NASA’s news release reports that Perseverance cored a mudstone in Jezero Crater whose chemistry and textures may record potential biosignatures—not proof of life, but suggestive clues. What follows are my thoughts about why, if we struggle to define “life” at all, the attempt to infer past life from such clues is far more complex—and why that complexity is a feature, not a flaw, of rigorous science.
1) What Perseverance actually found (and what it didn’t)
In July 2024, Perseverance drilled a core called Sapphire Canyon from a reddish mudstone nicknamed Cheyava Falls in the Bright Angel formation, an ancient river valley feeding the Jezero crater lake. Instruments (PIXL and SHERLOC) mapped sub-millimeter textures—“leopard-spot” reaction fronts—and a suite of iron phosphate and iron sulfide minerals consistent with vivianite and greigite, alongside organic carbon, sulfur, oxidized iron, and phosphorus. NASA characterizes these as potential biosignatures: patterns and minerals that, on Earth, can arise from microbially mediated redox reactions, yet may also form abiotically. The press release is explicit: this is not a claim of life, but a peer-reviewed case that warrants deeper testing—ideally with Earth-based laboratories.
Independent reporting likewise struck the balance: the rock’s textures and chemistry are among the strongest yet hints for ancient habitability and possible biology, but alternative non-biological pathways must be evaluated; definitive evaluation would benefit from sample return.
Those sentences already foreshadow the core challenge. Even before we ask whether a Martian mineral fabric was biogenic, we face a prior problem: What do we even mean by “life”?
2) The definition problem
For decades, NASA’s working definition has been pragmatically compact: “Life is a self-sustaining chemical system capable of Darwinian evolution.” It is useful because it anchors to chemistry and heredity without committing to Earth-specific biochemistry. But it is also famously leaky at the edges—do viruses count? What about sterile hybrids? Minimal RNA replicators? Philosophers and biologists have catalogued these boundary cases precisely to remind us that any definition will be operational, not metaphysical. The definition guides experiments; it does not settle ontology.
This matters because biosignatures are not organisms. They are traces—molecules, minerals, isotopic ratios, textures, morphologies—interpreted as evidence for life according to some definition. NASA’s own public materials draw a crucial distinction between a biosignature and a potential biosignature: a candidate that might be biological but for which abiotic pathways have not yet been reasonably excluded. That second term is where Perseverance’s finding now lives.
3) From “life” to “past life”: multipliers of difficulty
Inferring contemporary life is already hard; inferring ancient life multiplies the uncertainty for at least five reasons.
- Time degrades evidence. Signals fade. Organic matter is oxidized, heated, shocked, irradiated, weathered, mixed with later fluids, recrystallized. Even on Earth, interpreting the earliest putative fossils and stromatolites is contentious because geological processes have overwritten original textures and chemistries. On Mars, four billion years of surface exposure only intensify the problem.
- Abiotic mimics abound. Minerals we associate with biology—like greigite or vivianite—can also form without life under specific redox, temperature, acidity, and ion-availability regimes. Some textures that look “cellular” or “microbial” emerge from purely chemical self-organization. The literature on false biosignatures is deep for good reason.
- Context is king. A molecule or mineral is rarely dispositive in isolation. Interpreting it requires the geologic and geochemical context—sedimentary setting, diagenetic history, temperature/pressure constraints, timing of fluids, sources and sinks of carbon, sulfur, phosphorus, and iron. The Nature paper on Bright Angel emphasizes precisely this: the chemistry is consistent with low-temperature, redox-driven reactions involving organic matter—but that conclusion hangs on context.
- Instrument limits in situ. Rovers are miracles of engineering, but the instruments that fit on a one-ton robot cannot match the sensitivity, selectivity, and cross-calibration of Earth laboratories (nano-SIMS, clumped isotopes, enantiomeric excess, trace contaminant profiling, etc.). Hence NASA’s repeated refrain: sample return is the royal road from “suggestive” to “dispositive.”
- Our sample size of “life” is one. Definitions, training datasets, and intuitions are Earth-centric. Biosignatures for unfamiliar biochemistries—different solvents, different energy currencies—may be invisible to our methods. Conversely, Earth’s biological patterns may be too readily projected onto non-biological Martian processes. The risk runs both ways.
4) Hard-won caution: lessons from ALH 84001
In 1996, a Martian meteorite—ALH 84001—was announced to harbor possible microfossils, magnetite grains, and organics consistent with life. Decades of scrutiny followed. Some features could be explained by terrestrial contamination; others, by abiotic processes; others remained ambiguous. The most valuable residue of that debate was not “yes” or “no,” but the development of methods, controls, and skeptical discipline that now inform Mars work today. Perseverance’s team repeatedly signals they have learned that lesson.
5) Why NASA speaks in rungs, not declarations
In 2018 and 2021, NASA-affiliated scientists urged the community to adopt structured confidence scales—the “Ladder” or Confidence of Life Detection (CoLD)—so that claims move in steps: detect a possible signal; rule out contamination; establish biological plausibility; reject known abiotic sources; obtain independent, convergent lines of evidence; survive future scrutiny; and, finally, obtain independent confirmation. The recent press release explicitly gestures to these frameworks and to a community “standards of evidence” effort. The point is not to bureaucratize discovery; it is to communicate uncertainty honestly and to plan investigations that climb the ladder deliberately.
Under such a rubric, Bright Angel sits neither at the bottom (mere detection) nor at the top (confirmation). It marks a credible ascent: a plausible environment, intriguing chemistry, and textures consistent with redox processes involving organics—alongside an explicit, disciplined consideration of non-biological pathways.
