In recent days, Curiosity has accomplished a number of firsts, including the first use of its laser to zap a nearby rock and its first short drive. Many more such firsts lie ahead. But as the rover prepares to head off on a journey of discovery across previously unexplored territory, it seems like a good time to pause and remind ourselves just what it was that Curiosity was sent to Mars to do. MSL (Mars Science Laboratory) is not a life-detection mission. Its goal is not to look not for life, but rather to investigate whether Mars was ever capable of supporting life. It is searching for signs of habitability.

“I don’t think we have broad enough consensus in the scientific community about how to define life so universally that we would recognize it on Mars,” says Pan Conrad, deputy principal investigator for Curiosity’s SAM (Sample Analysis at Mars) instrument. “I can’t think of a single thing that we would see that any of us would say, ‘Aha, we have found life.’”

Indicators of microbial life can be difficult to nail down. For instance, debates still rage within the scientific community about whether certain textures and chemical signatures in 3.5-billion-year-old Australian rocks, some of the oldest rocks on Earth, are the remains of ancient biology or mere evidence of abiotic chemical reactions.

“We’re not ready to really target the biology,” says Mary Voytek, senior scientist for astrobiology at NASA headquarters and deputy program scientist for MSL. Although in recent years significant work has been done on DNA-detection chips, the scientific-instrument package for MSL was decided on before such chips were available. Even if MSL did contain a DNA detector, says Voytek, “We’d be asking the question, ‘Is there life on Mars that looks exactly like the life that’s here on Earth?’” But life on Mars, if it ever took hold, may have followed a different chemical trajectory than terrestrial life. So rather than looking for evidence of the specific chemistry that characterizes life on Earth, MSL is tasked with studying a more abstract question: habitability.