This week a major geochemistry conference heard an argument for life on Earth having originated on Mars, but does this hold up to scrutiny?
The idea that a young Mars, some four billion years ago, was a far more hospitable and temperate place is not particularly controversial - although it is certainly not understood in any great detail. Now, at the annual Goldschmidt conference on geochemistry, the notion that it was Mars, not the Earth, that was the better place for life's origins has been getting some attention. Indeed, as reported in this piece at BBC News, it sounds like there might be quite a robust case for martian beginnings.
The new argument, in a nutshell, is that young Mars had a more oxygen-rich atmosphere than Earth did 4 billion years ago, and was drier. This would have resulted in a different mineralogical surface environment, one that could have provided a catalyst for the assembly of key RNA molecules and all the biotic chemistry leading to life as we know it. Subsequent asteroid impacts on Mars would have spewed chunks of rock into space, carrying life (presumably in microbial form) through the solar system and onto the surface of a youthful Earth.
The transfer of life is certainly not a new idea. We know that pieces of Mars end up on the Earth, many are found in Antarctica where ice flow and exposure gathers and reveals them to keen human eyes. And we know that four billion years ago the rate of asteroid impact on the inner worlds of the solar system was significantly higher than it is today, part of the tailing off of planetary assembly and orbital evolution. Recent experiments and studies of what survives passage down through Earth's atmosphere certainly suggest that viable organisms could make it, and impact driven material could have fast transit times through interplanetary space.
However, one problem lies in our understanding of the steps between basic molecules on a planet's surface and the formation of a complex broth of RNA and proteins that could lead to cell structures and DNA. The hypothesis put forward at Goldschmidt rests on the idea that there must have been an inorganic, mineral catalyst to encourage basic molecules to assemble into the first RNA structures. Specifically, these were minerals built around boron and oxidized molybdenum, minerals that would have probably dissolved away in Earth's early oceans, or simply not existed, but could have fared much better on Mars.
But the truth is that our theories about these first steps are themselves hotly contested. For example, other laboratory work (such as this study reported in Nature in 2009) indicates that other, common, catalysts (stuff like manganese or magnesium compounds, also sometimes helped along by ultraviolet light) can set in motion complex chemical networks that very naturally pop out the sort of stuff that's going to make RNA, and even rudimentary cell membrane structures.
In other words, the precise nature of Earth's early geo-chemistry might not be quite as critical, nor might that of Mars.
Another problem is that this idea assumes young Mars was significantly less wet than young Earth - or else stuff like the boron compounds would have dissolved and dispersed to useless concentrations there too. Except we know that ancient Mars had transient and perhaps even long-term liquid surface water, it was a far wetter place than it is now.
It's also not clear that RNA is actually the only pixie dust on the road to life. The common emphasis on making RNA comes in part from the concept of an 'RNA-world', a precursor chemical universe where self-replicating and catalyzing RNA molecules operated before being 'replaced' by the modern DNA, RNA, and protein world. But not everyone agrees that this works, at least not in a pristine form. Early RNA may have operated within a more complex mess of other molecules. And there are suggestions that RNA is actually more of an evolutionary outcome - the winner in a stew of other polymers. A great discussion of some of this is here.
So, did Mars seed life on Earth? We simply don't know. However, by exploring the possibility we certainly stand to gain information from the fact that young Earth and young Mars probably were rather different - chemically and environmentally. Two natural test-tubes rather than one.
And there's an even wilder possibility that I'll throw out here for fun; although it's entirely unsubstantiated at the moment. Perhaps, just perhaps, the to-and-fro of small chunks of material between these planets itself played a role in stirring the pre-biotic chemical pots of both worlds...