HiSET Science: The Solar System

Terrestrial Planets
The term terrestrial planets refers to the four planets closest to the Sun (Mercury, Venus, Earth, and Mars). They are classified together because they share many similarities that distinguish them from the giant planets. The terrestrial planets have high densities and atmospheres that constitute a small percentage of their total masses. These atmospheres consist mostly of heavy elements, such as carbon dioxide, nitrogen, and water, and are maintained by the gravitational field of the planets (which could not prevent hydrogen from escaping). These planets exhibit magnetic fields of varying intensity. An important characteristic that distinguishes the terrestrial planets from the giant planets is the evidence of various levels of internally generated activity, which caused these planets to evolve from their original states. These processes are thought to have been caused by constant meteoritic impacts during the first few hundred million years of the planets' existence. Radioactive decay of certain isotopes increased the internal temperatures of these planets, leading to volcanic activity on all of the terrestrial planets except Venus.

Mercury
Mercury, the smallest interior planet, is the least well known of the four. This is due to its close proximity to the Sun and high temperatures. Mercury's atmosphere is not very dense; this means that the planet's surface experiences wide temperature differentials from day to night.
Mercury's density is close to that of Earth. As the smallest planet known to have experienced planetary evolution, Mercury's internal activity ceased (it became extinct) thousands of millions of years ago. The size of the planet is relevant because less massive bodies cool more quickly than larger ones after cessation of radioactivity. Mercury's surface is characterized by craters produced by meteoritic impact.

Venus
Venus is comparable to Earth in both mass and density. Venus is the brightest planet in the sky (partially due to the fact that it is proximate to the Sun), which makes exploration of its surface difficult. This planet's atmosphere consists mainly of carbon dioxide, with trace amounts of water and carbon oxide molecules, as well as high levels of sulfuric, nitric, and hydrofluoric acids in the clouds that characterize this atmosphere. The concentration of clouds, coupled with the chemical makeup of Venus's atmosphere, result in a strong greenhouse effect at the planet's surface. This surface consists of large plains (thought to be created by either volcanic activity, which remains unproven, or by meteoritic impacts) and large impact craters. The materials that compose Venus's surface are highly radioactive. Some astronomers have suggested past single-plate tectonic activity; again, however, the planet's dense atmosphere makes valid surface observation quite difficult.

Mars
Mars and Earth exhibit many similarities. For example, Mars has an internal structure that includes a central metallic core, a mantle rich in olivine and iron oxide, and a crust of hydrated silicates.
Martian soil consists largely of basalts and clay silicate, with elements of sulfur, silicon oxide, and iron oxide. The planet's surface belies high levels of past volcanic activity (though, due to its relatively small mass, it is probably extinct). In fact, Mars is home to the largest known volcano in the solar system. The Martian landscape also includes two major basins, ridges and plateaus, and, most notably, apparent evidence of fluvial (water-based) erosion landforms, such as canyons and canals. It is possible that the past pressures and temperatures on Mars allowed water to exist on the red planet. Some have gone so far as to suggest that this planet was a site of biochemical evolution.
So far, however, no evidence of life has been found.

Mars's Satellites
Two Martian satellites have been observed: Phobos and Deimos. Each of these bodies is ellipsoidal; the circular orbits of the two satellites lie in Mars's equatorial plane. The gravitational forces between this planet and Phobos and Deimos have caused both satellites to settle into synchronous rotation (the same parts of their surfaces are always facing Mars). This feature exerts a braking force on
Phobos's orbit. In other words, its orbit is decreasing in size. The relationship between Deimos and Mars is similar to the Earth-Moon system, in which the radius of the satellite's orbit is gradually growing. The differential compositions and densities of Mars and its satellites indicate that Phobos and Deimos probably did not break off from Mars.

Giant Planets
The large diameters of Jupiter, Saturn, Uranus, and Neptune gave rise to the name of the category into which they fall. The hypothetical icy cores of these planets cause them to exhibit primary atmospheres, because the large levels of mass they accreted prevented even the lightest elements from escaping their gravitational pulls. The atmospheres of the giant planets thus consist mostly of hydrogen and helium. The giant planets do not have solid surfaces like those of the terrestrial planets. Jupiter probably consists of a core (made of ice and rock) surrounded by a layer of metallic hydrogen, which is covered by a convective atmosphere of hydrogen and helium.
Saturn is believed to have the same type of core and hydrogen mantle, enriched by the helium missing from the atmosphere, surrounded by a differentiation zone and a hydrogenic atmosphere. Uranus and Neptune probably have the same type of core, surrounded by ionic materials, bounded by methane-rich molecular envelopes.
Uranus is the only giant planet that exhibits no evidence of internal activity.

Rings
Each of the four giant planets exhibits rings. These are flat disks of fragmented material that orbit just next to their respective planets. Many of the giant planets' smaller satellites are embedded in these rings. There are two main hypotheses regarding the formation of such rings. One theory suggests that the tidal force exerted on a satellite by its planet may surpass the Roche limit (the point at which particle cohesion is no longer possible) and break the satellite into fragments, which then collide and become smaller. This material then spreads out and forms a ring. An alternate theory of the formation of the rings of the giant planets suggests that there was unaccreted material left over after the formation of these planets. Below the Roche limit (within a certain vicinity to the planet), these particles could not join together to form satellites and would consequently settle into orbital rings.

Satellites
Each of the giant planets possesses a number of satellites.
Jupiter has over 50 known satellites—they are grouped according to size.
Each of the four largest satellites of Jupiter exhibits evidence of internal activity at some point in their evolutions. In fact, Io, the densest satellite and the one closest to Jupiter, is the only celestial body besides Earth known to be currently volcanically active. Saturn has 21 satellites. Titan, the second-largest known satellite, has its own atmosphere. The other six largest of Saturn's satellites all have icy surfaces; some of these show evidence of past internal activity. The smaller 14 are relatively unknown. Uranus has five satellites. Each of them displays evidence of geological activity, in the form of valleys, smoothed surfaces, cliffs, mountains, and depressions. Neptune has eight known satellites. The larger, Triton, is similar to Titan in that it has an atmosphere. The other seven satellites of Neptune are relatively unknown.

Pluto and Charon
Though Charon was originally considered a satellite of Pluto, the ninth planet in the solar system, it now appears that the two are more accurately described as a double-planet system (largely because of the similarity in the sizes of the two). It is believed that these bodies formed from the solar nebula like most other objects in the solar system. Pluto has a highly irregular orbit, which places it closer to the Sun than Neptune for periods of time. In sharp contrast to its giant neighbors, this planet's density is higher than that of water ice. The surface of Pluto consists of high levels of methane absorbed into ice, with trace amounts of carbon oxide and nitrogen. Charon resembles the major Uranian satellites more so than it does Pluto. It consists of water ice with a siliceous or hydrocarbonate contaminant.

Kepler's Laws
Kepler's laws are a collection of observations about the motion of planets in the solar system. Formulated by Johannes Kepler in the 1600s, these laws are still vital to our understanding of the way the universe works. Kepler's first law states that each planet moves in its own elliptical path and that all of these orbits have the Sun as their singular focal point. Before Kepler's discovery, astronomers had assumed that planetary orbits were circular (because the heavens were assumed to be geometrically perfect). Kepler's second law says that a straight line between a planet and the Sun sweeps out equal areas in equal time. In other words, planets move quickest in the part of their orbit that is closest to the Sun, and vice versa.
Kepler's third law states that the further a planet is from the Sun, the longer its orbital period will be. In mathematical terms, the square of a planet's period is inversely proportional to the cube of the radius of its orbit.