Through the geological processes of Earth, impact craters are recycled back into the mantle, or covered over by lava. Earth slowly erases its own history.
The moon has visibly older and younger sections, the old marked by scars of impacts, the young with less craters as lava flows have gone over and erased them.
Venus has a random distribution of craters all over its surface.
That may not sound strange, but it is actually very odd. Venus, if it were like other terrestrial planetary bodies, should have sections where the craters have been erased, and others where they remain. Instead, the uniformity of craters suggests that Venus’ surface has the same age everywhere, has since been dotted with craters, and been relatively inactive since. This idea is called ‘catastrophic resurfacing’: a time of intensive volcanic activity in which all of Venus was resurfaced. The planet would have essentially turned itself inside out around five hundred million years ago, leaving a young surface by geological standards.
As you might imagine, that idea sounds a touch crazy. Geological processes happen slowly over millions of years on the Earth. Why wouldn’t Venus be the same? The planet is about the same size and density as Earth: logically, it would behave similarly.
In actuality, Venus is very different from Earth. Clouds of sulfuric acid are so thick that we can’t see through it, and radar is needed to look past the clouds. The pressure would crush you, and the temperatures melt metals. The surface is dry and barren, and it rotates ‘backwards’ compared to the other planets.
So in short? Venus is not actually very Earth-like. It probably does have an Earth-like interior, but otherwise, there are few similarities. So perhaps its geological history is not similar, either. We already know that Venus does not appear to have plate tectonics.
In an alternate version of Venus history than that presented by catastrophic resurfacing is the equilibrium model. Perhaps Venus was resurfaced in a way that balanced the rate of impact craters, with resurfacing either occurring relatively frequently over short areas, or more slowly over large areas.
You can get an idea of which model of resurfacing is more correct by looking at the distribution of craters and how many of them are embayed (surrounded) by either of the two main types of plains of Venus. Regional plains are forty-three percent of the surface and are smooth. Eighty percent of the area covered by regional plains are from three sections. Their origin is not known. Lobate plains are only nine percent of the surface, and are lava flows younger than the regional plains. Lobate plains are formed from multiple lava flows, indicating that Venus was geologically active during their formation. The presence of these two types of plains suggests two different geological eras in Venus’ history.
Published in Planetary and Space Sciences in 2014, “Volcanically embayed craters on Venus; testing the catastrophic and equilibrium resurfacing models” by M.A. Ivanov and J.W. Head is an article that does looks at the craters and where they are situated with respect to regional and lobate plains.
Imagine a surface with an even, uniform distribution of craters. With catastrophic resurfacing, the activity on Venus is intense; while a few craters survive to be embayed, most are erased. The rest appear after resurfacing and are superimposed on top of the plains. However, if resurfacing was instead closer to the equilibrium model, some craters would be erased by the planet’s activity, but some would have lava flow around them, making them embayed. The activity is a lot less intense, and many more craters survive.
Only three percent of craters are embayed by regional plains. The percentage of embayed craters is thirty-three for lobate plains.
Taking both of these together, the equilibrium model would suggest that the distribution of craters would not be random, but we know that it is random. Taken separately, the percentage of craters embayed by lobate plains work with the predictions of the equilibrium model, but this does not follow for craters embayed by regional plains.
In other words, the surface of Venus does not appear to be entirely formed by equilibrium resurfacing. Catastrophic resurfacing also does not appear to give the whole picture based off the observations we have made of Venus’ surface. The authors present a logical possibility. The regional plains are from wide swaths of volcanic flooding, formed in something along the lines of catastrophic resurfacing. In a different era of Venus’ history, lobate plains were formed slowly over time with successive volcanic flows, erasing fewer craters so that a higher percentage became embayed.
Venus still holds a lot of mysteries. Even Earth, as vastly more studied than Venus, still keeps some secrets. In that way, the planets are not so different after all.
Astronomy Picture of the Day. “APOD: 2005 September 3 – Venus Unveiled.” N.p., 3 Sept. 2005. Web.
The picture used in the middle of the paper, illustrating Venus’ surface.
Bennett, Jeffrey O., ed. The Cosmic Perspective. 7th ed. Boston: Addison-Wesley, 2014. Print.
One of my astronomy textbooks, used for reference on what we know of Venus.
Ivanov, M. A., and J. W. Head. “Volcanically embayed craters on Venus: testing the catastrophic and equilibrium resurfacing models.” Planetary and Space Science 106 (2015): 116-121.
The main paper for today.
Reich, Eugenie. “Venus Crater Debate Heats up.” Nature. N.p., 30 Aug. 2010. Web.
A short article discussing the debate about the catastrophic resurfacing theory.
Turcotte, Donald Lawson, Gerald Schubert, and Donald Lawson Turcotte. Geodynamics. 2nd ed. Cambridge ; New York: Cambridge University Press, 2002. Print.
My textbook for geophysics, used for reference about Venus.