¹Rachel Y.Sheppard,²Ralph E.Milliken,²Kevin M.Robertson
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115083]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States of America
²Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, United States of America
Copyright Elsevier

The martian crust is often viewed through the lens of its dominant secondary minerals, Noachian phyllosilicates and Hesperian sulfates, based on orbital spectral observations. However, the effects of surface exposure on the spectra of these hydrous minerals are not fully understood. We use an environmental chamber to measure changes in near-infrared (NIR) spectral absorptions related to H2O in smectite (montmorillonite) and Mg-sulfate under different temperature, pressure, and relative humidity conditions with relevance to the surface of Mars. Observed spectral differences are attributed to changes in water content (hydration state), mineral phase, and degree of crystallinity. It is observed that even minor changes in hydration state and phase (for Mg sulfate) cause perceptible changes in NIR H2O absorption features when measured in a controlled laboratory setting under dry Mars-like conditions. Based on these results and the known ability of smectite to rehydrate under increased RH, smectites exposed at the surface of Mars are expected to exchange water with the martian atmosphere under specific conditions, making them active participants in the present-day hydrological cycle of Mars, and in theory these hydration-dehydration processes should be detectable using NIR reflectance spectroscopy. However, some of the spectral changes associated with these hydration changes are subtle and may not be detectable with orbital or landed VNIR spectrometers. Furthermore, we find that the presence of clay minerals can spectrally mask the presence of Mg sulfates under a range of hydration states if the clay minerals are above ∼10 wt% abundance. Random noise was added to the laboratory spectral data to simulate orbital-quality reflectance data, and it is observed that expected changes related to hydration state and crystallinity are likely difficult to detect in current orbital VNIR data such as CRISM and OMEGA. This highlights the importance of future in situ NIR reflectance observations to accurately determine the extent to which hydrous minerals exposed as the surface cycle water with the martian atmosphere under present-day environmental conditions and to properly assess the role of hydrous minerals in the martian water budget.

^1,2J.A.Cartwright,²K.V.Hodges,²M.Wadhwa
Earth and Planetary Science Letters 590, 117576 Link to Article [https://doi.org/10.1016/j.epsl.2022.117576]
¹Department of Geological Sciences, University of Alabama, Tuscaloosa, AL 35487-0338, USA
²School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-6004, USA
Copyright Elsevier

Impact events on planetary surfaces can leave significant volumes of melt, archived in planetary regoliths, which provide important information regarding the timing and nature of these events. For example, an observed ca. 3.9-4.1 Ga age cluster within lunar samples has been interpreted as indicative of a significant Solar System-wide event: the so-called “Late Heavy Bombardment”. Here, we report data from a laser ablation microprobe 40Ar/39Ar study of clasts within two unpaired howardite meteorites (NWA 1929 and Dho 485) to explore the impact history of their asteroid parent body – (4)Vesta. Laser microprobe dates for the howardites varied broadly between 3.5 to 4.5 Ga (NWA 1929) and 2.5 to 4.5 Ga (Dho 485), but show no clear cluster in ages at ca. 3.9-4.1 Ga. Consistent with previously reported U-Pb dates for HED meteorites, our data suggest an extended impact bombardment period on (4)Vesta as compared to the distribution of 40Ar/39Ar impactite dates for available samples from the Apollo and Luna sample archives. The impact history of Vesta revealed here highlights that current models of the impact flux in the inner Solar System based on the Late Heavy Bombardment hypothesis require refinement.

¹Edward D.Young,²Catherine A.Macris,¹Haolan Tang,²Arielle A.Hogan,^1,3Quinn R.Shollenberger
Earth and Planetary Science Letters 589, 117575 Link to Article [https://doi.org/10.1016/j.epsl.2022.117575]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, United States of America
²Earth Sciences, Indiana University–Purdue University Indianapolis, United States of America
³Lawrence Livermore National Laboratory, United States of America
Copyright Elsevier

We use new experiments and a theoretical analysis of the results to show that the isotopic fractionation associated with laser-heating aerodynamic levitation experiments is consistent with the velocity of flowing gas as the primary control on the fractionation. The new Fe and Mg isotope data are well explained where the gas is treated as a low-viscosity fluid that flows around the molten spheres with high Reynolds numbers and minimal drag. A relationship between the ratio of headwind velocity to thermal velocity and saturation is obtained on the basis of this analysis. The recognition that it is the ratio of flow velocity to thermal velocity that controls fractionation allows for extrapolation to other environments in which molten rock encounters gas with appreciable headwinds. In this way, in some circumstances, the degree of isotope fractionation attending evaporation is as much a velocimeter as it is a barometer.

