Mars, Old and New: A Personal View by Bruce Murray
Posted by Jennifer Vaughn
03-09-2013 18:07 CDT
This article was originally published in the July/August 2001 issue of The Planetary Report.
In early June , Planetary Report editor Charlene Anderson and I sat down with our resident expert, Planetary Society president and cofounder Bruce C. Murray, to talk about how his view of Mars has evolved over nearly four decades of exploration. Murray has been a key player in the exploration of Mars since the first American mission to the planet: the Mariner 4 flyby in 1965. Since then, he has been part of nearly every mission to the Red Planet, both American and Russian. Our two-hour exchange produced plenty of insights, explanations, and personal stories.
The Planetary Report: Clearly, a "New Mars" seems to be emerging from Mars Global Surveyor [MGS] data. How have your views of Mars changed?
Bruce C. Murray: Mars Global Surveyor has, for me, renamed Mars the land of broken paradigms. MGS not only produced stunning results but also revealed that we can't explain them, as we thought we would be able to.
When you look at Mars up close, you don't see the signatures of what you're expecting to see. Instead you see the signatures of something you don't recognize. I'll give you an example. Everybody had expected the planet's south polar layered terrains, where Mars Polar Lander was to touch down, would be smooth. They're smooth on the scale that we could see from earlier spacecraft. They seemed smooth in radar observations. So you can imagine our surprise when the first Mars Orbiter Camera [MOC] pictures showed a granulated, ridged surface. We don't know what causes these ridges, and there is no terrestrial analogy I can think of to try to explain them. Evidently there is some process occurring on the polar regions of Mars that we don't understand.
On the Moon's surface there are very few morphological features that are not easily attributable to impact. The Moon has a debris layer increasing at about 1 meter in thickness per billion years -- so a 3-billion-year-old lunar mare [volcanic plain] has a debris layer about 3 meters thick. We know that because you can see the layering in the impact craters. Astonishingly, Mars lacks a debris layer.
Back in 1997, Pathfinder landed on Mars, looking for the debris from ancient floods. At the time I thought this was a dumb idea, because any such debris would be covered up by regolith. [Regolith is a layer of surface material fractured and pulverized by repeated impacts.] But there were the targeted sedimentary remnants from giant floods that occurred billions of years ago, lying on the surface as if they were produced yesterday. Now that's astonishing. An explanation at the time was "Well, there probably was a sand dune protecting the area that moved off later." But here comes MGS and especially Mike Malin's MOC images that show us many, many parts of the planet likewise with no regolith.
Along with the lack of regolith, Mars is deficient in small craters. It is as though something has been operating to either protect the surface or scrape it clean, but we don't know what that something is. And the explanation of protective sand dunes that come and go doesn't work for the whole planet. So what's the answer?
Allow me to mention indications of another broken paradigm. Because MGS suffered a solar panel problem, aerobraking maneuvers were unable to be performed as planned. An extra year was spent going from an eccentric, loose orbit to a tight circular orbit. During this period, the spacecraft came very close to Mars' surface -- less than 150 kilometers [90 miles] above the surface instead of up higher, as originally intended. Why is this important? Well, mostly because the onboard magnetometer and electrometer obtained some intriguing data results.
The magnetometer measures how strong the magnetic field is and in which direction it appears to be pointing. The electrometer measures the direction in which the electrons, the charged particles, are coming into the system. The reason that's important is you perceive not only the magnetic field at the elevation you're measuring but also the electron path, so you can infer where the electrons came from and, to some extent, reconstruct the field down below. But this is only possible when you're below the ionosphere. Above the ionosphere, the electrons can't get through. The aerobraking maneuver brought the instruments below the ionosphere long enough to map the magnetic field.
The data were great -- much better than anyone expected -- but all of a sudden we discover there are huge anomalies on Mars' surface. We see crustal magnetism [magnetic fields from solid rocks in the crust; most of Earth's magnetic field originates much deeper and is due to molten rock in the core], originating in the upper, say 200 or 300 kilometers [120 to 190 miles], and it's bewildering. First of all, because the anomalies are so large -- ten to a hundred times the size of crustal rock anomalies on the Earth. And on Mars we have no clue what causes them. Second, the anomalies are mainly in the southern hemisphere, not in the north. That's a clue to something if only we are smart enough to figure it out. And then there's Hellas -- a huge basin, almost 2,000 kilometers [1,250 miles] across, with no anomalies. One interpretation is that Hellas is younger than the phenomena that created the anomalies. Perhaps when the Hellas basin formed, the shock and heat demagnetized the crust in that area. The problem is that Hellas has got to be close to 4 billion years old, meaning these darn anomalies have survived for a very long time. This is a major mystery.
