Dr. Jonathan Mitchell


"Understanding Titan from the perspective of Earth's climate"

I will summarize our understanding of the climate of Titan based on observations, theory, and modeling. The nature of observed phenomena, such as summer-hemisphere methane clouds, the vertical profile of methane in the lower atmosphere, high-latitude hydrocarbon lakes and low-latitude dunes, can generally be understood in analogy to Earthís tropics. The slow rotation rate and small size of Titan relative to Earth dictate a global overturning tropical circulation, or Hadley cell, which acts to homogenize atmospheric temperatures. Though Titan's thick haze allows less than 1/600th of the solar forcing at Earth to reach the surface, the global Hadley cell focuses this energy in the form of latent heat of methane into one or two convective updrafts, producing the observed isolated clouds. Unlike on Earth where the tropical oceans integrate out most of the seasonal cycle, Titanís solid, low-heat-capacity surface allows the Hadley cell to seasonally oscillate in latitude. The resulting dry climatology (net surface evaporation) of the low latitudes sustains deserts observed by Cassini; the mechanism sustaining these deserts is directly analogous to that of the subtropical deserts on Earth. Surface methane evaporated from high-latitude lakes is roughly conserved during transport to lower latitudes by the Hadley cell where temperatures are higher, thus producing the observed low methane relative humidities. A layer of condensing methane forms over much of the summer hemisphere at the top of the Hadley cell, which explains the low-latitude stratiform cloud observed by Huygens and ground-based telescopes. The sensitivity of the simulated atmospheric circulation to methane inventory indicates almost all of the methane is currently in the atmosphere, as Cassini RADAR measurements have confirmed.

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Received January 25, 2008