Classification of F Ring
Features Observed in
Cassini UVIS Occultations
Bonnie Meinke
UVIS team meeting
January 5, 2010
Overview
• Observations
• Classification
• Comparative observations
• Influence of Prometheus
• Conclusions
UVIS Observations
• 103 F ring occultation profiles in this study
• 39 statistically significant features observed
• Search Method:
– “Poisson Test” identifies statistically significant
features
– “Persistence Test” requires features have  > 0.4
Classification becomes
necessary
• As number of features grows, classification
becomes useful
• Classification based on shape of features in
the occultations independent of location
• 3 Classes:
– Moonlets
– Cores
– Icicles
Classification becomes
necessary
• 3 Classes:
– Moonlets
– Cores
– Icicles
• Note on categories
– Names reflect shape of feature as seen in
occultation profile
– Category names are not intended to identify the
physical object associated with the feature
– Some categories can be broken up into subcategories, but these 3 are the most straightforward way to approach any kind of classification
Typical shape of
occultation profile
Typical Core
“U-shape”
100 km
Two Observed Moonlets
1. “Mittens”
•
•
•
•
•
600 m radial width
~3 km outside core
Comparatively sharp
edges
Stellar signal goes to
background level
Opaque  A possible
consolidated object
139915
139916
139917
Radial distance (km)
139918
Two Observed Moonlets
2. “Sylvester”
•
•
•
•
•
270 m radial width
~5 km inside the
secondary core-like
feature
Near-Sharp edges
Stellar signal goes to
background level
Opaque  A possiblyconsolidated object
29.0
139900
139920
Radial distance (km)
139940
29.2
29.4
29.6
Radial distance (km)
29.8
30.0
Observed: unusuallyshaped core regions
Observed: icicles
Currently 2 subclasses:
Simple
•Intermediate opacity
•Simple, symmetric dip
in signal
Multiple
•Intermediate opacity
•Usually composed of
several adjacent dips in
signal
•Resembles uneven
overlap of simple icicles
-15km
-15km
0
0
15km
15km
Observed: select closer
views
Observed: outliers
Comparative observations
and models
• Profiles of images
– Different objects associated with icicles, w-cores
• Important to note that UVIS f ring feature categories are
not meant to predict the type of object observed (except
for the moonlet), but comment on how it appears in an
occultation
– Imaging doesn’t have same resolution UVIS does,
so we see more detail
• Icicles are usually ~100 m, but these are smeared out in
images to less-opaque 1 km features
– UVIS is only 1D, so images add extra dimension
that helps make sense of what we’re seeing
• For example, the extra dimension can distinguish an
icicle from a W-core
Comparative observations and
models
Comparative observations
and models
• Both image and UVIS occ’n can see similar ~10 kmwide box-cores and 1 km multi-icicles
• Only UVIS sees the small (~100 m) icicle, which
reveals more information about what shapes the ring
takes when perturbed
Distribution of observed
features
Cumulative size
distribution does
not match
Barbara and
Esposito (2002)
prediction
Power law of ~-1,
not bimodal
Observed
BE02
Evolution of classes?
• Evolution of icicles to
moonlets?
– Natural to imagine
moonlets as later
evolutionary stage of
icicle
– Optical depth is
indicator of clumping
because more-densely
aggregated material
blocks more light
– Looser clumps of
material compact to
form a feature that
appears opaque in
occ’n moonlet
• Occurance of
moonlet formation?
– Rare compaction state
or low survival time:
• Icicle is largest class
• Moonlet is smallest
class
– Prometheus may
stimulate clumping
and compaction
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
To Saturn
Trailing
Trailing:
Leading
Prometheus
Prometheus recently
encountered feature and
F Ring
surrounding material
Direction of orbit
Increasing L
Leading:
Prometheus has not
encountered feature in a
long time
(synodic period = 68 days)
Longitudinal separation from
Prometheus
Obvious correlation of
optical depth
with longitude
Pearson correlation
coefficient
r = -0.490
-180
-90
0
90
LFeature - LPrometheus
180
Clump optical depth grows after
Prometheus encounters
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
0
20
40
60
Days since Prometheus encounter
One
synodic
period
68
days
Optical depth largest after
Prometheus encounter
•
•
•
We infer a higher optical depth to mean
denser clumping
Optical depths of features is lower if they
have not encountered Prometheus in a while
We interpret this as evidence that
Prometheus causes material to clump as it
passes nearby
Moonlets and Icicles vs.

QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Pearson correlation
coefficient:
r = -0.406
Clumping of material
is more likely in
the trailing
hemisphere
(shortly after
Prometheus
encounter)
-180
-90
0
90
LFeature - LPrometheus
180
Conclusion
• We classify significant F ring features as 3 general
types:
• Moonlets (2 of 39)
• Cores (9 of 39)
• Icicles (28 of 39)
• Optical depth correlates to the relative position
of Prometheus
• Higher when recently encountered
• Lower when far away
• Influence of Prometheus causes identifiable
features that are likely short-lived compared to a
synodic period
• Thus, we infer that Prometheus stimulates
clumping
Observed: All Features
Features 1-13
Event # Nickname
Occ (Rev)
UTC
1 Mittens
Alp Leo (9)
2005-159T05:06:35.005
2 Tiger 1
126 Tau (8)
2005-139T14:17:08.995
30
3 Tiger 2
126 Tau (8)
Alp Sco (13)
Egress
Alp Vir (34)
Egress
Alp Vir (34)
Egress
Gam Ara (37)
Ingress
Gam Ara (37)
Ingress
Gam Ara (37)
Ingress
2005-139T14:17:09.025
10 Puma
Chi Cen (39)
2007-056T20:13:26.915
180
11 Mitzi
The Ara (41)
2007-078T19:51:47.614
12 Schmutz
Bet Per (42)
13
Zet Oph (26)
4 Pywacket
5 Butterball
6 Fluffy
7 Snowball 1
8 Snowball 2
9 Snowball 3
Radial Width (m)
Peak norm OD
600 >0.8
Radial Distance (km)
Longitude
Distance to core (km)
category
139918
-9.5
4.290 moonlet
1.6
140070
-132.8
0.570 multi-icicle
60
1.2
140070
-132.8
0.660 multi-icicle
2005-232T14:17:45.817
500
0.4
140554
10.0
10.630 simple icicle
2006-337T04:05:24.217
1100
0.7
140162
89.9
-1.400 w-core
2006-337T04:05:24.217
1400
0.8
140162
89.9
0.000 w-core
2007-022T01:40:20.015
40
1.2
140292
-111.6
3.400 simple icicle
2007-022T01:40:20.215
1020
2.2
140291
-111.6
2.660 multi-icicle
2007-022T01:40:21.215
9000 >4.9
140287
-111.6
0.000 box-core
0.4
139909
-101.5
-13.000 simple icicle
180
0.6
140404
59.8
3.000 simple icicle
2007-098T15:47:57.314
60
0.5
140237
46.6
178.400 simple icicle
2006-206T01:19:54.217
260
0.8
139896
0.1
0.000 v-core
Features 14-26
Event # Nickname
Occ (Rev)
UTC
14 Garfield
SAO205839
(57)
Radial Width (m)
Peak norm OD
Radial Distance (km)
Longitude
Distance to core (km)
category
2008-026T14:54:12.997
580
0.7
140376
-173.1
0.000 multi-icicle
15 Heathcliff 1
Alp Ara (63)
2008-092T12:09:56.869
323
1.9
140571
26.1
-1.370 multi-icicle
16 Heathcliff 2
Alp Ara (63)
2008-092T12:09:56.869
1048
2.2
140571
26.1
0.000 multi-icicle
17 Heathcliff 3
Alp Ara (63)
2008-092T12:09:56.869
242
1.4
140571
26.1
1.690 simple-icicle
18 Heathcliff 4
Alp Ara (63)
2008-092T12:09:56.869
81
1.7
140571
26.1
2.500 simple-icicle
19 Fang 1
Eps Cen (65)
2008-110T10:16:43.118
348
2.7
140169
-157.1
-0.320 W-core
20 Fang 2
Eps Cen (65)
2008-110T10:16:43.118
448
2.9
140169
-157.1
0.370 W-core
21 Whiskers 1
Bet Cen (75)
2008-188T20:49:46.078
1078
1.2
139838
173.0
1.391 multi-icicle
22 Whiskers 2
Bet Cen (75)
2008-188T20:49:46.218
193
1.5
139836
173.0
0.310 simple-icicle
23 Whiskers 3
Bet Cen (75)
2008-188T20:49:46.303
234
0.8
139836
173.0
-0.344 simple-icicle
24
Alp Ara (86)
2008-268T17:34:43.138
89
0.4
140490
138.5
25 Silvester
Alp Ara 1 (90) 2008-298T01:39:13.471
268
2.4
139930
139.1
26
Alp Ara 2 (90) 2008-298T06:32:57.934
11913
2.1
139964
37.4
0.000 V-core
-5.016 Double, moonlet*
2.969 W-core
Features 27-39
Event # Nickname
27 Felix
28 Tabby
29 Mr Bigglesworth
30 Socks 1
31 Socks 2
32 Socks 3
33 Socks 4
34 Socks 5
35 Socks 6
36 Socks 7
37 Hobbes
38
39 Fritz
Occ (Rev)
UTC
Radial Width (m)
Peak norm OD
Radial Distance (km)
Longitude
Distance to core (km)
category
Del Cen (98)
Alp Ara (98)
Ingress
Alp Ara (98)
Ingress
Alp Ara (98)
Egress
Alp Ara (98)
Egress
Alp Ara (98)
Egress
Alp Ara (98)
Egress
Alp Ara (98)
Egress
Alp Ara (98)
Egress
Alp Ara (98)
Egress
Eps Cas
(104)Ingress
Eps Cas (104)
Egress
Eps Cas (104)
Egress
2008-359T00:42:29.215
1273
0.7
139994
-166.0
-2.947 simple-icicle
2008-360T06:24:18.826
53
0.5
140320
134.5
0.000 simple-icicle
2008-360T06:24:19.446
318
1.6
140316
134.5
-3.281 simple-icicle
2008-360T11:55:46.925
54
0.5
140530
57.4
-2.780 simple-icicle
2008-360T11:55:47.445
1189
4.2
140533
57.4
0.000 simple-icicle
2008-360T11:55:47.975
54
0.5
140536
57.4
2.830 simple-icicle
2008-360T11:55:48.045
54
0.4
140536
57.4
3.200 simple-icicle
2008-360T11:55:48.155
54
0.4
140537
57.4
3.790 multi-icicle
2008-360T11:55:48.225
54
0.4
140537
57.4
4.170 multi-icicle
2008-360T11:55:48.275
54
0.4
140537
57.4
4.430 multi-icicle
2009-058T10:44:00.272
40
0.9
139278
62.9
2009-058T17:39:09.231
122
2.1
140189
137.4
2009-058T18:09:20.948
44
0.9
147868
141.2
-588.375 simple-icicle
-3.766 W-core
7674.800 simple-icicle