by Carl Sagan
Recent vulcanism on Mars implies outgassing, whether or not the clouds that we see at the summits of these volcanoes are signs of outgassing. When hot lava flows to the surface, it carries with it a significant amount of gas – on the Earth, mainly water, but with a significant amount of other materials. Thus, the volcanoes that we see on Mars must have made an important contribution to the Martian atmosphere. In part, at least, the air has come out of these holes in the ground. Because Mars is so cold today, water can be trapped in many forms, such as ice, and not remain in the atmosphere. Much more gas could have been produced by these volcanoes than we see in the Martian atmosphere today. If there is life on Mars, it will almost surely be based on the exchange of material with the atmosphere – just as on Earth, where the cycle of green-plant photosynthesis and animal respiration is predominant. If there is life on Mars, these volcanoes may – at least indirectly – have played an important role in its present development.
After the dust storm cleared, Mariner 9 was moved into a higher orbit, to facilitate the geological mapping originally planned. The spacecraft worked many times longer than its designers had expected. Complete geological coverage of the planet has been accomplished down to a resolution of half a mile. The resulting geological maps reveal an enormous array of linear ridges and grooves that surround the Tharsis Plateau – as if a third or a quarter of the whole surface of Mars were cracked in some colossal recent event that lifted Tharsis. The most spectacular of these quasilinear features is an enormous rift valley in a region called Coprates. It runs 80 degrees of Martian longitude and is almost exactly as long as the largest rift valley on the Earth, the East African Rift Valley, which runs up the entire east coast of Africa to the Dead Sea. Since Mars is a smaller planet, the Coprates Rift Valley is, relatively speaking, a much more impressive feature.
The East African Rift Valley occurs because of sea floor spreading and continental drift. The African and Asian continents are slowly moving away from each other, and the chasm that is developing there is the East African Rift Valley. But continental drift is thought to be due to the slow circulation of material in the mantle of the Earth. Should we then conclude that Mars, despite its smaller size and lower internal temperatures, also has mantle convection and continental drift? Or is it possible that different processes produce similar features on the two planets?
Whatever the answer, we cannot help but learn a great deal more about the old Earth-bound science of geology – with its practical future disciplines, such as earthquake prediction and control – by examining the geology of our neighboring planet Mars.
18. The Canals of Mars
In 1877 (as in 1971) the planet Mars was close – forty million miles from Earth. European astronomers, with newly developed telescopes, prepared for what was then Man's most detailed look at our planetary neighbor. One of them was Giovanni Schiaparelli, an Italian observing in Milan and a collateral relative of the present couturier and perfume enterpriser.
Generally speaking, the telescopic view of Mars was blurred and fuzzy, interrupted by the variable turbulence in the Earth's atmosphere that astronomers call "seeing." But there were moments when the Earth's atmosphere steadied and the true detail on the disc of Mars seemed to flash out. Schiaparelli was astonished to see a network of fine straight lines covering the disc of Mars. He called these lines canali, which in Italian means "channels." However, canali was translated into English as "canals," a word with a clear imputation of design.
Schiaparelli's observations were taken up by Percival Lowell, a diplomat once posted in Chosen, the present Korea. A Boston Brahmin, the brother of the president of Harvard University and of an even more famous personage, the poetess Amy Lowell (for some reason renowned for smoking little black cigars), Lowell established a private observatory in Flagstaff, Arizona, to study the planet Mars. He found the same canali that Schiaparelli had. He extended their description and elaborated an explanation.
Mars was, Lowell concluded, a dying world on which intelligent life had arisen and accommodated itself to the perils of the planet. The chief peril was the dearth of water. The Martian civilization, Lowell imagined, had constructed an extensive network of canals to carry water from the melting polar caps to the habitations in more equatorial climes. The turning point of the argument was the straightness of the canals, some of them following great circles for thousands of miles. Such geometrical configurations, Lowell thought, could not be produced by geological processes. The lines were too straight. They could only have been produced by intelligence.
This is a conclusion with which we all can agree. The only debate is about which side of the telescope the intelligence was on. Lowell believed that the penchant for Euclidian geometry was on the distant end of the telescope. But the difficulties in drawing a great deal of mottled fine detail in a few seconds of good seeing are so great that the eye-brain-hand combination is sorely tempted to connect such disconnected features into straight lines. Many of the best visual astronomers observing Mars between the turn of the century and the dawn of the space age found that, while they could see canals under conditions of good but not superb seeing, they were able in the extremely rare moments of perfect seeing to resolve the straight lines into a multitude of spots and irregular detail.
Then it was found that at least the vast bulk of the polar caps are carbon dioxide and not frozen water. The atmospheric pressure was discovered to be much less than on Earth. Liquid water was found to be entirely impossible. The idea of advanced forms of life and canals on Mars died. And yet …
As the planet-wide dust storm cleared in 1971, the Mariner 9 spacecraft began to photograph a region called Coprates by the classical observers. Coprates was one of the largest canali found by Lowell, Schiaparelli, and their followers. Toward the end of the dust storm, Coprates was revealed to be an enormous rift valley running three thousand miles east to west near the Martian equator, fifty miles wide in spots and a mile deep. It was not perfectly straight – it was certainly not an engineering work; but it was a vast gash proportionately longer than any such feature on Earth.
