1. XXM Status, Priorities, Plans, and Activities Icy Satellite Science
C. J. Hansen, A. Hendrix
January 2013

2. XXM Status Icy satellite observations executed since last team meeting
Two Dione, one Rhea occultations
ICYPLU (Enceladus)
ICYLON – Mimas, Tethys, Rhea
Upcoming observations in next 6 months
Rev 183 DOY March Rhea flyby for gravity
Rev 188 DOY April Dione occultation
Enceladus plume observations
Mimas PIEs
Low phase observations of Mimas, Enceladus, Tethys and Dione

3. Satellite Flyby Hiatus

4. Solstice Mission Enceladus Flybys
All Enceladus flyby’s in XXM have executed except E21
E20 and E22 are relatively distant flybys on Rev 223 and Rev 228, resp., in 2015

5. Proximal Mission Phase Priorities
Two more potential Enceladus stellar occs in 2016
One is equatorial
22 June :19 Rev 280 Mean anomaly = 116
The other requires a trajectory tweak – occ rayheight is currently 3000 km)
11 March :57 Rev 233 Mean anomaly = 220
The other main priority for UVIS are the Dione stellar occ opportunities
MAG sees mass-loading in the magnetosphere at ~7 kg/sec (Enceladus supplies ~200 kg/sec)
Dione plume won’t go as high or lose as much gas (escape velocity is higher because Dione is larger, gravity higher)

6. Looking for plumes
Does Dione have jets / plumes similar to Enceladus but on a more confined scale?
Evidence for mass-loading, but at a much lower level
Some numbers:
Dione gravity ~ 2x Enceladus
Dione escape velocity = 510 m/sec
Max plume height for v = 500 m/sec = ~540 km
Given lower level of mass-loading we should probably assume velocity is much lower than escape velocity
Plumes won’t go as high
At T = 100 K, thermal velocity = 342 m/sec, which would give a plume height of ~250 km
Height goes as square of velocity

7. The occultation collection Dione
Planned
2013 Rev 188
2015 Rev 217
2015 Rev 220
2016 Rev 242
2016 Rev 252
Acquired
2005 Rev 19
2007 Rev 45
2009 Rev 118
New
2012 Rev 169
2012 Rev 170

8. Locations probed to-date
Circles are ~250 km radius

9. Future Occultations – PIEs In
Rev
Star
Lat
Lon
188
Alpha Lyr
ingress
-33.3
154.1
egress
-54.5
208
217
Alpha Vir
-50.9
53.5
72.3
177.1
220
Alpha Eri
37.6
224.2
-27.5
338.5
242
Sig Sgr
55.8
199
31.5
304.6
252
Zeta Ori
-53.6
35.6
-9.7
211.7

10. Locations to-date plus planned
Circles are ~250 km radius

11. Future Occultations – no PIEs yet – in Proximal and F ring orbits
Rev
Star
Lat
Lon
260
Gam Ori
ingress
-23.9
132.7
egress
58.5
299
264
2.1
65.1
35.6
253.8
279
Beta CMa
16.1
93.6
31.7
286.9
293*
-2.0
107.6
19.2
283.2
* After s/c demise?

12. Locations to-date plus planned
Circles are ~250 km radius

13. Backup

14. HSP Ingress 0.002 integration summed to 1 sec
See from Josh,
0.002 integration summed to 1 sec
Very interesting dip in signal 1 sec before ingress
Probably just due to diffraction…

15. Detection limits for Atmosphere
From stellar occs
O2: 1.3 x 1015 cm-2
From emission features
Atomic oxygen solar scattering: 1.5 x 1013 cm-2
O2 electron impact dissociation: 1.6 x 1014 cm-2
For scale length of 100 km bulk density is 1.5 x 1013 / 105 = 1.5 x 108 m-3
INMS detects 3 x 1010 m-3

16. Backup Slides

17. Events in F-Ring/Proximal Mission Phases
Notes: Compiled from the SOST PIE list, Digit, and Horizons Pink is must have; orange is should have.
Blue are occs – don’t need Rhea
Last four of Enceladus are of equal priority – don’t need all?
Observations of small satellites would give estimate of amplitude of libration

18. System Scans - Summary of Request
June 2012
OR August hr
August hr
August hr
January resume 4 per year 64 hr cadence

19. Hello Helene!
Helene is Dione’s leading co-orbital; one of 4 of Saturn’s moons in the Lagrangian L4 point
Phoebe-size: 36 x 32 x 30 km
Sub-Saturnian side
36x32x30 km (like Phoebe), imaged June 18 from 7000 km

20. XXM Planning Status Requested PIEs for all occultations by:
Dione and Tethys
Look for volatiles being released, supplying E ring
Rhea
Look for rings or other evidence of volatile release
15 occs requested
8 in as occ PIEs
2 “in” SOST but in conflict
4 out
1 not scheduled yet
Iapetus in apoapsis Rev 196
Now looking at occs in proximal orbit

21. Objectives from the Traceability Matrix#
Summary
Comments
IC1a
Identify long-term secular and seasonal changes at Enceladus, through observations of the south polar region, jets, and plumes. Also focus on well-illuminated N. pole to understand why it is so fundamentally different from S. pole. (Opportunity is ideal because illumination conditions are reversed from the primary mission.)
Green = nature of prox. orbit; Red=illumination; Blue=long baseline
IN1a
Test for the presence of an ocean at Enceladus as inferred from plume composition, and constrain the mechanisms driving the endogenic activity by long-term remote sensing of plumes and thermal emission.
IC2a
Determine whether Dione exhibits evidence for low-level activity, now or in recent geological time.
IN2b
Determine whether Tethys contributes to the E-ring and the magnetospheric ion and neutral population.
Get rid of
IN2a
Explore the mechanisms behind the unique thermal and optical properties of Mimas discovered by Cassini with remote sensing, especially the N. pole, at the highest resolution possible.
IN1b
Observe selected small satellites to quantify the movement of Enceladus material through the system, the history and formation of satellites, including collisions/breakup, interaction with ring material as indicated by surface properties/composition, and cratering rates and geologic history deep in the Saturnian system

22. New Data Dione ICYEXO Rev 169
July 2012

23. HSP Occultation of alpha Vir Occultation on 2012-205T03:00
0.002 integration summed to 1 sec
Look at how quickly HSP count rate recovered after egress

24. FUV
5 sec integration summed over all wavelengths, spatial rows 12 to 14
Very interesting dip in signal before ingress, but have to be cautious now due to 5 sec integration time
Nothing on egress

25. FUV Ingress Zooming in…
Probably just due to beginning of occultation starting within time record 96 5sec integration
Ingress lat / lon: / 137.9
Egress lat / lon: / 321

26. Dione ICYEXO Rev 170 12 Aug 2012 Kappa Ori 2012-225
Ingress lat / lon: -32 / 104
Egress lat / lon: 1.5 / 287

27. FUV No signal attenuation in HSP at ingress
5 sec integration summed over all wavelengths, spatial rows 12 to 14
Decrease in signal on ingress and egress appears to be just due to star going behind the limb within the 5 sec integration period
Counts ingress
270 19
Counts egress
300 23
No signal attenuation in HSP at ingress