Of the thousands of specimens of meteorites found on Earth and known to science, only about 100 are igneous; that is, they have undergone melting by volcanic action at some time since the planets were first formed. These igneous meteorites are known as achondrites because they lack chondrules—small stony spherules found in the thousands of meteorites (called “chondrites”) composed primarily of unaltered minerals that condensed from dust and gas at the origin of the solar system. Achondrites are the only known samples of volcanic rocks originating outside the Earth-Moon system. Most are thought to have been dislodged by interbody impact from asteroids, with diameters of from 10 to 500 kilometers, in solar orbit between Mars and Jupiter.
Shergottites, the name given to three anomalous achondrites so far discovered on Earth, present scientists with a genuine enigma. Shergottites crystallized from molten rock less than 1.1 billion years ago (some 3.5 billion years later than typical achondrites) and were presumably ejected into space when an object impacted on a body similar in chemical composition to Earth.
While most meteorites appear to derive from comparatively small bodies, shergottites exhibit properties that indicate that their source was a large planet, conceivably Mars. In order to account for such an unlikely source, some unusual factor must be invoked, because the impact needed to accelerate a fragment of rock to escape the gravitational field of a body even as small as the Moon is so great that no meteorites of lunar origin have been discovered.
While some scientists speculate that shergottites derive from Io (a volcanically active moon of Jupiter), recent measurements suggest that since Io’s surface is rich in sulfur and sodium, the chemical composition of its volcanic products would probably be unlike that of the shergottites. Moreover, any fragments dislodged from Io by interbody impact would be unlikely to escape the gravitational pull of Jupiter.
The only other logical source of shergottites is Mars. Space-probe photographs indicate the existence of giant volcanoes on the Martian surface. From the small number of impact craters that appear on Martian lava flows, one can estimate that the planet was volcanically active as recently as a half-billion years ago—and may be active today. The great objection to the Martian origin of shergottites is the absence of lunar meteorites on Earth. An impact capable of ejecting a fragment of the Martian surface into an Earth-intersecting orbit is even less probable than such an event on the Moon, in view of the Moon’s smaller size and closer proximity to Earth. A recent study suggests, however, that permafrost ices below the surface of Mars may have altered the effects of impact on it. If the ices had been rapidly vaporized by an impacting object, the expanding gases might have helped the ejected fragments reach escape velocity. Finally, analyses performed by space probes show a remarkable chemical similarity between Martian soil and the shergottites.
21. The passage implies which of the following about shergottites?
I. They are products of volcanic activity.
II. They derive from a planet larger than Earth.
III. They come from a planetary body with a chemical composition similar to that of Io.
(A) I only
(B) II only
(C) I and II only
(D) II and III only
(E) I, II, and III
22. According to the passage, a meteorite discovered on Earth is unlikely to have come from a large planet for which of the following reasons?
(A) There are fewer large planets in the solar system than there are asteroids.
(B) Most large planets have been volcanically inactive for more than a billion years.
(C) The gravitational pull of a large planet would probably prohibit fragments from escaping its orbit.
(D) There are no chondrites occurring naturally on Earth and probably none on other large planets.
(E) Interbody impact is much rarer on large than on small planets because of the density of the atmosphere on large planets.
23. The passage suggests that the age of shergottites is probably
(A) still entirely undetermined
(B) less than that of most other achondrites
(C) about 3.5 billion years
(D) the same as that of typical achondrites
(E) greater than that of the Earth
24. According to the passage, the presence of chondrules in a meteorite indicates that the meteorite
(A) has probably come from Mars
(B) is older than the solar system itself
(C) has not been melted since the solar system formed
(D) is certainly less than 4 billion years old
(E) is a small fragment of an asteroid
25. The passage provides information to answer which of the following questions?
(A) What is the precise age of the solar system?
(B) How did shergottites get their name?
(C) What are the chemical properties shared by shergottites and Martian soils?
(D) How volcanically active is the planet Jupiter?
(E) What is a major feature of the Martian surface?
26. It can be inferred from the passage that each of the following is a consideration in determining whether a particular planet is a possible source of shergottites that have been discovered on Earth EXCEPT the
(A) planet’s size
(B) planet’s distance from Earth
(C) strength of the planet’s field of gravity
(D) proximity of the planet to its moons
(E) chemical composition of the planet’s surface
27. It can be inferred from the passage that most meteorites found on Earth contain which of the following?