And running out of Coprates were features that were very curious indeed – sinuous channels, meandering through the highlands above the Coprates Valley and graced with beautiful little tributaries. If such channels had been seen on Earth, they would unhesitatingly have been attributed to running water. But on Mars the surface pressures are so low that liquid water would instantly vaporize, just as the pressures on Earth are so low that liquid carbon dioxide vaporizes instantly. On Earth we have solid carbon dioxide and gaseous carbon dioxide, but not liquid carbon dioxide. On Mars this absence of the liquid phase is true as well for water.
But as the Mariner 9 photographic mission continued, a variety of additional channels were discovered: Channels with second- and third-order tributary systems, channels without a crater at their beginning or end, channels with teardrop-shaped islands in their midst, channels with braided termini, like those cut on Earth by episodic flooding.
There seems to be little doubt that most of the several dozen longest such channels (the longest are hundreds of miles long), and hundreds of smaller ones, were cut by running water. But since there can be no liquid water on Mars today, the channels must have been cut in a previous epoch of Martian history – when the total pressures were larger, the temperatures higher, and the availability of water greater.
The channels revealed by Mariner 9 speak eloquently of the possibility of massive climatic change on Mars. In this view, Mars is today in the throes of an ice age, but in the past – no one knows just how long ago – it possessed much more clement and Earth-like conditions.
The reasons for such dramatic climatic changes are still being hotly debated. Before the Mariner 9 launch, I proposed that such climatic changes leading to episodes of liquid water might occur on Mars. They might be driven by the precession of the equinoxes, a well-known motion akin to the slow, drifting precession of a rapidly spinning top. The precessional periods on
Mars are something like fifty thousand years. If we are now in a precessional winter, with an extensive North polar ice cap, twenty-five thousand years ago may have been the precessional winter with an extensive polar ice cap in the South.
But twelve thousand years ago may have been the epoch of precessional spring and summer. The dense atmosphere of that time is now locked away in the polar caps. Twelve thousand years ago may have been a time on Mars of balmy temperatures, soft nights, and the trickle of liquid water down innumerable streams and rivulets, rushing out to join mighty, gushing rivers. Some of these rivers would have flowed into the great Coprates Rift Valley.
If so, twelve thousand years ago was a good time on Mars for life similar to the terrestrial sort. If I were an organism on Mars, I might gear my activities to the precessional summers and close up shop in the precessional winters – as many organisms do on Earth for our much shorter annual winters. I would make spores; I would make vegetative forms; I would go into cryptobiotic repose; I would hibernate until the long winter had subsided. If this is indeed what Martian organisms do, we may be arriving at Mars twelve thousand years too early – or too late!
But there is a way to test these ideas. One way the hypothetical Martian organisms would know that the precessional spring has arrived is by the reappearance of liquid water. Therefore, as Linda Sagan has mentioned, the recipe for detecting life on Mars is "Add water." And this is just what the U. S. Viking biology experiments, scheduled to land on Mars in 1976 and search for microbes, will do. An automatic arm will drop two samples of Martian soil into liquid water. A third sample will be inserted into a chamber with no liquid water. If the first two experiments give positive biological results, and the third experiment does not, some support will be given to this idea that Martian organisms are waiting out the long winter.
But it is entirely possible that the designs of these experiments have been too Earth-chauvinist. There may be Martian organisms that enjoy the present environment and are drowned in liquid water. The idea of Martian organisms as sleeping beauties, awaiting a somewhat wet kiss from Viking, is a long shot – but a fascinating one.
By no means do all of the channels correspond to the positions of the classical canali drawn by Lowell and Schiaparelli. Some, like Ceraunius, appear to be ridges. Others correspond to no detail that can now be made out. But some, like Coprates, are grooves in the Martian terrain. There are channels on Mars. They may have biological implications, of a different sort than Lowell imagined (as the long-winter model suggests), or they may have no connection with Martian biology at all.
The canals of Lowell do not exist, but the canali of Schiaparelli are there to be seen, more or less. One day in the future, perhaps, the channels will again be filled with water and, for all we know, with visiting gondoliers from the planet Earth.
19. The Lost Pictures of Mars
The Mariner 9 mission to Mars radioed back to Earth 7,232 photographs that revolutionized our knowledge about the planet. Many hundreds of these pictures were devoted to studying variable features, the time changes in the relative configurations of bright and dark markings on the surface of the planet now known to be due largely to wind-blown dust. We have found thousands of bright and dark streaks, beginning in local impact craters and stretching across tens of miles of Martian surface. They point in the direction of the prevailing winds. We think they are produced by high winds carrying dust out of the craters and depositing it on the surface beyond the crater ramparts. These streaks are natural wind-direction indicators and, perhaps, anemometers laid down on the Martian surface for our edification and delight. We have discovered dark irregular patches or splotches, mostly residing in the interiors of craters, which tend to lie on the leeward walls of the craters. Thus, the splotches as well as the streaks are wind indicators. Some of the splotches have been resolved by Mariner 9 into enormous fields of parallel sand dunes.
