arXiv subjects

Larry R. Nittler

Publications and source records attributed to Larry R. Nittler.

At least 19 recordsLinked to original sources

arXiv2026

Constraints for Nuclear Astrophysics from an Unusual Presolar Silicate-Oxide Aggregate Grain Found in Primitive Ordinary Chondrite Meteorite Hills 00526

We report O, Mg-Al, Si, Ca, and Ti isotopic data for an unusual presolar oxide/silicate aggregate grain, M526-69, previously reported in the primitive ordinary chondrite Meteorite Hills 00526. The $\approx 1μ$m aggregate consists of a Mg- and Ca-rich silicate, a Al-rich oxide, and a tiny TiO$_2$ grain. A large $^{18}$O depletion and high inferred $^{26}$Al/$^{27}$Al classifies M526-69 as a Group 2 grain. Both low-mass (LM) and intermediate-mass (IM) asymptotic giant branch (AGB) stars are considered viable candidate parent stars of Group 2 grains based on their O isotopes and inferred $^{26}$Al/$^{27}$Al ratios. The lack of a large $^{30}$Si excess in M526-69 strongly supports an LM-AGB origin for it and other Group 2 grains. The stable Mg, Ca, and Ti isotopes all reflect the initial composition of the parent star, set by galactic chemical evolution (GCE) processes. Presolar O-rich grains provide a better measure of the GCE trends for Ti isotopes than presolar SiC grains as the latter are also affected by neutron capture reactions in the parent stars. Most of the Mg, Ca, and Ti isotopic ratios in M526-69 are consistent with its parent star having metallicity lower than solar. However, small excesses in stable non-radiogenic $^{26}$Mg, $^{46}$Ti, and $^{44}$Ca do not fit this pattern and instead point to heterogeneous GCE processes, though quantitative modeling is needed to test this hypothesis. Multi-phase presolar grains are extremely valuable for nuclear astrophysics as they can both provide isotopic compositions for multiple elements that must be matched at a single time and place in a single star.

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arXiv2025

Clues for Solar System Formation from Meteorites and their Parent Bodies

Understanding the origin of comets requires knowledge of how the Solar System formed from a cloud of dust and gas 4.567 Gyr ago. Here, a review is presented of how the remnants of this formation process, meteorites and to a lesser extent comets, shed light on Solar System evolution. The planets formed by a process of collisional agglomeration during the first hundred million years of Solar System history. The vast majority of the original population of planetary building blocks (~100 km-scale planetesimals) was either incorporated into the planets or removed from the system, via dynamical ejection or through a collision with the Sun. Only a small fraction of the original rocky planetesimals survive to this day in the form of asteroids (which represent a total of ~0.05% of Earth's mass) and comets. Meteorites are fragments of asteroids that have fallen to Earth, thereby providing scientists with samples of Solar System-scale processes for laboratory-based analysis. Meteorite datasets complement cometary datasets, which are predominantly obtained via remote observation as there are few cometary samples currently available for laboratory-based measurements. This chapter discusses how analysis of the mineralogical, elemental, and isotopic characteristics of meteorites provides insight into (i) the origin of matter that formed planets, (ii) the pressure, temperature, and chemical conditions that prevailed during planet formation, and (iii) a precise chronological framework of planetary accretion. Also examined is the use of stable isotope variations and nucleosynthetic isotope anomalies as constraints on the dynamics of the disk and planet formation, and how these data are integrated into new models of Solar System formation. It concludes with a discussion of Earth's accretion and its source of volatile elements, including water and organic species.

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arXiv2025

A Pyroxenite Mantle on Mercury? Experimental Insights from Enstatite Chondrite Melting at Pressures up to 5 GPa

