{"id":267,"date":"2017-08-08T13:12:25","date_gmt":"2017-08-08T17:12:25","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/chapter\/9-1-general-properties-of-the-moon\/"},"modified":"2021-05-01T01:56:06","modified_gmt":"2021-05-01T05:56:06","slug":"9-1-general-properties-of-the-moon","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/chapter\/9-1-general-properties-of-the-moon\/","title":{"raw":"9.1 General Properties of the Moon","rendered":"9.1 General Properties of the Moon"},"content":{"raw":"<div class=\"bcc-box bcc-highlight\">\r\n<h3>Learning Objectives<\/h3>\r\n<p id=\"fs-id1168584078329\">By the end of this section, you will be able to:<\/p>\r\n\r\n<ul id=\"fs-id1170326087619\">\r\n \t<li>Discuss what has been learned from both manned and robotic <span class=\"no-emphasis\">lunar exploration<\/span><\/li>\r\n \t<li>Describe the composition and structure of the Moon<\/li>\r\n<\/ul>\r\n<\/div>\r\nThe <span class=\"no-emphasis\">Moon<\/span> has only one-eightieth the mass of Earth and about one-sixth Earth\u2019s surface gravity\u2014too low to retain an atmosphere. The Moon may be seen in <a class=\"autogenerated-content\" href=\"#OSC_Astro_09_02_Moon\">Figure 1<\/a>. Moving molecules of a gas can escape from a planet just the way a rocket does, and the lower the gravity, the easier it is for the gas to leak away into space. While the Moon can acquire a temporary atmosphere from impacting comets, this atmosphere is quickly lost by freezing onto the surface or by escape to surrounding space. The Moon today is dramatically deficient in a wide range of <em>volatiles<\/em>, those elements and compounds that evaporate at relatively low temperatures. Some of the <span class=\"no-emphasis\">Moon\u2019s properties<\/span> are summarized in <a class=\"autogenerated-content\" href=\"#fs-id1170324252656\">Figure 2<\/a>, along with comparative values for <span class=\"no-emphasis\">Mercury<\/span>.\r\n<figure id=\"OSC_Astro_09_02_Moon\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Two Sides of the Moon.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_09_02_Moon-1.jpg\" alt=\"The Two Sides of the Moon. The left image shows part of the Moon\u2019s hemisphere that faces Earth; several dark maria and rayed craters are visible. The right image shows part of the Moon that faces away from Earth; it is dominated by highlands and is more heavily cratered.\" width=\"975\" height=\"459\" \/> <strong>Figure 1.<\/strong> The left image shows part of the hemisphere that faces Earth; several dark maria are visible. The right image shows part of the hemisphere that faces away from Earth; it is dominated by highlands. The resolution of this image is several kilometers, similar to that of high-powered binoculars or a small telescope. (credit: modification of work by NASA\/GSFC\/Arizona State University)[\/caption]<\/figure>\r\n<table id=\"fs-id1170324252656\" class=\"span-all aligncenter\" summary=\"This table is titled \u201cProperties of the Moon and Mercury\u201d and contains 3 columns and 8 rows. The first row is a header row and it labels each column, \u201cProperty\u201d, \u201cMoon\u201d and \u201cMercury\u201d. Under the \u201cProperty\u201d column are the values, \u201cMass (Earth = 1)\u201d, \u201cDiameter (km)\u201d, \u201cDensity (g\/cm3)\u201d, \u201cSurface gravity (Earth = 1)\u201d, \u201cEscape velocity (km\/s)\u201d, \u201cRotation period (days)\u201d and \u201cSurface area (Earth = 1)\u201d. Under the \u201cMoon\u201d column are the values, \u201c0.0123\u201d, \u201c3476\u201d, \u201c3.3\u201d, \u201c0.17\u201d, \u201c2.4\u201d, \u201c27.3\u201d and \u201c0.27\u201d. Finally, under the \u201cMercury\u201d column are the values, \u201c0.055\u201d, \u201c4878\u201d, \u201c5.4\u201d, \u201c0.38\u201d, \u201c4.3\u201d, \u201c58.65\u201d and \u201c0.38\u201d.\">\r\n<thead>\r\n<tr>\r\n<th colspan=\"3\">Figure 2. Properties of the Moon and Mercury<\/th>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<th>Property<\/th>\r\n<th>Moon<\/th>\r\n<th>Mercury<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr valign=\"top\">\r\n<td>Mass (Earth = 1)<\/td>\r\n<td>0.0123<\/td>\r\n<td>0.055<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Diameter (km)<\/td>\r\n<td>3476<\/td>\r\n<td>4878<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Density (g\/cm<sup>3<\/sup>)<\/td>\r\n<td>3.3<\/td>\r\n<td>5.4<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Surface gravity (Earth = 1)<\/td>\r\n<td>0.17<\/td>\r\n<td>0.38<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Escape velocity (km\/s)<\/td>\r\n<td>2.4<\/td>\r\n<td>4.3<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Rotation period (days)<\/td>\r\n<td>27.3<\/td>\r\n<td>58.65<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Surface area (Earth = 1)<\/td>\r\n<td>0.27<\/td>\r\n<td>0.38<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<section id=\"fs-id1170326319309\">\r\n<h1>Exploration of the Moon<\/h1>\r\n<p id=\"fs-id1170326160178\">Most of what we know about the Moon today derives from the US <span class=\"no-emphasis\">Apollo program<\/span>, which sent nine piloted spacecraft to our satellite between 1968 and 1972, landing 12 astronauts on its surface. Before the era of spacecraft studies, astronomers had mapped the side of the Moon that faces Earth with telescopic resolution of about 1 kilometer, but lunar geology hardly existed as a scientific subject. All that changed beginning in the early 1960s. Initially, Russia took the lead in lunar exploration with Luna 3, which returned the first photos of the lunar far side in 1959, and then with Luna 9, which landed on the surface in 1966 and transmitted pictures and other data to Earth. However, these efforts were overshadowed on July 20, 1969, when the first American astronaut set foot on the Moon.<\/p>\r\n<p id=\"fs-id1170326068257\"><a class=\"autogenerated-content\" href=\"#fs-id1170326114334\">Figure 3<\/a> summarizes the nine Apollo flights: six that landed and three others that circled the Moon but did not land. The initial landings were on flat plains selected for safety reasons. But with increasing experience and confidence, NASA targeted the last three missions to more geologically interesting locales. The level of scientific exploration also increased with each mission, as the astronauts spent longer times on the Moon and carried more elaborate equipment. Finally, on the last Apollo landing, NASA included one scientist, geologist Jack Schmitt, among the astronauts. He is shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_09_01_Scientist\">Figure 4<\/a>.