Planetary Science

C

Rock Samples

Planetary geologists analyze rock samples from Earth and other worlds to determine the chemical compositions of planets in the solar system, which gives important clues regarding the origins and evolution of planetary bodies. Astronauts from the Apollo Moon missions brought back rock samples from six different sites on the moon, and robot landing craft sent to the Moon by the former Soviet Union brought back soil samples from three other sites. Geologists have also collected thousands of meteorites, which are fragments of interplanetary debris that have landed on Earth. Since the lunar and Martian landing missions of the 1970s, planetary scientists have determined by chemical analysis that about a dozen of the known meteorites originated on the Moon and about a dozen more came from Mars. These planetary fragments appear to have been blasted from the surfaces of these worlds by the impact of large asteroids that originated in the asteroid belt between Mars and Jupiter. The remaining meteorites appear to be asteroid fragments that came directly to the Earth from the asteroid belt after being shattered and knocked out of orbit by collision with other asteroids.

III

Origins and Compositions of Planets

Astronomers believe that planetary systems are formed of elemental materials that were created in the interiors of giant stars. Some of this material comes from giant stars that shed material into space as they age. Most of the matter to form planets, however, comes from stars that explode as supernovas and spread debris enriched with the heavier chemical elements into space. According to the currently accepted views, the most likely first stage in the evolution of a planetary system is a later supernova near the clouds of interstellar dust and gas. A shock wave from the supernova explosion may compress a nearby cloud to a sufficiently high density so that the weak attractive force of gravitation is made strong enough to cause the cloud to collapse in on itself. The gravitational attraction of particles for each other and collisions between the particles of the cloud cause the cloud to form a large central body known as a protostar, encircled by a thin disk of dust, gas, and debris known as a planetary disk. In the case of Earth’s solar system, the protostar eventually became the Sun and the planetary disk broke up into the planets of the solar system. By studying our own planetary system, planetary scientists gain insight into the general mechanisms that determine the structure of planetary systems.

A

Formation of Planets

As an interstellar cloud begins to contract into a star, any random swirling motion in the cloud becomes more orderly and translates into a general rotation of the entire cloud. As the cloud continues to contract, its speed of rotation increases, just as figure skaters spin faster as they pull in their arms. The physical principle for this is known as the conservation of angular momentum, and it means that the total angular momentum of the cloud must remain constant. Because angular momentum depends on the distance of the mass from the center of rotation and the speed of rotation, as the distance decreases, the speed must increase to compensate and keep the momentum constant. In an interstellar cloud, this means that as distant parts of the cloud move closer to the center of rotation, the speed of the cloud’s rotation must increase.

A nonrotating cloud of interstellar gas and dust would contract into a sphere at the center of mass of the cloud, but the vast majority of objects in space rotate. Frictional drag within the cloud and other dynamic interactions cause the outer parts of the rotating cloud to flatten into a disk that surrounds the central spherical body. Planetary systems, such as our own solar system, form from material in these so-called planetary disks. Observations suggest that planetary disks surround as many as 60 percent of the new stars in young star clusters.

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A planetary disk heats up as it forms. Once a star forms in the center, the rest of the disk cools by radiation. As it cools, solid mineral grains and ice crystals condense, much as snowflakes condense in cooling air. As the grains collide, they stick together to form larger grains that sweep up other grains ever more quickly, a process called accretion. The disk around a newly forming star quickly becomes a sort of factory in which dust grains and ice crystals aggregate and grow into asteroid-sized bodies called planetesimals (small planets). The planetesimals gather more material through gravitational attraction and collision until eventually only a few planet-sized bodies are left.

In Earth’s solar system, the planet-forming process apparently happened relatively quickly. The planets reached their present sizes and arrangement probably within 10 million to 50 million years after the Sun’s ignition. In this view, the giant planets formed when their cores reached 10 to 15 times the mass of Earth, sufficient to attract hydrogen-rich gas from the solar nebula. In an alternative view, another more direct process may form gas giant planets such as Jupiter. A region of dust and gas becomes gravitationally unstable and quickly collapses into a large body that retains much of the gas that might otherwise be blown off by solar radiation.

B

Compositions of the Solar System’s Planets

The compositions of the planets of Earth’s solar system follow directly from the materials that condensed at different distances from the protostar that became the Sun. Near the Sun, condensed mineral grains were made of rocky material, and they formed four rocky planets: Mercury, Venus, Earth, and Mars. These planets are collectively known as the terrestrial planets. The name is derived from terra, the Latin name for Earth, and it refers to the inner planets’ similarity to Earth.

