Setting up the Terrestrial Environment

Earth from Apollo 17

As the proto-Earth grew, it heated up. This happened from some combination of accretion energy - the heat of impacts - and internal radioactivity. Worlds which have melted internally exhibit differentiation, a sinking of denser material to the center and floating of light rock to the surface. The Earth shows this clearly, with a core rich in iron and nickel and surface rocks rich in silicon and oxygen. Planetologists tell us that the single most dominating feature of a planet is how its heat gets out. The terrestrial crust solidified (for the last of what were likely several times) about 3.9 billion years ago, represented by the oldest rocks.

Our atmosphere is of the secondary variety, since the Earth is too hot and with too little mass to hang on to the original hydrogen/helium envelope. It is unique in being dominated not by CO2, but rather by free nitrogen. Carbon dioxide, formed from cosmically abundant elements, is the major constituent of the other terrestrial planets' atmospheres. The oceans (and perhaps subduction of seafloor rocks in plate tectonics) may be important in keeping this gas out of our atmosphere, and thus keeping us from going the way of Venus. Our atmosphere has had an eventful history. The high abundance of free oxygen traces back only to when organisms started releasing it - up until that time, completing about 1.6 billion years ago, iron was deposited in rock. Afterwards it rusted first. This also led to the development of a UV-absorbing layer of triatomic oxygen molecules (AKA ozone).

Atmospheric composition controls the surface temperature, through the greenhouse effect (which keeps the mean temperature from being -40 C). It is interesting that the Earth's surface temperature has stayed so constant, especially since the Sun has increased in luminosity by about 30% over its lifetime (perhaps telling us that the level of atmospheric CO2 has declined as it was incorporated into rock or dissolved in water). The Earth has gone the ways of neither Hoth nor Tatooine, telling us that its temperature never fell (on average) below freezing or too hot for liquid water. Some climate variations do appear to be driven either by small changes in solar output (i.e. the Maunder minimum or "Little Ice Age") and in the parameters of the Earth's orbit (the Milankovitch cycles which match the timing of recent ice ages).

The Big Deal about the terrestrial environment is, of course, water. Lots and lots of it. In fact, looking at a picture of the planet, we clearly give it the wrong name. And all that water coexists in all three phases of solid, liquid, and vapor. Where did it come from? Some probably was part of volcanic outgassing. Impacts from comets may also have contributed. Water is a remarkable solvent, as well as having fascinating thermal properties. Its crystalline structure makes it float on freezing, and its enormous heat capacity drives much of our weather. It's no wonder that whenever we see evidence for water (frozen or past) on other worlds, the idea of life isn't far behind. The temperature range for liquid water is interesting for all manner of organic chemistry, making reactions go fast enough without dissociating the molecules.

The Earth's surface remains geologically active, controlled by processes of plate tectonics that are not present now on other planets. This gives a continuous cycling of (mostly seabed) material between crust and mantle, as well as continental drift and its implications for habitats and competition as the biosphere developed.

The Moon also plays a continuing role. Its driving of the tidal rhythm is important for coastal organisms, and has in fact slowed the Earth's rotation (at least from a value near 18 hours). Its tidal influence may also help stabilize the orientation of the Earth's axis, preventing the kinds of wild climate variations over geological time that Mars seems to be subject to.

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Up: The cosmic stage for life on Earth

More details:

  • Evidence for water on Mars and Jupiter's moon Europa
  • Life on Mars?
  • Quick summary of Milankovic cycles (from Bonnie Blackwell, CCNY)
  • World tectonic activity map (live and digital)
    Last changes: 8/2002