Reliable spectroscopic identification of minerals associated with serpentinization: Relevance to Mars exploration

1,2Wen-Ping Liu,1,2Wei Yin,3Bin-Long Ye,1,2Tian-Lei Zhao,4Qi-Zhi Yao,1,3Yi-Liang Li,5Sheng-Quan Fu,1,2,6Gen-Tao Zhou
Icarus (in Press) Link to Article []
1Deep Space Exploration Laboratory, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
2CAS Key Laboratory of Crust-Mantle Materials and Environments, University of Science and Technology of China, Hefei 230026, China
3Department of Earth Sciences and Laboratory for Space Research, University of Hong Kong, Hong Kong 999077, China
4School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, China
5Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China
6CAS Center for Excellence in Comparative Planetology, Hefei 230026, China
Copyright Elsevier

Mars has become the preeminent target of astrobiology due to its many Earth-like features. Serpentinized environments on Mars are increasingly of astrobiological interest because they imply the presence of several of the “key elements” for life. The Mars 2020 rover carries a compelling set of spectral instruments with the intent to characterize past habitable serpentinized environments, search for potential biosignatures, and collect samples for potential return to Earth. Reliable spectroscopic identification of serpentinization minerals is, of course, a prerequisite for mission accomplishment. The current assignment of spectroscopic features is based on the databases derived from pure minerals. However, many studies have confirmed that mineral assemblage can complicate spectrum identification, often leading to misinterpretation of the data. Therefore, a rock-based library should be built, which will increase our capability to interpret the Martian spectroscopic data. As such, we performed a comprehensive mineralogical and spectroscopic survey of several rocks sampled from an ophiolite complex in Qaidam Basin, one of the largest Mars analogs on Earth, to build an ophiolite spectral database. X-ray fluorescence (XRF), visible and near-infrared (VNIR), Raman spectroscopy, and XRD were used to identify minerals in the rocks. The results show that serpentine in the rocks with talc could be misinterpreted as sepiolite only relying on the Raman vibrations, while the VNIR spectra can identify serpentine well in all rocks. In addition, the camera and Raman spectrometer on the Mars rover should work together to identify different polymorphs of serpentine, i.e., antigorite, lizardite, and chrysotile. Raman and/or VNIR spectroscopy is effective for other minerals associated with serpentinization, including brucite, dolomite, magnesite, magnetite, talc, and quartz. Our study provides a framework for detecting serpentinization minerals on Mars with spectrometers and can be used for data interpretation by the Mars 2020 mission. All the spectral data presented in the supplementary material facilitate further comparison with future in situ and orbital measurements on Mars.

Origin of the superchondritic carbon/nitrogen ratio of the bulk silicate Earth − an outlook from iron meteorites

1Damanveer S.Grewal,1Paul D.Asimow
Geochimica et Cosmochimica Acta (in Press) Link to Article []
1Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA
Copyright Elsevier

Disagreement regarding the origin of the bulk silicate Earth’s (BSE) superchondritic carbon/nitrogen (C/N) ratio is due, in part, to the unknown C/N ratios of differentiated planetesimals − the building blocks of Earth-like rocky planets. In this study we report solid-liquid metal partitioning experiments for C and N that allow us to reconstruct, from the C and N contents of iron meteorites, the C/N ratios of the cores of the earliest formed planetesimals. Due to their siderophile character, most of the C and N retained in these bodies after differentiation resides in their cores. Therefore, estimates of the bulk C and N contents and C/N ratios of the cores yield confident estimates of these quantities in the complete parent bodies of iron meteorites. Our experimental data, at 1 GPa and 1200-1400 °C, show that C and N are incompatible in solid metal relative to S-poor liquids but compatible in solid metal relative to S-rich liquids. Crucially, N is approximately an order of magnitude more compatible than C in S-rich systems. S itself is incompatible in solid metal and so the late-crystallizing liquids persisting at the end of core freezing were S-rich for most cores. Although these late-crystallizing liquids are unsampled by iron meteorites, we infer that their N contents and C/N ratios were generally lower and higher, respectively, than those in iron meteorites. Depending upon the fraction of unsampled late-crystallizing liquids as well as their S contents, the C/N ratios of the bulk cores and complete parent bodies are either similar to or higher than those measured in iron meteorites. The reconstructed C/N ratios of most of the parent bodies of iron meteorites are chondritic, except that the volatile-rich IC and IIC groups have superchondritic C/N ratios. Importantly, the C/N ratio of the parent body of the IC iron meteorite group lies within the estimated range of the BSE, whereas the C/N ratios of all other groups are distinctly lower. Correlated depletion of moderately volatile elements like Ge and Ga with C and N, variations in metallographic cooling rates, and Pd-Ag isotope systematics suggest that the parent cores of the volatile-depleted iron meteorite groups were likely affected by volatile degassing. If volatile-rich iron meteorites like the IC group better capture the C and N inventories of the parent cores of the earliest formed planetesimals, then delivery of C and N via such planetesimals makes the superchondritic C/N ratio of the BSE a natural consequence of the Earth’s accretion history. Otherwise, poorly constrained processes like atmospheric erosion or C and N delivery by exotic materials are required to explain the superchondritic C/N ratio of the BSE.

