Follow the Extraterrestrial Water

Avi Loeb
5 min readMay 4, 2023


The chemistry of life as-we-know-it occurs in liquid water. However, the liquid phase cannot exist in vacuum. When water ice is warmed up by sunlight, it sublimates directly into vapor in the absence of ambient pressure. This sublimation process generates a coma around icy comets as they approach the Sun.

To retain atmospheric pressure, a rocky planet must have a surface gravity comparable or higher than that of Earth. A borderline case is Mars, where the surface gravity is 38% of the Earth’s g=9.8 meters per second squared. At around the middle of its lifespan, Mars lost its atmosphere and consequently its liquid oceans, rivers and lakes.

It is certainly possible that Mars developed microbial life in its early water reservoirs. But over the billions of years that elapsed since it became a cold desert, its ancient biomass degraded rapidly. In addition to volcanic activity, asteroid impacts deposited the equivalent energy of a few tens of Hiroshima-bomb explosions per kilometer squared on Mars over the last two billion years. As a result, it is challenging to find traces of ancient life on the Martian surface. Recently, the NASA’s Perseverance rover dropped its first rock sample down onto the Martian surface at the Jezero Crater — one such location of a giant impact, to await retrieval and return to Earth in the next decade.

It is possible that Nature accomplished NASA’s mission of Martian sample return billions of years ago through natural panspermia. Early Martian life could have been delivered to Earth through rocks. In that case, terrestrial forms of life represent its remaining descendants. This possibility is supported by the discovery of the Martian rock ALH84001, whose magnetic properties imply that it was not heated to more than 40 degrees Celsius, allowing for most bacteria or eukarya to survive its interplanetary journey.

If life has been delivered from early Mars to Earth, Martian microbes were the first living astronauts to have travelled between the neighboring planets. In that case, the dream of Elon Musk to die on Mars was preceded by tiny ego-less organisms a few billion years before he was born. Once again, the Universe teaches us a sense of cosmic humility.

Without an atmospheric blanket to moderate temperature variations, a summer day on Mars may reach 20 degrees Celsius near the equator, while at night the temperature can plummet to minus 73 degrees Celsius. In addition, without an atmosphere the Martian surface is exposed to steady bombardment by harmful cosmic-rays. Fortunately, nature offers protection from these harsh conditions by infrastructures under the Martian surface.

Similarly to volcanic caverns on Earth, Mars has lava tubes that formed when basaltic lava flows developed a crusted roof by surface cooling above the subsurface lava stream. The flow eventually drains out of the tube, leaving a void under the surface. Because of its low surface gravity, 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. 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.

If the development of animals was accelerated in time by a factor of two on Mars compared to Earth, there might be paintings on the walls of lava tubes from cave dwellers that inhabited them several billion years ago, when Mars possessed an atmosphere and liquid water. Moreover, if these lava tubes ever hosted interstellar campers, we might find technological debris on their floors. Given the protection they offer to microbial life, lava tubes may be the ultimate frontier of both astrobiology and archaeology.

Aside from enabling life, water can also serve to fuel rockets. Electricity derived from solar panels can split the water into hydrogen and oxygen. When the separate hydrogen and oxygen gases are recombined and ignited, they burn or explode — giving out rapidly the sunlight energy that they consumed during the splitting process.

In the context of fueling resources, reservoirs of water ice on the Moon or Mars are as valuable as oil reservoirs on Earth. As soon as humanity establishes sustainable bases on the Moon or Mars, the local economies will assign a great financial value to water ice assets in order to accommodate their energy needs.

Given the above perspective, water is a remarkable source for both life and rocket fuel. Therefore, the search for large water reservoirs is an exciting frontier for both fundamental science and commercial space exploration, as envisioned by the Copernicus Space Corporation that I co-founded with Dr. Frank Laukien.

Interestingly, the first civilizations on Earth formed along the banks of rivers. The most notable examples are the ancient Egyptians on the Nile, the Mesopotamians on the Tigris and the Euphrates rivers, the ancient Chinese on the Yellow River, and the ancient Indians on the Indus river.

As humanity becomes a multi-planet species, it will continue to follow the water. Not only as it searches for extraterrestrial life and the relics it left in water-rich locations, but also as a fueling resource for supporting life wherever we choose to settle and for propelling humanity’s journey to new destinations.

Without a doubt, water will remain the focal point of scientific exploration and commercial benefits for billions of years to come.


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.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".