In the late 1970s, Saturn’s odd moon Titan, a hazy orange world, was expecting visitors — first, NASA’s Pioneer 11 probe, then the twin Voyager spacecraft.
Most moons are airless or boast little more than gauzy, gaseous veils. But Titan is cloaked in a blanket of nitrogen and methane so thick that, with a pair of wings and a running start, astronauts on the frosty satellite could fly just by flapping their arms.
A few years after the probes zipped past Titan, Kevin Zahnle (opens a new tab), a planetary scientist at NASA’s Ames Research Center, was mulling over the moon’s atmosphere when he found himself asking a deceptively simple question about how planets work: “Why is there air?”
Most scientists thought atmospheres around planets — and the odd moon like Titan — were a question of starting materials. If a growing planet gobbled up enough easily vaporized material, it would have an atmosphere. Otherwise, it wouldn’t. Scientists also knew that atmospheres cling to worlds because of gravity, and that the very smallest worlds lack the heft to hold onto air. But then observations of Mars suggested that, surprisingly, it too had lost substantial amounts of air.
As Zahnle considered the data from across the solar system, he started to wonder if atmospheric loss, rather than starting materials, might determine whether worlds held onto their air. So he plotted dozens of solar system bodies on a simple graph that compared a world’s escape velocity — a measure of its gravity — with the amount of sunlight it receives, as atmospheres wither away in the sun. The plot revealed a neat line separating our solar system’s bare rocks and snowballs from worlds wreathed in gas, a boundary he called the cosmic shoreline. “I was consciously trying to invoke an image that Carl Sagan would have drawn,” he said.
At first, Zahnle’s cosmic shoreline was largely ignored. Escape, he said, was not very popular — scientists mostly focused on how planets got atmospheres, not on how they lost them. But decades later, the discovery of thousands of worlds beyond our solar system has breathed new life — and billion-dollar stakes — into the overlooked idea. The search is on for habitable alien worlds, and eventually for signs of life in their atmospheres. To succeed, alien hunters will need to find planets with air. And the cosmic shoreline, if it truly is cosmic and extends to other star systems, could show them where to start.
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