After Landing on Mars
For Viking 1, the original landing site seemed, after the scientists examined orbiter photographs and late-breaking Earth-based radar data, unacceptably risky. For a while Carl Sagan was worried that Viking 1 had been condemned, like the legendary Flying Dutchman, to wander the skies of Mars forever, never to find safe heaven. Eventually the scientists found a suitable spot, still in Chryse but far from the confluence of the four ancient channels. The delay prevented them from setting down on July 4, 1976, but it was generally agreed that a crash landing on that date would have been an unsatisfactory two hundredth birthday present for the United States. The scientists deboosted from orbit and entered the Martian atmosphere sixteen days later.
After an interplanetary voyage of a year and a half, covering a hundred million kilometers the long way round the Sun, each orbiter/lander combination was inserted into its proper orbit about Mars; the orbiters surveyed candidate landing sites; the landers entered the Martian atmosphere on radio command and correctly oriented ablation shield, deployed parachutes, divested coverings, and fired retro-rockets. In Chryse and Utopia, for the first time in human history, spacecraft had touched down, gently and safely, on the red planet. These triumphant landings were due to considerable part to the great skill invested in their design, fabrication and testing, and to the abilities of the spacecraft controllers. But for so dangerous and mysterious a planet as Mars, there was also at least an element of luck.
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| One way or another, Mars is a world to which we will return. Image : The End by © Megan Jorgensen (Elena) |
Immediately after landing, the first pictures were to be returned. The scientists knew thay had chosen dull places. But they could hope. The first picture taken by the Viking 1 lander was of one of its own footpads – in case it were to sink into Martian quicksand, the humans wanted to know about it before the spacecraft disappeared. The picture built up, line by line, until with enormous relief the scientists saw the footpad sitting high and dry above the Martian surface. Soon other pictures came into being, each picture element radioed individually back to Earth.
Carl Sagan remembered being transfixed by the first lander image to show the horizon of Mars. This was not an alien world, he thought. He knew places like it in Colorado and Arizona and Nevada. There were rocks and sand drifts and a distant eminence, as natural and unselfconscious as any landscape of Earth. Mars was a place. Sagan would, of course, have been surprised to see a grizzled prospector emerge from behind a dune leading his mule, but at the same time the idea seemed appropriate. Nothing remotely like it ever entered his mind in all the hours he spent examining the Venera 9 and 10 images of the Venus surface.
The combination of Soviet successes of landing on Venus and Soviet failures on landing on Mars naturally caused some concern about the US Viking mission, which had been informally scheduled to set one of its two descent craft gently down on the Martian surface on the Bicentennial of the United States, July 4, 1976.
Like its Soviet predecessor, the Viking landing maneuver involved an ablation shield, a parachute and retro-rockets. Because the Martian atmosphere is only 1 percent as dense as the Earth’s, a very large parachute, eighteen meters in diameter, was deployed to slow the spacecraft as it entered the thin air of Mars. The atmosphere is so thin that if Viking had landed at a high elevation there would not have been enough atmosphere to brake the descent adequately: it would have crashed. One requirement, therefore, was for a landing site in a low-lying region. From Mariner 9 results and ground-based radar studies, we knew many such areas.
To avoid the probable fate of Mars 3, the Americans wanted Viking to land in a place and time at which the winds were low. Winds that would make the lander crash were probably strong enough to lift dust off the surface. If the scientists could check that the candidate landing site was not covered with sifting, drifting dust, they would have at least a fair chance of guaranteeing that the winds were not intolerably high. This was one reason that each Viking lander was carried into Mars orbit with its orbiter, and descent delayed until the orbiter surveyed the landing site. They had discovered with Mariner 9 that characteristic changes in the bright and dark patterns on the Martian surface occur during times of high winds. The scientists certainly would not have certified a Viking landing site as safe if orbital photographs had shown such shifting patterns.
But the guaranties could not be 100 percent reliable. For example, the American scientists could imagine a landing site at which the winds were so strong that all mobile dust had already been blown away. The scientists would then have had no indication of the high winds that might have been there. Detailed weather predictions for Mars were, of course, much less reliable than for Earth. (Indeed one of the many objectives of the Viking mission was to improve the understanding of the weather on both planets).
Because of communication and temperature constraints, Viking could not land at high Martian latitudes. Farther poleward than about 45 or 50 degrees in both hemispheres, either the time of useful communication of the spacecraft with the Earth or the period during which the spacecraft would avoid dangerously low temperatures would have been awkwardly short.
