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Mercury: Space Environment, Surface, and Interior (2001)
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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.