The Solar System

410 PART 4^ |^ THE SOLAR SYSTEM

If planets formed by accretion of planetesimals and were

later melted by radioactive decay and heat of formation, then

Earth’s early atmosphere may have consisted of a combination

of gases delivered by planetesimal impacts and released from

crushed rock. Such a relatively soft soil layer on the surfaces of
larger planetesimals may have been eff ective in trapping
smaller bodies.
Th e largest planetesimals would grow the fastest because
they had the strongest gravitational fi eld. Th eir stronger gravity
could attract additional material, and they could also hold on
to a cushioning layer to trap fragments. Astronomers calculate
that the largest planetesimals would have grown quickly to
protoplanetary dimensions, sweeping up more and more
material.
Protoplanets had to begin growing by accumulating solid
material because they did not have enough gravity to capture
and hold large amounts of gas. In the warm solar nebula, the
atoms and molecules of gas were traveling at velocities much
larger than the escape velocities of modest-sized protoplanets.
Th erefore, in their early development, the protoplanets could
grow only by attracting solid bits of rock, metal, and ice. Once
a protoplanet approached a size of 15 Earth masses or so, how-
ever, it could begin to grow by gravitational collapse, the
rapid accumulation of large amounts of in-falling gas from the
nebula.
Th e theory of protoplanet growth into planets supposes that
all the planetesimals had about the same chemical composition.
Th e planetesimals accumulated to form a planet-sized ball of
material with homogeneous composition throughout. Once the
planet formed, heat would begin to accumulate in its interior
from the decay of short-lived radioactive elements.
Th e violent impacts of in-falling particles would also have
released energy called heat of formation. Th ese two heating
sources would eventually have melted the planet and allowed it
to diff erentiate. Diff erentiation is the separation of material
according to density. Once a planet melted, the heavy metals
such as iron and nickel, plus elements chemically attracted to
them, would settle to the core, while the lighter silicates and
related materials fl oated to the surface to form a low-density
crust. Th e scenario of planetesimals combining into planets that
subsequently diff erentiated is shown in ■ Figure 19-9.
Th e process of diff erentiation depends partly on the presence
of short-lived radioactive elements whose rapid decay would
have released enough heat to melt the interior of planets.
Astronomers know such radioactive elements were present
because very old rock from meteorites contains daughter isotopes
such as magnesium-26. Th at isotope is produced by the decay
of aluminum-26 with a half-life of only 0.74 million years. Th e
aluminum-26 and similar short-lived radioactive isotopes are
gone now, but they must have been produced in a supernova
explosion that occurred shortly before the formation of the solar
nebula. In fact, some astronomers suspect that supernova explo-
sions could have triggered the formation of the sun and other
stars by compressing interstellar clouds (see Figure 11-2 and
11-3). Th us, our solar system may exist because of a supernova
explosion that occurred about 4.6 billion years ago.

■ Figure 19-9

This simple model of planet building assumes planets formed from accre-

tion and collision of planetesimals that were of uniform composition,

containing both metals and rocky material, and that the planets later dif-

ferentiated, meaning they melted and separated into layers by density and

composition.

Planetesimals

contain both rock

and metal.

A planet grows slowly

from the uniform

particles.

The resulting planet

is of uniform

composition.

Heat from radioactive decay

and planetesimal in-fall

causes differentiation.

The resulting planet

has a metal core and

low-density crust.