{"id":365,"date":"2017-05-15T12:13:47","date_gmt":"2017-05-15T16:13:47","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/chapter\/9-5-mercury\/"},"modified":"2018-06-02T12:38:48","modified_gmt":"2018-06-02T16:38:48","slug":"9-5-mercury","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/chapter\/9-5-mercury\/","title":{"raw":"9.5 Mercury","rendered":"9.5 Mercury"},"content":{"raw":"<div class=\"bcc-box bcc-highlight\"><h3>Learning Objectives<\/h3><p id=\"fs-id1168583455898\">By the end of this section, you will be able to:\n\n<ul id=\"fs-id1170324280821\"><li>Characterize the orbit of Mercury around the Sun<\/li><li>Describe Mercury\u2019s structure and composition<\/li><li>Explain the relationship between Mercury\u2019s orbit and rotation<\/li><li>Describe the topography and features of Mercury\u2019s surface<\/li><li>Summarize our ideas about the origin and evolution of Mercury<\/li><\/ul><\/div>The planet <span class=\"no-emphasis\">Mercury<\/span> is similar to the Moon in many ways. Like the Moon, it has no atmosphere, and its surface is heavily cratered. As described later in this chapter, it also shares with the Moon the likelihood of a violent birth.\n\n<section><h1>Mercury\u2019s Orbit<\/h1><p id=\"fs-id1170326290877\">Mercury is the nearest planet to the Sun, and, in accordance with Kepler\u2019s third law, it has the shortest period of revolution about the Sun (88 of our days) and the highest average orbital speed (48 kilometers per second). It is appropriately named for the fleet-footed messenger god of the Romans. Because Mercury remains close to the Sun, it can be difficult to pick out in the sky. As you might expect, it\u2019s best seen when its eccentric orbit takes it as far from the Sun as possible.\n\n<p id=\"fs-id1170326037300\">The semimajor axis of <span class=\"no-emphasis\">Mercury\u2019s orbit<\/span>\u2014that is, the planet\u2019s average distance from the Sun\u2014is 58 million kilometers, or 0.39 AU. However, because its orbit has the high eccentricity of 0.206, Mercury\u2019s actual distance from the Sun varies from 46 million kilometers at perihelion to 70 million kilometers at aphelion (the ideas and terms that describe orbits were introduced in <a href=\"\/contents\/e2171dce-08e7-4d5d-9f87-ae6d454b6831\" class=\"target-chapter\">Orbits and Gravity<\/a>).\n\n<\/section><section id=\"fs-id1170326273973\"><h1>Composition and Structure<\/h1><p id=\"fs-id1170326275662\">Mercury\u2019s mass is one-eighth that of Earth, making it the smallest terrestrial planet. Mercury is the smallest planet (except for the dwarf planets), having a diameter of 4878 kilometers, less than half that of Earth. Mercury\u2019s density is 5.4 g\/cm<sup>3<\/sup>, much greater than the density of the Moon, indicating that the composition of those two objects differs substantially.\n\n<p id=\"fs-id1170321648838\"><span class=\"no-emphasis\">Mercury\u2019s composition<\/span> is one of the most interesting things about it and makes it unique among the planets. Mercury\u2019s high density tells us that it must be composed largely of heavier materials such as metals. The most likely models for Mercury\u2019s interior suggest a metallic iron-nickel core amounting to 60% of the total mass, with the rest of the planet made up primarily of silicates. The core has a diameter of 3500 kilometers and extends out to within 700 kilometers of the surface. We could think of Mercury as a metal ball the size of the Moon surrounded by a rocky crust 700 kilometers thick (<a href=\"#OSC_Astro_09_05_Mercury\" class=\"autogenerated-content\">[link]<\/a>). Unlike the Moon, Mercury does have a weak magnetic field. The existence of this field is consistent with the presence of a large metal core, and it suggests that at least part of the core must be liquid in order to generate the observed magnetic field.<a name=\"footnote-ref1\" href=\"#footnote1\"><sup>1<\/sup><\/a>\n\n<figure id=\"OSC_Astro_09_05_Mercury\"><div class=\"title\" style=\"text-align: center\"><strong>Mercury\u2019s Internal Structure.<\/strong><\/div>[caption id=\"\" align=\"aligncenter\" width=\"383\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astrojbrewer\/wp-content\/uploads\/sites\/468\/2017\/05\/OSC_Astro_09_05_Mercury-1.jpg\" alt=\"Cutaway Illustration of Mercury. This diagram shows the huge metallic core of Mercury as a yellow sphere surrounded by the thin, rocky crust drawn in light red.\" width=\"383\" height=\"383\"> <strong>Figure 1.<\/strong> The interior of Mercury is dominated by a metallic core about the same size as our Moon.[\/caption]<\/figure><div class=\"example\"><div class=\"textbox shaded\"><p id=\"fs-id1170326063512\"><strong>Densities of Worlds<\/strong>\nThe average density of a body equals its mass divided by its volume. For a sphere, density is:\n\n<div class=\"equation unnumbered\">$$\\text{density}=\\frac{\\text{mass}}{\\frac{4}{3}\\pi{R}^{3}}$$<\/div>Astronomers can measure both mass and radius accurately when a spacecraft flies by a body.