Mars: Was There Once Life?

Mars is the world we know best after our own — and for over a century, it has fueled our imagination about extraterrestrial life. From Percival Lowell's "canals" to David Bowie's Ziggy Stardust, Mars has been humanity's go-to candidate for alien neighbors.

Today, the question is more refined but no less exciting: we're not looking for little green men. We're asking whether microbial life once existed during Mars's warmer, wetter past — and whether traces of it might still survive underground.

The Wet Planet

Billions of years ago, Mars was a very different world. Orbital imagery and rover data have revealed overwhelming evidence that liquid water once flowed abundantly:

  • Jezero Crater, where NASA's Perseverance rover currently operates, was once a lake fed by river deltas — exactly the kind of environment where life thrives on Earth
  • Valles Marineris, a canyon system stretching 4,000 km, shows evidence of ancient water erosion
  • Mineral deposits including clays, sulfates, and carbonates that only form in the presence of liquid water
  • Polar ice caps and subsurface ice confirmed by multiple missions

Mars also had a thicker atmosphere in its early history, potentially warm enough to support liquid water on the surface for hundreds of millions of years. That's more than enough time for life to emerge — on Earth, life appeared within the first 500 million years.

💡 Key Concept

Mars lost most of its atmosphere over billions of years because it lacks a global magnetic field. Without this protective shield, the solar wind gradually stripped away atmospheric gases, causing temperatures and pressures to drop until surface water could no longer exist as liquid.

The ALH84001 Controversy

In 1996, a team led by David McKay at NASA's Johnson Space Center made a bombshell announcement: a meteorite from Mars, found in Antarctica and labeled ALH84001, appeared to contain evidence of ancient Martian life (McKay et al., 1996).

The meteorite, blasted off Mars's surface by an asteroid impact millions of years ago, contained:

  • Carbonate globules formed in the presence of water
  • Mineral structures resembling those produced by bacteria
  • Polycyclic aromatic hydrocarbons (PAHs) — organic molecules associated with biological decay
  • Tiny elongated structures that looked like fossilized nanobacteria

The announcement made global headlines and even prompted a presidential statement. But the scientific community quickly pushed back. Each line of evidence, critics argued, could be explained by non-biological processes. The "nanobacteria" were far smaller than any known living cell. The PAHs could have come from contamination or abiotic chemistry.

⚠️ Scientific Debate

ALH84001 remains controversial to this day. While most scientists believe the evidence is insufficient to confirm past Martian life, the meteorite demonstrated that Mars rocks can preserve organic chemistry over billions of years — which is itself a significant finding.

Perseverance and Sample Return

NASA's Perseverance rover, which landed in Jezero Crater in February 2021, represents our most sophisticated attempt to search for ancient Martian life. The rover carries instruments designed to:

  • Detect organic molecules in rock samples using UV spectroscopy and Raman spectroscopy
  • Map mineral composition to identify rocks formed in habitable environments
  • Photograph rock textures at microscopic scales, looking for structures that might be biological in origin
  • Collect and cache samples in sealed tubes for future return to Earth

The sample return component is critical. Perseverance has been carefully selecting and caching rock cores from the most scientifically promising locations. A future Mars Sample Return mission — a joint NASA/ESA endeavor — aims to bring these samples back to Earth, where they can be analyzed with laboratory instruments far more powerful than anything a rover can carry.

💡 Why Sample Return Matters

Definitive detection of ancient biosignatures likely requires Earth-based laboratory analysis. No rover instrument can match the sensitivity and resolution of terrestrial mass spectrometers, electron microscopes, and genetic sequencing tools. Bringing Mars rocks home is the gold standard.

The Underground Question

Even if Mars's surface is now sterile — bombarded by UV radiation, oxidizing chemicals, and extreme cold — life might persist underground. Several factors support this possibility:

  • Subsurface liquid water may exist where geothermal heat melts ice
  • Perchlorate salts in Martian soil can lower water's freezing point, allowing brines to form at surprisingly cold temperatures
  • Methane has been detected in Mars's atmosphere by multiple missions, with seasonal variations that are difficult to explain through geology alone

On Earth, microbes have been found living kilometers underground in rock fractures, surviving on chemical energy from water-rock interactions. Mars's subsurface could harbor similar communities, completely isolated from the hostile surface.

🔮 Speculative Biology

If life once existed on Mars's surface and retreated underground as conditions deteriorated, it may have evolved to survive in extremely low-energy environments — perhaps metabolizing so slowly that a single cell division takes centuries. Detecting such life would require drilling several meters or more below the surface.

What Would Ancient Martian Life Tell Us?

Finding fossilized microbes in Martian rocks would be groundbreaking, but the real scientific jackpot would be understanding their biochemistry. Did Martian life use DNA? The same amino acids as Earth life? Or something completely different?

If Martian life shares biochemistry with Earth life, it could suggest that life was transferred between the two planets via meteorites — a process called panspermia. If it's biochemically distinct, it would prove that life can originate independently, making it likely that biology is common throughout the universe.

Either answer would transform our understanding of life's place in the cosmos.

Sources

  • McKay, D.S. et al. (1996). "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001." Science, 273(5277), 924–930.
  • NASA Mars 2020 Perseverance Mission. mars.nasa.gov/mars2020
  • Orosei, R. et al. (2018). "Radar evidence of subglacial liquid water on Mars." Science, 361(6401), 490–493.
  • Webster, C.R. et al. (2018). "Background levels of methane in Mars' atmosphere show strong seasonal variations." Science, 360(6393), 1093–1096.