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*SP-345 Evolution of the Solar System*
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* *

*INTRODUCTION*

 

[*3*] *1.1. FUNDAMENTAL APPROACHES TO THE PROBLEM*

How our solar system was formed is a question that today attracts as
much interest as the problem of the Creation did in the past. In many
theories advocated today, the basic approach to this problem remains
remarkably similar to what it was in ancient times: The author
hypothetically assumes some specific primordial configuration of matter
and then deduces a process from which some significant features of the
present state emerge. When the basic assumption is unrelated to actually
observed phenomena, chances are that the result will be the same as over
thousands of years: a model which, by definition, is a myth, although it
may be adorned with differential equations in accordance with the
requirements of modern times.

A realistic attempt to reconstruct the early history of the solar system
must necessarily be of a different character. It is essential to choose
a procedure which reduces speculation as much as possible and connects
the evolutionary models as closely as possible to experiment and
observation. Because no one can know a priori what happened four to five
billion years ago, we must start from the present state of the solar
system and, step by step, reconstruct increasingly older periods. /This
actualistic principle/, which emphasizes reliance on observed phenomena,
is the basis for the modern approach to the geological evolution of the
Earth; "the present is the key to the past." This principle should also
be used in the study of the solar system. The purpose of this monograph
is to show how this can be done.

We proceed by establishing which experimentally verified laws are of
controlling significance in the space environment. To achieve this, we
must rely on the rapidly increasing information on extraterrestrial
processes that modern space research is providing, and on laboratory
studies of these processes under controlled conditions. If the large
body of available empirical knowledge is interpreted strictly in terms
of these laws, the speculative ingredient of cosmogonic theories can be
significantly reduced.

When analyzing the origin and evolution of the solar system, we should
recognize that its present structure is a result of a long series of
complicated processes. The final aim is to construct theoretical partial
models of all [*4*] these processes. However, there is often a choice
between different partial models which a priori may appear equally
acceptable. Before the correct choice can be made, it is necessary to
define a framework of boundary conditions which these models must
satisfy. We consider this to be a main task of this monograph.

* *

*1.2. PLANETARY SYSTEM-SATELLITE SYSTEMS*

Theories of the formation of the solar system must also account for the
satellite systems in a manner consistent with the way in which the
planetary system is treated. In certain respects the satellite systems
provide even more significant information about evolutionary processes
than does the planetary system, partly because of the uncertainty about
the state of the early Sun.

Observing that the highly regular systems of Jupiter, Saturn, and Uranus
are in essential respects similar to the planetary system, we aim at /a
general theory of the formation of secondary bodies around a primary
body/. This approach contrasts with that of the Laplacian-type theories
in which the postulated processes for planetary formation fail to
explain the structure of the satellite systems. Although it is desirable
to avoid excessive terminology, we will frequently make brief reference
to this specific aspect of our analytical method by using the term
/hetegony/ (from the GreekGreek word, companion, and Greek word , generate).

The theoretical framework we try to construct should, consequently, be
applicable both to the formation of satellite systems around a planet
and to the formation of planets around the Sun. Through this
requirement, we introduce the postulate that these processes are
essentially analogous. Our analysis supports this postulate as
reasonable. Indeed, we find evidence that the formation of the regular
systems of secondary bodies around a primary body- either the Sun or a
planet-depends in a unique way on only two parameters of the primary
body, its mass and spin. It is also necessary to assume that the central
bodies were magnetized, but the strength of the magnetic field does not
appear explicitly; it must only surpass a certain limit.

* *

*1.3. FIVE STAGES IN THE EVOLUTION*

Applying the actualistic and hetegonic principles, we find that the
evolutionary history of the solar system can be understood in terms of
five stages, in part overlapping in time:

 

    (1) Most recently-during the last three to four billion years-a
    /slow evolution/ of the primeval planets, satellites, and asteroids
    which produced [*5*] the present state of the bodies in the solar
    system. By studying this latest phase of the evolution
    (post-accretional evolution), we prepare a basis for reconstructing
    the state established by earlier processes. 
      
    (2) Preceding this stage, an /accretional evolution/ of condensed
    grains, moving in Kepler orbits to form planetesimals which, by
    continuing accretion, grow in size. These planetesimals are the
    embryonic precursors of the bodies found today in the solar system.
    By clarifying the accretional processes, we attempt to reconstruct
    the chemical and dynamic properties of the early population of grains. 
      
