The year was 1936. The first episode of The Green Hornet was heard on WXYZ radio in Detroit. The first radioactive element was produced synthetically. Adolf Hitler announced the first Volkswagen Beetle®. And Norman Horowitz arrived at Caltech in Pasadena, California. It was the beginning of an auspicious career at both Caltech and the Jet Propulsion Laboratory. He was a biologist by training, but his eyes was trained on the stars…and in particular, the planet Mars.
“By 1959, it was definite that the Jet Propulsion Laboratory was going to be a planetary science lab, and people began coming down [to Caltech] from JPL to see if there was any interest here in planetary exploration.
“I thought [life on another planet] was a plausible idea. Everything that was known about Mars at that time later turned out to be wrong, but [at the time] suggested that there was a good possibility of life on Mars. I had a choice of going into something…taking this golden opportunity to get involved in a new program. And that's what I did. It turned out to be very exciting. Of course, we didn't find life on Mars, but I'm glad I did it.”1
Horowitz made a decision then and there that would affect not just his life, but the entire search for life on Mars. His move to JPL placed him in the Center for Planetary Exploration, where he would become one of the lead members of the Viking life-sciences team.
“The exploration of Mars became the key idea for a planetary program, for obvious reasons, and JPL set up a bio-sciences section to plan for the biological exploration of Mars, with an eventual lander. They asked me to come up and be chief of their section, which I did in 1965. There was a lot of work going on up there in trying to design instruments to fly to Mars for a biological search, and I got involved in that planning. Two of the instruments that eventually flew on Viking came out of that group. The Gas Chromatograph/Mass Spectrometer, which was probably the most important single instrument on the lander, was designed at JPL.
“When I went up there, that was already in process—it had been anticipated that this would be a useful instrument to have on Mars. What I did get involved with in connection with that instrument was making sure that there was a lot of ground-based experience with it. The instrument is based on empirical patterns of breakdown of organic compounds. You take an organic compound and you heat it until it pyrolizes—it breaks into smaller fragments due to the heating. These fragments can be identified by a combination of analytical steps called gas chromatography and then mass spectrometry. The only thing you have to identify the original compound you started with is the pattern of its breakdown products, and you try to infer the nature of the original compound from these breakdown products. There's not much general principle or general theory you can go on; you just have to have a library of results you can compare your actual results with. We did a lot of that during the years that I was there.”
And this was the key to the search for life on Mars—trying to find a way to identify the building blocks of life by remote observation. To do this, Horowitz's team would have to build up a large database of similar reactions working here on Earth. It was not a trip to Mars, for which many of them would have gladly gambled their lives, but was the next best thing to going there.
“Another thing I did was to get the idea for the second biological instrument that JPL had on the Viking lander. NASA called it the pyrolitic release experiment; we used to call it the carbon assimilation experiment. It was an experiment that I developed with two collaborators, George Hobby and Jerry Hubbard. The point of this experiment was to carry out a biological test on Mars under actual Martian conditions. It's hard to convey in a few words the total commitment people had in those days to an Earth-like Mars. This was an inheritance from Percival Lowell. It's amazing: in pre-Sputnik 1 days, in fact, up till 1963, well into the space age, people were still confirming results that Lowell had obtained, totally erroneous results. It's simply bizarre!”
And that was the challenge. Horowitz knew by the time he moved to JPL that Mars was not Earth-like in ways that counted toward supporting life, but sometimes he felt that he had trouble getting others to understand it. Oh, they might pay lip service to the thin atmosphere, the extreme temperatures, and the voluminous solar radiation, but living deep in their hearts was a very different image of the Red Planet.
“A lot of people thought Venus was covered by an ocean. But that was speculative; in the case of Mars, they were making measurements and coming up with the wrong answers. Measurements were made on the 200-inch telescope by…a well-known astronomer—and they were completely wrong. This is just one example. And this was all based on the desire of people to believe that Mars was an Earth-like planet. It wasn't until 1963 that this began to unravel; the first step in the de-Lowellization of Mars occurred in 1963.
“[That] was one infrared photograph taken at Mount Wilson. It was an unusually excellent photograph, showing the infrared spectrum of Mars. It must have been a very dry night above Mount Wilson, a very calm night. They got this marvelous single plate, and it was interpreted by Lew Kaplan, who was at JPL, and Guido Munch, who was a professor of astronomy here…and Hyron Spinrad, a young postdoc working on Mount Wilson at the time. They showed, first of all, the total atmospheric pressure on Mars….”
But even the coldest scientific data must meet with an emotionally charged challenge when presented to the broader community, and history had something to say about the subject: “Back around 1900 Lowell had estimated [Mars's atmospheric pressure to be] 85 millibars…so when the space program started, it was generally accepted that the surface pressure on Mars was 85 millibars, and that carbon dioxide was a small fraction of this; the rest of it was assumed to be mostly nitrogen, as on the Earth.
