The Saturn s Atmosphere

Transcription

1 The Saturn s Atmosphere Ben Wei Peng Lew May 2, Introduction Saturn is 9.5 AU away from the Sun. Given a radius of km (0.84 R Jup ) and mass of kg (0.28M Jup ), the surface gravity is around 11 kgms 2. It is tilted degree to the orbit with 10 h 32 min 45 s ± 46s rotation period based on the gravitational field and oblateness (Helled et al., 2015). The hue yellow color seen in Saturn chromosphere is possibly caused by spectrally featureless hydrocarbon (West et al., 2009). Saturn s effective temperature is around 125 K at 9.5 AU orbital distance from the Sun.The corresponding scale height is kt/µg=43km, where µ 2 for hydrogen dominated atmosphere. 2 Composition The 5 most abundant gases in Saturn atmosphere are hydrogen, helium (8%, Fletcher et al. (2007a)), methane (0.45%, Courtin et al. (1984)), ammonia (NH 3 ) (0.05%, Briggs & Sackett (1989)) and ethane (C 2 H 4 ) ( %, Courtin et al. (1984)). The composition of Saturn atmosphere are mainly estimated based on radio occultation and infrared spectroscopy. Radio occultation measures the refractivity of atmospheres as a function of altitude. Based on the measured refractive index, assumed atmospheric composition (as well as mean molecular mass) and ideal gas law (p= ρ k b µ T ), we can know the T(p) profile and corresponding infrared spectra from radiative transfer model. The assumed atmospheric composition is revisited if the estimated infrared spectra does not match with the observation. The helium to hydrogen mixing ratio that measured by Voyager is underestimated because of the instrumental systematics. A self-consistent helium-to-hydrogen mixing ratio is measured by Cassini, giving volume mixing ratio [He]/[H 2 ]=0.08 (mass fraction of 0.13, or a mole fraction of 0.07) (Gautier et al., 2006) by only considering H 2,He and CH 4 (Assume[CH 4 ]/[H 2 ]= ) in the atmosphere. The abundance of trace gases is more uncertain due to incomplete understanding of clouds. By using a series of possible mixing ratio of gases, including continuous absorption caused by clouds or haze and pressure-broadened spectral lines, forward radiative transfer 1

2 Figure 1: The infrared spectra calculated based on the T/µ profile measured from radio occultation method by Cassini.Dougherty et al. (2009) modeling was used by Courtin et al. (1984) to obtain the best fit of methane, ammonia, ethane, phosphine and other trace gasses composition. 3 Temperature Profile Both radio occultation and thermal infrared spectra can provide indirect measurement of Saturn s temperature profile. Thermal infrared spectra of Saturn atmosphere that covers from 10 to 1400 cm 1 (1000 to 7 µm) is observed by using Composite Infrared Spectrometer (CIRS) on Cassini. The molecular absorption features that are sensitive to temperature and pressure are used to infer the temperature at certain pressure level. The most useful IR features and pressure sensitivities are listed as below (Fletcher et al., 2007a): Collision induced continuum features that span from cm 1 (see also figure 1) that is sensitive to pressure until around 800mbar. In order to get the temperature, the fraction of para-hydrogen has to be known as well. Mid-IR ( cm 1 ) has higher spatial resolution and is used to fine-tune the temperature profile in mbar region. By assuming methane volume mixing ratio of q CH4 = (Flasar et al., 2005), the blended pseudo-continuum of CH 4 emission line in cm 1 region is used to update the temperature at around 1mbar region. Assuming the aerosol (ammonia) haze is evenly distributed from 80 to 1800 mbar with a standard size distribution of 1.0 ± 0.2µm and using pre-tabulated collision-induced absorption (CIA) coefficients of H 2 H 2, H 2 He, H 2 CH 4, CH 4 CH 4 pairs, (Fletcher 2