Mass-independent Sn isotope fractionation and radiogenic 115Sn in chondrites and terrestrial rocks

1,2Alessandro Bragagni,1Frank Wombacher,1,3Maria Kirchenbaur,1,4Ninja Braukmüller,1Carsten Münker
Geochimica et Cosmochimica Acta (in Press) Link to Article []
1Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Str. 49b, 50674 Köln, Germany
2Dipartimento di Scienze della Terra, Università degli studi di Firenze, via La Pira 4, 50121 Firenze, Italy
3Institut für Mineralogie, Leibniz Universität Hannover, Callinstraße 3, 30167 Hannover, Germany
4Institut für Geologische Wissenschaften, Freie Universität, Malteserstr. 74-100, 12249, Berlin, Germany
Copyright Elsevier

Tin has ten stable isotopes, providing the opportunity to investigate and discriminate nucleosynthetic isotope anomalies from mass-dependent and mass-independent isotope fractionation. Novel protocols for chemical separation (based on TBP-resin) and MC-ICP-MS analyses are reported here for high precision Sn isotope measurements on terrestrial rocks and chondrites. Relative to the Sn reference standard (NIST SRM 3161a), terrestrial basalts and chondrites show isotope patterns that are consistent with mass-dependent and mass-independent isotope fractionation processes as well as with 115Sn radiogenic ingrowth from 115In.

Two different mass-independent isotope effects are identified, namely the nuclear volume (or nuclear field shift) and the magnetic isotope effect. The magnetic isotope effect dominates in the two measured ordinary chondrites, while repeated analyses of the carbonaceous chondrite Murchison (CM2) display a pattern consistent with a nuclear volume effect. Terrestrial basalts show patterns that are compatible with a mixture of nuclear volume and magnetic isotope effects. The ultimate origin of the isotope fractionation is unclear but a fractionation induced during sample preparation seems unlikely because different groups of chondrites show distinctly different patterns, hence pointing towards natural geo/cosmochemical processes. Only the carbonaceous chondrite Murchison (CM2) shows a Sn isotope pattern similar to what expected for nucleosynthetic variations. However, this pattern is better reproduced by nuclear volume effects. Thus, after considering mass-independent and mass-dependent effects, we find no evidence of residual nucleosynthetic anomalies, in agreement with observations for most other elements with similar half-mass condensation temperatures.

Most chondrites show a deficit in 115Sn/120Sn (typically -150 to -200 ppm) relative to terrestrial samples, with the exception of one ordinary chondrite that displays an excess of about +250 ppm. The 115Sn/120Sn data correlate with In/Sn, being consistent with the β- decay of 115In over the age of the solar system. This represents the first evidence of the 115In-115Sn decay system in natural samples. The radiogenic 115Sn signature of the BSE derives from a suprachondritic In/SnBSE, which reflects preferential partitioning of Sn into the Earth’s core.

The photochemical evolution of polycyclic aromatic hydrocarbons and nontronite clay on early Earth and Mars

1Nina Kopacz et al. (>10)
Icarus (in Press) Link to Article []
1Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands
Copyright Elsevier

The photochemical evolution of polycyclic aromatic hydrocarbons (PAHs), an abundant form of meteoritic organic carbon, is of great interest to early Earth and Mars origin-of-life studies and current organic molecule detection efforts on Mars. Fe-rich clay environments were abundant on early Earth and Mars, and may have played a role in prebiotic chemistry, catalyzing the breakdown of PAHs and freeing up carbon for subsequent chemical complexification. Current Mars is abundant in clay-rich environments, which are most promising for harboring organic molecules and have comprised the main studied features by the Curiosity rover in search of them. In this work we studied the photocatalytic effects of the Fe-rich clay nontronite on adsorbed PAHs. We tested the effect of ultraviolet radiation on pyrene, fluoranthene, perylene, triphenylene, and coronene adsorbed to nontronite using the spike technique, and in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy in a Mars simulation chamber. We studied the infrared vibrational PAH bands with first order reaction kinetics and observed an extensive decrease of bands of pyrene, fluoranthene, and perylene, accompanied by the formation of PAH cations, while triphenylene and coronene remained preserved. We further analyzed our irradiated samples with nuclear magnetic resonance (NMR). Our study showed certain PAHs to be degraded via the (photo)Fenton mechanism, even under a dry, hypoxic atmosphere. Using solar spectra representative of early Earth, early Mars, and current Mars surface illumination up to 400 nm, the processes occurring in our set up are indicative of the UV-induced photochemistry taking place in Fe-rich clay environments on early Earth and Mars.