\n\n<p id=\"fs-id1170326558642\">Using the information in this chapter, we can calculate the approximate average density of the Moon.\n\n<p id=\"fs-id1170326106123\"><strong>Solution<\/strong>\nFor a sphere,\n\n<div id=\"fs-id1170326578283\" class=\"equation unnumbered\">$$\\text{density}=\\frac{\\text{mass}}{\\frac{4}{3}\\pi{R}^{3}}=\\frac{7.35\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}{10}^{22}\\phantom{\\rule{0.2em}{0ex}}\\text{kg}}{4.2\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}5.2\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}{10}^{18}\\phantom{\\rule{0.2em}{0ex}}{\\text{m}}^{3}}=3.4\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}{10}^{3}\\phantom{\\rule{0.2em}{0ex}}{\\text{kg\/m}}^{3}$$<\/div><div class=\"equation unnumbered\"><\/div><p id=\"fs-id1170326487901\"><a href=\"\/contents\/fa6f6f04-2ea7-4c0a-b792-565198157034#fs-id1170324252656\" class=\"autogenerated-content\">[link]<\/a> gives a value of 3.3 g\/cm<sup>3<\/sup>, which is 3.3 \u00d7 10<sup>3<\/sup> kg\/m<sup>3<\/sup>.\n\n<strong>Check Your Learning<\/strong>\nUsing the information in this chapter, calculate the average density of Mercury. Show your work. Does your calculation agree with the figure we give in this chapter?\n\n<div class=\"note\"><div class=\"title\"><strong>Answer:<\/strong><\/div><p id=\"fs-id1164754811280\">$$\\text{density}=\\frac{\\text{mass}}{\\frac{4}{3}\\pi{R}^{3}}=\\frac{3.3\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}{10}^{23}\\phantom{\\rule{0.2em}{0ex}}\\text{kg}}{4.2\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}1.45\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}{10}^{19}\\phantom{\\rule{0.2em}{0ex}}{\\text{m}}^{3}}=5.4\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}{10}^{3}\\phantom{\\rule{0.2em}{0ex}}{\\text{kg\/m}}^{3}$$\n\n<\/div><p id=\"fs-id1170326133891\">That matches the value given in <a href=\"\/contents\/fa6f6f04-2ea7-4c0a-b792-565198157034#fs-id1170324252656\" class=\"autogenerated-content\">[link]<\/a> when g\/cm<sup>3<\/sup> is converted into kg\/m<sup>3<\/sup>.\n\n<\/div>&nbsp;\n\n<\/div><\/section><section id=\"fs-id1170326146611\"><h1>Mercury\u2019s Strange Rotation<\/h1>Visual studies of <span class=\"no-emphasis\">Mercury<\/span>\u2019s indistinct surface markings were once thought to indicate that the planet kept one face to the Sun (as the Moon does to Earth). Thus, for many years, it was widely believed that <span class=\"no-emphasis\">Mercury\u2019s rotation<\/span> period was equal to its revolution period of 88 days, making one side perpetually hot while the other was always cold.\n\nRadar observations of Mercury in the mid-1960s, however, showed conclusively that Mercury does not keep one side fixed toward the Sun. If a planet is turning, one side seems to be approaching Earth while the other is moving away from it. The resulting Doppler shift spreads or broadens the precise transmitted radar-wave frequency into a range of frequencies in the reflected signal (<a href=\"#OSC_Astro_09_05_Doppler\" class=\"autogenerated-content\">[link]<\/a>). The degree of broadening provides an exact measurement of the rotation rate of the planet.\n\n<figure id=\"OSC_Astro_09_05_Doppler\"><div class=\"title\" style=\"text-align: center\"><strong>Doppler Radar Measures Rotation.<\/strong><\/div>[caption id=\"\" align=\"aligncenter\" width=\"669\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astrojbrewer\/wp-content\/uploads\/sites\/468\/2017\/05\/OSC_Astro_09_05_Doppler-1.jpg\" alt=\"Illustration of How Doppler Radar Measures Rotation. At left is a drawing of a portion of the Earth, with an oversized radar dish on the surface pointing upward toward a target planet to the right. A transmitted signal is drawn leaving the dish toward the planet. The reflected signal from the planet is shown as five arrows pointing back in the direction of Earth. At far right are four panels plotting radar intensity versus frequency, with frequency increasing toward the right. The upper panel, labeled \u201cTransmitted signal\u201d, shows the transmitted signal as a tall, narrow spike. The panel below, labeled \u201cReturn with no rotation\u201d, plots the return signal if the target planet did not rotate: it is a tall, narrow spike just like the transmitted signal. The next panel is labeled \u201cReturn with slow rotation\u201d. This curve is wider at the base and not as tall as the previous curves. The bottom panel is labeled \u201cReturn with faster rotation\u201d. This curve is very wide at the base and much shorter than the previous plots.\" width=\"669\" height=\"444\"> <strong>Figure 2.