Saturday 25 July 2020

Determining the ages of presolar silicon carbide grains from the Murchison Meteorite.

Interstellar dust is an important component of our galaxy. It influences star formation as well as the thermal and chemical evolution of the galaxy. Although dust only presents about 1% of the mass in the interstellar medium, it carries a large fraction of the elements heavier than helium, including the elements that form terrestrial planets and are essential for life. Thus, interstellar dust is a key ingredient of stars and habitable planetary systems, making increased knowledge about its composition and lifecycle desirable. Compositional, structural, and size information of interstellar dust can be obtained through astronomical spectroscopic observations, but dust lifetime estimates mainly rely on sophisticated theoretical models. These models, however, focus on the more common small dust grains and are based on assumptions with large uncertainties. These uncertainties mainly pertain to the residence time of the dust in various regions of the interstellar medium, which exhibit different rates of dust destruction through sputtering and collisions in supernova shock waves. Most of these models currently predict an average lifetime of interstellar grains on the order of 100 million years. However, more recent models and a few models for larger grains predict much longer survival times in the interstellar medium of up to billions of years.

In a paper published in the Proceedings of the National Academy of Sciences of the United States of America on 13 January 2020, Philipp Heck, Jennika Greer, and Levke Kööp of the Robert A. Pritzker Center for Meteoritics and Polar Studies at the Field Museum of Natural History, and the Chicago Center for Cosmochemistry and Department of the Geophysical Sciences at the University of Chicago, Reto Trappitsch of the Nuclear and Chemical Sciences Division at Lawrence Livermore National Laboratory, Frank Gyngard of the Physics Department at Washington University, and the Center for NanoImaging at Harvard Medical School, Henner Busemann and Colin Maden of the Institute of Geochemistry and Petrology at ETH Zürich, Janaína Ávila of the Research School of Earth Sciences at the Australian National University, Andrew Davis, also of the obert A. Pritzker Center for Meteoritics and Polar Studies at the Field Museum of Natural History, the Chicago Center for Cosmochemistry and Department of the Geophysical Sciences, and of the Enrico Fermi Institute at the University of Chicago, and Rainer Wieler, also of the Institute of Geochemistry and Petrology at ETH Zürich, present a laboratory-based approach of determining the interstellar lifetimes of individual large presolar silicon carbide stardust grains.

Presolar silicon carbide grain morphology. Scanning electron microscope images (secondary electrons) of representative samples of the two morphological types of presolar silicon carbide grains studied here. Grain L3_01 has a euhedral shape indicating it evaded shattering; (A) before and (B) after pressing into gold and after nanoscale secondary ion mass spectrometry and Sensitive High Resolution Ion Micro Probe analysis but before laser extraction of noble gases. Grain L3_20 has a shard-like appearance with fractures (C) before pressing and (D) got fractured further upon pressing into the gold substrate. Heck et al. (2020).