¹Chi Ma,¹John R. Beckett,²François L. H. Tissot,¹George R. Rossman
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13826]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, 91125 USA
²The Isotoparium, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, 91125 USA
Published by arrangement with John Wiley & Sons

Paqueite (Ca3TiSi2[Al,Ti,Si]3O14; IMA 2013-053) and burnettite (CaVAlSiO6; IMA 2013-054) are new refractory minerals, occurring as euhedral to subhedral crystals within aluminous melilite in A-WP1, a type A Ca-Al-rich inclusion, and CGft-12, a compact type A (CTA) from the Allende CV3 carbonaceous chondrite. Type paqueite from A-WP1 has an empirical formula of (Ca2.91Na0.11)Ti4+Si2(Al1.64Ti4+0.90Si0.24V3+0.12Sc0.07Mg0.03)O14, with a trigonal structure in space group P321 and cell parameters a = 7.943 Å, c = 4.930 Å, V = 269.37 Å3, and Z = 1. Paqueite’s general formula is Ca3TiSi2(Al,Ti,Si)3O14 and the endmember formula is Ca3TiSi2(Al2Ti)O14. Type burnettite from CGft-12 has an empirical formula of Ca1.01(V3+0.56Al0.25Mg0.18)(Si1.19Al0.81)O6. It assumes a diopside-type C2/c structure with a = 9.80 Å, b = 8.85 Å, c = 5.36 Å, β = 105.6°, V = 447.7 Å3, and Z = 4. Burnettite’s general formula is Ca(V,Al,Mg)AlSiO6 and the endmember formula is CaVAlSiO6. Paqueite and burnettite likely originated as condensates, but the observed grains may have crystallized from local V-rich melts produced during a later thermal event. For CGft-12, the compositions of paqueite, clinopyroxene, and perovskite suggest that type As drew from two distinct populations of grains. Hibonite grains drew from multiple populations, but these were well mixed and not equilibrated prior to incorporation into type A host melilite.

¹S.A.Singerling,²L.R.Nittler,²J.Barosch,³E.Dobrică,⁴A.J.Brearley,¹R.M.Stroud
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.05.007]
¹U.S. Naval Research Laboratory, Code 6366, Washington, DC 20375, USA
²Carnegie Institution of Washington, Washington, DC 20015, USA
³University of Hawai’i at Mānoa, Honolulu, HI, 96822, USA
⁴University of New Mexico, Albuquerque, NM, 87131, USA
Copyright Elsevier

We investigated six presolar grains from very primitive regions of the matrix in the unequilibrated ordinary chondrite Semarkona with transmission electron microscopy (TEM). These grains include one SiC, one oxide (Mg-Al spinel), and four silicates. This is the first TEM investigation of presolar grains within an ordinary chondrite host (in situ) and the first TEM study to report on any presolar silicates (in or ex situ) from an ordinary chondrite. Structural and elemental compositional studies of presolar grains located within their meteorite hosts have the potential to provide information on conditions and processes throughout the grains’ histories.

Our analyses show that the SiC and spinel grains are stoichiometric and well crystallized. In contrast, the majority of the silicate grains are non-stoichiometric and poorly crystallized. These findings are consistent with previous TEM studies of presolar grains from interplanetary dust particles and chondritic meteorites. The individual silicates have Mg#’s ranging from 15 to 98. Internal compositional heterogeneities were observed in several grains, including Al in the SiC, Mg and Al in the spinel, and Mg, Si, Al, and/or Cr in two silicates. We interpret the poorly crystalline nature, non-stoichiometry, more Fe- rather than Mg-rich compositions, and/or compositional heterogeneities as features of the formation by condensation under non-equilibrium conditions.

Evidence for parent body alteration includes rims with mobile elements (S or Fe) on the SiC grain and one silicate grain. Other features characteristic of secondary processing in the interstellar medium, the solar nebula, and/or on parent bodies, were not observed or are better explained by processes operating in circumstellar envelopes. In general, there was very little overprinting of primary features of the presolar grains by secondary processes (e.g., ion irradiation, grain-grain collisions, thermal metamorphism, aqueous alteration). This finding underlines the need for additional TEM studies of presolar grains located in the primitive matrix regions of Semarkona, to address gaps in our knowledge of presolar grain populations accreted to ordinary chondrites.

