Astronomers who study planet formation once believed that comets-because they remain mostly in the distant Oort cloud, where temperatures are close to absolute zero-must be pristine relics of the material that formed the outer planets. The conceptual shift away from seeing comets as pristine relics began in the 1970s, when laboratory simulations revealed there was sufficient ultraviolet radiation reaching comets to darken their surfaces and there were sufficient cosmic rays to alter chemical bonds or even molecular structure near the surface. Nevertheless, astronomers still believed that when a comet approached the Sun-where they could study it-the Sun's intense heat would remove the corrupted surface layer, exposing the interior. About the same time, though, scientists realized comets might contain decaying radioactive isotopes that could have warmed cometary interiors to temperatures that caused the interiors to evolve.

Considering all that apply

1.According to the passage,astronomers recognize which of the following as being liable to cause changes to comets?

A.Cosmic rays
B.Radioactive decay
C.Ultraviolet radiation


Considering all that apply

2. According to the passage, astronomers' belief that comets are pristine relics was

A. overturned by analysis of what happens when comets approach the Sun
B. supported by what observations revealed about the composition of the outer planets
C. based on consideration of the conditions that prevail where comets are located



3. The author suggests that the realization described in the final sentence of the passage had which of the following effects?

A. It introduced a new topic for study by astronomers interested in planetary formation.
B. It led astronomers to adopt a number of different strategies in trying to determine the composition of cometary interiors
C. It called into question an assumption that astronomers had made about comets
D. It cast doubt on astronomers' ability to study the interior parts of comets.
E. It caused astronomers to revise their account of the composition of the outer planets.


Considering all that apply

4. It can be inferred that the author would agree with which of the following statements about the "laboratory simulations"?

A. The simulations showed that despite the low temperatures in the Oort cloud, there was sufficient energy there to alter the comet.
B. Astronomers were initially reluctant to accept what the simulation showed about the composition of comets
C. The simulations themselves did not eliminate the possibility that comets contain pristine relics of material from the early solar system.