Currently, there are a number of hypothesis about the origin and formation pathways of organic compounds in small bodies. The major hypothesis is that organics in meteorite is derived from refractory organics formed by photochemical reactions in interstellar or solar nebula environments. On the other hand, some hypothesis supports Fischer-Tropsch type synthesis and/or irradiation of inner nebular gas, or hydrothermal synthesis on the parent body. Thus, we have not reached consensus yet.

One of the clues to this issue is to study the most primitive solar system materials available to us, that is, interplanetary dust particles (IDPs), Antarctic micrometeorites (AMMs), cometary dusts, and primitive carbonaceous chondrites (e.g., CR3 group). They are very primitive compared to types 1-2 carbonaceous chondritic meteorites, and are expected to retain the precursor compositions of organics in meteorites.

Recently, we carried out coordinated analyses (secondary ion mass spectrometry (SIMS), micro-x-ray absorption near edge structure (XANES) spectroscopy, and transmission electron microscope (TEM)) on the AMMs collected near Dome Fuji Station, Antarctica. According to the comparison of organic chemistry and mineralogy between anhydrous and hydrous AMMs, organics in anhydrous MMs contained nitrile, purine, and/or carboxyl groups, and are enriched in D (dD = 8000-10000 per mil) and ¹⁵N (d¹⁵N= 600-1000 per mil). Since amorphous silicates, so called glass with embedded metal and sulfides (GEMS), coexisted with the organic materials, it is very likely that the molecular and isotopic compositions of the anhydrous AMM organic materials were derived from the cold environments such as interstellar clouds or outer solar nebula, prior to parent body aqueous alteration.

In addition, we have studied an unusual type of AMM, an ultracarbonaceous micrometeorite (UCAMM). The extraterrestrial material contained very large organic matter in size of more than 15 micron, enriched in a variety of nitrogen-functional groups, such as imine, nitrile, heerocyclic nitrogen, and amide groups, while it contained only a few small mineral particles, such as GEMS-like object without accompanying Fe-Ni metal. Since interstellar photochemistry alone would not be able to explain the N/C ratio (= 0.15) of the UCAMM organics, we suggest that the UCAMM experienced a very weak degree of aqueous alteration, which is weaker than that of carbonaceous chondrites. Short-duration weak alteration probably caused by planetesimal shock locally melts cometary ice grains and releases water that dissolves organics, while the fluid unlikely mobilizes because of very low thermal conductivity of the porous icy body. This event allows formation of a large organic puddle of the UCAMM.