Astronomy
The Potential of the Martian Subsurface for Preserving Biosignatures
Quick fact
While the Martian surface experiences intense radiation that can destroy complex organic molecules within a few hundred million years, just a few metres of rock and regolith can shield biosignatures from the most damaging radiation for billions of years, making the subsurface the most promising archive of past life.
Why this is interesting
You've seen the red, dusty surface of Mars—but did you know that the most exciting clues to life may be hiding metres underground? What makes the deep interior so special?
Read the full explanation
Understanding The Potential of the Martian Subsurface for Preserving Biosignatures
Imagine a crime scene where the evidence has been bleaching in the sun for billions of years. The surface of Mars is bombarded by cosmic rays and ultraviolet light, which break apart organic molecules—the building blocks of biology. Additionally, chemical oxidizers like perchlorates react with and destroy organic material. But below the top few metres, the picture changes dramatically. The ground acts like a thick blanket, absorbing radiation. Cosmic rays lose energy and eventually stop; ultraviolet light is completely blocked. The temperature is more stable, and there is even subsurface water ice. This combination of shielding and relatively benign conditions means that if life ever existed, its chemical remains—biosignatures—could be preserved in the rocks below. This is why astrobiologists now look downward, rather than at the dusty surface, when searching for evidence of ancient life.
A deeper explanation
The preservation potential relies on the physics of radiation penetration and the chemistry of organic degradation. Ionizing radiation (galactic cosmic rays and solar energetic particles) strikes the surface with high energy, knocking electrons from atoms and breaking chemical bonds. Over time, this radiation can destroy organic molecules through a process called radiolysis. In addition, the surface is bathed in ultraviolet radiation, which is even more effective at breaking carbon bonds. The top metre or so is completely sterilized. However, radiation intensity decreases exponentially with depth. Below a few metres, the radiation dose is low enough that organic molecules can persist for billions of years. Furthermore, perchlorates, which are common on Mars, are strong oxidizers that can react with organics when exposed to ultraviolet or heat, but this reaction is less efficient in the cold, dark subsurface. Thus, the subsurface offers a triple shield: it reduces radiation, blocks UV, and limits oxidative chemistry. Even in the absence of active life, it can preserve molecular fossils, fatty acids, amino acids, and even DNA fragments if they ever existed. Understanding this mechanism is crucial for missions like ExoMars's Rosalind Franklin rover, which will drill up to two metres deep, and for selecting sample sites for Mars Sample Return.