The Lunar & Planetary Laboratory at the University of Arizona has an opening for a Postdoctoral Research Associate I in the field of modeling of planetary atmospheres. Research areas include hydrodynamic atmospheric escape from rocky exoplanets, nitrogen and sulfur cycling on early Mars and Earth, chemical kinetics of early Earth, Venus, Mars, and analogous exoplanets, and interior-atmosphere interactions on rocky worlds. Candidates are encouraged to reach out to Prof. Ranjan in advance of their application to cooperatively discuss specific research topics of mutual interest. Applicants should have previous experience modeling the atmospheres of planets or exoplanets, although a variety of backgrounds may be relevant. Candidates with expertise in 1D hydrodynamic escape of rocky planet atmospheres and/or in modeling early terrestrial or Martian geochemistry are especially encouraged to apply. The position is a year-to-year appointment for a period of up to three years, renewable annually contingent on funding, department needs, and satisfactory performance. Position is opened until filled. First review of applications will begin 12/15/2025.