Venus

At is closest approach the planet Venus can have an angular diameter just slightly larger than 1/60th of a degree. This angular size translates to a physical diameter of 7,520 mi (12,104 km), making the planet about 95% the size of Earth. Given the near similar sizes of Earth and Venus, it might be expected that the surface of Venus has been shaped by the same geological processes that operate on Earth. This expectation is only partly true.

While radar maps of the surface of Venus were constructed during the 1970s, the most detailed topographic maps obtained to date are those from the Magellan spacecraft mission. The Magellan spacecraft was placed into Venusian orbit by NASA in 1990, and a powerful on-board imaging radar system was used to map the entire Venusian surface to a resolution of a few hundred meters.

The Magellan radar data showed that Venus is remarkably flat, and that some 80% of the planet's surface is covered by smooth volcanic plains, the result of many lava out-flows. The altitude map constructed from Magellan data has revealed the existence of two large continent-like features on Venus. These features are known as Ishtar Terra (named after the Babylonian Goddess of love), and Aphrodite Terra (named after the Greek Goddess of love). Ishtar terra, which measures some 621 mi (1,000 km) by 931 mi (1,500 km), lies in Venus's northern hemisphere, and has the form of a high plateau ringed with mountains. The largest mountain in the region, Maxwell Montes, rises to a height of 7 mi (11 km). Aphrodite Terra is situated just to the south of the Venusian equator and is some 9,936 mi (16,000 km) long by 1,242 mi (2,000 km) wide. It is a region dominated by mountainous highlands and several large volcanoes.

Venus has three major terrains, based mainly upon elevation (above mean planet radius). The lowlands are rolling volcanic plains (~ 60% of the planet) with under 1,600 ft (500 m) of relief. The uplands are intermediate in elevation and represent a transition between lowlands and highlands. Elevations range from 0 to 1.2 mi (0 to 2 km) in the uplands. The highlands have up to 3 mi (5 km) relief and are quite mountainous in some places. The highlands have compression ridges and fractured rocks and comprise about 15% of the planet's surface.

Given the similarity in size of Venus and Earth, geologists had speculated on the possibility that plate tectonics, which is the primary agent for re-shaping Earth's surface, might operate on Venus. The Magellan probe found no evidence, however, for large scale tectonic activity on Venus. The reasons underlying the absence of any large-scale tectonic activity on Venus are presently unclear, but it may be indicative that the planet has a thinner and weaker lithosphere than Earth. The ridges and folds that cover many of the plain regions of Venus is indicative, however, of a certain amount of local tectonic activity on the planet.

The Magellan maps revealed many large craters on the Venusian surface. There is an apparent cut-off for craters with diameters less than a few kilometers. This is a selection effect imposed by the dense Venusian atmosphere. The atmosphere is in fact such a good filter of incoming meteoroids, that only those objects larger than a few kilometers in diameter survive their passage through the atmosphere, with sufficient mass, to produce a crater at the Venusian surface.

The relative age of different regions on a planet's surface can be gauged by counting the number of craters that appear per unit area. This method of crater counting has proved very useful for dating the various regions on our Moon. The essential idea being exploited in crater count dating is that, assuming the cratering rate is the same over the whole planet, if one region has fewer craters per unit area than another, it implies that some resurfacing, e.g., by a lava flow, has taken place in the region with fewer craters. The re-surfacing has in effect erased the older craters and re-set the cratering clock.

The number of craters in the Venusian plains is typically about 15% of the crater counts for the lunar maria. This observation indicates that the Venusian plains have an age roughly equivalent to 15% the age of the lunar maria. From the lunar rock samples that were returned from the Apollo Moon landing missions we know that the lunar maria are about 3.2 billion years old, and consequently the likely age of the Venusian plains is about 500 million years.

The main agent for re-surfacing the Venusian plains is believed to be aperiodic but widespread volcanism. Certainly, many (apparently) extinct volcano's were mapped by Magellan during its five-year survey. Some lava flow regions observed by Magellan are believed to be no more than ten millions years old, and they may be much younger.

Some of the more remarkable surface features discovered by Magellan were the pancake-shaped volcanoes. These flat-topped, circular volcanoes are unique to Venus, and it is thought that they are probably formed through the surface extrusion of a very thick and viscous lava.

In recognition of Venus being named in honor of the Greek Goddess Aphrodites, whom the Romans called Venus, the International Astronomical Union assigns only female names to the planet's surface features. Craters, for example, can be named after any famous women; linear features are named after Goddesses of War, while plains are named in honor of mythological heroines.