Welcome to Mars.
Since the textbook coverage is quite comprehensive, I will focus these notes on a few aspects of particular interest, some of which are only superficially discussed in the text. I will describe the interesting history of our understanding of Mars; the discoveries made by the space probes visiting Mars; and
with the last of these topics presented in a separate section of the notes.
Let us remind ourselves of our expectations. Mars is smaller than the Earth and Venus, so has weaker gravity; but on the other hand, it is farther from the sun, so somewhat cooler. Thus you might expect Mars to be able to hang onto an atmosphere, although probably a thin one. Moreover, you might expect it to be geologically intermediate in activity between Mercury (now essentially inert) and the Earth (very active). Are these expectations borne out? We will see.
Historical Perspectives.The text describes the problem of studying Mars from the Earth -- it is small, and even at its closest it shows very few details. An extra problem is introduced by the fact that the orbit of Mars is distinctly eccentric. This means that there are times when it is overhead at midnight -- that is, there are times at which the Earth lies directly between Mars and the sun -- but still so far away from us that it cannot be studied in any real detail, even with a powerful telescope. (The top-left part of the figure on page 202 of the text will help you understand this.) On the other hand, there can be favourable oppositions during which Mars is overhead at night and relatively close, so that it is a better target; this occurred in 1988, for example. Despite these difficulties, early observations revealed unambiguously the existence of white polar caps which change in size as the seasons progress. (The Martian year is not quite twice the length of our own year.) Moreover, there are dark regions on the face of the planet which come and go according to the seasons. This seems clear evidence for the existence of an atmosphere (since the polar caps presumably melt in the summer season and reappear as the winter returns, with at least some of the frozen material appearing in the form of a vapour). The dark patches, however, were a bit of a mystery. Early speculation included the notion that these regions were great areas of vegetation, like huge grasslands or forests, proof of life on Mars. As it happens, we now realize that the dark patches are simply areas of dark rock which can be exposed or covered by the wind-blown dust on Mars. As the text explains, windstorms on Mars come and go in a seasonal way which explains the regularity in these changes. In fact, enormous windstorms on Mars have long been known. Sometimes the entire face of the planet is obscured by dust which has been whipped into the air by the wind. Even at the very best, however, Mars is a small object upon which one can scarcely hope to see any details of consequence. The very largest mountains imaginable, for instance, would not be easily seen over such a great distance, and hopes of the direct study of the surface, and the detection of any signs of life, were not great a century ago. But then along came Percival Lowell.
The Legacy of Percival Lowell.We have already met Percival Lowell, in the context of . Lowell was fascinated to learn that an Italian astronomer named Schiaparelli had reported the discovery of `canali' on Mars. As it happens, the Italian word 'canali' means nothing more than ``channels'' , a word which could imply something like the English Channel -- a geographical feature of natural origin. Understandably, however, the word was mis-translated into the English word `canals', which seems to imply artifacts, features built by intelligent creatures. The impact of this apparent discovery was heightened by the state of human technology at the time. A century or more ago, canals were seen as almost the greatest imaginable engineering achievements of mankind -- the Suez canal, the Panama canal, the canal systems in England and New England. It is little exaggeration to say that the discovery of canals on Mars had an impact which might only be equalled today by the unexpected discovery of space centers (like the Kennedy Center in Florida) on Mars. People got very excited to learn that the supposed Martians were `just as technogically advanced as us,' apparently able to build great networks of canals. Prompted by this, Percival Lowell became single-minded about carrying out further observations. As I have already noted in my discussion of the search for Pluto, at this stage he made one undeniably important contribution to astronomy. He recognized, for the first time, the need for exceptionally fine astronomical images, and selected a site for an observatory on the basis of the quality of the seeing and transparency of the atmosphere. So was built the Lowell Observatory near Flagstaff, Arizona. The establishment of this new observatory led to a scientific impasse. When Lowell was able to `see' canals on Mars that others could not, he simply shrugged off the negative results with the explanation that their telescopes were not so well placed as his and that they simply could not resolve the subtle