The Primordial Porridge

摘要

The solar system formed from the collapse of a cloud of gas and dust called the solar nebula, through a series of energetic events that have left few traces. Studies of planetary bodies including meteorites have demonstrated that matter was efficiently mixed at an atomic scale before solids were formed. The isotopic compositions of some of the light elements such as hydrogen and nitrogen, however, vary dramatically across the solar system. Mass balance considerations indicate that these isotope heterogeneities are not inherited from previous nucleosynthesic processes in stars. Instead, these isotope variations are likely to have formed in the solar nebula or in interstellar space. Until now, the largest hydrogen and nitrogen isotopic variations havebeen found in interplanetary dust particles (IDPs), which are micrometer-sized particles flowing in interplanetary space that have been collected in the high atmosphere by NASA stratospheric planes. IDPs have deuterium (D) and nitrogen isotope anomaliesfound in organic matter, which suggests that IDPs are among the most primitive type of matter in the solar system and could be, in some cases, cometary in origin (1). Carbonaceous chondrites, which are volatile-rich meteorites originating from planetarybodies that never melted and differentiated, also exhibit enrichments in deuterium and ~(15)N associated with organic matter, but to a much lesser extent (2). These differences are in line with the more "primitive" character of IDPs compared with carbonaceous chondrites. Now, observations by Busemann et al. reported on page 727 of this issue (3) shed a different light on this view.