The Inner Planets
Before we entered what Wiley Ley called ‘The Third Era of Astronomy’—that is, robot probes and men exploring the other worlds—there were all kinds of speculation about what the surface of Mercury and Venus was like. Many believed the surface of Mercury–that we thought at the time continuously faced the Sun– would be smooth and featureless, having been melted smooth by the extreme heat. (We now know it faces the Sun only 2/3rds of Mercury’s 88-day orbit)
The total cloud cover that wrapped Venus led many to conjure up a dense, lush tropical rain forest, perhaps harboring life.
Our robots have destroyed those visions. We know now that Mercury is pock marked with craters, just as is our moon, and the side that faces the Sun most of the time is no different than the 'dark' side. There is also evidence of volcanic activity.
Did volcanoes exist on Mercury at one time. Are they still active? We may answer those mysteries in 2011 when the Messenger spacecraft goes into orbit about the planet. And we may learn if there is water ice at the poles, which are always in shadow. Flybys have detected patches brighter than the rest of the surface at both poles. A sign of ice?
And the clouds shrouding Venus do not hide a lush tropical jungle. The clouds are almost all carbon dioxide. The surface of the planet is hot, dry and ragged, with a temperature exceeding 470 degrees C. The robots that have landed on the surface have lasted only minutes.
But Venus may not always have been this way. German researchers have identified rocks in some areas that look like granite. Granite is formed when basalt is forced beneath plates by tectonic forces, mixes with water and is pushed back to the surface by volcanic activity.
Did Venus once have oceans and continents? If so, why did they disappear and the atmosphere become carbon dioxide? It will take another robot (or more) to solve that mystery.