Using Stellar Occultations to
look for Plumes from Dione
Status Report
C. J. Hansen
June 2013
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
The occultation collection
Dione
Acquired
Planned
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•
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2005 Rev 19
2007 Rev 45
2009 Rev 118
2012 Rev 169
2012 Rev 170
New
• 2013 Rev 188
2015 Rev 217
2015 Rev 220
2016 Rev 242
2016 Rev 252
Dione ICYEXO
Rev 188
28 April 2013
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Alpha Lyrae (Vega)
2013-118
Ingress lat / lon: -33.3 / 154.1
Egress lat / lon: -54.5 / 208
Pictures from ISS
Rev 188 Occultation
HSP
Some signal attenuation in
HSP at ingress?
Not really certain
FUV
•
Main pixel with star has interesting
structure
•
Star primarily in one pixel, but signal
also in adjacent pixel
•
If structure is real, should show up there
too
•
But it doesn’t
•
Also, no sign of water in spectrum
Locations probed previously
Circles are ~250 km radius
Future Occultations – PIEs In
Rev
Star
188
Alpha Lyr
Now
Executed
217
220
242
252
Alpha Vir
Alpha Eri
Sig Sgr
Zeta Ori
Lat
Lon
ingress
-33.3
154.1
egress
-54.5
208
ingress
-50.9
53.5
egress
72.3
177.1
ingress
37.6
224.2
egress
-27.5
338.5
ingress
55.8
199
egress
31.5
304.6
ingress
-53.6
35.6
egress
-9.7
211.7
Locations to-date plus planned
Circles are ~250 km radius
Future Occultations – no PIEs
Yet – in Proximal and F ring
orbits
Rev
Star
260
Gam Ori
264
279
293*
Gam Ori
Beta CMa
Beta CMa
* After s/c demise?
Lat
Lon
ingress
-23.9
132.7
egress
58.5
299
ingress
2.1
65.1
egress
35.6
253.8
ingress
16.1
93.6
egress
31.7
286.9
ingress
-2.0
107.6
egress
19.2
283.2
Locations to-date plus planned
Circles are ~250 km radius
Backup
HSP Ingress
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•
•
0.002 integration summed to 1 sec
Very interesting dip in signal 1 sec before ingress
Probably just due to diffraction…
Dione
Goal: Search for volatiles - MAPS instruments detect massloading of magnetosphere near Dione, similar to Enceladus but
down more than an order of magnitude? Look for small plumes.
May be very localized.
Result: No evidence for gas absorption (yet)
Occultations Observed
Observation start
Orbit 19
2005-358T16:40
Orbit 45
2007-147T12:12
Orbit 118
2009-263T17:00
Orbit 19
Star
66 Oph
alpha Leo
epsilon Cma
Ingress lat/long
-53.8 / 148.1
48.5 / 94.9
11.5 / 2.5
Egress lat/long
-52.4 / 327.9
61.9 / 304.3
26.3 / 163.1
Dione
Counts / 100 msec
Orbit 45
Orbit 118
New Data
Dione ICYEXO
Rev 169
July 2012
HSP
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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
FUV
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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
FUV Ingress
•
•
Zooming in…
Probably just due to beginning of occultation starting within time record 96 5sec
integration
•
•
Ingress lat / lon: -4.7 / 137.9
Egress lat / lon: -4.5 / 321
Dione ICYEXO
Rev 170
12 Aug 2012
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•
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Kappa Ori
2012-225
Ingress lat / lon: -32 / 104
Egress lat / lon: 1.5 / 287
FUV
•
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
Counts egress
250
255
260
265
270
300
305
310
315
320
34900
34674
34557
21938
19
23
29221
34869
35103
34559
No signal attenuation in
HSP at ingress