For the September 1972 issue, Popular Science sat down for an interview with Carl Sagan about our rusty neighbor, Mars. For several months before then, NASA spaceprobe Mariner 9 had been sending back thousands of photographs of the planet’s surface which raised more questions than they answered. The man who reminded us we are all made of starstuff dissected some of Mars’ mysteries in his trademark educational and awe-inspiring way. Happy Carl Sagan Day!

The original Q&A is republished in its entirety below.

The Changing Face Of The Red Planet: Close-Up Photos Reveal a Turbulent Mars

By Arthur Fisher

What is Mars like? Until very recently, the best answer, based on the desolate, eroded, crater-strewn vistas viewed by Mariners 4, 6, and 7, was that Mars was a dead planet, a fossil world whose geological activity lay in the remote past, waterless and incapable of sustaining life, more akin to our moon than anything else.

Now Mariner 9 has changed all that. In almost 7000 spectacular photos taken from mid-November to early April and several hundred since June, it has confronted scientists with a dramatically different planet, a turbulet world of super winds and swirling dust storms, towering volcanoes, huge chasms including a gorge far longer, wider, and deeper than the Grand Canyon, chaotic landscapes signaling massive upheavals in the recent geological past. Most astonishing of all: Not only are there many features that seem to have been hewn and molded by torrents of running water; there is evidence that the polar ice caps consist at least partly of frozen water. The question of water-dependent life on Mars is thus alive once again.

For an interpretation of this remarkable turnabout, PS sought a uniquely qualified scientist. Dr. Carl Sagan is Director of the Laboratory for Planetary Studies and Professor of Astronomy at Cornell’s Center for Radiophysics and Space Research. Now on leave at Caltech, he heads the Variable Features Working Group of Mariner 9’s television experiment team. Besides serving on many advisory groups to NASA, he is: Editor of ICARUS, the international journal of solar-system studies; an officer of the planetary commission of the International Astronomical Union; and VP of the working group on the moon and planets of the International space organization COSPAR. The following interview was conducted in Washington, D.C. this spring.

Fisher: Dr. Sagan, was there any singularly dramatic moment during the picture retrieval at Jet Propulsion Laboratory?

Sagan: They were all dramatic. I mean every day you’d arrive and there would be 72 new pictures for you to look at. At least a few of those pictures would show phenomena that you had never seen before and never guessed existed on the planet Mars. It’s a time of extremely high scientific excitement. That time is not yet over.

Fisher: Which of the Mariner 9 picture results have been the most significant to you personally?

Sagan: Well, I think the widespread evidence of dramatic changes due to windblown dust; the compelling evidence for volcanic activity on a massive scale; the evidence suggestive of running water in some not-so-distant time in Martian history; the details of the recession of the polar cap showing that there is a remnant of the south polar cap which just doesn’t go away in southern summer; and the first closeup pictures of Phobos and Deimos, the two moons of Mars. I would say that those are the highlights in my mind.

Panoramic view of Mars stitched from photos taken by Mariner 9 from January to March 1972

Mars equatorial region

Panoramic view of Mars stitched from photos taken by Mariner 9 from January to March 1972


Fisher: What about variable features?

Sagan: Bright and dark markings have been seen on Mars since men began looking at it through a telescope. It was observed, in the middle 19th century, that these markings sometimes seemed to change seasonally. Around 1870, a popular theory had it that the apparent seasonal changes in the Martian bright-and-dark markings were due to vegetation darkening the landscape in spring and summer, when the polar caps, thought to be frozen water, partially melted as the climate warmed. The sort of model people had in the back of their minds was an algal bloom, or perhaps the spring flowering of the arctic tundra in Canada and Siberia. Now, since this 19th century model, whether the changes are truly seasonal has been called into question. But, as we’ll see, widespread changes in the Martian surface have certainly been verified by Mariner 9.

Fisher: What about the so-called canals?

