Scientists looking for ways to search for life on Mars – or to explore safe abodes for long-term bases on the Moon – are using a NASA robot to explore underground tunnels in the high desert of northeastern California.

The work, says the project’s principal investigator, Jennifer Blank, an astrobiologist with the Blue Marble Space Institute of Science, is being carried out in Lava Beds National Monument, a 190-square-kilometre protected area that contains 780 known lava caves, the highest concentration of such caves in North America.

Sometimes called lava tubes, they are caverns that form beneath lava flows, where they serve as conduits for low-viscosity (runny) lavas.

When the eruption ceases, the lava flows out the lower end of the tube, leaving a long, noodle-like cavern in its wake. The longest one known on Earth, Kazumura Cave in Hawaii, stretches an incredible 32 kilometres.

Blank’s project is a NASA astrobiology mission called BRAILLE (Biologic and Resource Analog Investigations in Low Light Environments), which used a four-wheeled NASA test rover to explore nine caves in Lava Beds as practice for exploring on Mars and the Moon.

The goal wasn’t to gain major insights about the earthly caves – all nine are easily accessible to earthly scientists. Rather, it was to put the rover through its paces and figure out how to conduct such a mission from millions of kilometres away.

Planetary scientists, Blank says, know that such caves exist on both the Moon and Mars because we can peer into them via collapses in their ceilings, known as skylights.

“We’ve seen skylights on the Moon,” she says. “We’ve seen skylights on Mars and many [other] features implying that there may be many, many lava tubes on Mars.”

On the Moon, lava tubes may offer safe havens for Moon bases, shielded from dangerous radiation by the overlying rocks.

On Mars, they offer the same opportunity for shelter, but may also preserve signs of ancient microbes that once lived beneath the surface, “eating” energy-supplying chemicals in the rocks, much as ecosystems surrounding Earth’s deep-ocean hydrothermal vents “eat” chemicals bubbling up from the Earth’s interior.

“If there is life there,” Blank says, “those tubes are a good place to look. And if there was life in Mars’s ancient past, that’s where it’s most likely to be preserved.”

To see what might be detectable by a rover, Blank’s team used rover-mounted instruments to sample water seeping into the cave through fractures, examine the cave walls for minerals that might have been deposited by long-deceased microbes, and test the DNA of the microbes they found.

Earth, of course, is a living planet, so they found plenty of life signs. But that wasn’t the point. “We’re simulating a mission,” Blank says.

That said, there are still some technological hurdles to overcome before a lava-tube explorer can be deployed to Mars.

Lava tubes like the ones explored by BRAILLE tend to have smooth floors and are easy to either walk around or drive across, once you get into them. But getting into them is a different matter.

In Valentine Cave, which BRAILLE visited, the main entrance is via a stairway built to create easy access for tourists: not exactly something you could drive down with a 120-kilogram rover.

Instead, Blank’s team was forced to disassemble the rover at the top of the stairs, carry it into the cave, and reassemble it. Once there, however, it was free to explore 1000 metres of easily navigable cave passages.

On Mars or the Moon, stairs won’t be the problem, but it will be necessary, Blank says, to develop a way to lower a rover – probably smaller and lighter than the one used in the BRAILLE project – into a cave through a skylight.

There are a number of proposals for how this can be done, she says, but those are beyond the scope of the BRAILLE project. The key thing, she says, is that BRAILLE has proven that lava tubes can be explored by a remotely operated rover.

Due to Mars’s harsh surface environment, she adds, “I think more and more people are dubious that people will find signs of life preserved on the surface.” And lava caves, she says, may provide a much more accessible entry to the subsurface than simply trying to drill into it. “Drilling is hard,” she says.