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

The Mars Reconnaissance Orbiter (MRO) performs radio occultation (RO) measurements on selected orbits, generally once per day. We have retrieved atmospheric profiles from two subsets of data, yielding a variety of new results that illustrate the scientific value of the observations. One set of measurements sounded the tropics in northern summer at a local time ∼1 h before sunrise. Some of these profiles contain an unexpected layer of neutral stability with a depth of ∼4 km and a pressure at its upper boundary of ∼160 Pa. The mixed layer is bounded above by a temperature inversion and below by another strong inversion adjacent to the surface. This type of structure is observed near Gale Crater, in the Tharsis region, and at a few other locations, whereas profiles in Amazonis Planitia and Elysium Planitia show no sign of a detached mixed layer with an overlying inversion. We supplemented the measurements with numerical simulations by the NASA Ames Mars General Circulation Model, which demonstrate that water ice clouds can generate this distinctive type of temperature structure through their influence on radiative transfer at infrared wavelengths. In particular, the simulations predict the presence of a nocturnal cloud layer in the Tharsis region at a pressure of ∼150 Pa (∼10 km above the surface), and the nighttime radiative cooling at cloud level is sufficient to produce a temperature inversion above the cloud as well as convective instability below the cloud, consistent with the observations. The second set of measurements sounded mid-to-high northern latitudes in spring, when carefully coordinated observations by the MRO Mars Climate Sounder (MCS) are also available. The differences between the RO and MCS temperature profiles are generally consistent with the expected performance of the two instruments. Within this set of 21 comparisons the average temperature difference is less than 1 K where the aerosol opacities are smaller than 10 - 3 km - 1 , at pressures of 10–50 Pa, whereas it increases to ∼2 K where the aerosol opacities exceed this threshold, at pressures of 50–300 Pa. The standard deviation of the temperature difference is ∼2 K, independent of pressure. The second set of RO measurements also provides unique information about the stability of the annual CO 2 cycle and the dynamics near the edge of the seasonal CO 2 ice cap.