New evidence from meteorites suggests that the basic building blocks of life are present on Mars.
The study found that carbon present in 10 meteorites, spanning more than four billion years of Martian history, came from the planet and was not the result of contamination on Earth.
Details of the work have been published in the journal Science .
But the research also shows the Martian carbon did not come from life forms.
A team of scientists based at the Carnegie Institution for Science, based in Washington DC, found "reduced carbon" in the meteorites and says it was created by volcanic activity on Mars.
Reduced carbon is carbon that is chemically bonded to hydrogen or itself.
They argue this is evidence "that Mars has been undertaking organic chemistry for most of its history."
The team's leader Dr Andrew Steele told BBC News: "For about the last 40 years we have been looking for a pool of what is called 'reduced carbon' on Mars, trying to find where it is, if it's there, asking "does it exist?"
"Without carbon, the building blocks of life cannot exist... So it is reduced carbon that, with hydrogen, with oxygen, with nitrogen make up the organic molecules of life."
He says the new analysis has answered the first question.
"This research shows, yes - it does exist on Mars and now we are moving to the next set of questions.
"What happened to it, what was its fate, did it take the next step of creating life on Mars?"
He hopes the next mission to land on the Red Planet - the Mars Science Laboratory, also known as the "Curiosity" rover - will shed more light on the big question.
"The question 'are we alone' has been a big driver of science but it relates back to our own origins on this planet. If there is no life on Mars why? It allow us to make a more informed hypothesis about why life is here."
So does Dr Steele think there was, or is, life on Mars?
He laughs: "Get me some rocks back, I'll have a look and let you know."