I'm attending the third landing site selection meeting for the Mars Science Laboratory (MSL) rover today and tomorrow. The talk I most enjoyed today was the first one I saw, by Roger Buick, who presented on the challenges of locating the most ancient life in early Earth rocks, as analogues for the challenges of searching for past habitable environments and even life on early Mars.
To begin with, he did address the "street light syndrome" question that is often brought up (at least I always bring it up) about using Earth as an analogue for Mars life. Why "street light syndrome?" It's an old joke. Here's a version of the story that I like because it comes from a blog at "streetlightblog.blogspot.com" and because it makes fun of economists.
Anyway, Buick addressed it by saying, "Assuming that mars was inhabited, early Earth fossils are the best available analogue, and everything else is just speculation." Another speaker, later, argued that if you really care about life on Mars, "water is the only game in town," and that there was sufficient carbon that it seems unnecessary to assume anything more exotic like arsenic- or silicon-based life forms may once have existed.
Getting back to Buick's talk, he lamented the challenge of finding early Earth life. Only a few percent of the continental crust (and none of the oceanic crust) dates from the first half of Earth's history, much less from the earliest part of history. And, of course, the older the rocks, "the greater the probability the rocks have been screwed over by heat or pressure or they've been buried or they've been eroded. The odds of finding life in any particular rock are low."
He split fossils up into six types. There are "dead bodies," microfossils, the actual cells of ancient life. There are trace fossils, like tracks and burrows and, importantly for the early Earth, stromatolites. There are molecular signatures, also known as biomarkers. There are what he called "atomic fossils," which mostly means looking for isotopic signatures of microbial metabolisms. There are biominerals, which he also called "teeth and bones," none of which, he said, have been found for certain in early Earth history. And there is remineralization -- the alteration of rocks and minerals that occurs as a result of microbial activity.
Then he went through a huge number of studies in each type, beginning with the oldest claimed discovery, and analyzing how good a case he thinks they make. "We actually have a diverse record of early life, but a lot of the older ones have question marks -- in other worlds, people have claimed to find early life, and it's either been refuted or questioned."
A common problem with the oldest life claims is that these old rocks have often been metamorphosed -- altered as a result of deep burial under more rocks, which raises the pressure and, more importantly, the temperature. For example, the earliest claim of a microfossil was from Marble Bar, in northwest Australia, where there are very fossily-looking microscopic things seen in the rocks. But, he said, those rocks are greenschist facies -- this means that they contain minerals that signify that the rocks were heated to at least 350 Celsius for long enough actually to change their original minerals into new ones, notably chlorite, which is a green platy mineral. (That's how you tell it's greenschist.) He said that if you heated actual carbon-containing critters to that temperature, they should look black, but these don't, so he says the fossils might be contaminants, bugs that came in to the rock at a later time.
I won't detail all the different studies, but generally speaking, there are lots of claims for ancient life being found in rocks around 3.2 to 3.5 billion years old, but for the most part Buick said that these were questioned, and generally the oldest fossils that everyone agreed were actually fossils were around 2.7 or 2.6 billion years old. With the questioned claims, they often have only one line of evidence for life, and can't eliminate inorganic processes as possible explanations. With the claims that people agree on, there are multiple lines of evidence -- chemical, morphological, and so on.
After summarizing all these studies, he stepped back and generalized about where geologists have had success finding ancient Earth life. My shorthand notes of what he said (which was much more articulate than what I quote here): "In general, where do they occur? Almost exclusively in sedimentary rocks. I can't emphasize this enough. Mostly in carbonates, preferentially where they have been silicified. But also in sulfate evaporites, clastic sediments (sandstones, shales), rarely in water-laid tuffs. Only in fine-grained sediments. Only aqueous settings, but not from ephemeral water bodies like rivers. Largely preserved in reducing, not oxidizing environments. Stable tectonic settings. Not near volcanoes, mountain belts, impact craters. In an area without active and prolonged weathering."
How best to search? "To optimize the search, you want full information on minerals and lithology," that is, the types of rocks present. He also said we need a good understanding of the stratigraphic history -- the relative timing of the rocks' formation. He went on: "You need good geochronology. A complete post-depositional paragenesis -- what the history of metamorphic, deformational, diagenetic events are -- this is hard to get for Mars, I know." Diagenesis is how sediment (clay, sand, etc.) gets turned to rock. During other presentations today, many people made the point that if you want to see any organic materials from putative ancient Mars life preserved in rocks, diagenesis must happen fast, very soon after deposition; otherwise it's too easy for water to run around inside the unconsolidated sediment and carry away or alter all the interesting chemistry. Finally, he said, you need "a whole lot of luck. I'm famous to be the only paleontologist not to have found a single fossil throughout his career."
He said, "Go somewhere with diverse sedimentary rocks, diverse lithology, deposited over a range of times, with long-lived water bodies, rapid lithification, fine grain size (phyllosillicates are wonderful things). Little evidence of oxidation or acidic alteration. Somewhere with little susbsequent disturbance -- Mars is a good place for that. Somewhere only recently exposed -- a little crater. Somewhere where the basic geology is pretty well understood.
"Perhaps you will get lucky on Mars. Good luck, all of you.