Persistence Pays Off
Mariner 4 was an extraordinary accomplishment. Those 21 tiny little framelets -- each 200 by 200 pixels -- that Mariner 4 delivered revolutionized the view that Mars was somehow like the Earth. Mars has great big craters, 300 kilometers [185 miles] wide, which had to have formed at the same time the Moon formed similar craters, roughly 4 billion years ago. Mariner 4 also discovered that Mars has no appreciable atmosphere, which was a big surprise. With no appreciable atmosphere, you can't have liquid water. Talk about shattering paradigms.
After Mariner 4 I participated in Mariner 6 and 7 in 1969; on Mariner 9 I headed the polar studies group. It wasn't until 1985 that I came back to Mars. [During this apparent break from Mars, Bruce served as director of the Jet Propulsion Laboratory and oversaw the Viking missions.] I was selected as a participating scientist on Mars Observer, which was scheduled to reach Mars in August 1993. And, of course, Mars Observer failed.
In the meantime I had become involved in a similar capacity with the television cameras on the Soviets' Phobos 1 and 2. Phobos 1 failed shortly after launch, and Phobos 2 got into Mars orbit, in a very clever mission co-orbiting Phobos itself, before it failed.
I then joined the Mars '96 mission -- the Russians' Battlestar Galactica, with penetrators and small stations and radar antennas and everything. Unfortunately the upper stage of the Proton launcher failed, and the spacecraft ended up in the Pacific Ocean or the jungles of Bolivia, depending whom you ask.
We got little out of Phobos, and nothing out of Mars '96 or Mars Observer. So to some extent you could say I'm a jinx. But it gets worse.
I became involved with Mars Polar Lander because I knew something about the polar regions. In the meantime I had a hand in developing the Deep Space 2 probes. I was also selected as a participating scientist on Mars Climate Orbiter. Well, they all all failed! Incidentally, I had no connection whatsoever with Pathfinder, which succeeded brilliantly.
But because of my work on Mars Observer, I got grandfathered into being a participating scientist on Mars Global Surveyor. That permitted me to develop the view that we're in the midst of another paradigm shift about Mars -- in many ways as big a paradigm shift as Mariner 4. MGS offered such an extraordinary view of Mars, not just the camera, but also the Mars Orbiter Laser Altimeter Altimeter and Thermal Emission Spectrometer data. The long life of the orbit, the mapping . . . it was just incredible. It still is incredible.
So the reward for this long tour down the "shadow of the valley of death," or whatever you want to call it, is that we were finally able to get good data, and lots of it. -- BCM
TPR: How has Mars Orbiter Laser Altimeter [MOLA] data added to the new view of Mars?
BCM: What MOLA has done is equivalent to mapping the topography of all the land areas of the Earth to 1-meter precision [i.e., relative to a nearby point] and 10 meters absolute [i.e., accuracy of all data, for example, relative to a fixed point like the center of the Earth]. Plus, MOLA is always taking data, day and night, every orbit. So what results is an incredible set of data that are just beginning to be explored. We are using the topographic data at Caltech [the California Institute of Technology] right now to find new craters, believe it or not, that are not visible on the Mariner, Viking, or even the MOC wide-angle camera images.
What has been discovered? For one thing, the Hellas basin is much deeper than we thought. But if it's 4 billion years old, as we calculate, how could it have stayed empty so long? Valles Marineris is also deeper in places than we imagined.
But probably most surprising is that the north polar basin, which is a huge area, is not only, as had been expected, very low, but it's extremely smooth. So smooth, many people are saying, "Hey, there must have been a standing body of water here." Shorelines I'm not so sure of, but I do think this was the place where liquid water on Mars was directed and accumulated. The southern hemisphere is much higher than the north, and the ancient flood features that we see all point from south to north, so the destination of water must have been the north polar basins. So it's now pretty obvious there was a lot of water on Mars, though we still have no clear explanation for what happened to it or how it ever could have existed in the first place.
TPR: Do you believe it's possible that Mars had a warm, wet, maybe even Earth-like period?
BCM: I don't see any evidence that an Earth-like environment ever existed, at least not in the last 4 billion years. While there was a lot of water on the surface, I think it was ice covered. Here on Earth there are ice-covered lakes in the Antarctic, and the Arctic Ocean is ice covered much of the year. So ice-covered oceans or ice-covered bodies of water on Mars make a lot of sense. Whether there was any potential life habitat, who knows?
