Searching for biosignatures in sedimentary rocks from early Earth and Mars

The recognition of past habitable environments on Mars has increased the urgency to understand biosignature preservation in and characterize analogues of these environments on Earth. In this Review, we examine the detection and interpretation of potential biosignatures preserved in deposits rich in carbonates, silica and clay. Many of the earliest chemical, textural and morphological evidence of life on Earth are found in carbonates and carbonate-hosted phases. Early diagenetic chert within carbonate deposits can exceptionally preserve microbial body fossils, and clay minerals that form in ultramafic terrains can protect organic matter. On Mars, similar deposits older than 3.5 billion years could contain biosignatures or remnants of prebiotic processes that have long been erased from Earth. Terrestrial analogues for the deposition of magnesium carbonate minerals in Jezero crater, Mars, present patterns that can guide the collection of samples with the highest astrobiological potential by the Perseverance rover. Continued characterization of terrestrial analogue sites and rigorous examination of the processes that impact the preservation of isotopic signals, organic compounds, and microbial textures and fossils will advance the interpretation of Martian deposits. With the landing of the Perseverance rover and the search for biosignatures on Mars, it is important to understand how these signs of life can be preserved and interpreted. This Review describes these signatures on Earth and how terrestrial analogues can inform the search for signs of life on early Mars.