<\/strong> When a radar beam is reflected from a rotating planet, the motion of one side of the planet\u2019s disk toward us and the other side away from us causes Doppler shifts in the reflected signal. The effect is to cause both a redshift and a blueshift, widening the spread of frequencies in the radio beam.[\/caption]<\/figure><p id=\"fs-id1170326245084\">Mercury\u2019s period of rotation (how long it takes to turn with respect to the distant stars) is 59 days, which is just two-thirds of the planet\u2019s period of revolution. Subsequently, astronomers found that a situation where the spin and the orbit of a planet (its year) are in a 2:3 ratio turns out to be stable. (See <a href=\"#fs-id1170326024292\" class=\"autogenerated-content\">[link]<\/a> for more on the effects of having such a long day on Mercury.)\n\n<p id=\"fs-id1170324155047\">Mercury, being close to the Sun, is very hot on its daylight side; but because it has no appreciable atmosphere, it gets surprisingly cold during the long nights. The temperature on the surface climbs to 700 K (430 \u00b0C) at noontime. After sunset, however, the temperature drops, reaching 100 K (\u2013170 \u00b0C) just before dawn. (It is even colder in craters near the poles that receive no sunlight at all.) The range in temperature on Mercury is thus 600 K (or 600 \u00b0C), a greater difference than on any other planet.\n\n<div id=\"fs-id1170326024292\" class=\"note astronomy making-connections\"><div class=\"textbox shaded\"><div class=\"title\"><strong>What a Difference a Day Makes<\/strong><\/div><p id=\"fs-id1170326137292\">Mercury rotates three times for each two orbits around the Sun. It is the only planet that exhibits this relationship between its spin and its orbit, and there are some interesting consequences for any observers who might someday be stationed on the surface of Mercury.\n\n<p id=\"fs-id1170326137361\">Here on Earth, we take for granted that days are much shorter than years. Therefore, the two astronomical ways of defining the local \u201cday\u201d\u2014how long the planet takes to rotate and how long the Sun takes to return to the same position in the sky\u2014are the same on Earth for most practical purposes. But this is not the case on Mercury. While Mercury rotates (spins once) in 59 Earth days, the time for the Sun to return to the same place in Mercury\u2019s sky turns out to be two Mercury years, or 176 Earth days. (Note that this result is not intuitively obvious, so don\u2019t be upset if you didn\u2019t come up with it.) Thus, if one day at noon a Mercury explorer suggests to her companion that they should meet at noon the next day, this could mean a very long time apart!\n\n<p id=\"fs-id1170326142449\">To make things even more interesting, recall that Mercury has an eccentric orbit, meaning that its distance from the Sun varies significantly during each mercurian year. By Kepler\u2019s law, the planet moves fastest in its orbit when closest to the Sun. Let\u2019s examine how this affects the way we would see the Sun in the sky during one 176-Earth-day cycle. We\u2019ll look at the situation as if we were standing on the surface of Mercury in the center of a giant basin that astronomers call Caloris (<a href=\"#OSC_Astro_09_05_Basin\" class=\"autogenerated-content\">[link]<\/a>).\n\n<p id=\"fs-id1170323907842\">At the location of Caloris, Mercury is most distant from the Sun at sunrise; this means the rising Sun looks smaller in the sky (although still more than twice the size it appears from Earth). As the Sun rises higher and higher, it looks bigger and bigger; Mercury is now getting closer to the Sun in its eccentric orbit. At the same time, the apparent motion of the Sun slows down as Mercury\u2019s faster motion in orbit begins to catch up with its rotation.\n\n<p id=\"fs-id1170324155275\">At noon, the Sun is now three times larger than it looks from Earth and hangs almost motionless in the sky. As the afternoon wears on, the Sun appears smaller and smaller, and moves faster and faster in the sky. At sunset, a full Mercury year (or 88 Earth days after sunrise), the Sun is back to its smallest apparent size as it dips out of sight. Then it takes another Mercury year before the Sun rises again. (By the way, sunrises and sunsets are much more sudden on Mercury, since there is no atmosphere to bend or scatter the rays of sunlight.)\n\n<p id=\"fs-id1170326314575\">Astronomers call locations like the Caloris Basin the \u201chot longitudes\u201d on Mercury because the Sun is closest to the planet at noon, just when it is lingering overhead for many Earth days. This makes these areas the hottest places on Mercury.