The presolar grains analyzed in Henk et al.'s study were isolated by chemical methods from the Murchison CM2 Meteorite, where they had remained unaltered since their incorporation into the meteorite parent body in the early Solar System 4.6 billion years ago. These grains are identified as presolar by their large isotopic anomalies that exclude an origin in the Solar System. Presolar stardust grains are the oldest known solid samples available for study in the laboratory, represent the small fraction of material that formed in circumstellar environments, and survived processing in the interstellar medium and Solar System. The presolar stardust grain abundance in our parent interstellar cloud was a few percent of all interstellar dust present in this cloud, with the other dust having condensed in the interstellar medium. In the solar nebula, more dust condensed from the cooling gas and presolar stardust became an even more minor component. Most presolar grains were subsequently destroyed after accretion in their parent bodies during thermal metamorphism and aqueous alteration. Thus, their abundance in the most primitive Solar System materials that evaded destructive parent body processing is a few parts per million to about 200 parts per million, except for interplanetary dust presumably from comet Grigg-Skjellerup dust, which contains up to 1% presolar materials. Henk et al. used mass spectrometry to analyse the abundance of nuclides produced in the grains by spallation reactions with galactic cosmic rays, which comprise mostly high-energy protons and α-particles, during their residence in the interstellar medium. When these high-energy particles hit a grain, small fractions of the target nuclides break up. The resulting atomic fragments accumulate in the grain, and their concentrations are proportional to the timespan the grains were irradiated. Suitable daughter elements to study are those with a very low initial abundance in the grains such that the cosmic rayproduced ('cosmogenic') fraction becomes detectable. This is the case for helium, neon, and lithium. Silicon carbide is the best-suited interstellar phase for cosmogenic nuclide dating, due to its relatively large grain size, high retentivity of cosmogenic nuclides, and durability. Even though silicon carbide is only a small fraction of the total amount of interstellar dust, due to its durability, we consider it a useful tracer. In the most common silicon carbide grains, the ones that originate from low-to-intermediate-mass asymptotic giant branch stars, the initial helium and neon isotopic compositions, incorporated from their parent stars, are well known, so the cosmogenic fraction can be readily identified. Improved knowledge of production and retention of such cosmogenic nuclides enabled Henk et al. to obtain ages with improved reliability. While radiometric dating based on the uranium-lead decay system can provide ages with high accuracy and is often the method of choice for samples of Solar System materials, it has not yet been successfully applied to presolar grains. These grains have masses that are orders of magnitudes smaller than samples dated so far. Furthermore, presolar grains have large isotopic anomalies in essentially every element, so each grain may have a distinct initial lead isotopic composition, uranium isotope ratio, and age, robbing the uranium-lead system of some of its most desirable characteristics for geochronology. Until these obstacles are overcome, exposure age dating is the preferred method for determining presolar ages of individual stardust grains.

The first such studies were made on assemblages of thousands of silicon carbide grains from chemical separates. Ages of about 10 to 100 million years were derived from neon²¹, but it was suggested that individual grains might show much higher presolar ages of up to 2 billion years. However, it has been shown that much of the measured neon²¹ was not cosmogenic but was implanted neon from the helium shell of the parent asymptotic giant branch star, while, at the same time, they concluded that losses of cosmogenic neon²¹ upon production due to recoil out of the grains were much larger than assumed. It has been deduced much lower presolar ages for bulk silicon carbide assemblages of a few times 100 million years only, based on cosmogenic xenon, for which recoil losses are smaller. The first interstellar exposure ages on individual exceptionally large (about 5 to 60 μm) silicon carbide grains have been reported based on lithium isotopes and with helium and neon. The large grains contain greater amounts of cosmogenic nuclides, and, more importantly, require a smaller recoil correction. These studies both reported ages of between a few megayears to about 1 billion years, but the average of the lithium-based ages was considerably higher than the average noble gas age. It has been suggested that the many ages of less than 200 Ma may be explained by increased dust production after a galactic starburst 1 to 2 billion years prior to the birth of the Sun.

The Murchison meteorite at the National Museum of Natural History. Wikimedia Commons.

Henk et al. provide presolar ages based on cosmogenic neon isotopes, significantly increasing the total number of presolar grain ages. They also present re-evaluated ages from previously published data. This will enable us to further advance our understanding of the lifetimes of interstellar dust. Previous interstellar production rates of cosmogenic nuclides were based on fluxes deep within the heliosphere that were extrapolated to interstellar space. Henk et al. use, instead, improved interstellar production rates that were determined with a purely physical model that uses a state-of-the-art nuclear cross-section database and an interstellar galactic cosmic ray spectrum based on data collected by NASA’s Voyager 1 space probe at the edge of the heliosphere. Voyager 1 recorded  the low-energy part of the galactic cosmic ray spectrum, something that is not possible deeper within the heliosphere. To correct for recoil losses of cosmogenic nuclides from silicon carbide grains grains, Henk et al. use a physical recoil model that considers the energies of galactic cosmic ray protons and α-particles from the new cosmic ray spectrum.

