Hydrothermal fluid activity on asteroid Itokawa

The presence of NaCl crystals within the sample gathered from S-type asteroid Itokawa and genetically linked to the surrounding plagioclase indicates the presence of an active hydrothermal system within Itokawa, contrary to the assumption that parent asteroids of ordinary chondrites were relatively water-depleted. Carbonaceous chondrites contain widespread mineralogical evidence for water-rock interactions, indicating that the C-type asteroids from which they are derived had active hydrothermal systems. In comparison, ordinary chondrites contain secondary minerals that are predominantly anhydrous, suggesting that their parent S-type asteroids were relatively dry. The returned particles from the Hayabusa Mission allow us to probe directly the alteration history of S-type asteroid Itokawa. Here we report nanometre-sized NaCl crystals identified in the interior of an Itokawa particle. These crystals are intimately associated with secondary albitic plagioclase, indicating coupled formation. The NaCl most likely formed through precipitation from an aqueous fluid prior to complete metamorphic dehydration on asteroid Itokawa. Our results therefore imply that asteroid Itokawa supported an active hydrothermal system and suggest that the once-hydrated S-type asteroids could have potentially delivered water to terrestrial planets.