Earth reached nearly its present size with the impact of a mars-sized body 4.5 billion years ago that was so catastrophic as to give birth to the moon. This impact probably was also responsible for the 23 degree tilt of Earth's rotation axis with respect to the orbital plane that gives rise to the seasons of the year. Evidence for this has been provided by a statistical analysis of the heavily cratered surface of the moon and dating of geologic specimens returned from the moon. Earth's two outer layers, the atmosphere and hydrosphere, thus were born from the waning stages of the rain of comets and meteorites, which continued to fall long after Earth had reached 99 percent of its present size. Only a small portion of Earth's present atmosphere and hydrosphere are thought to have been derived from volcanic outgassing from the interior.
S waves can only propagate through solids.
An S wave can be induced in a solid object such as a steel bar by striking a glancing blow against one end. The reflections, refractions and abrupt velocity changes with depth of these seismic waves provide the basic scientific information about Earth's inner anatomy. In recent years, three dimensional images of the variations in seismic-wave velocities have been obtained using acoustic tomography, a technique similar in principle to the CAT scans used by physicians to image the body's interior. Deep Earth conditions can now be simulated in the laboratory using a diamond-anvil pressure cell. The heart of this apparatus consists of two large gem-quality diamonds with flat tips.