N2 and Hydrocarbon
latitudinal/longitudinal/vertical
structure from UVIS occultations
D. Shemansky
6/3/2007
Observed Properties
• Stellar and solar observations to date provide a statistically
small sample of hydrocarbon vertical structure as a
function of latitude and longitude, possibly mixed with
temporal variability.
• Vertical hydrocarbon partitioning is strongly variable with
apparent differences at different latitudes and longitudes.
• Extinction by aerosols is measured from 300 km to 1000
km. Significant variability in aerosol abundance is evident
at different latitudes only below 500 km.
Aerosol distribution
• The TB λSco and T21 αEri occultations
were obtained at ~ -35o and show identical
vertical abundance distributions. αEri was
in the sunlit atmosphere, and λSco was on
the darkside.
• The aerosol distributions from TB λSco (lat
-36o) and αVir (lat 58o – 45o) show
significant differences only below 500 km.
Species investigated
•
•
•
•
•
•
N2
CH4 methane
C2H2 acetylene
C2H4 ethylene
C2H6 ethane
C4H2 diacetylene
•
•
•
•
•
C6H6 benzine
C6N2 dicyanodiacetylene
C2N2 cyanogen
HCN hydrogen cyanide
HC3N cyanoacetylene
UVIS occultations
Event
Latitude (deg)
Longitude (deg)
TB λSco
-36
318
TB αVir
58 -- 45
310 -- 330
T10 solar
-62 -- -51
0
T26 solar
-76
0
T21 αEri
-34
116
Impact Parameter Longitude (deg)
0
90
180
270
360
occ_TB_T10_t26_t21_lat_vs_h
1500
Lat_αVir
Long_αVir
Lat_λSco
Long_λSco
Lat_αEri
Long_αEri
Lat_T10_sun
Long_T10_sun
Lat_T26_sun
Long_T26_sun
1300
h (km)
1100
900
700
500
300
UVIS TB (αVir, λSco), T21 (αEri), T10, T26 (solar) occultations
100
-80
-60
-40
-20
0
20
Impact Parameter Latitude (deg)
40
60
80
spic_tran_lsco_018e_lst_03
1100
αVir Lat 58o -- 45o Long 310o -- 330o
αVir
λSco Lat -36o Long 318o
CH4X 10-1
λSco
1000
900
[CH4] X 10
CH4_λSco X 10
CH4_αVir X 10-1
C2H2_αVir
C2H4_αVir
C4H2_αVir
-7
800
h (km)
CH4X 10-1
-1
700
600
500
400
300
11
C2H2_λSco
C2H4_λSco
C4H2_λSco
aerosol_λSco
CH4_λSco_model
12
13
14
15
Log([X] cm-2)
16
17
18
CH4_tau_1
abundance mixing ratios to CH4.
αVir
λSco
Lat.(deg)
48.7
-36
tau_1 (km) (1200A)
982.0
1023.2
tau_CH4. (1216A)
0.83
0.71
CH4.
1
1
C2H2.
2.2E-02
4.8E-02
C2H4.
8.2E-03
3.4E-02
C2H6.
8.0E-03
1.8E-02
HCN
3.6E-02
1.0E-01
C4H2.
2.6E-03
5.7E-03
HC3N
4.2E-03
1.0E-02
C2N2
3.0E-03
4.5E-03
Summary: Longitude dependent
vertical structure
• At latitude ~ -35o the sunlit atmosphere above 880 km is
depleted relative to the dark atmosphere in CH4 and higher
order species by factors of 2 to 4.
• Below 880 km CH4 abundance (based on a single pair of
occultations) has no longitude dependence, but higher
order species remain depleted in the subsolar atmosphere.
• Aerosol extinction is identical from 300 km to 900 km at
latitude ~ -35o , independent of longitude and the 2 year
interval between occultations.
Summary: Latitude dependent
vertical structure
• Darkside abundance of CH4 1000 km – 700 km is
unchanged in magnitude and vertical distribution between
latitude 58o – 45o and –36o.
• Higher order hydrocarbons such as acetylene are depleted
above 700 km by factors as large as 2 in the darkside
occultations between latitude 58o – 45o and –36o.
• North/south differences in aerosol extinction below 500
km are significant in structure and magnitude, with
extinction below 425 km substantially smaller in the north
latitude region.
Summary: Vertical structure
• Acetylene roughly follows the atmospheric scale
of methane between 650 km and 1000 km.
• Peak densities of the higher order hydrocarbons
are indicated in the range 550 km to 700 km.
• Spectral absorption shapes in methane, acetylene,
ethylene, and diacetylene show reduced
atmospheric temperatures below 800 km and
north/south differences.
The T10 and T26 solar
occultations
• T10: 2006 015, Lat –62 -- -51o
• T26: 2007 069, Lat –76o
• Photometric analysis shows small differences in
N2 and hydrocarbon extinction at high altitude.
Mixed differences appear at lower altitudes. No
inferred significant differences in N2 abundance
and altitude scale; N2 model for T10 applies
satisfactorily to T26.
T10_t26_ch4_n2_data_comp_01
comparison of T10 and T26 solar occultation phtotometric reductions
-0.0
-0.1
-0.2
-0.3
ln(I/I0)
-0.4
T10 lat -62o - -51o
T26 lat -76o
-0.5
-0.6
_1120_1130_T26
_1120_1130_T10
_HLyγ_T26
_HLyγ_T10
_584_T26
_584_t10
-0.7
-0.8
-0.9
-1.0
800
1000
1200
1400
h_bar (km)
1600
1800
2000
T10_t26_ch4_n2_data_comp_00
comparison of T10 and T26 solar occultation phtotometric reductions
_1120_1130_T26
_1120_1130_T10
_HLyγ_T26
_HLyγ_T10
_584_T26
_584_t10
-1
-2
ln(I/I0)
T10 lat -62o - -51o
T26 lat -76o
-3
-4
-5
800
1000
1200
1400
h_bar (km)
1600
1800
2000
N2_t10_rv1_2_tb_abnd_model_vs_data
T10 N2 solar occultation analysis compared to 150 K model
T10 N2 solar data
T10 N2 _12_8 model
TB λSco N2 _7_2 model
2000
T10:
1500
T∞ = 199 K
h (km)
TB λSco: T∞= 149 K
1000
500
0
12
14
16
18
20
Log([N2] (cm-2))
22
24
26
Possible impact of aerosol
extinction on the solar
occultation reductions
• Extraction of aerosols and hydrocarbons
from the solar occultations is complicated
by the relatively subtle differences in cross
section shape below 1100 A.
• The inferred shape of the tholin cross
section indicates possible significant effects
on the analysis at lower altitudes.
avir_lsco_tholin_abnd
Tholin abundances from λSco and αVir occultations
1100
αVir_Tholin
λSco_Tholin
CH4_model
1000
900
800
h (km)
700
600
CH4 X 10-7
500
400
300
200
100
9
10
11
12
13
14
Log([th] (cm-2))
15
16
17
18
Tholin_khare_xsec_shape
The shape of the Tholin cross section inferred from Khare et al
400
σ (Mb)
300
200
100
0
500
1000
1500
λ (A)
2000
Procedure for inclusion of
aerosols in solar occultation
reduction
• The stellar occultations indicate no measurable
dependence on latitude above 500 km.
• On this basis the EUV stellar occultations will be
used as a control for inferring aerosol cross section
shape below 1100 A.
• These results will then be applied to the solar
occultation analysis to indicate the impact on the
current solar occ results. This is not expected to
change the N2 results at high altitude.