The 8 Planets – Part 8: Neptune

LAST STOP: NEPTUNE!

Neptune

Neptune

The last planet Neptune, is quite a spectacle, essentially a blue marble. As with Uranus, methane (trace amount) gives Neptune its blue coloration. Named after the Roman god of the seas Neptune was noted by Galileo in 1612, but discovered as a planet by Urbain Le Vernier, John Couch Adams and Johann Galle on September 23, 1846. Neptune has a very elliptical orbit, and was further than Pluto between 1979 and 1999. Uranus and Neptune are usually paired together as “ice giants.” Uranus is light blue, named after the god of the sky, while Neptune is dark blue, named after the god of the seas. Unlike Uranus’s bland surfaces, Neptune’s ephemeral storms make up the planet’s active atmosphere. The Great Dark Spot is comparable to Jupiter’s Great Red Spot, but the Great Dark Spot comes and goes. With the strongest gales in the solar system, winds (rotates opposite of the planet’s rotation direction) on Neptune have speeds up to 2,100 kph— almost reaching supersonic flow! Winds called the scooter that speed across Neptune reach up to 3000 kph! Although Neptune’s atmosphere is one of the coldest places of the solar system, Neptune has a faint, fragmented ring system called arcs discovered during the 1960s and confirmed during the 1989. The rings give off a faint red hue, comprising mainly of ice and carbon-based materials. Like that of Uranus, Neptune’s magnetosphere is also relatively tilted (47º). The pressure on Neptune is so great that it rains diamonds there! On Neptune, that pole facing the Sun is 10ºC hotter than the other pole, so when the seasons change, frozen methane warm up and leak out into space.

MOONS

Triton and Nereid

Triton and Nereid

Neptune has 13 known moons, the largest are Triton and Nereid. In mythology, Triton and Nereid are Neptune’s sons. Interestingly, Triton has a retrograde orbit (spins east to west), which suggests the Neptune gravitationally pulled Triton into its orbit. In fact, in 3.6 million years, Neptune will pull Triton past the Roche Limit (past this limit, all moons are doomed to crash into the planet), and the moon will crash into Neptune! Neptune’s second largest natural satellite, Nereid, an irregular moon, has one of the most eccentric (elliptical) orbits in the solar system.

MISSIONS: Voyager 2

OVERVIEW

  • Order in Solar System: #8
  • Number of Moons: 13
  • Orbital Period: 164.8 years
  • Rotational Period: 16.11 hours
  • Mass: 1.0243 x 10^26 kg ( 17.147 Earths)
  • Volume: 6.254 x 10 ^13 km³ (57.74 Earths)
  • Radius: 24,764 km (3.883 Earths)
  • Surface Area: 7.6183 x 10^9 km² (14.98 Earths)
  • Density: 1.638 g/cm³
  • Eccentricity of Orbit: 0.0112
  • Surface Temperature (Average): 72 K
  • Escape Velocity: 23.5 km/s
  • Apparent Magnitude: 8.02 to 7.78
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The 8 Planets – Part 7: Uranus

NEXT STOP: URANUS!

Uranus

Uranus

Unlike any other planet in the solar, Uranus (Ur-uh-nus)’s name derives from Greek mythology, namely the Greek god of the sky. Uranus preceded Jupiter and Saturn in mythology as he and Gaia created the sky and earth. Named planets long after the ancient planets (Mercury, Venus, Mars, Jupiter, and Saturn), Uranus (Sir William Herschel, 1781) and Neptune are sometimes in a separate category called the “ice giants.” The two planets’ icy blue coloration comes from a primary composition of more heavier elements, “ices” such as water, ammonia, and methane. Like Venus, Uranus spins in a retrograde motion with a tilt of 97.77°! So, while other planets spin like spinning tops, Uranus spins like a rolling ball. A large object may have knocked Uranus on its side! Uranus’ rings spin parallel to its axis of rotation. Because of its unusual axial tilt, Uranus has unusually long seasons— each pole gets 42 years of sunlight followed by 42 years of darkness. Near the time of equinoxes, however, Uranus’ day-night cycle reaches that of those on other planets. Even Uranus’ magnetic field, with a tilt of 59º, is abnormal and does not line up to Uranus’ axis, with the north side strong and the south side comparatively weak. The second least dense planet, Uranus comprises of a rocky core, icy mantle, and an outer hydrogen and helium envelope. Because Uranus’ atmosphere is mainly methane, the planet is very smelly, like cow pastures. Uranus’ faint rings were mainly formed from scattered moons. Unlike the other gas giants, Uranus radiates hardly any heat; the planet’s core may have been depleted in an high-mass impact. Though Uranus is bland, dark spots like those usually found on Neptune, have recently been found on Uranus.

