It may not look like much — just an old rock with dust on it. But after careful examination of this and other images taken of the Martian surface, a geobiologist is now claiming that these distinctive surface features could have only come from one source: microbes.
This isn't proof that life once existed on Mars. Far from. But it's certainly one of the more intriguing hypotheses we've seen in quite some time.
Working under the (fairly safe) assumption that Mars once featured extensive and persistent surface water, Old Dominion University geobiologist Nora Noffke examined photos taken by Curiosity as the rover traveled through the Gillespie Lake outcrop in Yellowknife Bay in hopes of finding signs of MISS — microbially-induced sedimentary structures.
These geological layers formed from "microbial mats" and can be found here on Earth in any number of environments, including tidal flats, lagoons, riverine shores, and lakes. Billions of years ago, these highly diverse microscopic communities of bacteria became trapped and were rearranged in shallow bodies of water. Their fossilized remnants are still identifiable today.
Noffke reasoned that if Mars once harbored early-stage microbial life, it probably looked a lot like Earth's early-stage microbial life, and that it would have left behind these tell-tale remnants.
Indeed, the Gillespie Lake outcrop would be a great place to look. It's one of many playas on the Red Planet — a dried up lakebed that was filled with water about 3.7 billion years ago. Noffke took a look at Curiosity's Gillespie photographs hoping to find similarities between similar structures here on Earth, specifically 3.48-billion-year-old MISS that she herself discovered in Western Australia's Dresser Formation. It's considered the oldest ecosystem ever found, and about 300 million years younger than previous finds. She also compared the images to sedimentary features found in Germany, the United States, and other locations in Australia.
In the ensuing study, Noffke shows dramatic similarities between Martian sedimentary structures found in the lakebed to microbial structures found on Earth. These features included geological remnants like cracks, chips, pits, pockets, domes, and roll-ups.
Looking at the overlay sketch above, Noffke documented various sedimentary features, such as an elevated planar surface (defined by a 3-4 cm "cliff" with triangle-shaped protrusions), flat clasts, several round-shaped depressions (many of which are surrounded by half-moon-shaped ridges), and larger lineations. These and other features, say Noffke, display a variety of sedimentary structures that have equivalent features to MISS that formed on Earth in similar environmental settings.
Here's a side-by-side comparison of Martian and terrestrial features. These images compare the elevated and planar surface portion visible on a rock bed.
Above is a comparison of sedimentary structures on a Martian rock bed (in a different location on the Gillespie playa) with those on Earth. As Noffke writes:
The rock bed surface on Mars [shown in A] displays two depressions associated with triangular structures and ridges; linear ripple-like features occur in the base of the larger depression. Terrestrial erosional pockets [shown in B] produced by erosion of a microbial mat–covered sedimentary surface in a modern setting. Note the elongated roll-up structure of the original microbial mat on the left side of the erosional pocket in the center of the photograph.
Here we see Martian features compared to microbial terrestrial erosion found at a site at Carbla Point, Western Australia.
Noffke concedes there could be other explanations, writing that "it is important to consider that the Martian structures may simply represent products of abiotic processes."
Pits, for example, could simply be wind-formed artifacts — the result of saltation of rock beds by pebbles blown in by strong winds. Flat clasts, fragments, and flakes of rock could have emerged from a process known as insolation. Cracks could have formed as the result of the precipitation of minerals, and so on. But as she points out in the paper, the similarities of so many features would "constitute an extraordinary coincidence."
To verify the hypothesis, Noffke has presented a four-step process to help find MISS on Earth and other planets:
- Detection: the search for aquatic environments where microbial mats could have formed and been preserved
- Identification: the ability to distinguish actual MISS from abiotic structures
- Confirmation: the search for microscopic textures or structures in candidate features
- Differentiation: the comparison of candidate structures with possible abiotic, but physically and chemically similar structures
"These steps were developed in order to cover the criteria of biogenicity of MISS that is required to prove definitively whether a candidate structure is of biological origin," she writes. "This search requires rover capability and could be conducted during the Curiosity mission as well as a part of any other astrobiology-focused rover mission to Mars."
Reservations aside, Noffke's hypothesis has a number of people excited. As noted in an Astrobio article:
"The fact that she pointed out these structures is a great contribution to the field," says Penelope Boston, a geomicrobiologist at the New Mexico Institute of Mining and Technology. "Along with the recent reports of methane and organics on Mars, her findings add an intriguing piece to the puzzle of a possible history for life on our neighboring planet."
"I've seen many papers that say 'Look, here's a pile of dirt on Mars, and here's a pile of dirt on Earth,'" says Chris McKay, a planetary scientist at NASA's Ames Research Center and an associate editor of the journal Astrobiology. "And because they look the same, the same mechanism must have made each pile on the two planets.'"
McKay adds: "That's an easy argument to make, and it's typically not very convincing. However, Noffke's paper is the most carefully done analysis of the sort that I've seen, which is why it's the first of its kind published in Astrobiology."
And as Noffke is quoted in the Astrobio article, "All I can say is, here's my hypothesis and here's all the evidence that I have, although I do think that this evidence is a lot."
Read the entire study at Astrobiology: "Ancient Sedimentary Structures in the < 3.7 Ga Gillespie Lake Member, Mars, That Resemble Macroscopic Morphology, Spatial Associations, and Temporal Succession in Terrestrial Microbialites".