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*Cryotic bedrock and regolith **Cryotic rock

*Cryotic bedrock and regolith **Cryotic rock cryogenic phenomena in addition to water, according to I.Ya. Baranov. Thus for example comparison of the spectra of the giant planets shows that the spectral lines of ammonia are gradually weakening while the lines of methane are intensifying in going from the spectrum of Jupiter to the spectrum of Neptune, resulting from the ammonia freezing out with the decrease of temperature. It is evident that glacial and other ice covers must be formed on planets in the same sequence (in accordance with gaseous-phase crystallization at lower temperatures). Actually while H20 ice prevails in Earth's cryolithosphere, H20 ice and C02 ice prevail in the cryolithosphere of Mars, so one must also follow a change to formation of ice from carbon dioxide and ammonia with water, and then from methane, proceeding towards the most distant planets of the Jupiter group. Consequently one might distinguish between the planets and their satellites by the development of the cryogenic processes of water (Earth), carbon dioxide-water (Mars), water-methane (Triton - satellite of Neptune), methane (Pluto) and of other types (Table 1) within the Solar system. For example it is common knowledge that both the poles of Mars are covered with thick ice 'caps'. Thus the northern polar cap consists of H20 ice and contains a great amount of dust. The upper layer of the southern polar cap consists mainly of C02 ice and does not contain any dust. According to R.O. Kuz'min's opinion (1983) the Martian cryolithosphere has a three-layered structure with permafrost thickness varying in the range from 1 or 2 to 4 or 5 km (Fig. 1). Surface deposits (a few kilometers thick) representing glacial formations of a peculiar kind consisting of C02 ice and H20 ice stratifications, gas hydrates and loose (dusty) sedimentary rocks (Fig. 2), were found within the polar regions of that planet with the help of images obtained by the 'Mariner 9' space vehicle.

Investigation of the Solar system's celestial bodies has allowed American researchers to publish in recent years interesting data on the peculiar features typical of the planets and their satellites. Thus the Galilean satellite of Jupiter, Io, is the unique celestial body of the Solar system. It is supposed that the greater part of the surface of this satellite is composed of solidified sulphur and solid S02 or S02 ice. The surfaces of the other Galilean satellites such as Europa, Ganymede and Callisto consist mainly of H20 ice which, probably, is polluted by fine foreign material from fragments of meteorites. Study of the spectral reflecting power of Tethys, Dione, Rhea, Iapetus and Hyperion (satellites of Saturn) allowed the scientists to conclude that the surface of these satellites, except for the dark side of Iapetus, consists of all but pure H20 ice with a less than 1% admixture of mineral dust. It is not inconceivable that the ice can include a great amount of gas.

Fig. 1. Diagram of section (north pole - equator) of the Martian cryolithosphere along a meridian (after R.O. Koz'min): 1 - H20 ice; 2 - H20 ice and gas hydrate; 3 - H20 ice and C02 liquid; 4 - CO, ice; 5 - supercooled water and gas hydrate.

Fig. 2. Flat layered terrain on Mars lacking craters: a - an oval 'layered' plateau, 40 x 20 km in size; b - alternating glacial formations (with separate layers from 50 to 100 m in thickness).

Fig. 2. Flat layered terrain on Mars lacking craters: a - an oval 'layered' plateau, 40 x 20 km in size; b - alternating glacial formations (with separate layers from 50 to 100 m in thickness).

Titan (a satellite of Saturn) is likely to have an ethane-methane ocean and its surface consists mainly of'volatile' ices such as NH3 and CH4. It is supposed that there is a combination of H20 ice and NH3 hydrates in the composition of the surface of Enceladus (a satellite of Jupiter). This satellite has the

Fig. 3. The surface of Enceladus (from pictures taken from the spacecraft ' Voyager V and ' Voyager 2'). Scale 1:5 000 000.

highest reflecting power in the Solar system. It is possible that its surface is covered at all times with H20 ice and NH3.2H20 ice formed as a result of liquid water flowing out of the satellite interior. The largest part of the surface of the satellite is a plain with furrows (Fig. 3). Enceladus is the most active and the youngest satellite of Saturn from the geological standpoint with the density of craters being less than that on the other satellites even within the areas with the greatest crater concentration.

