Propellers in Saturn's rings -...
Transcript of Propellers in Saturn's rings -...
Propellers in Saturn's rings
M. Sremčević G.R. StewartN. AlbersL.W. Esposito
2014 UVIS team meetingJune 17, 2014
Propellers: traces embedded bodies N-body
simulation (w/o SG)
Bleriot: the largest known trans-Encke propeller.
(N1586641169 and N1586641255, 3km/pixel, lit geometry)
Zet Per vs Bleriot geometry uncertainty
● Occ dR <= 0.5km (ring edges) (~100ms down-the-track)
● Occ dL??? pessimistic 10 x dR <= 10km
● Bleriot dR <=50m (mean motion very precise)
● Bleriot dL <=10km (max scatter)
Bleriot in Zet Per: wake model
Stewart (1991) simple wake model R
Bleriot=200m predicts too many wakes!
gap1st wake
2nd wake
What are the odds to obtain UVIS Occultation of Bleriot?
● Zet Per was not planned:P= N
occ * 2 * 100km / (2*PI * a
Bleriot) < 4%
● Alp Lyr no planned either: P < 20%
Discovered propellers
● A ring propellers:1. Double wing (or “S”) shape2. 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)
Corrotating longitude is L = λ – λ0 – n (t – t
0) where λ is
longitude, λ0 logitude at epoch t
0, and n is pattern speed.
Color-scale represents brightness I/F.
Mean radial and azimuthal profiles subtracted.
Cassini ISS B ring panoramas (lit)
Color-scale represents brightness I/F.
Mean radial and azimuthal profiles subtracted.
Cassini UVIS occultations
● 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 – λ
0 – n (t – t
0)
λ is longitude, λ0 logitude at epoch t
0.
● Feature observed in 14 profiles at |L|<22o.
Asymmetric propeller shape Kingsford Smith (trans-
Encke 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.
N1544813989phase=64.465deg litb=-56.230degphi=-81.293degb_sun=-14.617degphi_sun=-15.586deg2.7 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.
Another inner B ring example
N1496890652tau=1.6(2.259,7.188)km/pixr=96710.455kmphase=11.162deg lit
●b=-18.201deg●phi=-87.994deg●b_sun=-21.456deg●phi_sun=-99.348deg●
●N1586641169 and N1586641255, 3km/pixel, lit geometry)
More inner B ring examples (lit geom.)
N1504581211N1504581211 N1536501056N1536501056
N1541711536N1541711536
N1536501056N1536501056
N1541711708N1541711708
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
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=0.5
P=1e-20
P=1e-6 P=3e-3
Interpretation: a gap + flanking wake!