94 95 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Spectroscopy Group Beamline B18 Analysing deep Antarctic ice to understand aMartianmineral Related publication: Baccolo G., Delmonte B., Niles P. B., Cibin G., Di Stefano E., Hampai D., Keller L., MaggiV., Marcelli A., Michalski J., Snead C. & Frezzotti M. Jarosite formation in deep Antarctic ice provides a window into acidic, water-limitedweathering onMars. Nat. Commun. 12 , 436 (2021). DOI: 10.1038/s41467-020-20705-z Publication keywords: Antarctica; Mars, Jarosite; Geochemistry; Ice cores; Mineral dust J arositeisacommonmineralonthesurfaceofMars,butrelativelyrareonEarth.SincetheOpportunityroverreportedwidespreadjarosite at Meridiani Planum in 2004, the mineral has been repeatedly identified on Mars. On Earth, jarosite forms through the weathering of iron-bearing minerals in water-limited settings. However, scientists have struggled to understand how the mineral may have formed on Mars. An international group of researchers have therefore studied whether glaciers played a role in the precipitation of jarosite on Mars in its geologic past. The ‘ice-weathering model’ suggests that the precipitation of jarosite on Mars could be related to the presence of massive ice deposits developed during ancient glacial ages. To test this hypothesis, the researchers investigated the properties of deep Antarctic ice, as an Earth analogue for paleo-ice deposits on Mars. Diamond Light Source’s B18 beamline is dedicated to X-ray Absorption Spectroscopy (XAS). This technique can characterise geochemical and mineralogical properties, even on tiny mineral dust samples extracted from Antarctic ice. The team measured more than 50 samples retrieved from the Antarctic Talos Dome ice core. They observed jarosite present in deep ice and depicted a coherent scenario to describe the formation of this mineral. Their scenario is consistent with the ice-weathering model, supporting the hypothesis that glaciers had a fundamental role in shaping the geologic and geochemical past of Mars. In 1987 it was predicted that jarosite, a ferric-potassium hydrated sulphate, would have been stable on Mars 1 . The confirmation came about 20 years later from the Opportunity rover, which identified jarosite in the layered sediments outcropping at Meridiani Planum 2 (Fig. 1). The identification of this mineral was a major scientific result, since its formation requires liquid water. Finding jarosite on Mars was thus regarded as evidence for the presence of liquid water on the planet, at least in its geologic past 3 . Since then jarosite has been observed at multiple sites on Mars, but scientists are still struggling to understand how it formed. Jarosite not only requires liquid water to form, it also needs iron-rich minerals and an acidic-oxidative environment. Several hypotheses have been made to describe the environment where such conditions could have been found on Mars. Most of them deal with the presence of evaporative basins resulting from the upwelling of acidic groundwater, or with volcanic and/or hydrothermal processes. An additional hypothesis, known as the ‘ice-weathering model’, proposed that Martian jarosite formed within massive paleo-ice deposits, rich in mineral dust and acidic volcanic aerosols 4 . According to it, in the deepest part of such deposits, high pressure and ice-metamorphism would have promoted the formation of micro-pockets where impurities and liquid water interacted, favouring the precipitation of jarosite. No evidence was available until now to test this model, neither on Mars, nor in other planetary contexts, Earth included. This study tried to fill the gap, looking for an appropriate Earth analogue suitable to assess if the englacial formation of jarosite is geochemically reliable. From many points of view, Antarctica is the region of Earth more similar to Mars. This is even more true when considering glaciers. The conditions found in the depths of Antarctic ice sheets and the ones characterising ancient Martian glaciers are expected to be comparable from several perspectives. Antarctic ice contains mineral dust, acidic atmospheric species and salts, exactly what the ice-weathering model predicted for Martian paleo-ice. To assess if jarosite is present in deep Antarctic ice, about 50 samples of mineral dust extracted from an Antarctic ice core were prepared at the University of Milano-Bicocca (Italy) and measured at beamline B18. Samples were obtained from the Talos Dome ice core, a 1620 m long ice core extracted from a peripheral region of the East Antarctic ice sheet, near the Ross Sea and the Transantarctic mountains. This site was selected because it is influenced by the local mountains outcropping from ice, which provide to Talos Dome mineral dust consisting in micrometric fragments of basaltic rocks 5 . Such conditions make the site suitable for a comparison with Mars, since Martian atmospheric dust is mostly produced from the alteration of basaltic outcrops. The core encompasses more than 150,000 years of Earth climatic history. Samples were analysed at B18 through X-ray Absorption Spectroscopy; this is a powerful technique allowing the gathering of accurate pieces of information about selected chemical elements, even on extremely diluted samples, such as the impurities present in Antarctic ice. The study focused on iron, a major constituent of jarosite. Over some years, a clean protocol suited to measure such extreme samples - each one consisting of a few micrograms of dust deposited on a filter - was developed at B18 5 . Samples were irradiated with an X-ray beam to induce X-ray fluorescence. A detector recorded the signal, and in particular the small variations