V.A. Dorofeeva

The Saturn system is a very complex. It contains a several rings, and many satellites both regular (24) and captured (38). They all differ in the structure, physical properties and chemical composition. But at the same time, all of these objects have one common property: they all contain water ice. This fact is extremely important because it allows to consider some important problems of cosmogony. In particular, to analyze the conditions of the formation and evolution of the Saturn system objects.

5 10
-4
D/H
6 comets from the Oort cloud
4 10
-4
Enceladus
3 10 -4
2 10
-4
Titan
1 10
-4
VSMOW
103P/Hartley 2
Jupiter family
C-chondrites
(OH )
Jupiter
Saturn interstellar clouds
Solar
Nebula
Water ice in the Solar system has two major characteristics: structure and D/H ratio.
In the outer regions of the Solar system water ice has an amorphous structure and a high
D/H ratio as in the interstellar molecular clouds and in the water from the Oort cloud comets.
In the inner regions of the Solar system water ice has a crystal structure. D/H ratio as in ice, as in liquid water is more that two times lower than in Oort cloud comets, such as in the Earth's oceans (VSMOW - Vienna Standard Mean Ocean Water), OH group of carbonaceous chondrites and in some of the Jupiter family comets, such as comet Hartley 2. It is the result of evaporation of amorphous ice in warm inner region of gas-dust protoplanetary solar disk and the isotope exchange reaction with molecular hydrogen according reaction HDO gas
H
2
O gas
+ H
2
=
+ HD. During the subsequent nebula cooling crystalline ice was formed. Crystalline ice of various modifications was also formed in the large ( R > 30 km) rock-ice bodies in the process of their differentiation as a result of the internal heating.

Saturn's rings.
According to the «Cassini» research, the most massive of A and B rings, with a mass of
(0.5-0.7)

10 22 and (4-7)

10 22 g respectively consist of a set of bodies of pure crystalline water ice ranging in size from 10 cm to 10 m.
Such bodies could not have been formed by condensation or accumulation. We assumed that the icy mantle of the differentiated rock-ice body was its original source. The body was formed in the first 1.5-2 million years of evolution of the Solar system and has passed the stage of melting water ice due to radioactive heat 26 Al and differentiated to form the icy crust. There could be several bodies, but their mass should be sufficient to keep the water in the liquid state during the time, which is necessary for the complete differentiation.

Left Fig. Computer simulation results of heating the ice-rock body with D = 200 km.
Right Figs. The thickness of the crust to radius ratio (H core
/R), maximum temperature and existence time of liquid water in the ice-rock bodies of different size: from 20 to 600 km in D.
We suppose, that forming of the A and B Saturn rings is the result of destroy the differentiated body by the tidal influence of the Saturn. The most likely diameter of the bodies was 100-300 km, having crystallization time < 100 million years.
100
Hcore/R, %
D = 200 km
T max
= 293 K
50
0
300
Tmax, K
273.15 K

3
250
300
200
100 liquid, million years
0
20 30 50 100 200 400 600 km

Saturn's moons.
The density

is one of the most important characteristic of cosmic bodies. The density value of Saturn’s moon characterize the ice/rock mass ratio in these bodies.
Let us compare those with the values of
 obtained for condensed phases in the condition of equilibrium of the solar cooling system. In our calculation we used the Solar Abundances tables by Katharina Lodders , 2010, density and composition of different phases according to the table. Calculations were carried out using the formula:
 m i



 
 m i i





 x
1
 i



1
Component of ice-rock body
Silicates (MgO, SiO
2
, CaO, Al
2
O
3
, Na
2
O, …)
Metal (FeS, FeO)
Organic compounds (CHON,
C
100
H
100
O
42
N
10
( Pollack et al., 1994 )
Crystalline ice Н
2
О
Density, g/cm 3 (Johnon, Lunine, 2005)
3.36
4.88
1.70
0.944

According to the results of equilibrium calculations in a system of the solar composition at low T and P ( T < 600K, P <10 -7 bar), C is found in a form of CH the rock compounds is in a form of H
2 and the resulting density of the body as

1.45 g/cm 3 .
4
, and all oxygen not being included in
O. In this case, the rock/ice mass ratio will be as

