¹Jingyi Mah,¹Ramon Brasser
¹Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114052]
¹Earth Life Science Institute, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8550, Japan
Copyright Elsevier

Combining isotopic constraints from meteorite data with dynamical models of planet formation proves to be advantageous in identifying the best model for terrestrial planet formation. Prior studies have shown that the probability of reproducing the distinct isotopic compositions of the Earth and Mars for both classical and Grand Tack models is very low. In the framework of the Grand Tack model, for Mars to be isotopically different from the Earth, it had to form under very specific conditions. Here, we subjected a fairly new and unexplored model—the depleted disc model—to the test. It presupposes that the region in the inner protoplanetary disc from Mars’ orbit and beyond is depleted in mass such that Mars is left with insufficient material to grow to a larger size. Our aim is to test the whether the distinct isotopic compositions of the Earth and Mars are a natural outcome of this model. We found that the terrestrial planets accrete material mostly locally and have feeding zones that are sufficiently distinct. The Earth and Mars, and by extension, Venus, can have distinct isotopic compositions if there is an isotopic gradient in the terrestrial planet region of the protoplanetary disc. Our results suggest that the material in the inner Solar System most likely did not undergo substantial mixing that homogenised the potential isotopic gradient, in contrast to the Grand Tack model where the feeding zones of the terrestrial planets are nearly identical due to the mixing of material by Jupiter’s migration.

¹Daniel P.Glavin et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13560]
¹NASA Goddard Space Flight Center, Greenbelt, Maryland, 20771 USA
Published by arrangement with John Wiley & Sons

The Asuka (A)‐12236 meteorite has recently been classified as a CM carbonaceous chondrite of petrologic type 3.0/2.9 and is among the most primitive CM meteorites studied to date. Here, we report the concentrations, relative distributions, and enantiomeric ratios of amino acids in water extracts of the A‐12236 meteorite and another primitive CM chondrite Elephant Moraine (EET) 96029 (CM2.7) determined by ultra‐high‐performance liquid chromatography time‐of‐flight mass spectrometry. EET 96029 was highly depleted in amino acids and dominated by glycine, while a wide diversity of two‐ to six‐carbon aliphatic primary amino acids were identified in A‐12236, which had a total amino acid abundance of 360 ± 18 nmol g−1, with most amino acids present without hydrolysis (free). The amino acid concentrations of A‐12236 were double those previously measured in the CM2.7 Paris meteorite, consistent with A‐12236 being a highly primitive and unheated CM chondrite. The high relative abundance of α‐amino acids in A‐12236 is consistent with formation by a Strecker‐cyanohydrin dominated synthesis during a limited early aqueous alteration phase on the CM meteorite parent body. The presence of predominantly free glycine, a near racemic mixture of alanine (d/l ~0.93–0.96), and elevated abundances of several terrestrially rare non‐protein amino acids including α‐aminoisobutyric acid (α‐AIB) and racemic isovaline indicate that these amino acids in A‐12236 are extraterrestrial in origin. Given a lack of evidence for biological amino acid contamination in A‐12236, it is possible that some of the l‐enantiomeric excesses (lee ~34–64%) of the protein amino acids, aspartic and glutamic acids and serine, are indigenous to the meteorite; however, isotopic measurements are needed for confirmation. In contrast to more aqueously altered CMs of petrologic types ≤2.5, no l‐isovaline excesses were detected in A‐12236. This observation strengthens the hypothesis that extensive parent body aqueous activity is required to produce or amplify the large l‐isovaline excesses that cannot be explained solely by exposure to circularly polarized radiation or other chiral symmetry breaking mechanisms prior to incorporation into the asteroid parent body.