We have discovered many cases of dark streaks and splotches varying through the mission in outline or extent. The positions and variabilities of these dark features correspond well to the classical dark markings of Mars, observed by ground-based astronomers for more than a century and most often attributed by them to seasonally changing dark vegetation on the Martian surface. But our Mariner 9 evidence points unambiguously to a meteorological, rather than a biological, explanation of the Martian seasonal changes.
This in no way excludes life on Mars. It merely means that if there is life on Mars, it is not easily detectable over interplanetary distances. The same is true in reverse: Photographic detection of life on Earth in daylight from the vantage point of Mars is impossible, as we have found by studying several thousand orbital photographs of our own planet. But the time-varying streaks and splotches on the Martian surface are a new and most exciting Martian phenomenon, which cries out for further study.
Since the Martian changes occur slowly, the variable-features objectives required very long time intervals between two pictures of the same region to see what changes had occurred. At the very end of the mission, fifteen photographs were successfully taken by the Mariner 9 cameras of regions in Syrtis Major and Tharsis, important for understanding the long-term variations. But when the time came to point the high-gain antenna of Mariner 9 to the Earth, so that these pictures could be transmitted by playing back the spacecraft's tape recorder, the last of the attitude-control gas was used up, Earth-lock could not be acquired, and playback did not occur. The spacecraft had literally run out of gas.
About a year before the Mariner 9 mission was launched, the possibility was raised that the spacecraft would run out of control gas. A solution was proposed: That the propulsion tanks be connected to the attitude-control gas system – a kind of spacecraft anastamosis. Excess propulsion gas could then be used for attitude control in case the attitude-control nitrogen was exhausted. This possibility was rejected – largely because of its expense. It would have cost $30,000. But no one expected Mariner 9 to last long enough to use up its attitude-control gas. Its nominal lifetime was ninety days – and it lasted almost a full year. The engineers had been overly conservative in assessing their superb product.
In retrospect, it sounds very much like false economy. With an adequate supply of attitude-control gas, the spacecraft might have lasted another full year in orbit around Mars. About $150 million of science might have been bought for $30,000 of pipe. Had we known that the spacecraft would die from a lack of nitrogen, I am almost certain that the planetary scientists involved would have raised the $30,000 themselves.
In fact, there are many such critical junctures in the space program where the addition of only a small amount of money can greatly increase the scientific return from a given mission. But NASA, severely limited by funding limitations imposed by Congress, the White House, and the Office of Management and Budget, has not had such small increments of money. If it were possible, and if a generous donor could be found, this would be a superb use for private philanthropy.
But these are idle musings. No anastamosis was performed; the final playback was not accomplished. Sitting there still on the Mariner 9 tape recorder are fifteen vital photographs of the planet. They will never be returned under Mariner 9's own power. It has now also lost solar lock; sunlight is no longer being converted to electricity on its four great solar panels, and there is no way to reactivate it. We may never know what Tharsis and Syrtis Major looked like around the beginning of November 1972 from the vantage point of Martian orbit.
Or perhaps we will. Mariner 9 is in an orbit that is slowly decaying in the Martian atmosphere. But the decay is so slow that the spacecraft will not crash into Mars for another half century. Long before then there should be manned orbital flights around Mars. Rendezvous and docking maneuvers are reasonably well developed in manned missions even now. Perhaps, then, sometime around 1990, as a small side-trip in a grand manned-orbital exploration of Mars, there will be a rendezvous with Mariner 9. The old and battered spacecraft will be taken aboard a large manned station and r
eturned home – perhaps to be put in the Smithsonian Institution; perhaps to prevent terrestrial micro-organisms on Mariner 9 from reaching Mars; but perhaps, also, to rescue and read off the fifteen lost pictures of the Mariner 9 mission.
20. The Ice Age and the Cauldron
On our tiny planet, spinning in an almost circular orbit at a nearly constant distance from our star, the climate varies, sometimes radically, from place to place. The Sahara is different from the Antarctic. The Sun's rays fall directly on the Sahara and obliquely on the Antarctic, producing a sizable temperature difference. Hot air rises near the equator, cold air sinks near the poles – producing atmospheric circulation. The motion of the resulting air current is deflected by Earth's rotation.
There is water in the atmosphere, but when it condenses, forming rain or snow, heat is released into the atmosphere, which in turn changes the motion of the air.
Ground covered by freshly fallen snow reflects more sunlight back to space than when it is snow-free. The ground becomes colder yet.
When more water vapor or carbon dioxide is put into the atmosphere, infrared emission from the surface of the Earth is increasingly blocked. Heat radiation cannot escape from this atmospheric greenhouse, and the Earth's temperature rises.
There is topography on Earth. When wind currents flow over mountains or down into valleys, the circulation changes.
