Methane Distribution in Titan’s Atmosphere Spica + Shaula Occultations. Candidate Observations Symmetrical Methane Distribution Flatfield Issues Asymmetrical Distribution Why a difference? Changes over extent of mission?

T-B: Lamba-Scorpio Occultation 1/e depth at 988 ± 20km With Shaula Lyman-alpha between km

T-B: Alpha-Virgo Occultation 1/e depth = 986 km ± 10 With km background 1/e depth = 1004 km ± 15 With km background

Candidate Titan Observation Criteria for Lyman-Alpha Occultations Sufficient off-target pixels to derive local unattenuated ISM Lyman-alpha counts. Coverage of all latitude ranges during a single observation. Sufficient pixels within each latitude bin to improve statistics. Variety of illumination and observational geometries.

Titan UVIS Observation List T0:TEMPMAP203_CIRS TEMPMAP001_CIRS FIRNADCMP001_CIRS TA:EUVFUV001_VIMS EUVFUV002_PRIME TB:EUVFUV002_PRIME EUVFUV001_PRIME TWOSTOCCS001_PRIME T3: EUVFUV001_PRIME MIDIRTMAP002_CIRS MONITORNA001_ISS MIRLIMBINT002_CIRS COMBINED001_ISS EUVFUV001_PRIME NIGHTWAC003_ISS FIRNADCMP002_CIRS MIDIRTMAP003_CIRS T4:FIRNADCMP002_CIRS EUVFUV001_PRIME FIRNADCMP003_CIRS T5:FIRNADCMP002_CIRS EUVFUV001_PRIME HIGHRESNA001_ISS T6: FIRNADCMP002_CIRS MIRLMBMAP002_CIRS FIRNADCMP002_CIRS FIRNADCMP004_CIRS MIDIRTMAP007_CIRR T8:GLOBMAP001_ISS MEDRES001_VIMS HIGHRESNA001_ISS MIRLMBMAP001_CIRS FIRNADCMP003_CIRS MIDIRTMAP005_CIRS T9:FIRNADCMP003_CIRS MEDRES001_VIMS GLOBMAP001_ISS EUVFUV001_PRIME T10:NIGHTNAC001_ISS EUVFUV001_PRIME GLBMAPNLP001_ISS MIRLMBINT002_CIRS FIRNADCMP002_CIRS MIDIRTMAP010_CIRS T11:MEDRES001_VIMS MONITORNA001_ISS MONITORNA002_ISS EUVFUV002_PRIME FIRNADCMP002_CIRS T12:FIRNADCMP003_CIRS GLOBMAPNA001_ISS REGMAP004_VIMS AURORA002_VIMS FIRNADCMP008_CIRS T13:EUVFUV002_PRIME T14:EUVFUV001_PRIME EUVFUV001_ENGR EUVFUV002_PRIME T15:EUVFUV002_PRIME T17:FIRNADCMP003_CIRS EUVFUV001_PRIME FIRLMBINT002_CIRS FIRLMBAER002_CIRS HIGHRESNA001_ISS REGMAPNA001_ISS GLOBMAPNA001_ISS MONITORNA001_ISS T18:EUVFUV001_PRIME T19:MIRLMBINT002_CIRS MIRLMBINT003_CIRS COMPMAP007_CIRS T20:CLOUDMAP201_VIRS COMPMAP001_VIMS T21:EUVFUV001_PRIME COMPMAP023_CIRS T22:FIRNADCMP003_CIRS FIRNADMAP002_CIRS REGMAPNA001_ISS FIRNADMAP003_ISS FIRNADCMP002_CIRS

