1
D. E. Shemansky
*
, J. Tew Hallett
*
, X. Liu
*
, L. W. Esposito
χ
, A. I. F. Stewart
χ
, & The
Cassini UVIS team
*University of Southern California
χ
University of Colorado
The microscopic particle populations in the magnetospheres of Saturn and Jupiter are in distinctly different physical states. The Jupiter system contains a fully ionized energetic plasma, while at Saturn the dominant magnetospheric species are neutral. An extensive distribution of atomic hydrogen was found in the Saturn magnetosphere in data from the Voyager Ultraviolet Spectrometer (UVS) experiment in 1980
1
, which is asymmetric and fixed in local time
2
. Following a prediction based on inferred plasma cooling rates
2
, the neutral hydroxyl radical was discovered near the orbits of the inner satellites in 1992
3
. We report the discovery of atomic oxygen in photometric images of the Saturn magnetosphere from the Cassini Ultraviolet Imaging Spectrometer experiment (UVIS)
4
, along with definitive observations of the of the atomic hydrogen distribution. Based on the large abundance of OI, the very different states of the Jupiter and Saturn magnetospheres is explained as a plasma quenching property of water chemistry.
The probable source for OI and OH, estimated > 8.5 X 10
27
s
-1
, is located in processes on or near the satellites Mimas, Enceladus and Tethys.
The Cassini NASA/ESA spacecraft was launched October 7, 1997, and began the Observatory Period of the prime mission program December 25, 2003 from a range
~85 Mkm. Cassini regularly produces spectral imaging mosaics of the Saturn system

2 using the UVIS
4
spectrographs. The photometric images reported here are derived from data gathered December 25, 2003 through January 6, 2004, and February 6, through
March 14, 2004. These images, have spatial information in 64 1 mr (milli-radian) pixels
(px) and spectral information in 1024 0.25 mr px, in the EUV from 56 to 118 nm, and in the FUV from 112 to 191 nm, accumulated simultaneously in all detector pixels. For these observations the spatial resolution at Saturn was 1 to 1.6 Saturn radii (R
S
). Mosaic exposures were 200 s to 1000 s at successive slit positions in a mapping process covering approximately 30 X 50 R
S
. This Letter discusses analysis of Saturn magnetosphere data. Two prominent FUV spectral features originate in the magnetosphere, the atomic hydrogen Rydberg resonance transition at 121.57 nm (H
Ly
α
) , and the atomic oxygen resonance multiplet at 130.4 nm. These two emission features are stimulated mainly in fluorescence by solar radiation. Excitation by plasmasphere electrons is very small in comparison
2,3
. Images from the 2004 DOY 37 –
74 period accumulated ~40000 s of exposure per spatial element, with resolution ~1
R
S
, and are thus a ~1 month average of the magnetosphere emission.
Figure 1 gives the Saturn system viewing geometry seen from Cassini. The subspacecraft latitude is 13.4
o
south. The solar flux geometry is indicated by a simulated shadow thrown by Saturn across the system. The Figure shows a torus distributed roughly as inferred from the image of the atomic oxygen emission. The solar phase of the observation line of sight (LOS) is 62.5
o
. The sub-solar point on the planet is at a latitude of 21.3
o
south. When UVIS views Saturn, we recognize south polar auroral emission in hydrogen, dayglow, and solar reflection spectra.
Figure 2 shows the Saturn system in H Ly
α emission, covering a range of ~
±
30
R
S
in the north-south (N-S) dimension and ~
±
30 R
S
east-west (E-W). The distribution is asymmetric as in the Voyager observations
2
with a more extended distribution on the dark side of the planet. There is no indication of a gas torus as may be expected from a

3 satellite source such as Titan. The most general characteristic of the distribution is increasing density leading into the planet. The peak appearing in the orbital plane LOS abundance on the dark side at ~3 R
S
is not present on the dayside. This LOS, however, passes through the Saturn shadow and the bulge in abundance is interpreted as located on the far side of the system. No gas density estimate is given here because the emission region is very large and at this time we have only a single observation direction. The emission peak near planet centre is produced by a combination of south pole aurora and dayglow. The large vertical extent of the distribution (out of the orbital plane) indicates other mechanisms beyond the dayglow mechanism discussed by Shemansky and Hall
2
, such as charge capture by protons accelerated along auroral field lines. We estimate a total magnetosphere atomic hydrogen content of ~2. X 10
35
atoms .
Figure 3 shows the atomic oxygen emission. This observed oxygen has a latitudinal full width at half maximum (FWHM) of ~ 3.6 R
S
and ~16.2 R
S
in the orbital plane. The image obtained at the beginning of the observatory period shows a smaller object with the same peak brightness as in Figure 3, indicating episodic changes may occur on a time scale of ~1 month (see Table 2). The intensity distribution in the orbital plane is asymmetric with larger values on the anti-solar (east) side of the planet, possibly caused by solar radiation pressure on longer lived components of the gas, and consistent with the earlier observations in 2003 DOY 352 – 2004 DOY 6. The viewing geometry (Figure 1) requires that the toroidal distribution be extended in the orbital plane by about 4 R
S
to encompass the orbits of Mimas, Enceladus, and Tethys, while the distribution latitudinally must be restricted to a minor radius of ~ 1 R
S
. The peak oxygen density at 4. R
S
east is calculated to be 600 cm
-3
. Table 1 shows the derived quantities for HI, OI, and OH, including calculated loss rates. HI is the most abundant gas but very much more extensively distributed in space. The OI distribution shows a significantly more extensive distribution than OH (Table 1). The OH quantities are obtained from reanalysis of the HST 1994 observations
5,6
in the present work.

