The Hayabusa2 mission, executed by the Japan Aerospace Exploration Agency (JAXA), provided direct access to some of the most pristine astromaterials investigated on Earth thus far. These materials were collected from asteroid Ryugu and returned to Earth on December 6, 2020. Preliminary measurements indicated that Ryugu was genetically linked to Ivuna-type (CI) carbonaceous chondrites, which have a bulk composition believed to represent the starting composition of our solar system. CI chondrites have experienced a very high degree of aqueous alteration on their parent bodies. To evaluate aqueous alteration and thermal processing on Ryugu, Takashi Nakamura from the Tokyo Institute of Technology and colleagues studied the petrographic characteristics and physical properties of several samples collected during the Hayabusa2 sample return mission. The team found that the samples showed varying degrees of aqueous alteration. They then performed chemical equilibrium and thermal modeling of solid-water-gas reactions to evaluate the conditions under which processing occurred on the planetesimal from which Ryugu originated.
The thermal model assumed an initial temperature of 70 K based on an estimated radius of 50 kilometers for the original planetesimal, with the temperature then increasing to ≤ 323 K due to the decay of radioactive 26Al in the interior. During chemical modeling, it was assumed that the aqueous fluids contained CO2 and HCl based on studies of trapped fluids in sulfide minerals within the samples. The team showed that aqueous alteration initiated with an Mg-Na-Cl-rich liquid in an H2O-CO2-rich gas, which evolved to become a Na-Cl alkaline brine in an H2-rich gas. Chemical modeling of aqueous alteration under temperatures determined through thermal modeling (aqueous alteration is initiated at 273 K from melting of trapped ice) reproduced the varying degrees of alteration and petrographic characteristics seen across the measured Ryugu samples. READ MORE