After conducting this Virtual Astronomy Laboratory, the learner will be able to …

We can learn about the interior of Earth by studying the transmission of seismic waves through Earth. These waves are most often produced by earthquakes, but can also be caused by impacts or explosions.

Seismologists identify two different types of relevant seismic waves:

Generate and observe each type of wave using the simulator below.

When an earthquake occurs, both P waves and S waves propagate through Earth. Both types of waves may undergo reflection and refraction at the boundaries of layers that have different densities. Both waves also refract (or bend) as they travel through Earth; this is due to the increase in density with depth.

In general, P waves move faster than S waves. Another difference between the two is that pressure (compression) waves can travel through either solid or liquid material, whereas shear waves cannot propagate through liquids.

Seismograms are collected from all over the world when earthquakes occur. They allow us to study whether or not both types of waves have been detected. They also reveal the relative arrival times of the two types.

From P and S wave measurements and other data, seismologists can identify four distinct regions of Earth's interior. From the center outward, they are:

The illustration below depicts convection currents in the outer (liquid) core. Like boiling molasses on a stove top, this layer of hot, dense material has rising currents that transport thermal energy outward.

This circulation of the liquid iron-nickel alloys constitutes an electric current. Because of this, the outer-core convection is thought to be responsible for Earth's magnetic field. When coupled with Earth's rapid spin, this process gives rise to an electromagnetic phenomenon known as the dynamo effectThe process by which a rotating and convecting mass of electrically conductive matter can generate a magnetic field..

You may wonder how it can be that Earth's hotter inner core is solid, while the relatively cooler outer core is liquid. Remember that the particular phase of matter (solid, liquid, or gas) depends on both temperature and pressure. The material in the inner core is under much higher pressures than that of the outer core and, thus, has a much higher melting point.

Although Earth's mantle is composed of solid material, it is in a "plastic" state that flows when under stress. When hot plumes of mantle material rise to the surface, they push against the crust and then descend, creating slow convection currents of their own. These currents shift pieces of crust about in a process known as plate tectonicsThe constant destruction and renewal of Earth's surface by the motion of sections of crust..

The animated global map below shows how Earth's landmasses have moved about since the early Jurassic Period. Note, toward the end, the relatively recent collision between the Indian subcontinent and Eurasia, giving rise to the world's highest mountains—the Himalayas. Note, too, the especially tight fit of Africa's western coastline and the east coast of South America.

Regions where two plates are separating from each other are called rift zones. Molten rock rises to fill the gap between the plates. It then solidifies, forming new crust. Rift zones are commonly found in the oceans and, thus, cause the oceans to grow.

Subduction zones are regions where one plate is being forced beneath another. The amount of crust created at rift zones roughly equals the amount destroyed at subduction zones. In the animated illustration above, subduction zones lie underneath coastal mountain ranges. Note that the convection currents dive down at those locations, carrying crustal material with them.

Plates move only a few centimeters per year, but interactions between them can cause dramatic changes to Earth's surface. Volcanoes and earthquakes are two of the phenomena that owe their origins to mantle convection and continental drift.

When a plate is subducted, it is forced downward into a region of higher temperature and pressure. Ultimately it melts a few hundred kilometers beneath the surface. Molten rock often breaks through the surface near subduction zones to form volcanoes and, eventually, new mountain ranges.

Taken from the International Space Station, the photo below shows ash and steam rising from Sarychev Volcano in Russia's Kuril Islands, northeast of Japan. This volcano lies near an active subduction zone.

Earthquakes occur near subduction zones and faults: locations where two plates slide past one another. Occasionally, frictional forces will cause the plates on either side of a fault to stick together and halt the motion. Pressure will build up over time until, suddenly, the plates rip free and resume their original motion. This sudden motion is what causes an earthquake.

Each of the terrestrial planets had a certain amount of heat energy at the time of its formation, and each has gained some additional energy because of the decay of radioactive elements. Both of these heat sources depend on the volume of material present—that is, on the size of the object, from center to edge.

The surface area from which the object can cool (radiate away this heat) also increases with size. However, as the size of an object increases, its volume grows more rapidly than its surface area. Consequently, small solar-system objects have cooled much more rapidly than larger ones.

Use the interactive calculator below to visualize the ratio of volume to area for a planet-sized sphere. Note that this ratio has dimensions of length (kilometers in this case). For every square kilometer of surface, a large spherical body will contain more cubic km of material—and each of those carries a fixed amount of thermal energy that ultimately will find its way into outer space.

Our Moon cooled very rapidly because of its small size. Measurements made by NASA's Lunar Prospector suggest that the Moon has a very small metallic core. This core is solid out to 440 km from the center and is partially molten at radii between 440 and 700 km. This notion of a smaller core is consistent with the fact that the Moon has no global magnetic field.

