1. Can you choose one or a few surface examples of Venus and explain what is known about those areas?
Below is a map of the surface of Venus. Venus has two "continents," Ishtar Terra in the north (about the size of Australia) and Aphrodite Terra (about the size of South America) near the equator. Please note that Ishtar "looks" larger only because of the map projection used. It was thought that these "continents" are tectonic features similar to what we see with the Himalayas, compression of the crust that creates mountains. Ishtar also has the tallest mountain on Venus, Maxwell Mons, 11 km above the mean surface (similar to Earth's sea level) of Venus. For comparison, the top of Mt. Everest is 8.8 km above sea level and the top of Mauna Loa is 9.2 km above the ocean floor. Most of Venus is relatively flat, compared to the Earth, with more than 50% of it within 500 meters of the mean surface. Venus has more craters than we see on Earth (more erosion on Earth), but no craters smaller than about 3 km due to the effects of the atmosphere (slows them down or breaks them up before they hit the surface).
2. The major catastrophic event that is a potential explanation for the low numbers of craters on Venus-- would that be at about the same time (same event) as the event that changed the direction of rotation of Venus?
Simply, no. If it was a collision that caused Venus to rotate clockwise (opposite of the Earth), this probably happened long ago at the same time that the planets were still forming and that a Mars-sized object hit the Earth and formed the Moon. It is likely, based on what we see now, that what happened on Venus 500 million years ago was due to a turning over of the surface. Without the water to allow for plate tectonics, it is harder for Venus to get rid of its internal heat. So, the mantle gets hotter and eventual melts the crust, forming a whole new surface. This process probably takes about 100 million years and is probably when most of the volcanoes on Venus formed—very little erosion to weather them away.
3. Is Venus like the Earth, in terms of its thick atmosphere, in Earth’s early history?
We do not think so. It might have been the other way around: early Venus was similar to the early Earth before a runaway greenhouse turned Venus into what it is today. Early Earth probably had a hydrogen-rich atmosphere and may not have had oxygen until plant life in the oceans started producing oxygen.
4. Does Venus have H2SO4 (sulfuric acid) rain? If so, wouldn’t that cause a lot of resurfacing of the surface?
Yes, there is acid rain on Venus, but it is like virga that we see in Tucson—rain that evaporates before it reaches the ground because it is too hot and dry near the surface. On Venus, there is sulfuric acid rain, but it evaporates long before reaching the ground, at an altitude of about 25 kilometers.
5. What are the differences in tectonic activity between Venus and the Earth?
Venus does have tectonic activity: faults, folds, volcanoes, mountains, and rift valleys. However, it does not have global tectonics as there is on Earth—plate tectonics. This is thought to be due to the fact that Venus is hot and dry. To have true plate tectonics, you need to have subduction zones so that one plate can ride over the other. This happens on Earth, but not on Venus.
6. How does Venus have a different style of tectonics?
Tectonics is a very general word for the processes that go on to “building” a surface: volcanoes and earthquakes, for example. These existed/exist on Venus. On Earth, we have these and much of this is due to plate tectonics which is the movement of the plates which gives us the processes discussed above.