GMAT 考满分题库

Unlike Mercury and Mars, Venus has a dense, opaque atmosphere that prevents direct observation of its surface. For years, surface telescopes on Earth could glean no information about the surface of Venus. In 1989, the Magellan probe was launched to do a five-year radar-mapping of the entire surface of Venus. The data that emerged provided by far the most detailed map of the Venusian surface ever seen.

The surface shows an unbelievable level of volcanic activity: more than one hundred large shield volcanoes, many more than Earth has, and a solidified river of lava longer than the Nile. The entire surface is volcanically dead, with not a single active volcano. This surface is relatively young in planetary terms, about 300 million years old. The whole surface, planet-wide, is the same age: the even pattern of craters, randomly distributed across the surface, demonstrates this.

To explain this puzzling surface, Turcotte suggested a radical model. The surface of Venus, for a period, is as it is now, a surface of uniform age with no active volcanism. While the surface is fixed, volcanic pressure builds up inside the planet. At a certain point, the pressure ruptures the surface, and the entire planet is re-coated in lava in a massive planet-wide outburst of volcanism. Having spent all this thermal energy in one gigantic outpouring, the surface cools and hardens, again producing the kind of surface we see today. Turcotte proposed that this cycle repeated several times in the past, and would still repeat in the future.

To most planetary geologists, Turcotte's model is a return to catastrophism. For two centuries, geologist of all kinds fought against the idea of catastrophic, planet-wide changes, such as the Biblical idea of Noah's Flood. The triumph of gradualism was essential to the success of geology as a serious science. Indeed, all features of Earth's geology and all features of other moons and planets in the Solar System, even those that are not volcanically active, are explained very well by current gradualist models. Planetary geologists question why all other objects would obey gradualist models, and only Venus would obey a catastrophic model. These geologists insist that the features of Venus must be able to be explained in terms of incremental changes continuously over a long period.

Turcotte, expecting these objections, points out that no incremental process could result in a planet-wide surface all the same age. Furthermore, a slow process of continual change does not well explain why a planet with an astounding history of volcanic activity is now volcanically dead. Turcotte argues that only his catastrophic model adequately explains the extremes of the Venusian surface.