\n\n<p id=\"fs-id1170324076403\">We bring all this up not because the exact details of this scenario are so important but to illustrate how many of the things we take for granted on Earth are not the same on other worlds. As we\u2019ve mentioned before, one of the best things about taking an astronomy class should be ridding you forever of any \u201cEarth chauvinism\u201d you might have. The way things are on our planet is just one of the many ways nature can arrange reality.\n\n<\/div><\/div><\/section><section id=\"fs-id1170326129853\"><h1>The Surface of Mercury<\/h1><p id=\"fs-id1170326274756\">The first close-up look at <span class=\"no-emphasis\">Mercury<\/span> came in 1974, when the US spacecraft Mariner 10 passed 9500 kilometers from the surface of the planet and transmitted more than 2000 photographs to Earth, revealing details with a resolution down to 150 meters. Subsequently, the planet was mapped in great detail by the <span class=\"no-emphasis\">MESSENGER<\/span> spacecraft, which was launched in 2004 and made multiple flybys of Earth, Venus, and Mercury before settling into orbit around Mercury in 2011. It ended its life in 2015, when it was commanded to crash into the surface of the planet.\n\n<p id=\"fs-id1170326524111\"><span class=\"no-emphasis\">Mercury\u2019s surface<\/span> strongly resembles the Moon in appearance (<a href=\"#OSC_Astro_09_05_MercuryTopo\" class=\"autogenerated-content\">[link]<\/a> and <a href=\"#OSC_Astro_09_05_Basin\" class=\"autogenerated-content\">[link]<\/a>). It is covered with thousands of craters and larger basins up to 1300 kilometers in diameter. Some of the brighter craters are rayed, like Tycho and Copernicus on the Moon, and many have central peaks. There are also <em>scarps<\/em> (cliffs) more than a kilometer high and hundreds of kilometers long, as well as ridges and plains.\n\n<p id=\"fs-id1170326449545\">MESSENGER instruments measured the surface composition and mapped past volcanic activity. One of its most important discoveries was the verification of water ice (first detected by radar) in craters near the poles, similar to the situation on the Moon, and the unexpected discovery of organic (carbon-rich) compounds mixed with the water ice.\n\n<div id=\"fs-id1170324139787\" class=\"note astronomy link-to-learning\"><div class=\"textbox shaded\">Scientists working with data from the <a href=\"https:\/\/openstax.org\/l\/30MESSmercuryrt\">MESSENGER mission<\/a> put together a rotating globe of Mercury, in false color, showing some of the variations in the composition of the planet\u2019s surface. You can watch it spin.<\/div>&nbsp;\n\n<\/div><figure id=\"OSC_Astro_09_05_MercuryTopo\"><div class=\"title\" style=\"text-align: center\"><strong>Mercury\u2019s Topography.<\/strong><\/div>[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astrojbrewer\/wp-content\/uploads\/sites\/468\/2017\/05\/OSC_Astro_09_05_MercuryTopo-1.jpg\" alt=\"False Color Image of Mercury\u2019s Topography. Data from the MESSENGER spacecraft was used to compile this detailed image of Mercury\u2019s northern hemisphere. The lowest regions are shown in purple and blue, and the highest regions are shown in red. The difference in elevation between the lowest and highest regions shown here is roughly 10 kilometers.\" width=\"975\" height=\"504\"> <strong>Figure 3.<\/strong> The topography of Mercury\u2019s northern hemisphere is mapped in great detail from MESSENGER data. The lowest regions are shown in purple and blue, and the highest regions are shown in red. The difference in elevation between the lowest and highest regions shown here is roughly 10 kilometers. The permanently shadowed low-lying craters near the north pole contain radar-bright water ice. (credit: modification of work by NASA\/Johns Hopkins University Applied Physics Laboratory\/Carnegie Institution of Washington)[\/caption]<\/figure><figure id=\"OSC_Astro_09_05_Basin\"><div class=\"title\" style=\"text-align: center\"><strong>Caloris Basin.<\/strong><\/div><figcaption><\/figcaption>[caption id=\"\" align=\"aligncenter\" width=\"325\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astrojbrewer\/wp-content\/uploads\/sites\/468\/2017\/05\/OSC_Astro_09_05_Basin-1.jpg\" alt=\"Photograph of the Caloris Basin on Mercury. The circular, flat plain of Caloris Basin is surrounded by cratered highlands and rough terrain. A few impact craters are scattered over the smooth surface of the basin.\" width=\"325\" height=\"325\"> <strong>Figure 4.<\/strong> This partially flooded impact basin is the largest known structural feature on Mercury. The smooth plains in the interior of the basin have an area of almost two million square kilometers. Compare this photo with [link], the Orientale Basin on the Moon. (credit: NASA\/Johns Hopkins University Applied Physics Laboratory\/Carnegie Institution of Washington)[\/caption]<\/figure><p id=\"fs-id1170326120983\">Most of the mercurian features have been named in honor of artists, writers, composers, and other contributors to the arts and humanities, in contrast with the scientists commemorated on the Moon. Among the named craters are Bach, Shakespeare, Tolstoy, Van Gogh, and Scott Joplin.