The Mars landscape is stark and red and lovely: boulders thrown out in the creation of a crater somewhere over the horizon, small sand dunes, rocks that have been repeatedly covered and uncovered by drifting dust, plumes of fine-grained material blown about by the winds. Where did the rocks come from? How much sand had been blown by wind? What must the previous history of the planet have been to create sheared rocks, buried boulders, polygonal gouges in the ground? What are the rocks made of? The same materials as the sand? Is the sand merely pulverized rock or something else? Why is the sky pink? What is the are made of? How fast does the wind blow? Are there marsquakes? How do the atmospheric pressure and the appearance of the landscape change with the seasons?
For every one of these questions Viking has provided definitive or at least plausible answers. The Mars revealed by the Viking mission is of enormous interest – particularly when we remember that the landing sites were chosen for their dullness. But the cameras revealed no sign of canal builders, no Barsoomian aircars or short sword, no princesses or fighting men, no thoats, no footprints, not even a cactus or a kangaroo rat. For as far as we could see, there was not a sign of life.
(There was a brief flurry when the Uppercase letter B, a putative Martian graffito, seemed to be visible on a small boulder in Chryse. But later analysis showed it to be a trick of light and shadow and the human talent for pattern recognition. It also seems remarkable that the Martians should have tumbled independently to the Latin alphabet. But there was just a moment when resounding in Sagan’s head was the distant echo of a word from his boyhood – Barsoom).
Perhaps there are large life-forms on Mars, but not in the two first landing sites. Perhaps there are smaller forms in every rock and sand grain. For most of its history, those regions of the Earth not covered by water looked rather like Mars today – with an atmosphere rich in carbon dioxide, with ultraviolet light shining fiercely down on the surface through an atmosphere devoid of ozone. Large plants and animals did not colonize the land until the last 10 percent of Earth history. And yet for three billion years there were microorganisms everywhere on Earth.
Very little effort was made to calibrate the experiments with plausible inorganic Martian surface materials. Mars is not the Earth. As the legacy of Percival Lowell, who saw canals on Mars, reminds us, we can be fooled. Perhaps there is an exotic inorganic chemistry in the Martian soil that is able by itself, in the absence of Martian microbes, to oxidize foodstuffs. Perhaps there is some special inorganic, nonliving catalyst in the soil that is able to fix atmospheric gases and convert them into organic molecules.
Recent experiments suggest that this may indeed be the case. In the great Martian dust storm of 1971, spectral features of the dust were obtained by the Mariner 9 infrared spectrometer. In analyzing theses spectra, O.B. Toon, J. B. Pollack and Carl Sagan found that certain features seem best accounted for by montoillonite and other kinds of clay, Subsequent observations by the Viking lander support the identification of windblown clays on Mars. Now, A. Banin and J. Rishpon have found that they can reproduce some of the key features – those resembling photosynthesis as well as those resembling respiration – of the “successful” Viking microbiology experiments if in laboratory experiments they substitute such clays for the Martian soil. The clays have a complex active surface, given too adsorbing and releasing gases and to catalyzing chemical reactions. It is too soon to say that all the Viking microbiology results can be explained by inorganic chemistry, but such a result would no longer be surprising. The clay hypothesis hardly excludes life on Mars, but it certainly carries us far enough to say that there is no compelling evidence for microbiology on Mars.
Even so, the results of Banin and Rishpon are of great biological importance because they show that in the absence of life there can be a kind of soil chemistry that does some of the same things life does. On the Earth before life, there may already have been chemical processes resembling respiration and photosynthesis cycling in the soil, perhaps to be incorporated by life once it arose. In addition, we know that montorillonite clays are a potent catalyst for combining amino acids into longer chain molecules resembling proteins. The clays of the primitive Earth may have been the forge of life, and the chemistry of contemporary Mars may provide essential clues to the origin and early history of life on our planet.
Landing On Mars
The combination of Soviet successes of landing on Venus and Soviet failures on landing on Mars naturally caused some concern about the US Viking mission, which had been informally scheduled to set one of its two descent craft gently down on the Martian surface on the Bicentennial of the United States, July 4, 1976.
Like its Soviet predecessor, the Viking landing maneuver involved an ablation shield, a parachute and retro-rockets. Because the Martian atmosphere is only 1 percent as dense as the Earth’s, a very large parachute, eighteen meters in diameter, was deployed to slow the spacecraft as it entered the thin air of Mars. The atmosphere is so thin that if Viking had landed at a high elevation there would not have been enough atmosphere to brake the descent adequately: it would have crashed. One requirement, therefore, was for a landing site in a low-lying region. From Mariner 9 results and ground-based radar studies, we knew many such areas.
