Cassini UVIS stellar occultation
measurements of the Saturn
atmosphere
D. E. Shemansky
J. T. Hallett
X. Liu
Cassini UVIS team
12
Saturn UVIS star occultations to date
11
Data statistical quality
10
signal rate quality
photometric cutoff
9
8
7
6
5
4
3
2
1
0
-40
-20
-0
Latitude
20
40
UVIS stellar occultations:
50
Latitude v s time of observ ation
40
30
Spectral
Photometric
Latitude
20
10
-0
-10
-20
-30
-40
-50
2005.0
2005.5
2006.0
Date
2006.5
2007.0
•
•
•
•
δ-Ori occultation at Saturn Apr
13, 2005; Latitude 40o S
Sunlit atmosphere
UVIS EUV and FUV spectrographs
Pointing stabilized on reaction wheels
No apparent pointing drift
Analysis based on non-LTE model
synthesis of discrete H2 absorption spectrum
in the EUV. Hydrocarbon absorption
properties derived from the FUV spectrum.
Cassini UVIS Delta Orionis occultation at Saturn
2005 Apr 13 16:34
Latitude –40o
H2 non-LTE model analysis
• Forward modeling using individual ground state
absorption vectors into the B, C, B’, and D
electronic states. Rotation populations assumed
thermal, as predicted by our H2 physical chemistry
model .
• Transition probabilities based on calculations
including perturbations in electronic structure
(Abgrall et al., 1993, 1994, 1997; Liu et al., 1998, 2000,
2002, 2003).
• Rotational levels to J=12.
• Confined to EUV spectrum.
Hydrocarbon absorbers
Measured species:
• CH4 Methane
• C2H2 Acetylene
• C2H4 Ethylene
• C2H6 Ethane
EUV2005_DELTOR_6rec_FIT1330_V10_345_reca
UVIS EUV transmission data vs Non-LTE H2 absorption model
1.2
1.1
Alt: 1330 km
T = 330 K; (H2)l = 4.3x1018 cm-2
1.0
I/Io
0.9
0.8
0.7
0.6
0.5
900
950
1000
1050
λ (A)
1100
1150
EUV2005_DELTOR_6rec_FIT840_v10_345_reca_pres
UVIS EUV transmission data vs Non-LTE H2 absorption model
1.2
Alt: 840 km
T = 200 K; H2l = 2.2x1020 cm-2
1.0
I/Io
0.8
0.6
0.4
0.2
0.0
900
950
1000
1050
λ (A)
1100
1150
FUV2005_DELTOR_10REC_II0A_660_reca_pres
UVIS FUV transmission: impact parameter 660 km
1.2
1.0
I/Io
0.8
0.6
H = 660 km:
CH4: 2x1016 cm-2
0.4
_046_14641
CH4 model
Non-LTE H2 model
0.2
0.0
1100
1200
1300
1400
1500
λ (A)
1600
1700
1800
1900
FUV2005_DELTOR_10REC_II0A_568_reca_pres
UVIS FUV transmission: impact parameter 568 km
1.2
1.0
H = 568 km:
CH4: 8.x1017 cm-2
I/Io
0.8
C2H2: 4x1016 cm-2
C2H4: 1.2x1016 cm-2
0.6
C2H6: 1.7x1017 cm-2
0.4
0.2
0.0
1100
1200
1300
1400
1500
λ (A)
1600
1700
1800
1900
H2 and CH4 los abundances
Cassini UVIS rev6; Voyager 2 (Smith etal 1983)
1450
H2 Cassini UVIS Model Fit
H2 Cassini UVIS Observation-Derived
H2 V2 Smith et al. (1983)
ch4_uvis_abundances
CH4 V2 Smith et al. (1983)
1350
Cassini UVIS
1250
Altitude (km)
1150
Latitude 40o S
Voyager UVS
Latitude translated to equator
1050
950
850
750
650
550
1016
1017
1018
1019
[H2]l (cm-2 )
1020
1021
1022
H2_syn_comp_04
Derived H2 densities from rev 6 stellar occultation
2000
Cassini UVIS rev6
LINDAL_85_F3_EGR
smith_etal_83
FESTOU_ATREYA_82_N
Hubbard et al., 1997
1800
1600
h (km)
1400
1200
1000
800
600
400
200
0
7
9
11
13
15
log([H2]) (cm-3)
17
19
21
H2_syn_comp_06_T
Derived temperature distribution from rev 6 occultation
smith_etal_83
LINDAL_85_F4_ING
LINDAL_85_F4_EGR
FESTOU_ATREYA_82_T
Cassini UVIS rev 6
Hubbard etal 1997
2000
1800
1600
h (km)
1400
1200
Festou & Atreya, 1982 δ-Sco
1000
Latitude 3.84o N
800
Smith et al., 1983
600
Equator derivation
400
Latitude 40o S
200
Hubbard et al., 1997
Cassini UVIS δ-Ori
Equator model
0
0
100
200
300
T (K)
400
500
Top of thermosphere temperature
• Voyager UVS 1981:
Smith et al.(1983), 420 K, Equatorial
Festou & Atreya (1982), 800 K, Equatorial
• Cassini UVIS 2005:
320 K, Latitude 40o S
Temperature profile
• Top of thermosphere: T= 320 K
• Peak: T= 343 K, h=1186 km,
[H2] = 2.2X1010 cm-3
• Mesopause: T= 100 K, h= 570 km,
[H2] = 2.4X1014 cm-3 [CH4]~6.X109 cm-3
Energy deposition rate
• Inferred energy deposition rate from the
temperature profile required to maintain the
observed temperature peak at 1200 km is 0.12 erg
cm-2 s-1 (About ½ the value from Voyager results)
• The observed dayglow (Hallett et al., 2005, P11C0127) deposits 0.01 erg cm-2 s-1, too low in
magnitude and too low in the atmosphere to
contribute to the energy deposition at the top of
the thermosphere.
Conclusions
• The thermosphere in 2005 is significantly colder than it
was in 1981, possibly related in some way with the fact
that the major solar cycle was at a maximum in 1981 and
near minimum in 2005.
• The altitude of the hydrocarbon homopause is 300 km
lower relative to the 1 bar pressure level, in 2005,
compared to the results from the 1981 experiments. It is
not clear that this can be explained by latitudinal and
temperature differences. The earlier measurements were
handicapped by inaccurate H2 physical parameters, and
Hubbard et al.(1997) using Earth based stellar occultations
regarded the lower 200 km of the Smith et al. (1983)
profile as incompatible with the Earth based results.
Conclusions continued
• The energy deposition required to heat the upper
thermosphere cannot be explained by the input
directly related to the dayglow or auroral emission
in 2005. Upper ionospheric electrons with
temperatures below 20000 K could provide the
heat deposition through the radiationless
dissociation of H2 and populate the magnetosphere
with the observed extensive atomic hydrogen
cloud.
UVIS Pre SOI image of atomic hydrogen in the Saturn system
Cassini UVIS image of atomic hydrogen in the Saturn system