Martian rocks photographed by Curiosity look tantalizingly similar to those created by microbial action on Earth. The observation is far from proof that the rocks have a living origin, but it is nevertheless one of the most promising signs yet found by the Martian missions.
Microbially Induced Sedimentary Structures (MISS) represent some of the oldest evidence for life on Earth, stretching back 3.5 billion years. They are similar to, but flatter and less noticable than, the more famous stromatolites.
MISS occur when colonies of bacteria form microbial mats that precipitate calcium ions to form layered structures that compact down to become rocks. Old Dominion University's Dr. Nora Noffke, who discovered the oldest known MISS on Earth, noticed the outline of a rock bed at Gillespie Lake in images taken by Curiosity. The similarity is so striking it can be seen even by those who have not spent years studying fossilized microbial populations, at least once Noffke pointed it out.
Noffke explains her suspicions in Astrobiology. Intriguingly, the sandstone beds she points to may be 3.7 billion years old, slightly more ancient than those she found in Western Australia.
"Without any evidence for life on Mars, reconstructing microbial ecosystems on the Red Planet (and how they might have changed temporally) is pure speculation,” she writes. “It is worth noting, however, that microbial biofilms and often laminated mats on Earth develop on nearly every solid substrate exposed periodically to water.”
“Three lithofacies of the Gillespie Lake Member sandstone display centimeter- to meter-scale structures similar in macroscopic morphology to terrestrial MISS that include 'erosional remnants and pockets,' 'mat chips,’ ‘roll-ups,’ ‘desiccation cracks,’ and ‘gas domes,’” Noffke writes. Moreover, she notes they are not randomly distributed, but arranged in ways that “indicate they changed over time.” One might almost dare to say evolved.
Image credit: Noffke/Astrobiology. An overlay of a sketch on the photograph above to assist in the identification of the structures on the rock bed surface
Noffke does more than note the similarities, although she does that in great detail in the 23 page open access paper. She also outlines “a strategy for detecting, identifying, confirming and differentiating possible MISS during current and future Mars missions.”
The wave of evidence for ancient surface water on Mars produced by Spirit, Opportunity and Curiosity has bolstered hopes that Mars may once have had abundant life, which may survive in underground pockets today. Noffke is not the first to see evidence of microbial action in Martian rocks, but her extensive experience with similar terrestrial formations gives her work credibility.
The playa Noffke observed shows signs of being formed by an “alternating wet and dry paleoclimate” that gradually became drier like many on Earth. In some locations, Noffke sees signs of the “establishment and subsequent decomposition of microbial mats.” In others, she sees additional stages, with gasses trapped beneath the mats that formed domes that eventually busted through, leaving holes behind. These match with different structures in a range of formations around the world, particularly the oldest ones known.
Credit: NASA/Noffke. Comparison of cracks in the Gillespie Lake outcrop on Mars to the modern microbial mat in Bahar Alouane, Tunisia
Noffke acknowledges that non-biological forces can form similar structures to those observed, but argues the extent of the similarities would require “an extraordinary coincidence.”