightened the debate. Scientists had expected the GCMS to find at least some trace of organic compounds of nonbiological origin, such as materials from meteorites. In fact, that was a concern surrounding the GCMS—how to tell biologic organics from nonbiologic. But now, with the GCMS recording absolutely no evidence of organics in Martian surface soils, the search for life on Mars became for some a search for processes that could reconcile the discovery of an evidently lifeless Mars with the biology results.
On September 3, Viking 2 settled down on to the Utopia Planitia, nearly halfway around the planet, some 6,400 kilometers distant from the Viking 1 landing site and about 25° farther north. The biology experiments and the GCMS were soon up and running, investigating soils that appeared to be slightly moister than samples from the Chryse site. Again, results from the biology experiments gave positive responses that appeared to be more indicative of chemistry to some, and the GCMS found no trace of organic carbon. Again, the results caused a stir, with some investigators holding out for biology, others chemistry. Again, the results highlighted a basic problem: the Vikings could perform four experiments and only four, and three were saying “maybe life,” while the other was saying “very doubtful.” If the soil samples had been in a terrestrial lab, dozens of additional experiments could have been performed to resolve the argument definitively. On Earth, the samples ould even have been incubated in a culture medium and the results observed directly with a microscope. But in Viking’s limited four-experiment lab on Mars, none of this was possible. In essence, we were left with contradictory results. In the words of writer Leonard David, “Viking went to Mars and asked if it had life, and Mars answered by replying ‘Could you please rephrase the question?’”