MOONS

Uranus' moons

Uranus’ moons

Uranus has 27 known moons named after characters from Shakespeare’s and Alexander Pope’s masterpieces. Uranus’ five main moons are Miranda, Ariel, Umbriel, Titania, and Oberon. These moons are comparatively and dull (brightness), comprising of 50% rock and 50% ice. Of the satellites, Ariel is the youngest with few impact craters and Umbriel is the oldest. Miranda has canyons, layers, and many variations in surface features caused by tidal heating (push and pull of the moon’s interior caused by gravitational pull) within the moon.

MISSIONS: Voyager 2

OVERVIEW

  • Order in Solar System: #7
  • Number of Moons: 27
  • Orbital Period: 84 years
  • Rotational Period: 17 hours
  • Mass: 8.6810 x 10^25 kg (14.536 Earths)
  • Volume: 6.833 x 10 ^13 km³ (63.086 Earths)
  • Radius: 25,559 km (4.007 Earths)
  • Surface Area: 8.1556 x 10^9 km² (15.91 Earths)
  • Density: 1.27 g/cm³
  • Eccentricity of Orbit: 0.044405586
  • Surface Temperature (Average): 76 K
  • Escape Velocity: 21.3 km/s
  • Apparent Magnitude: 5.9 to 5.32

The 8 Planets – Part 6: Saturn

NEXT STOP: SATURN!

Saturn

Poor Saturn is neither the largest nor the most massive. But this planet may be most eccentric— memorable in its appearance and properties. Named after the Titan of Time, Saturn was the Roman king of the Titans and father of Jupiter. Saturn is the least dense planet, even less dense than water! How does this happen? Saturn is only 1/8 the density of Earth, but with its large volume, is over 95 times more massive than Earth. Comprising mainly of the lightest element, hydrogen, Saturn is very “light” for its size. Saturn’s mass is 95 times that of Earth, but its volume is 764 times that of Earth. Since density = mass/ volume, Saturn’s large volume and relatively small mass equates to a very small density (0.687). So, if you build an enormous bathtub and fill it with H2O, Saturn would bobble around on the surface like a rubber duckie! In contrast to Jupiter’s myriad of colorful bands and zones, Saturn’s upper atmosphere of mainly ammonia crystals gives the planet a bland yellow-brown coloration. Once every 30 years, Saturn exhibits ephemeral storms on its banded surface, one known as the Great White Spot. At its North Pole, Saturn has a weird hexagon-shaped storm that may be a novel aurora or a wave pattern. Underneath that banal surface, winds reach up to 1,100 mph, faster than those on Jupiter! Unlike its ever-changing gaseous layers, Saturn’s core may be solid iron, nickel, and rock. Reaching up to 11,700 °C at the core, Saturn radiates 2.5 times more energy than received from the Sun by the Kelvin-Helmholtz mechanism of slow gravitational compression and the “raining out” of droplets of helium in its interior. Accumulating into a helium shell surrounding the core, the helium droplets release heat by friction passing though low density hydrogen. Layers of metallic hydrogen (deep), liquid hydrogen and liquid helium (intermediate), and hydrogen gas (outer) blanket the core. Electrical currents within the metallic hydrogen caused Saturn’s weak magnetic field to form. Effective at deflecting solar wind particles, Saturn’s magnetosphere also produces aurorae. Saturn has magnificent, highly reflective ice rings, perfectly visible with a telescope. All gas giants have rings, but with nine main continuous rings, three discontinuous arcs if ice particles, rock debris, and dust, Saturn and its rings are truly inseparable. In 1655, Christiaan Huygens suggested Saturn was surrounded by a ring. Since then, astronomers have named the main rings from A to G. The Cassini Division is a large gap between rings A and B, and the Roche Division is a gap between rings A and F. Some moons, like Pan and Prometheus, are shepherd moons that prevent Saturn’s rings from expanding.

