Encyclopedia of the Solar System 2nd ed

36 Encyclopedia of the Solar System

FIGURE 10 Oxygen isotopic composition of various bodies in
the solar system. The x and y axes show increasing 170 and 180
abundances, respectively. The oxygen isotopic composition of the
components in chondrites, in particular CAIs, is highly
heterogeneous for reasons that are unclear. The net result of this
variability is that different planets possess distinct oxygen
isotopic compositions that define as individual mass fractionation
lines as shown here for eucrites, howardites, and diogenites,
which come from Vesta and SNC meteorites, thought to come
from Mars. The Moon is thought to have formed from the debris
produced in a giant impact between the proto-Earth when 90%
formed and an impacting Mars-sized planet sometimes named
“Theia.” The fact that the data for lunar samples are collinear
with the terrestrial fractionation line could mean that the Moon
formed from the Earth, or the planet from which it was created
was formed at the same heliocentric distance, or it could mean
that the silicate reservoirs of the two planets homogenized
during the impact process, for example by mixing in a vapor
cloud from which lunar material condensed. (From A. N.
Halliday, 2003, The origin and earliest history of the Earth, in
“Meteorites, Comets and Planets” (A. M. Davis, ed.), Vol. 1,
“Treatise of Geochemistry” (H. D. Holland and K. K. Turekian,
eds.), pp. 509–557, Elsevier-Pergamon, Oxford.

in different parts of the nebula seem to have specific

oxygen isotope compositions. This makes it possible

to link all of martian meteorites together for example

(Fig. 10). These meteorites are specifically linked to

Mars because nearly all of them are too young to have

formed on any asteroid; they had to come from an ob-

ject that was large enough to be geologically active in

the recent past. This was confirmed by a very close

match between the composition of the atmosphere

measured with theVikinglander and that measured

in fluids trapped in alteration products in martian me-

teorites. In fact, martian meteorites provide an aston-

ishing archive of information into how Mars formed

and evolved as discussed in Section 6. To date, only

FIGURE 11 Iron meteorites are the most abundant kind of

meteorite found because they are distinctive and because they

survive long after other kinds of meteorites are destroyed by

weathering. In contrast, chondrites are the most abundant class

of meteorite observed to fall. Some iron meteorites are thought

to represent disrupted fragments of planetesimal cores. Others

appear to have formed at low pressures, probably as metal-rich

pools formed from impacts on asteroids. The Henbury meteorite

shown here is a type IIIAB magmatic iron that fell near Alice

Springs, Australia, about 5000 years ago. The texture shown on

the sawn face are Windmanstatten patterns formed by slow

cooling, consistent with an origin from a core located deep within

a meteorite parent body. (Photograph courtesy of Drs. M. Grady

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

one asteroidal source has been positively identified:

Vesta, whose spectrum and orbital location strongly

suggest it is the source of the howardite, eucrite, and

diogenite (HED) meteorites.

3. Irons(see Fig. 11) are largely composed of iron,
   nickel (about 10% by mass), and sulfides, together
   with other elements that have a chemical affinity for
   iron, calledsiderophile elements. Like chondrites,
   irons can be grouped according to their likely parent
   body, and several dozen groups or unique irons have
   been found. The textures of mineral grains in iron me-
   teorites have been used to estimate how quickly their
   parent bodies cooled, and thus the depth at which
   they formed. It appears that most irons are samples
   of metallic cores of small asteroidal parent bodies,
   10–100 km in radius. These appear to have formed
   very early, probably within a million years of CAIs,
   when there was considerable heat available from de-
   cay of short-lived radioactive isotopes (see Section 6).
   Other irons appear to have formed by impact melting
   at the surface of asteroids, and these formed later. A
   rare class of stony-iron meteorite (amounting to about
   5% of all nonchondritic meteorites) called pallasites
   contains an intricate mixture of metal and silicate
   (Fig. 12). It is thought these come from the core–
   mantle boundary regions of differentiated asteroids
   that broke up during collisions.