The Planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
THE INNER PLANETS
The innermost planet of the solar system, Mercury was named after the Roman God Mercury, messenger of the Gods. Mercury has the most elliptical orbit and a heavy core filled with iron. Although Mercury’s density is about the same as Earth’s, its day temperature (400°C) is high enough to melt zinc. Mercury’s surface is carved by craters, inter-crater plains, and the Caloris Basin. With some signs of tectonic disturbance, Mercury is filled with weird terrain. When the planet cooled, its lithosphere cracked and formed faults and scarps. Its core consists of iron 50% in volume and 70% in mass. With a weak magnetic field due to its small size, Mercury has a negligible atmosphere with amounts of hydrogen, helium, oxygen, argon, and sodium all from solar wind. Mercury has a period of rotation of 59 days and a period of revolution of 88 days. Due to lack of atmosphere, Mercury has a low albedo (more light received than reflected) and its sun intensity is 6.3x Earth’s.
Often called the sister planet of Earth due to similarities in size, Venus is named after the Roman Goddess Venus, goddess of love. With a beautiful coloration yet clouds covering its surface due to the greenhouse effect (carbon dioxide traps infrared radiation beneath the atmosphere), Venus is truly heaven and hell. Its clouds consist of sulfuric acid droplets; its atmosphere is 96% CO2; its surface pressure is 90 times that of Earth; its surface temperature is the highest in the solar system at 480°C. Unlike most planets, Venus spins in a retrograde motion, or clockwise. Like Earth in extreme conditions, Venus has many volcanoes, mountains taller than Earth’s, the Giant Canyon (largest in the solar system), and craters featuring coronae, or large and crown-like volcanic features (largest corona on Venus is Artemis Corona. Venus’ rotation period is 243 days and revolution period is 224.65 days.
Green with forests and fields, brown with mountains, and blue with oceans, Earth is the only planet in the solar system that supports life. With an iron and nickel core, an active mantle (asthenosphere), and the crust (lithosphere), Earth has a density of 5.5g/cc. The source of interior heat is natural radioactivity and geothermal energy. Because the lithosphere floats on a soft layer, the movement of Earth’s tectonic plates causes volcanoes, formation of mountain ranges, and earthquakes. Earth’s atmosphere contains 79% nitrogen, 20% oxygen, <0.1% water vapor, and 0.1% carbon dioxide. The layers of the atmosphere are troposphere (where living organisms live), stratosphere (weather, where airplanes fly), mesosphere, ionosphere, and thermosphere. Because Earth’s magnetic field cause Van Allen Belts (two doughnut shaped areas that tap particles from solar wind and Earth’s atmosphere), auroras appear at night in places near the poles. Earth’s rotation is 24 hours and revolution is 356.25 days.
Planet Earth is approximately 4.54 billion years old. But how did scientists determine this? With the radiometric dating of meteorites and the ages of the oldest known minerals. While the oldest meteorites found on Earth are approximately 4.5 billion years old, the oldest known mineral, zircon, discovered by Jack Hills in Australia is at least 4.4 billion years old. One meteorite used was the Canyon Diablo meteorite (4.55 billion years old) aged by C. C. Patterson. Since most of Earth’s minerals have undergone change in the core, mantle, and crust by plate tectonics, weathering, and hydrothermal circulation (circulation of hot water), scientists usually cannot use them in dating Earth. However, scientists used ancient Archaean lead ores of galena (natural mineral form of lead II sulfide), the earliest formed homogenous lead isotope, which very precisely dated Earth at 4.54 billion years. Furthermore, inclusions rich in calcium and aluminum in meteorites were formed within the solar system about 4.567 billion years age. As the oldest known solid component of meteorites, these Ca-Al inclusions determine the age of the solar system and set the upper limit of the age of Earth. Scientists do not known the time of Earth’s accretion (growth by gravitationally attracting more matter), but believe it started some after the Ca-Al inclusions formed.
In fact, scientists have long debated over and calculated the age of Earth. People had estimated Earth at just hundred of thousands of years! Later, scientists extended their estimates with more evidence. However, it wasn’t until Charles Darwin, who proposed the theory of natural evolution, that scientists began to make closer estimates. Using the molecular clock and the rate of genetic divergence, scientists estimates the last universal ancestor of all organisms at 3.5-3.8 billion years old.
Earth is the first terrestrial planet that has a natural satellite orbiting it. The moon is barren, dry, airless, with gravity one-sixth, size one-fourth, and mass one-eightieth that of Earth. Lunar features include maria (lava beds), craters formed by impact, mountain ranges, rilles, scarps, valleys, and highlands. The Terminator is the dividing line between the dark and bright side of the moon. The moon has an iron-rich core, silica-rich mantle, and crust thicker at the near side than at the far side. The moon revolves around Earth in 29.5 days with respect to the Sun, and its sidereal period (time required for a celestial body of the solar system to revolve once with respect to the fixed stars) is 27 1/3 days. Due to gravitational interlock, the synchronous rotation (rotational period and revolutionary period in a certain ratio) for the moon to Earth is one to one.
Named after the Roman God of war, Mars is the last terrestrial planet. Reddish-brown due to iron oxide, Mars is approximately half the size and 10% mass of Earth. Mars has a smaller core and thicker crust than Earth. Its density at 4 g/cc is less than those of Earth and Venus. Its axis of rotation at a 25 degree tilt to the ecliptic accounts for seasons. In the winter, polar ice caps appear. In the spring, the polar ice caps shrink and dark material gather and spread at more temperate zones, caused by massive dust storms. The four major regions on Mars’ surface are volcanoes (largest volcanic mountain = Olympus Mons), canyons (largest canyon = Valles Marinaris), craters, and terraced areas near poles. Unlike Earth, Mars has no continental drift and its surface pressure is only 1% of Earth’s. Mars’ thin atmosphere contains 90% carbon dioxide and 3% nitrogen. Although water most probably once flowed on Mars, present water only exists in polar ice caps containing mostly frozen carbon dioxide and permafrost. When carbon dioxide is in water, water freezes more easily as temperature drops. Mars’ two natural satellites are Phobos (fear) and Deimos (panic). Mars remains a mystery. One question remains unsolved: Did life once exist on Mars?