Pristinity and petrogenesis of eucrites

1,2Jasmeet K. Dhaliwal,1James M. D. Day,3Kimberly T. Tait
Meteoritics & Planetary Science (in Press) Link to Article []
1Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, 92093-0244 USA
2Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, California, 95064 USA
3Department of Natural History, Royal Ontario Museum, Toronto, M5S 2C6 Canada
Published by arrangement with John Wiley & Sons

New petrography, mineral chemistry, and whole rock major, minor, and trace element abundance data are reported for 29 dominantly unbrecciated basaltic (noncumulate) eucrites and one cumulate eucrite. Among unbrecciated samples, several exhibit shock darkening and impact melt veins, with incomplete preservation of primary textures. There is extensive thermal metamorphism of some eucrites, consistent with prior work. A “pristinity filter” of textural information, siderophile element abundances, and Ni/Co ratios of bulk rocks is used to address whether eucrite samples preserve endogenous refractory geochemical signatures of their asteroid parent body (i.e., Vesta), or could have experienced exogenous impact contamination. Based on these criteria, Cumulus Hills 04049, Elephant Moraine 90020, Grosvenor Range 95533, Pecora Escarpment 91245, and possibly Queen Alexander Range 97053 and Northwest Africa 1923 are pristine eucrites. Eucrite major element compositions and refractory incompatible trace element abundances are minimally affected by metamorphism or impact contamination. Eucrite petrogenesis examined through the lens of these elements is consistent with partial melting of a silicate mantle that experienced prior metal–silicate equilibrium, rather than as melts associated with cumulate diogenites. In the absence of the requirement of a large-scale magma ocean to explain eucrite petrogenesis, the interior structure of Vesta could be more heterogeneous than for larger planetary bodies.

Northwest Africa 8418: The first CV4 chondrite

1G. J. MacPherson,2K. Nagashima,1A. N. Krot,3S. M. Kuehner,3A. J. Irving,4K. Ziegler,5L. Mallozzi,6C. Corrigan,7D. Pitt
Meteoritics & Planetary Science (in Press) Link to Article []
1Smithsonian Institution, Washington, District of Columbia, 20560 USA
2University of Hawai‘i, Mānoa, Honolulu, Hawaii, USA
3University of Washington, Seattle, Washington, 98195 USA
4Institute for Meteoritics, University of New Mexico, Albuquerque, New Mexico, 87131 USA
5Stony Brook University, Stony Brook, New York, 11794 USA
6Smithsonian Institution, Washington, District of Columbia, 20560 USA
7Maine Mineral and Gem Museum, Bethel, Maine, 04217 USA
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 8418 is an unusual chondrite whose properties do not exactly match those of any other known chondrite. It has similarities to the CV (Vigarano group), CK (Karoonda group), and CL (Loongana group) chondrites, but its abundance of large calcium-aluminum-rich inclusions (CAIs) and the low NiO content (<0.2 wt%) of its matrix olivine ally it most closely with the CV group. The absence of grossular, monticellite, wollastonite, and sodalite from the alteration products of the CAIs; the magnesium-rich nature of the matrix olivines (Fa38) relative to that of the CV3 chondrites (~Fa50); and the presence of secondary Na-bearing plagioclase and chlorapatite indicate a metamorphic temperature >600 °C. NWA 8418 contains kamacite, taenite, and troilite, and lacks magnetite and pentlandite. We propose that NWA 8418 be reclassified as a reduced CV4 chondrite, which makes it the first CV chondrite of petrologic type 4.

