Touring our solar system (astronomy)


Published on

touring our solar system, grade 7 lesson

Published in: Education
1 Comment
No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Touring our solar system (astronomy)

  1. 1. Touring Our Solar System Prepared by: Victor R. Oribe
  2. 2. How Did the Planets Form?The Sun and planets formed at the same time froma large rotating cloud of interstellar dust and gascalled the Solar Nebula.
  3. 3. As the solar nebula contracted, the vast majorityof materials collected in the center to form the hotprotosun.
  4. 4. The remainderformed a flattened,spinning disk. With this spinning disk, matter gradually formed clumps of material that collided, stuck together and grew into asteroid- sized objects called planetesimals.
  5. 5. The composition of these planetesimalsdepended largely on their locations withrespect to their protostar.Temperature were greater near the protosunand much lower in the outer reaches of thedisk.This was critical since only those materialsthat could condense in a particular locationwould be available to form planetesimal.
  6. 6. Near the present orbit of Mercury onlymetallic grains condense- it was too hot foranything else to exist.Farther out, near the Earth’s orbit, metallicas well as rocky substances condensed, andbeyond Mars, ices of water, carbon dioxide,ammonia, and methane formed.
  7. 7. From clumps of matter ( ices of water, carbon dioxide,and methane) the planetesimals formed and throughrepeated collisions and accretion (sticking together) grewinto eight protoplanets and their moons.It took roughly abillion years afterthe protoplanetsformed togravitationallysweep the solarsystem clear ofinterplanetarydebris.
  8. 8. The period of intense bombardment isclearly visible on the Moon and elsewhere inthe solar system.Only a small amount of the interplanetarymatter escaped capture by a planet or moonand became the asteroid, comets, andmeteoroids.
  9. 9. Terrestrial and Jovian PlanetsThe Terrestrial (Earth-like) Planets
  10. 10. The Jovian (Jupiter-like) PlanetsPluto was recently demoted to a dwarf planet - a newclass of solar system objects that have an orbit around theSun but share their space with other celestial bodies.
  11. 11. The most obvious difference between the terrestrial andthe Jovian planets is their SIZE. The largest terrestrial planets (Earth and Venus) have a diameter only one- quarter as great as the diameter of the smallest Jovian planet (Neptune)The masses of Earth and Venus are only 1/17 as great asNeptune. Hence, the Jovian planets are also calledGIANTS.
  12. 12. The four Jovian planets are also referred to as theOUTER PLANETS, because of their relative locations.The terrestrial planet are also called INNER PLANETS.
  13. 13. Other dimension in which the terrestrial and the Jovianplanets differ include: DENSITY, CHEMICALMAKEUP, and the RATE OF RORATION.The density of the Terrestrial planets average about FIVEtimes the density of water.The density of the Jovian planets averages only 1.5 timesof water.One of the outer planets, Saturn, has a density only 0.7times that of water, which means that Saturn would floatif placed in a large enough water tank.
  14. 14. The Composition of the PlanetsThe substances that make up the planets are divided intothree compositional groups: GASES, ROCKS, and ICES,based on their melting point.1. Gases Hydrogen and Helium, are those with melting points near absolute zero (0K). Hydrogen and Helium, are the most abundant constituents of the solar nebula.
  15. 15. 2. Rocks Rocks present in the planets are usually silicate minerals and metallic iron, which have a melting points that exceed 7000C (13000 F)3. Ices Ices in the planets includes ammonia, methane, carbon dioxide, and water. This gases have intermediate melting points (for example, water has a melting point of 00C (320 F)
  16. 16. The terrestrial planets are dense, consistingmostly of rocky and metallic substances, withminor amounts of ices.The Jovian planets, on the other hand, containlarge amounts of gases (hydrogen and helium)and ices (mostly water, ammonia, andmethane). This accounts for their lowdensities.Jovian planets also contain substantial amountsof rocky and metallic materials, which areconcentrated in their central cores.
  17. 17. The Atmosphere of the PlanetsThe Jovian planets have a very thick atmospheres ofhydrogen, helium, methane, and ammonia. The Terrestrial planets have meageratmosphere at best.
  18. 18. Reasons for the Differences in the Atmosphere of the Planets1. Location of each planet within the solarnebula during its formation. The outer planets formed where the temperature was low enough to allow water vapor, ammonia, and methane to condense into ices, thus, the Jovian planets contain large amounts of these volatile gases. While the inner regions of the developing solar system, the environment was too hot for ices to survive.
