The Role of Isotopes in Understanding of the Formation and Evolution of the Solar System

Comparative planetology via isotope geochemistry has played a critical role in advancing our understanding of the inventories of both volatiles and organics in the atmospheres of solar system bodies. For example, the 12C/13C measured from atmospheric methane in Titan provided upper limits on the amount of time that methane has been present in the atmosphere, demonstrating that methane was only present in Titan’s atmosphere during at most the last 1 billion years (Gyr). As another example, measurements of 14N/15N in atmospheric HCN and N2 provide direct evidence of how photochemistry influences stable isotopes. These observations were used to determine that Titan’s nitrogen originated as NH3 in the protosolar nebula, thus constraining the origin and delivery of volatiles to the Saturnian moon Titan. These measurements of 12C/13C and 14N/15N in Titan’s atmosphere were obtained with remote and in situ observations from the Cassini spacecraft in Saturn orbit. However, combining spacecraft observations like these throughout the solar system with Earth-based telescope observations, including both ground-based and from orbiting telescopes, produces far more powerful results. For example, the lower limit obtained for 14N/15N in HCN in Pluto’s atmosphere by ALMA combined with New Horizons observations of the atmospheric composition provides a valuable tool for determining the origin of nitrogen for Pluto if the influences of condensation and aerosol trapping on isotopes can be constrained. Isotopes measured at Jupiter and Saturn and their moons, combined with composition measurements from comets provide information on giant planet formation processes and primordial conditions when the solar system was forming. In all of these studies, measurements of isotope ratios in comets have served as a reference for processes in the protosolar nebula.