Hypothetical predawn on ancient Mars, more than three billion years ago. The dimmer sun, shining through a thicker layer of warmer air, gently stirs the morning mist above shallow lakes as they greet the new sol.
When we see giant, colorful images of Mars today, we are tempted to place them in familiar context. Sunny desert, rocky crags, 24-hour days with seasonally cold nights and dry gulches, not unlike rugged vistas dotting the U.S. from Arizona to Wyoming.
Do not be fooled! Mars is not the desert southwest. The stippled, cratered surface is chemically ripped asunder by harsh solar and cosmic radiation, and the thin, turbulent air offers little protection from meteorites large and small. The tortured rock formations are scoured by a hail of biting, toxic dust carried by sub-Arctic winds of bone-dry carbon-dioxide and nitrogen at gale force. The open surface of Mars today is a forbidding, alien, instantly lethal place.
But over the last few decades, researchers have learned it wasn't always so. There was a time when Mars was warmer, its air thicker, and most important of all, parts of its surface covered in liquid water, much like Earth at the same time. A number of researchers have speculated on what this might mean for early Mars: Could life have caught on there, for however long?
Using the latest analysis and the best images from Mars Curiosity, one researcher has offered what may be new, testable lines of evidence that over three billion years ago, maybe some life did catch on:
Sandstone beds of the <3.7 Ga Gillespie Lake Member on Mars have been interpreted as evidence of an ancient playa lake environment. On Earth, such environments have been sites of colonization by microbial mats from the early Archean to the present time. Terrestrial microbial mats in playa lake environments form microbialites known as microbially induced sedimentary structures (MISS). On Mars, three lithofacies of the Gillespie Lake Member sandstone display ... structures similar in macroscopic morphology to terrestrial MISS ...
Join me below to review the findings, couched in layman's terms, hopefully, and a rockin-rolling, crowd-sourcing jubilee from our own community.
The paper begins with an introductory thesis using the terms playa for preserved lake bed, and microbialites, which form microbially induced sedimentary structures (MISS), such as stromatolites on Earth:
On Mars, ancient playa lake settings are quite common. ... On Earth, and presumably on Mars, playas are typical for a semiarid climate zone. During the periods of subaerial exposure of sedimentary surfaces in a playa, salt minerals precipitate atop and within the sediment. On Earth, clastic-evaporitic playas are known from the oldest sedimentary rock record 3.48 Ga to the present time. Throughout Earth history, playa sediments have been colonized by benthic microbial mats dominated by filamentous cyanobacteria that interact with the physical sediment dynamics at their sites of colonization to form characteristic microbialites known as microbially induced sedimentary structures (MISS).
The next part of the paper talks about MISS on Earth, noting their living and rocky, or lithofied features, are well-studied and understood. In the second section, the paper looks more closely at ancient Mars and the area Curiosity landed in, with a focus on the Lake Gillispie region, giving it an upper age of 3.7 billion years.
It's worth noting here just how diverse and unfriendly these MISS environments were and remain here on Earth. We have mircobes thriving in boiling-hot water at the bottom of the ocean with no sunlight; microbes that colonize tiny pores in solid, frozen rocks; microbe colonies in toxic sand alternately moistened and freeze dried; mounds and snotties and vases of microbes that cannot live unless bathed in mineral solutions so active or acid so strong they would chemically dissolve less-specialized organisms in minutes— they eat compounds of hydrogen, sulfur and even iron. Many form colonies with characteristics so distinctive that preserved remains can be easily seen with the unaided eye and quickly verified with a modest hand lens.
On Mars we are lucky explorers. Virtually the entire Martian landscape was frozen solid and dried at exactly the right time, in detail, and sections of it went on to survive subsequent events such as impacts, volcanoes, and weathering. Mars Curiosity was designed to look "for the ingredients of life" in those kinds of places. In retrospect, the Curiosity team could hardly have located and landed in a better surviving piece of real estate than Gale Crater and the Lake Gillispie playa to carry out that mission.
Drawing on all this, the paper assumes that microbes and the structures created by them, including the kind that thrived on Archean Earth over three billion years ago, could have probably survived on a wet Mars virtually unchanged in some locations. We can infer here today that they would have existed in number enough, with enough biological plasticity, to adapt to environmental variation and Martian idiosyncrasies over time, just as they have on Earth in scores of hospitable and extreme environments for billions of years. So the big question is, did they?
An overlay of sketch on a Mars photograph from above to assist in the identification of the structures on the rock bed surface used in a study by geobiologist Nora Noffke in the journal Astrobiology.
This is where the real action begins to happen in the paper. It examines large scale lithography, i.e., the actual features in the rocky surface material captured by Curiosity's sensitive eyes. Parts five through seven examine that issue in rigorous, professional detail. These include items like pits, clasts, ridges, and spatial grouping of candidate fossil assemblies, where serial positive comparisons are noted with terrestrial analogues. The paper then rather simply concludes with the identification of biological origin as a possibility and briefly discusses alternative formation from non-living processes. So far, Mars Curiosity Team members have said they're interested in any research from the rover, but for now they're more inclined to existing, non-biological explanations for the rocky features.
There is good evidence life appeared on Earth in quantity right on the heels of a hail of large impacts swarming through the inner solar system called Late Heavy Bombardment—a coincidence that seems awfully peculiar given the likely magnitude of the destruction. The barrage tapered off right as these ancient Martian playa were coming into their own, just over 3.8 billion years ago. But since life did appear, or somehow persevered on Earth through the bombardment, and since the Martian environment at the same time was probably similar to ours, we have to leave open the possibility that life could have developed there independently.
It's also fun to speculate that the Martian surface was arguably even more friendly by modern standards. Earth, shortly after bombardment, with its greater gravity, higher greenhouse gas pressures, and holding a greater heat of formation, may have have been a pitch-black smoking hellscape for tens of millions of years longer than Mars. Several analysis pegged Archean ocean temperature around 40º C or higher (+100º F). Life could have hitched a ride from a more hospitable Mars to a raging hot Earth for all we know. If life is ever confirmed off world—on Mars or anywhere else—without better data, a lot of data, and no small amount of luck finding it, that question may never be resolved.
The question of life on Mars will continue to fascinate us. But what's so exciting about this paper is how it demonstrates that everyone, inside or outside of NASA, can contribute to finding out as much as we can. That's partly because our ability to extract data from so far away is getting so good. The Curiosity Team was able to land safely on autonomous cables—unreeling from a crane suspended on rocket motors, mind you—and thanks to them we now have a roving, photo-geo-chemical laboratory on the surface of an alien world, with the power to investigate traces of ancient organics or possible microbes that seeped into alien rock over three billion years ago.
Our unmanned space program is the biggest bang for the science buck. Those capabilities are increasing at the speed of Moore's Law. If there is or was life in the solar system, these are the people who can find it for us. If only we have the wisdom to let them continue trying.