The Inner Planets: Mercury and Venus

Mercury

Mercury from Gustav Holst's The Planets

The Basic Facts:

average distance from sun is 0.387 AU or 58 million km
highly eccentric orbit e = 0.2056
orbital period 87.969 d
rotational period 58.646 d
inclination of axis 0 degree
average density 5.4 g/cc
surface temperature -173 C to 430 C

Mercury is a difficult planet to observe and only those who make a special effort have seen the planet even though it is very bright. The problem, of course is that Mercury is always situate close to the sun in either the evening or morning sky and is usually lost in the glare of the sun. At one time ancient astronomers thought that Mercury was two different planets. Telescopic observations reveal an essentially featureless disk.

Greatest Elongation

The inner planets are always seen near the sun - simply because they are situated between us and the sun. The largest distance in the sky that a planet can appear from the sun is called the Greatest Elongation . For Mercury this works out to be 28 degrees . It is at these times that we can see the planet most easily.

Mercury's Landscape

Mercury presents a very rugged and un-inviting landscape. In fact a casual glance at a satellite photo of Mercury (or even a studied one) would likely lead you to think that you were looking at our moon. The surface of Mercury is heavily cratered - in fact more so than the is the lunar surface. Temperatures range from a searing 700 K on the bright side of the planet to a truly frigid 100 K on the dark side. The very small inclination (<< 1 degree ) has even made it possible that Mercury has a polar cap if recent radar observations are correct!! If this is true then the polar cap would probably represent the total of the remaining volatile elements on the planet. For the most part the gravitational pull of the planet is too weak to hang on to any atmosphere which renders Mercury a lifeless and bleak world. Note that a weak atmosphere of Hand He is present - the result of trapping solar wind. This rarefied atmosphere is constantly boiling off the planet and being replenished by the solar wind.

Orbital Resonance

Mercury exhibits an orbital phenomenon that we will encounter with the asteroids and Jovian planets. The orbital period of Mercury is 97.97 days which is 1.5 (3/2) times the rotational period of 58.65 days. The very low rotation rate is a puzzle but the fact that it is precisely 2/3 the period of the planet is very important. This is called rotational resonance or spin-orbit coupling and results from the concentration of mass on one side of the planet. As the planet approaches the sun the gravitational tidal tug ensures that either the heavy side is directly towards or directly away from the sun at perihelion (closest approach).

Structure and Composition

Mercury has, in all probability, an extensive and hot core consisting of heavy elements (Fe, Ni, Co). Recent flyby missions (Mariner 10) reveal that despite the slow rotation rate Mercury has a magnetic field. This supports the suggestion that Mercury's core is very large. The surface is essentially mantle material made up of silicates.

Surface analysis reveals that like the moon, Mercury has experienced intense asteroidal impacts and extensive lava flow activity in its history and continues to be struck by meteoritic material. The Caloris Basin area of the planet is the result of a particularly violent impact that - by seismic focusing - produced a very distinctive and jumbled region on the opposite side of the planet. Planetologists - at a loss for a better term - call this a weird landscape! Another peculiar feature of the Mercurial landscape are the lobate scarps (curved cliff-like structures) that are believed to have formed as the planet cooled and shrunk slightly.

Seeds: Chp 22

If You Don't Stop Smoking You'll End Up Like Venus!

Venus from Gustav Holst's The Planets

The next planet in the tour is Venus. 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 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
average density 5.24 g/cc
surface temperature 472 C

Venus can only be seen in the morning or evening (48 degrees greatest elongation). The planet reveals little to an earth based telescopic other than its planetary phases. Our knowledge of the Venusian atmosphere and interior has been increased enormously by recent satellite missions to that world.

Orbital Quirks - the "Velikovsky Affair"

In 1950 the iconoclastic writer Immanuel Velikovsky, following years of study primarily of ancient historical records, published the book Worlds in Collision. The scientific community tried to ignore the work as "Crank Science". Velikovsky's ideas are "crazy" and certainly incorrect but they do contain a very small grain of "truth" and arise from some peculiarities of Venus' orbit. The most puzzling of these is the retrograde (opposite) rotation of Venus. Velikovsky explains this in the following way:

Venus is really a comet that was ejected from Jupiter, nearly hit the earth twice and eventually was captured by the sun to orbit were now see Venus.
The retrograde rotation is the result of interaction with the earth. Earth stopped rotating briefly (Numbers 11!).
The reaction against Venus was to introduce a spin in the opposite sense and ....

This is really painful for a trained scientist to read or discuss. In fact the Velikovsky affair teaches us a number of very valuable lessons:

people want to hear what they want to hear
scientists are often not very good at expressing ideas to the public
pseudo-science sells!
scientists have an obligation to present science fairly and honestly to the public. This includes pointing out possible flaws or shortcomings in current ideas.

Where is the tiny "grain of truth" in Velikovsky? Can you propose a theory - consistent with the solar nebula hypothesis - that could explain retrograde rotation?

The Greenhouse Effect

The heavy CO2 content of Venus' atmosphere accounts for the enormous surface temperature of 750 K. Light penetrating the cloud layer and heating the surface is re-radiated as infrared light. The CO2 in the atmosphere does not transmit these wavelengths and retains the heat close to the planet surface.

The Atmosphere of Venus

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.

CO2: 96.5%
N2: 3.5%
SO2: 0.015%
Ar: 0.007%
H2O: 0.002%
CO: 0.0017%
He: 0.0012%
Ne: 0.0007%

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. We will not dwell on this but should point out that the slow rotation rate of Venus allows us to understand another planetary feature - no detectable magnetic field.

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 are Earth are extremely important for this reason alone.

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!

Applets that help explain Greenhouse Gases and the Greenhouse Effect


Applet that shows how CO2 acts as a greenhouse gas	Applet showing how CFC's respond to electromagnetic waves of different wavelength	Applet that shows the absorption of various greenhouse gases in the IR part of the spectrum

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