You could be excused for thinking that the photo above is of Earth's Moon: its rugged mountains and bright, round craters look eerily familiar. Yet this is an image of our solar system's innermost planet, Mercury, taken in 2009 by the MESSENGER spacecraft. Neither object has enough mass to have retained an atmosphere, which would slow down, deflect, or break up many of the small solid objects that carved out the craters. Because of this, Mercury and our Moon both provide a valuable record of our solar system's early history, when orbiting chunks of rubble were much more numerous than they are today.

Let's study the Moon first. There are two very distinct types of terrain on the Moon. The light-colored areas are known as highlands. They consist largely of low-density, light-colored rocks. These materials rose to the surface when all was molten rock, and then solidified. The image below shows a typical highlands landscape. Because they solidified before the time of the Late Heavy Bombardment—a temporary surge in the impact rate, about 4 billion years ago—the highlands are heavily cratered.

While highlands cover essentially all the Moon's far side (the side that never faces Earth), the near side is dominated by dark areas known as maria (Latin for "seas"). These are huge, flat plains of hardened lava. When this lava flowed onto the surface of the Moon three billion years ago, the low-lying basins, produced by very large impacts, were filled in. However, the lava never reached the highlands.

The distribution of the craters tells us a lot about the history of the Moon. Note how the amount of cratering in the highlands is much greater than that in the maria. The highlands preserve the record of an earlier time, when the solar system was filled with debris. By the time the maria formed, much of the debris from planet formation had been swept up by the planets or ejected from the solar system. As a result, the maria are only sparsely cratered.

In the view from Apollo 15's Lunar Module shown below, the Command and Service Module assembly appears in front of the Sea of Fertility (Mare Fecunditatus).

Mercury is very similar to the Moon in that it has both extensive cratering and maria. It also cooled rapidly, due to its small size (2,400 km, compared to the Moon's 1,700 km). Another consequence of these bodies' small sizes is the lack of an atmosphere: neither had enough gravity to hold onto one.

Mercury's rapid cooling gave rise to some very interesting geologic features. Known as lobate scarps, these are giant, curved cliffs, as high as 3 km and hundreds of kilometers long. They formed when Mercury's crust wrinkled because of rapid cooling, much like the drying out of a raisin. (The image below shows a scarp curving upward across the lower-right quadrant, casting a shadow where the planet's surface descends abruptly.)

Although Earth has the highest density (5.5 g/cm³) of any planet in the solar system, this was achieved over time, through gravitational compression. Earth's uncompressed density would be about 4.0 g/cm³. Mercury's density of 5.3 g/cm³ has been achieved with very little compression and likely implies a large iron core.

The Mariner 10 spacecraft detected a magnetic field when it flew by Mercury decades ago. Although 200 times weaker than the Earth's magnetic field, Mercury's is stronger than any of the other terrestrial planets' magnetic fields. This provides additional indirect evidence for an iron core.

Mars is intermediate in size between the Moon and Mercury (on the small side) and Earth and Venus (on the large side). One would therefore expect Mars to have been internally active for an intermediate length of time as well.

There is considerable evidence of past volcanic activity on Mars, but evidence of recent activity can be seen only in the Tharsis region, where there are several huge volcanoes. These are all shield volcanoes created by hot spots poking through the crust. (The Hawaiian island chain is a row of shield volcanoes created as the Pacific Plate slides across a hot spot beneath the Pacific Ocean.)

On Earth, crustal plates don't stop moving long enough to allow a truly gigantic shield volcano to grow. However, Mars has never had plate tectonics. In addition, the crust of Mars must also be considerably thicker than Earth's in order to support the weight of these large volcanoes.

The scarcity of cratering on the volcano Olympus Mons (shown above) indicates that it has been active within the last 200 million years. Although fresh volcanic activity has not been observed on Mars, there is no reason to assume that it won't occur again in the Tharsis region.

Most of what we know about Venus comes from the Magellan space probe, which created radar maps of almost the entire surface. Its maps resolve features as small as football fields. The image below shows the planet's surface, as mapped by Magellan's radar. (The color is not intended to show what the surface would look like to the human eye. Rather, it was chosen to highlight differences in texture.)

Very few impact craters are found on Venus. It appears that all crater evidence was erased about half a billion years ago, in an episode of volcanic resurfacing. Models of the interior suggest that Venus may undergo brief periods of intense volcanic activity every couple of hundred million years, with more calm periods in between. (Venus has more than 500 volcanoes with diameters greater than 20 km, and many smaller ones as well.)

Venus shows no evidence of a crust broken into plates. In fact, the crust motion appears to be largely vertical. Most of the planet's geological features have been created by rising plumes of magma that distort the crust.