    (3) To account for grains moving in Kepler orbits around the Sun and
    the protoplanets, /transfer of angular momentum/ from these primary
    bodies to the surrounding medium must have occurred in the stage of
    evolution preceding accretion. 
      
    (4) /Emplacement of gas and dust/ to form a medium around the
    magnetized central bodies in the regions where the planet and
    satellite groups later accreted. 
      
    (5) /Formation of the Sun/ as the first primary body to accrete from
    the source cloud of the solar system. 

* *

*1.4. PROCESSES GOVERNING THE EVOLUTIONARY STAGES*

Each of the five main stages in the sequence discussed above was
governed by physical and chemical processes which may be characterized
in the following way:

* *

*1.4.1. Post-Accretional Evolution; Effects of Tides and Resonances*

The most striking result of the analysis of this stage, which has lasted
for about four billion years, is that there has been very little change.
The Earth-Moon system and the Neptune-Triton system have evolved due to
tidal effects, but otherwise the primary-secondary systems exhibit a
high degree of stability. This high degree of stability is shown not
only by the dynamic state of planets and satellites, but also by certain
structures in the asteroidal belt and the Saturnian rings. The
complicated pattern of resonances between the bodies in the solar system
is probably a major cause of this stability.

Comets and meteoroids are exceptions; they are in a state of rapid
change. Information on the changes in these populations can be derived
from studies of their orbital characteristics and to some extent can be
inferred from the structure and irradiation history of meteorites and
lunar materials.

[*6*] An evolution, although much slower, has also taken place in the
asteroid belt, resulting in changes in asteroidal jet streams and families.

* *

*1.4.2. Accretional Evolution; Viscosity-Perturbed Kepler Motion and the
Evolution of Protosatellites and Protoplanets From Jet Streams*

Since the planets and, even more so, the satellites are too small to
have formed by gravitational collapse, planetesimal accretion is the
only feasible theory of formation; that is, the planets and satellites
have been formed by accretion of small bodies (planetesimals and,
initially, single grains). The conditions in those regions of space
where planets or satellites formed must, at a certain stage in
development, have borne similarities to the present state in the
asteroidal region. Studies of the present asteroidal region consequently
provide information on the processes governing planet and satellite
accretion. The isochronism of asteroidal and planetary spin periods
gives strong support to the planetesimal model which leads to a
promising theory of spin.

The phenomenon basic to a study of accretion of planets and satellites
is the Kepler motion perturbed by viscosity (mutual collisions between
bodies). It is surprising that in all earlier cosmogonic theories the
basic properties of such a state of motion have been misunderstood. It
has been believed that a population of mutually colliding grains
necessarily diffuse out into a larger volume. This is not correct.
Because the collisions are essentially inelastic and the collision
frequency less than the orbital frequency, the diffusion of a population
in Kepler motion is /negative/, meaning that the orbits become
increasingly similar.

This negative diffusion lends to formation of /jet streams/,
self-focusing streams of bodies orbiting around a gravitating central
body. Such jet streams are likely to constitute an intermediate stage in
the accretion of celestial bodies.

Focused streams observed today in the asteroidal region and meteor
streams may be held together by the same effect. If this is confirmed,
studies of the essential properties of jet streams under present-day
conditions could reduce the speculative element of hetegonic theories.

* *

*1.4.3. Processes Relating to the Angular Momentum Transfer and Emplacement*

The motion of a dispersed medium under space conditions can obviously
not be treated without hydromagnetics and plasma physics as a basis. The
criterion for justified neglect of electromagnetic effects in the
treatment of [*7*] a problem in gas dynamics is that the characteristic
hydromagnetic parameter *L* (defined in eq. (15.1.1)) is much less than
unity. In cosmic problems involving interplanetary and interstellar
phenomena, *L* is usually of the order 10^15 -10^20 . In planetary
ionospheres it reaches unity in the *E* layer. Planetary atmospheres and
hydrospheres are the only domains in the universe where a
nonhydromagnetic treatment of fluid dynamic problems is justified.

Nonetheless, the misconception is still common that if only a cosmic
cloud is "cold" enough, and stellar radiation is absorbed in its outer
layer a nonhydromagnetic treatment is legitimate. In the interior of
cold, dark clouds, the factor *L* is certainly /much smaller/ than in
most other regions in space, but ionization by cosmic radiation, by
natural radioactivity, and especially by currents associated with
magnetic fields which are not curl-free is still sufficient to make it
/much larger/ than unity. *L* may possibly reach values as low as 10^6
in such environments, but this still means that by ignoring
hydromagnetic processes one neglects effects which are many orders of
magnitude larger than those considered.

