Propellers in Saturn's rings

M. Sremčević G.R. StewartN. AlbersL.W. Esposito

2014 UVIS team meetingJune 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

Bleriot motion: 2nd sine component

Bleriot motion: linear fit around Zet Per

Zet Per R42 occ

Bleriot: Zet Per geometry

Zet Per R42Gap position

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)

Zet Per geometry: 1st hi-res example

Zet Per geometry: 2nd hi-res example

Bleriot: Zet Per R42 occultation

Interpretation: a gap + TWO flanking wakes!

Bleriot in Zet Per: wake model

Stewart (1991) simple wake model R

Bleriot=200m predicts too many wakes!

gap1st wake

2nd wake

Bleriot in Zet Per: wake model

RBleriot

=800m predicts too few wakes!

gap1st wake

2nd wake

Bleriot in Zet Per: wake model

RBleriot

=400m looks like a possible solution

gap1st wake

2nd wake

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= N

occ * 2 * 100km / (2*PI * a

Bleriot) < 4%

● Alp Lyr no planned either: P < 20%

The opaque B ring

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.

Feature in Cassini UVIS

Cassini UVIS locations

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.

Inner B ring example

β Cen R96 propeller in ISS images

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!

Application of T-test: Bleriot (A ring)

Interpretation: a gap + flanking wake!

Summary● The only explanation for observed feature: B ring propeller (embedded body ~ 1.5km)

● Possible dozen to 100 other propeller bodies in B ring.

● Novel explanation of B ring irregular structure: shepherded by propellers!