1:1,
-4 lg(x ) P = 10 bar
Nonequilibrium system
-2
N
2
CO
But there are two problems
1. It was shown by experimental data, the gas-phase reaction of CO reduction by H
2
(CO+3H
2
=CH
4
+Н
2
О) is kinetically inhibited at T <~ 700 K.
2. The rock phase can contain the refractory organic compounds
(mainly CHON), which may include up to 50%

C (of total mass of C).
-6 CO
2
-8
-10
NH
3
CH
4
Therefore we calculated the density of ice-rock bodies depending on mol fraction of CO in gas phase of system and on part of carbon in organic compounds.
300 400 500 600
Temperature, K
700 800
Composition CHON was accepted
C
100
H1
00
O
42
N
10
( Pollack et al., 1994 )

3
2.0
1.8
1.6
C-org/ C
0
0.1
0.2
0.3
0.4
0.5
1.4
0.0
0.2
0.4
0.6
0.8
1.0
г/
3
1.45 – 1.6 г/см 3

d of an ice-rock body, g/cm
3
2.0
Titan
1.8
1.6
1.4
1.2
1.0
Enceladus
Dion
Rhea
Tethys
C-org/ C
0
0.5
This interval includes

Dione ( r
= 6.29 R
Sat
) values. But the density of two other of major regular Saturn’s moons - Rhea and Tethys is much lower than calculated the interval. These moons are sufficiently large ( D
= 1000-1500 km), so we can assume that they have zero porosity. Additionally, they are located relatively close to Dione
(within 5-9 R
Sat
). These arguments allow us to assume that Rhea and Tethys
 value needs clarification.
0.0
0.2
0.4
0.6
CO/ C
0.8
1.0
Enceladus density value (1.61 g/cm 3 ) may indicate that some of the H
2
O was lost during the eruption of water plumes. Part of this water may be form the Saturn’s F ring.
It is clear from our results, that it is impossible to obtain the
 rock-ice condensate value in the solar composition system equivalent to the
 of the largest Saturn’s moon – Titan, whose density = 1.88 g/cm 3 .

d of an ice-rock body, g/cm
3
2.0
Titan
1.8
Enceladus
1.6
Dion
1.4
Rhea
1.2
1.0
Tethys
0.0
0.2
0.4
0.6
0.8
1.0
CO/ C
Titan 20.36 5150 1.88
Enceladus 3.97 500 1.61
Tethis 4.91 1060 0.97
Dion 6.29 1120 1.48
Rhea 8.78 1530 1.23
Iapet 50.36 1460 1.08
C-org/ C
0
0.5
1.
 value of the Enceludus and the Dion are in agreement with their bulk composition having the solar proportions. Their formation is assumed not to be accompanied by the differentiation of icerock fractions.
2.
 values of the Rhea and the Tethys are to be supported as their dimensions and positions are approximately near to those of the Dion.
3.
 value of the Titan is sufficiently above the one calculated from the equilibrium models.
This points to the differentiation of ice
– rock processes that occur during the formation of the moon.

The differentiation of rock and ice components provides for the evaporation of water ice. And we have evidences that evaporation of the amorphous water ice in the region of the Titan formation took place.
1.
We know that accretion of Titan volatiles could take place with the participation of crystalline water ice only.
2.
It is possible that CH
4 formation occurred with hydrogen of water (by CO reduction). According “Cassini” data D/H
СН4 in Titan’s atmosphere = 1.58
2 
10 -4 is equivalent to D/H in the inner region of the Solar system were water ice was that evaporated.
Therefore, we can assume that the formation of Titan rock embryo occurred in the first 1 – 1.5 million years evolution of the Solar system when T in Saturn region was warm enough and this region was inside in snowline.
It was found that the relations of gases in the Enceladus water plumes and high value of D/H in water (D/H
Н2О
= 2.9

10 -4 , being near to cometary value) indicate the cometary composition of the Enceladus matter. Thus the Enceladus may have been formed at the latest stage evolution of the Solar gas-dust disk (nebula) and Saturn subdisk after the matter transport from grater radial distances up to the region of the
Saturn formation.
The study was supported in part by RAS Presidium Program №22