But there are no Mars samples now — except those that arrive as long-traveling meteorites — so astrobiologists have to conduct their search for life using other methods and teasing out hidden evidence.

The search for water on Mars, for instance, goes back decades and many missions. But scientists were never certain that the carved canyons and deltas were results of water running long ago, or perhaps lava or frozen carbon dioxide. Because of Curiosity, there is now a wide consensus that early Mars had much water.

This conclusion has been difficult to square with climate models, which point to a colder early Mars with a thin atmosphere that could not have supported large bodies of standing water, or rivers that ran for millions of years. But faced with mounting evidence of longstanding water and consequently warmer conditions, the climate scientists have gravitated toward two interwoven explanations — both with implications for early life.

The first is that frequent volcanoes and meteorite impacts heated the planet substantially; volcanoes also emit gases known to synthesize into organic compounds. The second is that to explain the substantial water cycle required to keep many Martian lakes filled and rivers flowing, the planet needed a substantial ocean in its northern half. Large swaths of Mars north of its equator are one to three miles lower than the so-called southern highlands, and scientists have proposed that an ocean may have filled and molded the vast depression. Others disagree on several grounds, including that no remnant shoreline has been detected.

Image John Grotzinger, project scientist for the Curiosity rover mission. Credit... Michael Tullberg/Getty Images

“We don’t have hard evidence of a northern ocean, but our models require that much water to explain what the geologists have now confirmed,” said Michael A. Mischna of the Jet Propulsion Laboratory, another Curiosity team member. “What Curiosity has done is to bring together atmospheric and climate information with the findings of the geologists and geochemists, and created a broad and consistent story of a very wet early Mars.”

While the evidence for water has become increasingly clear, the question of organic compounds is in flux. Such chemicals fall onto Mars all the time in interstellar dust and meteorites, as they do onto Earth. Yet none have been definitively detected.