Another summer, another trip to Iceland! However, instead of sampling the usual volcanic and hydrothermal deposits and environments, this visit was focused on one thing only: sediments. Sedimentary deposits have become a key focus of ongoing Mars exploration. This is because of their association with long-lived liquid water habitats (such as fluvial-lacustrine deposits), and preservation of organic matter (such as fine-grained clay-rich deposits). It is looking increasingly likely that the answer to whether or not organic matter/biosignatures exist in the ancient Martian rock record lies within sedimentary material lain down billions of years ago. As such, ancient sediment terrains on Mars, along with areas rich in hydrothermal mineral deposits, are being flagged-up for NASA’s next rover mission to Mars in 2020.
But to get to grips with sediments on Mars requires finding suitable sedimentary analogues on Earth to study. While there are numerous and incredible sedimentary terrains on Earth, most of these differ to Mars in one key way – the source rocks. The majority of sediments on Earth are sourced from continental crust (the landmass that pokes up above the oceans), which is typically made up of rocks rich in sodium and aluminium silicates. These elements are concentrated into the continental crust via plate tectonics, a planetary phenomenon unique to Earth. The Martian crust however lacks plate tectonics and is largely basaltic, and rich in Fe and Mg silicates. Therefore, even when sedimentary deposits form in similar environments on both planets (e.g. a lake environment), the resulting rock types are different. Different mineralogy means different geochemistry, which will have an effect on how organic matter and biosignatures are preserved and detected. Iceland fits into this story because, like Mars, the majority of the rocks that make up Iceland are basaltic. As such, the resulting sediments generated by weathering and erosion, and deposited in environments relevant to Mars exploration (such as lakes) are rich in basaltic minerals and their alteration products. These provide an ideal lithological analogue to sediments on Mars.
So back to Iceland we went, primarily to supplement the fluvial (river) sediments sampled during last years campaign with lacustrine (lake) sediments. Sedimentary terrains in Iceland take a bit of finding compared to the ubiquitous volcanoes and lava flows that make up most of the country, but where they do exist, they produce beautiful finely laminated sequences of grey sediment, rich in clays and other goodies. These are no-where near as old as the sedimentary terrains targeted on Mars (ten thousand years old vs a few billion years old!), but their comparable lithologies provide a valuable resource for Mars analogue research.
Aside from the challenge of finding secret sediments, this trip was comparatively easy compared to last year. No mountain huts, cold showers, stale food or steep glacier hikes. Instead, plenty of cake, whisky, and long drives characterised this trip (not in that order). Even the weather played along, giving us dry days and sunshine pretty much the whole trip.
In addition to the older lake sediments (above), we set off to sample younger sediments from a lake that has been retreating over the past few thousand years. This gave us access to the actual lake bed surface, enabling us to sample these sediments that have not been long exposed. This day also entailed the first real bit of ‘hiking’ of the trip, across a large lava flow, and accordingly this was the first day we had rain. The large, pale expanse of the now-exposed lake bed forms a stark contrast to the surrounding basaltic mountains and lava flows.
As well as the lake sediment samples we also collected a fine suite of recent outwash sediments, fluvial sediments, and present-day lake shore sediments. Next step is to move onto the analysis – smashing up and zapping with x-rays, amongst other types of scientific interrogation…
A big thanks to my trusty field assistant, navigator, and fellow geologist at St. Andrews – Sami Mikhail, and to the Royal Society of Edinburgh who funded this fieldwork as part of their ongoing (and much appreciated!) support during my RSE research fellowship.