Another aspect that was not considered in previous studies is the potential exposure of presolar grains to the enhanced particle flux of the early active sun. Large excesses of cosmogenic noble gases in single olivine grains in some primitive meteorites have been attributed by some workers to a high flux of energetic particles from the early Sun, although others contested this conclusion. Recently, however, unambiguous evidence for an enhanced exposure of hibonite (an aluminum−calcium oxide), possibly the earliest solar nebula condensate, to energetic particles from the early active sun has been reported. This implies that some of the presolar grains Henk et al. studied might have been exposed to the same enhanced solar particle flux. They were, therefore, also required to estimate the upper limit of cosmogenic nuclides concentrations produced in the early Solar System rather than in the interstellar medium.

Henk et al. processed their noble gas data from 27 silicon carbide grains and reprocessed data from published results from 22 silicon carbide grains to calculate an internally consistent set of presolar cosmic ray exposure ages for nearly 50 grains with the improved cosmogenic nuclide production rates and nuclear recoil corrections. The cosmogenic neon component can be clearly resolved from the two other main components, nucleosynthetic neon and adsorbed atmospheric neon based on distinct isotopic neon compositions. Nucleosynthetic neon is implanted into circumstellar grains from the hot post-asymptotic giant branch star wind emanating from the exposed helium shell, and its concentrations decrease with increasing grain size. Using carbon, nitrogen, and silicon isotopes, all but three grains have been classified as mainstream silicon carbide grains, originating in the outflows of low- to intermediate-mass (post) asymptotic giant branch stars. The three other grains are of AB type, based on their low carbon¹²/carbon¹³ ratios. All newly analysed grains are mainstream silicon carbide.

Henk et al. determined helium³ and neon²¹ exposures ages of 30 and 24 grains, respectively, and obtained upper age limits for 12 (helium³) and 16 (neon²¹) grains. For 18 grains, Henk et al. obtained both helium³ and neon²¹ exposures ages. Nominal recoil-corrected helium³ exposures ages for 16 out of these 18 grains are higher than recoil-corrected neon²¹ exposures ages, whereas uncorrected ages show an opposite trend. Helium is more easily lost through heating and through recoil than neon, so both effects would result in lower nominal helium³ than neon²¹ exposures ages before a recoil correction. Hence, a recoil correction will be larger for helium³ than for neon²¹. For grains of less than 10 μm, nominal cosmogenic helium³ recoil losses are over 94% for the smallest grains analysed by Henk et al., whereas corresponding losses for neon²¹ are over 40%. Hence, any uncertainties in recoil corrections will result in a larger uncertainty of helium³ exposures ages. Heating of grains to high temperatures (at leasr 900 K) would result in near-complete helium loss. Helium loss works in the opposite direction of the trend seen in the data. This implies that, while some helium loss cannot be excluded, no significant loss occurred; otherwise, much more helium than neon would have been lost, and even overcorrected helium³ exposures ages would be smaller than neon²¹ exposures ages. The helium³ exposures ages are less reliable than neon²¹ exposures ages, mainly because of larger uncertainties in the helium³ recoil correction. The 16 recoil-corrected helium³ exposures ages exceeding recoil-corrected neon²¹ exposures ages, consequently, indicate an overestimation of the recoil loss for helium³. The reason for this may be that these grains were actually irradiated in the interstellar medium as parts of larger grains or as grain aggregates, or the grains were coated with large mantles of ices and organics while in the interstellar medium. Henk et al. estimate the original sizes of the irradiated objects in the interstellar medium by varying the grain size and modeling the resulting recoil correction until the recoil-corrected helium³ and neon²¹ exposures ages match. The estimated object diameters during irradiation are factors of about 3 × up to about 30 × higher than those of the analysed grains. This results in ages of 44 to 85% of the original recoil-corrected ages. In principle, it would be possible to test this result with cosmogenic xenon that has a much smaller recoil loss. Unfortunately, the amounts of cosmogenic xenon produced are below current detection limits for single-grain analyses, due to the low amounts of suitable target elements for xenon production in silicon carbide. Bulk analyses of silicon carbide give mixed signals and are not useful in this regard, as these do not resolve cosmogenic gas contributions from grains with different lifetimes. Seventy-five percent of the 16 analysed grains that were part of much larger objects have euhedral shapes, which indicates they are not fragments of larger grains and were more likely parts of aggregates. The remainder look like they are shattered fragments of larger silicon carbide grains grains, but, given the large object sizes estimated during interstellar medium irradiation, larger than any known presolar silicon carbide grain, they were likely also part of aggregates. Aggregates of minerals, suspected by some to be presolar, in an organic material matrix were recently observed in interplanetary dust particles. A previous study observed organic coatings on about 60% of pristine presolar silicon carbide that were physically separated from their host meteorite without the use of chemical reagents. However, no aggregates or clustering of larger presolar grains have yet been observed during the in situ ion imaging searches of polished sections of meteorites. The lack of such clustering of larger grains could be due to preferred breakup of larger clusters of several dozen to hundred micrometers during accretion onto planetesimals in the early Solar System, while smaller clusters composed of smaller grains which have lower inertia, stayed intact. Henk et al. propose that grains in the size range we analyzed formed in the outflows of (post) asymptotic giant branch parent stars and coagulated there with organic matter to form larger aggregates. While large silicon carbide dust grains are rare in the interstellar medium, they are consistent with observations of circumstellar dust around asymptotic giant branch and post-asymptotic giant branch stars. Far-infrared excess associated with such dust may indicate the presence of up to millimeter-sized grains. Up to 5-mm-large dust grains were proposed to explain radio observations of dust around the Egg Nebula, a post-asymptotic giant branch star. Another possibility is that the high-density winds from post-asymptotic giant branch stars are the sources of the large presolar silicon carbide grains, such as the size fraction studied by Henk et al.

