Almost a year since that white-knuckle airdrop landing inside Gale Crater, the Curiosity rover is embarking on its biggest Martian road trip yet. It's driving toward the base of Mount Sharp, a conical mound taller than any peak in the continental 48 states, where it'll study past Martian environments. Curiosity already found evidence that its new home once held water, probably a lot of it, and that the landing area known as Yellowknife Bay could have been habitable for Martian microbes. But what else could it find out?
It's really an open question, according to scientists working on the Curiosity team. The rover is in the middle of a two-year initial mission, and while the team has several specific questions, Curiosity is all about discovery.
"There's an aspect of this that is unknowable," Joy Crisp, deputy project scientist for the rover mission at the Jet Propulsion Laboratory, told DVICE. "We always get surprised."
As someone once said, there are "known unknowns" and there are "unknown unknowns." Curiosity’s science team can guess about the former, but not really the latter. For instance, a known unknown is a question like, "was the water on Mars acidic?" Scientists are aware they don't know the answer, and they can devise more detailed questions to find out. But an unknown unknown would be a question we can't even conceive of.
What is Curiosity Looking For?
Curiosity's main goal is to characterize the environment inside Gale Crater, and find out whether it could have (at some point) been a home for life. The rover is also trying to understand how water may have changed the surface of Mars over time, and in doing that, uncover the planet's history. Mount Sharp may turn out to be like a Grand Canyon for Mars, with multiple layers of exposed rock that each tell a different tale.
Layers at the base of Mount Sharp show the layered geologic history of Mars. Image Credit: NASA/JPL-Caltech/MSSS
Mars orbiters have seen promising signs of hydrated minerals on the mountain, which change as elevation increases, for instance. Clays dominate at the base, which could mean water there was favorable for life, but sulfates are found higher up, which could mean higher-altitude water was more acidic or maybe more scarce.
"But what we'll actually find, who knows?" Crisp said. "It's akin to being a geologist on Earth and you started with pictures taken from an airplane or satellite. You decide to go and take a closer look. When you get there, you have to be prepared for finding something you didn't expect. ... You might say, 'gosh, I didn't know I’d find this.'"
So far, Curiosity has found some surprises. Its landing site in Gale Crater didn't look like much from orbit, but once on the ground, Curiosity quickly found an ancient streambed and plenty of evidence for ancient water. By contrast, scientists thought they saw signs of methane from orbit, but Curiosity hasn't detected any from the surface.
As for the known unknown about acidity? Curiosity and its older brethren, Opportunity, have already found answers. Roving on the other side of Mars, Opportunity found rocks that would have formed in water with a neutral pH — "water you can drink," as Opportunity principal investigator Steve Squyres put it. Curiosity also found similar evidence in Gale Crater. Is drinkable water the most interesting thing they'll find? Not necessarily, but it's a clue that paints a larger picture, and that’s the overall goal, Crisp said.
"The hope is we learn significantly new things about Mars, its past, and the way things work on the surface. It sounds simple, but it requires some luck in finding the clues that are diagnostic of processes," she said.
Opportunity is roving on the opposite side of Mars from Curiosity, its much larger sibling. Opportunity and its twin, Spirit, landed on Mars 10 years ago. Spirit stopped communicating with Earth in 2009. Image Credit: NASA/JPL
Could Curiosity Find Life?
This is not a question with a simple answer. The rover isn't designed to look for Martian life per se, but there's a small chance it could find evidence that life once existed. Scientists think if Mars did play host to life, odds are they were (or are) microbes, which are tiny and difficult to see even on Earth. Curiosity doesn't have a powerful microscope, for instance. Even if ancient microbial colonies formed stromatolites — essentially fossilized biofilms — it would be difficult to prove their identity, because other geologic activity can cause similar-looking structures.
"It's like trying to go to a three-billion-year-old rock on Earth and looking for microbial life evidence. That's just very very hard to do," Crisp said.
On the other hand, Curiosity has some powerful chemistry instruments, which could detect the presence of certain molecules like organic carbon. The real issue is how many secrets the rocks will tell, said Katie Stack, a graduate student working at the Jet Propulsion Laboratory.
"Whether life has existed is one question, but whether life is preserved is a whole other question," she told DVICE.
In some ways, the unknown unknowns are the most interesting things the rovers will find.