The Geology of Venus

Mars is clearly visible in the early morning sky at about 6AM. It is close to the star Regulus in the constellation Leo.

Once again, reason for this is the prominence of Venus in the evening sky. Go look at it and think about its description in this 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.

>>>>>> transparency of Venus, Figure 9.4.

View from Surface of Venus

Good quote from book: 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 Figure 9.5. 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 (recall the situation with extinctions on Earth).

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.

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 book implies on p198 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.