Enstatite chondrites (EC) are potential source material for the accretion of Mercury due to their reduced nature and enrichment in volatile elements. Understanding their melting properties is therefore important to better assess a scenario where Mercury formed from these chondrites. Here, we present experimental data on the partial melting of a modified EH4 Indarch EC, which was adjusted to have 18\% more metallic Si than SiO$_2$ in mass, yielding an oxygen fugacity of 3.7 below the iron--wüstite redox buffer and 12 wt\% Si in the metal. Experiments were performed from 0.5 to 5 GPa. Results indicate that the stability field of enstatite expands relative to olivine. This expansion is likely due to the presence of Ca--S and Mg--S complexes in the silicate melt, which enhance SiO$_2$ activity and promote enstatite crystallization. Additionally, sulfides show enrichment in Mg and Ca, up to 22 and 13 wt\% respectively, the main remaining cations being Fe, Cr, and Mn. These high Mg and Ca contents are observed at low temperatures and high silica content in the silicate melt, respectively. High-pressure melts (2 to 5 GPa, 160--400 km depth in Mercury) are Mg-rich, similar to those in Mercury's high-magnesium region (HMR), while low-pressure melts (0.5 to 1 GPa, 40--80 km depth) are Si-rich, comparable to the northern volcanic plains (NVP). Results suggest that a large fraction of Mercury's surface aligns compositionally with these melts, implying that Mercury's mantle could predominantly have a pyroxenitic composition. However, regions with differing compositions, such as aluminum-rich areas like the Caloris basin, suggest local variability in mantle geochemistry. Overall, our results show that if Mercury formed from materials similar to EC, batch melting of its primitive pyroxenite mantle would yield magmas with compositions resembling those of most rocks observed on the surface.

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arXiv2023

Explosive Nucleosynthesis in Core-Collapse Type II Supernovae: Insights from new C, N, Si, and Al-Mg isotopic compositions of presolar grains

We report C, N, Si, and Al-Mg isotope data for 39 presolar X silicon carbide (SiC) and four silicon nitride grains - a group of presolar grains that condensed in the remnants of core-collapse Type II supernovae (CCSNe) - isolated from the Murchison meteorite. Energy dispersive X-ray (EDX) data were used to determine the Mg and Al contents of the X SiC grains for comparison with the Mg/Al ratios determined by secondary ion mass spectroscopy (SIMS). Previous SIMS studies have used O-rich standards in the absence of alternatives. In this study, the correlated isotopic and elemental data of the X SiC grains enabled accurate determination of the initial 26Al/27Al ratios for the grains. Our new grain data suggest that (i) the literature data for X grains are affected to varying degrees by asteroidal/terrestrial contamination, and (ii) the Al/Mg ratios in SiC are a factor of two (with +/-6% 1 sigma uncertainties) lower than estimated based on the SIMS analyses that used O-rich standards. The lowered Al/Mg ratios result in proportionally higher inferred initial 26Al/27Al ratios for presolar SiC grains. In addition, the suppression of asteroidal/terrestrial contamination in this study leads to the observation of negative trends for 12C/13C-30Si/28Si and 26Al/27Al-30Si/28Si among our CCSN grains. We discuss these isotope trends in the light of explosive CCSN nucleosynthesis models, based on which we provide new insights into several non-traditional CCSN nucleosynthesis processes, including explosive H burning, the existence of a C/Si zone in the outer regions of CCSNe, and neutrino-nucleus reactions in deep CCSN regions.

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arXiv2023

Chromium on Mercury: New results from the MESSENGER X-Ray Spectrometer and implications for the innermost planet's geochemical evolution

Mercury, the innermost planet, formed under highly reduced conditions, based mainly on surface Fe, S, and Si abundances determined from MESSENGER mission data. The minor element Cr may serve as an independent oxybarometer, but only very limited Cr data have been previously reported for Mercury. We report Cr/Si abundances across Mercury's surface based on MESSENGER X-Ray Spectrometer data throughout the spacecraft's orbital mission. The heterogeneous Cr/Si ratio ranges from 0.0015 in the Caloris Basin to 0.0054 within the high-magnesium region, with an average southern hemisphere value of 0.0008 (corresponding to about 200 ppm Cr). Absolute Cr/Si values have systematic uncertainty of at least 30%, but relative variations are more robust. By combining experimental Cr partitioning data along with planetary differentiation modeling, we find that if Mercury formed with bulk chondritic Cr/Al, Cr must be present in the planet's core and differentiation must have occurred at log fO2 in the range of IW-6.5 to IW-2.5 in the absence of sulfides in its interior, and a range of IW-5.5 to IW-2 with an FeS layer at the core-mantle boundary. Models with large fractions of Mg-Ca-rich sulfides in Mercury's interior are more compatible with moderately reducing conditions (IW-5.5 to IW-4) owing to the instability of Mg-Ca-rich sulfides at elevated fO2. These results indicate that if Mercury differentiated at a log fO2 lower than IW-5.5, the presence of sulfides whether in the form of a FeS layer at the top of the core or Mg-Ca-rich sulfides within the mantle would be unlikely.