4
The calculated loss probabilities and lifetimes of the three neutral species are given in Table 2, based mainly on the rate process quantities given by Shemansky &
Hall
2
and Shemansky et al.
3
Table 3 contains energy budget quantities derived from
Shemansky and Hall
2
adjusted for the current observational results. Several remarkable properties of the oxygen content lead to new critically important conclusions on the state of the Saturn magnetosphere. First, the large abundance of neutral gas in the region of Mimas, Enceladus, and Tethys implies a very large loss rate of the gas through charge exchange with the atomic ions in the plasma-sheet. The earlier estimate of atomic loss rates
3
were already too large to be explained by ion sputtering of the satellites
7,8,9
, and the present evaluation is at least 4 times larger (Table 1). This now has a major impact on our understanding of the evolution of the Saturn system since a much larger source is required. This may well be related to the strong energetic particle absorption phenomenon observed in the vicinity Enceladus at Voyager 2 encounter
10 perhaps by a temporary ring. Our results now point to a large mass transport associated with the ring phenomenon for example, episodic creation of puffs of ice grains from collisions within the E ring. See Barbara & Esposito
11
for a model of such transient phenomena in the F ring. Second, the strong suppression of plasma development in the
Saturn magnetosphere relative to Jupiter is explained by the basic properties of water chemistry. The primary reaction responsible for quenching the plasma is charge exchange of the plasma atomic ions (which are intrinsically long lived) with the large population of OH, followed by rapid recombination of OH
+
, loss of the neutral products from the system, and quenching of the plasma. That is,
O
+
+ OH
→
OH
+
+ O (1)
OH
+
+ e
→
O + H (2)
Third, the substantially larger extent of the OI distribution than that of OH indicates that the production process is not entirely caused by water chemistry.

5
Dissociation of OH does not provide kinetically hot OI needed to explain the extent of the broad distribution. The dissociation of O
2
, however, through both photon and electron impact
12,13
, produces atoms with kinetic energy of a few eV required to account for the distribution. The implication is that O
2
may be derived from the satellites or ring grains (see Noll et al.
14
). The variation in OI image size found here may be a symptom of complexity in the source process including the transient phenomenon discussed above.
Fourth, the energy budget shown in Table 3 indicates that the present plasmasheet model does not contain the energy required to maintain the plasma by a substantial factor. Because the plasma model
7,15
is insufficient, and cannot yield our observation the actual mass loss is even larger than the numbers calculated here.
Given the inference of ring system collisional breakup as a plausible episodic source for the large gas production, the Saturn magnetosphere may be appropriately described as having the properties of a comet with a magnetic field.
Received: April 2, 2004
1. Broadfoot, A.L., et al. Extreme ultraviolet observations from Voyager 1 encounter with Saturn. Science.
212, 206-211 (1981).
2. Shemansky, D.E. & Hall, D.T. The Distribution of Atomic Hydrogen in the
Magnetosphere of Saturn. J. Geophys. Res.
97, 4143-4161 (1992).
3. Shemansky, D.E., Matheson, P., Hall, D.T., Hu, H.-Y. & Tripp, T.M. Detection of the hydroxyl radical in the Saturn Magnetosphere. Nature.
363, 329-331 (1993).

4. Esposito, L. W., et al., The Cassini Ultraviolet Imaging Spectrograph Investigation,
Sp. Sci. Rev., in press, 2004, Ed. Russell, C. T., Kluwer Acad. Pub., Dordrecht.
5. Richardson, J.D. Eviatar, A., McGrath, M.A. & Vasyliunas, V.M. OH in Saturn’s magnetosphere: Observations and implications. J. Geophys. Res.
103, 20245-20255
(1998).
6. Matheson, P. L., & Shemansky, D. E., The distribution of OH in the Saturn magnetosphere, XXI General Assembly, Int. Union Geod. and Geophys., Boulder CO ,
July 2-14(1995).
7. Richardson, J.D. Thermal plasma and neutral gas in Saturn’s magnetosphere. Rev.
Geophys.
36, 501-524 (1998).
8. Jurac, S., Johnson, R.E., Richardson, J.D. & Paranicas, C. Satellite sputtering in
Saturn’s magnetosphere. Planet. Space Sci.
49, 319-326 (2001).
9. Jurac, S., Johnson, R.E. & Richardson, J.D. Saturn’s E Ring and Production of the
Neutral Torus. Icarus. 149, 384-396 (2001).
10. Paranicas, C., & Cheng, A. F., Amodel of Satellite microsignatures for Saturn,
Icarus , 125 , 380-196(1997).
11. Barbara, J. M., & Esposito, L. W., Moonlet collisions and the effects of tidally modified accretion in Saturn’s F ring. Icarus , 160 , 161-171(2002).
12. Huebner, W.F., Keady, J.J. & Lyon, S.P. Solar photo rates for planetary atmospheres and atmospheric pollutants. Astrophys. Space Sci. 195, 1-294 (1992).
13. Garrett, B. C., Redmon, L. T., McCurdy, C. W., & Redmon, M. J., Electronic excitation and dissociation of O2 and S2 by electron impact, Phys. Rev. A , 32 , 3366-
3375(1985).
14. Noll, K.S., Roush, T.L., Cruikshank, D.P., Johnson, R.E. & Pendleton, Y.J.
Detection of ozone on Saturn’s satellites Rhea and Dione. Nature.
388, 45-47 (1997).
6

