The tiny hollows were just a centimeter or so tall and a couple of millimeters across, and formed air-tight sheets, which were inflated by methane gas, generated by decaying organic matter, bubbling through the sediments.
This methane could have been a nutrient for the bacteria, or it could have allowed water and nutrients to flow through the cavity. Over time layer upon layer of material built up and formed the laminate fossils Rasmussen's team discovered.
Living in underground cavities allowed the bacteria to escape the UV radiation that scoured the surface. Here was a tiny, protected environment for life.
The team's finding, published this month in the journal Geology, helps substantiate the hypothesis that bacteria colonies might still be hiding out within caves on Mars. Today the Martian surface is desolate, and far too harsh to support life , but recent discoveries of buried water ice revealed by meteor strikes raise tantalizing promise of hidden niches for a simple and hardy life.
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