<\/p>\r\n\r\n<table id=\"fs-id1170326114334\" class=\"span-all\" summary=\"This table is titled \u201cApollo flights to the Moon\u201d and contains 4 columns and 10 rows. The first row is a header row and it labels each column, \u201cFlight\u201d, \u201cDate\u201d, \u201cLanding Site\u201d and \u201cMain Accomplishment\u201d. Under the \u201cFlight\u201d column are the values, \u201cApollo 8\u201d, \u201cApollo 10\u201d, \u201cApollo 11\u201d, \u201cApollo 12\u201d, \u201cApollo 13\u201d, \u201cApollo 14\u201d, \u201cApollo 15\u201d, \u201cApollo 16\u201d and \u201cApollo 17\u201d. Under the \u201cDate\u201d column are the values, \u201cDec. 1968\u201d, \u201cMay 1969\u201d, \u201cJuly 1969\u201d, \u201cNov. 1969\u201d, \u201cApr. 1970\u201d, \u201cJan. 1971\u201d, \u201cJuly 1971\u201d, \u201cApr. 1972\u201d and \u201cDec. 1972\u201d. Under the \u201cLanding Site\u201d column are the values, \u201c-\u201c, \u201c-\u201c, \u201cMare Tranquillitatis\u201d, \u201cOceanus Procellarum\u201d, \u201c-\u201c, \u201cMare Nubium\u201d, \u201cMare Imbrium \/ Hadley\u201d, \u201cDescartes\u201d and \u201cTaurus-Littrow highlands\u201d. Finally, under the \u201cMain Accomplishment\u201d colun are the values, \u201cFirst humans to fly around the Moon\u201d, \u201cFirst spacecraft rendezvous in lunar orbit\u201d, \u201cFirst human landing on the Moon; 22 kg of samples returned\u201d, \u201cFirst Apollo Lunar Surface Experiment Package (ALSEP); visit to Surveyor 3 lander\u201d, \u201cLanding aborted due to explosion in command module\u201d, \u201cFirst \u2018rickshaw\u2019 on the Moon\u201d, \u201cFirst \u2018rover\u2019; visit to Hadley Rille; astronauts traveled 24 km\u201d, \u201cFirst landing in highlands; 95 kg of samples returned\u201d and \u201cGeologist among the crew; 111 kg of samples returned\u201d.\">\r\n<thead>\r\n<tr>\r\n<th colspan=\"4\">Figure 3. Apollo Flights to the Moon<\/th>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<th>Flight<\/th>\r\n<th>Date<\/th>\r\n<th>Landing Site<\/th>\r\n<th>Main Accomplishment<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr valign=\"top\">\r\n<td>Apollo\u00a08<\/td>\r\n<td>Dec.\u00a01968<\/td>\r\n<td>\u2014<\/td>\r\n<td>First humans to fly around the Moon<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Apollo\u00a010<\/td>\r\n<td>May\u00a01969<\/td>\r\n<td>\u2014<\/td>\r\n<td>First spacecraft rendezvous in lunar orbit<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Apollo\u00a011<\/td>\r\n<td>July\u00a01969<\/td>\r\n<td>Mare Tranquillitatis<\/td>\r\n<td>First human landing on the Moon; 22 kilograms of samples returned<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Apollo\u00a012<\/td>\r\n<td>Nov.\u00a01969<\/td>\r\n<td>Oceanus Procellarum<\/td>\r\n<td>First Apollo Lunar Surface Experiment Package (ALSEP); visit to Surveyor 3 lander<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Apollo\u00a013<\/td>\r\n<td>Apr.\u00a01970<\/td>\r\n<td>\u2014<\/td>\r\n<td>Landing aborted due to explosion in command module<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Apollo\u00a014<\/td>\r\n<td>Jan.\u00a01971<\/td>\r\n<td>Mare Nubium<\/td>\r\n<td>First \u201crickshaw\u201d on the Moon<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Apollo\u00a015<\/td>\r\n<td>July\u00a01971<\/td>\r\n<td>Mare Imbrium\/Hadley<\/td>\r\n<td>First \u201crover;\u201d visit to Hadley Rille; astronauts traveled 24 kilometers<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Apollo\u00a016<\/td>\r\n<td>Apr.\u00a01972<\/td>\r\n<td>Descartes<\/td>\r\n<td>First landing in highlands; 95 kilograms of samples returned<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Apollo\u00a017<\/td>\r\n<td>Dec.\u00a01972<\/td>\r\n<td>Taurus-Littrow highlands<\/td>\r\n<td>Geologist among the crew; 111 kilograms of samples returned<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<figure id=\"OSC_Astro_09_01_Scientist\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Scientist on the Moon.<\/strong><\/div>\r\n<figcaption><\/figcaption>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"487\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_09_01_Scientist-1.jpg\" alt=\"Scientist on the Moon. Photograph of geologist Harrison Schmitt collecting samples near a large boulder on the Lunar surface. Schmitt is at lower left, dwarfed by the massive boulder.\" width=\"487\" height=\"491\" \/> <strong>Figure 4.<\/strong> Geologist (and later US senator) Harrison \u201cJack\u201d Schmitt in front of a large boulder in the Littrow Valley at the edge of the lunar highlands. Note how black the sky is on the airless Moon. No stars are visible because the surface is brightly lit by the Sun, and the exposure therefore is not long enough to reveal stars.[\/caption]<\/figure>\r\n<p id=\"fs-id1170326542486\">In addition to landing on the lunar surface and studying it at close range, the Apollo missions accomplished three objectives of major importance for lunar science. First, the astronauts collected nearly 400 kilograms of samples for detailed laboratory analysis on Earth as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_09_01_MoonRock\">Figure 5<\/a>. These samples have revealed as much about the Moon and its history as all other lunar studies combined. Second, each Apollo landing after the first one deployed an Apollo Lunar Surface Experiment Package (ALSEP), which continued to operate for years after the astronauts departed. Third, the orbiting Apollo command modules carried a wide range of instruments to photograph and analyze the lunar surface from above.<\/p>\r\n\r\n<figure id=\"OSC_Astro_09_01_MoonRock\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Handling Moon Rocks.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"487\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_09_01_MoonRock-1.jpg\" alt=\"Photograph of Technicians Examining a Lunar Sample. Two scientists look at a Moon rock through the glass of a scientific \u201cglove box\u201d.