The structures of the terrestrial planets are similar. Each planet has a layered structure that developed through a process called differentiation. After the small bodies (called planetesimals) clumped together and melted, the gravity of the newborn planets pulled heavier elements to their centers, while lighter elements floated toward the surface.
Although we see only the top of the crust, by studying mountain ranges, volcanoes, basins, canyons, and fault zones, scientists can infer the structure of the rest of the crust, mantle, and even the core of a planet. And by measuring the motions of spacecraft near the planets, scientists have determined their densities and, in some cases, the thicknesses of their outer layers.
Detection of magnetic fields by spacecraft has made it possible to infer the deeper structures of planets, too. The cores of the terrestrial planets are all about the same size in relation to their total diameters, but the mantle of Mercury is proportionally smaller than the mantles of the other three planets.
Volcanic activity was part of the early geological history of all the terrestrial planets plus the Moon. The Moon and Mercury have been geologically "dead" for at least a billion years. Spacecraft discovered indirect evidence of recent volcanic activity on Venus, but have not observed any active volcanic eruptions. Giant volcanoes on Mars may have erupted in the last few million years, and a few may be dormant.
The atmospheres of the terrestrial planets were created by three processes: outgassing (carbon dioxide and water vapor released by volcanoes), evaporation from ices at the surface, and bombardment by icy comets. The outer layers of each planet contained the water vapor, carbon dioxide, and other gases that have become the present-day atmospheres of the terrestrial planets. Since these planets are not very massive, their gravity could not hold onto most of the lighter gases, such as hydrogen and helium, or even to the heavier gases in their original warm atmospheres. Molecules of oxygen, nitrogen, carbon dioxide, and water vapor are heavy enough to be retained in their present atmospheres, which are much cooler.
Because the mass and distance from the Sun is different for each world, their atmospheres have evolved quite differently during the solar system's history. The Moon and Mercury have only thin wisps of atmosphere. The Martian atmosphere is less than one percent as thick as Earth's, while Venus' atmosphere is 90 times thicker than Earth's.
Weather and climatic processes have significantly changed the geological record we see on the planets. Wind and water erosion on Earth have wiped out most of our planet's impact craters, for example. On Mars, we see evidence of ancient rivers and perhaps even a global ocean, but no liquid water.
The different factors that have shaped each of the terrestrial planets not only make each world unique, they help scientists learn more about Earth by studying how its evolution has varied from that of the other planets.
The Soviet Union's Luna 3 solved an ancient mystery in 1959 when it snapped the first pictures of the far side of the Moon (left). The images showed a hemisphere with plenty of craters and jumbled mountains, but few of the dark volcanic plains that are visible on the side that faces Earth. Later, Luna 9 made the first soft landing on the Moon, while Luna 10 became the first craft to enter lunar orbit.
In 1962, the American Mariner 2 staged the first successful encounter with another planet when it flew past Venus. Its instruments recorded Venus' surface temperature and pressure, and the composition of its atmosphere and the clouds that envelope the planet. The next successful mission in the series, Mariner 4, snapped the first close-range pictures of Mars in 1965. The images revealed a Moon-like landscape.
Surveyor 1 snapped more than 10,000 pictures after it became the first American probe to land on the Moon, in 1966. Apollo 11 astronauts Neil Armstrong and Edwin Aldrin made the first manned landing on July 16, 1969, and five more landings followed. Samples of rock and soil helped scientists learn about the Moon's birth and history. Apollo 12 astronaut Pete Conrad poses beside Surveyor 3, which had landed two years earlier.
Only one spacecraft has visited Mercury. Mariner 10 flew by the planet three times in 1974 and 1975, and mapped about half of its battered, Moon-like surface, which is dominated by impact craters. Mariner found that Mercury's interior consists of a large iron-rich core, which generates a weak magnetic field around the planet.
The Soviet Venera 9 became the first spacecraft to transmit pictures from the surface of another planet when it touched down on Venus in 1975. And in a mission that began 15 years later, the American Magellan used cloud-penetrating radar to map more than 98 percent of Venus' surface. Its images revealed hundreds of volcanoes, giant lava flows, cracked domes of volcanic rock, and long canyons.
In 1976, NASA's Viking 1 and 2 landers became the first spacecraft to touch down safely on Mars. From opposite sides of Mars, they photographed and analyzed their environments (left), finding a surface of iron-rich clay and a frigid atmosphere battered by giant dust storms. Experiments designed to look for signs of life produced inconclusive results, which scientists interpreted as meaning the Martian surface is sterile.
Building on the success of the 1997 Sojourner rover, the Spirit and Opportunity rovers landed on Mars in 2003. Although their planned mission was just 90 Mars days long (about 92 Earth days), both rovers were still operating in 2007. Both found evidence that water had helped shape the Martian landscape in the distant past. Several American and European craft found similar evidence from Mars orbit.