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by Academician Alexei MARAKUSHEV, RAS Institute of Experimental Mineralogy
The solar system is comprised of eight planets orbiting the luminary. The astronomical bodies gravitationally bound to the sun are divided into two groups: the relatively small ferrolithic ones like Mercury, Venus, Earth and Mars, and the giants like Jupiter, Saturn, Uranus and Neptune. The latter ones consist of molten ferrosilicate cores and giant fluid shells or mantles-helium-hydrogen ones of Jupiter and Saturn, which make them similar to the sun, and aqueous ones of Uranus and Neptune similar to that of comets. The solar system is surrounded by comets consisting of ice nuclei with in-frozen cosmic ferrosilicate dust. These comets periodically intrude into the solar system along strongly elongated orbits. This giant environment is divided into cloud-like formations-the Oort, Hills and Kuiper clouds. Belonging to the latter one, which is also the nearest, is Pluto-also consisting of water ice and cosmic dust. It used to be regarded as the ninth planet in the Solar system, but in two of his monographs (1992, 1999) the author of this article has substantiated his view that it rather belongs to the comet environment. This was proved in 2001 by photographs of Pluto taken by a US interplanetary station.
PRE-PIANETARY SUN HYPOTHESES
In 1796 the French scientist and Honorary Member of the St. Petersburg Academy, Pierre Simon de Laplace, put forward his hypothesis suggesting that the evolution of the solar system began with the formation of a rapidly rotating sun from which rings of matter split off in the equatorial plane which later provided the basis for the formation of planets. This assumption led to the conclusion about substantial degassing of the nebular (consisting of gas and cosmic dust) disk in the immediate environment of the sun, and the formation of planets around it. At first sight this offered a good explanation of the structure of the solar system: the presence therein of near-solar planets of the earth group made up of ferrolithic material, and of the remote fluidal ones.
Since then it has been suggested that the former appeared by way of direct accumulation of ferrolithic material (meteorites, cosmic dust) in the vacuum of space. And in the 20th century Academician Otto Schmidt in this country formulated his meteorite theory of their formation which was later developed by his numerous followers-Victor Safronov (1969), Andrei Vityazev (1990)*, Oleg Sorokhtin (1999) and others.
The departure of the meteorite hypothesis from the vacuum version is typical of scientists of the Japanese astronomical school led by K. Khayashi. They regard as the main factor the accretion of the earth (growth of its mass through gravitational capture) in the protosolar nebula. This, in their view, should have produced fluidal shells, or mantles, in planets of the earth group, which prevented the loss of heat generated during accretion; that promoted their melting and layering, or lamination, into cores and silicate mantles.
In essence, this is close to the traditional concepts about the origination of the giant planets; according to meteoritic hypotheses it started from the formation of ferrolithic cores which, having reached a critical mass, caused the accretion of gas under the effect of gravity. According to the present-day models of the inner structure of Jupiter and Saturn, their core masses are much smaller than was earlier assumed-they are comparable in size to the earth. However, the gravitational field of the comparable ferrolithic cores of fluidal planets is clearly not enough in order to retain the vast mass of molecular hydrogen contained in their mantles. Scientists discovered what they called a pulsed process of degassing of the earth core; grandiose volumes of hydrogen are thus released unto the surface of the planet. But almost all of it escapes into outer space, and only the heavier gases are left behind to form the earth's atmosphere.
Standing in contradiction with the hypothesis of the pre-planetary formation of the sun is the recent (1995-2000) discovery of several tens of stars, similar to the sun, which are surrounded by planets comparable to Jupiter in mass. These were discovered by Paul Butler of the Anglo-Australian Observatory, Geoffrey Marcy of Berkeley University, USA, and several of their colleagues. I shall be dwelling on their observations later; but I would like to stress now that all of these giant planets are in a position near the earth which is occupied in the solar system by planets of the earth group. Such astronomical facts attest to a complex evolution of the discov-
Diagram of the solar system. Hatched are fluidal mantles of giant planets: helium-hydrogen (H2) and aqueous (H2O); these are shown out of scale.
* See: A. Vityazev, "In the Beginning There Was...", Science in Russia, No. 5, 1994.- Ed.
Solar system with the Kuiper comet cloud and the position of the elongated Pluto orbit (traced in color) crossing the ecliptic plane in which planets are located at an angle of 17. Conventional signs: I-Saturn, II-Uranus, III-Neptune, IV-Pluto, V-Charon, VI-QB object.
ered planets incompatible with the hypothesis of their post-stellar formation.
SUN WAS BORN AFTER PLANETS
The hypothesis of an earlier (pre-solar) formation of the planets was formulated by the German scientist and Honorary Member of the St. Petersburg Academy Immanuel Kant in his book Universal Natural History and Theory of Heaven (1755). He assumed that in the present organization of space, with circulating spheres, there is no material cause which could have triggered off, or even directed their movement. The conclusion which followed: the planetary world used to be different- filled with matter capable of setting into motion the bodies therein and make their movements coordinated among themselves. In the final analysis gravitational pull "cleared" this space, gathering all matter into an isolated mass-thus giving birth to the sun.
However, according to Kant, the mass of the nebular disc, matching the sun, was not enough to set the planets into a rapid and coordinated motion. In reality the initial size of the disc is determined not by our luminary, but by its precursor-a giant star with a mass tens of times bigger than that of the sun. This is proved by the chemical composition of the bodies of the solar system: they could have inherited it only from a giant star with a short lifetime, capable of synthesizing all of the chemical elements down to the heaviest ones (uranium, thorium, etc.). These bodies originated as a result of its final explosion-a supernova flare (and within the sun itself-a yellow dwarf-only light elements are synthesized which remain within its bowels inaccessible for observations). The gas cloud thus produced, cooled down turning into the rapidly rotating disc of the protosolar nebula, as a result of which the forces of attraction could not collect matter in the center. Concentrated in the sun is but a minor part of the mass and the kinetic energy of the nebular disc, and that determined the small size of our luminary, its slow rotation and lifetime-of 5 bin years already-among the rapidly moving planets and dynamic comet clouds.
Yet the main mass of the dense nebular disc remained in the changing system around the sun wherein planets and comets, by way of accretion of ice water-hydrogen planetesimals (intermediate formations from a protoplaneraty gas and dust cloud), were formed. The main reason for that was a developing instability (collapse) in the rotating disc-like systems of planetesimals. Accumulated in a similar way (and simultaneously) was the mass of the sun. Turning into a small star, it actively affected by means of a strong solar wind the surrounding and rapidly rotating dense nebular disc, "sweeping out" gases from it. As a result, the solar system lost its main mass- mostly helium and hydrogen which migrated out into space. And the planets remained in a vacuum, having inherited nevertheless a con-
Uranus (diameter of 51,800 km), stratified into molten iron, ferrosilicate core and fluidal mantle, and its satellites-Miranda (235 km), Ariel (580 km), Umbriel (585 km), Titania (799 km), Oberon (770 km). Shown in comparison with the comet system are Pluto (2,320 km) and Charon (1,270 km).
Diagram of comparison of systems "Earth-Moon" and "Dense Jupiter Core-Its Satellites". The direction of movement of the latter is marked by arrows on diagram of their orbits (top right). I-IV-groups of satellites listed in the supposed sequence of their formation: I-II-remote with a reverse (I) and normal (II) orbital movement; III-IV-near-planetary massive dense satellites (III) and of low density (IV) framing the rings system of Jupiter.
siderable fraction of the kinetic energy of the protosolar nebular disc.
Thus the interplanetary space in the solar system was "cleared up" not by gravity, as Kant believed, but by an oppositely oriented force of the solar wind. And the amounts of helium and hydrogen which escaped into space were tens of times greater than the mass of our luminary.