\n\nThere is no evidence of plate tectonics on Mercury. However, the planet\u2019s distinctive long <span class=\"no-emphasis\">scarps<\/span> can sometimes be seen cutting across craters; this means the scarps must have formed later than the craters (<a href=\"#OSC_Astro_09_05_Discovery\" class=\"autogenerated-content\">[link]<\/a>). These long, curved cliffs appear to have their origin in the slight compression of Mercury\u2019s crust. Apparently, at some point in its history, the planet shrank, wrinkling the crust, and it must have done so after most of the craters on its surface had already formed.\n\n<p id=\"fs-id1170323797705\">If the standard cratering chronology applies to Mercury, this shrinkage must have taken place during the last 4 billion years and not during the solar system\u2019s early period of heavy bombardment.\n\n<figure id=\"OSC_Astro_09_05_Discovery\"><div class=\"title\" style=\"text-align: center\"><strong>Discovery Scarp on Mercury.<\/strong><\/div>[caption id=\"\" align=\"aligncenter\" width=\"487\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astrojbrewer\/wp-content\/uploads\/sites\/468\/2017\/05\/OSC_Astro_09_05_Discovery-1.jpg\" alt=\"Large Scarp on Mercury. This long cliff structure crosses both flat lands and craters as it extends down the right hand side of this image.\" width=\"487\" height=\"495\"> <strong>Figure 5.<\/strong> This long cliff, nearly 1 kilometer high and more than 100 kilometers long, cuts across several craters. Astronomers conclude that the compression that made \u201cwrinkles\u201d like this in the plank\u2019s surface must have taken place after the craters were formed. (credit: modification of work by NASA\/JPL\/Northwestern University)[\/caption]<\/figure><\/section><section id=\"fs-id1170326036946\"><h1>The Origin of Mercury<\/h1><p id=\"fs-id1170326273827\">The problem with understanding how <span class=\"no-emphasis\">Mercury<\/span> formed is the reverse of the problem posed by the composition of the Moon. We have seen that, unlike the Moon, Mercury is composed mostly of metal. However, astronomers think that Mercury should have formed with roughly the same ratio of metal to silicate as that found on Earth or Venus. How did it lose so much of its rocky material?\n\n<p id=\"fs-id1170326036144\">The most probable explanation for Mercury\u2019s silicate loss may be similar to the explanation for the Moon\u2019s lack of a metal core. Mercury is likely to have experienced several giant impacts very early in its youth, and one or more of these may have torn away a fraction of its mantle and crust, leaving a body dominated by its iron core.\n\n<div id=\"fs-id1170326088167\" class=\"note astronomy link-to-learning\"><div class=\"textbox shaded\">You can follow some of <a href=\"https:\/\/openstax.org\/l\/30NASAresmercu\">NASA\u2019s latest research on Mercury<\/a> and see some helpful animations on the MESSENGER web page.<\/div><\/div><p id=\"fs-id1170326283206\">Today, astronomers recognize that the early solar system was a chaotic place, with the final stages of planet formation characterized by impacts of great violence. Some objects of planetary mass have been destroyed, whereas others could have fragmented and then re-formed, perhaps more than once. Both the Moon and Mercury, with their strange compositions, bear testimony to the catastrophes that must have characterized the solar system during its youth.\n\n<\/section><section id=\"fs-id1170323910953\" class=\"summary\"><h1>Key Concepts and Summary<\/h1><p id=\"fs-id1170326114606\">Mercury is the nearest planet to the Sun and the fastest moving. Mercury is similar to the Moon in having a heavily cratered surface and no atmosphere, but it differs in having a very large metal core. Early in its evolution, it apparently lost part of its silicate mantle, probably due to one or more giant impacts. Long scarps on its surface testify to a global compression of Mercury\u2019s crust during the past 4 billion years.\n\n<\/section><section id=\"fs-id1170324108804\" class=\"further-exploration\"><h1>For Further Exploration<\/h1><section id=\"fs-id1170326091444\"><h2>Articles<\/h2><section id=\"fs-id1170326023791\"><h3><em>The Moon<\/em><\/h3><p id=\"fs-id1170324010712\">Bakich, Michael. \u201cAsia\u2019s New Assault on the Moon.\u201d <em>Astronomy<\/em> (August 2009): 50. The Japanese Selene and Chinese Chang\u2019e 1 missions.\n\n<p id=\"fs-id1170326204621\">Beatty, J. \u201cNASA Slams the Moon.\u201d <em>Sky &amp; Telescope<\/em> (February 2010): 28. The impact of the LCROSS mission on the Moon and what we learned from it.