¹Devin L.Schrader,¹Jemma Davidson
Earth and Planetary Science Letters 589. 117552 Link to Article [https://doi.org/10.1016/j.epsl.2022.117552]
¹Buseck Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, 781 East Terrace Road, Tempe, AZ 85287, United States of America
Copyright Elsevier

The most abundant group of meteorites currently falling to Earth, ordinary chondrites, originate from S-type (Si-rich) asteroids and are thought to have originated in the inner Solar System. These asteroids typically underwent only minor aqueous alteration but experienced varying degrees of thermal metamorphism that altered their primary compositions and textures. However, some rare members remain unaltered and retain the pristine compositions they obtained in the protoplanetary disk prior to accretion of their parent asteroids. In contrast, comets formed in the icy reaches of the outer Solar System. Here we report on silicate minerals in pristine ordinary chondrites that are compositionally distinct from those in all other known chondrites but show similarities to those found in comet samples returned from Comet Wild 2 by NASA’s Stardust mission and those sourced from an unknown number of comets represented by interplanetary dusty particles. The identification of this material suggests that comets may have formed from diverse far-flung Solar System materials, including grains that migrated from the inner Solar System to the comet-forming region between ∼1 Myr and potentially ⪆3 Myr after the first Solar System solids formed. This finding suggests that migration from the inner to the outer Solar System lasted for millions of years and that comets are composed of residual materials from the entire early Solar System.

¹S.Iannini Lelarge,^1,2L.Folco,¹M.Masotta,³R.C.Greenwood,⁴S.S.Russell,⁴H.C.Bates
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.05.003]
¹Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, Pisa, Italy
²CISUP, Centro per l’Integrazione della Strumentazione Università di Pisa, Lungarno Pacinotti 43 Pisa, Italy
³Planetary and Space Sciences, Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
⁴Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom
Copyright Elsevier

The cosmochemistry of meteorites provides unique clues on asteroids accretion, differentiation, collisional break-up and reassembly, processes of critical importance for understanding planet formation in the early solar system. Mesosiderites are a complex group of achondrites whose nearly 50:50 metal-silicate composition is interpreted in the literature as resulting from the mixing of core and crustal materials derived from differentiated asteroid(s). Because of their complex nature and contrasting geochemical, isotopic, and spectroscopic data, the formation mechanism of mesosiderites is still poorly understood and is open to a large variety of planetary differentiation scenarios and collisional histories. In this study, based on new petrographic and geochemical data of 16 mesosiderites, we investigate in detail the proposal that mesosiderites are related to the howardite-eucrite-diogenite (HED) meteorite group, whose parent body is widely considered to be asteroid 4 Vesta (∼500 km diameter), the target of the recent NASA’s Dawn mission. We present the first high precision oxygen isotope analyses on the matrix of a set of mesosiderite samples, coupled with new chemical and petrographic analyses of mesosiderites Um Hadid, Estherville, and Mount Padbury. Concordant Δ17O values between mesosiderites (–0.241 ± 0.015 (2σ)) and howardite-eucrite-diogenites (−0.241 ± 0.017‰ (2σ)) indicate that they derived from the same oxygen isotope reservoir, but petrological evidence, in particular the distinctly lower Fe/Mn ratios and the larger lithological diversity in mesosiderites, indicates that they formed within different parent bodies. This suggests that mesosiderites and howardite-eucrite-diogenites originated in distinct parent bodies that accreted in the 4 Vesta source region but experienced different geologic evolution in terms of crustal differentiation and impact history, which were more complex and catastrophic in the mesosiderite parent body.

