Mars and the Birth of Gaia

A substantial legacy of Viking is that scientists have gained a better sense of the finely tuned ballet of biogeochemical cycles that sus­tain Earth’s vibrant biosphere. The Viking missions unexpectedly became integral to the late-twentieth-century view of the Earth’s biosphere as a self-sustaining system produced by biota interacting with the planet’s geochemistry. Brian Skinner and Barbara Murck assert that future historians will consider discovery of the com­plex interactions between Earth’s biota and geologic, hydrologic, and atmospheric cycles among the most significant scientific con­tributions of the twentieth century.55 This sea change in thinking about our planet began to emerge in the 1960s and was sparked by British scientist James Lovelock’s Gaia hypothesis. That idea later

evolved into Gaia Theory and the academic discipline referred to as Earth System Science.

In the early 1960s, Lovelock was working for NASA’s Jet Pro­pulsion Laboratory with other planetary scientists to develop the experimental means for determining whether microbial life might exist on Mars. NASA’s plan for a mission to robotically explore Mars in search of life was initially titled Voyager, not to be con­fused with the interplanetary mission launched in 1977. That plan was subsequently scrubbed and reconfigured into what became the Viking mission. In 1965, while helping to develop life detection experiments for the Mars landers, Lovelock came to the sudden realization that Earth’s atmosphere must be a natural extension, and a by-product, of Earth’s biota. This became the basis for the Gaia hypothesis and a paper Lovelock published in the prestigious journal Nature that year.56

In his first book, Gaia: A New Look at Life on Earth (1979), Lovelock, then sixty years old, explained that as he mulled over ways one could detect organisms in the Martian soil, he turned to our own planet and began to imaginatively “look at the Earth’s atmosphere from the top down, from space.” In the opening sen­tence, Lovelock observed: “As I write, two Viking spacecraft are circling our fellow planet Mars, awaiting landfall instructions from the Earth. Their mission is to search for life, or evidence of life, now or long ago. This book also is about a search for life.”57 The organisms on Lovelock’s mind, however, were those on Earth.

Lovelock thought it might be possible to answer the question of whether Mars harbored microbes by simply examining the com­position of the atmosphere. If a planet supported life, Lovelock posited, its atmosphere would be shaped in part by biota “bound to use the fluid media—oceans, atmosphere, or both—as conveyer belts for raw materials and waste products. . . . The atmosphere of a life-bearing planet would thus become recognizably different from a dead planet.”58 A mix of reactive atmospheric gases like ox­ygen and methane, Lovelock surmised, would be a biosignature of life as on Earth. Moreover, in 1965, researchers at the Pic du Midi Observatory in France reported that the atmospheres of Venus and Mars were largely made of carbon dioxide.59 Lovelock knew that Earth’s atmosphere, by comparison, contained reactive gases that dissipate if not continuously replenished by Earth’s biota. The Pic du Midi observations seemed a sure confirmation to Lovelock, who was then collaborating and publishing with NASA colleague Dian Hitchcock on analyses of infrared surveys of the Martian atmosphere.60 Methane in our atmosphere “has been fairly con­stant as ice-core analyses prove, for the past million years, as has oxygen,” notes Lovelock, who highlights the fact that “for such constancy to happen by chance is infinitely improbable” and there­fore must be sustained by life.61 While Lovelock set the wheels in motion for a systems approach to understanding the biosphere, it was his collaboration with microbiologist Lynn Margulis that formalized the Gaia hypothesis.

Margulis was just then developing her theory of symbiogen – esis, which posits that cell structures, and ultimately organisms, evolved from symbiotic relationships between progenitor cells or organisms. She is celebrated for her research in early cell evolution and was first to identify bacteria as the antecedent of chloroplasts and mitochondria in eukaryotic cells. Margulis also was interested in how bacteria and other microorganisms might impact their en­vironment. As it happened, Lovelock shared an office at JPL with planetary scientist Carl Sagan (figure 2.4). Lovelock biographers John and Mary Gribbin comment: “Margulis had independently become intrigued by the oddity of the Earth’s oxygen-rich atmo­sphere and asked her former husband, Carl Sagan, whom she ought to discuss the puzzle with. Sagan knew just the man, and put her in touch with Lovelock.”62 Upon Sagan’s recommendation, Lovelock and Margulis began exploring the question of how the highly reactive gas oxygen in our atmosphere has been sustained at a consistent level over billions of years.