Imagine a Mars that isn’t a deserted wasteland. Picture a patchwork of rivers crisscrossing the planet, perhaps emptying into a large freshwater lake. Or glaciers, flowing across the surface and carving channels into the rock.

And now the big question: Did Mars ever have the environmental conditions to support life?

Launched on November 26, 2011, the Curiosity Rover’s mission is to discover evidence that could suggest Mars was capable of supporting microbial life. Equipped with more than a dozen instruments, Curiosity acquires rock, soil, and air samples for onboard analysis.

The Dynamic Albedo of Neutrons (DAN) tool mounted on the back of the Rover measures hydrogen in Mars’ soil. Subsurface hydrogen, which can be detected by DAN up to one meter below the planet’s surface, might reveal the presence of water in the soil. And water could indicate the means to support life.

“A neutron generator shoots neutrons into the ground underneath the Rover. Then, neutron detectors listen to the reflections of the neutrons that come back,” explains Craig Hardgrove, of Arizona State University’s School of Earth and Space Exploration and member of the DAN team.

“We're mapping things like clay minerals that have bound water, hydrated amorphous materials, and other hydrated silicates,” he says. These are substances that formed in the presence of water, possibly billions of years ago.

Interestingly, Hardgrove notes that the neutron detecting technology the team is using didn’t originate with a desire to explore the cosmos. It’s actually a byproduct of the nuclear arms race.

“The code was developed at Los Alamos National Lab, and it was intended to model the particles emitted by nuclear weapons,” says Hardgrove. “But it has evolved since then and the planetary science community now uses it to model our work.”

Creating a map of ancient Mars

In the eight years since it landed on the surface of the red planet, the Curiosity Rover has traveled approximately sixteen kilometers—and taken measurements all along the way.

“Each DAN measurement is acquired at one location. Then the Rover drives a certain distance and you take another measurement,” says Hardgrove. “It's like a sounding experiment. And it’s not just subsurface hydrogen that affects the neutron signal, but the composition of the rocks and the subsurface distribution of hydrogen. If you find a correlation between the two DAN measurement results through modeling the subsurface variables, then you can draw a line between the two and say, ‘There's something buried here.’”

But taking such frequent measurements and variables means Hardgrove and his colleagues have to grapple with large data sets.

“For each measurement, we fire the neutron generator tens of thousands of times,” says Hardgrove. “Each time, every interaction that that neutron undergoes with the Mars surface that we've modeled has to be calculated.”

Calculations were run on ASU’s Agave computing cluster, using more than 6 million CPU hours on over 10,000 cores.

While we can learn a lot from instances of water on Mars, the main goal has remained similar across much of Curiosity’s experimentations – figuring out what ancient Mars was like.

“The goals of the instrument are in line with the goals of the Rover missions,” says Hardgrove. “We're trying to identify historically habitable environments on Mars. And we have already identified that in Gale crater, Mars was habitable about three billion years ago. Now we're trying to figure out what happened.”

When exploring space is your day job

For Hardgrove, looking for signs of life on a distant planet connects to a lifelong interest in worlds beyond our own.

“I was in a grade school when I saw the first pictures from Voyager, and it was just fascinating,” he says. “The fact that there are these other planets out there that we can go to is enticing. I just want to know what is out there.”

His interest only grew as time went on. After seeing the first Rover prototypes on TV and watching interviews with the people who worked on it, Hardgrove realized for the first time that space exploration was something you could do for a living.

“I think my younger self would be happy that I'm in this position,” says Hardgrove. “I've had a lot of cool opportunities. I remember the first time I was doing Rover planning and I got to decide where the Rover drove – it was just the coolest. I'm like ‘Man, I got to pick a spot and there's going to be wheel tracks there forever.’”