Characteristics and Origins of the Solar System

Lecture 8, September 14, 2001

The Earth as a Planet

>> Let’s start with putting this section of the course in context. So far, we have given a rough sketch of the solar system, described how its structure is manifest in the night sky (and how we can find the object), and the way planets move around in the solar system (orbital motion).

Now let’s start talking about solar system objects. We start with the Earth.

Purpose:

• To study the Earth within the context of the other solar system objects.
• To learn things about the solar system which are unavailable from other astronomical objects (so far). To get into the spirit of things, look at Figures 7.1 and 7.3 from your textbook.

Material from next few lectures from Chapter 7 of textbook, take it out and sing along!

Note in particular the reference to Professor Van Allen in your textbook.

First: “Just the facts, Maam” This information is from Table 7.1 of book.

Characteristic	Value
Distance from Sun	1.00 astronomical unit
Radius	6378 kilometers
Mass	5.98 ´ 10^24 kilograms
Density	5.52 grams/cc

An intermezzo on density. Density is the amount of material per unit volume in a substance. It is defined as mass per volume. The most convenient units are grams/cubic cm. The density of water is 1 g/cc. Typical rocks have densities of about 3 gm/cc.

>>> Illustration with substances of different densities.

>>> First appearance of the famous picture.

Structure of the Earth

A few kilometers down and we have almost no direct measurements. Almost like astronomy in general. Best technique is seismology, ringing of Earth to Earthquakes.

Basic structure as shown in Figure 7.2 of book. Structure of Earth divided into three parts

Terra in partes tres divisa est.

• Crust, the outer part we live on, very thin
• Mantle, large layer of molten rock
• Core, or inner metallic part of Earth

>>> Transparency eius.

The crust is the part that most of geology deals with. However, it is a thin skin on the remainder of the planet.

Properties of the Earth’s Crust

• Extremely thin, and different in continents and oceans.
• Thickness 6 kilometers under oceans and 20 to 70 kilometers under continents.
• Rocks consist of basalts on ocean beds, granites in continents

Basalts and Granites are igneous rocks, associated with volcanic lava, hot flows.

Chemical compositions of these rocks is primarily silicon, oxygen, iron, aluminum, and magnesium. Different blends make different minerals.

An example of a mineral that is found on Earth and also is known to occur on other planets is Olivine . Its chemical structure is Fe₂SiO₄ or Mg₂SiO₄ .

>>>> “chocolate sampler” with different minerals.

An intriguing first result of this is that the chemical composition of the Earth differs so markedly from the Sun and the other stars. In those objects, the main elements are Hydrogen and Helium, followed at an abundance of about one atom in a thousand of oxygen, carbon, and nitrogen.

Types of Rocks

• Igneous (basalts and granites). Look at Figure 7.5 of your textbook.
• Sedimentary
• Metamorphic . These are defined in another book as “rocks originally formed in either of the two above modes (igneous or sedimentary) but changed to a new rock form by high pressure, high temperatures, or the addition of new chemicals”.
• Primitive (to an astronomer the most interesting. No examples on Earth. We will have a “laying on of the hands” of some samples from outer space.)

Characteristics of the Crust

Major fact is that it is fractured into tectonic plates, and that these are moving with respect to each other, in a process called continental drift. Book emphasizes importance of plate tectonics to field of geology, sic p151

“It (plate tectonics) is a concept as basic to geology as evolution by natural selection is to biology, or gravity is to understanding the orbits of the planets”

Personal aside: plate tectonics is actually in better shape experimentally than natural selection. It has been directly verified by measurements showing continental drift. In biology, we know evolution occurs, but natural selection is more of a suggestion as to why it occurs.

Look at Figure 7.6 to see where the plates are.

>>>>>> Map with tectonic plates.

Types of Plate Motions

Remember, in an astronomy class we are talking about all of this to grasp it as a potentially general, widespread planetary process! Transparency with these processes.

· Rift

· Subduction

· Mountain building and plate collision

· Faulting and plate shear

The plate motions are driven by convection of material in the mantle.

>>>The Final Point: Plate tectonics mean the continents float around on top of a magma ocean like rafts. The arrangement of the continents is different today than it was in the past. About 250 million years ago, all of the continents were together in a supercontinent called Panagea. The rest of the world was occupied by a gigantic ocean called the Panthalassic Sea.