Titan UVIS Observation List: Methane Abundance Mapping T0:TEMPMAP203_CIRS TEMPMAP001_CIRS FIRNADCMP001_CIRS TA:EUVFUV001_VIMS EUVFUV002_PRIME TB:EUVFUV002_PRIME EUVFUV001_PRIME TWOSTOCCS001_PRIME T3: EUVFUV001_PRIME MIDIRTMAP002_CIRS MONITORNA001_ISS MIRLIMBINT002_CIRS COMBINED001_ISS EUVFUV001_PRIME NIGHTWAC003_ISS FIRNADCMP002_CIRS MIDIRTMAP003_CIRS T4:FIRNADCMP002_CIRS EUVFUV001_PRIME FIRNADCMP003_CIRS T5:FIRNADCMP002_CIRS EUVFUV001_PRIME HIGHRESNA001_ISS T6: FIRNADCMP002_CIRS MIRLMBMAP002_CIRS FIRNADCMP002_CIRS FIRNADCMP004_CIRS MIDIRTMAP007_CIRS T8:GLOBMAP001_ISS MEDRES001_VIMS HIGHRESNA001_ISS MIRLMBMAP001_CIRS FIRNADCMP003_CIRS MIDIRTMAP005_CIRS T9:FIRNADCMP003_CIRS MEDRES001_VIMS GLOBMAP001_ISS EUVFUV001_PRIME T10:NIGHTNAC001_ISS EUVFUV001_PRIME GLBMAPNLP001_ISS MIRLMBINT002_CIRS FIRNADCMP002_CIRS MIDIRTMAP010_CIRS T11:MEDRES001_VIMS MONITORNA001_ISS MONITORNA002_ISS EUVFUV002_PRIME FIRNADCMP002_CIRS T12:FIRNADCMP003_CIRS GLOBMAPNA001_ISS REGMAP004_VIMS AURORA002_VIMS FIRNADCMP008_CIRS T13:EUVFUV002_PRIME T14:EUVFUV001_PRIME EUVFUV001_ENGR EUVFUV002_PRIME T15:EUVFUV002_PRIME T17:FIRNADCMP003_CIRS EUVFUV001_PRIME FIRLMBINT002_CIRS FIRLMBAER002_CIRS HIGHRESNA001_ISS REGMAPNA001_ISS GLOBMAPNA001_ISS MONITORNA001_ISS T18:EUVFUV001_PRIME T19:MIRLMBINT002_CIRS MIRLMBINT003_CIRS COMPMAP007_CIRS T20:CLOUDMAP201_VIRS COMPMAP001_VIMS T21:EUVFUV001_PRIME COMPMAP023_CIRS T22:FIRNADCMP003_CIRS FIRNADMAP002_CIRS REGMAPNA001_ISS FIRNADMAP003_ISS FIRNADCMP002_CIRS

End of Observation Beginning of Observation T-8: MIDIRTMAP005-CIRS

Background Lyman-alpha pixels are restricted to a ±1.5° region of space. Hence, the variability in Lyman-alpha during a single observation should be negligible. However, background Lyman-alpha count totals are highly variable. Mean of background counts = 54±7

T-8: MIDIRTMAP005-CIRS Incomplete Flatfielding affects the background Lyman-alpha levels. However, smoothing across many pixels effectively compensates. More tests remain to be done with new flatfield techniques and correction factors

T-8: MIDIRTMAP005-CIRS

Approximate error bars (+- 30 km)

T-9: EUVFUV001_PRIME

T-18: EUVFUV001_PRIME Beginning of Observation End of Observation

T-18: EUVFUV001_PRIME

T13: EUVFUV002_PRIME Typical ‘Asymmetrical’ Observation Observation geometry nearly identical to T-9: EUVFUV001_PRIME observation which showed symmetrical methane distribution.

T13: EUVFUV002_PRIME

‘Asymmetrical’ Observations Equatorial bins are generally symmetrical Additional features produce asymmetrical patterns Lyman-alpha ‘hump’ near exobase altitude unexplained ‘spikes’ seen at highest altitudes Overcorrection of RTG background

No Asymmetry N/S Methane ‘Asymmetry’ T8: MIDIRTMAP005_CIRS T9: EUVFUV001_PRIME T18:EUVFUV001_PRIME T3: EUVFUV001_PRIME MONITORNA001_ISS MIDIRTMAP002_CIRS MIDIRTMAP003_CIRS T6:MIDIRTMAP007_CIRS T10: EUVFUV001_PRIME MIDIRTMAP010_CIRS T13:EUVFUV002_PRIME T14:EUVFUV001_PRIME T17:EUVFUV001_PRIME MONITORNA001_ISS

No evidence for statistically significant latitudinal asymmetry in the distribution of methane in Titan’s atmosphere. Based on all Titan FUV observations to date. Observed ‘Asymmetry’ in Lyman-alpha absorption profiles due to additional factors. Incomplete flatfield correction. Unaccounted for Lyman-alpha emission near Titan exobase. Spurious or contaminated data. Possible changes in the global structure of Methane Distribution from February, 2005 – September, 2006 Need more global observations to confirm. Methane Distribution in Titan’s atmosphere.

Symmetric vs. Asymmetric Observations

T-9: EUVFUV001_PRIME

T-18: EUVFUV001_PRIME