We live in a special age - The robot spacecraft view more and more bodies in the Solar System close up, providing the type of data that geologists are used to, the more the study of the Solar System becomes a branch of geology - planetary geology. The information we get about these distant objects is crucial to our understanding of the Earth. Recent advances include the the discovery of giant caverns on Mars.

Information from the planets has greatly expanded our understanding of the Earth. But where does Earth come from?

Matter in the Universe:

Origin of stars: Stars from from clouds of interstellar gas and dust. Where does this material come from?

The result of all this is that huge clouds - nebulae of interstellar gas and dust are spread throughout the galaxy. Sometimes, that material can be compressed by the stellar winds of neighboring stars to the point that it contracts under its own gravity, leading to....
  • Formation of Solar Systems:

    Nebulae. Occasionally, conditions are such that clouds of gas and dust (leftovers of the big bang, material ejected from stars during their lifetimes, and the shrapnel of ancient supernovae) become sufficiently compressed that they form relatively dense clouds of material called nebulae (sing. nebula) like the Rosette Nebula at right. Particularly dense regions of nebulae can continue to contract as gravity sucks material to there centers, where it piles up forming a protostar. The Sun and Solar System formed in this way. The protostar becomes hot for at least two reasons.

  • Stages in origin of Sun: We now turn to what was going on in the disk of dust and gas surrounding the protosun when our solar system formed.

    The Sun and Planets to scale

    Origin of planets:

    A modern analog to this stage in the Solar System's development may be provided by the star Foumalhaut in whose accretion disk the first extrasolar planet ever directly to be imaged orbits.

    One class of meteorites represents material left over from the formation of the Solar System. They are 4.56 billion years old, so we infer that this is when the solar system condensed.

    Planetary types: A function of mass and distance from the sun. Exciting links to the new age of planetary wonders:


    Early history of the Earth: Remember those undifferentiated meteorites? The Earth was originally made of lots of that undifferentiated material. And yet, the oldest known rocks are very different from undifferentiated meteorites and from one another. What happened in between the formation of the Solar System and the formation of the earliest rocks that we can put our hands on to cause this differentiation?

  • Sources of heat for the early Earth.
  • Period of Differentiation:

    Differentiation yields three basic compositional layers of the Earth. (NOTE THE WORD "COMPOSITIONAL!" There are other ways of breaking down the Earth's layers that we will learn later.)

    • Crust: Light rocks rich in Si and Al. Anywhere from 10 to 70 km. thick.

    • Mantle: Bottom of crust down to 2880 km. Dense rocks rich in Fe and Mg.

    • Core: From 2880 km. to the center. Metallic - primarily of nickel and iron. (There is an outer liquid core and a solid inner core - same composition, different physical state. More on that later.)


    The origin of the Moon through the Giant Impact of a Mars-size planetesimal.

  • Sounds far-fetched, but three lines of evidence suggest that it is true. Two of these are surprising result of the Apollo program:

    To date, the only hypothesis that hasn't been falsified by good data is that the rocks that formed the moon were blown off the Earth's surface by a giant impact. Here's the sequence:


    Earth's stratification and other terrestrial planets. Like them, Earth has:

    However it also has these unique realms:

  • Hydrosphere: The realm of liquid water
  • Biosphere: The aggregate of living things - which interact strongly with other Earth systems. Earth's Difference from other planets stems from its being in the: Goldilocks zone - the region of the solar system not too hot, not too cold, but just right for liquid water. As we will learn, this water facilitates many geologic processes that are unique to Earth.

    These are the major subsystems of the dynamic Earth System.

    Earth cycles:

    Merck adds instructive example of carbon cycle.

    Compare Earth to Venus

    • Venus' average surface temperature is 840 deg F.
    • Venus' average atmospheric pressure is 90 atmospheres (What you would experience about 1 km. below the ocean surface). The atmosphere consists mostly of CO2.

    Venus, unlike Earth, has essentially no carbon cycle. On Earth,

    • Carbon enters the atmosphere as CO2 that erupts from volcanoes.
    • Atmospheric CO2 dissolves in oceans.
    • Eventually CO2 precipitates out of solution as carbonate minerals that become incoporated in the rocks rocks.
    • As they become buried deeply, they can melt, releasing carbon that, again, is erupted into atmosphere as CO2.

    Follow this link for a fictional but realistic cinematic imagining of Venus' surface. BBC's Voyage to the Planets - Venus. But on Venus there is no hydrosphere, so CO2 just gathers in the atmosphere. Venus has no proper carbon cycle.

    Digitally reworked Venera images showing Venus surface in normal perspective.

    A major source of Earth's uniqueness is its abundance of interacting cycles.

    But note: Up until a year ago, we thought that Earth was the only planet with an active hydrologic cycle. We now know of one other. Problem is, water is not involved.

    Key concepts and vocabulary: