A moon is a natural body that orbits around a planet. It is kept in its orbit by both the host planet’s gravity and the gravity of the moon itself. Some planets are circled by moons; some are not. In our own Solar System most of the moons are in orbit around the quartet of giant gaseous worlds in the outer limits. In dramatic contrast, the warmer and well-lit inner domain of our Solar System–where the four solid planets are located–is almost entirely barren of moons. Of the quartet of relatively small solid planets–Mercury, Venus, Earth and Mars–Mercury and Venus are moonless, and Mars is circled by a tiny duo of potato-shaped moons dubbed Phobos and Deimos. Of the four inner planets, only our Earth is circled by a large Moon.
More than 100 moons orbit planets in our own Solar System. Most of them are frozen oddballs, made up primarily of ices and rocky material. However, a few of these frigid moon-worlds may not be barren of life. Europa of Jupiter may contain a subsurface ocean of life-sustaining liquid water sloshing around beneath its cracked icy crust. This subsurface ocean would be warmed by tidal flexing, keeping water in its liquid phase. Life as we know it depends on the existence of liquid water. Where liquid water exists there is the possibility, though not the promise, of life existing as well. Indeed, primitive aquatic life-forms may be swimming around in Europa’s global subsurface ocean. Also, Titan of Saturn–the second largest moon in our Solar System after Ganymede of Jupiter–displays an environment that is eerily similar to that of our Earth before life emerged on our planet from a remarkable mixture of non-living substances. Lazy, large raindrops of liquid hydrocarbons shower down on the surface of this frigid, hydrocarbon-slashed moon-world, forming seas and lakes filled with liquid methane and ethane that play the same role as liquid water on Earth. It is possible that life, as we do not know it, can emerge and evolve using liquids other than water.
Our Solar System’s largest moon, Ganymede, is bigger than the planet Mercury–which is the smallest major planet inhabiting our Sun’s family. Like its sibling moon Europa, Ganymede is thought to contain a global ocean of swirling, sloshing liquid water hidden beneath its icy, rocky crust. Also, Enceladus of Saturn is a tiny frozen moon-world that launches geysers of ammonia-laced liquid water from fissures on its surface that astronomers refer to as its “tiger stripes.” For this reason, many scientists think that little Enceladus may harbor water beneath its devastatingly frigid, frozen surface of ice.
Astronomers have known since 1995 that our Solar System is not unique in the Cosmic scheme of things. It is generally thought that our Milky Way could be brimming with a multitude of distant exoplanets belonging to the families of remote stars–and that these alien planets may be circled by their own attendant moons. Some of these many, many moons could possess the mysterious and precious recipe that enables them to become the cradles of newly emerging life.
Moons come in a variety of sizes, shapes, and types. They are usually solid objects, and only a few of them sport atmospheres. Most of the planetary moons of our Sun’s family were probably born from disks of gas and dust swirling around young planets in the ancient Solar System. Some moons are big enough for their own relentless gravity to pull them into a spherical shape, while smaller moons appear to be captured asteroids–such as the tiny, shapeless Martian moons, Phobos and Deimos. Probably, the formation of many of the small, irregularly-shaped moons, had nothing to do with the formation and evolution of the worlds they orbit. The International Astronomical Union (IAU) lists more than 146 moons circling the planets inhabiting our own Solar System. This number does not include the moons that still await official confirmation and naming, the moons of the dwarf planets (such as Pluto), and the extremely small natural satellites that circle some asteroids and other relatively tiny celestial objects.
The enormous gas-giants, Jupiter and Saturn, and the smaller (but still huge) ice-giants Uranus and Neptune, are richly endowed with attendant moon-worlds. Gas-giant planets are composed almost entirely of atmosphere. Indeed, they may (or may not) contain small solid cores well-hidden beneath their heavy blankets of gas. In contrast, the ice-giants do contain large solid cores much bigger than those that Jupiter and Saturn may possess. The ice-giant duo, Uranus and Neptune–like their gas-giant siblings–are also heavily enshrouded by thick gaseous atmospheres. However, the atmospheres of the duo of ice-giants are not as thick as those swirling around Jupiter and Saturn. As these behemoth planets grew ever larger and larger in our primordial Solar System, they were able to snare objects with their gravitational snatching claws.
The banded behemoth Jupiter–by far the largest planet in our Sun’s family–is orbited by 79 known moons, while its somewhat smaller gas-giant sister, the beautiful ringed planet Saturn, is circled by 62 verified moons. Uranus has 27 moons–all icy–while Neptune has 14 moons. Many of Jupiter’s outer moons have highly elliptical orbits around their planet and circle it “backwards”, meaning that their orbits carry them opposite to the spin of Jupiter. This suggests that these outer moons were not born circling their planet, and that they are really captured objects born elsewhere. Saturn, Uranus, and Neptune also are circled by irregular moons, which are located in orbits far from their respective planets.
While Saturn has 62 verified moons, the chunks of sparkling ice and rock, spinning around within Saturn’s beautiful gossamer rings, are not classified as moons. However, there is a population of glittering frozen moonlets, called sheperd moons, embedded within Saturn’s rings, that perform the important function of keeping the rings in line. Saturn’s large moon, the hydrocarbon-tortured Titan, is the only moon in our Solar System that possesses a thick atmosphere.
Beyond Saturn, the greenish-blue ice-giant Uranus is circled by a population of inner moons that are half water ice and half rock. The Uranian moon Miranda is a true “oddball”, with a chopped up appearance that displays the tragic scars of impacts made by large rocky bodies. Neptune’s large moon Triton is about the same size as the dwarf planet Pluto, and it orbits its planet backwards compared with Neptune’s direction of rotation. Indeed, Triton has numerous characteristics that it shares with Pluto. Pluto is a large denizen of the distant Kuiper Belt, the frigid home of myriad icy comet nuclei and other frozen objects–both large and small. Triton may have been born in the Kuiper Belt, but had traveled inward, only to be snared by Neptune’s gravitational pull. In the future, Triton’s orbit will decay, and it will probably take a fatal plunge into its adopted parent-planet.
Pluto’s large moon, Charon, is approximately 50% the size of Pluto. For this reason, some planetary scientists consider Pluto/Charon to be a double-planet system. It has been proposed that Charon was born from debris left over from an ancient collision between an ill-fated impactor and Pluto. Astronomers using the Hubble Space Telescope have also discovered that Pluto has five additional frozen moons that are much smaller than Charon. Eris, a dwarf planet denizen of the Kuiper Belt, that is even more distant than Pluto, has a small moon of its own. 720p mkv movie download