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تسجيل مستخدم جديدPrimordial porous structure of chondrite parent bodies due to self-gravity
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The porosity of an asteroid is important when studying the evolution of our solar system through small bodies and for planning mitigation strategies to avoid disasters due to asteroid impacts. Our knowledge of asteroid porosity largely relies on meteorites sampled on Earth. However, chondrites sampled on Earth are suggested to be sorted by strength. In this study, we obtained an estimate of the most porous structure of primordial granular chondrite parent bodies based on measurements of the compaction behavior of chondrite component analogs. We measured compaction curves of dust and dust-bead mixture samples. The dust sample consisted of various spherical and irregular particles with diameters on the order of 10^0-10^1 $mu$m. The mixture sample consisted of dust and beads with different dust volume fractions (~0.2-1). We used 1.5 and 4.8 $mu$m particles as dust as a first step, although the typical size of materials in matrix may be much smaller. We approximated the compaction curve of each sample with a power-law form and calculated the porosity structure of the primordial chondrite parent bodies using the experimental results. Our results show that the primordial parent bodies are likely to have higher porosity than the chondrites. Moreover, the relatively higher volume fraction of the matrix may be one of the reasons why most meteorites with high porosity are carbonaceous chondrites.
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Ordinary chondrites (OCs) are by far the most abundant meteorites (80% of all falls). Their origin has long been the matter of a heated debate. About thirty years ago (e.g., Pellas 1988), it was proposed that OCs should originate from S-type bodies (
the most abundant asteroid spectral types in the inner part of the asteroid belt), but the apparent discrepancy between S-type asteroid and OC reflectance spectra generated what was known as the S-type--OC conundrum. This paradox has gradually been resolved over the years. It is now understood that space weathering processes are responsible for the spectral mismatch between S-type bodies and OCs. Furthermore, both telescopic observations and the first asteroid sample return mission (Hayabusa) indicate that most S-type bodies have mineralogies similar to those of OCs. Importantly, the S-type/OC link, which has remained sterile for more than 30 years, has been delivering fundamental constraints on the formation and evolution of planetesimals over the recent years.
Although petrologic, chemical and isotopic studies of ordinary chondrites and meteorites in general have largely helped establish a chronology of the earliest events of planetesimal formation and their evolution, there are several questions that cann
ot be resolved via laboratory measurements and/or experiments only. Here we propose rationale for several new constraints on the formation and evolution of ordinary chondrite parent bodies (and by extension most planetesimals) from newly available spectral measurements and mineralogical analysis of main belt S-type asteroids (83 objects) and unequilibrated ordinary chondrite meteorites (53 samples). Based on the latter, we suggest spectral data may be used to distinguish whether an ordinary chondrite was formed near the surface or in the interior of its parent body. If these constraints are correct, the suggested implications include that: i) large groups of compositionally similar asteroids are a natural outcome of planetesimal formation and, consequently, meteorites within a given class can originate from multiple parent bodies; ii) the surfaces of large (up to ~200km) S-type main-belt asteroids expose mostly the interiors of the primordial bodies, a likely consequence of impacts by small asteroids (D<10km) in the early solar system (Ciesla et al. 2013); iii) the duration of accretion of the H chondrite parent bodies was likely short (instantaneous or in less then ~10^5 yr but certainly not as long as 1 Myr); iv) LL-like bodies formed closer to the Sun than H-like bodies, a possible consequence of radial mixing and size sorting of chondrules in the protoplanetary disk prior to accretion.
Major components of chondrites are chondrules and matrix. Measurements of the volatile abundance in Semarkona chondrules suggest that chondrules formed in a dense clump that had a higher solid density than the gas density in the solar nebula. We inve
stigate collisions between chondrules and matrix in the surface region of dense clumps using fluffy aggregate growth models. Our simulations show that the collisional growth of aggregates composed of chondrules and matrix takes place in the clumps well before they experience gravitational collapse. The internal structure of chondrite parent bodies (CPBs) can be thereby determined by aggregate growth. We find that the aggregate growth generates two scales within CPBs. The first scale is involved with the small scale distribution of chondrules and determined by the early growth stage, where chondrules accrete aggregates composed of matrix grains. This accretion can reproduce the thickness of the matrix layer around chondrules found in chondrites. The other scale is related to the large scale distribution of chondrules. Its properties (e.g., the abundance of chondrules and the overall size) depend on the gas motion within the clump, which is parameterized in this work. Our work thus suggests that the internal structure of CPBs may provide important clues about their formation conditions and mechanisms.
The fall of the Annama meteorite occurred early morning (local time) on April 19, 2014 on the Kola Peninsula (Russia). Based on mineralogy and physical properties, Annama is a typical H chondrite. It has a high Ar-Ar age of 4.4 Ga. Its cosmic ray exp
osure history is atypical as it is not part of the large group of H chondrites with a prominent 7 - 8 Ma peak in the exposure age histograms. Instead, its exposure age is within uncertainty of a smaller peak at 30 pm 4 Ma. The results from short-lived radionuclides are compatible with an atmosperic pre-entry radius of 30 - 40 cm. However, based on noble gas and cosmogenic radionuclide data, Annama must have been part of a larger body (radius >65 cm) for a large part of its cosmic ray exposure history. The 10Be concentration indicates a recent (3 - 5 Ma) breakup which may be responsible for the Annama parent body size reduction to 30 - 35 cm pre-entry radius.
Northwest Africa (NWA) 11042 is a heavily shocked achondrite with medium-grained cumulate textures. Its olivine and pyroxene compositions, oxygen isotopic composition, and chromium isotopic composition are consistent with L chondrites. Sm-Nd dating o
f its primary phases shows a crystallization age of 4100 +/- 160 Ma. Ar-Ar dating of its shocked mineral maskelynite reveals an age of 484.0 +/- 1.5 Ma. This age coincides roughly with the breakup event of the L chondrite parent body evident in the shock ages of many L chondrites and the terrestrial record of fossil L chondritic chromite. NWA 11042 shows large depletions in siderophile elements (<0.01 times CI) suggestive of a complex igneous history involving extraction of a Fe-Ni-S liquid on the L chondrite parent body. Due to its relatively young crystallization age, the heat source for such an igneous process is most likely impact. Because its mineralogy, petrology, and O isotopes are similar to the ungrouped achondrite NWA 4284 (this work), the two meteorites are likely paired and derived from the same parent body.
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