3 Figure 2: The Saturn and Jupiter Spectra measured by CIRS Cassini (Burrows & Orton, 2009) et al., 2007a) used radiative transfer and non-linear optimal estimation algorithm (Rodgers, 1976; Irwin et al., 2004) 1 to retrieve the temperature and fraction of para-hydrogen as a function of pressure. The retrieval method also assumed that the upper limit of lapse rate is the dry adiabat (around 0.78Kkg 1 km 1 ). The temperature profile is relaxed to the a priori(initial guess) of retrieval model whenever no information is provided. 4 Clouds and Haze [!h] The composition and properties of clouds are normally obtained by fitting spectra or image with theoretical cloud models, which are highly depend on the assumed number of cloud layers, particulates size distribution and particle properties. In previous section, the retrieved temperature and composition is mainly from (Fletcher et al., 2007a) based on Cassini radio occultation and thermal spectra (CRIS). They compare and explore different simple haze layers in their model and concluded that haze model does not have significant effect to their result (see section in (Fletcher et al., 2007a)). Here I choose the latest work done by Sromovsky et al. (2013), which fits the Visual and Infrared Mapping Spectrometer (VIMS) spectra outside of storm of Saturn with 3 lay- 1 This algorithm minimize the cost function, which is sum of difference between model and data and the difference between model parameter and initial guess (a priori): φ = (y F(x)) T S 1 ε (y F(x)) + (x a) T S 1 x (x a) where y is the measured spectrum; F(x) is the spectrum from forward modeling; S ε is a covariance matrix, consist of measurement and modeling errors; x is a state vector, consist of a set of model parameters; a is a priori state vector, consist of the initial guess for forward modeling; S x is a priori covariance matrix, consist of the assumed error range for each parameter. 3

4 Figure 3: The a priori used for retrieval model from (Fletcher et al., 2007b). The assumed composition other than H 2, He is shown. They assume He/H 2 ratio is Figure 4: The T-P profile retrieved by Fletcher et al. (2007a) based on spectra from Composite Infrared Spectrometer (CIRS Composite Infrared Spectrometer (CIRS). ers cloud model. The first two layers are assumed to be formed via condensation process, though the exact composition of cloud layer is not clear. The third layer is stratospheric haze, which is optically thin and thought to be formed via similar process as suggested for Jupiter (see Figure 2). Again, (Sromovsky et al., 2013) used slightly different temperature (i.e. Lindal et al. (1985) and composition (e.g., H/He 2 ratio, CH 3 D, P H 3, NH 3 ) assump- 4

5 Figure 5: The volume mixing ratio used for (Sromovsky et al., 2013). Note that the P H 3 mixing ratio decrease slower compare to figure 3 tion. I also include the calculated result at similar pressure level from (Fouchet et al., 2009) and (Pérez-Hoyos et al., 2005) for comparison. 4.1 Particulates in upper troposphere The first two cloud layers suggested by Sromovsky et al. (2013): (i) An optically thick, deep cloud layer obscure most of thermal radiation from deeper layer but still thin enough to allow 5µm radiation. Cloud model fitting from citetfletcher2011 based on nightside µm VIMS spectra suggest that cloud base resides at bar level globally and advection raise the cloud base to higher altitude at equator. The composition of cloud is highly uncertain because of the strong opacity from the gases like P H 3 and the overlaying stratospheric aerosol layer. (ii) A middle cloud layer that extend from around mbar and have optical depth of at 2 µm. The particulates in this layer are around 0.6µm in radius with refractive index of i. Where are ammonia-ice clouds? Thermodynamics model predict that ammonia ice should exist in Saturn atmosphere (see Figure 7) but the corresponding spectral feature at 9.4 and 24 µm is not seen in Saturn atmosphere globally but only in giant storm where clouds are freshly formed. Several factors could partly explain this, including the overlapping absorption from methane and hydrogen gas, effect caused by non-spherical particulates, broaden and shallow line profile 5

6 Figure 6: The cloud model used by (Sromovsky et al., 2013). The cloud model in text is the Model C, which gives the best fit to VIMS data. Figure 7: Thermodynamics model predict that ammonia ice cloud should exist in Saturn s atmosphere. The cloud concentration is the upper limit in gram per liter. Schematic diagram is adopted from (Atreya & Wong, 2005) due to larger particle size, or contamination from upper or lower layers. However, West et al. (2009) reject all the later three factors. Fouchet et al. (2009) suggest it is actually diphosphine(p 2 H 4 ) haze that contribute to the significant opacity at deep layer, but no examination can be done without measuring the refractive index of diphosphine in lab first. 6

