Outgassing of H and OH from Saturn's rings and the possibility of detecting it are examined. The source gas around the rings is computed after which the products of ice, H2O, H, and OH and the physical mechanisms that produce these species are given. Production rates for the ice products are discussed

Blamont, J.

English

NASA Technical Reports Server

N75 17247
Jacques Blamont
THE "ATMOSPHERE"
OF THE RINGS OF SATURN
The question I would like to address is the outgassing of Saturn's rings and the
possibility of detecting it.
After discovery of the hydrogen Lyman-alpha emission around comet Tago
Sako Kosaka by OAO II, observations we made on comets Bennett and Encke
from OGO V in 1971 have shown that it is possible to deduce the source intensity
of the hydrogen atoms from the Lyman-alpha emission intensity.
The total source intensity was of the order of 1029 atoms of hydrogen per second
tor comet Bennett and something like 1027 atoms of hydrogen per second for Encke.
Comets may well be ice-covered bodies, and this is also probably true for
Saturn's rings.
.1 would like to make one suggestion. Dr. Fra.nklin mentioned yesterday that it
would be interesting to .obtain the scattering diagram of an ice particle in space.
Maybe one could obtain such a diagram from a comet nucleus, at a distance of the
order of two A.U. from the Sun when the comet, being far away from the Sun,
does not change from day to day. You just have to wait for a comet and there are
15 comets a year to try.
From our comet observations, we decided to look for the possibility of the same
outgassing of H and OH from Saturn's rings. This work was carried out by my
student, Michel Dennefeld.
First of all, the source of gas around Saturn's rings is computed. The products
of ice will be essentially H2O, H, and OH. There are different physical mechanisms
for producing these species, and the corresponding production rates are given in
table I. In order to decide if the source is significant, these rates can be com-
pared with the rate for comets, which is of the order of 1028 s"1; that is a significant
source and can be detected easily, but a source of 1020 s"1 cannot be detected.
Ice sublimation at a temperature of 80 K is a source of the order of 1020 s"1.
Centre National D*Etude» Spaliales.
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However, I heard yesterday that the temperature may be as high as 95 K. Then
we have a source intensity like 5 X 1026 s~', which is in the range of detectability.
If the temperature is 80 K, the most intense source is meteoroid bombardment.
We have used the Cook and Franklin (1970) value for meteoroidal bombardment.
In this mechanism, the meteoroid hits a particle, and local heating vaporizes H2O.
Solar wind bombardment does not have a large effect.
Interstellar gas (hydrogen, helium), which hits Saturn's rings, has a small
effect.
The most intense source of gas in the whole Saturnian system is the Titan
atmosphere, which produces 4X 1028 s~' of atomic hydrogen.
For OH, bombardment by solar wind creates 1024 radicals per second.
Once the source strength has been computed, the subsequent history of the
gases has to be determined.
Table II gives the lifetimes of those molecules and atoms in the vicinity of
Saturn. All the lifetimes are long compared to the orbital period of atoms, mole-
cules, or radicals around Saturn. For instance, the photodissociation time of
H2O, approximately 107 s, is 103 times the orbital period; therefore, something like
TABLE {.—Sources and produttion rates.
Species
11,0
H
OH
Source
Ice sublimation T = 80 K
T = 100 K
Meteoroid bombardment
Solar wind bombardment
Interstellar helium bombardment
Solar wind bombardment
Titan
Solar wind bombardment
Rate, s-1
1.4 x 1020
5 x 1026
4 x 1026
5 x 10"
5 x 1024
1023
4 x 1028
102<
TABLE H.-Lifetime at Saturn.
Atom or molecule
H,
H
H,0
OH
Orbital period
Lifetime, s
3X 10"
15 x 10"
9x 10"
6.5 X 10'
4x 104
Process
Photodissociation — » H + H
Photodissociation
Photodissociation
126 The Rings of Saturn
1000 encounters between one molecule of FhO and the ring will take place before
the production of an atom of hydrogen.
The sticking coefficient of FhO over the ring is unknown since the very nature
of the ring is unknown. It has to be of the order of 10~2 if we want to have a serious
production of hydrogen. However, if we obtain a large source by having a higher
temperature of the rings (about 100 K), we may, even with a larger sticking
coefficient, obtain quite a number of hydrogen atoms.
After the computation of the lifetime, the distribution of molecules and atoms
among the different orbits has been determined.
The number densities of different species are given in table III.
The orbits depend greatly on the formation processes, because the velocity is
a function of the energy of the atoms or molecules.
