stellar occultation measurements of particles in saturn's...
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Stellar Occultation Measurements of Particles in
Saturn's RingsRebecca Harbison & Philip Nicholson
August 14, 2014
VIMS Stellar Occultation Measurements of Particles in
Saturn’s A RingRebecca Harbison & Philip Nicholson
August 14, 2014
Stellar Ring OccultationsTr
ansm
issi
on1.0
Keeler Gap A Ring Edge
1.0
0.0
Distance from Saturn (kkm)136.4 137136.6 136.8
Stellar occultation at 2.9 microns.
(VIMS covers 0.9 to 5.2 microns.)
Diffraction Effects
Ring
Star
light
Obs
erve
r
Diffraction Effects
Ring
Star
light
Obs
erve
r
Diffraction Effects
Ring
Star
light
Obs
erve
r
VIMS pixel: 0.5 x 0.25 mrad Movement of star ~ 0.0003 mrad/point !
Diffraction @ 3 microns from 10-cm ring particle: 3 µm/0.1 m ~ 0.03 mrad
θ ~ λ/a
The Model: Particle-Size Distribution
Log
Num
ber D
ensi
ty
mm cm mdm
ring particle size
amin amax (Fixed)
slope = -qWe chose a
truncated power law (num. density
normalized by 𝜏) We are ignoring SG
wakes (for now)
Sample Fit: A Ring Edge
1.00
1.05
0 km 50 km 100 km
Tran
smiss
ion
Distance from Edge
Between 6 and 10 occs return good fits (as measured by chi squared < 1.5 per DOF)
Mean Power Law Index
Distance from Saturn (kkm)
Pow
er L
aw In
dex
2.0
3.0
4.0
𝜏
0.0
2.0
1.0
133 134 136 137
Voyager Radio Occultation (Marouf et al., 1983, Zebker et al., 1985)
Earth-based Stellar (28 Sgr) Occultation (French & Nicholson, 2000)Preliminary Cassini Radio Occultation (Marouf et al., 1998)
Encke Gap Keeler Gap & A Ring Edge
2.0
3.0
4.0* This Work
Mean Power Law Index
Distance from Saturn (kkm)
Pow
er L
aw In
dex
2.0
3.0
4.0
𝜏
0.0
2.0
1.0
133 134 136 137
Voyager Radio Occultation (Marouf et al., 1983, Zebker et al., 1985)
Earth-based Stellar (28 Sgr) Occultation (French & Nicholson, 2000)Preliminary Cassini Radio Occultation (Marouf et al., 1998)
Encke Gap Keeler Gap & A Ring Edge
2.0
3.0
4.0* This Work
The very edge of the A Ring shows a different
particle-size distribution than the
rest of the A ring.
Mean Minimum Particle Size
Distance from Saturn (kkm)
Min
Par
ticle
Siz
e
0.1 mm
1 cm
1m
133 134 136 137
Earth-based Stellar (28 Sgr) Occultation (French & Nicholson, 2000)Preliminary Cassini Radio Occultation (Marouf et al., 1998)
Encke Gap Keeler Gap & A Ring Edge
* This Work
0.1 mm
1 cm
1m
VIMS Solar Occultations (Harbison et al., 2013)
Mean Minimum Particle Size
Distance from Saturn (kkm)
Min
Par
ticle
Siz
e
0.1 mm
1 cm
1m
133 134 136 137
Earth-based Stellar (28 Sgr) Occultation (French & Nicholson, 2000)Cassini Radio Occultation (prelim results, Marouf et al., 1998)
Encke Gap Keeler Gap & A Ring Edge
* This Work
0.1 mm
1 cm
1m
VIMS Solar Occultations (Harbison et al., 2013)
The shaded area is where
light is scattered outside 1
VIMS pixel
Can our model still fit the data?
• Because amin still effects the amount of missing light, we might still be able to measure it when it is small.
• We created simulated data using our models plus Gaussian noise and ran that though our fitting routine to check.
We can’t fit independent parameters in Encke and Keeler Gaps
Min
Par
ticle
Siz
e
1 mm
3.02.5 3.5 4.0 4.50.75
1.50
Red
. Chi
Squ
.
2 mm
4 mm
3 mm
Power Law Index
But… We Don’t Just Have VIMS
• UVIS can also see diffraction from the A Ring; it has both a larger aperture and light scatters at smaller angles.
• VIMS is more sensitive to larger particles and has better signal to noise, but misses mm-sized particles (in stellar occultations).
Conclusions• The edge of the A Ring is unlike any other area of
Saturn’s rings, with power law index of q = 3.55 ± 0.27 and amin of 1.9 ± 0.5 mm
• Particles in the outer A Ring are sufficiently small (less than a few mm) that they scatter outside a VIMS pixel.
• Particle size distributions often can’t be defined by only one instrument.
Future Work• Fit the gaps and ringlets in the Cassini Division and C Ring.
• Care must be taken, as the Cassini Division has a shallow q (so amin is hard to fit)
• Test an optical depth model that includes the effects of self-gravity wakes in the A Ring.
• Preliminary results don’t improve the model fits, a change in optical depth can look like missing light from a small amin.
• Check parameter values in Encke and Keeler Gaps with the location of Pan and Daphnis.