Ring Spectroscopy and
Photometry
Todd Bradley
January 6, 2012
Outline
• Analysis of SOI observations
– Look for halos in UVIS data
– Determine albedo and compare to lit side
• Investigation of spectral dip at 184 nm
• Comparison of rings to icy moons
– Choose similar phase angles
– Take ratios above and below absorption feature
2
Nicholson et al 2008
Halo surrounding
Janus 5:4 and Mimas
5:3 density waves
3
SOI data
• Taken on July 1, 2004
• Unlit side of the rings
• Typical pixel size projected on the ring plane ~
200 km
• Phase angle ~ 60°
4
SOI observations
interpolated to 100
km resolution
5
SOI observations
interpolated to 220
km resolution
6
Albedo and Relative contamination
from SOI data
• Previously used observations of lit side over
wide range of phase angles to retrieve ring
particle albedo and phase function
• Bin SOI data to previously defined regions in A
ring
• Use retrieved phase function along with SOI
I/F to compute ring particle albedo
• Take ratio of I/F above and below absorption
edge
7
I/F from B3 region
8
Model discretely averaged spectra using
Chandrasekhar-granola bar model
I
o
 A* P*
1  exp n /  exp n / o 

F
4  o 
T  expn / 
S /W  H /W sin   


wake
To  expn / o 
 cot B 
S /W  1
S /W  H /W sin 


'


cot
B

o
wake
S /W  1
expgap / 
exp
gap
/ o 
9
Granola bar model
H
W
S
10
Assume power law phase function
P  Cn    
n
1
g
2
 P cos  sin d

0
Minimize D

n
1
2
D  (di  mi )
n i1
Where i = 1 to n is over a range of phase angles
11
12
Bond albedo from lit side observations
13
Albedo Comparison (180 nm)
14
Ratio above and below absorption edge
Phase angle of SOI unlit
observation ~ 60°
Phase angle of Lit side
observation ~ 30°
15
Investigate spectral dip at 184 nm
174 nm
184 nm
155 nm
16
Ratio of 174 / 155 nm
17
Ratio of 174 / 184 nm
18
19
20
21
Comparison of rings to icy moons
• Rings have finite optical depth, volume filling
factor between 0 and 1, wakes
• Cannot compare I/F directly because of wakes,
unknown volume filling factor
• Ratio I/F longward of the absorption edge to
I/F shortward of the absorption edge
• Choose similar phase angle observations
22
Outer A ring
Saturn
23
24
FUV2007_202_08_48_10_UVIS_048EN_ICYLON001_ISS
25
26
Iapetus
Contour of the brightness
averaged from 0.1750.185 m from UVIS
observations of Iapetus
made on September 10,
2007. The three boxed
regions represent areas
where reflectance spectra
have been averaged (dark
(leading
hemisphere),
intermediate
(trailing
hemisphere), and bright
terrain.
27
Iapetus
Reflectance for the three
boxed regions along with
reflectance spectra from the
A, B, and C rings at a similar
phase angle as the Iapetus
observations. The ring
reflectance was multiplied
by 10.
28
29
Brightness ratios of rings and icy moons
30
Summary
• Difficult to say if we can see halos
• Albedo computation for unlit side A ring is
consistent with lit side derivations
• Have to think about Chandrasekhar-granola
bar model for unlit side
• Small spectral dip at 184 nm appears to be
related to water ice
• Ratio of ring brightness is similar to Iapetus
bright and intermediate terrain
31