\n\n<p id=\"fs-id1170326169395\">Bell, T. \u201cWarning: Dust Ahead.\u201d <em>Astronomy<\/em> (March 2006): 46. What we know about lunar dust and the problems it can cause.\n\nDorminey, B. \u201cSecrets beneath the Moon\u2019s Surface.\u201d <em>Astronomy<\/em> (March 2011): 24. A nice timeline of the Moon\u2019s evolution and the story of how we are finding out more about its internal structure.\n\n<p id=\"fs-id1170326274468\">Jayawardhana, R. \u201cDeconstructing the Moon.\u201d <em>Astronomy<\/em> (September 1998): 40. An update on the giant impact hypothesis for forming the Moon.\n\n<p id=\"fs-id1170326450526\">Register, B. \u201cThe Fate of the Moon Rocks.\u201d <em>Astronomy<\/em> (December 1985): 15. What was done with the rocks the astronauts brought back from the Moon.\n\n<p id=\"fs-id1170326024132\">Schmitt, H. \u201cExploring Taurus\u2013Littrow: Apollo 17.\u201d <em>National Geographic<\/em> (September 1973). First-person account given by the only scientist to walk on the Moon.\n\n<p id=\"fs-id1170326056054\">Schmitt, H. \u201cFrom the Moon to Mars.\u201d <em>Scientific American<\/em> (July 2009): 36. The only scientist to walk on the Moon reflects on the science from Apollo and future missions to Mars.\n\n<p id=\"fs-id1170326118921\">Schultz, P. \u201cNew Clues to the Moon\u2019s Distant Past.\u201d <em>Astronomy<\/em> (December 2011): 34. Summary of results and ideas from the LCROSS and LRO missions.\n\n<p id=\"fs-id1170323821560\">Shirao, M. \u201cKayuga\u2019s High Def Highlights.\u201d <em>Sky &amp; Telescope<\/em> (February 2010): 20. Results from the Japanese mission to the Moon, with high definition TV cameras.\n\n<p id=\"fs-id1170326530121\">Wadhwa, M. \u201cWhat Are We Learning from the Moon Rocks?\u201d <em>Astronomy<\/em> (June 2013): 54. Very nice discussion of how the rocks tell us about Moon\u2019s composition, age, and origin.\n\n<p id=\"fs-id1170326035446\">Wood, Charles. \u201cThe Moon\u2019s Far Side: Nearly a New World.\u201d <em>Sky &amp; Telescope<\/em> (January 2007): 48. This article compares what we know about the two sides and why they are different.\n\n<p id=\"fs-id1170326093528\">Zimmerman, R. \u201cHow Much Water is on the Moon?\u201d <em>Astronomy<\/em> (January 2014): 50. Results from the LRO\u2019s instruments and good overview of issue.\n\n<\/section><section id=\"fs-id1170324015707\"><h3><em>Mercury<\/em><\/h3><p id=\"fs-id1170326419815\">Beatty, J. \u201cMercury Gets a Second Look.\u201d <em>Sky &amp; Telescope<\/em> (March 2009): 26. The October 2008 MESSENGER mission flyby.\n\n<p id=\"fs-id1170326287953\">Beatty, J. \u201cReunion with Mercury.\u201d <em>Sky &amp; Telescope<\/em> (May 2008): 24. The January 2008 MESSENGER encounter with Mercury.\n\n<p id=\"fs-id1170321783964\">\u201cMercury: Meet the Planet Nearest the Sun.\u201d <em>Sky &amp; Telescope<\/em> (March 2014): 39. Four-page pictorial introduction, including the new MESSENGER probe full map of the planet provided.\n\n<p id=\"fs-id1170326254441\">Oberg, J. \u201cTorrid Mercury\u2019s Icy Poles.\u201d <em>Astronomy<\/em> (December 2013): 30. A nice overview of results from MESSENGER mission, including the ice in polar craters.\n\n<p id=\"fs-id1170324016600\">Sheehan, W., and Dobbins, T. \u201cMesmerized by Mercury.\u201d <em>Sky &amp; Telescope<\/em> (June 2000): 109. History of Mercury observations and how amateur astronomers can contribute.\n\n<p id=\"fs-id1170323803403\">Talcott, R. \u201cSurprises from MESSENGER\u2019s Historic Mercury Fly-by.\u201d <em>Astronomy<\/em> (March 2009): 28.\n\n<p id=\"fs-id1170323845110\">Talcott, R. \u201cMercury Reveals its Hidden Side.\u201d <em>Astronomy<\/em> (May 2008): 26. Results and image from the MESSENGER mission flyby of January 2008.\n\n<\/section><\/section><section id=\"fs-id1170326295565\"><h2>Websites<\/h2><section id=\"fs-id1170326126801\"><h3><em>The Moon<\/em><\/h3><p id=\"fs-id1170326098030\">Apollo Lunar Surface Journal: http:\/\/www.hq.nasa.gov\/office\/pao\/History\/alsj\/. Information, interviews, maps, photos, video and audio clips, and much more on each of the Apollo landing missions.\n\n<p id=\"fs-id1170326256008\">Lunar &amp; Planetary Institute: http:\/\/www.lpi.usra.edu\/lunar\/missions\/. Lunar Science and Exploration web pages.\n\n<p id=\"fs-id1170326119274\">Lunar Reconnaissance Orbiter Mission Page: http:\/\/lro.gsfc.nasa.gov\/.\n\n<p id=\"fs-id1170326326771\">NASA\u2019s Guide to Moon Missions and Information: http:\/\/nssdc.gsfc.nasa.gov\/planetary\/planets\/moonpage.html.\n\n<p id=\"fs-id1170326177526\">Origin of the Moon: http:\/\/www.psi.edu\/projects\/moon\/moon.html. By William Hartmann, who, with a colleague, first suggested the giant impact hypothesis for how the Moon formed, in 1975.