Calspace Courses

 Climate Change · Part One
 Climate Change · Part Two
 Introduction to Astronomy

      Introduction to Astronomy Syllabus

    1.0 - Introduction
    2.0 - How Science is Done
    3.0 - The Big Bang
    4.0 - Discovery of the Galaxy
    5.0 - Age and Origin of the Solar System
    6.0 - Methods of Observational Astronomy
    7.0 - The Life-Giving Sun

  8.0 Planets of the Solar System
         · 8.1 - The Jovian Planets
         · 8.2 - The Terrestrial Planets

    9.0 - The Earth in Space
    10.0 - The Search for Extrasolar Planets
    11.0 - Modern Views of Mars
    12.0 - Universe Endgame

 Life in the Universe

 Glossary: Climate Change
 Glossary: Astronomy
 Glossary: Life in Universe

The Terrestrial Planets

Mercury (Source: NASA)
Going toward the Sun from Jupiter we find a totally different assemblage of planets. The biggest of the "inner" or "terrestrial" planets is Earth itself, at about 0.3 percent of the mass of Jupiter the 5th largest of the nine planets. Its atmosphere is quite unlike that of Jupiter and the outer planets in general. Instead of hydrogen and helium there is nitrogen and oxygen.

All the inner planets — Mercury, Venus, Earth and Mars — have solid surfaces with a sharp separation from their atmosphere. The solid matter consists of compounds of silicates ("minerals") rich in the elements magnesium, iron, calcium, sodium and potassium, besides the silicon and oxygen that provides the bulk for the various crystals. To account for the great density of the inner planets (more than 4 times higher than that of Jupiter) one must assume much enrichment with iron, over other elements. (Iron is what makes Mars red, and links it to blood, which is spilled in war; hence Mars has the name of the ancient war god.)

Mars (Source: NASA)
As far as the atmospheres of the inner planets, that of Earth is highly unusual, because it is chemically unstable (nitrogen and oxygen combine within lightning). By contrast, Mercury has no atmosphere. Its gravity is too low to hold on to gas heated by the Sun. (The same is true for the Moon, a body much like Mercury). Venus has an atmosphere almost a hundred times more massive than that on Earth. Its pressure corresponds to that in the ocean at about 1 km depth. But it is not water, it is carbon dioxide! At the surface it is hot enough to melt lead. Hydrochloric and sulfuric acids also are present. Only two percent of the sunlight penetrates to the surface. Most of it is reflected. (The high reflectivity, which is due to sulfuric clouds, makes Venus the brightest object in the sky, after the Sun and the Moon. Its beauty is recognized in its name.) The atmosphere of Mars has but one-hundredth the pressure of that of Earth. It also is almost entirely carbon dioxide, with some nitrogen and argon. The reason that the atmosphere of Mars is so thin is that the gravity of Mars, the smallest of the inner planets, is relatively weak.

Venus (Source: NASA)
In any event, our companion planets Venus and Mars have atmospheres made of carbon dioxide, while in the Earth’s atmosphere carbon dioxide is but a trace gas. The reason is that life processes have removed the carbon from the atmosphere into limestone rocks, coal, oil and gas (in the ground and as methane ice). Limestones, like the famous White Cliffs of Dover, were originally made of carbonate fossils. Coal is finely disseminated, mostly in sedimentary rocks, and made from plant materials. Oil and gas, too, are made from marine organic matter.

We are now using coal, oil and gas for production of energy (for the time reversing the long-term extraction of carbon from the atmosphere). Thus, carbon dioxide and methane are growing in abundance in Earth's atmosphere. Also, we are using limestones to make cement, which makes a modest contribution to carbon dioxide, as well. This recent change in the chemistry of the atmosphere of Earth is perhaps the most significant indicator of the presence of intelligent life in the solar system. (The emission of radio-waves carrying news about football games and stock prices is a close second.)

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