1. Cassini UltraViolet Imaging Spectrograph (UVIS) Observations of Enceladus’ Plume
C. J. Hansen, L. Esposito, A. Hendrix, J. Colwell, D. Shemansky, W. Pryor, I. Stewart, R. West
6 January 2006

2. Enceladus’ youthful surface implies active geological processes
Known since Voyager flyby
Prompted us to plan stellar occultations to search for tenuous atmosphere and/or eruptive plumes (see UVIS SSR paper)
ISS Color Mosaic Rev 11

3. Enceladus Occultation Geometries
February
Lambda Scorpii Occultation
July
Gamma Orionis Occultation
Egress
Ingress

5. Detection of Plume: High Speed Photometer (HSP) vs. Time
Clear indication of attenuation of signal during occultation ingress; egress is signature of HSP warmup
Start to sense atmosphere ~24 sec prior to hard limb occultation, maybe as much as 30 sec (FUV)
Ray height at –24 sec is ~ 155 km

6. FUV Data: Comparison of Occulted to Unocculted Spectra
FUV configuration:
spectral channels binned by 2 (512 spectral channels from nm to 191 nm)
low resolution slit width
5 sec integration time
full spatial resolution
Time record 33, the last full 5 sec integration prior to ingress, shows the deepest absorption. The ray altitude above Enceladus’ surface corresponding to time record 33 ranged from 30 to 7 km.
Clear signature of an atmosphere is present – both relatively narrow and broad absorption features

7. Composition of Plume is Water Vapor
I=I0 exp (-n*)
I0 computed from 25 unocculted samples
n = column density
 = absorption cross-section
The absorption spectrum of water (pink line) is shown compared to Enceladus’ plume spectrum (I/I0) for a column density of n = 1.5 x 1016 cm-2

8. What absorption features are real, what is noise?
To visualize the presence of noise relative to real spectral features I0 was computed for the unocculted star from the average of 25 spectra. Then each individual unocculted spectrum was ratioed to the average I0, shown in blue. The occulted spectra were then ratioed to the average I0, shown in red tones. At short wavelengths the absorption features < 0.9 are judged to be real. The shape of the broad absorption at longer wavelengths is also apparent.

9. Statistical Analysis using FITS

10. Statistical Analysis using FITS

11. CO Limit Used absorption cross-sections from Eidelsberg, 1992
“Require” 10% dip in signal for positive detection
I/I0 = 0.9 = exp (-nα)
for α = 820 x at Å
-> n = 1.3 x 1014 cm-2 upper limit

12. Localization of Enceladus’ Plume (Not a global atmosphere)
Ray intercepts were at latitude / west longitude:
15 / 300 Lambda Sco ingress (non-detection)
-31 / 141 Lambda Sco egress (non-detection)
-76 / 86 Gamma Ori ingress
-0.2 / 28 Gamma Ori egress (non-detection)
Consistent with localized plume or jet:
Enceladus’ gravity insufficient to hold gravitationally bound sputtered atmosphere
Also, the combination of other Cassini data sets are consistent with a plume of water vapor coming from Enceladus’ “Tiger Stripes” driven by the hot spot at the south pole detected by CIRS

13. CDA Peak
INMS Peak

14. This image shows the surprise that startled Cassini scientists on the composite infrared spectrometer team when they got their first look at the infrared (heat) radiation from the south pole of Saturn's moon Enceladus. There is a dramatic warm spot centered on the pole that is probably a sign of internal heat leaking out of the icy moon. The data were taken during the spacecraft's third flyby of this intriguing moon on July 14, 2005.
Based on data from previous flybys, which did not show the south pole well, team members expected that the south pole would be very cold, as shown in the left panel. Enceladus is one of the coldest places in the Saturn system because its extremely bright surface reflects 80 percent of the sunlight that hits it, so only 20 percent is available to heat the surface. As on Earth, the poles should be even colder than the equator because the sun shines at such an oblique angle there.
The right hand panel shows a global temperature image made from measurements of Enceladus' heat radiation at wavelengths between 9 and 16.5 microns. Cassini made the observation from a distance of 84,000 kilometers (52,000 miles) on the approach to Enceladus, and the image shows details as small as 25 kilometers (16 miles). Equatorial temperatures are much as expected, topping out at about 80 degrees Kelvin (-315 degrees Fahrenheit), but the south pole is occupied by a well-defined warm region reaching 85 Kelvin (-305 degrees Fahrenheit). That is 15 degrees Kelvin (27 degrees Fahrenheit) warmer than expected. The composite infrared spectrometer data further suggest that small areas of the pole are at even higher temperatures, well over 110 degrees Kelvin (-261 degrees Fahrenheit). Evaporation of this relatively warm ice probably generates the cloud of water vapor detected above Enceladus' south pole by several other Cassini instruments.
The south polar temperatures are very difficult to explain if sunlight is the only energy source heating the surface, though exotic sunlight-trapping mechanisms have not yet been completely ruled out. It therefore seems likely that portions of the polar region are warmed by heat escaping from the interior of the moon. This would make Enceladus only the third solid body in the solar system, after Earth and Jupiter's volcanic moon Io, where hot spots powered by internal heat have been detected.

