Deriving the N₂–CO Binary Phase Diagram Using Experimental Techniques and Thermodynamics

Abstract In the distant outer solar system, carbon monoxide (CO) and nitrogen (N 2 ) ices tend to be colocated in the same deposits due to their similar molecular weights and sublimation properties. For instance, these volatiles are abundant on the surfaces of Pluto and Triton, so knowledge of their phase behavior is necessary for understanding surface evolution and geology. However, it is presently unclear how mixing between CO and N 2 molecules affects the physical properties of such mixtures. Here, we measured the liquidus, solidus, and alpha–beta phase transitions for the N 2 and CO binary system. We observed the liquidus by using visual inspection. The solidus and alpha–beta transitions were measured by using Raman spectroscopy. The laboratory results were later compared to a thermodynamic model, CRYOCHEM 2.0. The liquidus and solidus were consistent with CRYOCHEM 2.0. However, the alpha–beta coexistence region is shown to be narrower in the laboratory results than in the thermodynamic model. Finally, we present a method for deriving the compositions of a sample using Raman spectroscopy (Appendices A.1 and A.2).