Saturn In Perspective I.
Ten years of Hubble Space Telescope
Observations (1994-2004):
Atmospheric Features and Zonal Winds
A. Sánchez-Lavega (1),
R. Hueso(1), S. Pérez-Hoyos (1), J. F. Rojas (1), R. G. French (2)
(1) Grupo Ciencias Planetarias, Dpto. Física Aplicada I
Universidad del País Vasco, Bilbao (Spain)
(2) Wellesley College, Dept. of Astronomy, Wellesley, MA (USA).
GIANT PLANETS WINDS
Saturn:
• Hazes & Upper Clouds (NH3, NH4SH, H2O)
(P~ 0.5-12 bar)
• Deep H2 atmosphere (~ 0.5 RS)
•Rapid rotation: Period: 10 hr 39 min 24 s
(S-III) Æ but PROBLEM ! ~ + 6 min (?)
•Internal Energy source (~ 2x Insolation)
Insolation Variability in Saturn’s Equator
(1) Orbital tilt ~ 27°
(2) Orbital eccentricity = 0.052
(3) Period (Saturn’s year) ~ 29.5 years
(4) Rings effects:
insolation shadowing,
reflection of incident sunlight,
thermal radiation
GWS 1994
GWS 1990
δFo/<FEQ> ~ ± 10 - 30 %
Equator
δFo/<FEQ> ~ ± 90 %
Large Equatorial Storms (Great White Spots)
Events in 1876, 1933, 1990, 1994 (secondary)
1994 event
GWS 1990: Onset & 1 month later
1994-95
SATURN’S FEATURES: HST 1994-2004
Polar Spots (CH4- 890 nm)
Temperate Storms (814 nm, 42°S)
Equatorial features (890 nm, 10°N – 20°S)
Bright Spots (439 nm, 29°S)
Mean Zonal Winds: 1980-81 versus 1994-2004
450
* Large Equatorial Wind “Drop”:
∆u ~ 200 ms-1 (Lat: 20°N to 20°S)
* Persistence of the jets outside
the equator
* New polar southern jet Æ
Saturn’s highly symmetryic wind
profile.
1994-2002
HST (points)
GB (circles)
Voyager 1 & 2
1980-81
Zonal Wind Velocity (m/s)
Mean Zonal Winds: 1980-81 versus 1994-2004
450
Equatorial
Jet
1994-2002
HST (points)
GB (circles)
* Large Equatorial Wind “Drop”:
∆u ~ 200 ms-1 (Lat: 20°N to 20°S)
* Persistence of the jets outside
the equator
* New polar southern jet Æ
Saturn’s highly symmetryic wind
profile.
Voyager 1 & 2
1980-81
Zonal Wind Velocity (m/s)
1- A. Sánchez-Lavega, J. F. Rojas and P. V. Sada, Saturn's Zonal Winds at Cloud Level. Icarus, 147, 405-420 (2000).
2- A. Sánchez-Lavega, S. Pérez-Hoyos, J. F. Rojas, R. Hueso and R. G. French. A strong decrease in Saturn's equatorial jet at cloud level. Nature, 423, 623625 (2003).
3- A. Sánchez-Lavega, R. Hueso, S. Pérez-Hoyos, J.F. Rojas and R. G. French. Saturn's cloud morphology and zonal winds before the Cassini encounter.
Icarus, 170, 519-523 (2004).
Reprints can be downloaded from http://www.ajax.ehu.es/grupo/
Storm - Zonal Flow Interaction
Wind changes by large convective events
Motions around storms
GWS 1990
N
TOTAL CHANGE IN THE
EQUATORIAL JET
390
km
∆U = 15-30 m/s
Global instability at the equator
S
W
510 km
E
Water ~ 4 solar
∆U = 200 m/s
Highly
unlikely
Wave Motions
Equatorial Rossby, no wind shear (Allison, 1990):
− βa 2 /n2
, ( gravest; j = 1)
c − u=
βa 2
1+ ( 2 j +1) 2
n gh(z)
For c - u = −200 ms −1; n = 2-4
⇒ Equivalent thickness: h ~ 44 - 58 km
Height
Longitude
c
c
Latitude
gh of
This
is nottrapping
a likely
scenario
+1)
Meridional
distance
(±20o );since
y2 = (2 jfeatures
very different sizes move at the same βwind
speed.
⇒ h = 64 km
Vertical Wind Shear
Height
u = 275 m/s
u = 475 m/s
T1
T2
∆T Ædu/dz
Latitude
Wind shear may be contributing to the “change”. However, the whole 200
m/s wind speed difference cannot be entirely due to a strong vertical shear.
Summary
1. SATURN’S EQUATORIAL REGION (~ ± 20°):
suffers intense dynamical changes
Prefered region for the Great White Spots (GWS):
Large-scale (20000 km) are rare events (1876, 1933, 1990),
periodic (?), evolve to planetary-scale disturbance.
(water moist convective storms).
The strongest eastward jet (500 ms-1) in planetary atmospheres
AND
Wind velocity variations (Æ ∆u ~ 200 m/s):
(1) Vertical (thermal wind) shear effects (partially likely).
(2) Wave motions (not true flow) (?)
(3) A true change in the mean zonal flow (?)
(a) Produced by the GWS & their large-scale dynamical
activity
(b) Seasonal insolation changes.
2. SATURN’S NON EQUATORIAL REGION:
The zonal jet system highly symmetric and long-term
Stable Æ Suggest deep rooted motions