details because of the blurriness imposed by the Earth's atmosphere. (He also often commented on the imperfect skills of the other observers). Lowell drew maps of the network of lines which he perceived on Mars. As the years passed, these maps became unbelievably intricate, with a fantastic tracery of canals, lines, and oases. Then, in trying to explain the origin of the canals, Lowell conjured up a remarkable and evocative image of a race of creatures who were desperately struggling to survive on a dying planet. According to him, Mars was drying up -- hence its red appearance, with sandy desert-like conditions prevailing almost everywhere -- and the Martians were using the canals to transport water from the polar caps to the arid equatorial regions. (If you think about it, you will see a parallel to the canals which are used to transport water to Los Angeles and environs from much farther north. The idea itself is not implausible.) As we will see in a bit, the space probes which were sent to Mars indeed revealed the presence of some very interesting geological findings -- a huge canyon much larger than the Grand Canyon, enormous extinct volcanoes, what appear to be ancient river beds, and so on. Just so there is no later ambiguity, let me emphasise that these features have essentially nothing in common with what Lowell imagined he was seeing. Essentially all of the features in his maps -- oases, canals, and all -- were in the eye of the beholder. Needless to say, this is a sad legacy. By the way, Lowell's claims about the canals on Mars were hotly contested by many, although not all, of the astronomers and physicists of his day. There were excellent reasons for doubting the reality of the canals: first of all, from the Earth you would not even be able to see a canal on Mars if it were comparable in size to the Suez or Panama Canal. Lowell got around this criticism by claiming that the lines he saw on the surface were in fact broad bands of irrigated vegetation, stretching for many tens of kilometers to each side of the canals. (We see something like this along the Nile, so it is a reasonable defence.) secondly, it was known already that the atmosphere of Mars was very thin, and because of the low Martian gravity one can easily show that the water in any open body -- a lake, a river, a canal -- would quickly evaporate off into the thin air. This is an almost insurmountable objection which Lowell, in effect, merely shrugged off. In the end, Lowell's self-advertisement and popular appeal persuaded the vast majority of lay people, partly because of the excitement he whipped up as a public speaker, even if most scientists were dubious. Of course, the acid test is in what we find when we finally visit Mars, a development which lay many decades in the future. (Lowell did not live to see the development of space flight.)
War of the Worlds: Moral Questions.Given Percival Lowell's widespread and evocative descriptions of a race of creatures on Mars struggling bravely to survive the extremes of a hostile and worsening climate, it is perhaps not surprising that people often think first of Mars when visualizing extraterrestrials - we speak, almost without active thought, of `creatures from Mars.' Lowell's treatment of them was sympathetic, or at least benign; but a strikingly different development came in the form of a novel by H.G. Wells, a British historian and science fiction writer. His story, `War of the Worlds' , imagined the Martians looking on the Earth with envy as a lush abode compared to the dying surface of Mars. They actually invade the Earth, and turn out to be loathsome octopus-like blood-drinking creatures of the most repellent kind. It was for dramatic effect, of course, that Wells chose to depict the invading Martians in the most repulsive way he could. If we were to make contact with a race of extraterrestrial creatures which look like puppy dogs or baby kittens, that would would provoke quite a different initial response from us than we would feel upon meeting a race of creatures which looked, say, like giant tarantulas or scorpions. (I hope, however, that we would eventually get over these initial atavistic responses, at least intellectually.) By the way, a scientific purist might point out that Wells made his story a little less plausible in describing the invading Martians as large rounded creatures as bulky as bears. (Probably he did not know the scientific arguments, or perhaps he considered that the impact provided by his description warranted his taking some liberties.) The point is again one of simple scaling arguments: in the reduced gravity of Mars, creatures which are about the size of us (or a bear) would need little structural rigidity to support themselves, and would likely have evolved to be rather spindly and long-limbed instead of bulky and rounded, although there is nothing that forbids a more bulky form. Still, that is a quibble in what is in fact a very powerful story. The Martians are eventually defeated, after all the resources of mankind fail, by disease, pure and simple: they have no resistance to the strains of bacteria found on the Earth, and die. Historically, you can see a parallel to the decimation