 |
| Titel |
Seasonal Temperature Variations in Saturn's Stratosphere: Radiative Seasonal Model vs. Observations. |
| VerfasserIn |
Thomas Greathouse, Julianne Moses, Leigh Fletcher, Glenn Orton, Sandrine Guerlet |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035762
|
|
|
|
|
|
| Zusammenfassung |
| The seasonal variations of temperatures in Saturn’s stratosphere are controlled by a
combination of several processes. The dominant driver of seasonal change is the variation of
insolation due to Saturn’s orbit and axial tilt. Radiative processes, originally thought to
dominate in the stratosphere, cool or heat the stratosphere depending on the relative
abundance of emitters and absorbers. To first order, photochemistry controls the abundances
of C2H2 and C2H6, the dominant coolants, which also depends on the variation of
insolation during Saturn’s year. Finally, dynamics could be responsible for the advection
of heat and transport of hydrocarbons such as C2H2 and C2H6. Transporting the
hydrocarbons affects the cooling rates and thus the temperatures of the affected
regions.
We present the results from a radiative seasonal climate model applied to Saturn. The
seasonal model performs the full radiative exchange calculation throughout the stratosphere
at all wavelengths (0-105 cm-1). We include the effects of aerosol absorption but not
scattering. The effects of dynamics are also not included. We implement vertically and
meridionally variable hydrocarbon abundances as measured by Greathouse et al. (2005) and
Guerlet et al. (2009), and compare them to a model with vertically and meridionally constant
abundances as was done in previous models (Bezard and Gautier, 1985; Conrath et al., 1990).
Finally, we compare the models to the TEXES and Cassini/CIRS observations described
below.
We have performed ground based observations of Saturn’s stratospheric emissions in
2002, 2004, 2005, 2007 and 2009 using the Texas Echelon Cross Echelle Spectrograph,
TEXES, mounted on the NASA Infrared Telescope Facility atop of Mauna Kea in Hawaii.
With a spatial resolution of - 1″ and a spectral resolving power R= λ-Îλ=80,000, we have
mapped the stratospheric emission due to H2, CH4, C2H2, C2H4, C2H6, C3H8 and CH3D.
From the observations of H2 and CH4, we infer the vertical, meridional and temporal
variations of Saturn’s stratospheric temperatures. Then, using the inferred temperatures, we
model the observations of the different hydrocarbons to retrieve their vertical, meridional,
and temporal abundance variations (Greathouse et al., 2005; Greathouse et al.,
2006).
Similar to the ground based observations, Cassini/CIRS observations (2004-2009) have
been used to infer the vertical, meridional, and temporal variations of Saturn’s stratospheric
temperature and hydrocarbon abundances (Flasar et al., 2005; Fletcher et al., 2008; Fletcher
et al., 2007; Fouchet et al., 2008; Guerlet et al., 2009; Howett et al., 2007). Ground based
observers are unable to observe through Saturn’s rings or behind the planet as viewed
from the Earth. On the other hand, Cassini has no such restriction and thus can
measure the global thermal and chemical state of Saturn for the duration of the
mission. An important goal is to bring the ground-based and Cassini observations
into agreement so that when the Cassini mission ends, continued ground-based
observations can be used to constrain seasonal and dynamic changes on Saturn
thereafter.
This work was funded by NASA PATM grant NNX08AL95G and NASA PAST grant
NNX08AW33G. |
|
|
|
|
|
|