  19. 19. Consequently, one of the long-standing questions for thenebular hypothesis was “How did Earth acquire waterand other volatile gases?”During the protoplanet stage, Earth was bombarded withicy fragments (planetesimals) that originated beyond theorbit of Mars.Like the inner planet, they (Mercury and Moon) surelywould have been bombarded by icy bodies.Why do Mercury and our Moon lack an atmosphere?
  20. 20. 2. More massive planets have betterchance of retaining their atmospheresbecause atoms and molecules need ahigher speed to escape. On the Moon the escape velocity is only 2.4 km/s compared with more than 11 km/s for Earth. Because of their strong gravitational fields, the Jovian planets have escape velocities that are mush higher than that of Earth, which is the largest terrestrial planet.
  21. 21. 3. The molecular motion of a gas istemperature-dependent. At the low temperatures of the Jovian planets even the lightest gases (hydrogen and helium0 are unlikely to acquire the speed needed to escape. By the contrast, a comparatively warm body with a small surface gravity, such a Mercury and our Moon, is unable to hold even heavy gases such as carbon dioxide and radon.
  22. 22. The Planets: A Brief Tour Mercury: The innermost and the smallestThe Innermost planet. Planet Larger than Earth’s moon. Mercury revolves quickly but rotates slowly. One full day-night cycle on Earth takes 24 hours, but on Mercury it requires 179 Earth-days. Night in Mercury lasts for about three months and is followed by three months of daylight.
  23. 23. Nighttime temperatures drop as low as -1730 C andnoontime temperature exceed 4270 C, hot enough tomelt tin and lead. Mercury has the greatest temperatur e extremes of any planets.
  24. 24. Mercury absorbs most of the sunlight that strikes it,reflecting only 6% into space.The low reflectivity of sunlight from Mercury ischaracteristic of terrestrial bodies that have virtually noatmosphere.Mercury has long scarps that cut across the plains andnumerous craters. The scarps are thought to be the result of crustal shortening as the planet cooled and shrank early in its history.
  25. 25. Venus: Second only to the Moon in brilliance in the night sky. The Veiled Planet Named for the goddess of love and beauty. It orbits the Sun in a nearly perfect circle once every 225 Earth-dayVenus is similar to Earth in size, density, mass, andlocation in the solar system, thus, it has referred to as“Earth’s twin”The surface of Venus reaches temperatures as great as4800 C, and its atmosphere is 97% carbon dioxide.
  26. 26. About 80% of the surface of Venus is consists of subduedplains that are covered by volcanic flows.Thousands of volcanic creatures have been identifiedmostly small shield volcanoes, although more than 1,500volcanoes greater than 20km across have been mapped. Only 8% of the Venus surface is consists of highlands that may be linked to continental areas of the Earth.
  27. 27. Mars: Mars is approximately half the size of The Red the Earth and revolves around the Sun in Planet 687 Earth-days. Martian atmosphere has only 1% the density of Earth’s and it is primarily carbon dioxide with tiny amounts of water vapor.Data from Mars probes confirm that the polar caps ofMars are made of water ice., covered by a thin layer offrozen carbon dioxide.Although the atmosphere of Mars is very thin, extensivedust storms occur and may cause the color changesobserved from Earth-based telescope.
  28. 28. Hurricane-force winds up to 270km per hour can persistfor weeks.Images from Viking 1 And Viking 2 revealed that aMartian landscape remarkably similar to a rocky deserton Earth. Most Martian surface features are old by Earth standards. Evidence suggest that weathering accounts for almost all surface changes during the last 3.5 billion years
  29. 29. Mariner 9, the first spacecraft to orbit another planet,reached Mars in 1971 amid a raging dust storm.When the dust cleared, images of Mars’ northernhemisphere revealed numerous large volcanoes.Olympus Mons, is the size of Ohio and 23km tall, nearly3 times higher than Mount Everest
  30. 30. Another surprising find made by Mariner 9 was theexistence of several canyons that dwarf even Earth’sGrand Canyon of the Colorado River.
  31. 31. Liquid water does not appear to exist anywhere on theMartian surface.However, poleward of about 300 latitude ice can be foundwithin a meter of the surface and in the polar regions itforms small permanent ice caps.In addition, considerable evidence indicates that in thefirst billion years of the planets’ history, liquid waterflowed on the surface creating valleys and relatedfeatures.
