Surface Temperature on Venus

The largest volcanic mountains of Mars are found in the Tharsis area, although smaller volcanoes dot much of the surface. The most dramatic volcano on Mars is Olympus Mons (Mount Olympus), with a diameter larger than 500 kilometers and a summit that towers more than 20 kilometers above the surrounding plains—three times higher than the tallest mountain on Earth. The volume of this immense volcano is nearly 100 times greater than that of Mauna Loa in Hawaii. Placed on Earth’s surface, Olympus would more than cover the entire state of Missouri.

(A) Images taken from orbit allow scientists to search for impact craters on the slopes of these volcanoes in order to estimate their age. (B) Many of the volcanoes show a fair number of such craters, suggesting that they ceased activity a billion years or more ago. (C) However, Olympus Mons has very, very few impact craters. (D) Its present surface cannot be more than about 100 million years old; it may even be much younger. Some of the fresh-looking lava flows might have been formed a hundred years ago, or a thousand, or a million, but geologically speaking, they are quite young. This leads geologists to the conclusion that Olympus Mons possibly remains intermittently active today—something future Mars land developers may want to keep in mind.

Q. In paragraph 2 there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?

Generally, the more craters that appear on a surface, the older that surface is.