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 ...