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Summary
In addition to organic molecules, the Curiosity rover has found another key chemical lurking in the martian soil: nitrates—the NO3-bearing compounds that are crucial ingredients for life on Earth and a chief component in fertilizer. The nitrogen is thought to come from the martian atmosphere, which, though depleted today, was much thicker in ancient times. On Earth, microbes tend to do most of the nitrogen "fixing": taking it out of the atmosphere and making it available in the soil to be used by life. On Mars, the nitrates could have been fixed without life—through the energy of asteroid impacts, for instance. But the discovery nonetheless means that another crucial nutrient was available in Mars's ancient past.
