A major gap in palaeontological research on Earth could doom two upcoming Mars expeditions to failure, researchers from Europe claim.
So, ahead of NASA’s planned Mars 2020 mission, and the European Space Agency’s ExoMars 2020 project – both of which will search for any traces of life, past or present, on the planet – the scientists are racing to fill in the blanks.
Writing in the journal Frontiers in Earth Science, Magnus Ivarsson, from the University of Southern Denmark, and colleagues, note that Earth’s fossil record – which at microbial scales forms a necessary comparison to any Mars evidence – is based entirely on findings made in sedimentary rocks.
Igneous rocks – those formed through the cooling of lava or magma – have traditionally been regarded as barren and have been the subject of little fossil-hunting interest. This poses a challenge for the NASA and ESA missions, given that the Martian surface is overwhelmingly made of igneous material.
That being the case, combined with the assumption that any biological life on Mars is broadly analogous to that on Earth, searching for sedimentary fossils in igneous rocks quite possibly represents a hiding to nothing.
However, a workable set of igneous comparisons is still possible, write Ivarsson and co-authors Therese Sallstedt, from the Swedish Museum of Natural History, and Diana-Thean Carlsson, from Germany’s University of Hamburg.
Far from being barren, the researchers suggest, the igneous rocks that lie below the ocean floor are likely to be rich and diverse in microbial life.
“The majority of the microorganisms on Earth are believed to exist in the deep biosphere of the ocean and continental crust,” says Ivarsson. “Yet we are just now beginning to explore – through deep drilling projects – this hidden biosphere.”{%recommended 3777%}
Such “subcrustal” ecosystems comprise fungi, bacteria and other microbes that exist in an environment subject to immense pressure and devoid of sunlight. The organisms colonise tiny cavities and fractures in the rock, forming self-sustaining food webs.
“Upon death, the microbial communities become fossilised on the walls of their rocky home,” Ivarsson continues. “These microfossils can provide a history of microbial life in volcanic rock.”
The researchers are part of a small community dedicated to uncovering fossil traces in deep sea rocks – a discipline made doubly challenging by the problems involved in securing samples from more than a kilometre underwater, and the nature of the evidence itself.
One controversial set of findings concerns, as Ivarsson and colleagues describe them, “conspicuous granular and tubular cavities in volcanic glass”. Some in the field define these as “ichnofossils” – the marks left behind perhaps billions of years ago by fungi boring through the rock.
Other scientists, however, dispute the conclusion, arguing that the cavities are the result of abiotic processes, such as radiation damage or fluid action.
Another type of evidence, however, concerning spherical or filamentous inclusions in excavated subsea basalt, has been less controversially identified as fossilised microorganisms.
And it is these, at the very least, that can form the basis of a reference set for Martian exploration.
The researchers propose the compilation of a “‘volcanic microfossil atlas’ to help select target sites for missions seeking evidence of extraterrestrial life”.
“Our aim is to be able to use the oceanic crust microfossil record as a model system to guide Martian exploration,” Ivarsson explains.
“Our review of existing knowledge is an important first step, but a more comprehensive understanding of the deep life is needed to show where and what to search for.”
