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What compels us to explore Mars?

We are seeking places where secrets are held, hoping to defy our isolation in the universe.

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One might fairly wonder why we have pinned our hopes for finding life in space to Mars. For the last couple of billion years, there has been no rain there. There are no rivers, no lakes, no oceans. Without the driving force of fluid erosion, scars left over millions of years by meteorites are strewn across the surface. Mars has no plate tectonics, no magnetic field, and little protective atmosphere. The terrain is quiet, exposed, and bewilderingly empty.

Yet long ago, before it rusted over, Mars was much more like Earth: smaller, but similar in size and elemental composition. In its early days, Mars was black with igneous rock. Untold piles of lava built the planet’s massive volcanic provinces, which bulged with enough basalt to flex the crust. The planet’s swollen side cracked opened as Mars cooled, with a fissure so deep that the Grand Canyon could disappear into a side channel. One of the largest mountains in the solar system was formed, towering over an escarpment that itself is nearly as tall as Everest.

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Those volcanoes lifted greenhouse gases into the air, wrapping the surface with a blanket of atmosphere. We know from the geologic record that the terrain was warm and wet, at least periodically. Around the time life may have been getting started here — conceivably in volcanic pools, in Darwin’s “warm little ponds” — water was present on Mars, pregnant with possibility. In fact, there may have been enough water to fill a northern ocean, still and deep, with a sea floor as smooth and flat as the abyssal plains of the great Pacific.

Then, between three and a half and four billion years ago, our planetary paths diverged, and Mars was laid bare. Almost all of the atmosphere disappeared, and so did the water. The planet slipped into a deep freeze, colder than the cold of Antarctica, leaving Mars the hyper-arid, frozen desert we know today, bathed in high-energy solar and cosmic radiation. Now a dust the consistency of red flour coats the surface, lofted by dust devils into the impossibly thin air.

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Yet life, we have learned, is stunningly resilient. It can adapt, it can wedge into a crevasse, it can hang on against all odds, and it can reveal itself in unlikely ways. Traces of biology hide in the most unexpected locations. It’s why I roam the terrain at the edge of the world, hunting for the subtlest fingerprints of life, learning how to look.

In the far reaches of Australia, there’s one particular lake that stands apart from the others, amid the rocks and dunes, past the Rabbit Proof Fence and Jilbadji Nature Reserve, past the derelict aerodrome. The surface is stippled with halite, a form of table salt that looks like freshly fallen snow. In the right place, with a good grip, you can pull out a crystal of gypsum, severed like a shark’s tooth from the jaw of the earth. The spear-tipped blades are as large as your hand. When you rinse away the red mud and hold it to the light, it flashes in the sun like a gemstone. Under a microscope, you can see the tiniest of pockets within it: glinting drops of lake water, sheathed in mineral hideaways. Life caught in a crystalline dagger.

These prismatic inclusions are just one of the many features we want to look for on Mars. We are seeking places where secrets are held, where traces of life might be preserved and protected. For over 50 years, we’ve been exploring Mars with telescopes, flyby missions, orbiters, landers, and rovers. We’ve scoured the surface for current life as well as indications of past life, for possibilities and actualities. The wild strangeness of the planet, with its tawny air and relentless red deserts, calls to us: With each mission, we grapple to understand a world that’s at once recognizable yet at the same time indescribably foreign. We return again and again, and the mysteries deepen.

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In the process, we’ve built an entire field of science around something we can barely see in the night. Four hundred years ago, Mars was still a blaze of light, no more than an idea. The earliest telescopes showed it about as large as a pea held at arm’s length, and even more modern telescopes gave us little to go on. We had no idea what the surface looked like or what it was made of, if there were mountains or valleys. We had only the crudest of maps. We didn’t know if there were clouds or what color the sky was. We started from almost nothing. We’ve gone careening down blind alleys and taken countless wrong turns, yet somehow, miraculously, the passion, ingenuity, and persistence we have brought to the enterprise have moved us toward a truer understanding of another world.

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In this way, the story of Mars is also a story about Earth: how we’ve sought another stirring of life in the universe, and what that search has come to mean. Mars has been our mirror, our foil, a telltale reflection of what has been deepest in our hearts. We have seen in Mars a utopia. A wilderness. A sanctuary. An oracle. With so few landmarks, guideposts, or constraints, all is possible; without data that could be used to cabin our inquiry or limit our imagination, Mars has been a blank canvas. And tenderly, our human seeking has rushed to fill it.

As a result, Mars has a human history inscribed upon its surface, even though no human has ever touched it. The modern scientists who have revealed the planet’s extraordinary natural history and the people from centuries past who inspired them all were seeking connection to something larger than themselves, some piece of evidence, some breakthrough observation that would show life could exist there. And they didn’t just seek it, they longed for it — I long for it — knowing that even the smallest glimpse of some greater, deeper, other realm might change everything. This is what sets Mars exploration apart: The quest to bring this distant world into focus, pursued over generations and on the frontiers of technological innovation, has always been about more than scientific knowledge. It has been an almost existential endeavor to confront our own limitations, to learn what life really is, and ultimately to defy our own isolation in the universe.

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Sarah Stewart Johnson is assistant professor of planetary science at Georgetown University. This essay is adapted with permission from her new book, “The Sirens of Mars: Searching for Life on Another World,” being released this week by Penguin Random House.