Exploration of the Solar System

Lecture 5

The Earth as a Planet

>> Keep an eye on the Moon this coming weekend as it continues moving to higher Right Ascension, through the constellations of Libra, Scorpius, and Sagittarius. By consulting your SC1 chart, you will clearly see that the Moon moves on a path which is close to the ecliptic, but inclined at about 15 degrees. Particularly pay attention on the night of September 19, when it is very close to the bright star Antares in Scorpius. This will help you get a better feel for the SC1 chart.

>> 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 objects), 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 Figure 8.9 of your textbook.

Material from next few lectures from Chapter 8 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 8.1 of the 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.

>>> Check the video clip for this course, showing substances of different densities.

>>> I think it is very useful to see how the Earth “stacks up” relative to other solar system objects. This is illustrated in Figure 10.1 of your book.

Structure of the Earth

A few kilometers down and we have almost no direct measurements. Almost like astronomy in general. The best technique is seismology, which is studying how the Earth “rings” after large earthquakes.

Basic structure as shown in Figure 8.15 of book. The structure of Earth is divided into three parts

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

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

Question: What evidence do we have that this picture is correct? In other words, what phenomena can you think of that are at least consistent with this picture?

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 . Another important mineral on the surface of Mars is Hematite, which is Fe₂O₃ . As we will see in the discussion on Mars, Hematite is a very important mineral because it forms in water. The presence of hematite on Mars, which has been amply measured by the Mars Exploration Rover spacecraft, is an intriguing hint that Mars probably had water on its surface in the remote past.

>>>>Once again, look at the video clip with a “chocolate sampler” of different minerals.

Types of Rocks

• Igneous (basalts and granites
• Sedimentary (these are rocks laid down by wind or water. An excellent example is the limestone deposits around the Coralville Resevoir and Lake McBride. These were ocean bottoms during the Devonian Period, about 370 million years ago.
• 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. Look at the video clips for pictures of some of these extraterrestrial, primitive rocks.

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. This is discussed on p164 of your textbook. Another book I have used emphasizes the importance of plate tectonics to field of geology in the following words:

“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 8.20 to see where the plates are.

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!

· Rift or divergence

· Subduction (convergence of plates)

· Faulting and plate shear

These are illustrated in Figure 8.21 of your book.

In addition to these three types of motion, there is another manifestation of plate convergence, which is plate buckling and pileup.

Question: What are some common phenomena (experiences) which correspond to these plate motions?

The plate motions are driven by convection of material in the mantle. This phenomenon is nicely illustrated in Figure 8.22 of your textbook.

>>>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.

This motion is illustrated in a little movie, which shows the location of the continents over the past 600 million years.

http://www.cmp.berkeley.edu/geology/anim1.html