7 4.2 Particulates in stratosphere (iii) Spectroscopic signatures for hydrocarbon haze, mainly at 3.4 µm, is observed at middle latitude via stellar occultation method by using Cassini VIMS (Nicholson et al. 2006). In polar region, benzene or polycyclic aromatic hydrocarbon (PAHs) could be more abundant. One of the most important properties of the stratospheric haze is the scattering phase function. The particle scattering phase function of Saturn at mid-latitude belt, measured by Pioneer 11Tomasko & Doose (1984), matches with the lab measurement of scattering angle of ammonia crystal, suggesting that the size of haze particle must be larger than visible wavelength. Strong forward scattering seen from polarization function Tomasko & Doose (1984) also indicates that the haze particle is not spherical but linear shape. Low UV reflectivity at large reflection angle near the limb of Saturn from Hubble Space Telescope (HST) indicates that the haze particle must extend to few millibar otherwise Rayleigh scattering at 10mbar level will cause Saturn s limb to be brighter than observed value (West et al., 2009). Pérez-Hoyos et al. (2005) used HST to measure the absolute reflectivity from center to limb in UV and near-ir wavelength region and fitted with twolayer haze models. They concluded that the stratospheric haze particle size is around µm, assuming they are spherical (note: this is actually contradict with observed strong forward scattering) with refractive index of The haze particle is optically thin, with optical depth ranges from 0.05 at around 0.9µm and 0.7 at 0.2µm. Figure 8: The photochemical network for the formation of stratospheric aerosols on Jupiter. Similar formation mechanism is seen expected on Saturn except NH 3, NH 2, N 2 H 4 are P H 3, P H 2, P 2 H 4 instead respectively. 7

8 5 Spatial and Time Variation of T-P profile There is significant variation in latitudinal distribution of haze. At polar region (> 70 deg), the UV absorption of haze particle is stronger, implying smaller particle size relative to other region (latitude < 70 deg). The image of equatorial region, especially at wavelength where methane has strong absorption, is seen to be darker than higher latitude region, suggesting that haze layer is thicker at lower latitude (West et al., 2009). Using ten years of HST images, Pérez-Hoyos et al. (2005) found that optical thickness of haze at short wavelength (255nm),which is most sensitive to stratospheric haze, is positively correlated with the insolation but the dependance varies at different latitudes. On the shorter timescale (months to 1-2 years), they noted optical thickness of haze layer can varies at a factor of 2, which likely attributed to atmospheric circulation. Figure 9: Interesting surface feature on Saturn: Upper left: False color image of Dragon Storm at 35 deg S in near-infared continuum and methane band filters from Cassini ISS in Red color is lower cloud tops, blue shades is higher cloud tops and white color is optically thick haze and clouds. Right: The Great White Storm from HST in Lower: False color images of a convective storm, which is looked like a inverse question mark, in Figures from Dougherty et al. (2009) 8

9 References Atreya, S. K., & Wong, A.-S. 2005, Space Science Reviews, 116, 121 Briggs, F. H., & Sackett, P. D. 1989, Icarus, 80, 77 Burrows, A., & Orton, G. 2009, ArXiv e-prints, arxiv: Courtin, R., Gautier, D., Marten, A., Bezard, B., & Hanel, R. 1984, ApJ, 287, 899 Dougherty, M. K., Esposito, L. W., & Krimigis, S. M. 2009, Saturn from Cassini-Huygens, doi: / Flasar, F. M., Achterberg, R. K., Conrath, B. J., et al. 2005, Science, 307, 1247 Fletcher, L. N., Irwin, P. G. J., Teanby, N. A., et al. 2007a, Icarus, 189, b, Icarus, 188, 72 Fouchet, T., Moses, J. I., & Conrath, B. J. 2009, 83 Gautier, D., Conrath, B., Flasar, M., et al. 2006, in COSPAR Meeting, Vol. 36, 36th COSPAR Scientific Assembly Helled, R., Galanti, E., & Kaspi, Y. 2015, Nature, 520, 202 Irwin, P. G. J., Parrish, P., Fouchet, T., et al. 2004, Icarus, 172, 37 Lindal, G. F., Sweetnam, D. N., & Eshleman, V. R. 1985, Astronomical Journal, 90, 1136 Pérez-Hoyos, S., Sánchez-Lavega, A., French, R. G., & Rojas, J. F. 2005, Icarus, 176, 155 Rodgers, C. D. 1976, Reviews of Geophysics and Space Physics, 14, 609 Sromovsky, L. A., Baines, K. H., & Fry, P. M. 2013, Icarus, 226, 402 Tomasko, M. G., & Doose, L. R. 1984, Icarus, 58, 1 West, R. A., Baines, K. H., Karkoschka, E., & Sánchez-Lavega, A. 2009, Clouds and Aerosols in Saturn s Atmosphere, ed. M. K. Dougherty, L. W. Esposito, & S. M. Krimigis (Springer), 161 9