An Organics-forward Approach To Searching For Life On Mars

The modern Martian surface is cold, dry, oxidizing, and perpetually bombarded with radiation, rendering it generally inhospitable to life as we know it.

However, geological evidence suggests ancient Mars was more Earth-like and characterized by warmer temperatures, widespread fluvial activity, a thicker atmosphere, and a protective magnetic field shielding the surface from radiation.

Habitable conditions continued until cessation of the Martian dynamo terminated the planetary magnetic shield, leading to solar wind-induced atmospheric erosion and the loss of surface liquid water.

Nevertheless, early environments inferred to be analogous to those found on Earth, common molecular feedstocks (including delivery of volatiles and organic matter by comets and asteroids), and plausible reactive pathways may have resulted in parallel origin of life events more than 3.4 billion years ago.

Organics as potential biosignatures on Mars

On Earth, life seemingly arose from a fortunate coalescence of chemistry and geology under placid conditions. Was this also the case on ancient Mars? If so, did increasingly cold and dry conditions result in the extinction of Martian life, or did it adapt to the changing environment?

Perhaps life never arose—if not, why? We believe these questions can be addressed with an organics-forward approach, because most of the building blocks of life form naturally through abiotic processes and are ubiquitous throughout the Solar System4–7. However, specific differences in their structures and distributions reveal crucial clues regarding their chemical origins.

Terrestrial biology preferentially synthesizes a small and specific set of building blocks to create complex biopolymers. These include, for instance: a homochiral suite of 20 amino acids, phospholipids containing predominantly C16 and C18 straight-chain carboxylic acids, and five nucleobases along with ribose (also with chiral preference) to build nucleic acid informational polymers (e.g., DNA and RNA).

In contrast, abiotic reactions within primitive accreting bodies (e.g., asteroids and comets) yield stochastically distributed molecular mixtures characterized by racemic amino acids, highly branched aliphatic carboxylic acids, hydrocarbons at low carbon numbers, and other molecular isomers not utilized by terrestrial biology.

Furthermore, some of these organics can persist throughout geological timescales, with the potential to serve as evidence for either extant or extinct life. On Earth, molecular fossils represent some of the most robust evidence of ancient life. If Mars experienced an independent genesis, similar organic remnants with structural preferences could persist in the geologic record today.

Astrobiology, Astrochemistry, Astrogeology,