\n\n<p id=\"fs-id1170326335326\"><em>Sky &amp; Telescope<\/em> magazine\u2019s observing guides and articles about the Moon: http:\/\/www.skyandtelescope.com\/observing\/celestial-objects-to-watch\/moon\/.\n\n<p id=\"fs-id1170324010150\">To the Moon: http:\/\/www.pbs.org\/wgbh\/nova\/tothemoon\/. PBS program on the Apollo landings.\n\n<p id=\"fs-id1170326448981\">We Choose the Moon: http:\/\/wechoosethemoon.org\/. A recreation of the Apollo 11 mission.\n\n<\/section><section id=\"fs-id1170326275417\"><h3><em>Mercury<\/em><\/h3><p id=\"fs-id1170326145193\">Mercury Unveiled by G. Jeffrey Taylor (summarizing the Mariner 10 Mission): http:\/\/www.psrd.hawaii.edu\/Jan97\/MercuryUnveiled.html.\n\n<p id=\"fs-id1170326445532\">MESSENGER Mission Website: http:\/\/messenger.jhuapl.edu\/.\n\n<p id=\"fs-id1170324331311\">NASA Planetary Data Center Mercury Page: http:\/\/nssdc.gsfc.nasa.gov\/planetary\/planets\/mercurypage.html.\n\n<p id=\"fs-id1170326054102\">Views of the Solar System Mercury Page: http:\/\/solarviews.com\/eng\/mercury.htm.\n\n<\/section><\/section><\/section><section id=\"fs-id1170321585226\" class=\"group-activities\"><\/section><div><h2>Footnotes<\/h2><ol><li><a name=\"footnote1\" href=\"#footnote-ref1\">1<\/a> Recall from the <a href=\"\/contents\/bc5ce180-f2b3-4469-a882-22bf96435c99\" class=\"target-chapter\">Radiation and Spectra<\/a> chapter that magnetism is an effect of moving electric charges. In atoms of metals, the outer electrons are easier to dislodge and they can form a current when the metal is in liquid form and can flow.<\/li><\/ol><\/div>","rendered":"<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<p id=\"fs-id1168583455898\">By the end of this section, you will be able to:<\/p>\n<ul id=\"fs-id1170324280821\">\n<li>Characterize the orbit of Mercury around the Sun<\/li>\n<li>Describe Mercury\u2019s structure and composition<\/li>\n<li>Explain the relationship between Mercury\u2019s orbit and rotation<\/li>\n<li>Describe the topography and features of Mercury\u2019s surface<\/li>\n<li>Summarize our ideas about the origin and evolution of Mercury<\/li>\n<\/ul>\n<\/div>\n<p>The planet <span class=\"no-emphasis\">Mercury<\/span> is similar to the Moon in many ways. Like the Moon, it has no atmosphere, and its surface is heavily cratered. As described later in this chapter, it also shares with the Moon the likelihood of a violent birth.<\/p>\n<section>\n<h1>Mercury\u2019s Orbit<\/h1>\n<p id=\"fs-id1170326290877\">Mercury is the nearest planet to the Sun, and, in accordance with Kepler\u2019s third law, it has the shortest period of revolution about the Sun (88 of our days) and the highest average orbital speed (48 kilometers per second). It is appropriately named for the fleet-footed messenger god of the Romans. Because Mercury remains close to the Sun, it can be difficult to pick out in the sky. As you might expect, it\u2019s best seen when its eccentric orbit takes it as far from the Sun as possible.<\/p>\n<p id=\"fs-id1170326037300\">The semimajor axis of <span class=\"no-emphasis\">Mercury\u2019s orbit<\/span>\u2014that is, the planet\u2019s average distance from the Sun\u2014is 58 million kilometers, or 0.39 AU. However, because its orbit has the high eccentricity of 0.206, Mercury\u2019s actual distance from the Sun varies from 46 million kilometers at perihelion to 70 million kilometers at aphelion (the ideas and terms that describe orbits were introduced in <a href=\"\/contents\/e2171dce-08e7-4d5d-9f87-ae6d454b6831\" class=\"target-chapter\">Orbits and Gravity<\/a>).<\/p>\n<\/section>\n<section id=\"fs-id1170326273973\">\n<h1>Composition and Structure<\/h1>\n<p id=\"fs-id1170326275662\">Mercury\u2019s mass is one-eighth that of Earth, making it the smallest terrestrial planet. Mercury is the smallest planet (except for the dwarf planets), having a diameter of 4878 kilometers, less than half that of Earth. Mercury\u2019s density is 5.4 g\/cm<sup>3<\/sup>, much greater than the density of the Moon, indicating that the composition of those two objects differs substantially.<\/p>\n<p id=\"fs-id1170321648838\"><span class=\"no-emphasis\">Mercury\u2019s composition<\/span> is one of the most interesting things about it and makes it unique among the planets. Mercury\u2019s high density tells us that it must be composed largely of heavier materials such as metals. The most likely models for Mercury\u2019s interior suggest a metallic iron-nickel core amounting to 60% of the total mass, with the rest of the planet made up primarily of silicates. The core has a diameter of 3500 kilometers and extends out to within 700 kilometers of the surface. We could think of Mercury as a metal ball the size of the Moon surrounded by a rocky crust 700 kilometers thick (<a href=\"#OSC_Astro_09_05_Mercury\" class=\"autogenerated-content\">[link]<\/a>). Unlike the Moon, Mercury does have a weak magnetic field. The existence of this field is consistent with the presence of a large metal core, and it suggests that at least part of the core must be liquid in order to generate the observed magnetic field.