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 c
arbon 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 int
o 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 t
o 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 t
ime, 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?’”