If we assume that the formation of the solar system took place in a
cloud of the same general character as the dark clouds observed today,
we can get observational indications of the minimum possible effects of
hydromagnetic and plasma processes in the hetegonic nebulae. Recent
observations of strong magnetic fields and of radio emission from
complex molecules in certain dark clouds give clues to the state of
matter in these clouds.

In an early nebula where, according to all theories, dispersed matter
was dissipating large amounts of energy, the inevitable hydromagnetic
effects must have been still more pronounced. A theory of the formation
of the solar system is obviously meaningless unless it is based on
modern plasma physics and magnetohydrodynamics.

Our analysis shows the controlling phenomena during the emplacement of
matter and transfer of angular momentum in the circumsolar region to be
as follows:

 

(1) /Critical velocity/, a plasma phenomenon which has been studied
extensively in the laboratory and also analyzed theoretically. It
defines the conditions under which neutral gas falling toward a
magnetized central body becomes ionized and stopped. The phenomenon is
sufficiently well understood for its importance in cosmic processes to
be recognized.

The application of the critical velocity phenomenon suggests an
explanation of the mass distribution in the planetary system as well as
in the satellite systems. It further accounts for some of the processes
of chemical] differentiation indicated by bulk properties of planets and
satellites and by the interstellar medium.

(2) /Partial corotation/, a state of revolution of a plasma surrounding
a rotating magnetized body. Evidence for the basic role of partial
corotation [*8*] in transfer of angular momentum in the primordial
planetary and solar nebular processes is found in the detailed structure
of the Saturnian rings and the asteroidal belt.

Transfer of angular momentum from a central body to a surrounding medium
is a process which is fundamental in the formation of secondary bodies
revolving around their primary. This process can be studied by
extrapolation of present-day conditions in the magnetosphere and in the
solar atmosphere. In fact, the electric current system in the auroral
region is now known to be of the same type as is necessary for the
transfer of angular momentum from a central magnetized body to a
surrounding plasma. Hence, this primordial process can be derived from
processes now studied by space probes.

(3) /Formation and plasma capture of grains/. The solid grains in the
solar system may have formed by condensation from the nebular plasma
emplaced in the circumsolar region. But it is also possible that much of
the solid material is interstellar dust condensed in other regions.

 

Infall of such preexisting grains may have been an important process
contributing material to the early solar system. This is suggested by
the present-day distribution of dust in dark clouds in interstellar
space and possibly by some of the chemical features of the material
preserved in meteorites. Since grains in space are necessarily
electrostatically charged, small dust particles of transplanetary origin
now found in bodies in nearcircular Kepler orbits are likely to have
been brought into corotation with the revolving magnetized plasma in the
circumsolar and circumplanetary regions.

* *

*1.4.4. Origin of the Sun*

Theories concerning the origin of the Sun and other stars clearly also
must have a foundation in hydromagnetics and plasma physics. Even if
appropriately constructed, such theories are by necessity speculative
and uncertain. For this reason, in the present study of the formation of
the solar system, we do not rely on any initial assumptions concerning
the primeval Sun or its history except that during the hetegonic era it
existed and was surrounded by a plasma. From our analysis of the solar
system, however, we can draw specific conclusions about the primeval
Sun: Its mass was approximately the same as today, but its spin and
magnetization were much larger.

* *

*1.5. MODEL REQUIREMENTS AND LIMITATIONS*

The completion of a set of quantitative, mutually consistent, and
experimentally supported models for the evolution of the solar system is
[*9*] obviously still in the remote future. We need much more
information from space data and from laboratory investigation to be able
to reduce to a manageable level the speculative element which such
models necessarily still contain.

As a first step, we have tried to identify those physical and chemical
laws that, at our present state of knowledge, emerge as most important
in controlling the processes in the solar system now and in early times.
Within the constraints obtained in this way, we have attempted to
develop a series of partial models which both satisfy the general
principles outlined above and also, when taken together, define what we
regard as an acceptable framework for theories of the evolution of the
solar system (ch. 23 <ch23.htm>). We construct a matrix (table 23.8.1)
complete in the sense that it comprises all the groups of planets and
satellites (with the exception of the tiny Martian satellites). The
general framework also includes asteroids, comets, and meteoroids.

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