\" width=\"487\" height=\"382\" \/> <strong>Figure 5.<\/strong> Lunar samples collected in the Apollo Project are analyzed and stored in NASA facilities at the Johnson Space Center in Houston, Texas. Here, a technician examines a rock sample using gloves in a sealed environment to avoid contaminating the sample. (credit: NASA JSC)[\/caption]<\/figure>\r\n<p id=\"fs-id1170326498390\">The last human left the Moon in December 1972, just a little more than three years after Neil Armstrong took his \u201cgiant leap for mankind.\u201d The program of lunar exploration was cut off midstride due to political and economic pressures. It had cost just about ?100 per American, spread over 10 years\u2014the equivalent of one large pizza per person per year. Yet for many people, the Moon landings were one of the central events in twentieth-century history.<\/p>\r\n<p id=\"fs-id1170326203298\">The giant Apollo rockets built to travel to the Moon were left to rust on the lawns of NASA centers in Florida, Texas, and Alabama, although recently, some have at least been moved indoors to museums as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_09_01_Rocket\">Figure 6<\/a>. Today, neither NASA nor Russia have plans to send astronauts to the Moon, and China appears to be the nation most likely to attempt this feat. (In a bizarre piece of irony, a few people even question whether we went to the Moon at all, proposing instead that the Apollo program was a fake, filmed on a Hollywood sound stage. See the <a href=\"#fs-id1170323819906\">Link to Learning<\/a> box below for some scientists\u2019 replies to such claims.) However, scientific interest in the Moon is stronger than ever, and more than half a dozen scientific spacecraft\u2014sent from NASA, ESA, Japan, India, and China\u2014have orbited or landed on our nearest neighbor during the past decade.<\/p>\r\n\r\n<div id=\"fs-id1170323819906\" class=\"note astronomy link-to-learning\">\r\n<div class=\"textbox shaded\">Read <a href=\"https:\/\/pirlwww.lpl.arizona.edu\/~jscotti\/NOT_faked\/\">The Great Moon Hoax<\/a> about the claim that NASA never succeeded in putting people on the Moon.<\/div>\r\n<\/div>\r\n<figure id=\"OSC_Astro_09_01_Rocket\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Moon Rocket on Display.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"731\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_09_01_Rocket-1.jpg\" alt=\"Moon Rocket on Display. Visitors stand in front of the five thrust nozzles of a massive Saturn 5 rocket outside NASA\u2019s Johnson Space Center in Houston.\" width=\"731\" height=\"549\" \/> <strong>Figure 6.<\/strong> One of the unused Saturn 5 rockets built to go to the Moon is now a tourist attraction at NASA\u2019s Johnson Space Center in Houston, although it has been moved indoors since this photo was taken. (credit: modification of work by David Morrison)[\/caption]<\/figure>\r\n<p id=\"fs-id1170326569797\">Lunar exploration has become an international enterprise with many robotic spacecraft focusing on lunar science. The USSR sent a number in the 1960s, including robot sample returns. <a class=\"autogenerated-content\" href=\"#fs-id1170326243317\">Figure 7<\/a> lists some of the most recent <span class=\"no-emphasis\">lunar missions<\/span>.<\/p>\r\n\r\n<table id=\"fs-id1170326243317\" class=\"span-all\" summary=\"This table is titled \u201cSome International Missions to the Moon\u201d and contains 4 columns and 13 rows. The first row is a header row and it labels each column, \u201cLaunch Year\u201d, \u201cSpacecraft\u201d, \u201cType of Mission\u201d and \u201cAgency\u201d. Under the \u201cLaunch Year\u201d column are the values, \u201c1994\u201d, \u201c1998\u201d, \u201c2003\u201d, \u201c2007\u201d, \u201c2007\u201d, \u201c2008\u201d, \u201c2009\u201d, \u201c2009\u201d, \u201c2010\u201d, \u201c2011\u201d, \u201c2013\u201d and \u201c2013\u201d. Under the \u201cSpacecraft\u201d column are the values, \u201cClementine\u201d, \u201cLunar Prospector\u201d, \u201cSMART-1\u201d, \u201cSELENE 1\u201d, \u201cChang\u2019e 1\u201d, \u201cChandrayaan-1\u201d, \u201cLRO\u201d, \u201cLCROSS\u201d, \u201cChang\u2019e 2\u201d, \u201cGRAIL\u201d, \u201cLADEE\u201d and \u201cChang\u2019e 3\u201d. Under the \u201cType of Mission\u201d column are the values, \u201cOrbiter\u201d, \u201cOrbiter\u201d, \u201cOrbiter\u201d, \u201cOrbiter\u201d, \u201cOrbiter\u201d, \u201cOrbiter\u201d, \u201cOrbiter\u201d, \u201cImpactor\u201d, \u201cOrbiter\u201d, \u201cTwin orbiters\u201d, \u201cOrbiter\u201d and \u201cLander\/Rover\u201d. Finally, under the \u201cAgancy\u201d column are the values, \u201cUS (USAF\/NASA)\u201d, \u201cUS (NASA)\u201d, \u201cEurope (ESA)\u201d, \u201cJapan (JAXA)\u201d, \u201cChina (CNSA)\u201d, \u201cIndia (ISRO)\u201d, \u201cUS (NASA)\u201d, \u201cUS (NASA)\u201d, \u201cChina (CNSA)\u201d, \u201cUS (NASA)\u201d, \u201cUS (NASA)\u201d and \u201cChina (CNSA)\u201d.\">\r\n<thead>\r\n<tr>\r\n<th style=\"width: 571.28125px\" colspan=\"4\">Figure 7. Some International Missions to the Moon<\/th>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<th style=\"width: 110px\">Launch Year<\/th>\r\n<th style=\"width: 135px\">Spacecraft<\/th>\r\n<th style=\"width: 130px\">Type of Mission<\/th>\r\n<th style=\"width: 148px\">Agency<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr valign=\"top\">\r\n<td style=\"width: 110px\">1994<\/td>\r\n<td style=\"width: 135px\">Clementine<\/td>\r\n<td style=\"width: 130px\">Orbiter<\/td>\r\n<td style=\"width: 148px\">US (USAF\/NASA)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td style=\"width: 110px\">1998<\/td>\r\n<td style=\"width: 135px\">Lunar Prospector<\/td>\r\n<td style=\"width: 130px\">Orbiter<\/td>\r\n<td style=\"width: 148px\">US (NASA)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td style=\"width: 110px\">2003<\/td>\r\n<td style=\"width: 135px\">SMART-1<\/td>\r\n<td style=\"width: 130px\">Orbiter<\/td>\r\n<td style=\"width: 148px\">Europe (ESA)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td style=\"width: 110px\">2007<\/td>\r\n<td style=\"width: 135px\">SELENE 1<\/td>\r\n<td style=\"width: 130px\">Orbiter<\/td>\r\n<td style=\"width: 148px\">Japan (JAXA)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td style=\"width: 110px\">2007<\/td>\r\n<td style=\"width: 135px\">Chang\u2019e 1<\/td>\r\n<td style=\"width: 130px\">Orbiter<\/td>\r\n<td style=\"width: 148px\">China (CNSA)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td style=\"width: 110px\">2008<\/td>\r\n<td style=\"width: 135px\">Chandrayaan-1<\/td>\r\n<td style=\"width: 130px\">Orbiter<\/td>\r\n<td style=\"width: 148px\">India (ISRO)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td style=\"width: 110px\">2009<\/td>\r\n<td style=\"width: 135px\">LRO<\/td>\r\n<td style=\"width: 130px\">Orbiter<\/td>\r\n<td style=\"width: 148px\">US (NASA)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td style=\"width: 110px\">2009<\/td>\r\n<td style=\"width: 135px\">LCROSS<\/td>\r\n<td style=\"width: 130px\">Impactor<\/td>\r\n<td style=\"width: 148px\">US (NASA)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td style=\"width: 110px\">2010<\/td>\r\n<td style=\"width: 135px\">Chang\u2019e 2<\/td>\r\n<td style=\"width: 130px\">Orbiter<\/td>\r\n<td style=\"width: 148px\">China (CNSA)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td style=\"width: 110px\">2011<\/td>\r\n<td style=\"width: 135px\">GRAIL<\/td>\r\n<td style=\"width: 130px\">Twin orbiters<\/td>\r\n<td style=\"width: 148px\">US (NASA)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td style=\"width: 110px\">2013<\/td>\r\n<td style=\"width: 135px\">LADEE<\/td>\r\n<td style=\"width: 130px\">Orbiter<\/td>\r\n<td style=\"width: 148px\">US (NASA)<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td style=\"width: 110px\">2013<\/td>\r\n<td style=\"width: 135px\">Chang\u2019e 3<\/td>\r\n<td style=\"width: 130px\">Lander\/Rover<\/td>\r\n<td style=\"width: 148px\">China (CNSA)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 110px\">2019<\/td>\r\n<td style=\"width: 135px\">Chandrayaan-2<\/td>\r\n<td style=\"width: 130px\">Lander<\/td>\r\n<td style=\"width: 148px\">India<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 110px\"><\/td>\r\n<td style=\"width: 135px\"><\/td>\r\n<td style=\"width: 130px\"><\/td>\r\n<td style=\"width: 148px\"><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/section><section id=\"fs-id1170326160434\">\r\n<h1>Composition and Structure of the Moon<\/h1>\r\n<p id=\"fs-id1170324016363\">The composition of the <span class=\"no-emphasis\">Moon<\/span> is not the same as that of Earth. With an average density of only 3.3 g\/cm<sup>3<\/sup>, the Moon must be made almost entirely of silicate rock. Compared to Earth, it is depleted in iron and other metals. It is as if the Moon were composed of the same silicates as Earth\u2019s mantle and crust, with the metals and the volatiles selectively removed. These differences in composition between Earth and Moon provide important clues about the origin of the Moon, a topic we will cover in detail later in this chapter.<\/p>\r\n<p id=\"fs-id1170326037233\">Studies of the <span class=\"no-emphasis\">Moon\u2019s interior<\/span> carried out with seismometers taken to the Moon as part of the Apollo program confirm the absence of a large metal core. The twin GRAIL spacecraft launched into lunar orbit in 2011 provided even more precise tracking of the interior structure. We also know from the study of lunar samples that water and other volatiles have been depleted from the lunar crust. The tiny amounts of water detected in these samples were originally attributed to small leaks in the container seal that admitted water vapor from Earth\u2019s atmosphere. However, scientists have now concluded that some chemically bound water is present in the lunar rocks.<\/p>\r\n<p id=\"fs-id1170326563475\">Most dramatically, water ice has been detected in permanently shadowed craters near the lunar poles. In 2009, NASA crashed a small spacecraft called the Lunar Crater Observation and Sensing Satellite (LCROSS) into the crater Cabeus near the Moon\u2019s south pole. The impact at 9,000 kilometers per hour released energy equivalent to 2 tons of dynamite, blasting a plume of water vapor and other chemicals high above the surface. This plume was visible to telescopes in orbit around the Moon, and the LCROSS spacecraft itself made measurements as it flew through the plume. A NASA spacecraft called the Lunar Reconnaissance Orbiter (LRO) also measured the very low temperatures inside several lunar craters, and its sensitive cameras were even able to image crater interiors by starlight.<\/p>\r\n<p id=\"fs-id1170326111910\">The total quantity of water ice in the Moon\u2019s polar craters is estimated to be hundreds of billions of tons. As liquid, this would only be enough water to fill a lake 100 miles across, but compared with the rest of the dry lunar crust, so much water is remarkable. Presumably, this polar water was carried to the Moon by comets and asteroids that hit its surface. Some small fraction of the water froze in a few extremely cold regions (cold traps) where the Sun never shines, such as the bottom of deep craters at the Moon\u2019s poles. One reason this discovery could be important is that it raises the possibility of future human habitation near the lunar poles, or even of a lunar base as a way-station on routes to Mars and the rest of the solar system. If the ice could be mined, it would yield both water and oxygen for human support, and it could be broken down into hydrogen and oxygen, a potent rocket fuel.<\/p>\r\n\r\n<\/section><section id=\"fs-id1170326445667\" class=\"summary\">\r\n<h1>Key Concepts and Summary<\/h1>\r\n<p id=\"fs-id1170326111929\">Most of what we know about the Moon derives from the Apollo program, including 400 kilograms of lunar samples still being intensively studied. The Moon has one-eightieth the mass of Earth and is severely depleted in both metals and volatile materials. It is made almost entirely of silicates like those in Earth\u2019s mantle and crust. However, more recent spacecraft have found evidence of a small amount of water near the lunar poles, most likely deposited by comet and asteroid impacts.