The advantage of caves is that they can provide protection from hazardous conditions on the planet’s surface, such as exposure to cosmic radiation or high temperatures. For example, it may be the case that caves deep beneath the Martian soil will be warmer, wetter, and more habitable.
“From my point of view, the king of the questions is our 1992 prediction that there may be remnants of a microbial biosphere in the depths of Mars,” Boston says.
What’s the best way to look for cave aliens?
Scientists have discovered this over the last few decades hundreds of caves on the Moon and Mars, often looking for “fireflies” that reveal cave entrances. In February, the team announced the discovery a colossal lava tube beneath the surface of Venus, several thousand feet high and wide.
Scientists also speculate that water-filled ice caves, called interstitial lakes, may be common in the frozen crusts of moons such as Europa orbiting Jupiter or Enceladus orbiting Saturn. Although these icy moons are notable for their subsurface oceans, lake environments could provide potentially habitable pockets that can receive even a secure amount of sunlight and that could be much easier to sample with future landers than the moons’ deep oceans.
These lakes are “the most inhospitable place for multicellular life on Earth imaginable,” Sebree says. “However, in space it is a completely protective environment where radiation from the Sun, Jupiter or any nearby planet will not cause DNA destruction. You are protected from the vacuum of space, so you can actually have the chemistry of liquid water.”
“The worst place to live on Earth is actually the safest place to live on another planet,” he adds.
To explore these caves, scientists will need to build advanced robots and carefully plan missions to optimize the best places for extraterrestrial exploration. On rocky planets like Mars, skylights can lead to deep pits, with no other connecting passages. With this in mind, it would be wise to target areas with a lot of apparent skylights to avoid getting stuck in dead ends.
And while access to confined or very remote spaces may be tough, they may be the most promising areas to look for signs of alien life, called biosignatures. Such signs of life can be very subtle, and it is unlikely that we will encounter a habitat full of Martian megafauna.
“If life ever evolved on Mars and still exists as extant life forms underground, it will be microbes,” says Wynne, who is an expert on bats (a terrestrial genus). “Although this statement breaks my heart, Martian bats will probably not be discovered.”
While the dream of finding Martian bats may be dashed, biosignatures in these caves could potentially be detected using specialized equipment such as spectrometers, which can reveal tantalizing traces of minerals and intricate compounds.
Sebree has used these instruments extensively in cave environments, most notably at Wind Cave in South Dakota, where he first caught a cave worm in 2019 – fortunately, in this case, not literally. “Seeing the cave from the trail is beautiful, but when I actually had to sweat and crawl to get to an even nicer place, I immediately became addicted and I still do it to this day,” he says.
Sebree and his colleagues apply spectrometers to identify traces of nutrients and other mineral deposits on cave walls that can keep ecosystems in the murky. Similar instruments could be placed on robots to look for biosignatures in all kinds of extraterrestrial caves.