7. Venus: The veiled planet
Planet-wide covering of lava
Spreading lava has flooded and filled the low-lying regions of Venus, creating extensive smooth plains that cover about 85 percent of Venusís surface. The volcanic nature of these lowland plains, each designated by the term planitia, is demonstrated in the Magellan images. You can practically see the molten rock spreading like heavy cream across these plains, often running for hundreds of thousands of meters down gentle slopes. The magma has risen from within canyons as the crust pulled apart, cooling and solidifying into lava flows that look like frozen river currents.
In other places, the molten material has burned paths in the preexisting lava deposits, following a narrow, sinuous smoothly curving course. They can meander for millions of meters across the planet's surface. Many end in outflows that look like river deltas. These river-like channels were formed not by water, but by lava that was hot enough to carve through solid rock, remaining hot and liquid over distances that are longer than the Nile, the longest river on Earth. The high surface temperature on Venus probably kept the lava liquid, and prevented the cooling flow front from damming up the molten rock behind it.
A relatively young surface
Venus, like all planets, has been subjected to a continual rain of meteorite bombardment over the aeons. The plains of Venus are uniformly peppered with impact craters, the scars of this bombardment, though nowhere near as liberally as on the surfaces of the Moon, Mars and Mercury. On Venus the craters of a given size are far fewer in number and more widely spaced than on the Moon. At one time Venus was probably as heavily pockmarked with large craters as the Moonís ancient surface is, but the scarcity of the craters now on Venus indicates that the surface we now see is much younger than the lunar surface. The average surface age on Venus is 750 million years.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University