The origin of isotope anomalies

The origin of isotope anomalies in meteorites and planets

For more than 30 years, cosmochemists have assumed that variations in the stable isotopic composition (isotope anomalies) of planetary materials reflect primordial heterogeneity inherited from the protosolar molecular cloud - the cloud of dust and gas that collapsed to form our solar system. However, a new study from an international team led by scientists from the Center for Stars and Planets suggests that this long-held view may, in fact, be incorrect. Their results were published in the internationally-recognized journal Science on April 17, 2009.

By precisely analysing the relative abundances of the five stable isotopes of titanium, they have demonstrated a strong correlation between the abundances of the least and most neutron-rich isotopes of titanium – namely 46Ti and 50Ti – for all materials in the inner solar system. Given that these two isotopes formed in very distinct stellar environments prior to their incorporation into the protosolar molecular cloud, their correlated abundances requires that they were thoroughly homogenized early in the evolution of the solar system. They postulate that the variability now observed is due to the variable removal of Ti-bearing phases enriched in 46Ti and 50Ti by thermal processing of a once homogeneous mixture, rather than an inherited heterogeneity.

The earlier view of inefficient mixing of pre-accretionary material has heavily influenced our current models describing the evolution of the protoplanetary disk and planetary accretion. The new view of early homogeneity and progressive thermally-driven unmixing to explain observed isotopic variability requires the construction of new astrophysical models for the formation and evolution of the proto-planetary disk. This challenge will serve as a central theme for the new Centre for Star and Planet Formation at the Natural History Museum of Denmark, which brings together cosmochemists, astrophysicists and astronomers.