The Planet Venus

Venus is often called the twin planet to Earth because 1) it has a similar radius/size, 2) is has a similar mass, 3) it has a similar density and 4) it has an atmosphere. However, the environment of Venus is very different from the Earth.

It also has the somewhat unusual property of rotating extremely slow (once per 243 Earth days) such that 1 Venusian day is longer than a Venusian year (225 Earth days). Even more suprising is that Venus spins in the opposite direction. That is, the Sun rises in the west and sets in the east. There are two differing theories explaining these orbital properties.

1)The slow rotation is a consequence of billions of years of tidal forces operating on the dense atmosphere causing the rotation rate of the atmosphere to slow down with time. In turn, the frictional drag exerted by the atmosphere onto the surface, slows the overall planetary rotation down to a crawl.

2)Venus may have been hit by a large planetesimal causing the anomalous rotation. Venus does have a slight orbital tilt indicative of some early event.

Observing Venus

The main problem with probing Venus is simply that its completely covered with clouds and a very thick atmosphere so that its surface is completely obscured.

The Soviet Union's Venera 8 and 9 missions (1975) represent the only landings on the Venusian surface. As the Venusian surface is fairly hostile (temperature ~ 450 Farenheit, atmospheric pressure 90 times greater than Earth, constant sulfuric acid rain), these spacecraft did not function long. But they did get a few pictures of the base of the spacecraft and a bunch of rocks:

However, radar can penetrate this atmosphere and return a map of topographic features. Radar mapping works by sending a pulse of radio waves that bounce off surface features. Differences in radar timing at different locations represent different elevations. For instance, a pulse bouncing off a large mountain would take less time to return to the space craft than a pulse bouncing off a low valley floor. In this way radar imaging can build up a topological map of Venus.

Much of our knowledge of Venus is now based on such data as gathered by the The Magellan Mission to Venus

The radar mosiac is shown below. The effective resolution of this image is about 3 kilometers. It was processed to improve contrast and to emphasize small features, and was color-coded to represent elevation.

The overall geological and structural features of Venus are the following: