Hydrogen Investigation of Asteroid Regolith Analogs Murchison and Aguas Zarcas in Anticipation of Asteroid Sample Returns

Abstract:

While meteorites have long represented humanity’s best resource for probing the chemical evolution of our solar system, regolith samples returned from asteroids will soon offer a much more accurate assessment of the effects of Earth’s atmosphere on extraterrestrial samples. Missions such as NASA’s OSIRIS-REx and JAXA’s Hayabusa 2 are scheduled to return samples from the C-type asteroids Bennu and Ryugu, respectively, in the mid-2020s. These samples will be free from alteration due to transit through Earth’s atmosphere and equilibration with the terrestrial environment. Sample return missions are therefore critical because even short term exposure of meteorites (~ 10¹ years) to Earth’s atmosphere can significantly alter their hydrogen inventories. Accurately determining the hydrogen inventories of these asteroids, probable analogs for planetary building materials, could potentially provide new information about the origin of Earth’s water and the likelihood of Earth-like planets around other stars. Using meteorite analogs to develop the protocols for studying such samples now will allow rapid assessments of Bennu and Ryugu, and will aid mission planning for the next generation of sample return missions.

The most suitable analogs for the regolith of Bennu and Ryugu are fresh meteorite falls of aqueously altered carbonaceous chondrites (e.g., CI and CM). We chose to investigate CM chondrites Aguas Zarcas and Murchison due to similarities in their proposed parent bodies, but vastly different terrestrial residence times (~50 years). These meteorite samples consist of organic-rich matrix enclosing silicate chondrules, fractured to euhedral grains isolated in the matrix, and sparse CAIs. A significant fraction of the chondrules in both samples have been partially or completely replaced by hydrated phases. We prepared fresh chips of each meteorite using dry polishing protocols, and mounted the meteorite chips in indium. We used nano scale secondary ion mass spectrometry (NanoSIMS) to measure the water concentration and hydrogen isotopic composition of olivine, orthopyroxene, and clinopyroxene in Aguas Zarcas and Murchison both in chondrules and in the matrix. Significant differences in the hydrogen isotopic compositions were found between grains within chondrules and those isolated in Murchison matrix.