PastCast: Carl Sagan uses Galileo to search for signs of life

Kerri Smith

This is the Nature PastCast, each month raiding Nature’s archive and looking at key moments in science. In this show, Nature is concerned with finding extra-terrestrial life.

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Voice of Nature: John Howe

Nature. Volume 365, 21 October 1993.

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David Kaiser

So, in October of 1993, Carl Sagan and several other colleagues published a research article in Nature with, really, a remarkable discovery. They found highly suggestive, perhaps conclusive, evidence for having found life on a planet in the Universe.

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Voice of Nature: John Howe

The Galileo spacecraft found evidence of abundant gaseous oxygen, a widely distributed surface pigment and atmospheric methane in extreme thermodynamic disequilibrium. Moreover, the presence of narrow band, pulsed, amplitude-modulated radio transmission seems uniquely attributable to intelligence.

David Kaiser

My name is David Kaiser. I teach physics and the history of science at the Massachusetts Institute of Technology. Remarkably enough, in the 1990s you could publish an article claiming you’d found life on Earth.

Voice of Nature: John Howe

A search for life on Earth from the Galileo spacecraft.

David Kaiser

Which was en route to Jupiter, but sort of fortuitously because of the particular orbital path that it would have to take to get there, it had to sort of linger relatively close to the Earth. So, it was in space and looking at its near neighbour – the planet Earth.

Charles Cockell

So, this is a quite classic paper that uses the Earth as a test case – can you detect life on the Earth? So, they used the Galileo spacecraft, spun it round, pointed it back to the Earth and asked a very simple question – can we see any signatures of life? And of course, they did.

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Charles Cockell

I’m Charles Cockell and I’m an astrobiologist at the University of Edinburgh. Now, we’re entering into a period of finding Earth-like planets around other stars. It’s obviously an area that should be of interest to biologists about the implications of finding Earth-like planets and looking for life on them.

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Voice of Nature: John Howe

Carl Sagan, W. Reid Thompson, Robert Carlson, Donald Gurnett and Charles Hord. These observations constitute a control experiment for the search for extra-terrestrial life by a modern interplanetary spacecraft.

David Kaiser

Sagan and his colleagues thought it was a great way to test general detection schemes for when they could be sending multiple probes further away from Earth. Could we indeed try to look at other objects in the Solar System, perhaps even some day even further away? And what would we need to know to look for to try to detect signs of life?

Voice of Nature: John Howe

In what follows, we do not assume properties of life otherwise known on Earth, but instead attempt to derive our conclusions from Galileo data and first principles alone.

David Kaiser

They find, just through the instruments on the spacecraft, not by prior knowledge, there is tremendously conclusive evidence that there’s lots of water on this new planet, the planet Earth. Some of the water is frozen – they could even tell that. Some of it was in humongous oceans – they could tell that from the patterns detected by these instruments on the spacecraft.

Charles Cockell

They saw methane and oxygen in the atmosphere which are two unusual gases to have in an atmosphere at the same time without life. That’s quite a strong signature that you’ve got a disequilibrium, a chemical disequilibrium which suggests that there’s something active going on on the planetary surface. It’s not just chemistry. In fact, biology producing oxygen.

Voice of Nature: John Howe

Once candidate disequilibria are identified, alternative explanations must be eliminated. Life is the hypothesis of last resort.

Carl Sagan extract

All my life I’ve wondered about life beyond the Earth. On those countless other planets that we think circle other suns, is their also life? Might the beings of other worlds resemble us, or would they be astonishingly different?

David Kaiser

Sagan was a remarkably multifaceted individual so he began life, his career, as basically an astronomer, astrophysicist, with strong interests in what we might now consider kind of Earth sciences or planetary sciences. So, not only astronomy as in looking through telescopes and seeing what one sees, but trying to understand how geographical formations on Earth might help us understand rocky objects in the sky – really a wide range of scientific interests in their own right. And then of course became a masterful explainer of esoterica to non-specialists. He had certainly a great passion for science policy throughout much of his career, for politics more broadly, and then he also had a successful career as a novelist, as a science fiction author. Right at this time, in fact, the time the paper was published, there were ferocious debates. In fact, now I say it, virtually the same week this was published in October 1993. There were ferocious debates in the US Congress over cutting all funding for a number of scientific projects, including what was called SETI, the Search for Extra-terrestrial Intelligence, with which Sagan had also been associated. This paper was not about SETI – it was about finding any forms of life or evidence for life – but SETI had been really raked over the coals, many historians would say unfairly or inappropriately in the US congress, a kind of political theatre. At a time not unlike today, a very harsh economic climate of hard decisions about priorities and what should get funding, but SETI – it was often called the search for little green men – suffered from the so-called giggle factor. Why is taxpayer money been spent to search for Martians? It was very easy to ridicule, especially in the US Congress. So, I think many people like Sagan had to be especially careful about their pronouncements about life on other planets, intelligent or otherwise, about their reasons to pursue this as real science.

