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Ever since scientists have had an orbiter's view of Mars, they've observed intriguing black spots on the surface so dark no one knows what's inside them. 

They're thought to be the mouths of deep caves where the sun doesn't shine, formed by ancient volcano vents. Within them could be existing Martian life, liquid water, or traces of long-dead bacteria or fungi that flourished eons ago. 

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With NASA's new moon-to-Mars Artemis space campaign, researchers are intent on developing technology to make spelunking the red planet possible. If astronauts one day fly to Mars, these underground lairs could be ideal places to create a makeshift home, sheltered from space radiation and dust storms. 

But the challenges are monumental: Even if engineers could make a robot that wouldn't get stuck on rocks, they would still have to figure out how to beam back pictures and data from the cave-diving rovers, which aren't likely to return. The answer, says Wolfgang Fink, a University of Arizona engineer, is to send a robot into a cave with no expectation that it will come back.

"So that it can finally attempt going to high-risk areas where the science really lies," he told Mashable. 

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But any Earthling who has tried to talk on a phone while driving through a tunnel or walking into an elevator knows exactly the problem Fink and his team have to solve: how to not drop that crucial call. 

A new paper in Advances in Space Researchdescribes a type of communication system that could enable small robots to venture into these hazardous environments without a heavy, tangle-prone cable. If implemented, the invention could not only assist explorers on Mars but on other planets, including this one. Think of rescue teams trying to find survivors trapped under rubble after an earthquake.

A small robot, connected to other rovers with wireless data connection, would enter one of these Martian skylights, thought to be a lava tube opening. On its back would be something akin to a Pez candy dispenser, loaded with little beacons, each about the size of a $1 coin. As the robot travels, it drops the beacons, or communication nodes, whenever the mother rover on the surface senses the signal between them is fading. 

'Breadcrumb-style' communication network

Fink compares the system to the breadcrumbs Hansel and Gretel dropped to find their way back out of the woods. But in this situation, it's not the robot that needs the breadcrumbs — it's the data.

"For the most part, we're connected wherever we go, but on Mars, for example, that's not the case because there's no such thing as a cell phone tower. There's no GPS system," Fink explained. "So basically, this is a communication infrastructure on the go." 

The cave robot would be expendable, designed to explore until its battery dies. 

Caves aren't straight lines. They can have corridors, curves, and thick slab walls. One of the novelties of the proposed communication system is that the robot would not drop beacons only when necessary, Fink said. Additionally, the robots would be outfitted with a light detection and ranging system, or LIDAR, to allow the mission team to map the underground passages in 3D. 

The same communication method could be used for exploring ocean worlds, according to the team's paper. A lake lander could serve as the mother craft on the surface. That robot could run a cable down to a submarine — except the beacons in this scenario would act as repeaters to boost the signal. 

Where are lava tubes found?

Scientists believe that underneath Mars, Venus, and even the moon are enormous, miles-long lava tubes, formed by flowing magma and covered in crystals. On the red planet, the atmosphere is thin and the climate is dry — not a hospitable environment for life.

But caves — as geologists and biologists have found on Earth — tend to have lives of their own. The pressure and climate are different, water is shielded from evaporation, and, through interactions with volcanic heat and minerals, they could support microbes. 

David Crown, a geologist with the Planetary Science Institute in Arizona, has been mapping the region around Alba Mons, which likely has the largest concentration of giant lava tubes on Mars. His team's recent study mapped over 300 segments, with systems extending up to about 250 miles. They estimate the caves could be 1.9 to 2.5 million years old. 

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The fact that they appear to be linked suggests they're the same age, formed through one major eruption that spewed massive amounts of lava. That could make it an enticing area for cave-diving robots. 

"Some lava tubes, you feel like you could drive a school bus through them: They're big and they're cleaned out and open. And other ones have big, collapsed areas, and the interior dimension can be small," Crown told Mashable. "We have to do some homework to pick the right tube." 

In the meantime, scientists are studying Earth's lava tubes so NASA will know what it's looking at when it sends robots into Martian caves. The space agency's BRAILLE project, which stands for Biologic and Resource Analog Investigations in Low Light Environments, is focused on developing tools to detect life on cave walls from afar. 

Possibility of life on Mars

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Earth caves host complex ecosystems, supported by bacteria that munch on rocks and convert the material into energy for life. Because of this, many astrobiologists have published papers on the merits of going underground to look for Martian life forms, including over 50 scientists in a paper published in Nature Astronomy. 

"The most likely place to find biosignatures of putative modern day extant life is in the subsurface, where groundwater (likely in the form of brines containing pure water mixed with salts) could still be stable," the authors said, calling Mars' lava tubes the "next frontier" for planetary and human exploration. 

Imagine that moment after lowering a robot into one of these Martian skylights, and it shines an ultraviolet light on one of the walls for the first time. What would it see?

"All of the sudden, you see some green-yellow sparkle," a fluorescent indicator of living organisms, Fink dreams. "That would be absolutely mindblowing."

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