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arXiv2023

Tracing the history of an unusual compound presolar grain from progenitor star to asteroid parent body host

We conducted a TEM study of an unusual oxide-silicate composite presolar grain (F2-8) from the unequilibrated ordinary chondrite Semarkona. The presolar composite grain is relatively large, has an amoeboidal shape, and contains Mg-rich olivine, Mg-Al spinel, and Ca-rich pyroxene. The shape and phase assemblage are reminiscent of amoeboid-olivine-aggregates and add to the growing number of TEM observations of presolar refractory inclusion-like (CAIs and AOAs) grains. In addition to the dominant components, F2-8 also contains multiple subgrains, including an alabandite-oldhamite composite grain within the olivine and several magnetite subgrains within the Mg-Al spinel. We argue that the olivine, Mg-Al spinel, and alabandite-oldhamite formed by equilibrium condensation, whereas the Ca-rich pyroxene formed by non-equilibrium condensation, all in an M-type AGB star envelope. On the other hand, the magnetite subgrains are likely the result of aqueous alteration on the Semarkona asteroidal parent body. Additional evidence of secondary processing includes Fe-enrichment in the Mg-Al spinel and olivine, elevated Al contents in the olivine, and beam sensitivity and a modulated structure for the olivine. Compound presolar grains record condensation conditions over a wide range of temperatures. Additionally, the presence of several different presolar phases in a composite grain can impart information on the relative rates and effects of post-condensation processing in a range of environments, including the interstellar medium, solar nebula, and the host asteroid parent body. The TEM observations of F2-8 provide insights across the lifetime of the grain from its formation by condensation in an M-type AGB star envelope, its transit through the interstellar medium, and aqueous alteration during its residence on Semarkona's asteroidal parent body.

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arXiv2022

Presolar Silicon Carbide Grains of Types Y and Z: Their Strontium and Barium Isotopic Compositions and Stellar Origins

We report the Sr and Ba isotopic compositions of 18 presolar SiC grains of types Y (11) and Z (7), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. We find that the Y and Z grains show higher 88Sr/87Sr and more variable 138Ba/136Ba ratios than mainstream (MS) grains. According to FRANEC Torino AGB models, the Si, Sr, and Ba isotopic compositions of our Y and Z grains can be consistently explained if the grains came from low mass AGB stars with 0.15 Zsun <= Z < 1.00 Zsun, in which the 13C neutron exposure for the slow neutron-capture process is greatly reduced with respect to that required by MS grains for a 1.0 Z8 AGB star. This scenario is in line with the previous finding based on Ti isotopes, but it fails to explain the indistinguishable Mo isotopic compositions of MS, Y, and Z grains.

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arXiv2022

Presolar stardust in asteroid Ryugu

We have conducted a NanoSIMS-based search for presolar material in samples recently returned from C-type asteroid Ryugu as part of JAXA's Hayabusa2 mission. We report the detection of all major presolar grain types with O- and C-anomalous isotopic compositions typically identified in carbonaceous chondrite meteorites: 1 silicate, 1 oxide, 1 O-anomalous supernova grain of ambiguous phase, 38 SiC, and 16 carbonaceous grains. At least two of the carbonaceous grains are presolar graphites, whereas several grains with moderate C isotopic anomalies are probably organics. The presolar silicate was located in a clast with a less altered lithology than the typical extensively aqueously altered Ryugu matrix. The matrix-normalized presolar grain abundances in Ryugu are 4.8$^{+4.7}_{-2.6}$ ppm for O-anomalous grains, 25$^{+6}_{-5}$ ppm for SiC grains and 11$^{+5}_{-3}$ ppm for carbonaceous grains. Ryugu is isotopically and petrologically similar to carbonaceous Ivuna-type (CI) chondrites. To compare the in situ presolar grain abundances of Ryugu with CI chondrites, we also mapped Ivuna and Orgueil samples and found a total of SiC grains and 6 carbonaceous grains. No O-anomalous grains were detected. The matrix-normalized presolar grain abundances in the CI chondrites are similar to those in Ryugu: 23 $^{+7}_{-6}$ ppm SiC and 9.0$^{+5.3}_{-4.6}$ ppm carbonaceous grains. Thus, our results provide further evidence in support of the Ryugu-CI connection. They also reveal intriguing hints of small-scale heterogeneities in the Ryugu samples, such as locally distinct degrees of alteration that allowed the preservation of delicate presolar material.

