Life is good in our terrestrial habitat, protected by an atmospheric blanket and a shielding magnetic field, which block harmful cosmic-rays, UV light or X-rays.
We should not take this natural shelter for granted. On our sister-planet nearby, Mars, this protective layer is absent. The atmosphere is over a hundred times thinner with only 0.6% of the terrestrial air pressure and where there is no magnetic shield. How could nature protect humans who aspire to reside on Mars in the decades to come?
The answer might be simple, known to prehistoric humans. They became cave dwellers in order to protect themselves from harsh weather and predators. Interestingly, this might be the best choice also for post-historic humans, like Elon Musk, who would like to live on Mars in the future.
Similar to volcanic caverns on Earth, Mars has lava tubes that formed when basaltic lava flows developed a hard crust roof by surface cooling above the subsurface lava stream. The flow eventually drains out of the tube, leaving a void space under the surface. Since gravity on Mars is 38% that on the Earth’s surface, the Martian lava tubes are often larger than their terrestrial analogs.
Martian lava tubes were discovered in the Viking orbiter images, and later identified in imagery from Mars Odyssey, Mars Global Surveyor, Mars Express, and Mars Reconnaissance Orbiter. Collapsed lava tubes appear as long sinuous troughs known as rilles, and cave “skylights” appear as nearly circular features on the surface of Mars. The skylight near the Martian volcano Pavonis Mons is estimated to have a diameter of 160–190 meters and a depth of at least 115 meters.
Given the potential use of Martian lava tubes as a shield for humans, it would be prudent to send precursor probes into them and map their interiors. Absent direct information from the interiors, we can only simulate or model their surface collapse and skylight morphology and compare these findings with terrestrial analogues. Preliminary studies imply that the volumes of lava tubes are orders of magnitude larger on Mars than on Earth, and their cumulative total volume exceeds a billion cubic meters.
There are many benefits to primitive forms of life in these tubes. First, the Martian surface exhibits extreme temperature variations between 20 °C on a summer day and −70 °C at night near the equator. Subsurface temperature changes are much more modest, making it easier for life to survive. Second, in addition to the shield they provide from energetic particles, lava tubes might trap valuable nutrients and water ice. As a result, a wide range of organisms could have survived under the Martian surface. Before Mars lost its atmosphere, bacteria which are known to exist under near-freezing conditions on Earth, could have thrived inside lava tubes. And of course, if animals inhabited them, we might find their skeletons.
Irrespective of whether traces of primitive life may already exist in the interior of Martian lava tubes, advanced technological life may reuse them as shelters in the future. Subsurface cavities that are tens of meters long, would protect human health and technology from the threat of micrometeorites.
Once again, nature was kind to us in providing the protective infrastructure that would save on payload mass and construction efforts needed to accompany crewed missions to Mars.
But we could also gain insights about the distant past, more than 2.5 billion year ago — a time when Mars had an atmosphere and liquid water on its surface. If Martian cave dwellers existed back then, we might find their tools on the ground or paintings on the walls of lava tubes.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, is scheduled for publication in August 2023.