- Phobos and Deimos
Both Mars’ attendants in mythology, Phobos (fear) and Deimos (panic), unsurprisingly, are Mars’ natural satellites― irregular chunks of rock. Phobos will collide with Mars in 30 million years, as Mars’ gravity gradually pulls the satellite closer to its surface. Because both are gravitationally locked to Mars, the moons will always have the same sides facing Mars. Similar to asteroids, Phobos and Deimos have low albedos with carbon-rich rocks.
THE OUTER PLANETS
The largest planet (2/3 of all planets’ masses combined) and the first outer planet, Jupiter was named after the king of the Roman Gods, Jupiter. Jupiter is a gaseous planet and oblate (bulges 7% more at the equator than at poles). The most noticeable feature is the Great Red Spot, a storm the size of Earth. Throughout Jupiter’s surface are belts and zones, or bands of rising and falling gases. Jupiter’s atmosphere contains 85% hydrogen and 15% helium, in addition to complex cloud patterns and convection that causes great storms. Its deep hydrogen atmosphere is 10 times Earth’s diameter. Furthermore, Jupiter’s interior contains a rocky iron and silicon core 20 times Earth’s mass, an outer layer of highly compressed liquid metallic hydrogen generating magnetic field, a liquid molecular hydrogen outermost layer, and an internal heat source (fuels convection) that radiates four times more heat than received from the Sun. Jupiter’s magnetic field is the strongest in the solar system (10 times Earth’s). Emitting radio waves, Jupiter has three Van Allen Belts, a magnetic axis tilted 10 degrees from the rotational axis. Jupiter’s barely visible ring is composed of small rock fragments and dust particles which absorb light. In 1994 after colliding with Jupiter during a two-month period, the Shoemaker-Levy comet broke up into 21 pieces. Although Jupiter has over 62 moons, the most important four moons, also termed Galilean moons, are Io, Europa, Ganymede, and Callisto. The four Galilean moons all have rocky cores with water, ice, and carbonaceous material.
- Io, Europa, Ganymede, and Callisto
Io is Jupiter’s most active moon, with its young surface constantly transformed by intense volcanic activity. This volcanism is caused by gravitational forces from Jupiter and others moons squeezing and un-squeezing Io. Io has an orange coloration due to high sulfur content. The brightest of moons, Europa has a cracked ice surface that indicates heat and a possible body of water. Because of the presence of water and warmth, Europa shows possible signs of life. The largest satellite in the solar system at 1.5 times the size of the moon, Ganymede harbors a large amount of water ice around a rocky core, many craters and grooved terrain, and faults similar to the San Andreas Fault in California. Geologically inactive, Callisto has some craters; its most famous feature is the Bull’s Eye Valhalla.
Named for the king of the Titans, Saturn has an easily distinguishable ring system and at least 62 moons. Its features of convection are similar to Jupiter’s. Saturn radiates 2.5 times more heat than received and convection causes spots of storms, belts and zones, and high winds. Saturn also has a magnetic field and Van Allen belts. Saturn is oblate by 10% and its density is less than 1 (thus, Saturn would be able to float in water if there existed a large enough ocean). With its surface and interior to Jupiter’s, Saturn has 92.4% hydrogen and 7.4% helium, ammonium, and methane in its atmosphere and 20% rocky material and 80% liquid metallic hydrogen in its core. Of Saturn’s spectacular ring system, the Roche Limit is about 2.5 times the radius of the planet; inside the limit, matter cannot be held together but are torn apart by tidal forces forming rings rather than a moon. Cassini’s Division is the gap between the brightest B and A rings, made up of ice particles that scatter light. Saturn’s Shepherd Moons are the faster inner moons and slower outer moons keeping ring particles within the rings. Spokes are radial spikes elevated above the plane of rings.
The largest of Saturn’s moons, Titan is mostly nitrogen in its atmosphere and its surface pressure is 1.5 times Earth’s. Titan contains methane lakes and oceans and reddish smog. Mimas, Titan’s main feature, is a huge crater and giant canyon.
Named after the Greek god of the Sky, Uranus appears blue-green due to methane. Because of its thick methane clouds, Uranus has high albedo as the clouds are highly reflective. Since auroras are visible, Uranus has a magnetic field. It has Van Allen Belts and its magnetic axis is tipped 60 degrees offset from the center. Like Venus, Uranus is retrograde. Uranus’ axis of rotation is almost perpendicular to the other planetary axes and its poles point to the Sun. Like the other outer planets, Uranus has 11 narrow rings, composed of small amounts of fine dust particles, with no albedo and no color about 1.7 to 2.1 times the radius of Uranus. Like Saturn, Uranus also has Shepherd moons. Uranus has 27 known moons.
Named after the Roman god of the seas, Neptune appears bluish due to methane and has many clouds and storms, including the Great Dark Spot. Neptune radiates 2.7 times more heat with a strong internal heat source. Its magnetic axis is offset and it has two thin rings and 2 broad rings. Neptune has 13 moons; the two largest satellites are Triton and Nereid.
- Triton and Nereid
In mythology, Triton and Nereid are Neptune’s sons. In astronomy, they are Neptune’s largest satellites. Triton has the most varied surface in the solar system, with cantaloupe-like terrain and ice volcanoes. Triton orbits in retrograde motion around Neptune.