Initial results from radio occultation measurements with the Mars Reconnaissance Orbiter: A nocturnal mixed layer in the tropics and comparisons with polar profiles from the Mars Climate Sounder

•We have begun to retrieve atmospheric profiles from MRO radio occultation data.•Some tropical profiles contain an unexpected low-altitude nocturnal mixed layer.•The mixed layer arises from radiative cooling in an overlying water–ice cloud.•Polar profiles agree closely with coincident retrievals by the Mars Climate Sounder.•Polar profiles confirm the stability of the annual carbon dioxide cycle.