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Voice of Nature: John Howe

During the Galileo fly-by, the plasma wave instrument detected radio signals, plausibly escaping through the nightside ionosphere from ground-based radio transmitters. Of all Galileo science measurements, these signals provide the only indication of intelligent, technological life on Earth.

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David Kaiser

For decades, the game in SETI had been to reason about particular parts of the radio wave spectrum that would be kind of golden areas to search for these unexpected communications.

Frank Drake

I’m Frank Drake. I’m a senior astronomer at the SETI Institute in Mountain View, California. I did my graduate work at Harvard University in the 1950s. I got my PhD there and went immediately on the staff of the National Radio Astronomy Observatory, then being established in Green Bank, West Virginia. When we first started talking about searching for extra-terrestrial intelligence signals, we were confronting a taboo that existed in astronomy at that time, and this had come about through the spurious reports of detections of life on Mars and canals on Mars and all of that, which took place in the early part of the twentieth century. This had tainted the subject, and I, being young, was a little foolish and was not dissuaded by that attitude and proceeded to propose that we search because at that time, for the first time we had instruments sensitive enough to detect the signals we were then transmitting from the nearest stars to the Sun. All the extra-terrestrials had to be was like us and we could detect them.

David Kaiser

Back then it was still really hypothetical. And so, Frank Drake wrote out this equation just to focus discussion actually at an upcoming conference. He was meant to say what are the issues to think about and can we put almost like placeholders, how many planets, what fraction of the planets would have conditions suitable for life, at least as we know it? That’s a smaller fraction than all the planets out there presumably. How many of those planets would the spark of life somehow have happened in ways perhaps what had happened to Earth or otherwise, and so on. And the last factor in the Drake equation was what’s the expected lifetimes of such civilisations if they ever did happen.

Frank Drake

And so, I organised that and I invited all the people in the world that I knew of that were thinking in this area, all twelve of them. Laughs. And we bought them to Green Bank in November of 1961. At that time, there’s only one factor we really knew which was there is a star formation in our galaxy. The other six were ones about which we had almost no observational data and even theories. You have to be optimistic because you have to believe that in fact, the other civilisations do as we do – develop a high technology, exploit it, and in the process, make their presence known. People who work in SETI are very optimistic that when we find other civilisations, they will turn out to be friendly civilisations. In fact, the unfriendly ones have all probably destroyed themselves.

Charles Cockell

Optimism is always a good thing. It drives people forward. There’s a very fine line between being optimistic and framing science in a way that your whole hope is to find extra-terrestrial life or extra-terrestrial intelligence and then when you don’t find it, you see that as a failure of science, which it’s not. But there’s nothing wrong with optimism. You need that to keep going, particularly when you’re trying to test a hypothesis and you haven’t got any data yet, which is the case for the search of extra-terrestrial intelligence. So, certainly, people who dedicate their lives to searching for extra-terrestrial intelligence and that’s their one focus need a lot of optimism. We don’t know what the probability of finding it is but if we did find it, of course, the implications would be huge, so it’s good that there are people doing searches for extra-terrestrial intelligence.

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Charles Cockell

So, I would say that the search for extra-terrestrial intelligence is now a sort of second order question that comes after finding Earth-like planets that might have any life at all. It’s inevitable that to be a species trying to answer the question of life elsewhere, you have to have originated from a planet where there was an origin of life and where there was the evolution of photosynthesis that produced all the organic carbon and oxygen that we use to respire and power our brains and allow us to build planet-searching telescopes. We just don’t know how common that is throughout the rest of the Universe.

David Kaiser

The latest amazing work going on in exoplanet astronomy is on objects of planets far outside our own Solar System, so within our galaxy, but the galaxy is a really big place, and so this is not a place where Earth-based spacecraft are flying by anytime soon.

Charles Cockell

But the expectation is that in the coming decades, maybe in the next two decades, that will change and there will be space telescopes and improved ground-based observations that will allow scientists to look at the atmospheric composition of rocky, Earth-like planets elsewhere, so we’ll be able to test this idea on other planets.

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Charles Cockell

The hypothesis is that there’s life elsewhere in the Universe, and we don’t care what the answer to that is. Certainly, it will be fascinating if the answer was yes, there’s life elsewhere but no life in the rest of the Universe would be a stunning scientific discovery. It just means that the negative outcome tells us something about how special biology is in the places where it does exist.

Voice of Nature: John Howe

Although a great deal more exploration remains to be done, our results are consistent with the hypothesis that widespread biological activity now exists, of all the worlds in this Solar System, only on Earth.

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Kerri Smith

You’ve been listening to the Nature PastCast, produced by me Kerri Smith, with contributions from Frank Drake, David Kaiser, and Charles Cockell. Next time, we go all the way back to Nature’s very first issue in November 1869.