Essentially, the basic element of the formation of all the objects of the solar system were planetesimals. Their closest analogues are small cometary bodies of the clouds of Oort, Hills and Kuiper which remained at a primitive level of development because of their size. They are held together by gravitational forces and produce rotating formations; they can be regarded only conditionally in the "planet-satellite" subordination. One such example is offered by the rotating Pluto-Charon system in which the periods of revolution of both bodies are equal (6.32 days) so that as a whole they have a common axis of rotation. Such systems, however, are basically different from the planetary-satellite ones where the size of the satellites is incomparably smaller as compared with the planets.
In planets, as different from comets, thanks to the accretion of planetesimals (their gravitational pulling together) incomparably greater masses of ice matter were concentrated.
The primary ice contractions possessed watery composition at the periphery of their nebular disc, where Neptune and Uranus were formed, and a helium-hydrogen one in the central zone of the accumulation of the sun and the nearby planets.
The pulling together of vast masses of ice matter was accompanied by their gravitational compression with a sharp rise of temperature (up to 20,000 K in Jupiter), complete melting and thermal emission which made the giant planets look like stars ("wondering stars"). And the formation of the ferrosilicate heavy cores of planets was attended by a pulsed-accelerated rotation of their giant fluidal shells. Separating from them
under the effect of centrifugal forces were fluido-silicate masses-the future satellites.
SATELLITES OF GIANT PLANETS
These are located in their equatorial plane in groups (I-IV) which differ by their mean density, distance from the mother-planets and the specifics of rotation around them. The most developed such system is of Jupiter which possesses the biggest fluidal shell with a diameter of 1.43 ? 10 5 km. Located at the periphery of this system are light ice satellites (Group 1) with an inverse rotation around Jupiter. At closer distances towards it the rotation of the satellites becomes coordinated with its own rotation, and their density grows successively (groups II->III); it reaches its maximum in the stone satellite Io (3.5 g/cm 3 ). Then the density decreases, and in the immediate proximity of the mother-planet there are moving light stone-ice satellites (Group IV).
The whole of this satellite system reflects the history of the development of Jupiter. The outer light satellites with reversed rotation characterize the stage of the evolution when the planet had not yet been formed and only an accretion disc of ice planetesimals was taking shape.
When the planet reached its critical mass, its gravitational compression and melting stabilized the direct rapid rotation accompanied by the separation of satellites (Groups II-III) whose density rose with reduced orbit radii; formed at the same time was the ferrosilicate core of the planet.
Closer to the present condition, the rate of Jupiter's spin slowed down so that it could cast off to small distances light satellites only (Group IV) which broke up forming Jupiter rings from the splinters. Each of these consists of fine silicate and ice particles, has an open-work structure and looks transparent-these are the residues of the dense and compact rings like the famous younger rings of Saturn and Uranus. The ability of the giant planets to generate satellites and rings of splinter material belongs to the natural manifestations of their inner activity. With Saturn and Uranus this may be still in progress even now.
Jupiter differs from other giant planets by its massive, so-called Galilean satellites (named in honor of their discoverer). These are of big size: Callisto-4,840 (1.88 ? 10 6 ), Ganymede-5,270 (1.07 ? 10 6 ), Europa-3,130 (6.71 ? 10 5 ), Io-3,640 (4.22-10 5 ). Given in brackets are the distances of these satellites from Jupiter (both parameters in km). With their approach to the planet the density of the satellites increases in the following way: 1.8->1.9->3.1->3.5 (g/cm 3 ), approaching the density of the ferrosilicate molten core of Jupiter itself (4.6 g/cm 3 at zero pressure).
