Highlights

Energetic Ion Interactions with Solar System Materials: Application of In Situ Ion Irradiation Experiments

Scientific Achievement

Solid-state processing of solar system materials by the energetic ions in space plasmas is a fundamental physical and chemical process in the evolution of the solar system.  Solid mineral grains sampled directly and indirectly from comets, the most undisturbed reservoirs of materials from the solar nebula, show evidence that processes such as ion-induced solid-state amorphization, preferential sputtering and ion mixing were all occurring in the solid grains exposed to the energetic ions coming from the early Sun.  In addition, some of these grains that appear to be interstellar in origin may have been radiation-processed in an interstellar environment that pre-dates the solar system.  Even after formation of the solar system and its evolution to the present day, energetic ions from the Sun continue to cause radiation processing to occur on the surfaces of all planets and moons that lack an atmosphere, with far reaching consequences for the chemical and physical properties of the surfaces of these bodies.

Using 1 MeV Kr ions in IVEM-Tandem facility in the EMC, we are systematically calibrating how key solar system minerals respond to ion radiation processing, with current emphasis on determining relative ion-dose thresholds for radiation-induced solid-state amorphization.  The long-suspected “radiation resistance” of Fe_1-xS pyrrhotite-structure sulfides, a key low-temperature phase formed in the early solar system, has been quantiatively confirmed for the first time, with this phase showing only long-range vacancy disordering, but no amorphization, at up to 1.4 x 10⁵ eV/nm³ of deposited nuclear collision energy (E_dep).  By comparison, important nebular condensate silicate phases, such as enstatite (MgSiO₃) and forsterite (Mg₂SiO₄) show abrupt, first-order type, transitions to complete amorphization at two orders of magnitude lower Kr ion dose, corresponding to E_dep = 3500-6000 eV.  The amorphous state in these silicates is being studied with additional techniques, such as FTIR, to obtain data relevant to astronomical observations of early solar systems.

Significance

Our results place constraints on the expected structural states of key silicate and non-silicate phases exposed to the space plasma environments in the early solar system and interstellar space. Details of the radiation-induced vacancy disordering in the sulfides are particularly interesting for their implications of the type of sulfide structures astronomers would expect to find in observations of circumstellar disks.  The silicate E_dep values have additional application in TRIM-based Monte Carlo calculations of the formation of amorphous, radiation damaged rims on silicate grains in space-exposed grains on the surface of the Moon and other airless bodies. Future work will extend our successful use of the IVEM-Tandem facility to better understand the additional effects of ion-mixing and recoil-implantation in driving nanoscale chemical changes in nebular and pre-solar grains as well as grains from the lunar soil.

Performers