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arXiv2022

TEM analyses of in situ presolar grains from unequilibrated ordinary chondrite LL3.0 Semarkona

We investigated six presolar grains from very primitive regions of the matrix in the unequilibrated ordinary chondrite Semarkona with TEM. These grains include one SiC, one oxide (Mg-Al spinel), and four silicates. 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. 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.

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arXiv2021

New Multielement Isotopic Compositions of Presolar SiC Grains: Implications for Their Stellar Origins

We report NanoSIMS Si and Mg-Al isotopic data (and C, N, and Ti isotopic data when available) for 85 submicron- to micron-sized presolar SiC grains from the CM2 Murchison meteorite, including 60 MS, 8 AB1, 8 X, 7 AB2, and 2 Y grains. The MS and Y grain data demonstrate that (1) C and N contamination mainly appears as surface contamination, and sufficient presputtering is needed to expose a clean grain surface for obtaining intrinsic C and N signals, and (2) Mg and Al contamination appears as adjacent grains and rims, and high-resolution imaging and the choice of small regions of interest during data reduction together are effective in suppressing the contamination. Our results strongly indicate that previous studies on presolar SiC grains could have sampled differing degrees of contamination for C, N, Mg, and Al. Compared to the literature data, our new MS and Y grains are in better agreement with carbon star observations for both the C and N isotopic ratios. By comparing our new, tighter distributions of 12C/13C, 14N/15N, and initial 26Al/27Al ratios for MS and Y grains with FRUITY AGB stellar models, we provide more stringent constraints on the occurrence of cool bottom processing and the production of 26Al in N-type carbon stars, classical asymptotic giant branch stars.

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arXiv2021

TEM analyses of unusual presolar silicon carbide: Insights into the range of circumstellar dust condensation conditions

Presolar silicon carbide (SiC) grains in meteoritic samples can help constrain circumstellar condensation processes and conditions in C-rich stars and core-collapse supernovae. This study presents our findings on eight presolar SiC grains from AGB stars (four mainstream and one Y grain) and core-collapse supernovae (three X grains), chosen on the basis of μ-Raman spectral features that were indicative of their having unusual non-3C polytypes and/or high degrees of crystal disorder. Analytical transmission electron microscopy (TEM), which provides elemental compositional and structural information, shows evidence for complex histories for the grains. Our TEM results confirm the presence of non-3C,2H crystal domains. Minor element heterogeneities and/or subgrains were observed in all grains analyzed for their compositions. The C/O ratios inferred for the parent stars varied from 0.98 to greater than or equal to 1.03. Our data show that SiC condensation can occur under a wide range of conditions, in which environmental factors other than temperature (e.g., pressure, gas composition, heterogeneous nucleation on pre-condensed phases) play a significant role. Based on previous μ-Raman studies, about 10% of SiC grains may have infrared (IR) spectral features that are influenced by crystal defects, porosity, and/or subgrains. Future sub-diffraction limited IR measurements of complex SiC grains might shed further light on the relative contributions of each of these features to the shape and position of the characteristic IR 11-μm SiC feature and thus improve the interpretation of IR spectra of AGB stars like those that produced the presolar SiC grains.

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arXiv2020

Cluster analysis of presolar silicon carbide grains: evaluation of their classification and astrophysical implications

Cluster analysis of presolar silicon carbide grains based on literature data for 12C/13C, 14N/15N, δ30Si/28Si, and δ29Si/28Si including or not inferred initial 26Al/27Al data, reveals nine clusters agreeing with previously defined grain types but also highlighting new divisions. Mainstream grains reside in three clusters probably representing different parent star metallicities. One of these clusters has a compact core, with a narrow range of composition, pointing to an enhanced production of SiC grains in asymptotic giant branch (AGB) stars with a narrow range of masses and metallicities. The addition of 26Al/27Al data highlights a cluster of mainstream grains, enriched in 15N and 26Al, which cannot be explained by current AGB models. We defined two AB grain clusters, one with 15N and 26Al excesses, and the other with 14N and smaller 26Al excesses, in agreement with recent studies. Their definition does not use the solar N isotopic ratio as a divider, and the contour of the 26Al-rich AB cluster identified in this study is in better agreement with core-collapse supernova models. We also found a cluster with a mixture of putative nova and AB grains, which may have formed in supernova or nova environments. X grains make up two clusters, having either strongly correlated Si isotopic ratios or deviating from the 2/3 slope line in the Si 3-isotope plot. Finally, most Y and Z grains are jointly clustered, suggesting that the previous use of 12C/13C= 100 as a divider for Y grains was arbitrary. Our results show that cluster analysis is a powerful tool to interpret the data in light of stellar evolution and nucleosynthesis modelling and highlight the need of more multi-element isotopic data for better classification.

