Sarah Stewart Johnson. Photo: Beowulf Sheehan.

Sarah Stewart Johnson grew up in Kentucky before becoming a planetary scientist. She now runs a research lab as a professor at Georgetown and works on NASA missions. Her writing has appeared in The New Yorker, the New York Times, the Los Angeles Times, The Harvard Review, and The Best American Science and Nature Writing. Her book, The Sirens of Mars: Searching for Life on Another World, was selected as one of The New York Times Book Review’s 100 Notable Books of 2020.

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An excerpt from The Sirens of Mars:

Mars, after all, is only our first step into the vast, dark night. New technologies are paving the way for life-detection missions to the far reaches of our solar system, to the moons of the outer planets, far from what we once considered the “habitable zone.” To worlds that hold stacks of oceans amidst shells of ice, floating like a layer cake. That spew out jets of briny water through cryovolcanoes. That have pale hills and dark rivers and hydrocarbon rain. And then there are also the planets around other stars. There could be as many as forty billion planets that could support life in the Milky Way alone, belted with moons and moonlets—potentially an entire solar system for every person on Earth. The idea of knowing these places intimately, of one day touching their surfaces, may seem ludicrous. The universe has a speed limit—it’s slow, and these worlds are very far away. What could we ever know about them, besides a few details about their orbits, perhaps some spectrographic measurements of their atmospheres? They are points of light and shadow at the very edge of our sight, far beyond our grasp. Then again, that is exactly how Mars seemed only a century ago.

As much as Mars feels like a place we understand, a place like Earth, it is still the alien other. One of my favorite things inside the box, tucked in a bent folder, is a set of pictures that Opportunity took in 2010. All those years ago, it seemed like such a marvel that the rover was still working. No one would have dared to believe it would have thousands more sols of science. The dust was building, the power dropping. It had been traversing the planet for six years and was already long past its ninety-day expiration date. But then a gust of wind whistled across Meridiani Planum and cleaned some of the fine particles off the solar panels. With the unexpected spike in electricity output, the team commanded the panoramic camera to take a series of pictures that could be strung together with time-lapse photography.

The flickering images captured by the rover are unforgettable. There, on an ancient plain near the equator of Mars, against an ochre sky on a dusty day, the sun is setting. A white circle of light is drifting down over the dark desert. The terrain is bare, and the sky is still in the half-light of dusk. And on the horizon, with the dust having scattered all the red light away, the sunset glows an eerie, baffling, incandescent blue.

The color makes no sense. It rattles the mind. It rips at the seams of the physical world. Scientifically, I understand it—the properties of the light, the microphysics of the system. There is no mystery to behold. And yet the mystery, like many others in our universe, is profound, nearly incomprehensible. That blue. So recognizable, yet so foreign. Shining in a halo around our shared star, calling us like a siren.