A benefit of caves is that they can provide protection from dangerous conditions on a planet’s surface, such as exposure to cosmic radiation or harsh temperatures. For example, it may be that caves deep under the Martian soil could be warmer, wetter, and more conducive to life.
“The queen of questions, from my point of view, is our prediction from 1992 that there could be a remnant microbial biosphere on Mars at depth,” says Boston.
What’s the Best Way to Search for Cave Aliens?
Over the past few decades, scientists have discovered hundreds of caves on the Moon and Mars, often by looking for “skylights” that expose cave entrances. In February, a team announced the discovery of a colossal lava tube under the surface of Venus that is several thousand feet in height and width.
Scientists have also speculated that ice caves filled with water, known as interstitial lakes, may be common in the frozen shells of moons like Europa, which orbits Jupiter, or Enceladus, which orbits Saturn. While these ice moons are famous for their subsurface oceans, the lake environments could provide potentially habitable pockets that may even receive a safe amount of sunlight, and which could be much easier to sample with future landers than the moons’ deep oceans.
These lakes are “the most inhospitable place you can think about for multicellular life on Earth,” says Sebree. “Yet in space, this is a completely protective environment where the radiation from the Sun or from Jupiter or whatever planet is nearby is not going to shred DNA. You’re protected from the vacuum of space, so you can actually have liquid water chemistry.”
“The worst place to live on Earth is actually the safest place to live on another planet,” he adds.
In order to explore these caves, scientists will need to build advanced robots and plan missions carefully to optimize the best sites for extraterrestrial spelunking. On rocky planets like Mars, skylights may lead to deep pits with no other connected passages. With that in mind, it would make sense to target regions with lots of visible skylights to avoid getting stuck with dead ends.
And while sealed-off or highly remote spaces may be difficult to access, they could be the most promising areas to search for signs of alien life, known as biosignatures. Such hints of life could be very subtle, and it’s unlikely we’ll stumble into a habitat packed with Martian megafauna.
“If life ever evolved on Mars and it still exists as extant life-forms underground, it’s going to be microbial,” says Wynne, who is an expert on bats (the Earth kind). “As much as it breaks my heart to say this, Martian bats are probably not going to be discovered.”
While the dream of finding Martian bats may well be dashed, biosignatures in these cave environments could potentially be detected by specialized equipment, such as spectrometers that can expose tantalizing mineral trails and complex compounds.
Sebree has extensively used these instruments in cave environments, especially Wind Cave in South Dakota, where he first caught the cave bug in 2019—fortunately not a literal one, in this case. “Seeing the cave from the tour trail is beautiful, but once you’ve actually had to do the sweat and the crawling to get somewhere even better looking, for me, that was an immediate addiction to it, and I’ve been doing it ever since,” he says.
Sebree and his colleagues use spectrometers to identify nutrient trails and other mineral buildups on cave walls that can sustain ecosystems in the dark. Similar instruments could be packed onto robots to look for biosignatures in extraterrestrial caves of all varieties.