VENUS

Venus is covered with a thick cloud layer which makes its surface impossible to see in visible light. As a result, it has been quite difficult to form conclusions about the processes and forces which have shaped Venus's surface. Because of Venus's similarity to the Earth, however, it was expected that the processes which shaped Venus's surface should be similar to the processes which shaped the Earth's surface. There are, however, some things which are different and expected to play roles in the evolution of Venus's surface.

• Venus's atmosphere of carbon dioxide is ~ 100 times more massive than the atmosphere of the Earth. This led to a Runaway Greenhouse Effect which raised its surface temperature to more than 800 F. This is not hot enough to melt rocks, but the heat can make surface rocks softer and less dense than those on the Earth may make the lithosphere of Venus weaker than on the Earth.

• The thick atmosphere will also destroy and slow down incoming meteoroids so that cratering by smaller objects should be less than found on the Earth; there is a lack of craters of diamter less than ~ 1.5 kilometers on Venus and the number of craters with diameters less than 30 kilometers is deficient.

• Venus's atmosphere has very little water. On Earth, if we spread the water around, it would cover the Earth to a depth of a few kilometers (the oceans). On Venus, if you were to cover the planet with all of its water it would cover the planet to a depth of a few centimeters. Venus is extremely dry. It is not clear what effect this has on tectonics (and if it has an effect on plate tectonics).

Magellan

Beginning in 1990 and continuing on until 1993, the Magellan spacecraft mapped the surface of Venus using a technique known as radar ranging. To get around the problem of the dense cloud layer, Magellan viewed the surface using microwave radiation. Microwave radiation is a type of light, it simply has a longer wavelength than does visible light. Microwave radiation is able to penetrate the cloud layer of Venus.

Magellan was in a polar orbit about Venus and let the planet turn underneath it. As Magellan swooped over the surface of Venus, it shone a beam of radiation onto the surface of the planet. The process of mapping the planet is quite simple. The craft simply measured the amount of time it took for the radiation to go to Venus and return to the craft. Then knowing that the radiation traveled at the speed of light, it was easy to figure out far the ground was from the craft at any given time. This allowed Magellan to map the changes in elevation (the ground) on Venus. In this manner, Magellan mapped the surface of Venus with a horizontal resolution of 100 - 300 meters and a vertical resolution of 80 meters. Magellan also measured how intense the returning radiation was. The reflectivity of the surface of Venus depends upon the composition of the surface material and how rough is the surface.

Venusian Topography and Volcanism

The surface of Venus has three types of terrain:

• rolling plains which cover about 65 % of the surface characterized by numerous craters and circular basins, many of which are apparently filled with lava and resemble lunar maria.

• highlands which cover about 8 % of the surface concentrated in three major regions, two of which Ishtar Terra (in the northern latitudes) and Aphrodite Terra (on the equator south and east of Ishtar Terra) are comparable in size to the continents of Africa and Australia. The third, Beta Regio, is much smaller. The maximum elevation above the mean surface level, the summit of a mountain called Maxwell Montes, is greater than that of Mt. Everest. There are adjacent parallel valleys and trenches in the highlnads, some forming systems up to 9,000 kilolmeters in length ( rifts? Probably not according to Magellan which finds no sign of spreading).

• lowlands which cover the remaining 27 % of the surface

It is clear that there are many volcanoes on Venus (indicating active geology in the past). However, Magellan saw no definitive evidence of current volcanism (as had been hoped). However, other indirect evidence suggests that volcanism may be still be ongoing:

• Sulfur dioxide is detected in Venus's atmosphere; the amount undergoes large fluctuations. Sulfur dioxide is put into the atmosphere via volcanism on the Earth. It is conceviable that the large observed fluctuations in sulfur dioxide are due to current volcanism on Venus

• The Pioneer and Venera probes detected bursts of radio emission in the atmosphere of Venus similar to types seen from the plumes of Terrestrial volcanoes.

Unfortunately, Magellan did not see a volcano in the act of erupting, so the issue is not settled. Magellan did see signs of recent volcanism, however, e.g.:

• Layered lava flows
• Sif Mons
• Gula Mons

Magellan also found widespread evidence of other types of geological activity; deformations indicating large internal stresse, e.g.,

• Coronae
• Faulting
• Arachnoids
• Chasmas

{So, as for Mars, the conclusion is that there is no definitive evidence for plate tectonic activity but there is ample evidence for an active geology on Venus.