Rolling Ironstones from Earth and Mars: Terrestrial Hydrothermal Ooids as a Potential Analogue of Martian Spherules

High-resolution images of Mars from National Aeronautics and Space Administration (NASA) rovers revealed mm-size loose haematite spherulitic deposits (nicknamed “blueberries”) similar to terrestrial iron-ooids, for which both abiotic and biotic genetic hypotheses have been proposed. Understanding the formation mechanism of these haematite spherules can thus improve our knowledge on the possible geologic evolution and links to life development on Mars. Here, we show that shape, size, fabric and mineralogical composition of the Martian spherules share similarities with corresponding iron spherules currently forming on the Earth over an active submarine hydrothermal system located off Panarea Island (Aeolian Islands, Mediterranean Sea). Hydrothermal fluids associated with volcanic activity enable these terrestrial spheroidal grains to form and grow. The recent exceptional discovery of a still working iron-ooid source on the Earth provides indications that past hydrothermal activity on the Red Planet is a possible scenario to be considered as the cause of formation of these enigmatic iron grains.

Capacity of Chlorate to Oxidize Ferrous Iron: Implications for Iron Oxide Formation on Mars

Chlorate is an important Cl-bearing species and a strong potential Fe(II) oxidant on Mars. Since the amount of oxychlorine species (perchlorate and chlorate) detected on Mars is limited (<~1 wt.%), the effectiveness of chlorate to produce iron oxides depends heavily on its oxidizing capacity. Decomposition of chlorate or intermediates produced during its reduction, before reaction with Fe(II) would decrease its effective capacity as an oxidant. We thus evaluated the capacity of chlorate to produce Fe(III) minerals in Mars-relevant fluids, via oxidation of dissolved Fe(II). Each chlorate ion can oxidize 6 Fe(II) ions under all conditions investigated. Mass balance demonstrated that 1 wt.% chlorate (as ClO3−) could produce approximately 6 to 12 wt.% Fe(III) or mixed valent mineral products, with the amount varying with the formula of the precipitating phase. The mineral products are primarily determined by the fluid type (chloride- or sulfate-rich), the solution pH, and the rate of Fe(II) oxidation. The pH at the time of initial mineral nucleation and the amount of residual dissolved Fe(II) in the system exert important additional controls on the final mineralogy. Subsequent diagenetic transformation of these phases would yield 5.7 wt.% hematite per wt.% of chlorate reacted, providing a quantitative constraint on the capacity of chlorate to generate iron oxides on Mars.

Modern Iron Ooids of Hydrothermal Origin as a Proxy for Ancient Deposits

We constrained the origin and genetic environment of modern iron ooids (sand-sized grains with a core and external cortex of concentric laminae) providing new tools for the interpretation of their fossil counterparts as well as the analogous particles discovered on Mars. Here, we report an exceptional, unique finding of a still active deposit of submillimetric iron ooids, under formation at the seabed at a depth of 80 m over an area characterized by intense hydrothermal activity off Panarea, a volcanic island north of Sicily (Italy). An integrated analysis, carried out by X-ray Powder Diffraction, Environmental Scanning Electron Microscopy, X-ray Fluorescence and Raman spectroscopy reveals that Panarea ooids are deposited at the seafloor as concentric laminae of primary goethite around existing nuclei. The process is rapid, and driven by hydrothermal fluids as iron source. A sub-spherical, laminated structure resulted from constant agitation and by degassing of CO₂-dominated fluids through seafloor sediments. Our investigations point the hydrothermal processes as responsible for the generation of the Panarea ooids, which are neither diagenetic nor reworked. The presence of ooids at the seawater-sediments interface, in fact, highlights how their development and growth is still ongoing. The proposed results show a new process responsible for ooids formation and gain a new insight into the genesis of iron ooids deposits that are distributed at global scale in both modern and past sediments.

Microscopic emission and reflectance thermal infrared spectroscopy: instrumentation for quantitative in situ mineralogy of complex planetary surfaces

The diversity of investigations of planetary surfaces, especially Mars, using in situ instrumentation over the last decade is unprecedented in the exploration history of our solar system. The style of instrumentation that landed spacecraft can support is dependent on several parameters, including mass, power consumption, instrument complexity, cost, and desired measurement type (e.g., chemistry, mineralogy, petrology, morphology, etc.), all of which must be evaluated when deciding an appropriate spacecraft payload. We present a laboratory technique for a microscopic emission and reflectance spectrometer for the analysis of martian analog materials as a strong candidate for the next generation of in situ instruments designed to definitively assess sample mineralogy and petrology while preserving geologic context. We discuss the instrument capabilities, signal and noise, and overall system performance. We evaluate the ability of this instrument to quantitatively determine sample mineralogy, including bulk mineral abundances. This capability is greatly enhanced. Whereas the number of mineral components observed from existing emission spectrometers is high (often >5 to 10 depending on the number of accessory and alteration phases present), the number of mineral components at any microscopic measurement spot is low (typically <2 to 3). Since this style of instrument is based on a long heritage of thermal infrared emission spectrometers sent to orbit (the thermal emission spectrometer), sent to planetary surfaces [the mini-thermal emission spectrometers (mini-TES)], and evaluated in laboratory environments (e.g., the Arizona State University emission spectrometer laboratory), direct comparisons to existing data are uniquely possible with this style of instrument. The ability to obtain bulk mineralogy and atmospheric data, much in the same manner as the mini-TESs, is of significant additional value and maintains the long history of atmospheric monitoring for Mars. Miniaturization of this instrument has also been demonstrated, as the same microscope objective has been mounted to a flight-spare mini-TES. Further miniaturization of this instrument is straightforward with modern electronics, and the development of this instrument as an arm-mounted device is the end goal.