Sagan: They were discovered in 1877 by Giovanni Schiaparelli, who observed, to his surprise, a network of fine straight lines interlacing and covering the surface of Mars. An American diplomat-turned-astronomer named Percival Lowell was quite excited by Schiaparelli’s findings and established an observatory in Flagstaff, Ariz., to pursue such observations. Schiaparelli described what he had seen as canali, Italian for channels or grooves. But Lowell and others mistranslated this word as “canals,” which had a clear implication of design. Lowell believed in a literal canal network that carried liquid water from the melting polar cap to the thirsty inhabitants of the equatorial cities of Mars.

There were a great many books written on this kind of speculation. In English, a set of more than a dozen novels by Edgar Rice Burroughs was based on a gentleman adventurer from Virginia named John Carter, who was able to get to Mars by standing in an open field and wishing hard at the planet. And when he arrived he found it populated by beings of all sorts, including ones who were very human. In Germany, a writer named Kurt Lasswitz wrote a similar piece of romantic fiction called “On Two Planets,” which played a role in helping a very young man named Wernher von Braun develop an avid interest in spaceflight. So, the Lowellian interpretation of what had been seen on Mars turns out to have played a significant role, not because it was right—it’s almost certainly dead wrong—but because it was dramatic and excited many boys and young men to study Mars who have in one way or another been involved in the most recent studies of the planet.

Fisher: Did you receive similar stimulation?

Sagan: Yes, I was similarly intrigued when I read the John Carter stories by Edgar Rice Burroughs. Well, the actual story on the canals appears to be that there are no lines “like those on a fine steel etching,” as Percival Lowell described them. The canals seem to be a sort of psychophysiological rather than an astronomical problem. The eye tends, when the “seeing,” or atmospheric turbulence is pretty bad, to string up disconnected fine detail, because it’s easier to remember a straight line than a patchy, disconnected matrix of blobs. The observation of the best visual observers in the last 50 years has been that they can see canals when the seeing is bad. But as the seeing improves to the best it ever gets, they are unable to resolve the straight lines into disconnected fine detail. So even before Mariner close-up photography of Mars, most astronomers studying the subject were prepared to believe that there was not actually a network of straight lines—exceptionally straight lines crossing the planet, going for thousands of kilometers following great circle routes and so on. And indeed Mariners 4, 6, and 7 found virtually not a trace of anything at all like such canals. Now Mariner 9 is providing the first full coverage of the Martian surface, where everything has been photographed with a resolution of one kilometer except for a little cap at the very north pole.

“People said the Martian environment was too severe for life…an exceedingly provincial conclusion.”

Well, it turns out that there are more or less straight lines on Mars in various cases. But they are not at all as Percival Lowell imagined; they are as Schiaparelli imagined, that is, channels and grooves, although not necessarily in the places that he drew them. Not only does Mars have channels and grooves, it has an enormous rift system like the east African rift system, which is involved in continental drift on the Earth. And the geological significance of such rift valleys on Mars is extremely high.

Photograph taken by Mars Express spacecraft in 2013

Hebes Chasma on Mars

Photograph taken by Mars Express spacecraft in 2013

Fisher: Does that mean these huge valleys are evidence of recent tectonic activity?

Sagan: Yes. The rift valleys tend to surround and come radially out from an extremely high region in the part of Mars called Tharsis, which contains volcanic calderas; very high mountains with holes in the top that have been formed by successive episodes of lava outwelling from the interior. The largest of them are larger than the island of Hawaii, which is the biggest such feature on Earth. We can see from the photos that they are very recent, on a geological time scale, because they are not battered by meteoric craters or worn down by other forms of erosion.

Fisher: Recent on a scale of billions of years?

Sagan: Yes. Whether they are 10 million years old or one million years old, we don’t yet know. This, together with other evidence, shows that Mars is not an ancient, dead planet, but is geologically active in recent times, geologically young in the sense that the Earth is. But let’s get back to variable features.

