Methane abundance on Titan, measured by the Space Telescope Imaging Spectrograph

Mark T. Lemmon, Peter H. Smith, Ralph D. Lorenz

Research output: Contribution to journalArticlepeer-review

28 Scopus citations


Although methane is the dominant absorber in Titan's reflection spectrum, the amount of methane in the atmosphere has only been determined to an order of magnitude. We analyzed spectra from the Space Telescope Imaging Spectrograph, looking at both a bright surface region (700-km radius) and a dark surface region. The difference between the spectra of the two regions is attributed to light that has scattered off the surface, and therefore made a round-trip through all of Titan's methane. Considering only absorption, the shape of the difference spectrum provides an upper limit on methane abundance of 3.5 km-am. Modeling the multiple scattering in the atmosphere further constrains the methane abundance to 2.63 ± 0.17 km-am. In the absence of supersaturation and with a simplified methane vertical profile, this corresponds to a surface methane-mole fraction near 3.8% and a relative humidity of 0.32. With supersaturation near the tropopause, the surface methane mole fraction could be as low as 3%.

Original languageEnglish (US)
Pages (from-to)375-385
Number of pages11
Issue number2
StatePublished - Dec 1 2002


  • Abundance, atmospheres
  • Atmospheres, composition
  • Satellites, atmospheres
  • Spectrophotometry
  • Titan

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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