Phyllosilicate formation on early Mars via open-system acid alteration of basaltic glass

Earth and Planetary Science Letters 603, 117987 Link to Article []
1Jacobs, NASA Johnson Space Center, Houston, TX 77058, United States of America
2NASA Johnson Space Center, Houston, TX 77058, United States of America
3University of Nevada, Las Vegas, NV 89154, United States of America
Copyright Elsevier

Smectites are widespread on Mars, but the water-rich, neutral-to-alkaline pH conditions favorable for smectite formation would be expected to have also produced abundant carbonates on early Mars, which are not observed. Smectite formation from basaltic glass on Mars could occur in acidic environments unfavorable for carbonate formation. Acidic smectite formation has been previously demonstrated in batch experiments (closed hydrologic systems), however, the mechanisms and octahedral composition of smectite forming in acidic flow-through (open hydrologic systems) environments are still not fully understood. We conducted hydrothermal (190 °C) alteration experiments on Stapafell basaltic glass at 0.01 and 0.25 mL min−1 flow rates corresponding to low and high water to rock ratio (W/R) flow conditions, and initial pH (pH0) values of 2, 3, 4 and 6. A batch low W/R experiment was conducted at pH0 2 for comparison to the open system experiment. Kaolinite, montmorillonite and chlorite formed at pH0 2 at low W/R; no phyllosilicates formed at pH0 2 at high W/R; and lizardite formed at pH0 ≥ 3 at both W/R ratios. Lizardite, kaolinite, and montmorillonite in these experiments formed by precipitation from solution and chlorite likely formed through alteration of montmorillonite and/or basalt. Saponite formed at pH0 2 in batch conditions by alteration of basaltic glass. Comparison of experimental data with martian observations of phyllosilicate assemblages indicated that smectite formation on Mars likely occurred under water-limited environmental conditions. Al-rich smectite could form in low W/R open system subsurface environments under a very narrow range of pH (pH < 3) while saponite could form in closed low W/R systems under acidic to alkaline conditions. The combination of open and closed hydrological regimes could be responsible for development of clay mineral stratigraphies observed on Mars. The acidic conditions required for formation of Al-rich smectite montmorillonite were unfavorable for carbonate precipitation, but carbonate precipitation could occur together with Fe/Mg-smectite saponite in closed systems at pH > 4. The lack of widespread carbonates on Mars could not therefore be explained solely by acidic conditions. Phyllosilicate formation under acidic conditions on Mars may affect biosignature stability in martian regolith as preservation capacity is lower in phyllosilicates that formed in or experienced acidic pH.

Sub-surface alteration and related change in reflectance spectra of space-weathered materials

1,2,3Kateřina Chrbolková,4Patricie Halodová,1,3Tomáš Kohout,2Josef Ďurech,5Kenichiro Mizohata,6Petr Malý,7Václav Dědič,8Antti Penttilä,6František Trojánek,4Rajesh Jarugula
Astronomy & Astrophysics 665, A14 Open Access Link to Article [DOI]
1Department of Geosciences and Geography, PO Box 64, 00014 University of Helsinki, Helsinki, Finland
2Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000 Prague, Czech Republic
3Czech Academy of Sciences, Institute of Geology, Rozvojová 269, 16500 Prague, Czech Republic
4Research Centre Řež, Hlavní 130, 250 68 Husinec–Řež, Czech Republic
5Department of Physics, PO Box 43, 00014 University of Helsinki, Helsinki, Finland
6Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Prague, Czech Republic
7Institute of Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Prague, Czech Republic
8Department of Physics, PO Box 64, 00014 University of Helsinki, Helsinki, Finland
Reproduced with permission (C)ESO

Context. Airless planetary bodies are studied mainly by remote sensing methods. Reflectance spectroscopy is often used to derive their compositions. One of the main complications for the interpretation of reflectance spectra is surface alteration by space weathering caused by irradiation by solar wind and micrometeoroid particles.

Aims. We aim to evaluate the damage to the samples from H+ and laser irradiation and relate it to the observed alteration in the spectra.

Methods. We used olivine (OL) and pyroxene (OPX) pellets irradiated by 5 keV H+ ions and individual femtosecond laser pulses and measured their visible (VIS) and near-infrared (NIR) spectra. We observed the pellets with scanning and transmission electron microscopy. We studied structural, mineralogical, and chemical modifications in the samples. Finally, we connected the material observations to changes in the reflectance spectra.

Results. In both minerals, H+ irradiation induces partially amorphous sub-surface layers containing small vesicles. In OL pellets, these vesicles are more tightly packed than in OPX ones. Any related spectral change is mainly in the VIS spectral slope. Changes due to laser irradiation are mostly dependent on the material’s melting temperature. Of all the samples, only the laser-irradiated OL contains nanophase Fe particles, which induce detectable spectral slope change throughout the measured spectral range. Our results suggest that spectral changes at VIS-NIR wavelengths are mainly dependent on the thickness of (partially) amorphous sub-surface layers. Furthermore, amorphisation smooths micro-roughness, increasing the contribution of volume scattering and absorption over surface scattering.

