Scientists believe that iron meteorites come from.the cores of asteroids that melted. But what happened.to the corresponding rocky material that formed the.mantles of these bodies? A few asteroids have spectra^1.that match those of mantle rocks, but they are very.rare. Some nonmetallic meteorites come from.asteroids that have partially or wholly melted, but.these do not match the minerals we would expect to.see in the missing mantles of the iron parent bodies..These exotic meteorites must come from some other.kind of parent body instead..The rarity of mantle rocks in our meteorite.collection and in the asteroid belt, known as the.“missing mantle problem,” is a long-standing puzzle..There are several reasons why iron fragments might.survive better than rocky fragments when asteroids.break apart. Iron lies in the core of a differentiated.asteroid, while rocky material lies near the surface..Thus, rocky material will be the first to be removed.when an asteroid is bombarded, while iron is the last.to be exposed. As a result, rocky fragments have to.survive in space for longer than iron ones. Most of the.rocky mantle may be peeled away in small fragments.—chips from the surface—while the iron core remains.as a single piece, making it harder to disrupt later. Last.and most important, iron is much stronger than rock:.a piece of iron is likely to survive in the asteroid belt at.least 10 times longer than a rocky fragment of the.same size..If most differentiated bodies broke apart early in.the solar system, perhaps all the mantle material has.been ground down to dust and lost over the billions of.years since then. This would mean that intact.differentiated asteroids are very rare in the asteroid.belt today. Perhaps Vesta [a differentiated asteroid.with a diameter of more than 300 miles] and a handful.of others are all that remain..However, collisional erosion cannot be the whole.story. Primitive asteroids, the parent bodies of.chondritic meteorites [the most common type of.meteorite found on Earth], are no stronger than the.mantle rocks from differentiated asteroids. How did.so many primitive asteroids survive when almost.none of the differentiated ones did? Part of the.explanation may simply be that differentiated bodies.were relatively rare to begin with and none have.survived. Still, if almost all differentiated bodies were.destroyed in violent collisions, how did Vesta survive.with only a single large crater on its surface?.Astronomer William Bottke and his colleagues.recently came up with a possible explanation: perhaps.the parent bodies of the iron meteorites formed closer.to the Sun, in the region that now contains the.terrestrial planets. Objects would have been more.tightly packed nearer the Sun, so collisions would.have been more frequent than in the asteroid belt..Many, perhaps most, differentiated bodies were.disrupted by violent collisions. Gravitational.perturbations from larger bodies scattered some of.these fragments into the asteroid belt. Both iron and.rocky fragments arrived in the asteroid belt, but only.the stronger iron objects have survived for the age of.the solar system. Later on, the parent bodies of.primitive meteorites formed in the asteroid belt. Most.of these objects survived, leaving an asteroid belt.today that is a mixture of intact primitive bodies and.fragments of iron..(^1) Characteristic wavelengths of light that asteroids reflectQ: The main purpose of the passage is to Answer Choices: (A)discuss a study intended to explain the high number of meteorites on Earth that have come from primitive asteroids. (B)describe competing hypotheses about the conditions under which primitive asteroids initially formed. (C)present a scientific debate about the prevalence of differentiated asteroids in the asteroid belt in the early solar system. (D)account for the scarcity of a component of differentiated asteroids in the asteroid belt and among meteorites on Earth.
A: Among A through D, the answer is
(A)discuss a study intended to explain the high number of meteorites on Earth that have come from primitive asteroids.
(B)describe competing hypotheses about the conditions under which primitive asteroids initially formed.
(C)present a scientific debate about the prevalence of differentiated asteroids in the asteroid belt in the early solar system.
(D)account for the scarcity of a component of differentiated asteroids in the asteroid belt and among meteorites on Earth.
Answer: 3