Encyclopedia of the Solar System 2nd ed

34 Encyclopedia of the Solar System

been similar to that of the Sun today. The challenge is there-
fore to explain how it is possible that a disk that formed gas
giant objects like Jupiter and Saturn, also generated rocky
terrestrial planets like the Earth (Fig. 4).
Most meteorites are thought to come from parent bod-
ies in the Main Asteroid Belt that formed during the first
few million years of the solar system. As a result, these
objects carry a record of processes that occurred in the so-
lar nebula during the formation of the planets. In a few
cases, the trajectories of falling meteorites have been used
to establish that they arrived on orbits coming from the As-
teroid Belt. Most other meteorites are deduced to come
from asteroids based on their age and composition. IDPs
are thought to come from both asteroids and comets. A
few meteorites did not originate in the Asteroid Belt. The
young ages and noble-gas abundances of the Shergottite–
Nakhlite–Chassignite (SNC) meteorites suggest they come
from Mars. A few dozen SNC meteorites have been found
to date, and a comparable number of lunar meteorites from
the Moon are also known.
The Earth is currently accumulating meteoritic material
at the rate of about 5× 107 kg/year. At this rate, it would
take more than 10^17 years to obtain the Earth’s current mass
of 5.97× 1024 kg, which is much longer than the age of
the universe. Even though it is thought that the Earth did
form as the result of the accumulation of smaller bodies, it
is clear that the rate of impacts was much higher while the
planets were forming than it is today.
Broadly speaking, meteorites can be divided into three
types: chondrites, achondrites, and irons, which can be dis-
tinguished as follows:

1. Chondritesare mixtures of grains from submicron-
   sized dust to millimeter- to centimeter-sized particles
   of rock and metal, apparently assembled in the solar
   nebula. Most elements in chondrites are present in
   broadly similar ratios to those in the Sun, with the ex-
   ception of carbon, nitrogen, hydrogen, and the noble
   gases, which are all highly depleted. For this reason,
   chondrites have long been viewed as representative
   of the dust and debris in the circumstellar disk from
   which the planets formed. So, for example, refractory
   elements that would have resided in solid phases in
   the solar nebula have chondritic (and therefore so-
   lar) relative proportions in the Earth, even though
   the volatile elements are vastly depleted. The non-
   metallic components of chondrites are mostly sili-
   cates such as olivine and pyroxene.Chondrulesare
   a major component of most chondrites (see Fig. 6).
   These are roughly millimeter-sized rounded beads
   of rock that formed by melting, either partially or
   completely. Their mineral-grain textures suggest they
   cooled over a period of a few hours, presumably in the
   nebula, with the heating possibly caused by passage

FIGURE 6 Chondrules are spherical objects, sometimes partly

flattened and composed of mafic silicate minerals, metal, and

oxides. They are thought to form by sudden (flash) heating in the

solar nebula. Some formed as much as 3 Ma after the start of the

solar system. (Photograph courtesy of Drs. M. Grady and S.

Russell and the Natural History Museum, London.)

through shock waves in the nebular gas. Some chon-

drules are thought to have formed later in collisions

between planetary objects. Most chondrites also con-

taincalcium-aluminum-rich inclusions(CAIs, see

Fig. 7), which have chemical compositions similar to

those predicted for objects that condensed from a

gas of roughly solar composition at very high tem-

peratures. It is possible that CAIs formed in the very

FIGURE 7 Calcium–aluminum refractory inclusions are found

in chondrite meteorites and are thought to be the earliest objects

that formed within our solar system. They have a chemical

composition consistent with condensation from a hot gas of solar

composition. How they formed exactly is unclear, but some have

suggested they were produced close in to the Sun and then

scattered across the disk. (Photograph courtesy of Drs. M. Grady

and S. Russell and the Natural History Museum, London.)