There were originally two spacecraft intended to be launched to Mars: Mariner 8 and Mariner 9 each with its own mission. Mariner 8 failed and landed in the Caribbean, where it is not even sending back oceanographic data! Since we have only one craft, Mariner 9, we do not have a mission that has been optimized for variable features. Nevertheless, we’ve found them much more easily than we expected. In fact, Mariner 9 has been a spectacular success.

One problem in looking for things changing is that you might look at the same object two weeks apart but under different lighting conditions, and if you forget about that you might think there’s been a real change when, in fact, it’s just a different sun or viewing angle. The spacecraft orbit has been arranged so that we can look at the same area on one-day turnarounds and on 19-day turnarounds, in both cases with the lighting angles all very closely constant. So, if we see any significant changes either a day apart or 19 days apart, then we can have fair confidence that that’s a real change and not a result of lighting conditions.

We have indeed found such variable features; they fall into several different categories. One category is what are called splotches. These are dark markings that weren’t there the last time we looked but are there now. Where they are smaller than craters there’s a certain tendency for them to appear in the inside of the crater. Where they are larger than craters they tend to wash over the craters.

We have some quite striking cases where we look at a given area and see an array of bright and dark features, we come back and look at it two weeks later or so and there are all the old features plus one new dark feature that just wasn’t there before. Then we continue looking at that area on every successive opportunity and only small changes appear. This kind of time scale is absolutely characteristic. In time scales of between several days and two or three weeks, features tend to change on Mars. Characteristic changes that we have been seeing are the appearance of dark features where they previously did not exist.

At the present time the conditions on Mars are certainly not too hostile for life to exist. We must merely keep an open mind until more data is in.

Fisher: Are we always talking about a feature that is at least a kilometer in diameter, something the size of Yankee Stadium, say?

Sagan: Yes. In fact, I’m talking about features that are ten or 15 kilometers in diameter that just suddenly appear. For example, there’s one feature called the Spearhead, because that’s what it looks like. In one picture it’s not there, and in the next picture it’s there, and in all subsequent pictures it just stayed there.

Fisher: What could possibly account for such a performance?

Sagan: I’ve mentioned one possibility, which is more than a century old: namely, that we’re seeing the growth of dark vegetation into a region previously not populated by this vegetation. There is another possibility that my co-workers and I have advocated for the last five years—that we are seeing a manifestation of wind-blown dust. We believe the appearance of a feature like the Spearhead is due to horizontal winds carrying fine bright dust off the surface, revealing underlying dark material. And that the so-called seasonal changes are due to seasonal wind patterns that cover and uncover underlying dark material by windblown bright material. And we can see from the existence of planet-wide storms, at least at certain times on Mars, that dust can easily be carried quite significant distances

Fisher: Would this mechanism account not only for the random appearance and disappearance of blotches, as you describe them, but also for what has been called the progressive wave of darkening?

Sagan: Okay. Now let me say a word about the wave of darkening. It’s been called that because some observers thought this seasonal change had a wavelike progression from the polar cap towards and across the equator at about 35 kilometers a day. Some years ago we did a statistical study to show that it was hardly an invariable wave; it does not work like clockwork. Sometimes an equatorial place darkens long before a polar place darkens. And so I think the phrase is probably a misnomer. To some degree the same kinds of darkenings occur every Martian year. I think that that is due to the repetition of the same wind patterns—which are tied, of course, to the seasons—covering and uncovering the dark stuff.


Fisher: At Jet Propulsion Laboratory last November you gave an analysis of the kinds of wind velocities that would be necessary to raise dust storms on Mars. Could you go into that?

Sagan: Sure. Let me first, before I do that, say something about another kind of variable feature on Mars—tails. Most commonly, tails emanating from craters. There’s a crater, and coming out of it for let’s say 10 or 20 crater diameters is a long, bright or dark tail. If there are other craters nearby, they generally have tails parallel to that first one. There are some cases where we have 30, 40, or 50 tails, all parallel, all emanating from craters, all going in the same direction. We think that at least some or most of the bright streaks represent bright material trapped in the craters that has since been blown out by winds—another piece of evidence for extensive windblown dust on the planet.

