Propellers in
Saturn's rings
M. Sremčević
G.R. Stewart
N. Albers
L.W. Esposito
2014 UVIS team meeting
June 17, 2014
Miodrag.Sremcevic@lasp.colorado.edu
Propellers: traces embedded bodies
N-body
simulation
(w/o SG)
Bleriot: the
largest known
trans-Encke
propeller.
(N1586641169 and
N1586641255, 3km/pixel,
lit geometry)
Bleriot: biggest
A ring propeller
Bleriot: biggest A ring propeller
Gap is nearly 5000km=~2deg in length!
Bleriot: Zet Per R42 occultation
Very dominant gap (even compared to dens. w.)
Bleriot: Zet Per R42 occultation
Interpretation: a gap + flanking wakes!
Bleriot motion: harmonic fit
Zet Per R42 occ
nd
Bleriot motion: 2 sine component
Bleriot motion: linear fit around Zet Per
Zet Per R42 occ
Bleriot: Zet Per geometry
Zet Per R42
Gap position
Zet Per vs Bleriot geometry uncertainty
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Occ dR <= 0.5km (ring edges)
(~100ms down-the-track)
Occ dL???
pessimistic 10 x dR <= 10km
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Bleriot dR <=50m (mean motion very precise)
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Bleriot dL <=10km (max scatter)
st
Zet Per geometry: 1 hi-res example
nd
Zet Per geometry: 2 hi-res example
Bleriot: Zet Per R42 occultation
Interpretation: a gap + TWO flanking wakes!
Bleriot in Zet Per: wake model
2nd wake
1st wake
gap
Stewart (1991) simple wake model
RBleriot=200m predicts too many wakes!
Bleriot in Zet Per: wake model
2nd wake
1st wake
gap
RBleriot=800m predicts too few wakes!
Bleriot in Zet Per: wake model
2nd wake
1st wake
gap
RBleriot=400m looks like a possible solution
Bleriot new detection: Alp Lyr R175
Bleriot new detection: Alp Lyr R175
GAP
Flanking Wake?
P~10%
Alternative statistical test
●
T-test: do two data sets have same mean or not?
Bleriot new detection: Alp Lyr R175
Bleriot @ Alp Lyr R175: UVIS vs VIMS
Bleriot @ Alp Lyr R175: UVIS vs VIMS
What are the odds to obtain UVIS
Occultation of Bleriot?
Zet Per was not planned:
P= Nocc * 2 * 100km / (2*PI * aBleriot) < 4%
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Alp Lyr no planned either:
P < 20%
The opaque B ring
Discovered propellers
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A ring propellers:
1. Double wing (or “S”) shape
2. Near Kepler motion
(trans-Encke propellers deviate by 10-5)
3. Lifetime > few weeks
(distinguish against other phenomena)
→ Criteria for discovery of propellers.
Cassini ISS B ring panoramas (unlit)
Color-scale
represents
brightness I/F.
Mean radial
and azimuthal
profiles
subtracted.
Corrotating longitude is L = λ – λ0 – n (t – t0) where λ is
longitude, λ0 logitude at epoch t0, and n is pattern speed.
Cassini ISS B ring panoramas (lit)
Color-scale
represents
brightness I/F.
Mean radial
and azimuthal
profiles
subtracted.
Cassini UVIS occultations
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●
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Feature at a0=112,921km has pattern speed
n=806.35864o/day (equal to Kepler speed at a0)
106 UVIS occultation cut at a0.
93 cuts have sufficient signal-to-noise
Corrotating longitude L:
L = λ – λ0 – n (t – t0)
λ is longitude, λ0 logitude at epoch t0.
Feature observed in 14 profiles at |L|<22o.
Feature in Cassini UVIS
Cassini UVIS locations
Asymmetric propeller shape
Kingsford Smith (transEncke propeller located at
a=134,337km) is in the
middle of Janus 6:5 ILR (at
134,266km).
Clearly demonstrates
asymmetric shape due to
the presence of the surface
mass density gradients.
N1544813989
phase=64.465deg
lit
b=-56.230deg
phi=-81.293deg
b_sun=-14.617deg
phi_sun=-15.586deg
2.7 x 3.2 km/pix
Object properties
1. At least 5 years lifetime.
2. Moves at Kepler speed (appropriate for its radial
location).
3.The feature is a partial gap (low optical depth in UVIS
occultations).
4. Radial offsets in UVIS are consistent with an
asymmetric propeller shape.
What it is not:
- B ring spokes or impact ejecta clouds [why 1. or 3.?]
- Plasma effects (Mach cones) [why 3. or 4.?]
- Resonances with outer moons or general resonant
effects: their pattern speed is significantly smaller than
Kepler speed. And there are no resonances within 1000km.
●
Inner B ring example
β Cen R96 propeller in ISS images
Another inner B ring example
N1496890652
tau=1.6
(2.259,7.188)km/pix
r=96710.455km
phase=11.162deg lit
More inner B ring examples (lit geom.)
N1504581211
N1536501056
N1541711536
N1541711708
Statistical significance
Voyager Uranus data & Cassini F ring: M test
based on Poisson statistics (J. Colwell)
●
But: M test is not applicable to main rings
(microstructure changes significantly statistics, e.g.
excess variance and autocorrelations)
●
Additional problems with M test:
- How to determine the background and its
uncertainty
- What is appropriate sampling of data, sliding bins
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Alternative statistical test
●
T-test: do two data sets have same mean or not?
Issues:
- T-test is devised for normal distribution
- Variable background will hide useful signal
(possible solution: partial de-trending)
- Sqrt9 compression (solution: binning data)
● Proposed significance level: p=1e-4
● Null test also needed (compare background with
background)
●
Application of T-test: B ring object
P=1e-20
P=0.5
P=1e-6
P=3e-3
Interpretation: a gap + flanking wake!
Application of T-test: Bleriot (A ring)
Interpretation: a gap + flanking wake!
Summary
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The only explanation for observed feature:
B ring propeller (embedded body ~ 1.5km)
Possible dozen to 100 other propeller bodies in B
ring.
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Novel explanation of B ring irregular structure:
shepherded by propellers!
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