Atomic Scale Analysis of Extraterrestrial Fluids By Atom Probe Tomography

Primitive astromaterials are fundamental record keepers of early solar system physical and chemical processes, preserving records usually destroyed through reprocessing in the solar system during solar nebula evolution. Studies of meteorites and interplanetary dust particles indicate some primitive parent bodies experience a significant degree of thermal and aqueous alteration. Direct measurements of aqueous fluid samples in primitive astromaterials are fundamental for constraining the nature, location, and timing of aqueous alteration on primitive parent bodies. However, direct measurements have been challenging due to the surviving trapped fluid inclusions usually being nanoscale and volatile, leaving the bulk of our understanding primarily constrained by theoretical modelling, analysis of icy grains from Enceladus during the Cassini mission and aqueous leaching experiments of carbonaceous chondrites. Consequently, this has left fundamental gaps in our understanding of early solar system aqueous fluids. The first direct analyses of aqueous fluids in primitive astromaterials were in halite crystals in ordinary chondrites (OC) Monahans (1999) and Zag, as these crystals were purple megacryst, distinguishing them from the matrix. Freezing experiments inferred halite bearing fluids comprised divalent cations, e.g., Fe2+, Ca2+ or Mg2+ in addition to Na+ and K+. These results were later strengthened by inductively coupled plasma mass spectrometry measurements which revealed Ca, Mg and Fe ions barely above background, later confirmed through the recent application of time-of-flight secondary ion mass spectrometry (ToF-SIMS). The introduction of x-ray computed nanotomography in correlation with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in the search for trapped fluids has since led to the discovery of fluid inclusions in carbonates, sulphides, olivine, enstatite and calcite within primitive meteorites. Here we present the first atom probe tomography (APT) study of fluid inclusions in extraterrestrial samples. The fluid inclusions are preserved in a pyrrhotite crystal returned to Earth by JAXA from Asteroid 162173 Ryugu, an asteroid essentially identical to Ivuna type (CI) carbonaceous chondritic meteorites. Collected during the Hayabusa 2 sample return mission, these samples are pristine, providing a unique opportunity to probe solar system fluids free from terrestrial influences. APT is a 3D sub-nanometer technique with a detection limit of 10 ppm or 0.001 at.%. Its application to fluid inclusions in astromaterials provides a lower detection limit and higher spatial resolution than achievable with current techniques.