The situation is more complex than I’ve indicated because there are also dark tails coming out of craters. And so, are we to imagine two kinds of materials, bright and dark dust, with dark dust settling in some places and bright dust in others? Or is it possible that the dark tails are not tails at all, but wind shadows? Say a big cloud of bright dust comes along and is deposited everywhere except downwind of obstacles. Then, looking at it from Mariner 9, we see a dark streak downwind of the crater wall, not because dark stuff has been blown out of the crater, but because the wall has prevented the bright stuff from being deposited where we see the tail. Some places we see streaks behind—not craters, but small hills showing that there are certainly cases of wind shadowing occurring on the planet. We think there’s a pretty good array of evidence that wind-blown dust is a very important aspect of the Martian environment. We arrive at Mars November 13 and see the entire planet obscured by dust; the dust settles out and then we see on the surface features that are changing, and streaks coming out of craters—both, likely, due to windblown dust.

Mars photo

Streaks on Mars, 2003

Now, what’s necessary to move dust around on Mars? The Martian atmosphere is extremely thin, much thinner than Earth’s. That means that you have to move the air much faster to get something to move forward. It turns out that to move the same-sized grain of sand on Mars that you are moving on the Earth requires winds 10 times faster. If you believe any dust is moved around at all on Mars you have to immediately assume winds…


Fisher: You’re saying, just to pick it off the surface?

Sagan: Just to make it roll over. You have a little grain that is projecting up slightly at some small angle above the Martian surface; the wind comes along and just makes it fall down. How fast does that wind have to be? According to present theory, you need winds of about 80 meters a second just to start dust grains moving on Mars. That’s around 180 miles an hour. It’s a very fast wind.

Fisher: How could such winds arise on Mars?

Sagan: On the Earth, the winds are driven primarily by the fact that the equator is hotter than the pole. Air rises from the hot equator and falls at the cold pole, and that produces a circulation in which the air returns along the ground from pole to equator and circulates aloft from equator to pole. On Mars, the equator-to-pole temperature difference is even larger than on the Earth. There are no seas to moderate temperatures. There’s also a longer year on Mars—687 days. So that temperature difference is larger and the resulting winds are larger. But they are not large enough, as far as we can tell. That’s not the source of the high-velocity winds that stir up the dust storms.

Fisher: I think you’ve said previously that they would contribute a wind of about 40 meters a second.

Sagan: Yes. That seems to be the way the calculation goes. So, if you have that wind going in the right direction and then another wind comes along parallel to it, then the two winds can add and reach your 80 meters a second or more. But there are two other kinds of winds which we think are important on Mars and may be more directly responsible for wind-blown dust. One of them is dust devils. They’re common in the American southwest—whirlwinds only a few meters across which stir up dust. Then that column of circulating dust itself slowly moves across the desert. The conditions on Mars are much more favorable for the production of dust devils than conditions in the American southwest, and it’s possible that the generation of the dust storm that we saw on Mars is produced by sets of such dust devils, even though none has actually been photographed on Mars.

A 12-mile-high twister as seen by NASA's Mars Reconnaissance Orbiter in 2012.

Martian dust devil

A 12-mile-high twister as seen by NASA’s Mars Reconnaissance Orbiter in 2012.

The other source of winds clearly capable of producing dust storms is what we call slope winds. And these occur in cases where the elevation differences are comparable to the characteristic thickness of the planet’s atmosphere—the scale height of the atmosphere, which on Mars is around nine kilometers. Now there are elevation differences on Mars of nine kilometers and in fact, twice nine kilometers. It’s a situation very different from that on Earth, where the elevation differences tend to be quite moderate compared to the atmospheric scale height. As a result, we calculate that on Mars there’s just a brand-new category of wind that runs along these high-elevation slopes. Since Mars has quite striking elevation differences, the wind velocities get to be very high. In fact, 80 or 100 meters a second, all by themselves.


Fisher: What kind of picture of atmospheric conditions on Mars do we get from this analysis?

