Venus has almost the same diameter as the Earth and is the next closest planet to the Sun. The similarity ends with the weather report, however. The surface temperature is more than 465 ^o C (870^o F) and atmospheric pressure is 90 times that of Earth. The surface is hidden from view by a dense blanket of clouds, so we must use radar systems to “see” the landscape below. Using a data processing technique called synthetic aperture radar, these systems create images that reveal information of great interest to geologists: how much rocky material is scattered across the surface, how thick is any dust layer, and what kinds of mountains, volcanoes, folds, or ridges exist? In the early 1990s, the Magellan spacecraft mapped nearly the entire planet with a resolution that could pick out features about the size of a football field. Even with this global view, there is much we do not understand about Venus, especially about what goes on at the surface. With such a large planet there is plenty of internal heat, so we expect that volcanoes might erupt, but Magellan could not linger in orbit long enough to watch for such changes. Since that time, Venus has kept its secrets, visited by the European Venus Express Mission but not mapped again by an orbiting radar sensor. Instruments on Venus Express can get a blurry view of the ground by imaging narrow bands in the electromagnetic spectrum where the atmosphere lets heat escape from the surface. From these maps, some volcanic areas seem to be less weathered than others, perhaps indicating more recent eruptions. The exact nature of what is happening cannot be understood without a long-term comparison between images that reveal the landscape in much finer detail.

Earth-based radar astronomy may fill that gap in our monitoring of Venus. In 1988, a team led by Donald Campbell of Cornell University used the radar system at Arecibo Observatory in Puerto Rico to map the side of Venus that faces Earth when the two planets are closest together in the sky (called an inferior conjunction). This map can distinguish features about a mile across—less detailed than Magellan but still adequate to see large lava flows from volcanoes. In 2012, a team led by Smithsonian scientists carried out a similar type of mapping, transmitting the radar signal from Arecibo Observatory but receiving the echoes from Venus at the Green Bank Telescope in West Virginia.

 Contrasting view of the mountainous Alpha Regio on Venus. The top radar map is from the Magellan spacecraft, and the bottom map was produced by Earth-based systems. The area covered is about 3600 km (2237 miles) across. The impact craters Stuart, Peggy, and Nadia are labeled. Fine-grained material from Stuart crater darkens both radar images within the white outline, but in the Earth-based view can be seen extending well into the highlands (orange outline).

It is painstaking to compare these images and search for evidence of change, but the work is ongoing. In the meantime, combining images from the two observing periods is yielding a wealth of insight about other processes that alter the surface of Venus. One source of change is through the formation of impact craters, which hurl dusty material above the atmosphere, from where it settles back to the surface up to hundreds of miles away. Over time, the gentle surface winds move this material around, piling it up against ridges or hills. The Earth-based data show that such blankets of fine material can cover large regions of the fractured, mountainous uplands called tesserae. These rugged highlands are likely places to find rocks that record the early history of Venus, and some evidence suggests that the tesserae share similarities with continents on Earth. Mapping the extent of crater deposits and other types of surface change will be crucial to finding the best landing sites for future spacecraft that venture into this very “unearthly” environment.