Desorption of Water from Cosmic Silicates

Abstract Recent JWST and ALMA observations of protoplanetary disks have identified water reservoirs in the inner regions of the disks. This would provide terrestrial planets forming therein access to a water reservoir. In situ formation of water molecules on grain surfaces was proposed as one possible formation process in the PDS 70 system, but transport by small silicate particles coupled to the gas could be another mechanism. Laboratory experiments on the thermally induced desorption of water molecules from different water-containing silicates at elevated temperatures provide another possibility for the enrichment of water molecules in the inner disk. Physically and chemically bound water in silicates can be released upon annealing processes. Water that is physically bound undergoes desorption at temperatures up to 473 K. In contrast, chemically bound water in the form of different silanol or Mg–OH groups in amorphous silicates or phyllosilicates can be released at higher temperatures, ranging from 773 to 923 K. The relevant temperatures are sufficient to trigger the solid-state recrystallization of amorphous silicates to Mg-rich orthopyroxene and olivine. The existence of these crystalline silicates has been substantiated through Spitzer and JWST-MIRI observations. Based on the experimental results, we postulate that the desorption of bound hydroxyl groups may serve as a mechanism for the formation of the water reservoir at temperatures up to 900 K, much higher than the water sublimation temperature of 100–200 K.