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arXiv2020

Presolar Stardust in Highly Pristine CM Chondrites Asuka 12169 and Asuka 12236

We report a NanoSIMS search for presolar grains in the CM chondrites Asuka (A) 12169 and A12236. We found 90 presolar O-rich grains and 25 SiC grains in A12169, giving matrix-normalized abundances of 275 (+55/-50, 1$σ$) ppm or, excluding an unusually large grain, 236 (+37/-34) ppm for O-rich grains and 62 (+15/-12) ppm for SiC grains. For A12236, 18 presolar silicates and 6 SiCs indicate abundances of 58 (+18/-12) and 20 (+12/-8) ppm, respectively. The SiC abundances are in the typical range of primitive chondrites. The abundance of presolar O-rich grains in A12169 is essentially identical to that in CO3.0 Dominion Range 08006, higher than in any other chondrites, while in A12236 it is higher than found in other CMs. These abundances provide further strong support that A12169 and A12236 are the least-altered CMs as indicated by petrographic investigations. The similar abundances, isotopic distributions, silicate/oxide ratio, and grain sizes of the presolar O-rich grains found here to those of presolar grains in highly primitive CO, CR and ungrouped carbonaceous chondrites (CCs) indicate that the CM parent body(ies) accreted a similar population of presolar oxides and silicates in their matrices to those accreted by the parent bodies of the other CC groups. The lower abundances and larger grain sizes seen in some other CMs are thus most likely a result of parent-body alteration and not heterogeneity in nebular precursors. Presolar silicates are unlikely to be present in high abundances in returned samples from asteroids Ryugu and Bennu since remote-sensing data indicate that they have experienced substantial aqueous alteration.

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arXiv2020

The Case for Non-Cryogenic Comet Nucleus Sample Return

Comets hold answers to mysteries of the Solar System by recording presolar history, the initial states of planet formation and prebiotic organics and volatiles to the early Earth. Analysis of returned samples from a comet nucleus will provide unparalleled knowledge about the Solar System starting materials and how they came together to form planets and give rise to life: 1. How did comets form? 2. Is comet material primordial, or has it undergone a complex alteration history? 3. Does aqueous alteration occur in comets? 4. What is the composition of cometary organics? 5. Did comets supply a substantial fraction of Earth's volatiles? 6. Did cometary organics contribute to the homochirality in life on Earth? 7. How do complex organic molecules form and evolve in interstellar, nebular, and planetary environments? 8. What can comets tell us about the mixing of materials in the protosolar nebula?

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arXiv2020

Formation and evolution of the local interstellar environment: combined constraints from nucleosynthetic and X-ray data

Several observations suggest that the Solar system has been located in a region affected by massive stellar feedback for at least a few Myr; these include detection of live $^{60}\text{Fe}$ in deep-sea archives and Antarctic snow, the broad angular distribution of $^{26}\text{Al}$ around the Galactic plane seen in all-sky $γ$-ray maps, and the all-sky soft X-ray background. However, our position inside the Galactic disc makes it difficult to fully characterise this environment, and our limited time baseline provides no information about its formation history or relation to large-scale Galactic dynamics. We explore these questions by using an $N$-body+hydrodynamics simulation of a Milky-Way-like galaxy to identify stars on Sun-like orbits whose environments would produce conditions consistent with those we observe. We find that such stars are uncommon but not exceptionally rare. These stars are found predominantly near the edges of spiral arms, and lie inside kpc-scale bubbles that are created by multiple generations of star formation in the arm. We investigate the stars' trajectories and find that the duration of the stay in the bubble ranges from 20 Myr to 90 Myr. The duration is governed by the crossing time of stars across the spiral arm. This is generally shorter than the bubble lifetime, which is $\sim 100$ Myr as a result of the continuous gas supply provided by the arm environment.