  32. 32. Tiny Phobos and Deimos are the two satellites of Mars. Phobos and Deimos are only 24 and 15 km in diameter.Phobos is nearer to its parent tan any other naturalsatellite in the solar system, only 5,500km and requiresjust 7 hours and 39 minutes for one revolution.Mariner 9 revealed that both satellites are irregularlyshaped and have numerous impact craters.
  33. 33. It is likely that these moons (Phobos and Deimos) areasteroid captured by Mars. The most interesting coincidence in astronomy and literature is the close resemblance of Phobos and Deimos to two fictional satellite of Mars described by Jonathan Swift in Gulliver’s Travels, written about 150 years before these satellites were actually discovered.
  34. 34. Jupiter: Lord Truly a giant among planets, has a mass two and half times greater thanof the Heaven the combined mass of all the remaining planets, satellites, and asteroids. Jupiter is ten times larger than the sun, it would evolved into a small star. Despite its great size, however, it is only 1/800 as massive as the sun. Jupiter revolves around the Sun once every 12 Earth- years, and rotates more rapidly than any other planet, completing one rotation in slightly less than 10 hours.
  35. 35. When viewed through a telescope or binoculars, Jupiterappears to be covered with alternating bands ofmulticolored clouds aligned parallel to its equator.The most striking feature is the Great Red Spot in thesouthern hemisphere. The great red spot has been a prominent feature since it was first seen more than three centuries ago. When Voyager 2 swept by jupiter in 1979, the Great Red Spot was the size of two Earth-size cicles placed side by side.
  36. 36. Images from Pioneer II as it moved near Jupiter’s cloudtops in 1974 indicated that the Great Red Spot is acounterclockwise-rotating storm caught between twoJetstream-like bonds flowing in opposite directions.This huge, hurricane-like storm makes a completerotation about once every 12 days.The winds on Jupiter are the product of different heating,which generates vertical convective motions in theatmosphere.Jupiter’s convective flow produces alternating dark-colored belts and light-colored zones
  37. 37. Jupiter’s atmosphere is composed mainly of Hydrogen(90% and Helium (10%) but also contains lesser amountsof methane, ammonia, and water which form cloudscomposed of liquid droplets or icy crystals.Atmospheric pressure at the top of the clouds is equal tosea-level pressure on Earth.The pressure in Jupiter increases rapidly toward itssurface.At 1,000 km below the clouds, the pressure is greatenough to compress hydrogen gas into a liquid.Less than halfway into Jupiter’s interior, extremepressures cause the liquid hydrogen to turn into liquidmetallic hydrogen.
  38. 38. The fast rotation and liquid metallic core are a possibleexplanation for the intense magnetic field surroundingJupiter.Jupiter is also believed to contain as much rocky andmetallic material as is found in the terrestrial planets,probably located in a central core.
  39. 39. Jupiter’s MoonJupiter’s satellite system, consistingof 63 moons discovered so far,resembles a miniature solar system.The four largest satellites,discovered by Galileo travel innearly circular orbits around theplanet, with a periods of from 2 to17 Earth-days. Callisto and Ganymede, surpass Mercury in size. Europa and Io, are about the size of Earth’s Moon.
  40. 40. Images from Voyager 1 and 2 revealed, to the surprise ofalmost everyone, that each of the four Galilean satellitesis a unique geological world.
  41. 41. The composition of each Jupiter’s satellite is strikinglydifferent, which implies a different evolution for eachone.Ganymede has a dynamic core that generates a strongmagnetic field not observed on the other satellites.Io, is perhaps the most volcanically active body in thesolar system, more than 80 active sulfurous volcaniccenters.The heat source for volcanic activity is tidal energygenerated by a relentless “tug of war” between Io andJupiter and the other Galilean satellites.
  42. 42. In addition, Jupiter has numerous satellites that are verysmall (about 20km in diameter), revolve in a directionthat is opposite (retrograde) to that of the largest moons,These satellites appear to be asteroids that passed nearenough to be captured gravitationally by Jupiter.One of the interestingaspect of the Voyager1 mission was a studyof Jupiter’s ringsystem.
  43. 43. By analyzing how theserings scatter light,researchers determinedthat the rings arecomposed of fine, darkparticles, similar in sizeto smoke particles. Furthermore, the faint nature of the rings indicates that these minute particles are widely dispersed.
