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6.29 Internal Heat and Geological Activity

Heat is the ultimate source of the energy that drives geological activity on any planet. The internal heat of the Earth, fueled by radioactivity, is the "engine" that drives the geological activity of the planet. Volcanoes, for example, erupt because the heat inside the Earth melts rock and creates supplies of molten magma; the magma is less dense than surrounding rock, so it tends to rise to the surface. Earthquakes are also caused by pressure associated with the movements of molten rock. Without radioactivity, the Earth’s interior would have cooled long ago. Volcanoes would be extinct, and there would be no earthquakes. Earth would be geologically dead.

Radioactivity is one of the main sources of heat inside the Earth. How do radioactive minerals inside the planet produce heat? Imagine an atom of a radioactive element buried inside Earth. When the atom breaks down during the process of radioactive decay, it shoots subatomic particles and photons of energy outward into the neighboring material. Just as fragments of an exploding billiard ball might set neighboring balls into motion, the "exploding" atom’s debris hits other atoms and increases their motions. Temperature is a measure of particles’ speed, so increased motion results in a higher temperature. Radioactive material is therefore an energy source, and it heats the interior of the planet in which it’s trapped.

The heating action of radioactive elements inside the Earth explains why early models of the Earth’s age, based simply on its cooling time, were far too short. As Newton and Buffon showed, the Earth would have cooled completely within millions of years if it had no radioactivity. But with this internal heating, the depths of the Earth have been kept hot for thousands of times longer - several billion years.

In general, planets produce heat according to their size. Radioactive atoms decay in the interior, and conduction and convection transport this heat from the interior to the surface. Bigger planets have more gravity, and the pressure due to gravity helps to create a molten interior that can drive geological activity. Also, the bigger the planet, the longer it takes internal heat to reach the surface. There’s no mystery here — if you pull a large rock and a small rock out of a campfire, the small one will cool off quickly, while the big one will stay warm longer. Planets are the same.

The simple idea of internal heat scaling with size tells us a lot about planetary evolution, since the geological activity on planets is driven by heat from the interior. A small planetary body cools off very quickly, and its interior produces little heat because there is less radioactive material. So the surface of a small planet will show countless crater scars from impacts that happened throughout geological time. On the other hand, a large planetary body retains its heat longer, and its interior produces a lot of energy from a larger mass of radioactive material. This internal heat will drive volcanism and tectonics, which constantly reshape the surface. Therefore, the surface of a large planet will be young and show fewer impact craters.

The rate and mode of heat transfer from the center of the planet to the surface controls the rate of geological activity on the surface. If the planet is small and has cooled off, the center is not much warmer than the surface, and the heat can be carried outward by conduction. However, if a planet is large and has a lot of energy from radioactive decay, the center is going to be much hotter than the surface. The strong temperature difference between the center and surface induces convection in the mantle, which is a sluggish mass movement of the hot, slightly plastic material. If they’re hot enough, the inner rock layers of a planet can actually flow slowly. Hot plumes of convicting material rising in the asthenosphere create "hot spots" under the lithosphere. Such hot spots produce volcanoes. The Hawaiian Islands are a terrestrial example, and the giant volcanoes on Venus and Mars may also be located over hot spots created by convection. A smaller planet with a colder interior has less internal heat to create convective movement in the mantle, and thus less internal energy to drive seismic activity, tectonics, or volcanoes.

Simple ideas of heat generation and transfer inside planets allow us to understand the differences in geological features from one planet to another. If we list the terrestrial planetary bodies in order of increasing size, we see exactly what we would expect, a progression toward younger surfaces and more geological activity:

• Deimos - Cold throughout - Heavily cratered.

• Phobos - Cold throughout - Heavily cratered.

• Moon - Mostly cold - Heavily cratered, some lava flows (3.5 billion years old).

• Mercury - Mostly cold - Heavily cratered, some old flows (3.5 billion years old?).

• Mars - Warm interior? - Half of the planet cratered; half is covered by young lava flows (1.4 billion years old?). Several very young volcanoes (500 million years old, possibly still active?).

This global view of the surface of?Venus?is centered at 180 degrees east longitude.?Magellan?synthetic aperture radar mosaics from the first cycle of Magellan mapping are mapped onto a computer-simulated globe to create this image. Data gaps are filled with?Pioneer Venus Orbiter?data, or a constant mid-range value. Simulated color is used to enhance small-scale structure. The simulated hues are based on color images recorded by the Soviet?Venera 13?and?14?spacecraft. The image was produced by the Solar System Visualization project and the Magellan science team at the JPL Multimission Image Processing Laboratory and is a single frame from a video released at the October 29, 1991, JPL news conference. It is important to note that Venus is completely shrouded in clouds. A bright elongated region in the center is Aphrodite Terra. Click here for original source URL.

• Venus - Still hot - Few impact craters. Intense volcanism, lava-covered surface (mean age 0.7 billion years).

Blue Marble Earth composite images generated by NASA in 2001 (left) and 2002 (right). Click here for original source URL.

• Earth - Still hot - Few impact craters. Active volcanism and plate tectonics (mean age of crust 400 million years).

The general rule that large planets stay hot for longer has nothing to due with solar heat reaching the planet from the outside. The Sun is the dominant heat source only for the top meter or so of the surface; this layer warms in the day and cools off at night. The Sun's heat doesn’t penetrate more than a few meters into the planet. As you may know, the inside of a cave stays about the same temperature all day long, and even all year long, because the surrounding rock layers insulate the cave. Likewise, the inside of a planet is insulated. The heat inside a planet that melts rock and creates volcanic lava comes almost entirely from the heat sources inside the planet itself, not from the Sun (although the radioactive elements producing the heat were once formed in a stellar explosion called a supernova).

To have significant internal heat, a planet must be large enough to have sufficient radioactive material, and large enough to provide some insulation to retain that heat. So small asteroids, for example, don’t have enough internal heat to be geologically active. However, there is also an energy source that has affected all planets, regardless of their size. Soon after it formed, each planet was partially heated by countless planetesimals crashing into it. This was material left over from the formation of the solar system, and the kinetic energy of the impactors was transformed into thermal energy when they collided. This explains why we find meteorites from undifferentiated asteroids that have been thermally altered — the asteroid they came from was not large enough to completely melt, but impacts melted portions of the surface.

Another source of thermal energy that’s independent of size is tidal heating. Io is the most geologically active body in the solar system, and it is about the same size as the Earth’s Moon. It’s too small to have significant radioactive heating. Instead, the massive gravity of Jupiter exerts tremendous stresses on Io’s interior. The resulting friction heats the satellite from within.