Venus - the Shrouded Planet

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At one time Venus was thought to be Earth's "twin" sister. It is almost identical in size and average density. How different a world it is however!. Its cloud shrouded surface has long been a mystery to earth-tied astronomers but recent orbiter and lander missions reveal an inferno of hot, corrosive gases. Venus demonstrates the "run away Greenhouse Effect" better than any other planet.

 

Basic Facts

average distance from the sun 0.7233 AU or 108 million km
orbital period 224.68 d
rotational period 243.01 d (CW!)
orbital inclination 177 degrees
radius = 6050 km
mass = 5240 kg/m3
albedo = 0.76
average density 5.24 g/cc
surface temperature 472 C
Table 17.5 Basic properties of Venus

Venus lies between us and the Sun so can only be seen in the morning or early evening sky . The planet is perpetually cloud covered and reveals little to an earth based telescope other than its planetary phases. Our knowledge of the Venusian atmosphere and interior has been increased enormously by recent satellite missions to this world.

 
Figure 17.26 Globe showing surface features of Venus. Image produced by radar-mapping.

The Atmosphere of Venus

Space probes that have entered the Venusian atmosphere have told us an interesting story. The atmosphere is dominated by CO2 (96%) and Nitrogen (3.5%). Most peculiar are the traces of highly corrosive compounds such as H2 SO4 (battery acid!!) or HCl (hydrochloric acid!). The clouds that can be seen in Venus' atmosphere consist primarily of sulfuric acid droplets!

The Venusian atmosphere is about 1000 times denser than Earth's.
CO2:
96.5%
N2:
3.5%
SO2:
0.015%
Ar:
0.007%
H2O:
0.002%
CO:
0.0017%
He:
0.0012%
Ne:
0.0007%
Table 17.6 Atmospheric composition of Venus (by weight).

Composition and Structure

Radar imaging of Venus' surface has allowed us to peer though the clouds and discover a planet that consists of rolling planes, impact craters and volcanoes. Evidence for volcanoes comes from a number of sources. Imaging reveals numerous volcano-like formations, lava flow channels are visible and most convincing is the fluctuating level of SO2 in the atmosphere. In fact there is good reason to believe that volcanoes are the primary agent in the production of atmospheres in general (Venus as well as Earth).

Structurally Venus would appear to be very similar to Earth but there are at least two very important differences:

  • There is no detectable magnetic field and yet Venus must have a molten core similar to Earth's. The for the difference is that Venus has too slow a rotation rate to produce the dynamo-effect necessary for the creation of the magnetic field.
  • The does not appear to be any plate activity on the surface of the planet. Reasons for this difference are poorly understood but some have speculated that it is related to the absence of liquid water at the surface of the planet. There still, however, is tectonic activity on the planet. Figure 17.27 shows a series of ridges and troughs formed by strong compressive forces in the crust.
Figure 17.27 Series of ridges and troughs formed by compressional deformation of the crust of Venus. (Image courtesy NASA)

One of the most dramatic differences between Venus and Earth is the scale of volcanic activity on Venus. More than 80% of the surface of Venus has been flooded by lava since the end of the era of bombardment in the solar system. Even though there are craters on the surface of Venus most have been covered by these extensive lava flows. As well, in the absence of plate movement the volcanoes can grow to enormous size and also form some very unusual structures unlike volcanoes on Earth. One such example are the "pancake domes" shown in Figure 17.28.

As well, the lack of plate activity on Venus means that the predominant kind of volcano on the planet is the shield volcano.

Even though several spacecraft have successfully landed on Venus their usable lifetimes were short and little additional information about the surface of the planet has been provided. Venus, in many ways, remains an enigma.
Figure 17.28 Pancake dome volcanoes (Image courtesy NASA)  

 

Earth and Venus: Why So Different

Venus is sometimes described as Earth's twin sister. Yet the two worlds are very different. The reason, primarily, is due to the different locations of the planets in the solar system. For example - as water formed on earth steam would form on Venus. (Water could come from "out gassing" of the planet or, as some suggest, from cometary impact). Water vapour in the atmosphere would shift the greenhouse effect enough that a "runaway" situation would ensue and the developmental paths of the two planets would quickly diverge. Without the benefit of large bodies of liquid water CO2 levels would continue to rise rather than be absorbed back into water. Oceans (liquid water) on Earth are extremely important for this reason alone. (To learn more about the role of greenhouse gases in determining planetary climate please consult the "take a closer look" section linked below.)

Is there a lesson in this? Greenhouse effects are real. Humanities continued abuse of earth through uncontrolled CO2 emission could have permanent, irreversible consequences. We merely need look in the early evening or morning sky for proof!

Example 17.9 Follow the "take a closer look" link above and use the applet GreenHouseGases to investigate the role of GHGs in the regulation of Earth's and Venus' climates. In particular contrast the size of the greenhouse effect on each planet.

Solution: Set the surface temperature to be either +15C for Earth or +470C for Venus, adjust the albedo and distance appropriate for each planet and adjust the GHG slider until radiative equilibrium is achieved. You should discover the following: 1) Earth the GHG "factor" is 0.43; 2) Venus the GHG factor is 0.99 - Both of these numbers are large and tell us that each planet has a significant greenhouse effect but the dense atmosphere of Venus leads to a dramatically different surface temperature!

Orbital Quirks

Venus is rotating backwards! In fact - you could correctly assert that on Venus a day is longer than a year! The orbital period for Venus is 224.68 days while the rotational period (Venusian day) is 243.01 d. Furthermore - the rotation is in the opposite direction. What could account for this?

One theory is that in the early stages of planetary development Venus underwent a catastrophic collision that altered its rotation. This may be true but more recent models of the tidal interaction between the Sun and the very dense atmosphere on Venus and the frictional interaction between the atmosphere and the planet could produce a braking effect and actually cause a reversal in the direction of rotation of the planet. This, along with many other aspects of Venus remain open questions and Venus remains an enigma!

Practice

  1. Explain why the absence of plate activity on Venus would mean that volcanic structure on the planet would be significantly different than those on Earth.
  2. Describe the fundamental role that liquid water plays in establishing climate on a planet.
  3. Discuss what it means to describe Venus as an example of a "Run-away Greenhouse Effect".
  4. Why would it be reasonable to expect that Venus has a molten core?
  5. Why is there no significant magnetic field on Venus?

 

 

 

 

Chp 22-1