Chapter 13: Cosmic Samples and the Origin of the Solar System

This chapter concludes our discussion of the solar system. We begin with a discussion of meteors and meteorites, examples of the primitive matter from which the solar system formed. Then I will summarize the things we've discussed from chapters 6 to 12. Finally I will consider the question of how the solar system formed and evolved, and the impact our understanding of planets, satellites, rings, asteroids, and comets has on this question.

13.1 Meteors

As comets approach the Sun, they are heated and material evaporates. This material remains in orbit around the Sun. During the Earth's orbit around the Sun we pass through clouds of comet material.

Observing Meteors

Meteors can be seen during any night by a careful observer. When pieces of material enter Earth's atmosphere they heat the gas in the upper atmosphere and the hot gas gives off light seen as meteors or shooting stars. (Note that shooting stars have no direct connection with actual stars!)

Meteor Showers

At certain points in its orbit, the Earth crosses places where comets previously passed. In these regions the comet leaves an extra amount of material that can enter the Earth's atmosphere and create a meteor shower. Meteor showers from the same comet remnants occur each year (like holidays). For instance, the Perseid shower that occurs in early August and the Leonid shower in mid-November are well known.

The tracks in a meteor shower seem to point back to a particular point in the sky. This occurs because the particles involved in the shower came from the same source and move roughly together.

13.2 Meteorites: Stones from Heaven

A meteorite is any fragment that survives its passage through the atmosphere and reaches the ground.

Extraterrestrial Origin of Meteorites

Though meteorites have been around for centuries, it wasn't until the late 18th century that we realized the extraterrestrial origin of meteorites.

Meteorite Falls and Finds

Meteorites are found in two ways.

Recently a third source has turned up: as ice flows and recedes in the Antarctic, meteorites are uncovered in the ice.

Meteorite Classification


Ages and Compositions of Meteorites

Scientists study the age and composition to learn about the material from which the solar system formed. Ages can be determined by radio-isotope dating. We find that the oldest meteorites are about 4.5 billion years old. Meteorites that came from the breakup of larger objects, or ejecta from impacts can be substantially younger.

The Allende and Murchison Primitive Meteorites


13.3 Formation of the Solar System

No let's assemble the information from the previous chapters into a theory for the formation of the solar system.

Observational Constraints

We have three types of constraints

These constraints are the facts that a theory must explain.

  1. Motional
    1. all the planets revolve around the Sun in the same direction and approximately the same plane as the Sun rotates
    2. most planets rotate in the same direction that they revolve
    3. most of the satellites rotate and revolve in the same sense as well
    4. the theory must also deal with the exceptions, like Venus's retrograde rotation
  2. Chemical
    1. Jupiter and Saturn have a similar composition the the Sun (and stars)
    2. other planets are lacking in hydrogen and helium
    3. the inner planets are metal rich, then come rocky planets, and finally ice dominated objects in the outer solar system
    4. temperature progression from inner to outer planets
    5. how to explain the presence of water on Earth and Mars where things were hot early on (came later from outer region?)
  3. Age
    1. Earth samples are at least 3.8 billion years old
    2. lunar samples are up to 4.4 billion years old
    3. primitive meteorites are 4.5 billion years old
    4. indicates that planets must have cooled rapidly
    5. little unaltered material left from before the formation of the solar system

The Solar Nebula

The facts are consistent with the theory that the solar system formed 4.5 billion years ago from a rotating could of hot material called the solar nebula. The composition of the nebula would be similar to the Sun (it does represent 99.8% of the material). As the nebula collapsed under its own gravity, the material heated (this is a result of physics, not speculation). The heat would have vaporized any solid stuff in the mix -- destroying most of the evidence of the material's original state.

As material falls inward, the rotation would become faster and result in a disk shape, consistent with the planetary orbits we have now. Observations of young stars (to be discussed in a later chapter) are consistent with this idea.

After the material has finished collapsing, it begins to cool. Except for the Sun which begins producing its own energy through fusion (to be discussed in chapter 15). The cooling material can form into molecules and then into droplets, finally to begin condensing into planets. In the inner, hotter region, only metals and other heavy compounds were able to condense. In the outer regions, water, methane and ammonia were able to condense. This will create differences in the composition of the planets.

Formation of the Terrestrial Planets

The model is that material in the inner solar system formed into small solid bodies, planetisimals, that then accreted into larger bodies, eventually resulting in protoplanets. As protoplanets accrete more material, they heat up, allowing differentiation to occur.

Formation of the Giant Planets

In the outer solar system a similar process occurred, but the cooler temperatures allowed ices to accrete along with metals and silicates. The masses that resulted were considerably larger.

Further Evolution of the System

After formation, evolution of the solar system would continue. For instance, this is when we presume that Mercury suffered a large impact that stripped away much of its mantle, Venus suffered an impact that reversed its rotation, and Earth suffered an impact that formed the Moon.

13.4 Planetary Evolution

Geological Activity


Elevation Differences

Mountains on the inner planets come from a number of sources. On the Moon and Mercury, the major mountains are ejecta from large impacts. The large mountains on Mars (including Olympus Mons) are volcanos. While there are volcanos on Earth and Venus, the largest mountains result from compression and uplift. On Earth this is a result of plate tectonics; on Venus a result of the force of subsurface magma.

Why is the highest mountain in the solar system on Mars? Mars doesn't have plate tectonics that can impede large volcanos. Mars has lower surface gravity than Earth or Venus, allowing the underlying material to more easily support the weight of the mountain. Mars has a thin atmosphere and less erosion than on Earth.


Mercury and the Moon have no atmospheres to speak of. The remaining inner planets probably started with similar atmospheres that evolved differently. Mars has an atmosphere too thin to trap heat. Its water froze. Venus has a thick atmosphere that retained too much heat, raising the surface temperature well past the boiling point. Only Earth had the right mix to allow liquid water. Life was able to begin on Earth, and through photosynthesis, remove CO2 from the atmosphere and add oxygen.

Earth's atmosphere is largely the result of life.


This brings to close our discussion of the solar system.

© Robert Harr 2001