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In 1995 the Galileo spacecraft captured data about Jupiter’s atmosphere—namely, the absence of
most of the predicted atmospheric water—that challenged prevailing theories about Jupiter’s structure.
The unexpectedness of this finding fits a larger pattern in which theories about planetary composition
and dynamics have failed to predict the realities discovered through space exploration. Instead of
normal planets whose composition could be predicted by theory, the planets populating our solar
system are unique individuals whose chemical and tectonic identities were created through numerous
contingent events. One implication of this is that although the universe undoubtedly holds other
planetary systems, the duplication of the sequence that produced our solar system and the development
of life on Earth is highly unlikely.
Recently planetary scientists have suggested that the external preconditions for the development
of Earth’s biosphere probably included four paramount contingencies. First, a climate conducive to life
on Earth depends upon the extraordinarily narrow orbital parameters that define a continuously
habitable zone where water can exist in a liquid state. If Earth’s orbit were only 5 percent smaller than
it is, temperatures during the early stages of Earth’s history would have been high enough to vaporize
the oceans. If the Earth-Sun distance were as little as 1 percent larger, runaway glaciation on Earth
about 2 billion years ago would have caused the oceans to freeze and remain frozen to this day.
Second, Jupiter’s enormous mass prevents most Sun-bound comets from penetrating the inner
solar system. It has been estimated that without this shield, Earth would have experienced
bombardment by comet-sized impactors a thousand times more frequently than has actually been
recorded during geological time. Even if Earth’s surface were not actually sterilized by this
bombardment, it is unlikely that any but the most primitive life-forms could have survived. This
suggests that only planetary systems containing both terrestrial planets like Earth and gas giants like
Jupiter might be capable of sustaining complex life-forms.
Third, the gravitational shield of the giant outer planets, while highly efficient, must occasionally
fail to protect Earth. Paradoxically, while the temperatures required for liquid water exist only in the
inner solar system, the key building blocks of life, including water itself, occur primarily beyond the
asteroid belt. Thus the evolution of life has depended on a frequency of cometary impacts sufficient to
convey water, as well as carbon and nitrogen, from these distant regions of the solar system to Earth
while stopping short of an impact magnitude that would destroy the atmosphere and oceans.
Finally, Earth’s unique and massive satellite, the Moon, plays a crucial role in stabilizing the
obliquity of Earths rotational axis, this obliquity creates the terrestrial seasonality so important to the
evolution and diversity of life. Mars, in contrast, has a wildly oscillating tilt and chaotic seasonality,
while Venus, rotating slowly backward, has virtually no seasonality at all.
2. It can be inferred from the passage that the planetary scientists would be most likely to agree with
which of the following statements concerning the development of complex life forms on Earth?
A. It might have occurred earlier in Earth’s history if cometary impacts had been less frequent than
they were.
B. It could have occurred if Earth’s orbit were 1 percent larger than it is but not if Earth’s orbit were 5
percent smaller
C. It probably follows a pattern common on other terrestrial planets that occupy planetary systems
containing gas giants.
D. Its dependence on the effect that Jupiter’s gravitational shield has on Earth was difficult to recognize
prior to 1995.
E. It has been contingent on conditions elsewhere in Earth’s solar system as well as on conditions on
Earth itself
我选的C，是根据第三段最后一句This suggests that only planetary systems containing both terrestrial planets like Earth and gas giants like Jupiter might be capable of sustaining complex life-forms.
但答案是E。百思不得其解，求大神解答！
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发表于 2018-12-17 19:29:04
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