of the indigenous people of North America who succumbed to diseases like measles and smallpox after the arrival of the Europeans. H.G. Wells' story, and the real-life North American tragedy, both point to a profound moral. We must be likewise attentive to the possibility that any samples of Martian soil returned to Earth might contain some micro-organism which could devastate life on our planet. (A modern reworking of this theme is to be found in the early novel `The Andromeda Strain' by Michael Crichton, the author of 'Jurassic Park.' ) This is not a danger to be taken lightly when future missions to Mars are planned. There are even speculations, some by the British astronomer Fred Hoyle, that the Earth may be occasionally be bombarded by `viruses from space,' perhaps carried in the tails of comets. Hoyle, who is not a biologist or epidemiologist, has tried to argue that the way in which flu epidemics start and persist can best be understood if the Earth's surface is randomly sprinkled all at once with a lot of viruses introduced in this way. His arguments are not taken very seriously by the experts in the field, however. You may know that considerations of contamination actually influenced part of the American space program. The Apollo 11 astronauts, the first to walk on the moon, were put into a strict quarantine following their return to Earth, just in case they had carried back some dangerous infection. Of course, that might have been a case of ``too little too late,'' in the sense that they had already splashed down into the ocean on their return. Could NASA really have bottled up any dangerous microbes in such circumstances, or would they have inevitably escaped into the biosphere? In any event, I don't know just how seriously the threat was considered, given that we believe the airless moon to be so very sterile. The quarantining of the astronauts was mostly a public-relations exercise. It is, however, just as important to recognize that the argument works in both directions. We will face a moral question of real importance in the not-too-distant future. When we finally go to Mars, we will almost surely eventually infect it with our own micro-organisms. The Viking landers, discussed later, were carefully sterilized before launch and while in space to prevent such an occurrence. But prolonged visits by living creatures like ourselves obviously make it impossible to guarantee the security of the indigenous biosphere of Mars. If there is indeed life on Mars which has sprung up completely independently, we face a very nice moral and ethical dilemma. However simple the life form may be, do we have the right to impose ourselves on it, and almost certainly destroy it in the process? Finally, you may know of the Hallowe'en trick played in 1938 by the actor Orson Welles (no relation to H.G. Wells): he broadcast the story of the invasion of the Earth by Martians over North American radio, and succeeded in persuading many listeners that the story was true. There was quite a lot of panic as a result.
Early Flybys of Mars.Considering the rich history of speculation about life on Mars, it had always seemed an important part of any investigation of Mars to test for its existence. But hopes were a little dashed when the first probes were sent past Mars in the 1960's: the images they sent back showed an apparently desolate planet, more like the moon than anything else. The probes that were sent were simply moving ballistically (coasting freely) as they passed Mars because they did not have rockets and fuel on board to permit them to go into orbit around the planet. (In the 1960's, there were no rockets powerful enough to launch a space probe big enough to carry all that extra mass all the way to Mars.) The probes were near Mars just long enough to take a series of photographs as they swept past; then the information was radioed back to Earth. By bad luck, the photographs showed regions of Mars which were geologically quite uninteresting. They looked very much like parts of the moon: barren, desert-like landscapes, with impact craters here and there on the surface. There was certainly no evidence for an elaborate system of canals (not that anyone seriously expected to see that), but it looked moreover as if the planet had been more or less geologically inactive for a very long time. The probes also revealed the discouraging news that the Martian atmosphere is very thin indeed - considerably less than one one-hundredth of the density of the Earth's. Earlier spectroscopic work from the Earth had correctly revealed the presence of carbon dioxide in the Martian atmosphere, but the working assumption had been that Mars had a lot of nitrogen as well, just as here on Earth - our atmosphere is eighty percent nitrogen - so that the two together would make up a moderately rich atmosphere. (The nitrogen would have been difficult to detect using Earth-based telescopes and equipment, so its presence or absence on Mars was not easily demonstrated.) But the probes revealed that there is no important nitrogen component in the Martian atmosphere, and that the air, almost pure carbon dioxide, is very thin. This seemed to further diminish the prospects for life.