Since Mariner 4 we've known that Mars has big craters that are very old, though they've not been eroded away entirely. They've got to be 3 to 4 billion years old, and the smaller ones look just like lunar craters, bowl-shaped with sharp edges. Meteor Crater in Arizona is only 20,000 years old, and it already has had a lake at the bottom and suffered eroded sides. But these craters on Mars look all fresh and new. So the crater evidence says Mars was never Earth-like. But, on the other hand, you see these giant flood features, so clearly water was present at times. Personally, I think you resolve the dilemma by saying ice covered any standing water existing on Mars' surface. Now that still doesn't explain where the water went -- there's not enough room up in the polar regions for all that water to exist there now as ice layers. So plenty of mystery remains.
TPR: What about the hematite that the Thermal Emission Spectrometer [TES] detected? [Hematite is a form of iron oxide that usually forms only when in contact with liquid water.] Isn't that an indication of a warmer, wetter period?
BCM: TES has had a hard time penetrating the atmosphere to pick out these very subtle mineral signatures on the surface. Remember, TES doesn't use reflected sunlight the way an ordinary spectrometer does -- instead it uses emitted thermal radiation. It senses the thermal radiation and then tries to get a spectral [emission] signature from that. This is hard enough to do on the Moon, where you have no atmosphere. When you have a carbon dioxide atmosphere with dust and water vapor, it gets even more difficult. So the TES team has worked hard to construct atmospheric emission and transmission models. And they have found some things. For one, the dark areas, presumed to be lava, in the northern hemisphere are more silica-rich than places in the southern hemisphere that we think are also lava.
That's big news because, on Earth, you don't get things that silica-rich without plate tectonics, and we have strong evidence that plate tectonics never happened on Mars. So some other process has been involved on Mars, which, once again, we have no idea about, some unearthly process that leads to geological and chemical differentiation. Another crustal mystery that we had no way of anticipating.
But getting back to hematite -- the crystalline form of rust -- it forms on Earth all the time, on ships, in pipes, and so on. It results from the natural interaction between any kind of iron-rich material, over time, with water and oxygen. What's surprising is to get a report of detection on Mars, and not just a little, but 300 kilometers' worth over a flat, oval-shaped area. What could possibly cause this? Did it precipitate out of a lake?
The detection of hematite is the only evidence we have on Mars of what we would call chemical weathering. After all our hopes for finding clays or calcium carbonates [which on Earth are usually formed in water], what we find instead is unweathered feldspar and pyroxene. Feldspar, which is a very common igneous mineral on Earth, weathers very quickly. You can get a piece of feldspar from a Hawaiian lava flow, and although it may only be a few decades old, you can already see the weathering. Add the combination of water and warmth, and boom, you've got clay. It happens in situ right there on the surface.
On Mars, what is being reported from the TES experiment is feldspar, not clay. It's astounding that you would get a signature that shows feldspar with no weathering. This stuff is 2 billion years old. That means there has been no significant water moisture available. So the same instrument, TES, detected hematite, which needs moisture to form, and also feldspar, which wouldn't exist if moisture were present. While there are those who still argue for a warm, wet period on Mars, to me, it's another mystery, another broken paradigm.
Breaking Paradigms Is Hard To Do
In 1955 I did my Ph.D. dissertation on the Lower Mississippian-Horton sedimentary layers in Nova Scotia. They include a rare sedimentary unit up there, an oil shale. There's no other unit like it in North America. But there is one at Spitzburgen in Norway and also one in Greenland, and I had read about them.
Then, while doing my fieldwork, I found a Ganoid fish fossil, which is like a sturgeon, with thick scales -- an early, primitive kind of fish. Apparently it had an air-breathing bladder. It lived in water so foul that it had to surface periodically and gulp the air.
Remember, this was before continental drift was accepted. So I said to myself, there's no way this fish could have swum across the ocean, between Spitzburgen and Nova Scotia, yet the fossils were found in both places.
Other people had encountered similar evidence suggesting the continents had once been joined, going back to the German paleontologist [Alfred] Wegener. But my colleagues in geophysics at the Massachusetts Institute of Technology, where I was a graduate student in geology, questioned how a continent could "slide." They stated that continental drift could not happen, because too much friction would be generated at the base of the crust. Of course, we didn't know then about the low-velocity layer, a peculiar situation that now helps explain the phenomenon. Most important, this was shortly before magnetic "striping" was found, which finally made people realize the seafloor was expanding.