Henk et al. also obtained lithium isotope data for 19 silicon carbide grains. Many of these grains have a lithium⁷/lithium⁶ ratio below the chondritic ('solar') value of 12.06, indicating the contribution of a cosmogenic lithium component (the end-member cosmogenic lithium⁷/lithium⁶ ratio is about 1.2). These observations could be due to a combination of contamination with terrestrial or Solar System lithium, matrix effects, or additional, unidentified lithium components that would have contributed to the measured lithium concentration. Because of low concentrations of cosmogenic lithiun and high abundance of normal lithiun, a reliable determination of cosmogenic lithiun is very difficult. Currently, lithium does not allow the obtaining of a reliable age.

Evidently, the neon ages are more reliable than the lithium and helium ages, and Henk et al. base their discussion mainly on neon²¹ ages. They range from 4 million years to 3200 million years, and upper limits range from 3 to 3300 million years. Henk et al. obtained neon²¹ ages for two out of three AB grains; the calculated ages, 65 million years and 260 million years, fit into the age range of the mainstream grains. No age was determined for the third AB grain, due to an insufficient gas amount; the 2-μm-sized grain was the smallest one analysed in Henk et al.'s study.

Overall, the neon²¹ age distribution trend is similar to what was previously reported for a smaller sample set, with most exposure ages below 300 million years (60%) and 50% below 200 million years. This is consistent with most theoretical lifetime estimates for much smaller, under 1 μm interstellar dust of 100 to 300 million years, but in contrast to the longer lifetimes expected for large grains. Assuming constant dust production rates from asymptotic giant branch stars and constant dust destruction rates, Henk et al. would expect to encounter younger grains more frequently than older grains simply because older ones have a higher probability of encountering a destructive process. However, their age distribution does not fit any of the assumed steady-state models for different average lifetimes. Having many large grains in a relatively narrow age range seems to require an explanation other than simply a lifetime effect, which would apply to small grains. Henk et al. propose that this age distribution can be explained by these large grains being late-stage products of asymptotic giant branch stars with initial masses of two times that of the Sun, that formed together. While less massive stars were more abundant, their evolutionary lifetimes were too long to reach the dust-producing asymptotic giant branch phase before the formation of the Solar System, and, hence, their dust has not been incorporated into meteorite parent bodies. The rarer, more massive asymptotic giant branch stars (with initial masses of over three times that of the Sun) are not likely to be a source of large silicon carbide grains, as their higher radiation pressures would have ejected circumstellar grains before they grew to the large grain sizes observed here. It was previously suggested that the grains’ parent stars originated in a presolar starburst that could have been triggered by a galactic merger, which has been proposed to explain the silicon isotopic compositions of presolar mainstream silicon carbide. Most observational and theoretical work on the history of the star formation rate of our galaxy does not see evidence of a large starburst event in presolar times as hypothesised previously, nor a flat star formation rate, but most studies conclude that the star formation rate only mildly fluctuated. These studies find a moderately enhanced star formation rate around 7 to 9 billion years ago. Several of the observational studies show that this broad peak consists of two peaks, with the more recent one close to 7 billion years ago. Recent modeling work based on observations of the chemical compositions of stars in the solar neighborhood reveals a moderately enhanced star formation rate that peaked around 7 billion years ago. In this model, this enhancement was caused by streams of cold matter that accreted onto the galactic disk from the