  44. 44. The main ring is composed of particles believed to be fragmentsblasted by meteorite impacts from the surface of Metis andAdrastea.Impacts on Jupiter’s moon’s Amalthea and Thebe are believed to bethe source of the outer ring Gossamer ring.
  45. 45. Requiring 29.46 Earth-years to SATURN: make one revolution. Elegant Planet Saturn is almost twice as far from the Sun as Jupiter, yet its atmosphere, composition, and internal structure are believed to be remarkably similar to Jupiter’s.The most prominent feature of Saturn is its system ofrings, first seen by Galileo in 1610.Their nature was determined 50 years later by the Dutchastronomer Christian Huygens.
  46. 46. Saturn’s atmosphere is quite dynamic with winds roaring at up to 1,500km/h. Cyclonic “storm” similar to Jupiter’s Great Red Spot occur in Saturn’s atmosphere as does intense lightning.Atmosphere is nearly 75% hydrogen, and 25% helium, theclouds are composed of ammonia, ammonia hydrosulfide,and water each segregated by temperature.
  47. 47. The central core of Jupiter is composed of rock and icylayered with liquid metallic hydrogen, and then liquidhydrogen.Like Earth, Saturn’s magnetic field is believed to be createdwithin the core. In this process, helium condenses in theliquid hydrogen layers, releasing the heat necessary forconvection.
  48. 48. The origin of planetary ring system is still being debated.Perhaps the rings formed out of a flattened cloud of dustand gases the encircled the parent planet.The ring formed at the same time and from the samematerials as the planets and moons.Perhaps the ring formed later, when a moon or largeasteroid was gravitationally pulled apart after staying tooclose to a planet.Another hypothesis suggest that a foreign body blastedapart one of the planet’s moons; the fragments of whichwould tend to jostle one another and form a flat, thin ring.
  49. 49. The Saturnian satellite system consists of 56 known moons.If you count the “moonlets” that comprise Saturn’s rings,this planet has millions of satellites.The largest, Titan, is bigger than Mercury and is thesecond-largest satellite in the solar system (after Jupiter’sGanymede).Titan and Neptune’s Triton are the only satellites in thesolar system known to have a substantial atmosphere.The atmospheric pressure at Titan’s surface is about 1.5times that of the Earth’s surface.
  50. 50. Cassini-Huygens probe determined the atmospherecomposition to be about 95% nitrogen and 5% methanewith additional organic compounds – similar to Earth’sprimitive atmosphere prior to the onset of life.Recent evidence suggests that Titan has Earth-likegeological landforms and geological processes, such asdune formation and fluvial erosion caused by methane rain.
  51. 51. URANUS and Neptune: The TwinsUranus and Neptune have similar structure andcompositions.They are less than 1% different in diameter (about 4x thesize of the Earth), and they are both bluish in appearance,which is attributable to the methane in their atmospheres.
  52. 52. Uranus and Neptune take 84 and 165 Earth-days,respectively, to complete one revolution around theSun.The core composition is similar to that of the othergas giants with a rocky silicate and iron core, butwith less liquid metallic hydrogen and more icethan Jupiter and Saturn.Neptune, however, is colder, because it is half againas distant from the Sun as in Uranus.
  53. 53. Uranus: The sideways PlanetUranus rotates on its sideways. Its axis of rotation,instead of being generally perpendicular to theplane of its orbit, like the other planets, lies nearlyparallel to the plane of its orbit.Its rotational motion, therefore, has the appearanceof a rolling ball, rather than the toplike spinning ofother planet.A surprise discovery in 1977 revealed that Uranushas a ring system
  54. 54. Spectacular views from Voyager 2 of the fivelargest moons of Uranus show quite varied terrains. Neptune: The Windy PlanetNeptune has a dynamic atmosphere, much likethose of Jupiter and Saturn.Wind exceeding 1,000km/h encircle the planet,making it one of the widest places in the solarsystem.
  55. 55. Neptune has an Earth-size blemish called the GreatDark Spot that is reminiscent of Jupiter’s Great RedSpot and is assumed to be a large rotating storm.About 5 years after theVoyager 2 encounter, when theHubble Space Telescopeviewed Neptune, the spot hadvanished, and was replaced byanother dark spot in theplanet’s northern hemisphere.
