(CNN) Exploring beneath the surface of other planets may be the key to determining whether life has ever existed outside of Earth.
As other missions, including that of NASA's InSight lander, have shown, drilling down through the surface of planets like Mars is tough -- a little too tough to get more than a few inches into the subsurface.
Recently, the Curiosity rover measured total organic carbon, a necessary ingredient in the molecules of life, in Martian rocks for the first time. But it doesn't prove that life ever existed on Mars, because carbon can also be produced by nonliving sources.
New research suggests that the best chance of finding past or present evidence of life on Mars requires going below its surface -- at least 6.6 feet (2 meters) below. Mars has an incredibly thin atmosphere, which means that the surface of the red planet is bombarded by high energy radiation from space, and that could quickly degrade substances like amino acids that provide fragile evidence of life.
Those harsh surface conditions also present a challenge for astronauts, which is one reason scientists have suggested that caves on other planets could be the key to future exploration. Vast cave systems on the moon and Mars could act as shelters for future space travelers.
Caves could also contain resources like water, reveal more about the history of a planet -- and be havens for evidence of microbial life. On Earth, there are a varied range of cave systems, many of which remain unexplored, and they support diverse groups of microorganisms. But caves are dangerous -- and since we've never peered inside a Martian cave, it's difficult to know what to expect.
Before humans land on Mars and explore its subsurface, a group of scientists want to send ReachBot -- a robot designed to crawl and climb through extraterrestrial caves.
The idea for ReachBot was born in 2018 when Marco Pavone, director of the Autonomous Systems Lab at Stanford University, and his students were brainstorming concepts for a Martian cave explorer.
They knew the robot would need to be able to grab anchor points so it could move without falling -- and if it couldn't find enough anchor points, it wouldn't get very far.
One of his students suggested the idea of a small robot with extendable arms that reach out like measuring tape, which could be used the same way Spider-Man slings webs to help him navigate the skyline of New York City.
The robot concept is between the size of a basketball and a toaster oven and covered in extendable booms equipped with spiny grippers that could grab objects and grasp or push off the steep, rocky surfaces of Martian caves. It would be able to anchor itself and crawl across long distances.
When the robot's booms aren't needed, they roll up to stay out of the way.
Pavone, who is also associate professor of aeronautics and astronautics at Stanford University's School of Engineering, and his students arrived at the idea of a robot with extendable booms. They created a proposal to submit to NASA's Innovative Advanced Concepts Program, which funds visionary concepts in the field of space robotics that could transform future missions.
The ReachBot concept received funding for Phase I, which the team used to conduct a round of studies that proved the concept was a feasible one, Pavone said.
Now, ReachBot has received funding for Phase II. The team will use the next two years to work on 3D simulations, a robot prototype, develop strategies that help the robot avoid risk, and test out ReachBot in a realistic mission environment -- likely a cave site in New Mexico or California. These tests will determine how ReachBot could be used for future exploration.
If ReachBot becomes its own mission, it will likely rely on a larger, more capable robot -- like a rover -- to access the caves it will explore. The rover will deliver ReachBot to the cave entrance or drop it off at a cliff face, which ReachBot would be able to scale.
ReachBot will likely be equipped with cameras, microscopes and a remote sensing method called LIDAR. But instruments require power and add weight, in addition to the power and communication system the robot will need.
The team envisions that ReachBot will be tethered to the surface-bound rover, which can provide power and act like a communications relay, said Stephanie Newdick, a doctoral student in aeronautics and astronautics at Stanford University's School of Engineering.
In addition to sending data to the rover, ReachBot may also have a conveyor belt system that allows it to collect samples and dispatch them to the surface. The rover will be larger and have instruments that can analyze the samples in detail, Newdick said.
Any initial findings from ReachBot can determine the next step for follow-up missions.
"Caves are risky environments, but they're scientifically interesting," Newdick said. "Our idea for this robot is to go far before people would get there to do interesting science and scope out the area."
Martian caves are just one possible opportunity for a robot like ReachBot. Pavone sees the potential for these robots to operate alongside humans in a place like the International Space Station, handling some tasks so astronauts can make better use of their time.
The Gateway, a planned lunar outpost that will exist between Earth and the moon, won't be crewed all the time like the space station. Robots like ReachBot could perform maintenance and upkeep, Newdick said. ReachBot could also crawl inside of lunar caves that may serve as a resource for astronauts exploring the moon.
In the future, the team believes that ReachBot could be customizable depending on its destination, influencing the design choices like its size and number of extendable arms, Pavone said.
The ends of ReachBot's arms could also be equipped with scientific instruments that can go inside tiny cracks and crevices where a robot wouldn't fit.
With so many capabilities, the team sees their creation as a way to further exploration across our solar system, going places where humans cannot yet tread.