Out of all the planets in the solar system, Earth is the only planet that scientists can study in detail. Atmospheric scientists can measure minute by minute atmospheric conditions (weather) from ground level to the “edge of space” by use of surface instruments and space vehicles. Geologists not only can detail surface features and how they change over time, but also can deduce Earth's structure to its very center. The division of Earth's interior into a core, mantle, and crust structure sets the context for how we study the other similar planets.
Only a small number of physical factors actually distinguish the various objects in the solar system. There are numerical quantities like the total mass, a measure of the size (for spherical objects we use the radius), density, gravitational acceleration, and escape velocity. Other, more general terms can be used to indicate the present of an atmosphere, the condition of the surface, and the nature of the interior. Earth and its satellite, the Moon, compare as in Table 1.
Topographically the Moon is very different from Earth. The Moon's surface is characterized by highlands and lowlands, mountains, and most notably, craters (bowl‐shaped cavities of meteoric origin). These craters are often marked by secondary craters and by rays from ejecta, or ejected matter from the meteor's impact. The Moon's dark regions, called maria, are lava‐filled basins up to 1,000 kilometers in diameter. Maria are sites of immense meteoric strikes early in lunar history that later were filled by molten lava seeping up from the interior. These maria are also the sites of gravity anomalies, or mascons, which are caused by the concentration of very dense material beneath the surface of the Moon. Mascons are found only on the near side of the Moon (the side of the Moon that faces Earth), suggesting that the influence of Earth's gravitation altered the trajectories of the impacting objects that produced these features.
Many of the lunar mountain ranges actually mark ancient crater rims. Unlike Earth, none of these features were formed by volcanism or by plate tectonic collisions. Rills and ridges that cross the lunar surface show evidence of surface contractions due to cooling of the rocky material of the lunar surface. The nature of the Moon's surface leads astronomers to the conclusion that it is basically original and was modified only by cratering and lava flows. By analyzing the Moon's physical features, therefore, we can deduce the early history of our solar system.
In contrast to the Moon, Earth's surface has an extremely varied topography. These differences can be attributed to two primary factors. First, as a larger object, Earth has cooled more slowly since it was formed. In fact, it is still cooling, with heat energy left over from the time of formation of Earth still slowly working its way outward. Energy always flows from hotter to cooler material; in Earth's interior the central heat in the core drives convection currents in the mantle that bring hot mantle material up toward the crust, and colder mantle and crustal rocks sink downward. At the Earth's surface this heat flow drives plate tectonics ( continental drift) ; large segments of the earth's crust (plates) separated along deep cracks called faults are forced into motion. When the plates collide, these powerful internal tectonic forces squeeze and fold solid rock, creating massive changes in Earth's crust (see Figure 1). Mountain uplift and associated volcanic activity where plates collide are only two aspects of the continual recycling and rebuilding of the crust.
Earth's changing surface. Earth's surface is in a constant state of change
due to factors such as convection currents, plate tectonics, and erosion.
The upwelling mantle material, driven by the flow of heat outward from the core of the planet, must spread out laterally beneath the crust, causing the continental plates to move apart. Because this movement occurs primarily in the denser surface rocks at the bottom of the oceans, it's termed sea‐floor spreading. The weakened crustal structure allows molten material to rise, creating new surface rocks and mid‐oceanic ridges, or mountain chains that can be traced for significant distances. The magnetic field patterns of oceanic sediments, symmetric on opposite sides of the mid‐oceanic ridges, and the relative youth and thinness of mid‐ocean sediments confirm continental drift. Researchers can also utilize radio astronomy techniques to directly measure motion showing, for example, that Europe and North America are drifting apart at a rate of several centimeters per year. The continents retain evidence of this drift, with shapes that resemble puzzle pieces that could be fitted together. The similarities between geological formations and fossil evidence show that indeed the present continents were once part of a single large land mass some millions of years ago.
Continental plates moving apart in one region means that elsewhere these plates must be colliding with other plates. Meanwhile, the denser ocean plates (heavier basalt) are moving under the lighter plates underlying the continental masses in subduction zones. These zones are marked by oceanic trenches, or mountain ranges caused by crumpling of continental materials to form mountain ranges, volcanism (for example, the Pacific ring of fire), and earthquake zones that obliquely dip below the continents.
Earth's surface is also constantly affected by the atmosphere (including wind and windblown sand and dust) and surface water (rain, rivers, oceans, and ice). Because of these factors, erosion of Earth's surface is an extremely rapid process. In contrast, the only erosive processes on the Moon are slow. There are the alternate heating and cooling of the surface during its month‐long day; expansion and shrinkage only very slowly alter the surface. There are also impacts and slow modification of surface rocks from the solar wind.
Temperature and energy
The overall average temperature of Earth and the Moon (as well as any other planet) is due to a balance between the energy that they receive from the Sun and the energy that they radiate away. The first factor, energy received, depends on the planet's distance from the Sun and its albedo (A), the fraction of light reaching the planet that is reflected away and not absorbed. The albedo is 0.0 if all the light is absorbed and 1.0 for a if all the light is reflected. The Moon has an albedo of 0.06 because its dusty surface absorbs most of the light hitting the surface, but Earth has an albedo of 0.37 because clouds and the ocean regions are reflective. A planet's temperature may also be influenced by the greenhouse effect, or the warming of a planet and its lower atmosphere caused by trapped solar radiation.
The energy a planet receives per second per unit area (solar flux) is L ⊙/4πR 2, where L ⊙ is the solar luminosity and R is the distance from the Sun (residual heat coming from the planet's interior, energy produced from radioactivity, and humanity's combustion of fossil fuels have no significant effect on Earth's surface temperature). The total energy a planet absorbs per second is the fraction that is not reflected and also depends on the cross‐sectional area of the planet, or L ⊙/4πR 2×(1‐A). At the same time, the Stefan‐Boltzman law ΣT 4 expresses the thermal energy emitted per second by each square meter of surface area. The total energy radiated per second is the Stefan‐Boltzman Law times surface area, or ΣT 4 × 4πR(planet) 2. In equilibrium, there is a balance between the two, which yields the following: L ⊙/4πR 2 = 4ΣT 4. For Earth, this yields an expected temperature of T = 250 K = –9°F (a number lower than Earth's actual temperature because of the greenhouse effect).
On a microscopic level, energy absorption and energy emission is more complicated. Any small volume in the atmosphere is affected not only by the local absorption of solar energy, but also by the absorption of radiation from all other surrounding regions, energy brought in by convection (air currents), and energy gained by conduction (at the surface, if the ground is hotter). The loss of energy is due not only to the thermal black‐body emission, but also by atomic and molecular radiation, energy taken away by convection, and energy removed by conduction (at the surface, if the air temperature is higher than the ground temperature). All these factors are responsible for the temperature structure of the atmosphere.