To avoid the probable fate of Mars 3, the Americans wanted Viking to land in a place and time at which the winds were low. Winds that would make the lander crash were probably strong enough to lift dust off the surface. If the scientists could check that the candidate landing site was not covered with sifting, drifting dust, they would have at least a fair chance of guaranteeing that the winds were not intolerably high. This was one reason that each Viking lander was carried into Mars orbit with its orbiter, and descent delayed until the orbiter surveyed the landing site. They had discovered with Mariner 9 that characteristic changes in the bright and dark patterns on the Martian surface occur during times of high winds. The scientists certainly would not have certified a Viking landing site as safe if orbital photographs had shown such shifting patterns.
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| Nobody wishes to land in too rough a place (Quotations from Megan Jorgensen). Image by Meg Jorgensen (Elena) A Wallflower |
Because of communication and temperature constraints, Viking could not land at high Martian latitudes. Farther poleward than about 45 or 50 degrees in both hemispheres, either the time of useful communication of the spacecraft with the Earth or the period during which the spacecraft would avoid dangerously low temperatures would have been awkwardly short.
Life on Mars
The Mars landscape is stark and red and lovely: boulders thrown out in the creation of a crater somewhere over the horizon, small sand dunes, rocks that have been repeatedly covered and uncovered by drifting dust, plumes of fine-grained material blown about by the winds. Where did the rocks come from? How much sand had been blown by wind? What must the previous history of the planet have been to create sheared rocks, buried boulders, polygonal gouges in the ground? What are the rocks made of? The same materials as the sand? Is the sand merely pulverized rock or something else? Why is the sky pink? What is the are made of? How fast does the wind blow? Are there marsquakes? How do the atmospheric pressure and the appearance of the landscape change with the seasons?
For every one of these questions Viking has provided definitive or at least plausible answers. The Mars revealed by the Viking mission is of enormous interest – particularly when we remember that the landing sites were chosen for their dullness. But the cameras revealed no sign of canal builders, no Barsoomian aircars or short sword, no princesses or fighting men, no thoats, no footprints, not even a cactus or a kangaroo rat. For as far as we could see, there was not a sign of life.
 |
| Viking Orbits Mars. To look for life on Mars, we must look for microbes.Image in public domain, by NASA. |
Perhaps there are large life-forms on Mars, but not in the two first landing sites. Perhaps there are smaller forms in every rock and sand grain. For most of its history, those regions of the Earth not covered by water looked rather like Mars today – with an atmosphere rich in carbon dioxide, with ultraviolet light shining fiercely down on the surface through an atmosphere devoid of ozone. Large plants and animals did not colonize the land until the last 10 percent of Earth history. And yet for three billion years there were microorganisms everywhere on Earth.
Absence of Life
Very little effort was made to calibrate the experiments with plausible inorganic Martian surface materials. Mars is not the Earth. As the legacy of Percival Lowell, who saw canals on Mars, reminds us, we can be fooled. Perhaps there is an exotic inorganic chemistry in the Martian soil that is able by itself, in the absence of Martian microbes, to oxidize foodstuffs. Perhaps there is some special inorganic, nonliving catalyst in the soil that is able to fix atmospheric gases and convert them into organic molecules.
Recent experiments suggest that this may indeed be the case. In the great Martian dust storm of 1971, spectral features of the dust were obtained by the Mariner 9 infrared spectrometer. In analyzing theses spectra, O.B. Toon, J. B. Pollack and Carl Sagan found that certain features seem best accounted for by montoillonite and other kinds of clay, Subsequent observations by the Viking lander support the identification of windblown clays on Mars. Now, A. Banin and J. Rishpon have found that they can reproduce some of the key features – those resembling photosynthesis as well as those resembling respiration – of the “successful” Viking microbiology experiments if in laboratory experiments they substitute such clays for the Martian soil. The clays have a complex active surface, given too adsorbing and releasing gases and to catalyzing chemical reactions. It is too soon to say that all the Viking microbiology results can be explained by inorganic chemistry, but such a result would no longer be surprising. The clay hypothesis hardly excludes life on Mars, but it certainly carries us far enough to say that there is no compelling evidence for microbiology on Mars.
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| Mars is not the Earth, don’t be fooled! People there may don’t be what they look. Image: © Sketch Drawing Megan Jorgensen (Elena) |
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