The Orbiters and the Martian Moons.In the early 1970s, NASA managed to put Mariner 9 into orbit around Mars. (It was the first artificial satellite ever to circle another planet.) I told you in class an interesting anecdote about the preparation for this mission. The astronomer Carl Sagan had suggested that the probe be equipped with a bit more ``attitude-contral gas'' (the gas which can be squirted out to make the satellite turn on its axis so that it can be made to face the right way for photographs to be obtained), but his suggestion had been turned down by NASA as a cost-saving measure. When Mariner 9 reached Mars, however, the planet was completely enshrouded in one of its big dust storms, so that nothing could be seen of the surface. NASA took the opportunity to photograph Deimos and Phobos, the tiny moons of Mars. By the time the dust had settled, Mariner might well have run out of the needed gas, since it gradually leaks out; indeed, the predictions were that it should have by that time. But by good fortune this did not happen. Still, it is a remarkable story because the saving in the cost of the gas was minuscule, relative to the cost of the whole project, yet it could have compromised the entire program. There is an interesting sequel to this story, since the photographs taken of the moons settled a rather peculiar suggestion which had been made about their origin. The moons of Mars, Phobos and Deimos (Fear and Panic), are named after the two horses which pull the chariot of Mars, the god of war, in the mythological tales. They are so tiny -- mere kilometers across -- that they were not even discovered until the 1800's. But in the early 1960's, before the space probes were sent to Mars, a suggestion was made by Schklovskii, a Russian astrophysicist, that they might not be moons at all, but perhaps ancient space stations! His reasoning was as follows: observations from the Earth appeared to show that Phobos and Deimos were slowing in their orbits, and might eventually spiral down to crash on the surface of Mars. (It is now realized, by the way, that those observations were wrong, or at least wrongly interpreted.) Schklovskii knew, as you all should by now, that a small body orbiting under only the gravitational influence of a bigger one should not have a decaying orbit - it should orbit essentially unchangingly for aeons - so there must be some other force acting, if the interpretation is correct. The obvious candidate is air resistance. The Martian satellites are indeed fairly close to Mars, and might be moving through the tenuous outer parts of the Martian atmosphere (although the atmosphere turned out to be even more tenuous than Schklovskii thought). The air resistance a body feels depends on how big a cross-sectional area it has (which is why downhill skiiers get into a crouch, to minimize their area as they knife into the wind). This resistance provides a force which will tend to slow the moving object, but a big solid chunk of rock like Phobos or Deimos has so much inertia (mass) that it should not be slowed perceptibly by the predicted tiny amount of air resistance. This seems to rule out that explanation. Schklovskii realised, however, that if the moons were much lighter, the same force might would slow them more noticeably. But how could these orbiting bodies be much lighter than we thought? (Naturally enough, it had been assumed from the start that they were lumps of rock.) Schklovskii's imaginative idea was to suggest that they were hollow! Since no natural process could readily produce a ten-kilometer chunk of hollow rock, he suggested further that they were artificial, built as space stations by a Martian civilization seeking a last safe retreat to allow their species to survive as the planet itself died around them. In this way, we came full circle to the picture of Martians as bravely struggling to carry on in a hostile environment - a return to the style of Percival Lowell. But this too is not correct. The moons are merely moons. (Look at the pictures on page 239 of the text.)
