Cabrol, Nathalie

astrobiologist and planetary geologist; director of the Carl Sagan Center for the Study of Life in the Universe at the SETI Institute; holds women's world records for altitude scuba and free diving

Nathalie Cabrol: Search for Alien Life

Lex Fridman Podcast · Dec 19, 2022

• Nathalie Cabrol
• SETI Institute
• search for life on Mars
• nature of life versus origin of life
• shadow biosphere
• high-altitude extremophile diving
• cyanobacteria and UV resistance
• UAP and UFO science
• Drake equation
• astrobiology fieldwork

Core ideas

Cabrol traces her research path from studying ancient water channels and lakes on Mars using early Viking-era imagery (which she notes was so low-resolution that scientists could only infer former lakes by locating impact craters) to realizing that understanding whether Mars was ever habitable required first understanding, in fine detail, the environments where analogous life survives on Earth today. She explains that impact craters are paradoxically both destructive and constructive for life — bringing violent energy and disruption, but also delivering many of the raw chemical building blocks (carbon, hydrogen, oxygen, nitrogen, phosphorus) that make up living organisms — and situates this within a broader discussion of panspermia, which she stresses is only a potential distribution mechanism for life's ingredients, not an explanation for life's origin itself. She argues that because roughly 82% of Earth's geological history elapsed between the emergence of simple microbial life and the rise of complexity (let alone intelligence), any life that arose on Mars — where surface conditions favorable to life may have lasted less than a billion years before the loss of its magnetic field and atmosphere — likely never advanced past a simple, microbial stage, and that if evidence remains, it would most plausibly take the form of morphological traces like stromatolites or subtle chemical signatures (she mentions ambiguous carbon-12 abundance findings) rather than anything resembling a fossil animal.

A recurring theme is her conviction that the deeper scientific prize is not merely finding life elsewhere but understanding the fundamental "nature of life" — which she frames, drawing on Erwin Schrödinger's and physicist Jeremy England's work, as fundamentally about the universe's drive to gather, exchange, and preserve information and thereby resist entropy. She draws an extended analogy between the hierarchical structure of biochemistry (atoms to molecules to RNA/DNA) and human language (letters to syllables to words needing a verb for direction), arguing that if life is best understood as a general information-processing and entropy-resisting process, then the search for extraterrestrial life might be reframed as a search for a universal signature not tied to any particular biochemistry — a perspective she says pushes her toward the idea that the universe itself may be best understood as an increasingly complex, information-gathering, and possibly self-aware process, echoing Carl Sagan's framing of humanity as "the universe trying to understand itself." She pushes back against a strict Fermi-paradox framing of alien absence as anthropocentric, and offers her own account of the "shadow biosphere" hypothesis — the idea that life may have originated multiple times on Earth via different, currently unrecognized biochemical pathways still present but undetected because our tests are built only to recognize life as we know it — as an intellectually valuable exercise in scientific humility about her own potential blind spots, regardless of whether the hypothesis itself holds up.

On UFOs and UAPs, Cabrol distinguishes her own SETI-affiliated work (searching for extraterrestrial life and technosignatures "in their natural habitat," largely privately funded due to decades of political stigma tracing back to Cold War-era "flying saucer" panic) from UAP investigations (assessing whether unexplained aerial phenomena represent incursions into human airspace), stating that roughly 95% of UFO reports are explainable natural or human phenomena, a further slice are likely classified programs, and a small residual category remains genuinely unexplained; she welcomes recent government interest in scientifically studying UAPs as a possible "course correction" after decades in which the field's association with unfounded folklore made it politically toxic to fund serious SETI research, while pushing back gently against Lex's framing that leaning into public fascination ("surfing the wave") is costless, noting the discipline paid a real price in lost funding and career opportunities for scientists.

Cabrol recounts in vivid detail her fieldwork diving in high-altitude volcanic lakes in the Andes (some over 20,000 feet, chosen as close terrestrial analogs to early Mars due to their aridity, high UV exposure, hydrothermal activity, and Mars-like coloration) as a way to study UV- and temperature-resilient cyanobacteria that have persisted largely unchanged for roughly 3.5 billion years; she describes her transition from a childhood-rooted fear of pressurized tanks that initially pushed her toward free diving, to eventually training with the U.S. Navy for three months to use military-grade oxygen rebreathers (eliminating decompression risk and mitigating altitude sickness) once free-diving time limits became scientifically limiting. She recounts a near-fatal free-diving incident in which trapped air in a dry suit left her stuck underwater and unable to surface for several attempts, describing training-induced calm rather than panic, and separately recounts leading a team through a magnitude-7.8 earthquake near a volcanic summit, followed by signs of a possible nearby eruption (color and composition of emitted gas serving as real-time danger signals relayed over radio), that forced an emergency descent — noting that her adrenaline response collapsed into complete physical exhaustion only once the danger had passed.

On the search for life on Mars specifically, she argues current orbital data describes habitability only at a planetary or regional scale, while what actually matters is habitability "at the scale of the microbe" — citing endolithic cyanobacteria that shelter inside translucent rocks, using the rock as UV shielding while still permitting photosynthesis, as a model for where Martian microbial life, if it exists or persists, might most plausibly be found (potentially not deep underground, but just beneath exposed surfaces). She proposes deploying networks of environmental monitoring stations on Mars that would simultaneously serve astrobiology, planetary protection (avoiding contamination in both directions), and future human exploration logistics. She closes with a personal and philosophical arc: describing a troubled adolescence including a suicide attempt and a near-fatal rescue attempt of a drowning stranger as formative experiences that permanently reoriented her toward valuing and pursuing life fully; an account of an intense feeling of unity and "no separation" she experienced while free-diving at a volcanic summit; reflections on her recently deceased husband (a 44-years-older partner she describes as a defining influence on living fully without dwelling on mortality); and a closing argument that humanity's stewardship of Earth's biosphere — amid what she calls an unfolding sixth mass extinction — should take priority over, and inform, any near-term ambitions toward settling other planets.

Excerpts