15. This image shows the warmest places in the south polar region of Saturn's moon Enceladus. The unexpected temperatures were discovered by Cassini's composite infrared spectrometer during a close flyby on July 14, The image shows how these temperatures correspond to the prominent, bluish fractures dubbed "tiger stripes," first imaged by Cassini's imaging science subsystem cameras. Working together the two teams were able to pinpoint the exact location of the warmest regions on Enceladus.
The composite infrared spectrometer instrument measured the infrared heat radiation from the surface at wavelengths between 9 and 16.5 microns within each of the 10 squares shown here. Each square is 6 kilometers (4 miles) across. The color of each square, and the number shown above it, describe the composite infrared spectrometer's measurement of the approximate average temperature of the surface within that square.
The warmest temperature squares, at 91 and 89 degrees Kelvin (minus 296 and minus 299 degrees Fahrenheit), are located over one of the "tiger stripe" fractures. They contrast sharply with the surrounding temperatures, which are in the range 74 to 81 degrees Kelvin (minus 326 to minus 313 degrees Fahrenheit). The detailed composite infrared spectrometer data suggest that small areas near the fracture are at substantially higher temperatures, well over 100 degrees Kelvin (minus 279 degrees Fahrenheit). Such "warm" temperatures are unlikely to be due to heating of the surface by the feeble sunlight striking Enceladus' south pole. They are a strong indication that internal heat is leaking out of Enceladus and warming the surface along these fractures. Evaporation of this relatively warm ice probably generates the cloud of water vapor detected above Enceladus' south pole by several other Cassini instruments. Scientists are unsure how the internal heat reaches the surface. The process might involve liquid water, slushy brine, or soft but solid ice.
The imaging science subsystem image is an enhanced color view with a pixel scale of 122 meters (400 feet) that was acquired at the same time as the composite infrared spectrometer data. It covers a region 125 kilometers (75 miles) across. The spacecraft's distance from Enceladus was 21,000 kilometers (13,000 miles). The broad bluer fractures that can be seen running from the upper left to the lower right of the image are 1 to 2 kilometers (0.6 to 1.2 miles) wide and more than 100 kilometers (60 miles) long. The fractures are thought to be bluer than the surrounding surface because coarser-grained ice (which has a blue color just as thick masses of ice, like glaciers and icebergs, do on Earth) has been exposed in the fractures. The color image was constructed using an ultraviolet filter (centered at 338 nanometers) in the blue channel, a clear filter in the green channel, and an infrared filter (centered at 930 nanometers) in the red channel.

16. Structure of the Plume
The increase in water abundance is best fit by an exponential curve – a comet-like evaporating atmosphere (1/R2) does not fit the data well, nor do global hydrostatic cases
The best fit scale length is 80 km

17. Statistical Analysis using FITS

18. Neutral Species in Saturn’s System
The Saturnian system is filled with the products of water molecules:
H detected by Voyager
OH detected by HST
Atomic Oxygen imaged by UVIS

19. “Search for the Missing Water Source”1
Neutral Species
Water and its products are lost from the system by collisions, photo- and electron- dissociation and ionization
Estimates of required re-supply rates, water molecules/sec:
2.8 x Shemansky, et al.
3.75 x Jurac, et al.
Jurac and Richardson
2 x Shemansky, et al.
E Ring
Saturn’s E ring is also subject to erosion and loss due to sputtering of water from the surface of the E ring’s dust particles and collisions of particles with Saturn’s moons
Estimate of required re-supply rate:
1 kg / sec Juhasz and Horanyi

20. Estimation of Water Flux from Enceladus
S = flux
= N * h2 * v
= n/h * h2 * v
= n * h * v
Where
N = number density / cm3
h2 = area
v = velocity
n = column density measured by UVIS
Estimate h from plume dimension, = 80 (from scale length) or 175 km (from horizontal distance traversed)
Estimate v from thermal velocity of water molecules in vapor pressure equilibrium with warm ice (41,200 at 145 K or 46,000 cm/sec at 180 K – note that escape velocity = 23,000 cm/sec) h v
S = 1.5 x 1016 * (80 or 175) x 105 * (41 or 46) x 103 = 0.5 to 1.2 x H2O molecules / sec
= 150 to 360 kg / sec

21. Summary of Results
The occultation of Gamma Orionis July 14 observed by UVIS during the third close Enceladus flyby has led to the following results:
Confirmation of the existence and composition of Enceladus’ plume
Water vapor fits the absorption spectrum best
Near surface abundance (line of sight) = 1.5 x 1016 cm-2
Upper limit for CO abundance ~ 1% of water column density
Localization of Enceladus’ plume
Enceladus’ “atmosphere” is not global, it has only been detected near the south pole
Gas absorption features were not detected on the ingress or egress of the Lambda Sco occultation in February 2005
Gas absorption features were detected on the ingress but not the egress of the gamma Orionis occultation
The water budget derived from the water vapor abundance is adequate to supply most if not all of the OH detected by HST, atomic oxygen in the Saturn system detected by UVIS, and to re-supply Saturn’s E ring

22. Conclusion
Using simple, conservative modeling of the water vapor flux from Enceladus’ plume we are able to conclude that Enceladus is a probable source of most (if not all) the water required to
Supply the neutrals in Saturn’s system
Re-supply the E ring against losses
We have found the “steaming” gun
A decades-long mystery has been solved

23. Backup Charts

24. Structure of the Atmosphere
The drop in signal is best fit by an exponential curve – a comet-like evaporating atmosphere (1/R2) does not fit the data well, nor do global hydrostatic cases (ruled out by our data anyway, but shown here for illustration).