For instance, an atom of hydrogen, when created by bombardment, will have a
very low velocity on the order of 1 km s"1; if it has been created by photodissocia-
tion of HiO, it will have a velocity around 18 km s"1. Therefore, hydrogen created
by bombardment will stay in the plane of the ring; if it has been created by photo-
dissociation of water, it will fill a sphere around Saturn.
After all processes have been taken into account, a number density of 10 atoms
of hydrogen per cm3 is formed from outgassing of Saturn's rings.
Table IV gives the luminosity expressed in Rayleighs (1 Rayleigh corresponds
to 106 photons per m2 emitted in 4?r steradians). Around the rings of Saturn, an
emission strength of 10 Rayleighs of Lyman-alpha and 10 Rayleighs of OH reso-
nance line has to be expected. The intensity of 500 Rayleighs of Lyman-alpha
emitted by the Titan torus has been computed in the case of an optically thin
TABLE III. — Number densities of species.
Species
H2O
OH
O
H (Saturn)
H (Titan)
Number density, cm~3
10-100
1-10
1-10
1-10
10-10"
TABLE IV. — Luminance.
Species (and source) Luminosity, Rayleighs
H (Titan)
H (Saturn's rings)
OH (Saturn's rings)
500-1000
1-10
10
"Atmosphere" of Saturn's Rings 127
- Rings
Saturn
FIGURE 1. — Schematic view of a cross section through the Saturn
system as seen in Lyman-alpha.
medium. Actually, the center of the torus is optically thick, and an exact computa-
tion for an optical thickness of 4 would provide an emission intensity of 100
Rayleighs.
This is a very easily detected phenomenon from MJS.
Figure 1 gives you some feeling for what it would look like. This would be the
system seen in Lyman-alpha. It has an odd shape because of the properties of
the orbits of the atoms of hydrogen around Titan, but it should be the most
spectacular phenomenon in the Saturn system in the ultraviolet. Around the rings
you have a very small emission of Lyman-alpha due to the atoms that have been
created by meteoroid bombardment, and all around the system you will have a
weak but detectable glow due to the atoms created by photodissociation.
DISCUSSION
James Pollack How did you compute the evaporation rate of the water vapor
from the ring particles?
Jacques Blamont It is a cocktail of basic thermodynamics and measurements
that have been published.
Charles Lillie What is the extent of that cloud around Titan?
Blamont It's of the order of 1 Mkm in diameter. As seen from the Earth, it
would have a diameter of 5 min of arc. The existence of the torus has been
proposed by Neil Brice (1973) and was discussed extensively last week at the
Titan atmosphere workshop held at the Ames Research Center.
William Irvine You are saying that Titan is the source for that?
Blamont Yes.
Irvine You have to assume things about the atmosphere of Titan to make that
prediction.
Blamont You have to assume an exospheric temperature of 100 K and a flux of
hydrogen, which I think almost everyone agreed on at the Titan atmosphere
workshop.
Brad Smith This would be a toroidal ring that would surround Saturn?
Pollack Yes, essentially because Titan loses hydrogen fairly rapidly. We have
evidence that there is a significant amount of hydrogen in its atmosphere, which
would suggest that a steady state has been established.
128 The Rings of Saturn
Lillie We had some observations of Saturn in Lyman-alpha with the satellite
OAO II. It is possible to set upper limits on the surface brightness around the
system. The photometer had a 10-min field of view, and it observed a surface
brightness, assuming it is filled, of about 59 Rayleighs, plus or minus about 30.
It is dominated by the noise of the system.
Blamont You mean around Titan or around Saturn?
Lillie This observation was centered on Saturn.
Blamont That is quite comparable to what I am talking about; I am thinking of
100 Rayleighs for Titan.
Lillie We are almost just about right. You are almost permitted.
Irvine Do you know if Titan was in the field of view?
Lillie Yes, I think it would have been. I don't know the distance of Titan from
the primary at the time. I don't know the radius of Titan's orbit.
Fred Franklin It is 20 Saturn radii.
Lillie All that has to be in the instrument field of view is the toroid; you don't
have to see Titan physically. As long as you hit the toroid, it's in the bag.
Blamont Yes, but the toroid is an extended source, with varying optical thickness
in Lyman-alpha.
Lillie I calculated that if we could get about a 70-Rayleigh toroidal cloud, we
would come out about right, which is permitted by the observation.
REFERENCES
COOK, A. F.; AND FRANKLIN, F. A.: The Effect of Meteoroidal Bombardment on Saturn's Rings. Astron.
J., vol. 75,1970, pp. 195-205.
McDoNOUCH, T. R.; AND BRICE, N. M.: New Kind of Ring Around Saturn? Nature, voL 242, 1973,
p. 513.
"Atmosphere" of Saturn's Rings 129


The atmosphere of the rings of Saturn

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