A laboratory technique for thermal emission measurement of hydrated minerals

Laboratory emission spectra are measured at Arizona State University's Mars Space Flight Facility for comparison to remotely sensed data from Earth and Mars. Such emission spectroscopy using an interferometric spectrometer measures the energy of the sample, including reflected and emitted background sources. The detector is uncooled at ambient temperature, which produces a very low signal when measuring the energy from a sample that has a temperature close to its own. In order to increase the energy difference between the sample and the detector, thereby increasing the signal received by the detector, samples are typically heated to between 60 and 80 degrees C for several hours prior to measurement. While this method is acceptable for most rock and mineral samples, some hydrous minerals dehydrate quickly at low relative humidity and temperatures above room temperature. This change is evident in both the physical appearance of the mineral and in the position and shape of its spectral absorptions. One solution to this problem is to heat samples to lower temperatures (e.g., 40 degrees C) for only a short time period. However, this approach results in a low signal from the sample and does not always avoid dehydration. For this reason, we have developed a technique for measuring and calibrating emission spectra of hydrated minerals that involves cooling samples to well below the temperature of the detector, which avoids dehydration, while creating a large delta temperature and a strong signal from the sample. Our method allows for accurate library spectra, with discrete, pronounced spectral features (high spectral contrast), of hydrated minerals that can be used for comparison to planetary surfaces.

Size-dependent structural transformations of hematite nanoparticles. 1. Phase transition

Using Fourier Transform InfraRed (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM), we characterize the structure and/or morphology of hematite (alpha-Fe(2)O(3)) particles with sizes of 7, 18, 39 and 120 nm. It is found that these nanoparticles possess maghemite (gamma-Fe(2)O(3))-like defects in the near surface regions, to which a vibrational mode at 690 cm(-1), active both in FTIR and Raman spectra, is assigned. The fraction of the maghemite-like defects and the net lattice disorder are inversely related to the particle size. However, the effect is opposite for nanoparticles grown by sintering of smaller hematite precursors under conditions when the formation of a uniform hematite-like structure throughout the aggregate is restricted by kinetic issues. This means that not only particle size but also the growth kinetics determines the structure of the nanoparticles. The observed structural changes are interpreted as size-induced alpha-Fe(2)O(3)<-->gamma-Fe(2)O(3) phase transitions. We develop a general model that considers spinel defects and absorbed/adsorbed species (in our case, hydroxyls) as dominant controls on structural changes with particle size in hematite nanoparticles, including solid-state phase transitions. These changes are represented by trajectories in a phase diagram built in three phase coordinates-concentrations of spinel defects, absorbed impurities, and adsorbed species. The critical size for the onset of the alpha-->gamma phase transition depends on the particle environment, and for the dry particles used in this study is about 40 nm. The model supports the existence of intermediate phases (protohematite and hydrohematite) during dehydration of goethite. We also demonstrate that the hematite structure is significantly less defective when the nanoparticles are immersed in water or KBr matrix, which is explained by the effects of the electrochemical double layer and increased rigidity of the particle environment. Finally, we revise the problem of applicability of IR spectroscopy to the lattice vibrations of hematite nanoparticles, demonstrating that structural comparison of different samples is much more reliable if it is based on the E(u) band at about 460 cm(-1) and the spinel band at 690 cm(-1), instead of the A(2u)/E(u) band at about 550 cm(-1) used in previous work. The new methodology is applied to analysis of the reported IR spectra of Martian hematite.

Spectral properties of Lake Superior banded iron formation: application to Martian hematite deposits

Several locations have been identified on Mars that expose bulk, coarsely crystalline gray hematite. These deposits have been interpreted as being sedimentary and formed in aqueous environments. Lake Superior Type (LST) banded iron formation (BIF) was investigated as a spectral and possible process analog to these deposits. In northern Michigan, LST BIF formed in a sedimentary, continental shelf or shallow basin environment under stable tectonic conditions, and the oxide facies contains gray, crystalline hematite. These deposits are Proterozoic in age and contain microfossils associated with the early diversification of life on Earth. Samples of the hematite-bearing oxide facies, as well as the carbonate facies, were collected and analyzed for their spectral and geochemical characteristics. Sample spectra were measured in the visible, near-infrared, and thermal infrared for comparison with remote and in situ spectra obtained at Mars. Thin section analysis, as well as X-ray diffraction and scanning electron microscopy measurements, were performed to determine detailed geochemistry. There is no evidence for BIF at Opportunity's Meridiani landing site, and the results of this work will provide useful data for determining whether BIFs exist elsewhere on Mars and are, thus, relevant to current and future Mars exploration missions.