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the protoplanet Mars was forming compared to where the proto-
planet Earth was forming. Evidence for this part of the model
is found in Mars today, with its greater abundance of iron. It is
the abundant iron oxides that make Mars the red planet.
All of the protoplanets might have started out somewhat
similarly as huge accumulations of a slushy mixture with an
A
atmosphere of hydrogen and helium gases. Gravitational at-
traction must have compressed the protoplanets as well as the
protosun. During this period of contraction and heating, gravi-
tational adjustments continued, and about one-fifth of the disk
B
nearest to the protosun must have been pulled into the central
body of the protosun, leaving a larger accumulation of matter in
the outer part of the accretion disk.

C
STAGE C
FIGURE 15.19 Formation of the solar system according to During stage C, the warming protosun became established as
the protoplanet nebular model, not drawn to scale. (A) The process a star, perhaps undergoing an initial flare-up that has been
starts with a nebula of gas, dust, and chemical elements from pre- observed today in other newly forming stars. Such a flare-up
viously existing stars. (B) The nebula is pulled together by gravity, might have been of such a magnitude that it blasted away the
collapsing into the protosun and protoplanets. (C) As the planets hydrogen and helium atmospheres of the interior planets (Mer-
form, they revolve around the Sun in orbits.
cury, Venus, Earth, and Mars) out past Mars, but it did not reach
far enough out to disturb the hydrogen and helium atmospheres
region of space as dust, gases, and chemical compounds, but of the outer planets. The innermost of the outer planets, Jupi-
hydrogen was still the most abundant element in the nebula that ter and Saturn, might have acquired some of the matter blasted
was destined to become the solar system. away from the inner planets, becoming the giants of the solar
system by comparison. This is just speculation, however, and
the two giants may have simply formed from greater concentra-
STAGE B tions of matter in that part of the accretion disk.
During stage B, the hydrogen gas, dust, elements, and chemical The evidence, such as separation of heavy and light mineral
compounds from former stars began to form a large, slowly ro- matter, shows that the protoplanets underwent heating early in
tating nebula that was much, much larger than the present solar their formation. Much of the heating may have been provided by
system. Under the influence of gravity, the large but diffuse, slowly gravitational contraction, the same process that gave the proto sun
rotating nebula began to contract, increasing its rate of spin. The sufficient heat to begin its internal nuclear fusion reactions. Heat
largest mass pulled together in the center, contracting to the proto - was also provided from radioactive decay processes inside the
star, which eventually would become the Sun. The remaining protoplanets, and the initial greater heating from the Sun may
gases, elements, and dusts formed an enormous, fat, bulging disk have played a role in the protoplanet heating process. Larger bod-
called an accretion disk, which would eventually form the planets ies were able to retain this heat better than smaller ones, which
and smaller bodies. The fragments of dust and other solid matter in radiated it to space more readily. Thus, the larger bodies under-
the disk began to stick together in larger and larger accumulations went a more thorough heating and melting, perhaps becoming
from numerous collisions over the first million years or so. All of completely molten early in their history. In the larger bodies, the
the present-day elements of the planets must have been present in heavier elements, such as iron, were pulled to the center of the
the nebula along with the most abundant elements hydrogen and now-molten mass, leaving the lighter elements near the surface.
helium. The elements and the familiar chemical compounds ac- The overall heating and cooling process took millions of years as
cumulated into basketball-sized or larger chunks of matter. the planets and smaller bodies were formed. Gases from the hot
Did the planets have an icy slush beginning? Over a period interiors formed secondary atmospheres of water vapor, carbon
of time, perhaps 100 million years or so, huge accumula- dioxide, and nitrogen on the larger interior planets.
tions of frozen water, frozen ammonia, and frozen crystals of Interestingly, the belt of asteroids was discovered from a
methane began to collect, together with silicon, aluminum, prediction made by the German astronomer Bode at the end of
and iron oxide plus other metals in the form of rock and the eighteenth century. Bode had noticed a pattern of regular-
mineral grains. Such a slushy mixture would no doubt have ity in the spacing of the planets that were known at the time.
been surrounded by an atmosphere of hydrogen, helium, and He found that by expressing the distances of the planets from
other vapors thinly interspersed with smaller rocky grains of the Sun in astronomical units, these distances could be approxi-
dust. Evidence for this icy beginning is found today in the Oort mated by the relationship (n + 4)/10, where n is a number in
cloud and Kuiper Belt. Local concentrations of certain minerals the sequence 0, 3, 6, 12, and so on where each number (except
might have occurred throughout the whole accretion disk, with the first) is doubled in succession. When these calculations were
a greater concentration of iron, for example, in the disk where done, the distances turned out to be very close to the distances

394 CHAPTER 15 The Solar System 15-18