¹Takaaki Noguchi et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13919]
¹Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto, 606-8502 Japan
Published by arrangement with John Wiley & Sons

Asteroids and comets are thought to form in the inner and outer solar systems, respectively. Chondritic porous and smooth interplanetary dust particles (CP IDPs and CS IDPs, respectively) in the stratosphere are regarded as dust grains from comets and hydrated asteroids, respectively. Here, we describe an Antarctic micrometeorite (AMM) composed of lithologies of both CP and CS IDPs. In addition to the CS IDP-like compact lithology that experienced severe aqueous alteration, the CP IDP-like porous lithology shows evidence of very weak aqueous alteration. The structure of the organic matter in the porous lithology varies from that in the CP IDPs to aromatic-rich organic matter. In contrast, the structure of the organic matter in the compact lithology is homogenous, which is consistent with higher degrees of aqueous alteration. Its structure is more similar to that of CP IDPs and Wild 2 samples than that of meteoritic insoluble organic matter, suggesting that the compact lithology formed from the porous lithology. Some CP IDPs are related to cometary dust streams, such as those originating from 26P/Grigg-Skjellerup. In addition, the presence of this AMM indicates an additional origin of the CP IDPs and their equivalent AMMs. The mineralogy and organic chemistry of this AMM suggest that its parent body was composed of the same building blocks as those of the comets, and later experienced incomplete aqueous alteration. The AMM probably formed as microbreccia in the regolith layer composed of materials from a CP IDP-like crust and a hydrated interior.

¹Jüri Plado,^2,3Ania Losiak,¹Argo Jõeleht,⁴Jens Ormö,⁵Helena Alexanderson,⁵Carl Alwmark,⁶Eva Maria Wild,⁶Peter Steier,²Marek Awdankiewicz,³Claire Belcher
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13914]
¹Department of Geology, University of Tartu, Ravila 14A, EE 50411 Tartu, Estonia
²Institute of Geological Sciences, Polish Academy of Sciences, Podwale 75, PL 50-449 Wrocław, Poland
³WildFIRE Lab, University of Exeter, Exeter, EX4 4PS UK
⁴Centro de Astrobiología CSIC-INTA, Instituto Nacional de Técnica Aeroespacial, 28850 Torrejon de Ardoz, Spain
⁵Department of Geology, Lund University, SE-22362 Lund, Sweden
⁶VERA Laboratory, Faculty of Physics, Isotope Physics, University of Vienna, Währinger Straße 17, A-1090 Vienna, Austria
Published by arrangement with Johne Wiley & Sons

Compared to intensive research on km-sized meteorite impact craters, fewer studies focus on smaller craters. The small craters are often hard or impossible to recognize using “classical” criteria like the presence of shatter cones, shocked quartz, and geochemical indicators. Therefore, a long list of candidate structures awaiting approval/disapproval of their origin has been formed over the last decades. One of them is the Tor structure in central Sweden. To test a hypothesis of an impact origin of this structure, we have performed topographical analysis, geophysical studies, 10Be exposure dating of boulders, and 14C dating of Tor-associated charcoal. None of the methods gave us a reason to claim the Tor structure is of impact origin. Thus, we support a recently suggested idea of Tor being formed by a grounded iceberg within a glacial lake.