¹Matthias van Ginneken,²Vinciane Debaille,³Sophie Decrée,⁴Steven Goderis,⁵Alan B. Woodland,¹Penelope Wozniakiewicz,³Marleen De Ceukelaire,³Thierry Leduc,³Philippe Claeys
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.13818]
¹Centre for Astrophysics and Planetary Science, School of Physical Sciences, Ingram Building, University of Kent, Canterbury, CT2 7NH UK
²Laboratoire G-Time, Université Libre de Bruxelles, Brussels, BE1050 Belgium
³Geological Survey of Belgium, Royal Belgian Institute of Natural Sciences, rue Vautier 29, Brussels, BE1000 Belgium
⁴Analytical, Environmental, and Geochemistry, Vrije Universiteit Brussel, Pleinlaan 2, Brussels, BE1050 Belgium
⁵Institut für Geowissenschaften, Goethe-Universität Frankfurt, Altenhöferallee 1, Frankfurt am Main, D-60438 Germany
Published by arrangement with John Wiley & Sons

Meteorites are prone to errestrial weathering not only after their fall on the Earth’s surface but also during storage in museum collections. To study the susceptibility of this material to weathering, weathering experiments were carried out on polished sections of the H5 chondrite Asuka 10177. The experiments consisted of four 100-days cycles during which temperature and humidity varied on a twelve hours basis. The first alteration cycle consisted of changing the temperature from 15 to 25 °C; the second cycle consisted of modifying both humidity and temperature from 35 to 45% and 15 to 25 °C, respectively; the third cycle consisted of varying the humidity level from 40 to 60%; and the fourth cycle maintained a fixed high humidity of 80%. Weathering products resulting from the experiments were identified and characterized using scanning electron microscopy–energy dispersive spectroscopy and Raman spectroscopy. Such products were not observed at the microscopic scale after the first cycle of alteration. Conversely, products typical of the corrosion of meteoritic FeNi metal were observed during scanning electron microscope surveys after all subsequent cycles. Important increases in the distribution of weathering products on the samples were observed after cycles 2 and 4 but not after cycle 3, suggesting that the combination of temperature and humidity fluctuations or high humidity (>60%) alone is most detrimental to chondritic samples. Chemistry of the weathering products revealed a high degree of FeNi metal corrosion with a limited contribution of troilite corrosion. No clear evidence of mafic silicate alteration was observed after all cycles, suggesting that postretrieval alteration remains limited to FeNi metal and to a lesser extent to troilite.

¹J.-Y.Chaufray,¹F.Leblanc,¹A.I.E.Werner,¹R.Modolo,²S.Aizawa
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115081]
¹LATMOS-IPSL, CNRS, Sorbonne-Université, Paris-Saclay, Paris, France
²IRAP, CNRS, Toulouse, France
Copyright Elsevier

We simulate the seasonal variations of the Mg 285.3 nm and Ca 422.7 nm brightness and compared our results to the MESSENGER/MASCS observations at dawn. Our results are consistent with the previous studies of Ca while for Mg we used another seasonal variation for the g -value (excitation frequency) at 285.3 nm. We find that both emissions are well reproduced from micrometeoroid impacts when the true anomaly angle (TAA) of Mercury is larger than 80°. For true anomaly angle lower than 80°, an additional source is needed to reproduce the Ca observations in agreement with previous studies, and possibly the Mg observations. We compare several solar spectra (observed or modeled) to study the Mg g-value and found that the seasonal variation of the g-value peaking near TAA = 60° used by previous studies to analyse the MESSENGER observations of Mg may be due to an artefact not present in the solar spectrum. The observed seasonal variations of the Mg brightness are better reproduced without this artefact. However, observations of the solar spectrum near 285.3 nm at a spectral resolution of ~20 mA would be needed to better estimate the seasonal variations of the Mg excitation frequencies and then to better understand the possible differences in the source of these two species in the exosphere of Mercury.

¹Prabhakar Alok Verma,¹Mamta Chauhan,¹Prakash Chauhan
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115075]
¹Indian Institute of Remote Sensing, Indian Space Research Organisation, Dehradun, India
Copyright Elsevier

This paper presents the methodology for correction of thermal emission from the Imaging Infrared Spectrometer sensor data onboard the recently launched Chandrayaan-2 spacecraft. The sensor provides extended spectral range of 0.8–5 μm and a spectral resolution of ~20 nm. The Imaging Infrared Spectrometer measures the reflected and emitted radiation of the Moon at a spatial resolution of ~80 m and has an advantage over the previous sensors in complete and clear characterization of OH/H2O feature. The thermal removal method follows calculation of the mean surface temperature at a fixed emissivity to obtain thermally corrected spectra. Further, the data have been utilized to generate temperature map of the Moon from the available data strips. The data after thermal correction can be further utilized to understand and derive the radiative and thermophysical properties of regolith and volatiles on the Moon.