6) How biosignature inference actually works
Stripped to essentials, inferring past life is a Bayesian enterprise. We refine our degree of belief by integrating multiple, independent constraints and by testing alternative hypotheses.
- Hypothesis set. For Bright Angel: (H1) microbially mediated redox cycling produced vivianite/greigite textures; (H2) purely abiotic reactions did, under low-temperature conditions; (H3) textures/minerals formed at higher temperatures or in acidic regimes later erased from other signals; (H4) organics are exogenous, later introduced, or catalytic but not biogenic. The Nature study inventories and weighs these possibilities.
- Discriminants. We look for fingerprints that are hard to mimic abiotically: – Isotopic fractionations (e.g., multiple-S, C, Fe) characteristic of metabolism; – Enantiomeric excess in organics; – Compound-specific isotopes tracing biological pathways; – Textural relationships (growth fronts, templating) that fit metabolic gradients; – Co-occurrence patterns across minerals and organics locked by timing. The National Academies syntheses and “false biosignature” reviews outline these discriminants and their pitfalls.
- Context priors. Ancient river-fed mudstones, rich in organics, sulfur, iron, and phosphorus, increase biological plausibility relative to basaltic lavas or high-T hydrothermal veins. Yet priors are not proofs; they only weight interpretations.
- Evidence accumulation. On Mars, we push in situ methods to their limits; to go further we need the lab bench. NASA’s own communication emphasizes that crossing higher CoLD rungs likely requires sample return—itself a programmatic challenge subject to budget and schedule.
7) Why “potential biosignature” is the right phrase—scientifically and philosophically
The modesty of the term often frustrates readers who yearn for a declarative headline. Yet it encodes three virtues.
First, it respects underdetermination. The same data may be explained by multiple mechanisms; acknowledging that is not weakness but epistemic hygiene. The Standards-of-Evidence community report was organized precisely to keep the discipline honest about this.
Second, it avoids definition traps. If we cannot concisely define life to everyone’s satisfaction, we can still define tests that make “life-like” explanations progressively more plausible than alternatives. The NASA working definition guides those tests without pretending to settle boundary questions.
Third, it keeps the public conversation calibrated. Astrobiology sits at a cultural fault line—immense public appetite, high stakes for credibility. Scaled claims, with explicit levels and caveats, protect both the science and the story. NASA’s recent release is exemplary in that regard.
8) What would move Bright Angel higher up the ladder?
Some practical next steps—most only feasible in Earth labs, hence the urgency of sample return.
- High-precision isotopes. Carbon (δ¹³C), sulfur (multiple-S), iron (δ⁵⁶Fe) at grain-scale, tied to textures, to test for fractionations characteristic of metabolism versus abiotic pathways.
- Organics at molecular resolution. Compound-specific isotopes, chain-length distributions, chirality, and degradation products that are statistically improbable under non-biological synthesis.
- Reaction-front microstratigraphy. Nano-SIMS maps aligning organics with mineral fronts; templating relationships that imply growth coupled to redox gradients.
- Thermal/chemical history constraints. Clumped isotopes and thermometry to rule out high-T phases that would favor abiotic formation of greigite or vivianite, matching the in situ low-temperature inference.
- Abiotic null models. Laboratory recreations of plausible Martian fluids, temperatures, and diagenesis producing similar textures without biology—then comparing statistics. The “false biosignature” literature encourages building the skeptic’s model first.
9) The long shadow of a meteorite—and why it helps
ALH 84001 is the cautionary tale we deserve. The early excitement—the “nanofossils,” PAHs, magnetite—was gradually wrapped in sober caveats about contamination, alternative mineral pathways, and formation temperatures. That protracted debate forged today’s conservatism. Bright Angel’s authors and NASA communicators now emphasize low-temperature context, multiple lines, and explicit alternatives—a posture learned, at least in part, from that 1990s storm.
10) So, how much more complex is it to infer past life?
Much more complex—and instructively so.
When we define life, we wrestle with borderline cases but enjoy the immediacy of living systems: metabolism, replication, evolution observable or at least proximate.
When we infer past life, we do historical science. We reconstruct causes from traces through intervening noise. The problem is not just classificatory; it is forensic. It demands stratigraphy, diagenesis, kinetics, geochemistry, and statistics. It demands humility about alternative mechanisms. It demands, above all, convergence: context + chemistry + texture + isotopes + replication of results by independent teams.
The Perseverance result is compelling precisely because it tightens several of those strands at once, while refusing to leap beyond the data. It tells us the right thing twice: this might be life, and we don’t know yet. Both statements are true; the second protects the first.
11) A brief coda on meaning
One seductive mistake is to believe that certainty is the only form of knowledge worth having. In frontier science, we live by graded confidence. The Confidence-of-Life-Detection ladder and the community’s standards-of-evidence project are intellectual technologies for holding that graded confidence in public view. They slow us down on purpose. If Mars once harbored microbes, history will not change while we do the work carefully. And if Mars did not, then the very process of eliminating biological explanations will have taught us how to search more wisely elsewhere.
12) Final synthesis
We struggle to define “life” because life is a cluster of properties: self-maintenance, metabolism, heredity, adaptation, ecological entanglement. The NASA working definition is useful scaffolding, not a verdict. Biosignatures are not life; they are arguments about life, built from traces. In Bright Angel, the argument is coherent, cautious, and incomplete. It is also the best kind of progress: the kind that tells us exactly what to do next.
