Olivines are silicate minerals containing varying proportions of magnesium and iron. Magnesium-rich olivines are common on Earth, being the primary component of the upper mantle.
Ferromagnesian minerals such as olivines react with water, releasing hydrogen, in a process known as serpentinization. Serpentinites are rocks composed predominantly of one or more serpentine group minerals, and serpentinization has played a significant role in the development of Earth's surface environments over time. Our current understanding of Mars, pieced together from our examination of meteorites, satellite images and data collected by NASA's rovers, is that the planet was once warm and wet. However, our understanding of serpentinization on Earth doesn't directly translate to a Martian context. The olivine minerals found on Mars are much richer in iron; the least iron-rich olivine ever observed in Martian rocks contains more than double the iron of common Earth olivines. As iron-rich olivines are rare on Earth, they have not been the focus of scientific research. In work recently published in Science Advances, researchers from the University of Calgary and the University of Cambridge undertook a detailed study of iron-rich olivines from Minnesota. Their results show that serpentinization reactions could have provided the vital boost needed to stabilise liquid water and promote habitability on early Mars.
The 1.1-billion-year-old Duluth Complex is a large igneous intrusion under much of north-eastern Minnesota, USA. It is part of a large structure known as the Midcontinent Rift, which formed when North America began, but ultimately failed, to split apart. Molten rock from the mantle rose through the rift and cooled to form a complex and heterogeneous body of rock. The olivines found there are iron-rich and similar in composition to Martian olivines, offering a close compositional analog we can study here on Earth.
Dr Benjamin Tutolo from the University of Calgary said;
There are two fundamental reasons we're interested in studying serpentinization reactions on early Earth and early Mars. One is that they generate lots of hydrogen, and perhaps through other reactions, hydrocarbons that could then be strung together to make the first cells and biomolecules. Serpentinization might be one of the key reactions for the origins of life on planetary surfaces. A second is that 4.5 billion years ago, the Sun was only about 70% as bright as it is today. That means that more greenhouse gases would have been needed to trap the Sun's heat and keep a planet warm, and climate simulations of early Mars tell us that carbon dioxide on its own isn't enough. It needs an extra kick from something, and hydrogen has been proposed as being that extra kick.
Hydrogen can combine with other atmospheric gases to generate a strong greenhouse effect. However, on Earth, serpentinization reactions in magnesium-rich olivines don't generate the quantities of hydrogen that would be needed. In this work, the researchers investigated whether serpentinization in the iron-rich olivines found on Mars would make a more significant contribution.
At Diamond's I18 beamline, Dr Tutolo used X-ray Absorption Near Edge Structure (XANES) to analyse the Duluth Complex rock samples. He explained;
XANES is useful for iron for two reasons. One is that it gives you redox (oxidation) states, at a scale that's impossible using any other technique. So we could map the redox state in our samples and get individual spot analyses of the redox state of iron in the rocks. And in so doing, you can also say something about the coordination state of the iron - whether it's in serpentine, and what kind of serpentine it is. So we could see how iron was partitioning during serpentinization reactions into the oxidized versus reduced state and how it was generating hydrogen.
Microfocus Spectroscopy beamline I18 allowed not only measuring redox state of iron which was important for this study, but also made it possible to measure it in individual mineral grains which could be as small as several micron in size. This is a unique capability of I18, and recent advances in data collection and processing made it possible to quickly survey the areas of interest in seconds and select rock grains of interest for more detailed analysis. These developments in hardware and software were made possible by a team effort of Diamond support groups and beamline staff.
Previous work by atmospheric physicists has calculated the rate at which hydrogen escapes from the Martian atmosphere into space. Combining the XANES results with other data allowed the research team to conclude that serpentinites on Mars would have generated about five times more hydrogen than serpentinites on Earth - enough to accumulate hydrogen in the atmosphere and warm the planet. Serpentinization could have provided the extra kick needed to stabilise liquid water and promote habitability on early Mars. Dr Tutolo said;
There's pretty decent evidence from climate models and rocks on Mars of cyclical warming and cooling on early Mars, and serpentinization seems to be key to that cycle.
This research could inform the future geological explorations of Mars, which could help us understand early Earth. Over time, the dynamic movements of tectonic plates have churned up Earth's surface, constantly dragging down older rocks and replacing them with newer ones. As a result, there are few places on Earth where geologists can study ancient rocks. That hasn't happened on Mars, so the planet provides us with a fantastic opportunity to investigate the solar system's early history.
And although iron-rich olivine rocks aren't abundant on Earth, it's possible that they could be used to produce hydrogen as a sustainable fuel source, meaning that the serpentinization processes that once kept Mars warm could help Earth keep its cool.
To find out more about the I18 beamline, or discuss potential applications, please contact Principal Beamline Scientist Konstantin Ignatyev: konstantin.ignatyev@diamond.ac.uk.
Tutolo BM and Tosca NJ. Observational constraints on the process and products of Martian serpentinization. Science Advances 9.5:eadd8472 (2023). DOI:10.1126/sciadv.add8472.
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