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arXiv2020

Cryogenic Cometary Sample Return

Comets likely formed in the outer regions of the protosolar nebula where they incorporated and preserved primitive presolar materials, volatiles resident in the outer disk, and more refractory materials from throughout the disk. The return of a sample of volatiles (i.e., ices and entrained gases), along with other components of a cometary nucleus, will yield numerous major scientific opportunities. We are unaccustomed to thinking of ices through a mineralogical/petrological lens, but at cryogenic temperatures, ices can be regarded as mineral components of rocky material like any other. This is truly Terra Incognita, as a sample from a natural cryogenic (10s of K) environment is unprecedented in any setting; currently, we can only make educated guesses about the nature of these materials on a microscopic scale. Such samples will provide an unparalleled look at the primordial gases and ices present in the early solar nebula, enabling insights into the gas phase and gas-grain chemistry of the nebula. Understanding the nature of the ices in their microscopic, petrographic relationship to the refractory components of the cometary sample will allow for the study of those relationships and interactions and a study of evolutionary processes on small icy bodies. The previous 2013-2022 Planetary Decadal Survey included a study of a Flagship-class cryogenic comet nucleus sample return mission, given the scientific importance of such a mission. However, the mission was not recommended for flight in the last Decadal Survey, in part because of the immaturity of critical technologies. Now, a decade later, the scientific importance of the mission remains and relevant technological advances have been made in both cryo instrumentation for flight and laboratory applications. Such a mission should be undertaken in the next decade.

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arXiv2020

Origin of large meteoritic SiC stardust grains in metal-rich AGB stars

Stardust grains that originated in ancient stars and supernovae are recovered from meteorites and carry the detailed composition of their astronomical sites of origin. We present evidence that the majority of large ($μ$m-sized) meteoritic silicon carbide (SiC) grains formed in C-rich asymptotic giant branch (AGB) stars that were more metal-rich than the Sun. In the framework of the slow neutron-captures (the s process) that occurs in AGB stars the lower-than-solar 88Sr/86Sr isotopic ratios measured in the large SiC grains can only be accompanied by Ce/Y elemental ratios that are also lower than solar, and predominately observed in metal-rich barium stars - the binary companions of AGB stars. Such an origin suggests that these large grains represent the material from high-metallicity AGB stars needed to explain the s-process nucleosynthesis variations observed in bulk meteorites (Ek et al. 2020). In the outflows of metal-rich, C-rich AGB stars SiC grains are predicted to be small ($\simeq$ 0.2 $μ$m-sized); large ($\simeq$ $μ$m-sized) SiC grains can grow if the number of dust seeds is two to three orders of magnitude lower than the standard value of $10^{-13}$ times the number of H atoms. We therefore predict that with increasing metallicity the number of dust seeds might decrease, resulting in the production of larger SiC grains.

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arXiv2020

Global Major-Element Maps of Mercury from Four Years of MESSENGER X-Ray Spectrometer Observations

The X-Ray Spectrometer (XRS) on the MESSENGER spacecraft provided measurements of major-element ratios across Mercury's surface. We present global maps of Mg/Si, Al/Si, S/Si, Ca/Si, and Fe/Si derived from XRS data collected throughout MESSENGER's orbital mission. We describe the procedures we used to select and filter data and to combine them to make the final maps, which are archived in NASA's Planetary Data System. Areal coverage is variable for the different element-ratio maps, with 100% coverage for Mg/Si and Al/Si, but only 18% coverage for Fe/Si north of 30 $^{\circ}$ N, where the spatial resolution is highest. The spatial resolution is improved over previous maps by 10-15% because of the inclusion of higher-resolution data from late in the mission when the spacecraft periapsis altitude was low. Unlike typical planetary data maps, however, the spatial resolution of the XRS maps can vary from pixel to pixel, and thus care must be taken in interpreting small-scale features. We provide several examples of how the XRS maps can be used to investigate elemental variations in the context of geological features on Mercury, which range in size from single $\sim$100-km-diameter craters to large impact basins. We expect that these maps will provide the basis for and/or contribute to studies of Mercury's origin and geological history for many years to come.

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