Comets and the Origin and Evolution of Life

32 A. Delsemme
happened during that “lost interval,” we must leave geophysics and look at
the astronomical evidence about the origin of the Solar System
2.2.2 The Origin of the Solar System
Laplace (1796) is the first who tried to solve the problem of the origin of the
Solar System by using observational facts. In the last pages of his “Exposition
du Système du Monde,” he lists first all the quasi-regularities of the Solar System,
and then proposes that originally, a “fluid of immense extent enveloped
the Sun as an atmosphere,” whose primeval rotation enforced all those quasiregularities.
For a long time however, 19th-century astronomers believed that
Laplace’s model was wrong, because the Sun turned much too slowly to result
from the central agglomeration of this “Solar Nebula” (dubbed this way by a
false analogy with what we now call spiral galaxies).
The problem of the present angular momentum of the Sun disappeared,
when the solar wind was discovered. Its spiral motion away from the Sun
produces a magnetic brake that slows the solar rotation continuously. The
apparent contradiction with a very fast primeval rotation had disappeared,
and Laplace’s hypothesis came back in favor some 50 years ago. However, it
still was a speculative scenario difficult to analyze numerically. For instance,
von Weiszäcker (1944) proposed to arrange the gas eddies in epicycles in
order to explain the distances of the planets (Bode’s law). Soon, his analysis
was shown to be in quantitative disagreement with fluid mechanics, but it
attracted the attention on a possible mechanism that had been neglected so
far: viscous turbulence can dissipate energy and redistribute momentum in
the primitive nebula.
To make a long story short, the basic theory of the viscous accretion disk
has been given by Lynden-Bell and Pringle (1974). Numerical models have
been developed, in particular by Cameron (1985), Lin and Papaloisou (1985),
Wood and Morfill (1988), Morfill (1988), and Morfill and Wood (1989). There
are still some difficulties with the model. For instance, the origin of the rather
high viscosity needed to dissipate the angular momentum has not been properly
clarified, although its most likely source is a strong convection driven by
gas turbulence. Gas turbulence in the disk is itself maintained by the gravitational
collapse of the interstellar cloud that feeds the disk. However, there are
other possible mechanisms. A gravitational torque (Larson, 1984) could also
dissipate angular momentum. Such a gravitational torque could be introduced
by a spiral structure in the disk, or in general by mass distributions that are
not axisymmetrical. If the gas is ionized enough, magnetic forces could also
play an important role, as the action of the solar wind on the present Sun
demonstrates. After all, the solar wind could still be the subsiding remnant
of the mechanism involved in the shedding of the disk 5 billion years ago.