Conclusions. Soon after exposure to the space environment, the appearance of partially amorphous sub-surface layers results in rapid changes in the VIS spectral slope. In later stages (onset of micrometeoroid bombardment), we expect an emergence of nanoparticles to also mildly affect the NIR spectral slope. An increase in the dimensions of amorphous layers and vesicles in the more space-weathered material will only cause band-depth variation and darkening.

Mafic mineralogy assemblages at the Chang’e-4 landing site: A combined laboratory and lunar in situ spectroscopic study

1C.Liu et al. (>10)
Astronomy & Astrophysics 658, A67 Link to Article [DOI]
1Shandong Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai 264209, PR China
Reproduced with permission (C)ESO

Context. Chang’e-4 (CE-4) provides unprecedented information about lunar materials exposed by the South Pole-Aitken (SPA) basin. Diverse results have been obtained from previous interpretations of CE-4 visible and near-infrared (VNIR) spectra. Some studies suggest that materials at the CE-4 landing site are dominated by olivine and orthopyroxene, but others argue that only a small amount of olivine should be exposed at the CE-4 landing site.

Aims. Laboratory spectroscopy studies using the Engineering Model of CE-4 Visible and Near-infrared Imaging Spectrometer (VNIS) are critical in constraining the accurate mineral proportions and composition of soils and boulders at the CE-4 landing site.

Methods. VNIR spectra of nine lunar analogs – prepared by mixing orthopyroxene (OPX), clinopyroxene (CPX), olivine (OL), and plagioclase – were acquired using the CE-4 VNIS Engineering Model. The spectral indices model and modified Gaussian model were developed to estimate CPX/(CPX+OPX) and OL/(OL+CPX+OPX) and are applicable to the in situ spectra acquired by the Yutu-2 VNIS spectrometer.

Results. The lunar rocks and regolith at the CE-4 landing site excavated by the Finsen impact are CPX-rich with limited OL (CPX:OPX:OL = 56:29:17). The mineral chemistries of the four lunar rocks show Mid-Ca, Fe pyroxene, and Mid-Mg OL (Fo60−79), providing critical constraints for mineral compositions in the SPA compositional anomaly. These rocks exhibit high M1 intensity ratios, indicating that they were crystallized at a high temperature (980–1300 °C) and a rapid-cooling magmatic system produced by impact melt differentiation or volcanic resurfacing events.

Diverse space weathering effects on asteroid surfaces as inferred via laser irradiation of meteorites

1,10P. Zhang (张鹏飞) et al. (>10)
Astronomy & Astrophysics 659, A78 Link to Article [DOI]
1State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, PR China<
10CNSA Macau Center for Space Exploration and Science, Macau, PR China
Reproduced with permission (C)ESO

Context. Space weathering (SW) is crucial to improve the understanding of the evolution of optical characteristics on airless bodies. The classical view based on research of the Moon suggests that SW decreases albedo (darkening) and steepens spectral slope (reddening) in visible to near-infrared (VIS-NIR) wavelengths, producing nanophase iron (npFe0). However, this conclusion is not perfectly applicable to asteroids.

Aims. In this study, we focus on investigating the space weathering spectral alteration effects (SWSAE) and the causes of spectral alteration on various types of asteroids after long-term continuous micrometeoroid bombardments.

Methods. We used a pulsed laser to irradiate eight meteorites at the same energy, namely, of 28 mJ, in ten shots, including ordinary chondrites (OCs), aubrite (Aub), enstatite chondrites (ECs), CO, CV, and CM carbonaceous chondrites. Then we measured and compared the virgin and irradiated VIS-NIR reflectance spectra of these meteorites. We further surveyed the causes of spectral alteration through a scanning electron microscope and transmission electron microscope.

Results. Three different SWSAE are shown: (1) darkening and reddening on OCs, Aub, CO, and CV chondrites; (2) brightening and reddening on ECs; (3) brightening and bluing on CM chondrite. After irradiation, npFe0 and nanophase iron-nickel particles were respectively found in CV and CO chondrites; thick amorphous layers without any nanophase particles were found in Aub; melting and sputtering of metal were observed in ECs; a great deal of vesicles or bubbles without any nanophase particles were found in CM chondrite.

Conclusions. The long-term SW via micrometeoroid bombardments can spectrally remodel asteroid surfaces in different ways: darken and redden anhydrous silicate asteroids (e.g., S-, E-, and K-types); brighten and redden metal-rich M-type objects. The SWSAE of volatiles-rich carbonaceous asteroids (e.g., Ch-, Cgh-, and D-types) is related to SW degree: darkening and bluing at low degree then brightening and continue bluing as the SW degree increases. The various spectral units on Ryugu, Bennu, and Phobos can be created by the heterogeneity of the degree of SW.