Between Mars and the next most distant planet, Jupiter, is a belt of rocky and carbon-rich planetesimals that never coalesced into a planet. This is called the asteroid belt, and the bodies composing it are known as asteroids. Gravitational disturbances caused by the massive, nearby planet Jupiter probably kept the asteroids from forming a planet. The asteroid belt vividly shows the transition in composition from the terrestrial planets to the outer, more carbon-rich planets. The asteroids nearest Mars, closest to the Sun, are composed primarily of the rocks, minerals, and metals of the terrestrial planets, but asteroids beyond the middle of the belt, closer to Jupiter, are colored black by sooty, carbon-containing material. All interplanetary bodies beyond this point show this dark coloration.

Beyond the asteroid belt, icy grains were added to rocky and carbon-rich materials. Out of this material formed the four major gaseous planets—Jupiter, Saturn, Neptune, and Uranus— collectively known as the Jovian planets because of their similarity to the planet Jupiter. The Jovian planets, also called gas giants, are all huge gaseous spheres of hydrogen and helium that surround relatively small cores of metallic and rocky material. The atmosphere of Jupiter is almost three-quarters hydrogen and one-quarter helium by weight, with traces of carbon dioxide (CO₂) and the more common hydrogen-rich compounds—for example water (H₂O), methane (CH₄), and ammonia (NH₃). This is very similar to the composition of the sun. To planetary scientists, this indicates that the outer disk was cold enough that the ices of carbon dioxide, methane, and ammonia could form. These ice compounds, which are far more common in the solar system and in the universe than the silicates and metals of the terrestrial planets, condensed into crystals that stuck together and rapidly formed very large bodies. When Jupiter and Saturn reached masses of about 15 Earth masses, their gravitational fields simply swept up the remaining dust and gases still floating free in the solar system, including the remaining hydrogen and helium. Uranus and Neptune are sometimes classified as ice giants, apart from the “gas giants” Jupiter and Saturn. Unlike Jupiter and Saturn, the two outermost planets are mainly made of water in a hot, compressed, slushy state that scientists refer to as “ice.”

The differences in composition of the planets show up directly in their mean densities, which can be determined by studying the motions of their own satellites and by spacecraft sent to them from Earth. The inner planets have densities characteristic of metal-bearing rock (about 3 to 5 g/cu cm), while the Jovian planets and their satellites have lower densities, characteristic of ices or ice/soil mixtures (about 1 to 3 g/cu cm). Saturn’s mean density is lower than water. If there were a large enough pool of water to place it in, Saturn would float.

Beyond the orbit of Neptune is the dwarf planet Pluto, once counted as the ninth planet in the solar system. Pluto almost certainly formed in the region of icy bodies called the Kuiper Belt, named for the astronomer who predicted its existence. Not much is known about Pluto because it is smaller than our Moon and far from Earth. Measurements of the motion of one of its moons, Charon, indicate that Pluto’s density is higher than the density of the Jovian planets and suggests that it is composed primarily of rock and a mixture of ices. Pluto also orbits the Sun in a plane that is about 17° off from the plane in which all the other planets orbit. Other icy bodies that appear similar to Pluto have been discovered in its vicinity. These objects are sometimes called Kuiper Belt Objects (KBOs). Many astronomers now classify Pluto as merely one of the largest members of the Kuiper Belt. If astronomers had known about KBOs, Pluto almost certainly would not have been called a planet after it was discovered in the 1930s. The International Astronomical Union reclassified Pluto as a dwarf planet in 2006 on the grounds that it had not cleared the neighborhood of its orbit of other bodies. Slightly larger and about 27 percent more massive than Pluto is the dwarf planet Eris, a KBO that has a more distant and steeply inclined orbit than Pluto.

IV

Structures and Features of the Terrestrial Planets

The terrestrial planets and the larger satellites are in a constant state of change and evolution. Worlds that have atmospheres show evidence of wind erosion and wind-driven transport of material, and still other worlds exhibit volcanism and other signs of motion and activity beneath their surfaces. Motion of material deep within a planet often creates a strong magnetic field. Even the geologically inactive worlds occasionally experience collisions with interplanetary debris that leave large impact craters as evidence.

It is impossible to “see” the interior of a planet, so planetary scientists must use indirect means of determining the processes that are at work on a planet. The presence or absence of craters on a planet’s surface is one of the most important clues available to planetary scientists. As a rule, surfaces showing sparse numbers of craters are half a billion years old or more, those with a moderate concentration of craters are a billion years old or older, and surfaces crowded with craters can be nearly as old as the solar system itself. Surfaces devoid of craters only exist on worlds that have active volcanoes, geologically active atmospheres, or other internal mechanisms for renewing the surface at intervals of a half billion years or less.

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