The Geology of Venus

Go outside before dawn in the next few days and look at Venus, and think about the description you find in your book.

We cannot see the surface of Venus from outer space. There are two ways of finding out about it. (1) Land probes there, (2) Bounce radar signals off the planet and map the surface that way. Most of what we know about Venus comes from radar studies,

Particularly the Magellan spacecraft which was in orbit around Venus. There were also important preliminary results from the Arecibo radio telescope in the 1970’s and 1980’s.

Let’s start with pictures from the surface, and deal with the radar data later. These were pictures returned from the Soviet Venera spacecraft in the 1960s and 1970s.

It is hard to know what to make of this picture without some contextual information.

To provide that context, we can look at radar maps of the surface of Venus, which give us a detailed view of that planet. Let’s take a look at the surface of Venus.

The brighter tones indicate higher elevations. The data from the Arecibo and Magellan spacecraft radars have allowed us to make a map of this planet, which is shown in Figure 10.20 of the book.

Another textbook on solar system astronomy has the following quote: since Venus has about the same size and composition as the Earth, we might expect the geology to be similar….

As the book describes, there are similarities, but the differences are important too.

One striking similarity is existence of basins (oceans on Earth) and thicker,more elevated parts of the crust (continents on Earth).
Venus has two continents, Aphrodite and Ishtar.
Rocks appear to be igneous, basaltic rocks.
Surface features seem to indicate importance of convection of magma in the planet’s mantle.

Nonetheless, there are some major differences in the geology we see on Earth and on Venus.

· Although there appears to be convection in the mantle, it apparently is not vigous enough to cause continental drift like we have on Earth. We do not see what appear to be trenches or ridges, subduction of plates, etc.

· Lacking water and strong winds, all processes of erosion of rocks and soil are absent. This means Venus retains evidence of its geology than does the Earth.

· It is like the Moon in this respect, although less pronounced.

Impact Craters are to be seen on Venus. Good pictures are to be seen in Figures 10.24 and 10.26.

The fact that we can see lots of craters on Venus, and that they are hard to see on Earth, means that Venus, like the Moon, has remained geologically static for a while.

We can use the information obtained from the Moon, i.e. the cratering rate as a function of time. Again, we are assuming Venus has been in the same part of the solar system as the Moon, and was hit by the same projectiles.

The density of craters is that which we would have if the surface had been repaved about 500 million years ago. This is taken as evidence that there was a planet-wide outburst of volcanism or lava flows about this time in the past .

It is intriguing to note that the End-Permian Extinction Event on Earth was apparently associated with a worldwide spasm of volcanic activity. Perhaps what we are seeing on Venus is a similar phenomenon to that on Earth, 250 million years ago, wiped out 90 % of all species.

There are also craters on Venus that appear like dark splotches on the surface (there are not any examples in your book). These are believed to be meteors that denotated in the atmosphere, creating a shock wave, but did not have an impactor make it to the ground. This is, again, an consequence of the heavy atmosphere.

The Runaway Greenhouse Effect

The most intriguing aspect of Venus is the fact that, despite its physical similarities with Earth as regards mass and radius, it has such hellish surface conditions. This difference is directly attributable to its massive, carbon dioxide atmosphere. Carbon Dioxide impedes the flow of infrared radiation to space, the process by which a planet cools itself off, and so the temperature is higher.

This can be illustrated in a diagram that was drawn for the Earth, but can be used for any planet that has an atmosphere. Energy absorbed from the Sun at visible wavelengths of light is balanced by re-radiation to space at infrared wavelengths. If one or the other flow is disrupted, the balance is upset, and the temperature of the planet increases or decreases. Carbon dioxide weighs in on the heating side. It impedes the flow of energy to space by making the atmosphere less transparent to infrared radiation, so the Earth has a harder time cooling itself off.

One of the scariest facts in astronomy is that the Earth and Venus have a similar amount of carbon dioxide in the environment close to the surface. On Venus, it is in the atmosphere. On the Earth, it is tied up in carbonate rocks in the Earth’s crust.

There is a theoretical possibility that a rise in the temperature of the Earth could release carbon dioxide from the rocks, enhancing the Greenhouse effect, making things still warmer, etc, which would end up in making Earth a Venus-like planet. This is well worth keeping in mind as we discuss the carbon dioxide emissions which are occurring today.

The historical record of the CO2 content of the Earth’s atmosphere is shown below.

The book implies that this happened on Venus, and states that it probably was more Earth-like in the early days of its history. This is highly speculative.

An equally, if not more likely history is one in which Venus was never cool enough to form liquid water oceans. The carbon dioxide emitted by volcanos build up over time and produced the hot dense atmosphere, and hot planet today. On Earh, by contrast, the oceans served to remove carbon dioxide from the atmosphere and “fix” it in carbonate rocks. Perhaps in the future we will be able to definitely choose between these two possibilities, or come up with another one.