As in the case of Jupiter's and Saturn's satellites, H20 ice was identified on the surface of all five satellites of Uranus (Miranda, Ariel, Umbriel, Titania, Oberon) (see Table 1). In this case the images from the 'Voyager 2' vehicle travelling between the orbits of Miranda and Ariel in 1986, show highly cratered areas as well as traces of tectonic activity in the form of extended faults and furrows on the surface of all the satellites.

American scientists proposed on the basis of experimental research that Neptune's satellite Triton is similar to Titan. It is thought that this satellite is shrouded in a mainly nitrogen atmosphere while there are oceans of liquid nitrogen with floating fragments of methane ice. It is possible that there is water hoar frost on the satellite's surface. It is supposed also that the planet most distant from the Sun in the Solar system, Pluto, contains a great amount of methane ice.

One can recognize a number of categories, classes, groups and varieties of planets, their satellites, asteroids and other celestial bodies (comets, meteorites, cosmic dust accumulations) with respect to conditions of development of frozen ground (granular or cemented with H20, C02, NH3, CH4 ice etc. or materials containing these varieties of 'ices') and with respect to their distribution over the planets.

Heat flux arrival at the surface of a celestial body from the interior q and from outside Q is of fundamental importance in such a classification. At the first approximation one can recognize the planets without (q = 0) and with (q > 0) heat flux to the surface of the planet from the interior (Table 2) in accordance with the state of the core of the planet (cold or hot) and physical and chemical exo- and endogenous thermodynamic processes and reactions. With respect to the heat arrival at the solid celestial body surface from a source of radiation (the Sun or from another celestial body) one can distinguish the planets without heat flux arrival at their surface from the outside (Q = 0 -when the planet is situated far away from the sources of thermal radiation) and with small (Q > 0) or great external heat flux (Q ยป 0 -when the planet is situated near the external thermal source). Various combinations of heat balance parameters cause a number of variations in the permafrost distribution over the planets and celestial bodies. In this respect completely cryogenic, incompletely cryogenic, deeply cryogenic, continuous-surface-cryogenic, discontinuous-surface-cryogenic and completely uncryogenic categories of planets and solid celestial bodies can be recognised (see Tables 1 and 2).

When the relationships between the periods of a planet's rotation around its axis xrot and of its revolution around the Sun xrev are also considered, one can recognize classes of planets and their satellites with symmetric (xrot > xrev) permafrost distribution and with asymmetric distribution (Trot = Trev)> with permafrost being developed over the night side only. Daily temperature fluctuations with the period being less than the annual revolution of the celestial body around the external source of radiation are typical of the planets of the symmetric type. Thus consideration of the nature of the heat balance (of relations between the values of internal and external thermal fluxes and of the specific nature of the heating of the planet for the period of its rotation) allows us to recognize nine classes of planets which vary greatly in cryogenic structure i.e. in the particular permafrost occurrence (in depth) and distribution (over the surface) within the planets and celestial bodies (see Table 2).

The particular nature of the water balance of a planet (or of the carbon dioxide, hydrogen, ammonia, etc. balance), with the presence or absence of

Table 2. Conditions of development and arrangement of planets of cryogenic type

With respect to the character of thermal and water balance

With respect to cryogenic stability and presence of seasonal freezing (thawing)

Community class

Group

Completely cryogenic q > 0

<2-> o continuous surface cryogenic q = 0

Incompletely cryogenic symmetric q = 0 2>>0

Deeply cryogenic q > 0

Discontinuous surface cryogenic q > 0 2>>0

symmetric vrot ^rev symmetric vrot ^rev symmetric

T s? T ^rot <v 'rot asymmetric symmetric cb o

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