<\/p>\r\n\r\n<\/section>","rendered":"<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<p id=\"fs-id1168584078329\">By the end of this section, you will be able to:<\/p>\n<ul id=\"fs-id1170326087619\">\n<li>Discuss what has been learned from both manned and robotic <span class=\"no-emphasis\">lunar exploration<\/span><\/li>\n<li>Describe the composition and structure of the Moon<\/li>\n<\/ul>\n<\/div>\n<p>The <span class=\"no-emphasis\">Moon<\/span> has only one-eightieth the mass of Earth and about one-sixth Earth\u2019s surface gravity\u2014too low to retain an atmosphere. The Moon may be seen in <a class=\"autogenerated-content\" href=\"#OSC_Astro_09_02_Moon\">Figure 1<\/a>. Moving molecules of a gas can escape from a planet just the way a rocket does, and the lower the gravity, the easier it is for the gas to leak away into space. While the Moon can acquire a temporary atmosphere from impacting comets, this atmosphere is quickly lost by freezing onto the surface or by escape to surrounding space. The Moon today is dramatically deficient in a wide range of <em>volatiles<\/em>, those elements and compounds that evaporate at relatively low temperatures. Some of the <span class=\"no-emphasis\">Moon\u2019s properties<\/span> are summarized in <a class=\"autogenerated-content\" href=\"#fs-id1170324252656\">Figure 2<\/a>, along with comparative values for <span class=\"no-emphasis\">Mercury<\/span>.<\/p>\n<figure id=\"OSC_Astro_09_02_Moon\">\n<div class=\"title\" style=\"text-align: center\"><strong>Two Sides of the Moon.<\/strong><\/div>\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_09_02_Moon-1.jpg\" alt=\"The Two Sides of the Moon. The left image shows part of the Moon\u2019s hemisphere that faces Earth; several dark maria and rayed craters are visible. The right image shows part of the Moon that faces away from Earth; it is dominated by highlands and is more heavily cratered.\" width=\"975\" height=\"459\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 1.<\/strong> The left image shows part of the hemisphere that faces Earth; several dark maria are visible. The right image shows part of the hemisphere that faces away from Earth; it is dominated by highlands. The resolution of this image is several kilometers, similar to that of high-powered binoculars or a small telescope. (credit: modification of work by NASA\/GSFC\/Arizona State University)<\/figcaption><\/figure>\n<\/figure>\n<table id=\"fs-id1170324252656\" class=\"span-all aligncenter\" summary=\"This table is titled \u201cProperties of the Moon and Mercury\u201d and contains 3 columns and 8 rows. The first row is a header row and it labels each column, \u201cProperty\u201d, \u201cMoon\u201d and \u201cMercury\u201d. Under the \u201cProperty\u201d column are the values, \u201cMass (Earth = 1)\u201d, \u201cDiameter (km)\u201d, \u201cDensity (g\/cm3)\u201d, \u201cSurface gravity (Earth = 1)\u201d, \u201cEscape velocity (km\/s)\u201d, \u201cRotation period (days)\u201d and \u201cSurface area (Earth = 1)\u201d. Under the \u201cMoon\u201d column are the values, \u201c0.0123\u201d, \u201c3476\u201d, \u201c3.3\u201d, \u201c0.17\u201d, \u201c2.4\u201d, \u201c27.3\u201d and \u201c0.27\u201d. Finally, under the \u201cMercury\u201d column are the values, \u201c0.055\u201d, \u201c4878\u201d, \u201c5.4\u201d, \u201c0.38\u201d, \u201c4.3\u201d, \u201c58.65\u201d and \u201c0.38\u201d.\">\n<thead>\n<tr>\n<th colspan=\"3\">Figure 2. Properties of the Moon and Mercury<\/th>\n<\/tr>\n<tr valign=\"top\">\n<th>Property<\/th>\n<th>Moon<\/th>\n<th>Mercury<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr valign=\"top\">\n<td>Mass (Earth = 1)<\/td>\n<td>0.0123<\/td>\n<td>0.055<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Diameter (km)<\/td>\n<td>3476<\/td>\n<td>4878<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Density (g\/cm<sup>3<\/sup>)<\/td>\n<td>3.3<\/td>\n<td>5.4<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Surface gravity (Earth = 1)<\/td>\n<td>0.17<\/td>\n<td>0.38<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Escape velocity (km\/s)<\/td>\n<td>2.4<\/td>\n<td>4.3<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Rotation period (days)<\/td>\n<td>27.3<\/td>\n<td>58.65<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Surface area (Earth = 1)<\/td>\n<td>0.27<\/td>\n<td>0.38<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<section id=\"fs-id1170326319309\">\n<h1>Exploration of the Moon<\/h1>\n<p id=\"fs-id1170326160178\">Most of what we know about the Moon today derives from the US <span class=\"no-emphasis\">Apollo program<\/span>, which sent nine piloted spacecraft to our satellite between 1968 and 1972, landing 12 astronauts on its surface. Before the era of spacecraft studies, astronomers had mapped the side of the Moon that faces Earth with telescopic resolution of about 1 kilometer, but lunar geology hardly existed as a scientific subject. All that changed beginning in the early 1960s. Initially, Russia took the lead in lunar exploration with Luna 3, which returned the first photos of the lunar far side in 1959, and then with Luna 9, which landed on the surface in 1966 and transmitted pictures and other data to Earth. However, these efforts were overshadowed on July 20, 1969, when the first American astronaut set foot on the Moon.<\/p>\n<p id=\"fs-id1170326068257\"><a class=\"autogenerated-content\" href=\"#fs-id1170326114334\">Figure 3<\/a> summarizes the nine Apollo flights: six that landed and three others that circled the Moon but did not land. The initial landings were on flat plains selected for safety reasons. But with increasing experience and confidence, NASA targeted the last three missions to more geologically interesting locales. The level of scientific exploration also increased with each mission, as the astronauts spent longer times on the Moon and carried more elaborate equipment. Finally, on the last Apollo landing, NASA included one scientist, geologist Jack Schmitt, among the astronauts. He is shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_09_01_Scientist\">Figure 4<\/a>.