The relative young age of the Io satellite is proved by its volcanic activity and the presence of its own magnetic field. On March 4, 1979, the US interplanetary probe Voyager-1 discovered upon it eight active volcanoes. Almost all of them remained active four months later during the Voyager-2 mission. The explosive activity of volcanism is unusually great with the erupted material ejected to an altitude of 70-280 km over the surface of Io. Spectroscopic data also established very high temperatures of eruptions-of the order of 1,500C. Io is unique in this respect-it is the only one of the planetary satellites in the solar system where intense volcanic processes are in progress now. Current activities of other massive satellites of Jupiter (Europa, Ganymede) are manifested only in the form of formation of their ice covers. As for the most remote Callisto, it has lost all of its activity and its magnetic field.
Another giant planet Saturn, has a smaller fluidal shell and generates satellites of a lower density (maximum in Titan-1.9 g/cm 3 ). Light satellites originating nearby disintegrate with the formation of rings of fragment material.
And even less perfect are the satellite systems of Uranus and Neptune.
ORIGIN OF THE MOON
The dense Galilean satellites could be produced by Jupiter alone with its truly giant fluidal mantle. This conclusion bears directly on our understanding of the origin of the moon. By its size and mean density it approaches lo (3.3 g/cm 3 ), and consequently, could have originated in the satellite system of the mother-planet- Protoearth, which had a fluidal shell like that of Jupiter.
But volcanic activity on the moon developed 4.6-3.2 bin years ago, whereas on Io it is still intense to this day Studies of our night luminary indicate that in heavenly bodies of this size the "fluidal store" is big enough for about 1.4 bin years of endogenic activity Thus present-day processes on Io attest to a considerable age difference of this satellite of Jupiter from the moon. But their formation proceeded in the same manner by way of their separation under the effect of centrifugal forces from the giant mantles of the rapidly rotating mother-planets. It is also quite clear that satellites consisting of such high-density matter like that of the Moon and Io could have been born only by planets no smaller than Jupiter in size. Consequently, the very fact of the earth having such a satellite reveals the specifics of its development in the bowels of the Protoearth which also possessed a giant helium-hydrogen mantle and was able to produce its own satellite and ring systems to which it supplied part of its silicate material.
Like the core of Jupiter, the Earth originated under the effect of the gravitational pull in the mother-planet thanks to the fact that hydrogen, iron and silicates in the molten state did not mix within its giant fluidal mantle. The formation of the core provided an impulse for its rapid rotation and separation of the Moon under the effect of centrifugal forces. Thus the Earth and the Moon "emerged" from one and the same source-the Protoearth. But because of its proximity to the Sun its subsequent evolution was different from that of Jupiter.
SUN AMIDST GIANT PLANETS
The mass of our luminary accumulated similar to that of the planets near it-through the gravitational pulling
Planets of the solar and other stellar systems depending on their mass (in units of Jupiter mass, Miup) and the mean distance to stars (astronomical units). Conventional signs: 1-stars and the sun; 2-brown dwarfs', 3-planets with giant fluidal mantles; 4-ferrolithic planets of the terrestrial group; 5-ice comet-like bodies. With the approach to stars (sun) one can trace a normal increase in the mass of planets, indicated by arrow, and a reverse process, indicated by connecting lines-anomalous decrease of their mass as a result of surf ace degassing under the effect of stellar wind.
together of the ice helium-hydrogen planetesimals. In the center of the rapidly spinning giant nebular disc the temperature was close to the absolute zero, which provided for the condensation and solidification of nearly all of the gases it contained, including hydrogen. Due to a low temperature much more icy matter accumulated in the center of the solar mass than in the giant planets. As a result of gravitational compaction, accompanied by heat release, the temperature rose to 1.6* 10 7 K. That was enough for the development of thermonuclear reactions, and the sun reached its stellar condition.
Under the effect of the solar wind emitted by the luminary a vast mass of hydrogen and helium present in the rapidly rotating nebular disc, which originally filled the intercometary and interplanetary spaces, escaped from the solar system. Its main genetic essence consists in the inability to amass in the central star the huge volumes of these gases. And this is explained by the excessively rapid rotation of the protosolar nebular disc. As a result there appeared-typical of the solar system- distribution of the momentum between the slowly rotating luminary and the planets speeding up around it along their orbits. At that stage of evolution the sun happened to be surrounded by giant planets being at a distance from it (Jupiter, Saturn, Uranus and Neptune) and in its immediate proximity (protoplanets of the earth group).
