Workshop on Planetary Atmospheres (2007) 9079.pdf

An Updated Radiative Seasonal Climate Model for the Saturnian Atmosphere. S. B. Strong1, T. K. Greathouse2, J. I. Moses3, J. H. Lacy, 1University of Texas, Austin (University of Texas at Austin, 1 University Blvd. C1400, Austin, TX 78712, [email protected]), 2 Southwest Research Institute (Southwest Research Institute, 6220 Culebra Rd., San Antonio, Texas 78238-5166), 3 Lunar and Planetary Institute (Lunar & Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058).

Introduction: Here, we present the results from an improved With an of 26.7o, exhibits seasonal cli- radiative seasonal climate model for Saturn's stratosphere (1 mate variations similar to Earth's. Saturn's atmosphere con- x 10-4 to 660 mbar) that includes transfer of radiation with tains less then 1% methane, and yet this hydrocarbon and its wavenumbers from 0 to 10.0 x 104 cm-1. We have included photochemical products play a dominant role in regulating vertically, latitudinally, and seasonally dependent hydrocar- the atmospheric seasonal temperature variability. Methane bon abundances, as determined by Moses et al. [10]. We photochemical byproducts, ethane and acetylene, are Sat- examine the effects of meridional seasonal temperature vari- urn's main stratospheric coolants while methane itself domi- ability with altitude and include the effects of ring attenua- nates in solar absorption. The amount of ultraviolet flux from tion and correctly averaged diurnal radiation. Our model the Sun regulates the hydrocarbon production rates. This flux implements multiplicative corrections to Irwin et al. [11] depends on such factors as latitude, , time of day, methane absorption coefficients derived from Huygens DISR distance from the Sun, and ring attenuation [1]. observations of [12]. Saturn's 29.5 year presents a challenge for Results and Conclusions: the observational monitoring of atmospheric temperature Implementing temporally, meridionally, and verti- variability. Dating back to 1973, during the last southern cally variable hydrocarbon abundances (TMV), rather then o summer solstice on Saturn (Ls = 270 ), and going through to solely vertically variable abundances (nominal) predomi- the present, observations have indicated enhanced hydrocar- nantly affects temperatures in the upper stratosphere, 1.1 bon emission at the south pole during southern summer sol- mbar and higher, by 1 to 10 K. The biggest temperature stice [2,3,4,5,6,7]. The 2002 observations by Greathouse et change occurs at the smallest pressure. The time-dependent al. [3], close to southern summer solstice, showed a 6 K in- seasonal lags for both nominal and TMV abundances are crease in temperatures from equator to south pole at 5 mbar. nearly identical except for the 0.01 mbar level at -80 degrees By 2004, this increase in temperature from equator to south latitude. The atmospheric response time is predicted to lag pole, ς, was +15 K. Similar ς values were derived by Howett the seasonal insolation at 5 mbar, at the pole, by tan-1 2π et al. [7], Flasar et al. [6], and Orton et al. [5] in 2004. 9.4/29.5o = 63.5o (Ls), or 5.2 Earth years [13]. We find this Cess et al. [8] and Bézard and Gautier [9] produced lag to be closer to 3 Earth years. From Conrath et al. [13], radiative seasonal climate models for Saturn. The Cess et al. the radiative response times in Saturn's south polar strato- [8] model replicated the pre-Voyager emission trends ob- sphere at 0.1, 1, and 5 mbar are 7, 9, and 9.4 years, respec- served by Tokunaga et al. [2] but predicted a negative value tively. We find faster response times. For 0.1, 1, and 5.6 of ς, such that temperatures at the south pole at 1 mbar were mbar the response times are 0, 2.5, and 3.3 years from south- warmer than the equator in 2002 by 6 K. Also, relatively no ern summer solstice, respectively. The value of ς as a func- seasonal lag as a function of thermal inertia was noted. Such tion of season and pressure corresponds well with current an effect is expected [3,13]. The multi-layer, monochro- Cassini and ground-based observations. matic Bézard and Gautier [9] model predicted the correct References: [1] Moses, J. I. and T. K. Greathouse magnitude of the observed factor of ς seen in recent observa- (2005) JGR, 110, E09007. [2] Tokunaga, A. et al. (1978) tions, but forecasted a 5 K decrease in temperatures at the 5- Icarus, 36, 216-222. [3] Greathouse, T. et al. (2005) Icarus, mbar level, from the equator to south pole during 2002. 177, 18-31. [4] Greathouse, T. et al. (2005) LPI, 1365. [5] Both the Cess et al. [8] and Bézard and Gautier [9] Orton, G. et al. (2005) Science, 307, 696. [6] Flasar, F. et al. models adopted latitudinally and vertically constant hydro- (2005) Science, 307, 1247. [7] Howett, C. et al. (2007) carbon mixing ratios. In the real atmosphere, mixing ratios Icarus, in press. [8] Cess and Caldwell (1979) Icarus, 38, the should vary with latitude and season due to transport 349. [9] Bézard, B. and D. Gautier (1985) Icarus, 61, 296- processes and photochemical production and loss [1]. The 310. [10] Moses, J. I. et al. (2007), this meeting. [11] Irwin, low methane mixing ratio used in the Bézard and Gautier [9] P. G. et al. (2006) Icarus, 181, 309. [12] Tomasko, M. et al. +1.0 -3 model, [CH4]/[H2] = 3.5 0.7 x 10 , may have also contrib- (2007) Icarus, submitted. [13] Conrath, B. et al. (1990) uted to discrepant temperatures. From Cassini CIRS observa- Icarus, 83, 255. tions, the methane mixing ratio was confirmed to be -3 [CH4]/[H2] = 4.5 +/- 0.9 x 10 [6]. These incorrect assump- tions and outdated inputs could account for the discrepancies between these models and current observations.