Sagan: That Mars is a dusty and very windy place, even though the atmosphere is quite thin; temperatures near the equator at noon are very comfortable by human standards, but the temperature at night or very early in the morning is extremely low, maybe 150 Fahrenheit degrees less than room temperature. There’s little oxygen in the atmosphere. There is very little ozone, so ultraviolet light from the sun is not absorbed as it is in our atmosphere and penetrates to the surface relatively unimpeded, if there’s no dust storm. This would pose a serious hazard for exposed organisms of a terrestrial variety.

Putting all those factors together, people in the past said the Martian environment was probably too severe for life. In my view that’s an exceedingly provincial conclusion. We, and others, have done experiments in which we simulate all these conditions in the laboratory. We found that even a wide variety of terrestrial organisms survive those conditions perfectly well. They survive the ultraviolet light when they’re under a tiny fragment of rock, and they even grow during the warm part of the day if there are small quantities of liquid water available in the soil, which is by no means out of the question on Mars today.

Fisher: What kinds of organisms are you talking about?

Sagan: I’m talking about microorganisms: bacteria—spore forming, or non-sporeforming bacteria.

Fisher: But nothing like lichens?

Sagan: Well, lichens are the kind of standby in the speculation about life on Mars because they are supposed to be hardy and all, but they’re not at all hardy under Martian conditions. If there is life on Mars, what’s clear is that—unless we’ve contaminated the planet by not sterilizing our spacecraft—life we find is going to be extremely different from life on the Earth. At least that’s my own belief. Life on Mars will have gone through 4½-billion years of independent biological evolution. There are so many arbitrary branch points in evolution…

Fisher: …that is, 4½-billion years ago…

Sagan: Yes, 4½-billion years ago. Well, life certainly is not arising now. The conditions on Mars today are much too perilous for the origin of life. That requires very protected conditions. But they are not too perilous for the maintenance of life. The origin of life on Mars, just like the origin of life on Earth, must have occurred a great interval of time ago in the past. I mean the origin of life on Earth could not have happened on the Earth, either, if conditions were like today’s—much too hostile. For example, we have an enormous poisonous atmosphere of oxygen which would prevent the origin of life. Mars is in fact better in that respect by not having much of this poison gas. Oxygen oxidizes organic compounds. It’s not a good thing to have around. Because we humans breathe it we think it’s terrific. That’s also a provincial point of view.

The conclusion I’d like to make about life on Mars is that there is certainly no compelling evidence for it, but there is equally certainly no compelling evidence against it. Mariner 9 was not designed to detect it nor has it detected life on Mars. The Viking mission will be the first serious attempt to find life on the planet. At the present time the conditions on Mars are certainly not too hostile for life to exist. We must merely keep an open mind until more data is in.

NASA's Curiosity Mars rover took this photo in 2014

Curiosity Selfie

NASA’s Curiosity Mars rover took this photo in 2014


Fisher: You mentioned water in connection with the possibility of life on Mars. I understand that Mariner 9 photos show some geological features that look as though they had been carved by water. Others show the recession of the south polar cap in a way that seems to suggest there is at least some water ice in the ice caps. What about the problem of water on Mars?

Sagan: Something around 100 years ago it was obvious that the Martian polar caps were made of ordinary ice, water ice. Because, what else could they be? Six years ago it was obvious the Martian polar caps were dry ice, frozen carbon dioxide. Today it’s clear that the situation is much more complex than that. The temperatures in Martian winter are certainly low enough to condense out CO2. We know CO2, is the major constituent of the atmosphere—it has to condense out. On the other hand, we now see in Martian summer that although the south polar cap recedes, a portion remains right through the summer, despite the fact that the temperatures are too high for carbon dioxide to remain in the frozen state. This suggests that the remnant we are looking at is a water cap. And so it’s not terribly astonishing that if you have both water and CO2, on the planet, and the polar temperatures are very low, that you condense out both CO2, and water at the polar cap. Then you heat things up and the one that goes away first is CO2, the one with the higher vapor pressure. Then you’re left in summer with the water cap.

