Identification of molecular-cloud material in interplanetary dust particles

Interplanetary dust particles (IDPs) collected in the Earth's stratosphere and meteorites are fragments of comets and asteroids. These are 'primitive' meteorites in part because they have preserved materials which predate the formation of the Solar System. The most primitive (least altered) meteorit es contain a few parts per million of micrometre-sized dust which formed in the atmospheres of giant stars(1). Some meteorites(2) have elevated D/H an d N-15/N-14 ratios that are attributed to surviving interstellar organic mo lecules which have probably been strongly diluted and altered by parent-bod y processes(2). Most IDPs are chemically, mineralogically, and texturally p rimitive in comparison to meteorites(3,4). Here I show that H and N isotopi c anomalies among fragile 'cluster' IDPs are far larger, more common, and l ess equilibrated than those previously observed in other IDPs or meteorites . In some cases, the D/H ratios that we measure reach the values of interst ellar molecules, suggesting that molecular-cloud material has survived inta ct. These observations indicate that cluster IDPs are the most primitive cl ass of Solar System materials currently available for laboratory analysis.