Titan mass density at 950 km altitude
from EUV/FUV occultation
measurements
D. Shemansky
08/21/06
Results from Voyager and Cassini
UVIS occultations
• The attached figures show the derived densities from the
Voyager solar occultation (published) and the Cassini
UVIS results to date for CH4 and N2 as a function of
latitude of the impact parameter at 950 km. The first figure
shows CH4 and N2 number densities. The second figure
shows mass densities and includes the results reported
from INMS.
• The UVIS results are from two stellar occultations at TB
and the solar occultation egress at T10. The values are
interpolated to 950 km and corrected for integration
interval.
uvis_occ_950_lat
Figure 1
Altitude 950 km
10.0
CH4 X 10-8
N2 X 10-10
Scaled number density (cm -3 )
9.0
8.0
Voy_Verv_ing
7.0
TB_lsco
6.0
5.0
4.0
TB_avir
T10_dat_phot
Voy_Verv_egr
3.0
Voy_Smith_ing
2.0
T10_sol_model
Voy_Verv_ing
Voy_Verv_egr
1.0
0.0
-90
-70
-50
-30
-10
10
Latitude
30
50
70
90
Summary of properties: Figure 1
• The CH4 values from the UVIS TB and T10 occultations
show variability of about 25%. There is no clear indication
of latitude dependence. The Vervack et al results near the
equator straddle the UVIS data. The ingress result at
Voyager is regarded as more reliable. The N2 densities
from Cassini and Voyager indicate no latitudinal variance
within measurement uncertainty. The higher N2 density
from Smith etal is probably caused by lack of correction
for the inclusion of CH4 extinction, and the Vervack et al
result at 2.7 deg latitude should be given higher weight.
mass_dens_lat
Figure 2
Altitude 950 km
16
INMS_TA
14
 (10 -10 kg m-3)
12
INMS_TB
Voy_Smith_ing
10
INMS_T16
8
Voy_Verv_ing
T10_sol_model
Voy_Verv_egr
6
INMS_T5_CA
4
2
0
-100
-80
-60
-40
-20
0
Latitude
20
40
60
80
100
Summary of properties: Figure 2
• The Voyager results near the equator (if the weight of the
values is placed on the more recent Vervack et al analysis)
show very little deviation from the UVIS value at –60o.
The occultation results in the –60o to 2.7o latitude range
fall between the in-situ INMS values above 70o latitude.
My conclusion from these results is that no clear long term
latitudinal dependence of mass density can be inferred
from the current collection of instantaneous derived values,
if the results shown here are taken uniformly seriously.
• Tabulated results from the UVIS occultations are attached
with notes.