Saturn in perspective II.
Ten years of Hubble Space Telescope observations
(1994-2004):
Vertical Cloud Structure.
S. Pérez-Hoyos1, A. Sánchez-Lavega1, R.G. French2, J. F. Rojas3
(1) Dpto. Física Aplicada I, ESI, Universidad del País Vasco, Bilbao
(2) Dpt. of Astronomy, Wellesley College, Massachussets (USA)
(3) Dpto. Física Aplicada I, EUITI, Universidad del País Vasco, Bilbao
INDEX
1. Introduction
a. Saturn quick facts
4. Long-term evolution
a. Tropospheric changes
b. Stratospheric changes
b. Saturn unknowns
c. Our contribution
2. Observations
a. Time base line
5. Atmospheric features
a. Equator
b. Middle-latitudes
c. South Polar Region
b. Geometrical aspects
c. Spectral coverage
d. Spatial resolution
3. Method
a. Numerical model
b. Strategy
c. Mean model
6. Short-term changes
a. General aspects
b. Changes in the SPR
7. Hemispheric asymmetry
a. Ring plane crossing epoch
b. The rings effect
8. Conclusions
JENAM Granada 13 – 17 Septiember 2004
INTRODUCTION
• The Lord of The Rings
SATURN
QUICK FACTS
• Dense atmosphere with zonal winds
• Cloudy atmosphere (NH3, NH4SH, H2O, …)
• Rotational axis tilt ~ 26.73º ; Period ~ 30 yr
• Two energy sources
• Internal rotational period
• Cromophore species
• Equatorial and non-equatorial storms
THE
PUZZLING
SATURN
?
• Equatorial jet slow down revealed!
• Seasonal behaviour of the atmosphere
• Cloud tracers height
• The problem of the energy budget
JENAM Granada 13 – 17 Septiember 2004
~
OUR
CONTRIBUTION
Still working…
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OBSERVATIONS
2000
SATURN
1996
α
EARTH
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SUN
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OBSERVATIONS
2000
SATURN
• Long time
baseline
• Wide espectral
coverage
1996
α
EARTH
SUN
Spectrum by Karkoschka (1994)
• Good spatial
resolution
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METHOD
Numerical model
• Scalar resolution of the radiative transfer
equations in a plane-parallel atmosphere with
the “doubling-adding” technique.
• Reproduces gases (absorption by CH4 and
scattering by H2, He), particles (with many
different phase functions) and mixtures of
both.
Strategy
• We retrieve the vertical
structure of a given
latitude at a given time
by fitting the observed
reflectivity curve at all
wavelengths
simultaneously.
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METHOD
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LONG-TERM EVOLUTION
• Which properties suffer the most important variations?
• How do the upper clouds and hazes follow the insolation changes?
• Optical properties of
the hazes are more
latitude- than timedependent.
• Optical depth of both
hazes is the most
variable parameter.
• Results strengthen
the idea of a clearer
summer atmosphere.
Tropospheric optical
depth suffers the most
intense changes,
decreasing steeply.
• Thermal processes
partially excluded.
?
• Photochemical
processes favoured.
JENAM Granada 13 – 17 Septiember 2004
Stratospheric changes
are more clear at the
pole, where optical
depth increases.
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ATMOSPHERIC FEATURES
• Equatorial features
are dark in the blue
and bright in the red
Æ height differences.
• Maximum increase
~ 8 mbar (7 km).
• Some convective
features at middlelatitudes.
• Storms at ~ 200 mbar.
• Other features show
color variations.
All atmospheric features
seem to be imbedded in
the upper troposphere,
no lower than 300
mbar.
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SHORT-TERM CHANGES
• Sometimes, in a month period, there are strong
changes in the contrast of belt/zones.
• No features apparently related with those
changes.
• Models frequently show variations in the color of
the tropospheric particles Æ sizes or composition?
• Regions of change seem to be correlated with
the peaks of the jets.
• Slower changes in the aspect of the South
Polar Region.
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HEMISPHERIC ASYMMETRY
• What happens in the Northern
Hemisphere at the same time?
• The shadow of the rings prevents the
analysis of most regions of the
hemisphere.
• At some years we can compare northern
and southern latitudes (see Muñoz et al.,
2004).
• We have to wait until ~ 2038, for the
next favourable ring plane crossing from
Earth.
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CONCLUSIONS
We have a model
that reproduces
observed
reflectivities in a
wide espectral
range within
expected
parameter values.
We have studied the
latitudinal and temporal
variations of the vertical
structure in a third of a
Saturn’s year, witnessing
the transition from a
“winter” to a “summer”
atmosphere.
We have constrained the main
properties of the cloud tracers
by using some simple plausible
models.
We have detected strong
contrast changes in the
belt/zone structure for some
years, interpreted as particle
variations.
Things to do:
1. Analyze and compare
available observations of the
Northern Hemisphere.
2. Model the expect solar heat
deposition at every pressure
level and study its influence
in the atmospheric structure.
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CONCLUSIONS
We have a model
that reproduces
observed
reflectivities in a
wide espectral
range within
expected
parameter values.
We have studied the
latitudinal and temporal
variations of the vertical
structure in a third of a
Saturn’s year, witnessing
the transition from a
“winter” to a “summer”
atmosphere.
We have constrained the main
properties of the cloud tracers
by using some simple plausible
models.
We have detected strong
contrast changes in the
belt/zone structure for some
years, interpreted as particle
variations.
Things to do:
1. Analyze and compare
available observations of the
Northern Hemisphere.
2. Model the expect solar heat
deposition at every pressure
level and study its influence
in the atmospheric structure.
JENAM Granada 13 – 17 Septiember 2004
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