15. Richardson, J.D. An extended plasma model for Saturn. Geophys. Res. Lett.
22,
1177-1180 (1995).
<supp> Style tag for supplementary information line ‘ Supplementary Information accompanies the paper on Nature ’s website (http://www.nature.com).
The authors wish to thank Prof. Bill Kurth for his determined effort in obtaining Cassini UVIS observation time in the Observatory Period. The authors thank Ms Alanna Borrowman and Ms Lucy
Hoag for essential work in data processing. This work is supported by the NASA Cassini Program.
Correspondence and requests for materials should be addressed to D. E. Shemansky (e-mail: dons@usc.edu).)
7
Figure 1 Image of the Saturn system. Viewing geometry from the Cassini
Spacecraft for the time frame of the UVIS Mosaics. The Saturn solar shadow is indicated. The torus image represents the approximate, inferred atomic oxygen distribution. Horizontal lines are separated by 1 R
S

8
Figure 2 Image of the H Lyman Alpha emission. Range: ~ ± 30 R s
in both the (N-
S) and (E-W) dimensions. Signal was accumulated by Cassini UVIS 2004 DOY
37 – 73. The initial and final spacecraft-planet distances were 7.06x10
7 km and
5.40x10
7 km, respectively. The signal was binned to a square grid size of (.93
R s
) 2 , given the spatial pixel dimensions of 1 mr x 1.5 mr and the varying spacecraft-planet distance. The signal peak at planet centre is comprised of dayglow and south pole aurora emission.

9
Figure 3 Image of the atomic oxygen 130.4nm emission feature. Range: ~ ± 10
R s
in both the (N-S) and (E-W) dimensions. Signal was accumulated simultaneously with H Ly α , using the same procedure described in Figure 2.
The asymmetric distribution peaks at a distance of ~4 R s
from Saturn centre on the anti-solar side.

10
Table 1 Derived neutral gas quantities in the Saturn magnetosphere*.
Species FWHM a
(R
S
E-W N-S 10
13
cm
-2 b
Peak density cm
-3
Total population
10
34
Loss rate c
10
27
s
-1
20.
1.8
1.3
5.3 O 16.4 3.6 0.73@4.0 E 605.@4.0 E
2.0
*Hydrogen and oxygen data from UVIS mosaic of 2004 DOY 37 – 73, Sub-S/C latitude –13.42
o
,
Solar phase 62.5
o
. Hydroxyl data from HST observations 1994. a
Full width at half maximum intensity (HWHM) in east-west and north-south directions. b
Peak measured abundance
(particles cm
-2
) at indicated east-west location (R
S
) with the line-of-sight passing through the orbital plane. c
Loss rate to charge capture and electron ionisation reactions.
Table 2 Species loss probabilities and lifetimes.
Species Loss a
(10
-8
s
-1
) Lifetime d e b
X c
O
+ a
Within the plasma-sheet volume. b
Electron impact ionisation. c
Charge capture. d
Hours or days as indicated.

11
Table 3 Energy budget.
Source
Flow to electrons O
+
Sink Rate coefficient Rate
10
-20 erg cm
3
s
-1 a
10
-16 erg cm
-3
s
-1 ec 8. 3.2 ec 503. 12.8 Flow to electrons eh
Total flow to ec
O 2.2
16.
8.4 ec to neutrals radiative ec to neutrals ionization O
OH
1.1
1.9
4.1
9.4 ec to neutrals radiative ec to neutrals ionization ec to neutrals radiative ec to neutrals ionization ec to ion radiative ec to ion ionization
H
O
+
O
+
OH
H
6.8
4.5
8.7
5 8
12.8
4.3
0.83
2.3
Total ec loss rate
Total ec loss
Plasma model heating of ec a
Mean electron density ec = 60., eh= 0.4 , T ec
= 4. eV, T he
= 0.4
0.006 0.0024
42.2
9.4 GW
3.5 GW