halo. Based on stellar main-sequence lifetime calculations, Henk et al. estimate that stars with about 1.6 to 1.9 times the mass of the Sun, that formed together during this enhanced star formation rate episode about 7 billion years ago, reached their dust-producing asymptotic giant branch phase between about 4.9 and about 4.6 billon years ago. These dust grains would then have been exposed to interstellar galactic cosmic rays for up to 300 million years before being shielded in the forming Solar System. What we are seeing in the silicon carbide age peak are the first arrivers of dust formed in the late stages of stars originating in the presolar enhanced star formation rate peak. The rest of the peak must be more recent than the start of the Solar System and was not sampled in the presolar grain population. Although speculative, this scenario is consistent with Henk et al.'s data and, barring another explanation, may be a plausible reason for the observed presolar silicon carbide age distribution for large grains with presolar ages of up to 300 million years. While Henk et al. see older grains, they do not see older peaks (other than from individual grains) in their age distribution. They explain this by two effects. First, grain destruction reduced the number of surviving old grains, and, second, the signal to noise ratios farther back in time are currently too low to show peaks within Henk et al.'s dataset. Older interstellar neon²¹ exposure ages obtained for at least 7 grains are over 300 million years and, for a few grains (3 out of 24, excluding those with upper limits; 5 out of 40 including upper limits), are consistent with what is expected for large grains. In particular, if these grains were over 100-μm aggregates in the interstellar medium, long lifetimes are expected. Erosion by sputtering is slower than the time the grain is exposed to shock-heated gas, but large grains can erode significantly when they get slowed down in the cooled postshock gas and experience rare collisions with other large grains. Gradual erosion by collisions with smaller grains would leave cratered surfaces, something that has not been observed with silicon carbide grains to date. Possible evidence of a microimpact crater was so far only found in a large presolar aluminum oxide grain. Some of the old grains could have been shielded from destructive processes in clumps. Such protective density inhomogeneities have been observed astronomically in shocked regions of the interstellar medium.

The oldest grains based on both helium³ and neon²¹ ages are the smallest, and an inverse trend between age and grain size is apparent onsistent with the preliminary trend observed in xenon bulk silicon carbide analyses. The trend persists in the recoil-corrected data and in the size-corrected subset but gets less prominent in the latter. Smaller grains are more abundant than larger grains in the interstellar medium, resulting in a higher number of smaller grains that are old compared to larger ones. We can exclude a sampling bias, as we have not disproportionally analyzed small grains; on the contrary, only 12 of the 49 grains are under 4 μm. 

It has been proposed that grains with presolar ages older than the sun’s galactic year (about 230 million years) might have had the time to radially migrate from the inner parts of the galaxy toward the galactocentric distance of the forming Solar System. Because of the compositional gradient within our galaxy, we would expect these grains to reflect the metallicity of their parent stars. However, we do not observe a correlation between age and silicon isotopic composition, which is a proxy for metallicity of stellar sources. Either our dataset is too small to reveal such a trend, the grains did not migrate as suggested, or there is no galactic gradient for silicon isotopic composition, in contrast to oxygen isotopic composition and (iron/hydrogen). Recent astronomical observations did not find a galactocentric silicon²⁹ proportion trend within about 200‰, a range that was less than expected from galactocentric variations in other isotope ratios but similar to the one measured in presolar silicon carbide mainstream grains.