“So at least [life] was plausible. The Martian environment appeared to be Earth-like, but a very cold and dry Earth-like environment, an extreme form…with all the same elements, with water available and enough pressure so that liquid water could exist at least transiently on the surface. This was a difficult point, to get enough liquid water to support life. With 85 millibars, there was a possibility that you could have liquified water, at least for part of the day.”
But the soon-to-be-infamous Mount Wilson data showed something very different.
“[Kaplan, Munch, and Spinrad's] findings showed that the surface pressure could not be 85 millibars. It looked more like 25 millibars to them. They also identified water vapor in the spectrum; that had never been seen before. They found very little water. And it was obvious that carbon dioxide was a big portion of the atmosphere and not a minor portion.
“Well, this turned out just to be the first step. The next big step came in 1965, when Mariner 4 flew by Mars and found that the surface pressure was more like 6 millibars! And that is the average pressure. And carbon dioxide is the principle gas in the atmosphere. Well, with 6 millibars, there's virtually no chance of having any liquid water.”
By now the great Martian cities and canals and pumping stations of Lowell and others had died a quick and merciful death at the hands of Mariner 4. But there was still a chance for something smaller, more simple, and more realistic: “There was [still a possibility for life]. The main point up until Viking was water. And there were enough theoretical mechanisms for getting some water of the surface of Mars to maintain the remote possibility—although by the time we launched Viking, it was very remote—that there were either pools of brine or, after snow or frost there might be enough meltwater at sunrise to sustain a population of microorganisms…. [T]he real interest was in the possibility of having microbial life.”
But, though the discoveries spoke loud enough for Horowitz and others like him to hear and adapt, it was a challenge to change the thought patterns of the broader scientific community.
“In spite of all these new discoveries, people were still building instruments to fly to Mars that were based on the terrestrial environment, and they were eventually approved by NASA. NASA was supporting these efforts. Around 1960, I got involved in one of them, one that actually later flew on Viking. We called it Gulliver at the time. It was invented by an engineer in Washington, named Gilbert Levin. It depended on an aqueous medium. Two other experiments that were being supported by NASA also involved aqueous solutions into which you would put the Martian soil and then use various ways of measuring the metabolism of the organisms. But after 1965, after the Mariner 4 flyby, it was obvious that the chance of liquid water on Mars was so remote that one had to plan for the contingency that there was no water—that if there was any life on Mars, it was living under conditions that were in no way terrestrial. So we designed an experiment that would work under Martian conditions and that involved no liquid water.”
Old notions die hard, and the persistent idea of some kind of earthly life on Mars was no exception: “I think most of [this] was [because] people didn't want to give up the idea. And I agreed that, now that we had the capability, we would never solve the problem by just looking at Mars from the Earth. This was a classical problem, part of Western culture, the idea of life on Mars has been around for three hundred years. And here was the first time we had the ability to test it.
“Mariner 9 found an objective argument for flying to Mars, because [it] saw that Mars once had water on it. There are dry streambeds, obviously cut by water. All the geologists agree that they're water cut; there was water on Mars at one time. And you could say that, if there was water on Mars, then there may have been an origin of life, and that life may still be surviving. Now Mariner 9 was an orbiter…and up to that point, up to the time Mariner 9 took its photographs, I would have said the a priori probability of life on Mars was close to zero. It would have really been an irrational act to fly to Mars before 1971 to look for life.”
But, as with his life-science experiments onboard Viking, studies on Earth would prove a valuable precursor to experiments on Mars. And few places on Earth were as close to the Martian environs as Antarctica.
“Another important thing I initiated at JPL [were] studies in the Antarctic. I never went to the Antarctic myself, but there was a microbiologist at JPL named Roy Cameron who studied microbial life of the world's deserts—he was traveling all the time. Just before I went up to JPL, I read a report of biological work that had been done in the Antarctic during the International Geophysical Year, around ’58…. [T]here are areas called the dry valleys, actually ice-free areas. A team of microbiologists…got in there during the International Geophysical Year and they found that a lot of their soil samples were sterile; they couldn't find any bacteria. These dry areas are as Mars-like as you can find on the Earth. They're very cold and they're very dry. Roy brought back tons of soil…. [These samples were] used for a long time as standards during the testing of the Viking instruments.”
But after all the things Horowitz has experienced and done, studying the most minute organisms on one world and seeking them on another, he still has a broad and revealing global perspective.
“I think that Mars exploration is quite important. If we are the only inhabited planet in the solar system, and there's only one form of life on Earth—I mean, when you look at the composition of living creatures and see that they all have the same genetic system and they all operate on DNA and proteins composed of the same amino acids with the same genetic code…then we're all related. The origin of life may have happened only once, and it happened here and no place else in the solar system. Or if it happened elsewhere, it didn't survive. I think this is a conclusion of really cosmic importance. If people become aware of this, then maybe they'll be less inclined to destroy the planet.”
The exploration of Mars may indeed serve many functions. Let's hope this is one of them.