PhobosIs the larger and innermost of the twonatural satellitesofMars, the other beingDeimos. Both moons were discovered in 1877 by American astronomerAsaph Hall.Phobos is a small, irregularly shaped object with a mean radius of 11km and is seven times more massive than Deimos, Mars's outer moon. Phobos is named after theGreek godPhobos, a son ofAres(Mars) andAphrodite(Venus) which was the personification ofHorror. The name "Phobos" is pronounced/fobs/foh-bs, or like the Greek.Phobos orbits 6,000km (3,700mi) from the Martian surface, closer to itsprimarythan any other known planetary moon. It is so close that it orbits Mars faster than Mars rotates, and completes an orbit in just 7 hours and 39 minutes. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 hours 15 min or less, and set in the east, twice each Martian day. Phobos is one of the least reflective bodies in the Solar System, and features a largeimpact crater,Stickney. The temperatures range from about 4C (25F) to 112C (170F), on the sunlit and shadowed sides respectively. Images and models indicate that Phobos may be arubble pileheld together by a thin crust, and that it is being torn apart bytidalinteractions.Phobos is drawing closer to Mars by 2 meters every one hundred years, and it is predicted that in 30 to 50 million years it will collide with the planet or break up into aplanetary ring.

DeimosDeimos is the smaller natural satellite of Phobos, approximately 15 km in diameter. However, a more distant orbit of Mars (approximately 20,000 km) and tend to be stable, no closer to the orbit of Mars as belonging to Phobos. Deimos orbits Mars at a period of 30 hours. Deimos has a flatter surface than Phobos. This is because most of its surface covered by a thin layer of dust, so that the relief surface detail is not so visible. Deimos orbits Mars together with Phobos. Deimos rotation speed is much slower than the speed of rotation of Phobos. Deimos was discovered by Asaph Hall on August 12, 1877 at about 07:48 UTC.

GanymedeIs the largestmoon of Jupiterandin the Solar System, and the onlymoonknown to have amagnetosphere. It is the seventh satellite outward fromJupiter,and third of theGalilean moons, the first group of objects discovered orbiting another planet. Completing anorbitin roughly seven days, Ganymede participates in a 1:2:4orbital resonancewith the moonsEuropaandIo, respectively. With a diameter of 5,268km (3,273mi), it is 8% larger than the planetMercury, but has only 45% as much mass.Its diameter is 2% larger than that of Saturn'sTitan, the Solar System's second-largest moon. At 2.02 times the mass of theMoon, it is the most massive planetary satellite.It is the 9th largest object in the Solar System, and the largest without a substantial atmosphere.Ganymede is composed of approximately equal amounts ofsilicate rockand water ice.It is a fullydifferentiatedbody with an iron-rich, liquid core, and aninternal oceanthat may contain more water than all of Earth's oceans together.Its surface is composed of two main types of terrain. Dark regions, saturated withimpact cratersand dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result oftectonicactivity brought about bytidal heating.Ganymede's magnetosphere was probably created throughconvectionwithin its liquid iron core.The meager magnetosphere is buried within Jupiter's much largermagnetic fieldand would show only as a local perturbation of thefield lines. The satellite has a thinoxygenatmospherethat includes O, O2, and possibly O3(ozone).Atomic hydrogenis a minor atmospheric constituent. Whether the satellite has anionosphereassociated with its atmosphere is unresolved.Ganymede's discovery is credited toGalileo Galilei, who was the first to observe it on January 7, 1610.The satellite's name was soon suggested by astronomerSimon Marius, for the mythologicalGanymede, cupbearer of theGreek gods.Beginning withPioneer 10, spacecraft have been able to examine Ganymede closely.TheVoyagerprobes refined measurements of its size, whereas theGalileocraft discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is theEuropean Space Agency'sJupiter Icy Moon Explorer(JUICE), due to launch in 2022.

MoonTheMoon(inGreek:Selene, inLatin:Luna) isEarth's onlynatural satellite. It is one ofthe largest natural satellites intheSolar System, and, among planetary satellites, the largest relative to the size of the planet it orbits (itsprimary).It is the second-densestsatellite among those whose densities are known (afterJupiter's satelliteIo).The Moon is thought to have formed approximately 4.5 billion years ago,not long after Earth. There are several hypotheses for its origin; the most widely accepted explanation is that the Moon formed from the debris left over after agiant impactbetween Earth and aMars-sized body calledTheia.The Moon is insynchronous rotationwith Earth, always showing the same face with itsnear sidemarked by dark volcanicmariathat fill between the bright ancient crustal highlands and the prominentimpact craters. It is the second-brightest regularly visiblecelestial objectin Earth's sky after theSun, as measured byilluminanceon Earth's surface. Although it can appear avery bright white, its surface is actually dark, with areflectancejust slightly higher than that of worn asphalt. Its prominence in the sky and its regular cycle ofphaseshave, since ancient times, made the Moon an important cultural influence onlanguage,calendars,art, andmythology.The Moon's gravitational influence produces theocean tides,body tides, and theslight lengtheningof the day. The Moon's current orbital distance is about thirty times the diameter of Earth, causing it to have anapparent sizein the sky almost the same as that of the Sun, with the result that the Moon covers the Sun nearly precisely in totalsolar eclipse. This matching of apparent visual size is a coincidence. The Moon's linear distance from Earth is currently increasing at a rate of 3.820.07 centimetres (1.5040.028in) per year, but this rate is not constant.IOIo/a.o/is the innermost of the fourGalilean moonsof the planetJupiter. It is thefourth-largest moon, has the highest density of all the moons, and is the driest known object in theSolar System. It was discovered in 1610 and was named after the mythological characterIo, a priestess ofHera.With over 400 activevolcanoes, Io is the most geologically active object in the Solar System.This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellitesEuropa,GanymedeandCallisto. Several volcanoes produce plumes ofsulfurandsulfur dioxidethat climb as high as 500km (300mi) above the surface. Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of Io's silicate crust. Some of these peaks are taller thanMount Everest.Unlike most satellites in the outer Solar System, which are mostly composed of water ice, Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. Most of Io's surface is composed of extensive plains coated with sulfur and sulfur dioxide frost.Io's volcanism is responsible for many of its unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various subtle shades of yellow, red, white, black, and green, largely due toallotropesand compounds of sulfur. Numerous extensive lava flows, several more than 500km (300mi) in length, also mark the surface. The materials produced by this volcanism make up Io's thin, patchy atmosphere and Jupiter's extensivemagnetosphere. Io's volcanic ejecta also produce a largeplasma torusaround Jupiter.Io played a significant role in the development of astronomy in the 17th and 18th centuries. It was discovered in January 1610 byGalileo Galilei, along with the other Galilean satellites. This discovery furthered the adoption of theCopernican modelof the Solar System, the development ofKepler'slaws of motion, andthe first measurement of the speed of light. From Earth, Io remained just a point of light until the late 19th and early 20th centuries, when it became possible to resolve its large-scale surface features, such as the dark red polar and bright equatorial regions. In 1979, the twoVoyagerspacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, and a young surface with no obvious impact craters. TheGalileospacecraft performed several close flybys in the 1990s and early 2000s, obtaining data about Io's interior structure and surface composition. These spacecraft also revealed the relationship between Io and Jupiter's magnetosphere and the existence of a belt of high-energy radiation centered on Io's orbit. Io receives about 3,600rem(36Sv) ofionizing radiationper day.