The Martian Surface Unveiled: The First Discoveries.As the dust clouds settled, NASA turned their cameras back to Mars and away from its moons. Right away, some amazing pictures were acquired, pictures which demonstrated that the surface of Mars is very different from the sterile moon-like body which had been implied by the flyby photos. Among other things, Mariner 9 returned pictures which showed signs of recent, or perhaps continuing, geological activity. These features included: Olympus Mons, a great extinct volcano three times the height of the largest mountains on Earth. As I noted in class, Carl Sagan had already calculated, using and the known strengths of materials, that such a mountain could exist in the reduced gravity of Mars: and here it was. Other volcanic cones, not much smaller, were found as well. There is an enormous `rift valley' , now called the Valles Marineris, several thousand kilometers in length. It is so big, in fact, that the whole Grand Canyon could easily sit in one of its side features. If transplanted to the Earth, Valles Marineris would cross the entire North American continent! It is reminiscent of the Rift Valley in East Africa, a geological feature which on Earth is caused by continental drift which is pulling one part of the crust away from another, opening the rift. Valles Marineris is apparently not geologically active now, but presumably has a similar origin. (The Grand Canyon, on the other hand, was sculpted by erosion: it is a river bed.) Even more interesting was the discovery of what looked like sinuous (i.e. winding) river beds, and numerous other features which seemed to have been caused in geologically recent times by the flow of large amounts of water. This is not absolutely clearcut. There are, for instance, a few features rather like this on the moon, features which are interpreted as being caused by the flow of particularly runny, low-viscosity lavas. But the features on Mars are so wide-spread, and so consistent with the behaviour of water, that there seems little doubt about the interpretation. Moreover, there is detectable water vapour in the atmosphere of Mars, and there is certainly some water ice in at least one of the polar caps of Mars (although most of the material is frozen carbon dioxide). Remote study of the polar caps revealed that they consist both of frozen water and of frozen carbon dioxide (``dry ice''). Clearly, there is now at least some water on Mars, although perhaps only a tiny amount, and the topographic and geological evidence seems to suggest that in the past there were large amounts of it actually flowing on the surface. Putting this all together, we deduce that at one time Mars had large amounts of liquid water, and a geology active enough to build huge volcanoes and to drive some incipient plate tectonics (hence the rift valley). The volcanoes in turn might have outgassed sufficient material to provide Mars with a thickish atmosphere, at least for a while (although it would eventually be lost to space, thanks to the planet's weak gravity). Such an atmosphere, rich in carbon dioxide, would have induced a mild greenhouse effect, pushing the temperatures on Mars up to some moderately warm level. In short, Mars might indeed have been a very pleasant place for some primitive life forms to come into existence and flourish. Given that, NASA's plans for the Viking landers due to reach Mars in 1976 were expanded to allow room for some simple - a topic which we will explore in detail in the next section of the notes. First, though, we consider the logistics of the missions which included plans to drop landers onto the Martian surface.
The Viking Landers.There were in fact two independent Viking landers, both of which were wonderfully successful. They were designed to reach the surface of Mars in stages, as follows: In each case, a moderately large capsule, mounted on top of a rocket, is launched towards Mars. On arrival, it is slowed in its trajectory (using retro-rockets - rockets that fire in the forward direction) so that it goes into orbit around the planet. Photographs and radar images of the surface are acquired by the orbiting capsule and studied so that NASA can refine its plans for the best landing site. There is, after all, no merit in dropping the lander into a field of big boulders if you can avoid it! This is not as simple as it sounds: from high orbit, it is very hard to study the ground in enough detail to make the decision clearcut. The limited resolution of the cameras on board, coupled with the fact that you are surveying the planet from many kilometers up, means that you would not, for instance, detect a boulder the size of a car - or a landing area strewn with