First Detection of Life

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Matt O'Dowd (1:02)

This episode is supported by LegalZoom. In 1990, an experiment conceived by Carl Sagan was performed using the Galileo spacecraft. The purpose? To detect life on a planet based on measurements by a space probe. The experiment was successful, and abundant life was unequivocally confirmed. That planet? The Earth.

Now, a quarter of a century later, we're on the verge of conducting that same experiment on a world orbiting another star.

In December of 1990, during its first gravity-assist flyby, the Jupiter-bound Galileo spacecraft turned its eye towards the Earth. In a plan devised by, among others, Carl Sagan, Galileo would measure the spectrum of Earth's atmosphere, take pictures, and look for radio emission during this brief flyby window. The aim would be to test if such a probe could positively detect life on a world using only data taken from space, and with as few prior assumptions as possible. The data and conclusions from these experiments were published in Nature in 1993 In this paper, Sagan et al. provides a framework for finding life on other worlds. Decades before our technology would allow a similar experiment beyond our solar system.

But that technology has finally arrived.

But first, let's talk about what life on Earth looks like to an observer in space. The easiest and so probably the first way we'll spot alien life is by its effect on this planet's atmosphere, in particular, the chemical content of that atmosphere. So let's look at what Galileo saw. Looking at the spectrum taken with its near infrared spectrometer, you can see these deep dips that result from molecules in Earth's atmosphere absorbing specific wavelengths of light from what would otherwise be the smooth heat glow of the atmosphere. Those dips, absorption features, are undeniable signatures of certain molecules. We see strong absorption from H2O and O2.

Apparently, there's water and oxygen in our atmosphere. That's a relief. Water is essential to life, and oxygen is essential to us, but also an indication of the presence of photosynthesis. Going to longer wavelengths, we see carbon dioxide, nitrous oxide, methane, ozone, and well, more water. While some of these molecules are known to arise from life on Earth, their presence isn't enough to confirm life on another planet. See, an important and necessary condition for claiming a detection of life is a clear departure from thermodynamic equilibrium. That is, the natural chemical balance that an atmosphere should fall into without something weird like life messing it up. It's definitely not enough to find molecules associated with life, like water, carbon dioxide, or methane. These can arise from nonbiotic processes. Geological processes alone can produce lots of interesting chemical reactions. A set of non-equilibrium chemical abundances must be observed that can't possibly be explained without life. As Sagan puts it, life is the hypothesis of last resort. It's never aliens until it's aliens.

As an example, methane in the presence of oxygen is a big giveaway. Given how quickly methane oxidizes into water and carbon dioxide, there should barely be a single methane molecule in an oxygen-rich atmosphere. Yet there is. Impossibly more than the thermodynamic equilibrium abundance. So where is all of that methane coming from? Well, we know that around half of it is from natural biological systems, like methane bacteria. Anthropogenic sources account for the other half, including burning fossil fuels and, as Sagan puts it, flatulence from domesticated ruminants.

Yep, Carl Sagan published a nature paper about how to detect alien cow farts.

Nitrous oxide also appears in high disequilibrium. It's destroyed by solar ultraviolet light in the atmosphere with a half-life of around 50 years. But that means it has to be continuously produced in order to be seen. While there are non-biological ways of making nitrous oxide, like lightning, these aren't nearly enough to account for the amount observed. On Earth, we know it comes from nitrogen-fixing bacteria and algae.