The reality of this condition of the solar system is confirmed by the analogues of protoplanets of the earth group-giant planets recently discovered in stellar systems similar to the solar one. Thanks to their closeness to the stars, they cause their oscillation in space which is detected on the earth with the help of precision Dopper instruments.
STELLAR ANALOGUES OF SOLAR SYSTEM
The sequence of Uranus-Satum-Jupiter in the solar system reflects the legitimate mass increase of the giant planets with their approach to stars. This regularity also fully applies to the giant planets discovered around stars like the sun most of which are more massive than Jupiter. With the approach to stars, planets give way to brown dwarfs whose thermal emis-
sion adds up to those of the stars caused by thermonuclear reactions on the basis of deuterium.
Coming back to the solar system, the position of the giant planets around the luminary dates back to the beginning of their evolution, when all of them were formed in the same manner and their mass increased with their approach to the sun. In the following period of its maximum activity, and under the effect of solar wind, these giant protoplanets underwent degassing on the surface and their ferrolithic cores turned into planets of the earth group-Mercury, Venus, Earth and Mars. In this condition the planetary system lost its influence upon the position of the sun in space. That is why stellar analogues of the present-day solar system are indiscernible from earth.
With the loss of its fluidal mantles planets of the earth group also lost their satellite systems; only fragments of them have been preserved-Moon near the earth and Phobos and Deimos near Mars. The loss of the external limiting pressure produced a stressed condition of the ferrolithic planets-thanks to the high pressure of hydrogen fluids concentrated in their molten iron cores at the protoplanetary stage of their development. Connected with this is the endogenic activity of planets of the earth group. At the present stage it continues only in the earth, which retains its molten core, with the rest of the near-sun planets (Mercury, Venus, Mars) being fully consolidated, having lost their activity and their own magnetic fields.
The surface degassing has proved to be most catastrophic for planets at a primitive level of development, which did not laminate into cores and strong silicate mantles (situated in orbits between Mars and Jupiter). These underwent explosive breakups, forming a splinter belt of asteroids serving as the source of meteorites. Partial degassing also occurred on Jupiter, which lost some of its lightest gas-hydrogen, which is proved by an abnormally high mean density of this planet (almost twice as high as of Saturn). But in the main, Jupiter, like other planets of its group, has retained its giant helium-hydrogen mantle thanks to its distance from the Sun of 5.2 astronomical units.
The grandiose process of degassing of the near-sun planets has an analogy in star systems; this consists in a smaller mass of planets with their proximity to stars. Thus the closer to the Upsilon Andromedae star its surrounding giant fluidal planets, the less massive they are. The biggest of them has almost folly preserved its fluidal mantle thanks to its great distance from the star (2.4 a. u.). This planet is an analogue of the protoplanets of the asteroid belt in the solar system (distance from the sun-2.2- 2.8 a. u.); the asteroids underwent complete surface degassing under the effect of the solar wind and explosive disintegration-under the pressure of helium-hydrogen fluids which concentrated in their cores during the period of protoplanetary development.
A partially degassed planet of the Upsilon Andromedae star, at a distance of 0.8 a. u. from it, correlated to some extent with Venus (0.7 a. u. from the sun), while another one, at a distance of 0.06 a. u., has no analogues among the planets of the solar system. But by its proximity to the star that planet, similar to Jupiter, is unique.
Thus we can trace in the planetary star systems only partial surface degassing of the near-star giant planets. This has been completed in the solar system leading to the formation of small ferrolithic planets of the terrestrial group. Tracing their analogues in the world of stars calls for a special approach-on the basis of a long-time monitoring of stars which will be able to detect the "shadows" of even small ferrolithic planets surrounding them.
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