Still, I had the evidence right in front of me. I had asked myself the right question, but I backed away from it because it challenged the conventional paradigm. That's how it is with Mars. A lot of paradigms are still going unchallenged. Some that we now know are wrong have been broken, but there are other problems we haven't yet been imaginative enough to puzzle out. That's what makes for a very exciting time, when we move from the broken paradigm phase to the new paradigm phase. -- BCM
TPR: Describe your view of the future of Mars exploration.
BCM: I liken exploring Mars to the historic exploration of the Antarctic -- however, the South Pole is a far nicer place to be than any place on Mars. Exploration is of course geographic, and it's about discovering things we didn't know were there. In my view, the first exploration of Mars was telescopic, and that, in Antarctic thinking, would be at best like Captain [James] Cook skirting around the edge of the icy continent and recognizing there was a land mass out there. The next phase began with Mariner 4, the start of very primitive robotic exploration, and continued on through MGS and the next decade of missions. This compares to the whalers reaching the Antarctic shore for the first time, the first people building a hut at McMurdo station, and then from there organizing expeditions to the interior. They did not initially have map coverage of the whole place. It wasn't until after World War II that the United States possessed affordable technology to fly over Antarctica and survey the continent from the air. Now it's done from satellites.
The first human occupancy in Antarctica followed in 1976. So, in the case of Antarctica, you can probably count 80 to 85 years of exploration before human occupancy begins. If you start with Mariner 4 back in 1965 as the beginning of Mars exploration, by scale, it wouldn't surprise me if it were at least 2030 before humans reach the surface of the planet.
So this endeavor, in which we are all involved, and which I have allegiance to, started before I came on the scene and will continue long after I leave. I think historians writing in 2201 will probably characterize the exploration of Mars as an effort that started in the 1960s, driven by Cold War rivalry, which developed into progressively greater robotic missions and landers, then established outposts -- robotic first, and eventually a permanent human base.
Right now, in 2001, the best thing we could do is more real exploration -- to maximize the chance to find things we couldn't imagine. That, in the case of Mars, means something like a Discovery Program for Mars. NASA just announced what is being called the Scout mission, planned for 2007. Already roughly 50 different groups have submitted study proposals, only one of which, unfortunately, will be chosen to fly. And currently there are no plans for another Scout mission, ever.
The good news is that, until the failure of Mars Polar Lander in 1999, there were no plans for any kind of exploratory mission like Scout at all. The overriding thrust was toward sample return and technological development. Now the balance has shifted. But I was hoping it would shift more. I was hoping there would be one Scout mission every other launch opportunity -- that is, every four years indefinitely. But at this point we know of only one shot. Now it's apparent from the number of groups that have shown interest in Scout there's a mismatch between scientific interest and NASA flight availability.
TPR: You mentioned the idea of Mars Outposts, which we featured in our last issue. Would you talk a little about your view of an outpost program?
BCM: An outpost phase in exploring Mars would alleviate the disconnect between those who want a human mission and those who consider that a threat to robotic exploration. We need both. We need the dream of human exploration and we need the practical manifestation of that dream. My view of exploring Mars is not Tom Paine's [former administrator of NASA and Planetary Society Board memberuntil his death in 1991] -- I don't see humans in space suits lowering themselves by rope down the sides of Valles Marineris. I see an advanced symbiosis of humans and machines. The outpost plan permits that to happen.
This goes back to my point that we're in an exploratory process very much paralleling that of the Antarctic. What's different is not the human factor. There's no astronaut any more resourceful or braver now than was Ernest Shackleton. The machines are what are getting better -- especially information technology. That's what's going to make the effort affordable, but it requires that future human explorers of Mars fall in love with machines.
TPR: Did we miss anything? Any more broken paradigms come to mind?
BCM: I've lost track of how many broken paradigms I've discussed here! A lot. Anyway, with MGS, we have enormously more data, and better data, and better coverage than we've ever had, and yet we have less knowledge. Now how can that be?
It's because the knowledge we thought we had was wrong. We were deceived, and I am guilty as charged. I'm probably a coconspirator in the process of misjudging what we've been seeing. I didn't grasp how complex the processes are that operate on Mars -- I was thinking in too simplistic terms. I was, however, in good company.
MGS is an incredibly elegant mission, it really is. The data set from MGS will probably be the data set on Mars for a full 30 years. But NASA's current Mars program is based on objectives that were in place before MGS.
We are very early in the exploratory process, much earlier than we even realized. That means exploration is the thing we should be doing, and a lot more of it.
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