MOONS

Saturn has the second most number of moons with 62. Inhabit Saturn’s rings, Saturn’s moons range from the hundreds of “moonlets” to its largest natural satellite Titan. Of its 62 known moons, Saturn has 53 with actual names, 13 with diameters larger than 50 km, 7 with hydrostatic equilibrium due to planetary mass, dense rings, and complex orbits of their own, 24 regular satellites (prograde orbits not greatly inclined) named after Titans and Titanesses, and 38 irregular satellites with farther orbits and high inclination orbits and named after Inuit, Norse, and Gallic mythological characters. There can be no objective boundary for the classification of Saturn’s moons, for Saturn’s rings contain objects from the microscopic to the largest object Titan.

TITAN

Titan

The most prominent is Titan. Larger than Mercury, Titan is the only moon to retain a substantial atmosphere. Titan produces white convective clouds in cold nitrogen and methane atmosphere. Its surface is relatively young with few impact craters, dark regions of frozen hydrocarbons, flow channels, volcanoes, and sand of frozen water or hydrocarbons. The only moon with large bodies of methane/ ethane lakes, Titan, like Ganymede and Europa (Jupiter’s moons) may have a subsurface ocean of water and ammonia. The largest lake on Titan, Kraken Mare, is larger than the Caspian Sea.

BRIEF TAKES ON OTHER PROMINENT MOONS

Saturn’s moons

MIMAS: smallest and least massive of inner round moons, large impact crater called Herschel, no known geologic activity

ENCELADUS: one of the smallest of Saturn’s spherical moons, smallest known body geologically active, diverse surface that includes ancient heavily cratered terrain and younger smoother areas, south pole unusually warm and emits jets of water vapor and dust that replenishes material in Saturn’s E Ring and is the main source of ions in Saturn’s magnetosphere, may have liquid water under south pole, pure ice and high reflective surface

TETHYS: third largest inner moon, large impact crater called Odysseus, cast canyon system called Ithaca Chasma, composed of mainly water ice with little rock

DIONE: second largest inner moon, heavily cratered old terrain, extensive system of troughs and lineaments named “wispy terrain” indicates tectonic activity

RHEA: second largest moon, only moon that has rings, two large impact basins called Tirawa and Inktomi (“The Splat”), a young crater which has butterfly-shaped bright rays, geologically dead

HYPERION: closest moon to Titan (when Titan makes four revolutions, Hyperion makes three), very irregular shape, sponge-like tan-colored icy surface, numerous impact craters, no well-defined poles or equator (chaotic rotation) which makes its rotational behavior unpredictable

IAPETUS: third largest moon, most distant large moon, greatest orbital inclination (orbits at a greater altitude, at 14.72°), one hemisphere is pitch-black (Iapetus’s leading hemisphere collides with dust and ice particles as it rotates, darkening its surface) and the other is bright as snow

MISSIONS: Cassini-Huygens, Pioneer 11, Voyager

OVERVIEW

  • Order in Solar System: #6
  • Number of Moons: 62
  • Orbital Period: 29.5 years
  • Rotational Period: 10.5 hours
  • Mass: 5.6846 x 10^26 kg (95.152 Earths)
  • Volume: 8.2713 x 10 ^14 km³ (763.59 Earths)
  • Radius: 60,268 km (9.4492 Earths)
  • Surface Area: 4.27 x 10^10 km² (83.703 Earths)
  • Density: 0.687 g/cm³ (less than water!)
  • Eccentricity of Orbit: 0.056
  • Surface Temperature (Average): 134 K
  • Escape Velocity: 35.5 km/s
  • Apparent Magnitude: +1.47 to -0.24