presence of layers of differing density, which are the result of differing temperature, chemical composition, or crystal structure. For example, part of the upper mantle is less viscous than the mantle rocks deeper down. This zone, named the asthenosphere, extends from just below the lithosphere (about 125 kilometers down, on average) to about 250 kilometers. This layer is thought to behave fairly plastically, possibly because it may also contain pockets of partially molten rock. The asthenosphere plays an important role in plate tectonics: it cushions the overlying lithospheric plates as they ride along. In effect, it decouples them somewhat from the convective action of the mantle, thus moderating the intensity of surface tectonic activity.

On Venus, geophysicists have had only one probe of the interior: a map of the planet's gravity field compiled during the Pioneer Venus mission. By recording minute Doppler shifts in the radio signals from the orbiting spacecraft, scientists were able to monitor subtle changes in the craft's motion and from this to construct a low-resolution gravity map. As on Earth, tiny variations in the strength of the field are detected, but the pattern of these variations is markedly different. On Venus, areas where the field strength is slightly greater than average, called positive gravity anomalies, occur wherever there is a topographic high such as Aphrodite or Ishtar. Negative anomalies (places where the field strength is slightly diminished) are detected within lowland basins like Lavinia Planitia. This is in sharp contrast to the situation on Earth, where gravity anomalies bear little relation to gross (continent-scale) topography and are assumed to be caused by density variations deep within the mantle. We know, however, that the gross topography of Earth's surface reflects variations in thickness within the upper reaches (i.e., the upper several tens of kilometers) of the crust, rather than the state of the deep mantle.

A more systematic way of comparing the gravity anomalies of Earth and Venus is provided by a measurement called the apparent depth of compensation (ADC). Consider an iceberg floating in the ocean. The small visible portion is supported by a much larger submerged icy mass that is in turn held up by isostatic pressure from the surrounding, higher-density fluid. If the iceberg is stable—if it is not sinking or rising—it is said to be isostatically compensated. Similarly, a topographic high such as a mountain must also be supported in some fashion, lest it sink under its own weight. The principle of isostatic compensation of mountains was discovered in 1865 by Sir George Airy, England's astronomer royal, who used it to explain a discrepancy between two sets of surveying measurements made in India.



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