<\/p>\n<table id=\"fs-id1170326114334\" class=\"span-all\" summary=\"This table is titled \u201cApollo flights to the Moon\u201d and contains 4 columns and 10 rows. The first row is a header row and it labels each column, \u201cFlight\u201d, \u201cDate\u201d, \u201cLanding Site\u201d and \u201cMain Accomplishment\u201d. Under the \u201cFlight\u201d column are the values, \u201cApollo 8\u201d, \u201cApollo 10\u201d, \u201cApollo 11\u201d, \u201cApollo 12\u201d, \u201cApollo 13\u201d, \u201cApollo 14\u201d, \u201cApollo 15\u201d, \u201cApollo 16\u201d and \u201cApollo 17\u201d. Under the \u201cDate\u201d column are the values, \u201cDec. 1968\u201d, \u201cMay 1969\u201d, \u201cJuly 1969\u201d, \u201cNov. 1969\u201d, \u201cApr. 1970\u201d, \u201cJan. 1971\u201d, \u201cJuly 1971\u201d, \u201cApr. 1972\u201d and \u201cDec. 1972\u201d. Under the \u201cLanding Site\u201d column are the values, \u201c-\u201c, \u201c-\u201c, \u201cMare Tranquillitatis\u201d, \u201cOceanus Procellarum\u201d, \u201c-\u201c, \u201cMare Nubium\u201d, \u201cMare Imbrium \/ Hadley\u201d, \u201cDescartes\u201d and \u201cTaurus-Littrow highlands\u201d. Finally, under the \u201cMain Accomplishment\u201d colun are the values, \u201cFirst humans to fly around the Moon\u201d, \u201cFirst spacecraft rendezvous in lunar orbit\u201d, \u201cFirst human landing on the Moon; 22 kg of samples returned\u201d, \u201cFirst Apollo Lunar Surface Experiment Package (ALSEP); visit to Surveyor 3 lander\u201d, \u201cLanding aborted due to explosion in command module\u201d, \u201cFirst \u2018rickshaw\u2019 on the Moon\u201d, \u201cFirst \u2018rover\u2019; visit to Hadley Rille; astronauts traveled 24 km\u201d, \u201cFirst landing in highlands; 95 kg of samples returned\u201d and \u201cGeologist among the crew; 111 kg of samples returned\u201d.\">\n<thead>\n<tr>\n<th colspan=\"4\">Figure 3. Apollo Flights to the Moon<\/th>\n<\/tr>\n<tr valign=\"top\">\n<th>Flight<\/th>\n<th>Date<\/th>\n<th>Landing Site<\/th>\n<th>Main Accomplishment<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr valign=\"top\">\n<td>Apollo\u00a08<\/td>\n<td>Dec.\u00a01968<\/td>\n<td>\u2014<\/td>\n<td>First humans to fly around the Moon<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Apollo\u00a010<\/td>\n<td>May\u00a01969<\/td>\n<td>\u2014<\/td>\n<td>First spacecraft rendezvous in lunar orbit<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Apollo\u00a011<\/td>\n<td>July\u00a01969<\/td>\n<td>Mare Tranquillitatis<\/td>\n<td>First human landing on the Moon; 22 kilograms of samples returned<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Apollo\u00a012<\/td>\n<td>Nov.\u00a01969<\/td>\n<td>Oceanus Procellarum<\/td>\n<td>First Apollo Lunar Surface Experiment Package (ALSEP); visit to Surveyor 3 lander<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Apollo\u00a013<\/td>\n<td>Apr.\u00a01970<\/td>\n<td>\u2014<\/td>\n<td>Landing aborted due to explosion in command module<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Apollo\u00a014<\/td>\n<td>Jan.\u00a01971<\/td>\n<td>Mare Nubium<\/td>\n<td>First \u201crickshaw\u201d on the Moon<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Apollo\u00a015<\/td>\n<td>July\u00a01971<\/td>\n<td>Mare Imbrium\/Hadley<\/td>\n<td>First \u201crover;\u201d visit to Hadley Rille; astronauts traveled 24 kilometers<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Apollo\u00a016<\/td>\n<td>Apr.\u00a01972<\/td>\n<td>Descartes<\/td>\n<td>First landing in highlands; 95 kilograms of samples returned<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Apollo\u00a017<\/td>\n<td>Dec.\u00a01972<\/td>\n<td>Taurus-Littrow highlands<\/td>\n<td>Geologist among the crew; 111 kilograms of samples returned<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<figure id=\"OSC_Astro_09_01_Scientist\">\n<div class=\"title\" style=\"text-align: center\"><strong>Scientist on the Moon.<\/strong><\/div><figcaption><\/figcaption><figure style=\"width: 487px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_09_01_Scientist-1.jpg\" alt=\"Scientist on the Moon. Photograph of geologist Harrison Schmitt collecting samples near a large boulder on the Lunar surface. Schmitt is at lower left, dwarfed by the massive boulder.\" width=\"487\" height=\"491\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 4.<\/strong> Geologist (and later US senator) Harrison \u201cJack\u201d Schmitt in front of a large boulder in the Littrow Valley at the edge of the lunar highlands. Note how black the sky is on the airless Moon. No stars are visible because the surface is brightly lit by the Sun, and the exposure therefore is not long enough to reveal stars.<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1170326542486\">In addition to landing on the lunar surface and studying it at close range, the Apollo missions accomplished three objectives of major importance for lunar science. First, the astronauts collected nearly 400 kilograms of samples for detailed laboratory analysis on Earth as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_09_01_MoonRock\">Figure 5<\/a>. These samples have revealed as much about the Moon and its history as all other lunar studies combined. Second, each Apollo landing after the first one deployed an Apollo Lunar Surface Experiment Package (ALSEP), which continued to operate for years after the astronauts departed. Third, the orbiting Apollo command modules carried a wide range of instruments to photograph and analyze the lunar surface from above.<\/p>\n<figure id=\"OSC_Astro_09_01_MoonRock\">\n<div class=\"title\" style=\"text-align: center\"><strong>Handling Moon Rocks.<\/strong><\/div>\n<figure style=\"width: 487px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_09_01_MoonRock-1.jpg\" alt=\"Photograph of Technicians Examining a Lunar Sample. Two scientists look at a Moon rock through the glass of a scientific \u201cglove box\u201d.\" width=\"487\" height=\"382\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 5.<\/strong> Lunar samples collected in the Apollo Project are analyzed and stored in NASA facilities at the Johnson Space Center in Houston, Texas. Here, a technician examines a rock sample using gloves in a sealed environment to avoid contaminating the sample. (credit: NASA JSC)<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1170326498390\">The last human left the Moon in December 1972, just a little more than three years after Neil Armstrong took his \u201cgiant leap for mankind.