So far, we only know life exists on one planet, Earth, but scientists don't know how it started or even if it had one or more false starts before ultimately taking hold.
"Since Earth remains for now the only instance of an inhabited planet, the search for life also requires that we further develop our understanding of life on Earth," NASA's lead scientist Ellen Stofan said.
"We know life is tough, tenacious, metabolically diverse and highly adaptable to local environmental conditions," she added.
Scientists have discovered microbial life that consumes what would be considered toxic to others and life that can withstand radiation, cold, heat and other extreme conditions.
"We do know that life evolved very rapidly here on Earth after conditions stabilized. That's a factor that makes us optimistic that there's life elsewhere in the solar system," Stofan said.
Clues about how life started on Earth may be preserved on the moon, which holds the geologic record of the first billion years of Earth.
"That's the time that life began on Earth. To understand what was happening geologically, we can do no better than turn to the moon," Cornell University's Jonathan Lunine said.
"We really have no laboratory model for how life began on the Earth," he added. "One of the reasons for going out to environments in our solar system where the conditions for life are apparently there and possible is to see whether life actually began, to do the experiment in the field rather than in the laboratory."
"It is remarkable that we have found four destinations in our solar system where life may actually exist, or have existed for quite some time in the past. Now is the time to actually go search," he said.
Photo: The far side of the moon, illuminated by the sun, crosses between the Deep Space Climate Observatory and Earth.