Ring Spectroscopy and
Photometry
Todd Bradley
June 5, 2013
Overview
• Retrieved ring particle albedo, AB, at a wider
range of wavelengths than before
• Looked into very short wavelength AB
• Investigated composition using a twocomponent Triton tholin model
• Investigated different types of mixtures
2
Review
• Found ring particle albedo and ring particle
asymmetry parameter at two wavelengths
using a single scattering Chandrasekhargranola bar model
• Composition may be retrieved using ring
particle albedo but albedo at more
wavelengths is needed
3
Example of spectra from central A ring and C ring
Long wavelength
Short wavelength
4
Model discretely averaged I/F with Chandrasekhar-granola bar model

I
o
 AB * P *
1  exp  n / exp  n / o 
F
4  o 


I/F at 180 nm
Also did I/F at 155 nm
(not shown)
Need to do this for
more wavelengths and
shorter wavelengths if
possible
5
Comparison of Ly-alpha Wings to Rings Spectra
• Lyman-alpha from IPH observation
• Scaled peak Lyman-alpha line to that from B ring
6
Temporal Variation of Solar Spectra
7
Compositional Modeling
Cuzzi and Estrada (1998) used a Van de Hulst to relate AB to ϖ
AB
1 S 1 0.139S 


1 1.17S
where S 
1 SI
  Qs  Se  1 SE 

1 SI 



1 r exp 
1 
1  g
g = regolith grain
anisotropy parameter
n 12  k 2
4
SE 

0.05
,
S

1

I
2
n  12  k 2
nn  1
 where r  1 
1
    2d /3
ri  exp  
   2di /3
i
i
i
 /   
 /   
n and k are the optical constants, d is the grain diameter, α = 4πk/λ and ς is the internal
scattering coefficient
8
Try Two Different Two Component Mixtures
Water Ice and Triton Tholin (irradiation of 0.999:0.001 N2:CH4;
bulk substance: C3H5N4
Linearly
add
optical
constants and get one
single scattering albedo;
a single regolith grain is a
well mixed combination
of components (Cuzzi
and Estrada, 1998)
ni  1 fe nio  fenie
nr  1 fe nro  fenre
Separate grains for each
component. Calculate single
scattering albedo for each.
Add together.
 H O   X

,
1 
2
 x  H o DH o
=
 H o  x Dx
2
2
2
μ = bulk density, ρ = solid density, D = size
Assume ρ is same for both (Hapke, 1993)
9
Single Grain, Linearly Add Optical Constants
• Free parameters:
– grain size
– asymmetry parameter
– water ice fraction
• Fit model to FUV data points then see how that fits
to long wavelength data
• Fit model to FUV + long wavelength data
• Single grain size for all wavelengths
• For following figures, red stars are AB and black
diamonds are model results
10
Model fit only to FUV
data points
Model fit to FUV + VIS
data points
11
Model fit only to FUV
data points
Model fit to FUV + VIS
data points
12
Model fit only to FUV
data points
Model fit to FUV + VIS
data points
13
Model fit to 338-862 nm only
14
Separate Grain Mixture
• Free parameters:
– water ice grain size
– tholin grain size
– asymmetry parameter
– water ice fraction
• Fit model to FUV data points then see how that
model fits to long wavelength data
• Fit model to FUV + long wavelength data
15
Model fit only to FUV
data points
Model fit to FUV + VIS
data points
16
Model fit only to FUV
data points
Model fit to FUV + VIS
data points
17
Model fit only to FUV
data points
Model fit to FUV + VIS
data points
18
Model fit to 338-862 nm only
19
Different Mixtures for Regolith Grains
Hapke mixture
Cuzzi mixture
Water ice
grains
Contaminant
grains
Water ice grains
Contaminant
grains
Water ice and contaminant are
thoroughly mixed within each grain
Combine fractions of
constants then calculate
single scatter albedo
optical
overall
Water ice and contaminant grains are
separate with their own single
scattering albedo but are thoroughly
mixed within the regolith.
Compute single scattering albedo for
each component and then combine
fractions of single scattering albedos
to get overall single scattering albedo 20
Conclusions and Discussion
• Lots of variables (grain size(s), asymmetry parameter,
fraction of water ice, contaminant, number of
contaminants, mixture)
• Mixtures may look different depending on
wavelength regime
• Also may need to use different grain sizes for
different spectral regimes
• In addition to new spectroscopic lab measurements,
we need to make laboratory measurements of
mixtures for different size scales of water ice grains
21