\u201d The program of lunar exploration was cut off midstride due to political and economic pressures. It had cost just about ?100 per American, spread over 10 years\u2014the equivalent of one large pizza per person per year. Yet for many people, the Moon landings were one of the central events in twentieth-century history.<\/p>\n<p id=\"fs-id1170326203298\">The giant Apollo rockets built to travel to the Moon were left to rust on the lawns of NASA centers in Florida, Texas, and Alabama, although recently, some have at least been moved indoors to museums as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_09_01_Rocket\">Figure 6<\/a>. Today, neither NASA nor Russia have plans to send astronauts to the Moon, and China appears to be the nation most likely to attempt this feat. (In a bizarre piece of irony, a few people even question whether we went to the Moon at all, proposing instead that the Apollo program was a fake, filmed on a Hollywood sound stage. See the <a href=\"#fs-id1170323819906\">Link to Learning<\/a> box below for some scientists\u2019 replies to such claims.) However, scientific interest in the Moon is stronger than ever, and more than half a dozen scientific spacecraft\u2014sent from NASA, ESA, Japan, India, and China\u2014have orbited or landed on our nearest neighbor during the past decade.<\/p>\n<div id=\"fs-id1170323819906\" class=\"note astronomy link-to-learning\">\n<div class=\"textbox shaded\">Read <a href=\"https:\/\/pirlwww.lpl.arizona.edu\/~jscotti\/NOT_faked\/\">The Great Moon Hoax<\/a> about the claim that NASA never succeeded in putting people on the Moon.<\/div>\n<\/div>\n<figure id=\"OSC_Astro_09_01_Rocket\">\n<div class=\"title\" style=\"text-align: center\"><strong>Moon Rocket on Display.<\/strong><\/div>\n<figure style=\"width: 731px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_09_01_Rocket-1.jpg\" alt=\"Moon Rocket on Display. Visitors stand in front of the five thrust nozzles of a massive Saturn 5 rocket outside NASA\u2019s Johnson Space Center in Houston.\" width=\"731\" height=\"549\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 6.<\/strong> One of the unused Saturn 5 rockets built to go to the Moon is now a tourist attraction at NASA\u2019s Johnson Space Center in Houston, although it has been moved indoors since this photo was taken. (credit: modification of work by David Morrison)<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1170326569797\">Lunar exploration has become an international enterprise with many robotic spacecraft focusing on lunar science. The USSR sent a number in the 1960s, including robot sample returns. <a class=\"autogenerated-content\" href=\"#fs-id1170326243317\">Figure 7<\/a> lists some of the most recent <span class=\"no-emphasis\">lunar missions<\/span>.<\/p>\n<table id=\"fs-id1170326243317\" class=\"span-all\" summary=\"This table is titled \u201cSome International Missions to the Moon\u201d and contains 4 columns and 13 rows. The first row is a header row and it labels each column, \u201cLaunch Year\u201d, \u201cSpacecraft\u201d, \u201cType of Mission\u201d and \u201cAgency\u201d. Under the \u201cLaunch Year\u201d column are the values, \u201c1994\u201d, \u201c1998\u201d, \u201c2003\u201d, \u201c2007\u201d, \u201c2007\u201d, \u201c2008\u201d, \u201c2009\u201d, \u201c2009\u201d, \u201c2010\u201d, \u201c2011\u201d, \u201c2013\u201d and \u201c2013\u201d. Under the \u201cSpacecraft\u201d column are the values, \u201cClementine\u201d, \u201cLunar Prospector\u201d, \u201cSMART-1\u201d, \u201cSELENE 1\u201d, \u201cChang\u2019e 1\u201d, \u201cChandrayaan-1\u201d, \u201cLRO\u201d, \u201cLCROSS\u201d, \u201cChang\u2019e 2\u201d, \u201cGRAIL\u201d, \u201cLADEE\u201d and \u201cChang\u2019e 3\u201d. Under the \u201cType of Mission\u201d column are the values, \u201cOrbiter\u201d, \u201cOrbiter\u201d, \u201cOrbiter\u201d, \u201cOrbiter\u201d, \u201cOrbiter\u201d, \u201cOrbiter\u201d, \u201cOrbiter\u201d, \u201cImpactor\u201d, \u201cOrbiter\u201d, \u201cTwin orbiters\u201d, \u201cOrbiter\u201d and \u201cLander\/Rover\u201d. Finally, under the \u201cAgancy\u201d column are the values, \u201cUS (USAF\/NASA)\u201d, \u201cUS (NASA)\u201d, \u201cEurope (ESA)\u201d, \u201cJapan (JAXA)\u201d, \u201cChina (CNSA)\u201d, \u201cIndia (ISRO)\u201d, \u201cUS (NASA)\u201d, \u201cUS (NASA)\u201d, \u201cChina (CNSA)\u201d, \u201cUS (NASA)\u201d, \u201cUS (NASA)\u201d and \u201cChina (CNSA)\u201d.\">\n<thead>\n<tr>\n<th style=\"width: 571.28125px\" colspan=\"4\">Figure 7. Some International Missions to the Moon<\/th>\n<\/tr>\n<tr valign=\"top\">\n<th style=\"width: 110px\">Launch Year<\/th>\n<th style=\"width: 135px\">Spacecraft<\/th>\n<th style=\"width: 130px\">Type of Mission<\/th>\n<th style=\"width: 148px\">Agency<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr valign=\"top\">\n<td style=\"width: 110px\">1994<\/td>\n<td style=\"width: 135px\">Clementine<\/td>\n<td style=\"width: 130px\">Orbiter<\/td>\n<td style=\"width: 148px\">US (USAF\/NASA)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td style=\"width: 110px\">1998<\/td>\n<td style=\"width: 135px\">Lunar Prospector<\/td>\n<td style=\"width: 130px\">Orbiter<\/td>\n<td style=\"width: 148px\">US (NASA)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td style=\"width: 110px\">2003<\/td>\n<td style=\"width: 135px\">SMART-1<\/td>\n<td style=\"width: 130px\">Orbiter<\/td>\n<td style=\"width: 148px\">Europe (ESA)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td style=\"width: 110px\">2007<\/td>\n<td style=\"width: 135px\">SELENE 1<\/td>\n<td style=\"width: 130px\">Orbiter<\/td>\n<td style=\"width: 148px\">Japan (JAXA)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td style=\"width: 110px\">2007<\/td>\n<td style=\"width: 135px\">Chang\u2019e 1<\/td>\n<td style=\"width: 130px\">Orbiter<\/td>\n<td style=\"width: 148px\">China (CNSA)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td style=\"width: 110px\">2008<\/td>\n<td style=\"width: 135px\">Chandrayaan-1<\/td>\n<td style=\"width: 130px\">Orbiter<\/td>\n<td style=\"width: 148px\">India (ISRO)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td style=\"width: 110px\">2009<\/td>\n<td style=\"width: 135px\">LRO<\/td>\n<td style=\"width: 130px\">Orbiter<\/td>\n<td style=\"width: 148px\">US (NASA)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td style=\"width: 110px\">2009<\/td>\n<td