in the emission resulting from the changing energy of the incident beam. Analysing the variations, it was possible to infer about the following properties of iron: coordination, oxidation and mineralogy. To obtain this information it was necessary to measure not only Antarctic dust samples, but also mineralogical specimens and standard materials. The signature of jarosite was identified in X-ray absorption spectra, confirming that this mineral is present in Antarctic ice. The identification was also confirmed by an independent analysis carried out at the NASA Johnson Space Center (USA). However, jarosite is not uniformly distributed along the core. Results showed that the properties of dust are not stable with ice depth. Some oscillations are associated with the alternation of glacial and interglacial periods, but other trends are not linked to climate and seem more dependent on ice depth, as in the case of jarosite abundance. The mineral is present only deeper than 1000 m into ice, in the form of sub-micrometric crystals as it is possible to appreciate in Fig. 2. Below this depth jarosite firstly appears and its concentration rapidly increases, until becoming the most abundant iron- mineral present in the bottom sections of the ice core (Fig. 3a). Concurrently we observed a progressive oxidation of the iron fraction of dust (Fig. 3b). This is compatible with jarosite accumulation since this mineral is completely oxidised. Our evidence suggests that jarosite is not originally deposited with snow at Talos Dome, but that it forms into deep ice, supporting the Mars ice- weathering model. Considering other records and analyses, a coherent scenario describing the formation of jarosite in deep Antarctic ice was depicted. Deep ice is subjected to metamorphism, i.e. the re-crystallisation of ice grains. During this process, the impurities present in ice are expulsed from ice grains and accumulated at their junctions, favouring the interaction between mineral dust, acids and salts. The concurrent presence of these species lowers the local pressure melting point of ice, allowing for the occurrence of liquid water. Dust, iron minerals, acids and liquid water: the ingredients for jarosite! This scenario perfectly resembles the one described by the ice-weathering model to understand the formation of jarosite on Mars. This study is the first direct evidence supporting the hypothesis that Martian jarosite formed as the result of geochemical reactions occurred in past massive glaciers that were present on the planet. Once these glaciers disappeared through sublimation because of climate change, jarosite and other sublimation residues were accumulated and re-worked by winds, reaching wide sectors of the planet and allowing for the formation of layered rocks, such as the ones analysed by the Opportunity rover. This finding is extremely important with respect to our comprehension of Mars and of the geologic and geochemical processes that defined its present state. Martian glaciers have been more important than previously thought. References: 1. Burns R. G. Ferric sulfates on Mars. J. Geophys. Res. 92 , 570–574 (1987). DOI: 10.1029/jb092ib04p0e570 2. Klingelhöfer G. et al. Jarosite and hematite at Meridiani Planum from opportunity’s Mössbauer spectrometer. Science (80-. ). 306 , 1740–1745 (2004). DOI: 10.1126/science.1104653 3. Madden M. E. E. et al. Jarosite as an indicator of water-limited chemical weathering on Mars. Nature 431 , 821–823 (2004). DOI: 10.1038/ nature02971 4. Niles P. B. et al. Meridiani Planum sediments on mars formed through weathering in massive ice deposits. Nat. Geosci. 2 , 215–220 (2009). DOI: 10.1038/ngeo438 5. Baccolo G. et al. Regionalization of the Atmospheric Dust Cycle on the Periphery of the East Antarctic Ice Sheet Since the Last Glacial Maximum. Geochemistry, Geophys. Geosystems 19 , 3540–3554 (2018). DOI: 10.1029/2018GC007658 Funding acknowledgement: This work is part of the TALDEEP project funded by MIUR (PNRA18_00098). The Talos Dome Ice core Project (TALDICE), a joint European programme, is funded by national contributions from Italy, France, Germany, Switzerland and the United Kingdom. Primary logistical support was provided by PNRA at Talos Dome. This study was generated in the frame of Beyond EPICA. The project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 815384 (Oldest Ice Core). It is supported by national partners and funding agencies in Belgium, Denmark, France, Germany, Italy, Norway, Sweden, Switzerland, The Netherlands and the United Kingdom. Logistic support is mainly provided by PNRA and IPEV through the Concordia Station system. Corresponding author: Giovanni Baccolo, University of Milano-Bicocca, Italy,
[email protected] Figure 1: The ‘Burns’ formation on the flank of Endurance crater at Merdiani Planum; the photograph was captured by the Opportuniy rover. Jarosite and other iron-minerals are common constituents of the layered formations portrayed in the picture. Credits: NASA/JPL/ Cornell. Figure 2: A Scanning Electron Microscope image of a mineral grain extracted from the deep part of the Talos Dome ice core. The hexagonal platelets, as revealed by X-ray Absorption Spectroscopy and other techniques, are sub-micrometric jarosite crystals precipitated on mineral dust grains. Figure 3: The analysis of X-ray absorption data gathered at beamline B18 allowed identification of the presence of jarosite in the dust samples extracted from the Talos Dome ice core; (a) abundance of jarosite within the dust samples; (b) Fe K-edge energy along the core, this is a parameter related to iron oxidation state and to jarosite precipitation. Details can be found in a Nature publication 3 .