  56. 56. Six new satellite were discovered in the Voyagerimages, bringing Neptune’s family to 8, and morerecent observations bring the total to 13.Triton, Neptune’s largest moon, is a mostinteresting objects.It is the only large moon in the solar system thatexhibits retrograde motion.Triton exhibits one of the most amazingmanifestations of volcanism, the eruption of ices.
  57. 57. Minor Members of the Solar System1. Asteroids : PlanetesimalsAsteroids are small fragments (planetesimal) about4.5 billion years old left over from the formation ofthe solar system.The largest asteroid, Ceres, is 940km in diameter,but most of the 100,000 known asteroids are muchsmaller
  58. 58. Most asteroids lie roughly midway between theorbits of Mars and Jupiter in the region known asthe Asteroid Belt.
  59. 59. 2. Comet : Dirty SnowballsComets, like asteroids, are also left over from theformation of the solar system.Comets are composed of ices (water, ammonia,methane, carbon dioxide and carbon monoxide) thathold together pieces of rocky and metallicmaterials, thus the nickname “dirty snowballs”
  60. 60. Comets are among the most interesting andunpredictable bodies in the solar system.Many comets travel in very elongated orbits thatcarry them far beyond Pluto. These comets takehundreds of thousands of years to complete a singleorbit around the Sun.However, a few short-period comets (those havingorbital period of less than 200 years) such asHalley’s comet, make a regular encounters with theinner solar system.
  61. 61. When first observed, a comet appears very small,but as it approaches the Sun, solar energy begins tovaporize the ices, producing a glowing head calledCOMA. As the comet approach the Sun, some, but not all, develop a tail that extends for millions of kilometers. As a comet moves away from the Sun, the gas forming the coma recondense, the tail disappears, the comet returns to cold storage.
  62. 62. Most comets are found in two regions of the outersolar system. The short period comets are thoughtto orbit beyond Neptune in region called the Kuiperbelt, in honor of astronomer Gerald Kuiper, whohad predicted their existence.
  63. 63. Long period comets have orbits that are NOTconfined to the plane of the solar system.These comets appear to be distributed in alldirections from the Sun, forming a spherical shellaround the solar system called OORT CLOUD, Named after the Dutch astronomer Jan Oort. Millions of comets are believed to orbit the Sun at distances greater than 10,000 times the Earth-Sun distance.
  64. 64. The most famous short-period comet is Halley’sComet.Its orbital period averages 76 years, and every oneof its 29 appearances since 240 BC has beenrecorded by Chinese astronomers.When seen in 1910, Halley’s Comet had developeda tail nearly 1.6 million km long and was visibleduring daytime.Last perihelion Feb. 9, 1986, next aphelionDecember 9, 2023.
  65. 65. Halley’s comet is potato shaped and 16 by 8 km size. The surface is irregular and full of craterlike pits.Gases and dust that escape from the nucleus toform the coma and tail appear to gush from itssurface as bright jets or streams.Only about 10% of the comet’s total surface wasemitting these jets at the time of rendezvous.
  66. 66. Meteoroids: Visitors to EarthAlso called the “shooting star”The streak of light that lasts from an eyeblink toa few seconds occurs when a small solid particle,a meteoroid, enter Earth’s atmosphere frominterplanetary space.The remains of meteoroids, when found onEarth, are referred to as meteorites.
  67. 67. Most meteoroids originate from any one of thefollowing three sources: 1. Interplanetary debris that was not gravitationally swept up by the planets during the formation of the solar system. 2. Material that is continually being lost from the asteroid belt. 3. The solid remains of comets that once passed through Earth’s orbit.
  68. 68. Occasionally, meteor sighting increasedramatically to 60 or more per hour.These displays, called meteor showers, resultswhen Earth encounters a swarm of meteoroidstravelling in the same direction and at nearly thesame speed as Earth.
  69. 69. Major Meteor Showers Adapted from the book, Whats Out Tonight? Celestial Almanac & Astronomy Field Guide, 2000 to 2050 by Ken Graun SHOWER* Constellation Peak Date** Hourly Count***QUADRANTIDS**** Bootes Jan 3 60 - 200 LYRIDS Lyra Apr 22 15 - 20+ Eta AQUARIDS Aquarius May 5 60Delta AQUARIDS Aquarius Jul 29 20 PERSEIDS Perseus Aug 12 120 - 160 ORIONIDS Orion Oct 21 20 TAURIDS Taurus Nov 5 -13 5 LEONIDS Leo Nov 17 10***** GEMINIDS Gemini Dec 14 120 URSIDS Ursa Minor Dec 22 10+
  70. 70. A few very large meteoroids have blasted out craters on Earth’s surface that strongly resemble those on the lunar surface. The most famous is Meteor Crater in Arizona, about 1.2km across and 170meters deep and has an upturned rim that rises 50 meters above the surrounding countryside.More than 30 tons of ironfragments have been foundin the immediate area.Based on the amount oferosion, the impact likelyoccurred within the last50,000 years.