style=\"width: 135px\">LCROSS<\/td>\n<td style=\"width: 130px\">Impactor<\/td>\n<td style=\"width: 148px\">US (NASA)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td style=\"width: 110px\">2010<\/td>\n<td style=\"width: 135px\">Chang\u2019e 2<\/td>\n<td style=\"width: 130px\">Orbiter<\/td>\n<td style=\"width: 148px\">China (CNSA)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td style=\"width: 110px\">2011<\/td>\n<td style=\"width: 135px\">GRAIL<\/td>\n<td style=\"width: 130px\">Twin orbiters<\/td>\n<td style=\"width: 148px\">US (NASA)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td style=\"width: 110px\">2013<\/td>\n<td style=\"width: 135px\">LADEE<\/td>\n<td style=\"width: 130px\">Orbiter<\/td>\n<td style=\"width: 148px\">US (NASA)<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td style=\"width: 110px\">2013<\/td>\n<td style=\"width: 135px\">Chang\u2019e 3<\/td>\n<td style=\"width: 130px\">Lander\/Rover<\/td>\n<td style=\"width: 148px\">China (CNSA)<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 110px\">2019<\/td>\n<td style=\"width: 135px\">Chandrayaan-2<\/td>\n<td style=\"width: 130px\">Lander<\/td>\n<td style=\"width: 148px\">India<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 110px\"><\/td>\n<td style=\"width: 135px\"><\/td>\n<td style=\"width: 130px\"><\/td>\n<td style=\"width: 148px\"><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/section>\n<section id=\"fs-id1170326160434\">\n<h1>Composition and Structure of the Moon<\/h1>\n<p id=\"fs-id1170324016363\">The composition of the <span class=\"no-emphasis\">Moon<\/span> is not the same as that of Earth. With an average density of only 3.3 g\/cm<sup>3<\/sup>, the Moon must be made almost entirely of silicate rock. Compared to Earth, it is depleted in iron and other metals. It is as if the Moon were composed of the same silicates as Earth\u2019s mantle and crust, with the metals and the volatiles selectively removed. These differences in composition between Earth and Moon provide important clues about the origin of the Moon, a topic we will cover in detail later in this chapter.<\/p>\n<p id=\"fs-id1170326037233\">Studies of the <span class=\"no-emphasis\">Moon\u2019s interior<\/span> carried out with seismometers taken to the Moon as part of the Apollo program confirm the absence of a large metal core. The twin GRAIL spacecraft launched into lunar orbit in 2011 provided even more precise tracking of the interior structure. We also know from the study of lunar samples that water and other volatiles have been depleted from the lunar crust. The tiny amounts of water detected in these samples were originally attributed to small leaks in the container seal that admitted water vapor from Earth\u2019s atmosphere. However, scientists have now concluded that some chemically bound water is present in the lunar rocks.<\/p>\n<p id=\"fs-id1170326563475\">Most dramatically, water ice has been detected in permanently shadowed craters near the lunar poles. In 2009, NASA crashed a small spacecraft called the Lunar Crater Observation and Sensing Satellite (LCROSS) into the crater Cabeus near the Moon\u2019s south pole. The impact at 9,000 kilometers per hour released energy equivalent to 2 tons of dynamite, blasting a plume of water vapor and other chemicals high above the surface. This plume was visible to telescopes in orbit around the Moon, and the LCROSS spacecraft itself made measurements as it flew through the plume. A NASA spacecraft called the Lunar Reconnaissance Orbiter (LRO) also measured the very low temperatures inside several lunar craters, and its sensitive cameras were even able to image crater interiors by starlight.<\/p>\n<p id=\"fs-id1170326111910\">The total quantity of water ice in the Moon\u2019s polar craters is estimated to be hundreds of billions of tons. As liquid, this would only be enough water to fill a lake 100 miles across, but compared with the rest of the dry lunar crust, so much water is remarkable. Presumably, this polar water was carried to the Moon by comets and asteroids that hit its surface. Some small fraction of the water froze in a few extremely cold regions (cold traps) where the Sun never shines, such as the bottom of deep craters at the Moon\u2019s poles. One reason this discovery could be important is that it raises the possibility of future human habitation near the lunar poles, or even of a lunar base as a way-station on routes to Mars and the rest of the solar system. If the ice could be mined, it would yield both water and oxygen for human support, and it could be broken down into hydrogen and oxygen, a potent rocket fuel.<\/p>\n<\/section>\n<section id=\"fs-id1170326445667\" class=\"summary\">\n<h1>Key Concepts and Summary<\/h1>\n<p id=\"fs-id1170326111929\">Most of what we know about the Moon derives from the Apollo program, including 400 kilograms of lunar samples still being intensively studied. The Moon has one-eightieth the mass of Earth and is severely depleted in both metals and volatile materials. It is made almost entirely of silicates like those in Earth\u2019s mantle and crust. However, more recent spacecraft have found evidence of a small amount of water near the lunar poles, most likely deposited by comet and asteroid impacts.<\/p>\n<\/section>\n","protected":false},"author":9,"menu_order":2,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-267","chapter","type-chapter","status-publish","hentry"],"part":260,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters\/267","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/users\/9"}],"version-history":[{"count":8,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters\/267\/revisions"}],"predecessor-version":[{"id":2764,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters\/267\/revisions\/2764"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/parts\/260"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters\/267\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/media?parent=267"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapter-type?post=267"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/contributor?post=267"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/license?post=267"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}