NASA's Mars Science Laboratory and its Curiosity rover are set to land shortly after 1 a.m. ET Monday on the most advanced, most ambitious and most adventurous mission ever sent to the Red Planet.
The scientific return promises to be profound, but there is one thing Curiosity won't be looking for: little green men.
Or even tiny microbes, for that matter.
This $2.5 billion expedition is not a life-detection mission. The lab's instruments are the most sophisticated in the history of planetary space exploration, but they couldn't distinguish pebbles from protozoans.
"Curiosity is not set up to detect life directly," says Steven Lee, deputy manager of Mars Science Laboratory surface operations. "Now if we come across a trilobite or a dinosaur bone, yes, that would be pretty definitive. But if life ever developed on Mars, it was most likely small, microbial life. So we're not quite ready to make instruments small enough to do that detection directly on Mars."
This mission is all about what planetary scientists call habitability. Scientists are trying to determine whether Mars ever harbored all the ingredients key to the formation of life.
Liquid water, energy such as sunlight and "organics" -- carbon compounds -- are the fundamental ingredients of all life on Earth.
NASA's Martian orbiters and surface rovers over the past 15 years have beamed back ample evidence that Mars once was warmer and wetter -- a place that could have been hospitable to the formation of primitive life.
Still elusive: conclusive proof of carbon compounds in Martian rocks or soil.
What's more, evidence of organic matter is not a slam-dunk score.
"The real question is: Were those organics produced by life? Are they biological in origin?" says Matt Golombek, a landing site scientist for NASA's Mars Exploration Program.
"There are lots of organics produced without life whatsoever," he says.
Meteorites and comets crashing into the planet over eons could have distributed organic matter not of biological origin.
Any discovery would lead to debate over the origin of the organic material.
"That is a much more difficult question to answer," Golombek says. "I don't think we should be holding our hopes out for that. That would be a very, very difficult thing to establish."
But what if scientists could make that leap and determine that all the ingredients to form life once existed on a world that is now dry, acidic and constantly scoured by deep, penetrating cosmic radiation?
Driving the exploration of Mars and other planets are the most fundamental questions humanity can ask: Are we alone in the universe? Will life form anywhere that liquid water is present?
"Or did you need some one-in-a-trillion act of God, some accident, for life to have formed here on Earth?" Golombek says. "If life forms anywhere that liquid water is present, then Mars should have had life, and we ought to be able to go there and see that.
"Was there a second Genesis? Did life form somewhere else? Are we in fact alone in the universe? Or are we just an incredibly fortunate, or unfortunate, luck of the draw?
"If you can answer that sort of question, that kind of fundamental question that really goes to the heart of an exploring society, you can't measure that in dollars."
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