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Mercury: Space Environment, Surface, and Interior (2001) 8003.pdf SPECIFIC CHARACTER OF THE FORMATION OF MERCURY AS THE DENSEST PLANET. A. A. Marakushev, O. V. Chaplygin and A. V. Bobrov, Institute of Experimental Mineralogy (IEM RAS, 142432 Chernogolovka, Moscow Region, Russia; e-mail: [email protected]). Earth–type planets were initially formed as giant planets, similarly to the planets of the group of Jupiter and simultaneously with the solar mass by the accumulation of aqueous–hydrogen planetesimals, which were similar by physical state to Pluto, Charon, and comets. Gravitational compression was accompanied by melting of the planets whose size increases with the approach to the Sun, with which they also similar in composition. Development of hydrogen–water–silicate liquid immiscibility in planets was accompanied by their layering to heavy cores and gigantic fluid envelopes. The impulses of high–spin motion of these envelopes resulted in separation of satellites under the action of centrifugal forces. Satellite density varies widely, growing with the approach to dense iron–silicate cores of their parental giant planets. The maximum density of satellites of the peripheral, remote from the Sun planets (Neptune, Uranus, Saturn) does not exceed 1,9 g/cm3, but for near–solar giant planets it reaches 3,5 g/cm3 (Io, the satellite of Jupiter) and 3,3 g/cm3 (Moon, the satellite of proto–Earth). In contrast to peripheral near–solar giant planets underwent surface degassing under the action of solar wind. Jupiter in this case lost the part of hydrogen of its fluid envelope, which can be judged from its anomalously high density (1,3 g/cm3), almost two times higher than that of Saturn (0,7 g/cm3). The giant planets located nearer to the Sun, completely lost fluid envelopes being transformed into terrestrial planets. They lost satellite systems because of decrease of gravity. Only relicts of their satellites (Moon, Phobos and Deimos) remained. Several dozens of analogues of near–solar giant planets (figure) was revealed in the last years (1995–2001) in the surroundings of the stars, similar to the Sun [1, 2, 3, etc.]. The degree of surface degassing of these planets is traced by the effect of the decrease of their massiveness with the approach to the stars [2]. Terrestrial planets initially were formed as iron– silicate cores of parental giant protoplanets simultaneously with the separation of lighter silicate material into their satellite systems. Cores were formed in the protoplanets in the sequence of the development of chondrite systems, starting with the iron meteorites (I) of the corresponding genetic groups: I (type Netschaevo) – HH – H – L – LL. This development was determined by the increase of H2O/H2 ratio in the fluid envelopes of the protoplanets, which were undergoing degassing under the action of the solar wind. The development ceased at the different stages upon the complete degassing of planets in the sequence of moving away from the Sun. Proto–Mercury was the first giant planet, which lost its fluid envelope. Therefore Mercury has the most primitive composition, extremely rich in iron (I–HH). Furthermore the content in earth–type planets of nickel–iron phase is found in dependence on the degree of the development of their satellite systems. It increases with the approach to the Sun in the row: Neptune – Uranus – Saturn – Jupiter. The propagation of this regularity to the giant protoplanets of terrestrial group explains well the unique nature of Mercury, whose silicate phase to a considerable extent went away into the satellite system of its parental protoplanet, which the latter then lost. Figure. Planets of the Solar system and their satellites in comparison to the extrasolar planets. 1 – the Sun and analogous stars; 2 – brown dwarfs; 3 – fluid giant planets; 4 – terrestrial planets; 5 – molten iron–silicate core of Jupiter; 6 – the Jovian galilean satellites and the Moon; 7 – supposed lost satellites of proto–Earth and proto–Mercury; 8 – Pluto. MJup – mass of Jupiter; AU – astronomical unit. Normal growth of the massiveness of planets with the approach to the stars is marked by an arrow, the decrease of their massiveness as a result of the surface degassing under the action of the stars is shown with connodes. References: [1] Mayor, M. and Queloz, D. (1995) Nature, 378, N 6555, P. 355. [2] Butler R. P. (1999) Carnegie Inst. Washington, 110–112. [3] Marcy G. W. and Butler R. P. (1998) Ann. Rev. Astronomy and Astrophysics, 36, 57–98.