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.