The nature of the organic matter in interplanetary samples is central to elucidating the formation and early evolution of the Solar System. Although most meteorites derive from asteroids, micrometeorites mainly sample more remote objects. Ultra-carbonaceous Antarctic micrometeorites (UCAMMs), which have the highest carbon content among interplanetary samples, offer a unique window into cometary organics. Here we report a survey of the H, C and N isotopes in four UCAMMs, of which two are ¹⁵N-poor (δ¹⁵N ≃ −120‰), which suggests that their formation involved primordial N₂ (δ¹⁵N ≃ −380‰). Such a composition could be the result of Galactic cosmic ray irradiation of N₂ ices at the surface of cold small bodies in the outermost parts of the Solar System, possibly the Oort cloud. The two other UCAMMs exhibit higher δ¹⁵N (75‰ and 282‰), like those reported for carbonaceous chondrites and interplanetary dust particles. They may originate from parent bodies initially on lower heliocentric orbits in the Kuiper belt that have surfaces cold enough to retain N-bearing species, such as cyanides (δ¹⁵N ≥ 200‰), that are richer in ¹⁵N than primordial N₂. According to their elemental and isotopic composition, UCAMMs constitute a unique probe into the coldest objects of the Solar System, namely those in the Kuiper Belt and the Oort cloud, which are largely out of reach of current space exploration.

行星际样品中有机物的性质对于阐明太阳系的形成和早期演化至关重要。虽然大多数陨石来自小行星，但微陨石主要采样更遥远的物体。超碳质南极微陨石（UCAMM）是行星际样本中碳含量最高的，为了解彗星有机物提供了一个独特的窗口。在这里，我们报告了对四个 UCAMM 中 H、C 和 N 同位素的调查，其中两个是贫¹⁵ N (δ ¹⁵ N ≃ -120‰)，这表明它们的形成涉及原始 N ₂ (δ ¹⁵ N ≃ - 380‰）。这种成分可能是银河系宇宙射线照射太阳系最外层寒冷小天体（可能是奥尔特云）表面的 N ₂冰的结果。另外两个 UCAMM 表现出更高的 δ ¹⁵ N（75‰ 和 282‰），与碳质球粒陨石和行星际尘埃颗粒的报道类似。它们可能起源于最初位于柯伊伯带较低日心轨道上的母体，其表面足够冷以保留含氮物质，例如氰化物（δ ¹⁵ N ≥ 200‰），其¹⁵ N 含量比原始 N ₂更丰富。根据其元素和同位素组成，UCAMM 构成了对太阳系最冷天体（即柯伊伯带和奥尔特云中的天体）的独特探测器，而这些天体在当前的太空探索中很大程度上是遥不可及的。