boulders of such a size. Clearly, dropping the lander safely onto the surface requires some luck as well as good judgement. Next, a piece of the orbiter is detached and slowed (again with rockets) so that it starts a freefall towards the planet. As it enters the atmosphere, friction with the thin air slows it to an extent, but not nearly enough: it is, after all, falling from tens of kilometers up. For this reason, a large parachute is deployed to slow it to a safe landing speed. As the lander nears the ground, but while it is still fairly high, the parachute is ejected so that there is no danger that it will settle over and completely cover the machine when it finally comes to rest. (After all, the lander will need to see the surroundings, to get sunlight onto the solar panels for power, and to be able to aim its radio antenna to send news of its findings back to Earth.) But without the parachute the lander picks up speed again and is soon falling so fast that it might be smashed to pieces on impact. To avoid this fate, small rockets are turned on to slow the descent some more. This is an unwelcome (although enforced) solution, because the exhaust gases being squirted out might consist of organic compounds (if you are burning some simple petroleum fuel in the rocket engine) which will contaminate the soil beneath you and lead to confusing detections of substances in the soil. For this reason, the landers were provided with very special fuels, chosen to minimise any such contamination. Finally, to keep the surface soil as pristine as possible, the engines are shut off early so that the lander falls freely the rest of the way to the ground. The final impact of each lander is fairly vigorous, but is cushioned by shock-absorbing legs. And so they come to the surface of Mars. In the textbook, you will find reproductions of some of the first direct images sent back from the surface of Mars. Interesting measurements were made of the air pressure and temperature, and it was noted that the sky was not blue but pink in colour, thanks to the reddish dust thrown into the atmosphere by the winds. For me, however, the most interesting experiments carried out were the direct searches for evidence of life on Mars. In the lecture, I left you with the following thought. If you had to design a piece of equipment to search for life on Mars, what would it consist of? You might want to send along something like a powerful electron microscope, to search for microbes and viruses in a soil sample, but there is a crucial restriction. Whatever apparatus you send has to be housed in a box about one foot square (i.e. about the size of a recycling `blue box') and must not weigh a great deal if the Viking landers are going to carry it all the way to Mars. This is a real challenge, as you can imagine! In the next section, we will discover how this was accomplished, and learn about the interesting and somewhat perplexing findings which were made. Previous chapter:Next chapter
0: Physics 015: The Course Notes, Fall 2004 1: Opening Remarks: Setting the Scene. 2: The Science of Astronomy: 3: The Importance of Scale: A First Conservation Law. 4: The Dominance of Gravity. 5: Looking Up: 6: The Seasons: 7: The Spin of the Earth: Another Conservation Law. 8: The Earth: Shape, Size, and State of Rotation. 9: The Moon: Shape, Size, Nature. 10: The Relative Distances and Sizes of the Sun and Moon: 11: Further Considerations: Planets and Stars. 12: The Moving Earth: 13: Stellar Parallax: The Astronomical Chicken 14: Greek Cosmology: 15: Stonehenge: 16: The Pyramids: 17: Copernicus Suggests a Heliocentric Cosmology: 18: Tycho Brahe, the Master Observer: 19: Kepler the Mystic. 20: Galileo Provides the Proof: 21: Light: Introductory Remarks. 22: Light as a Wave: 23: Light as Particles. 24: Full Spectrum of Light: 25: Interpreting the Emitted Light: 26: Kirchhoff's Laws and Stellar Spectra. 27: Understanding Kirchhoff's Laws. 28: The Doppler Effect: 29: Astronomical Telescopes: 30: The Great Observatories: 31: Making the Most of Optical Astronomy: 32: Adaptive Optics: Beating the Sky. 33: Radio Astronomy: 34: Observing at Other Wavelengths: 35: Isaac Newton's Physics: 36: Newtonian Gravity Explains It All: 37: Weight: 38: The Success of Newtonian Gravity: 39: The Ultimate Failure of Newtonian Gravity: 40: Tsunamis and Tides: 41: The Organization of the Solar System: 42: Solar System Formation: 43: The Age of the Solar System: 44: Planetary Structure: The Earth. 45: Solar System Leftovers: 46: The Vulnerability of the Earth: 47: Venus: 48: Mars: 49: The Search for Martian Life: 50: Physics 015 - Parallel Readings.
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(Monday, 17 May, 2021.)