The large number of craters on the lunar highlands compared to those on the maria is evidence that
(a) the surface of the maria is liquid and craters quickly disappear there.
(b) the material composing the highlands is very soft and easily cratered.
(c) the bodies that struck the Moon and made the craters were clumped in such a manner that they missed hitting mare areas.
(d) the maria are much younger than the highlands.
(e) the maria are much older than the highlands.
The Correct Answer and Explanation is :
The correct answer is:
(d) the maria are much younger than the highlands.
Explanation:
The surface of the Moon has two major types of terrain: the lunar highlands and the maria. The highlands are heavily cratered, whereas the maria have far fewer craters. This disparity in crater density provides crucial evidence about the relative ages of these two regions.
- Cratering and Surface Age: Craters on the Moon are formed by impacts from asteroids, meteoroids, and comets. Over time, the number of craters on a surface increases. Therefore, a heavily cratered surface, such as the lunar highlands, indicates great age because it has been exposed to impactors for a long period. Conversely, a surface with fewer craters, such as the maria, is younger because it has been exposed for a shorter time.
- Formation of the Highlands and Maria: The highlands are the oldest parts of the lunar surface, formed over 4 billion years ago. They represent the original crust of the Moon that solidified as it cooled. In contrast, the maria were formed later, around 3 to 3.5 billion years ago, when massive impacts punctured the crust and allowed molten basaltic lava from the Moon’s interior to flood the surface. This volcanic activity resurfaced the maria, effectively “resetting” their cratering record.
- Comparison of Crater Densities: After the volcanic resurfacing, the maria were exposed to impacts, but only those that occurred after the lava solidified are visible. The highlands, being much older, show a greater density of craters because they have accumulated impacts for a longer period.
This difference in crater density is strong evidence for the maria being significantly younger than the highlands, making the correct answer (d).