National Geographic : 1993 Feb
RAIN OF METEOROIDS Bull's-eye marks the spot where an asteroid or comet slammed into Venus near Aphro dite Terra (facing page). The impact created a huge crater and a spectacular debris pat tern 230 miles long. Upon impact, ejecta is thrown out of the crater, but it does not go far because of the planet's intense atmospheric pressure. As surface rocks melt because of the heat of impact and the high surface tempera ture, the resulting flow spreads out and lines valleys in its path. Following impact (diagram), large objects leave craters much like those found else where in the solar system, often with flat floors and cen tral peaks. Smaller meteoroids explode in the thick atmo sphere before ever reaching the ground. The resulting blasts have left bizarre, craterless impact marks, called splotches, visible to Magel lan's sensitive radar. One 200-mile-long dark area (above) is believed by many scientists to be an example of this unusual phenomenon. It could be the result of an object's entering Venus's dense atmosphere at a very low angle and leaving a long shock path. Several craters at the eastern end of the streak may be from the remains of the incoming object. Magellan data show that meteoroids have been a major cause of the patchiness of Venus's veneer of sediment. The tremendous force of a body hitting the surface at several miles a second and the resulting violent winds quickly accomplish what the slow-moving surface winds do barely at all: break up surface rock and redistribute it. Sand dunes, in fact, are visible in some Magellan images. Scientists have a good idea of the number of impact cra ters that should appear in a given time and are thus able to estimate the age of Venus's surface. Magellan has logged more than 800 impact craters, which indicates a surface age of about 500 million years relatively young as planets go.