  71. 71. Meteorites are classified according to their composition: 1. Irons, mostly iron with 5-20 percent nickel. 2. Stony-silicate minerals with inclusions of other3. Stony-iron mixture. minerals.
  72. 72. The Dwarf PlanetsSince Pluto’s discovery in 1930, it has been a mysteryon the edge of the solar system.At first, Pluto was thought to be about as large as Earth,but as better images were obtained, Pluto’s diameter wasestimated to be a little than one half that of Earth.In 1978, astronomers discovered that Pluto has asatellite (Charon), whose brightness combined with itsparent made Pluto appear much larger than it really was.Recent images obtained by the Hubble Space Telescopeestablished the diameter of Pluto at only 2,300km. 1/5that of the Earth and less than half of Mercury.
  73. 73. Seven moons, including Earth’s moon are larger thanPluto.In 1992 astronomers discovered another icy body inorbit beyond Pluto. Soon hundred of these Kuiper beltobject were discovered forming a band of small objectssimilar to the asteroid belt between Mars and Jupiter.Many other planetaryobjects, some largerthan Pluto, are thoughtto exist in this belt oficy worlds foundbeyond orbits ofNeptune.
  74. 74. The International Astronomical Union, a group that hasthe power to determine whether or not Pluto is a Planet,voted August 24, 2006, to ad a new class of planetscalled dwarf planets.These include “celestial bodies that orbit around theSun, are essentially round due to their self-gravity, butare not the only object to occupy their area of space”Pluto is recognized as a dwarf planet and the prototypeof new category of planetary object.This is not the first time a planet has been demoted. Inthe mid-1800 astronomy textbooks listed as many as 11planets in our solar system, including the asteroidsVesta, Juno, Ceres, and Pallas.
  75. 75. SAQ’s1. By what criteria are the planets placed into either the Jovian or terrestrial group?2. What are the three types of materials thought to make up the planets? How are they different? How does their distribution account for the density differences between the terrestrial and Jovian planetary groups?3. Explain why the terrestrial planets have meager atmospheres, as compared to the Jovian planets.4. How is crater density used in the relative dating of features in the Moon?5. Briefly outline the history of the Moon.6. How are the Maria of the Moon THOUGHT TO BE SIMILAR TO THE Columbia Plateau?7. Venus has been referred to as “Earth’s twin”. In what ways are these two planets similar? How do they differ?8. What surface features does Mars have that are also common on Earth?9. Why are the largest volcanoes on Earth so much smaller than the largest ones on Mars?10. Why might astrobiologists be intrigued by the evidence that groundwater has seeped onto the surface of Mars?11. The two “moon” of Mars were once suggested to be artificial. What characteristics do they have that would cause such speculations?
  76. 76. 12. What is the nature of Jupiter’s Great Red Spot?13. Why are the Galilean satellites of Jupiter so named/14. What is distinctive about Jupiter’s satellite Io?15. Why are the outer satellites of Jupiter thought to have been captured rather thanhaving been formed with the rest of the satellite system?16. How are Jupiter and Saturn similar?17. What two roles do ring moons play in the nature of planetary ring system?18. How are Saturn’s satellite Titan and Neptune’s satellite Triton similar?19. What do you think would happen if Earth passed through the tail of a comet?20. Why are impact carters more common on the Moon than on Earth, even thoughthe Moon is a much smaller target and has a weaker gravitational field?21. Compare meteoroid, meteor, and meteorite.22. Where are most comet thought to reside? What eventually becomes that orbitclose to the Sun?23. What are the three main sources of meteoroids?24. Where are most asteroids found?25. It has been estimated that Halley’s comet has a mass of 100 billion tons.Furthermore, this comet is estimated to lose 100 million tons of materials during thefew months that its orbit brings it close to the Sun. With an orbital period of 76 years,what is the maximum remaining life span of Halley’s Comet?