Research

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First author papers

• The evolution of Titan's cold south polar cloud
  
  Hanson, L. E., French, R. S., Waugh, D., Barth, E., & Anderson, C. M. (2025). The evolution of Titan's cold south polar cloud. Geophysical Research Letters, 59, e2024GL113415. doi: 10.1029/2024GL113415.

  Abstract:

  Early during Titan's southern fall, images captured by Cassini's Imaging Science Subsystem revealed the formation of a large cloud above Titan's south pole. Subsequent analysis of Cassini data revealed the cloud contained HCN ice, but the cloud's evolution has not been examined. We reviewed imagery of Titan's south pole between 2012 and the end of mission at L_s = 93°. We find evidence of cloud formation as early as L_s = 32° (April 2012), one terrestrial month earlier than previously reported, after which the cloud persists until L_s = 79° (mid 2016). The cloud top altitude descended from about 320 km at L_s = 32° to below 230 km by L_s = 79°, at which point it became obscured by atmospheric methane absorption. The cloud also grew laterally, remaining just beyond the terminator, suggesting the cloud marks a region of cold air in the polar stratosphere. These measurements place new constraints on Titan's polar stratospheric temperature structure and circulation during southern fall.
• Investigation of Titan's South Polar HCN Cloud during Southern Fall Using Microphysical Modeling
  
  Hanson, L. E., Waugh, D., Barth, E., & Anderson, C. M. (2023). Investigation of Titan’s South Polar HCN Cloud during Southern Fall Using Microphysical Modeling. The Planetary Science Journal, 4, 237. doi: 10.3847/PSJ/ad0837

  Abstract:

  Ice clouds in Titan's polar stratosphere are implicated in radiative heating and cooling and in transporting volatile organic compounds from where they form in the upper atmosphere to the surface of the moon. In early southern fall, Cassini detected a large, unexpected cloud at an altitude of 300 km over Titan's south pole. The cloud, which was found to contain HCN ice, was inconsistent with the most recent measurements of temperature in the same location and suggested that the atmosphere had to be 100 K cooler than expected. However, changes to Cassini's orbit shortly after the cloud's appearance precluded further observations, and, consequently, the atmospheric conditions and the details of the formation and evolution of the cloud remain unknown. We address this gap in the observational record by using microphysical cloud modeling to estimate the parameter space consistent with published measurements. Based on the nearest available temperature profile retrievals and other observations, we hypothesize that the cloud forms around 300 km and then descends until it reaches the cold lower stratosphere by late southern fall. The observations can be simulated using a cloud microphysical model by introducing a descending cold layer with temperatures near 100 K. In simulations of this scenario, the precipitation from this cloud rapidly removes over 70% of the HCN vapor from the stratosphere. This result suggests that vapor descending into the polar stratosphere during early fall is mostly removed from the stratosphere before the onset of winter and does not circulate to lower latitudes.