Henk et al. highlight that, at the end of their interstellar journey, the presolar grains could have been exposed to enhanced particle radiation from the young Sun. Based on cosmogenic helium and neon concentrations in hibonite, an aluminum−calcium oxide, from the Murchison meteorite, the solar cosmic ray flux these grains might have been exposed to was orders of magnitudes higher than today, consistent with what is expected during the T Tauri phase of the Sun. Hibonites were among the first condensates in the protoplanetary disk and were transported to the disk surface far enough from the sun to evade significant heating, where they were irradiated by an enhanced solar cosmic ray flux. If the presolar silicon carbide grains had a similar exposure history to solar energetic particles in the protoplanetary disk as the hibonites, they would also have acquired a similar concentration of solar cosmic ray produced noble gases. The difference in irradiation time on the disk surface between presolar solar cosmic ray and hibonites is not known. Given that the high-temperature condensate hibonite was present very early in the disk, the short disk lifetime of a few megayears, and the exposure required to explain the cosmogenic hibonite data, we consider that the time difference between the hibonite and silicon carbide exposure duration was probably small. Henk et al.'s results show that the majority of the cosmogenic neon²¹ was acquired during presolar galactic cosmic ray exposure. Specifically, at least 80% of the cosmogenic neon²¹ for grains with neon²¹ ages greater than 100 million years was acquired by presolar galactic cosmic ray exposure. For these grains, the amount that might have been acquired during early Solar System formation is smaller than the uncertainty of the presolar exposure ages and, hence, not detectable. These findings only apply if the presolar grains were exposed to the early active sun at all. At most, the five grains with the lowest ages might have acquired all their cosmogenic neon²¹ in the early Solar System. Early Solar System exposure does not significantly affect our interpretation of presolar ages, except, possibly, for these five grains.

However, our observation has implications for the origin of hibonites that formed in the solar nebula: The cosmogenic nuclide concentrations in the hibonites are typically much lower than that observed in presolar silicon carbide grains, indicating that the irradiated hibonites are indeed early Solar System products and not of presolar origin.

Henk et al. note that a presolar exposure age of a silicon carbide grain is a nominal age and that the actual residence time in the interstellar medium might have been shorter if the grains were exposed to a high energetic particle flux from other nearby stars in addition to background galactic cosmic ray exposure. Henk et al. estimate that the chances of such a close encounter for the average interstellar silicon carbide are low and that such exposure could have also led to destruction of the grain. Modeling of this probability is difficult due to many unknowns and beyond the scope of this work.

With this study, Henk et al. have increased the number of presolar silicon carbide grain neon exposure ages, calculated with improved recoil corrections and cosmogenic nuclide production rates. Based on neon isotopes, they conclude that a majority (about 60%) of the large presolar silicon carbide grains analysed have interstellar cosmic ray exposure ages below 300 million years before the formation of the Solar System. This is compatible with most theoretical estimates of interstellar dust lifetimes of 100 to 200 million years. This age distribution is also consistent with the hypothesis that these grains originate from stars that initially formed during an enhanced stellar formation rate about 7 billion years ago and became dust-producing asymptotic giant branch stars between about 4.9 an 4.6 billion years ago. Furthermore, a significant fraction has presolar ages above 300 million years ago, with at least about 8% above 1 billion years, making them the oldest dated samples so far. These old ages require that these grains evaded destruction in supernova shockwaves, possibly in dense clumps that formed in such shockwaves. Based on a comparison of cosmogenic hellium and neon, it is clear that some grains were part of larger particles or aggregates and might have had large mantles of ices and organics during cosmic ray exposure in the interstellar medium.

The studied presolar grains might have acquired a small but, in most cases, undetectable fraction of their cosmogenic neon during exposure of energetic particles from the early active sun. However, only particularly young grains with very low interstellar residence times might have received a significant fraction of their cosmogenic nuclides in the early Solar System, before accretion onto planetesimals. The specifics of this exposure, such as the solar particle flux and exposure, are currently unknown.

We conclude that neon exposure age dating is currently the only viable method to date presolar grains. While the method provides ages relative to the start of the Solar System and suffers from relatively large uncertainties, it can provide unique information about the interstellar dust cycle and star-forming events in the Galaxy before the birth of the Sun.

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