8、The word "considerably" in the passage is closest in meaning to

Frequently

Significantly

Clearly

Surprisingly

9、According to paragraph 4, what is demonstrated by the fact that craters fill in much faster on Mars than on the Moon?

Erosion from meteoritic impacts takes place more quickly on Mars than on the Moon.

There is more dust on Mars than on the Moon.

The surface of Mars is a dry desert.

Wind is a powerful eroding force on Mars.

10、In the papragraph 4, why does the author point out that Mars has few ancient craters that are less than 5 kilometer?

To explain why scientists believe that the surface matter fill Martian craters is mostly dust

To explain why scientists believe that the impact craters on Mars were created by meteoroids

To support the claim that Martian atmosphere is an efficient erosive agent

To argue that Mans experienced fewer ancient I impacts than the Moon did

Paragraph 5

As on the Moon, the extent of large impact cratering (i.e.craters too big to have been filled in by erosion since they were formed) serves as an age indicator for the Martian surface. Age estimates ranging from four billion years for Mars's southern highlands to a few hundred million years in the youngest volcanic areas were obtained in this way.

11、According to paragraph 5, what have scientists been able to determine from studies of large impact cratering on Mars?

Some Martian volcanoes are much older ithan was once thought.

The age of Mars's surface can vary from area to area.

Large impact craters are not reliable indicators of age in areas with high volcanic activity.

Some areas of the Martian surface appear to be $Wer than they actually are.

Paragraph 6

The detailed appearance of Martian impact craters provides an important piece of information about conditions just below the planet's surface. Martian craters are surrounded by ejecta (debris formed as a result of an impact) that looks quite different from its lunar counterparts. A comparison of the Copernicus crater on the Moon with the (fairly typical) crater Yuty on Mars demonstrates the differences. The ejecta surrounding the lunar crater is just what one would expect from an explosion ejecting a large volume of dust, soil, and boulders.However, the ejecta on Mars gives the distinct impression of a liquid that has splashed or flowed out of crater. Geologists think that this fluidized ejecta crater indicates that a layer of permafrost, or water ice, lies just a few meters under the surface. Explosive impacts heated and liquefied the ice, resulting in the fluid appearance of the ejecta.

12、 According to paragraph 6, the ejecta of Mars's crater Yuty differs from the ejecta of the Moon's Copernicus crater in that the ejecta of the Yuty crater

Has now become part of a permafrost layer