NASA’s Mars rover Curiosity has turned up tantalizing clues of the planet’s complicated chemical evolution, a story that includes carbon, the first detailed analysis of the planet’s soil shows.

Scientists found traces of carbon in several compounds detected by the rover’s Sample Analysis at Mars, or SAM, instrument. They do not yet know if the carbon, which is a key building block for life, is contamination from Earth, was delivered to Mars by organics-rich comets or asteroids, or arose on Mars.

If indigenous, the carbon could be an indicator of geologic or biological activity.

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“We’re not really sure of where it comes from right now,” the mission’s lead scientist John Grotzinger told reporters during the opening day of the American Geophysical Union conference in San Francisco on Monday.

“Just finding carbon somewhere doesn’t mean that it has anything to do with life, or the finding of a habitable environment,” he said.

Life as we know it needs three basic ingredients to evolve — water, a energy source and carbon. Other building blocks include sulfur, oxygen, phosphorous and nitrogen.

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Curiosity, which is four months into a planned two-year mission on Mars, already has turned up evidence that its landing spot on the floor Gale Crater, was once covered in water. Minerals in the soil analysis also show a history of chemical interaction with water.

Unlike previous rovers and landers dispatched to Mars, Curiosity contains an onboard chemistry laboratory in an attempt to find the ingredients for microbial life, the environments that could have supported it and places where life could have been preserved.

In a trio of runs, SAM detected signs of an oxygen-chlorine compound, possibly perchlorate, and traces of carbon-containing chlorinated methane compounds.

Scientists specifically chose dry, fine-grained sand believed to be typical to the Martian surface for Curiosity’s first soil analysis.

“It’s not unexpected that this sand pile would not be rich in organics. It’s been exposed to the harsh Martian environment,” said lead SAM scientist Paul Mahaffy, with NASA’s Goddard Space Flight Center in Greenbelt, Md.

“What we’re seeing here are some very simple compounds, and it’s entirely possible they’re coming from the very reactive chlorine that’s released and picking up carbon from somewhere. We have to try to understand where that carbon is coming from,” he said.

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The rover’s prime target is a stack of layered deposits rising from the center of the crater.

“It’s a really going to be an exciting hunt over the course of this mission to find early environments that might be protected from this surface Mars environment and see what we can add to the carbon story,” Mahaffy said.

“We’re starting to find the spices that make a stew tasty,” Grotzinger told Discovery News. “There are the basic ingredients that you expect to be there, but it’s how you combine them and the minor ingredients that really turn out to be interesting.”