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39A comprehensive study of organic chemistry and mineralogy of an ultracarbonaceous 40 micrometeorite (UCAMM D05IB80), collected from near the Dome Fuji Station, Antarctica, 41 has been carried out in order to understand the genetic relationship among organic materials, 42 silicates, and water. The micrometeorite is composed of a dense aggregate of ~5-µm-sized 43 hollow ellipsoidal organic material containing submicrometer-sized phases such as GEMS 44 and mineral grains. There is a wide area of organic material (~15 x 15 μm) in its interior. 45Low-Ca pyroxene is much more abundant than olivine and shows various Mg/(Mg+Fe) ratios 46 from ~1.0 to 0.78, which is common to previous works of UCAMM. By contrast, GEMS 47 amide. The ratio of nitrogen to carbon (N/C) in the smooth region of the organics is ~0.15, 53 which is five times higher than insoluble organic macromolecules in types 1 and 2 chondritic 54 meteorites. In addition, the UCAMM organics is soluble in epoxy, and thus it has 55 hydrophilicity. These polar natures indicate that the organic material in the UCAMM is very 56 primitive. The surface of the organics is coated with an inorganic layer with a few nanometers 57 thickness, which consists of C, O, Si, S, and Fe. Sulfur is also contained in the interior, 58implying the presence of organosulfur moieties. There is no isotopic anomaly of D, 13 C and 59 15 N in the organic material. 60Since interstellar photochemistry alone would not be able to explain the N/C ratio of the 61 UCAMM organics, we suggest that very small amount of fluid on a comet must have been 62 necessary for the formation of UCAMM. The GEMS grains depleted in Mg and S in the 63 UCAMM prove a very weak degree of aqueous alteration, which is weaker than that of 64 carbonaceous chondrites. Short-duration weak alteration probably caused by planetesimal 65 shock locally melts cometary ice grains and releases water that dissolves organics, while the 66 fluid unlikely mobilizes because of very low thermal conductivity of the porous icy body. 67 This event allows formation of a large organic puddle of the UCAMM, as well as organic 68 matter sulfurization, formation of mineral membrane-like thin layers, and deformation of 69 5 organic nanoglobules. 70 (408 words (max. 500 words)) 71 72 9 124 2. Experimental 125 The Antarctic snow, collected by the 51st JARE team of the National Institute of Polar 126 Research (NIPR), was melted and filtered in a class 1000 clean room at Ibaraki University, 127 and the residual particles were manually picked up under a binocular microscope. Details of 128 the micrometeorite collecting method are described by Sakamoto et al. (2010). They were 129 observed with JEOL JSM-5600LV scanning electron microscope (SEM) equipped with 130 energy dispersive spectrometer (EDS) at Ibaraki University and micrometeorites were 131selected from terrestrial materials based on the morphology and EDS spectra with chondritic 132 composition rich in Si, Mg, Fe, and O (see electronic supplementary data, S1). About 90 133 micrometeorites ...

Ten micrometeorites weighing 0.14-18.5 μg, each retrieved from surface snow near the Dome Fuji Station, Antarctica (snow-AMMs), were studied to elucidate their noble gases, mineralogy, morphology, and chemical compositions. Low densities in the range of 0.2-1.4 g/cm 3 estimated for seven samples suggested a porous inner structure. Noble gases were extracted from each particle using stepwise heating with a laboratory manufactured miniature furnace. Isotopic ratios of He and Ne indicate that the light noble gases with high 4 He concentrations ranging from 10 −2 to 10 −4 cm 3 STP/g are mostly of solar origin. The higher concentrations of 4 He observed for several samples are comparable with those of IDPs enriched in solar He, but exceed those reported for ice-AMMs. In contrast to He and Ne, heavy noble gases Ar, Kr, and Xe are primordial ones resembling Q-gas trapped in chondrites, although a small contribution of solar Ar is indicated for some samples with higher 36 Ar/ 132 Xe ratios than that for the Q-gas. Three particles released appreciable amounts of He at temperatures lower than 800• C, suggesting heating temperatures lower than 700• C at the time of atmospheric entry. Other particles released at most 10% of total He at the temperatures up to 800• C. Based on their sizes, weights, and release profiles of 4 He, initial speeds of less than 14 km/s at atmospheric entry were indicated for the particles. The slow entry speeds imply that all the snow-AMMs studied in this work were likely derived from asteroids. The present work demonstrates that the miniature furnace can be applicable to noble gas analysis of tiny grains from the Itokawa asteroidal regolith materials returned by the Hayabusa mission.

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