We hope to look at the north polar cap in the summer. We know from ground-based observations that the size of the remnant of the cap is much larger in the north than in the south, indicating much more frozen volatiles (probably water and CO2,) in the northern cap than in the southern cap. In fact it looks as if there may be an immense quantity of frozen CO2, and water in the north polar cap. So much so that if you were able to vaporize all of it you could dramatically increase the total surface pressure on Mars and make it more likely that you could have running water on the planet and, through the greenhouse effect, provide an increase in surface temperatures and make conditions much more clement.

Whether in the course of Martian history there ever are natural events that free that CO2, and water is a question that many of us are debating. I propose that this does happen during the procession of the equinoxes on Mars and that there are epochs in Martian history in which conditions are very different from today, that in fact we are examining Mars in an ice age, and that some 10,000 years from now conditions may be much warmer and much wetter. Under such conditions we can understand the features on Mars that look as if they were carved by running water: things that look like dry arroyos, riverbeds, and very difficult to understand on any other terms. They pose a serious mystery because you cannot have extensive liquid water on the planet today. The pressures in the atmosphere are just not large enough. It’s the same reason you cannot have liquid CO2, on the Earth. You have dry ice and you have gaseous CO2. Now, the characteristic sign of things due probably to running water is tributaries, such as the ones you see in some of the Mariner 9 photos. Tributaries are not produced by flowing lava and pose serious difficulties for being understood in any terms but running water. They are the key to the water hypothesis.

Summer 2000

Mars southern polar cap

Summer 2000

Fisher: Well, what’s the current thinking? Are there any other hypotheses that explain these pictures?

Sagan: There is another hypothesis that goes as follows: I can’t believe that there was ever any liquid water on the planet: Therefore, there is some other cause, which I don’t understand, that produces that phenomenon.

Fisher: You call that hypothesis B?

Sagan: Yes, hypothesis B, brand X.

Fisher: Then as far as you personally are concerned, you think it is likely that the obvious explanation is the correct one?

Sagan: That’s right. I propose on other grounds that such conditions existed before the Mariner 9 photographs were in. I’m naturally attracted to this hypothesis.

Fisher: That would also go a longer way toward fostering the idea that life could have originated at some time in the planet’s past during a previous non-ice age?

Sagan: Yes. It also means that organisms on Mars may now be in hibernation. That is, if the period of time between successive epochs in which there is extensive liquid water, if that time is small in a geological sense, then it may make sense for organisms to just shut up shop for the long winter and wait for the spring to come. A way to test that is to drop Martian soil samples into liquid water, which would be the cue for hibernating organisms or spore formers that the precessional spring has come. At that time, the organisms should go do their stuff.

Fisher: Dr. Sagan, what can we expect next from Mariner 9?

Sagan: If the spacecraft works, we’re going to get just extraordinary pictures during the summer. There’s a great deal of picture processing to be done, which will bring out details that could not be seen before. There will be an enormous amount of interaction among the experiments. I believe that some of the most dramatic scientific payoff is going to come in another year or two, when we’ve done those things. So I don’t think the most exciting aspects of Mariner 9 are in yet at all.

Fisher: What kind of pictures will be taken when Mariner resumes?

Sagan: There will be some pictures taken for Viking landing-site selection. There will be, I hope, a great many pictures taken of variable-feature sites, because now we have a very long time baseline. Many, many months have elapsed. What has changed in that period of time? In fact, in the last few weeks before we went off the air in March things seemed to be changing all over the planet. I’d like to see if that’s continuing. I’m personally very excited to see what signs of variable features there are at end of summer. We’ll have our first look at the north polar cap, which for reasons I mentioned before is very important. We will begin to get some global coverage. Up to now we’ve been very close, just seeing little postage-stamp-size frames, and we’ve been mosaicking them together. There are a great many very exciting things coming up and I am just talking about TV, not the thousands of spectra and other very important data acquired by Mariner 9.

Fisher: Thank you, Dr. Sagan.