1. XM Status and Plans, XXM Activities Icy Satellite Science
C. J. Hansen, A. Hendrix, D. Shemansky
8 June 2009

2. XM Status, Priorities, Plans
To a major extent XM planning is complete for icy satellites
Still polishing icy satellite flyby timelines
Rev Rhea 1 March 2010
Rev Dione 6 April 2010
Close Enceladus flybys on revs 120, 121, 130, 131
Rev November 2
Rev November 21
Rev April 28
Rev May 18
Rev 120 has high resolution UVIS observation of plume with Saturn in the background
Will acquire solar occ on Rev 131
Continuing to collect icy satellite observations: ICYLONs, ICYMAPs, ICYECLs, ICYATMs, ICYOCCs…

3. Enceladus Flybys
March 12, 2008

4. Rev 120
Enceladus’ Plume against Saturn

5. Solar Occultation Rev 131
We have the opportunity to observe an occultation of the sun on Rev 131
New results are in the EUV, which gives us access to a different wavelength range than the FUV
The big scientific payoff is the chance to definitively detect / measure nitrogen in the plume - important for models of chemistry-driven dynamics in the interior

6. SM-7a
Tweaked a little closer?

7. Nitrogen in Enceladus’ Plume? Why is that important to know?
Evidence for liquid water in the interior
Compositional evidence for a hot interior: ammonia (Matson et al, 2007)
Existence of N2 implies decomposition of NH3
Titan data (lack of argon) implies NH3 is primordial material from Saturn’s disk
Decomposition of NH3 requires temperatures well above the melting point of water
Clathrate reservoir hypothesis (Kieffer et al, 2006)
CH4 and N2 trapped in water ice clathrates could decompose explosively when exposed to near-vacuum
Advective heat transfer, redistribution of latent heat is driving source of energy

8. XXM Status Requested PIEs for all occultations by: Dione and Tethys
Look for volatiles being released, supplying E ring
Rhea
Look for ring or other evidence of volatile release
SOST accepted all of these as PIEs, now going through integration process with other TWTs
Enceladus flyby allocations
Requested assignment of Enceladus flyby on 19 October 2011 for stellar occultation
No contest

9. SOST Occultation PIEs

10. XXM SM7a Tweak
Opportunity for dual stellar occ by Enceladus’ plume tweaked in,
19 October 2011, epsilon Orionis (blue) and zeta Orionis (white)

11. Backup Slides

12. Observation Suite
System scans show variable atomic oxygen in Saturn’s system
Localized scans across Enceladus give higher resolution picture of the distribution of oxygen
Execute routinely to monitor Enceladus’ activity remotely

13. XM Enceladus Flybys Executed in 2008: Enceladus Rev 80 August 11 50 km
Enceladus Rev 88 October 9 25 km
Enceladus Rev 91 October km
UVIS collected high resolution surface maps, but no more stellar occs

14. XM Status, Priorities, Plans
To a major extent XM planning is complete for icy satellites
Still polishing icy satellite flyby timelines
Collecting icy satellite observations: ICYLONs, ICYMAPs, ICYECLs, ICYATMs, ICYOCCs…
Amanda is working on reflectance spectroscopy, and what that tells us about E ring coatings on the surfaces of inner satellites, age of ice around the tiger stripes, thermal migration of volatiles on Iapetus
Candy is working on oxygen in Enceladus’ vicinity and whether that can be used as a proxy for plume activity, and analyzing stellar occs
Un-analyzed data continues to accumulate

15. Solar Occultation
How well can UVIS measure N2 with a solar occultation?
Abundance of H2O measured by UVIS = 1.5 x 1016 cm-2
Mixing ratio of mass 28 in the INMS experiment at Enceladus was [M28]/[H2O]=0.036
A solar occultation has been simulated for our H2O optical depth assuming a commingled mixture of H2O and N2 in the spectral region of the H Ly line
The ability to measure N2 in a mixing ratio of [N2]/[H2O] = is indicated, for an abundance of N2 = 1 x 1014 cm-2
How do we know?
N2 was measured above the exobase in the UVIS T10 solar occultation observation using the measured extinction of the sol H Ly line by the N2 b(3,0) band.

16. Solar Occultation

17. Solar Occultation Reduced amount of water vapor simulation
(Further from source so need to take this into account)

18. Solar Occultation
This is a simulation of the results we could get from a solar occultation by Enceladus’ plume
UVIS can detect N2 absorption near 972 Ang
Mixing ratio for blue curve, showing clear absorption, is 0.05, close to the INMS derived value of [M28/H2O] = 0.036
Green curve shows likely detection limit with an order of magnitude less nitrogen, or [M28/H2O] = 0.005