<a name=\"footnote-ref1\" href=\"#footnote1\"><sup>1<\/sup><\/a><\/p>\n<figure id=\"OSC_Astro_09_05_Mercury\">\n<div class=\"title\" style=\"text-align: center\"><strong>Mercury\u2019s Internal Structure.<\/strong><\/div>\n<figure style=\"width: 383px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astrojbrewer\/wp-content\/uploads\/sites\/468\/2017\/05\/OSC_Astro_09_05_Mercury-1.jpg\" alt=\"Cutaway Illustration of Mercury. This diagram shows the huge metallic core of Mercury as a yellow sphere surrounded by the thin, rocky crust drawn in light red.\" width=\"383\" height=\"383\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 1.<\/strong> The interior of Mercury is dominated by a metallic core about the same size as our Moon.<\/figcaption><\/figure>\n<\/figure>\n<div class=\"example\">\n<div class=\"textbox shaded\">\n<p id=\"fs-id1170326063512\"><strong>Densities of Worlds<\/strong><br \/>\nThe average density of a body equals its mass divided by its volume. For a sphere, density is:<\/p>\n<div class=\"equation unnumbered\">$$\\text{density}=\\frac{\\text{mass}}{\\frac{4}{3}\\pi{R}^{3}}$$<\/div>\n<p>Astronomers can measure both mass and radius accurately when a spacecraft flies by a body.<\/p>\n<p id=\"fs-id1170326558642\">Using the information in this chapter, we can calculate the approximate average density of the Moon.<\/p>\n<p id=\"fs-id1170326106123\"><strong>Solution<\/strong><br \/>\nFor a sphere,<\/p>\n<div id=\"fs-id1170326578283\" class=\"equation unnumbered\">$$\\text{density}=\\frac{\\text{mass}}{\\frac{4}{3}\\pi{R}^{3}}=\\frac{7.35\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}{10}^{22}\\phantom{\\rule{0.2em}{0ex}}\\text{kg}}{4.2\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}5.2\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}{10}^{18}\\phantom{\\rule{0.2em}{0ex}}{\\text{m}}^{3}}=3.4\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}{10}^{3}\\phantom{\\rule{0.2em}{0ex}}{\\text{kg\/m}}^{3}$$<\/div>\n<div class=\"equation unnumbered\"><\/div>\n<p id=\"fs-id1170326487901\"><a href=\"\/contents\/fa6f6f04-2ea7-4c0a-b792-565198157034#fs-id1170324252656\" class=\"autogenerated-content\">[link]<\/a> gives a value of 3.3 g\/cm<sup>3<\/sup>, which is 3.3 \u00d7 10<sup>3<\/sup> kg\/m<sup>3<\/sup>.<\/p>\n<p><strong>Check Your Learning<\/strong><br \/>\nUsing the information in this chapter, calculate the average density of Mercury. Show your work. Does your calculation agree with the figure we give in this chapter?<\/p>\n<div class=\"note\">\n<div class=\"title\"><strong>Answer:<\/strong><\/div>\n<p id=\"fs-id1164754811280\">$$\\text{density}=\\frac{\\text{mass}}{\\frac{4}{3}\\pi{R}^{3}}=\\frac{3.3\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}{10}^{23}\\phantom{\\rule{0.2em}{0ex}}\\text{kg}}{4.2\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}1.45\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}{10}^{19}\\phantom{\\rule{0.2em}{0ex}}{\\text{m}}^{3}}=5.4\\phantom{\\rule{0.2em}{0ex}}\\times\\phantom{\\rule{0.2em}{0ex}}{10}^{3}\\phantom{\\rule{0.2em}{0ex}}{\\text{kg\/m}}^{3}$$<\/p>\n<\/div>\n<p id=\"fs-id1170326133891\">That matches the value given in <a href=\"\/contents\/fa6f6f04-2ea7-4c0a-b792-565198157034#fs-id1170324252656\" class=\"autogenerated-content\">[link]<\/a> when g\/cm<sup>3<\/sup> is converted into kg\/m<sup>3<\/sup>.<\/p>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<\/section>\n<section id=\"fs-id1170326146611\">\n<h1>Mercury\u2019s Strange Rotation<\/h1>\n<p>Visual studies of <span class=\"no-emphasis\">Mercury<\/span>\u2019s indistinct surface markings were once thought to indicate that the planet kept one face to the Sun (as the Moon does to Earth). Thus, for many years, it was widely believed that <span class=\"no-emphasis\">Mercury\u2019s rotation<\/span> period was equal to its revolution period of 88 days, making one side perpetually hot while the other was always cold.<\/p>\n<p>Radar observations of Mercury in the mid-1960s, however, showed conclusively that Mercury does not keep one side fixed toward the Sun. If a planet is turning, one side seems to be approaching Earth while the other is moving away from it. The resulting Doppler shift spreads or broadens the precise transmitted radar-wave frequency into a range of frequencies in the reflected signal (<a href=\"#OSC_Astro_09_05_Doppler\" class=\"autogenerated-content\">[link]<\/a>). The degree of broadening provides an exact measurement of the rotation rate of the planet.<\/p>\n<figure id=\"OSC_Astro_09_05_Doppler\">\n<div class=\"title\" style=\"text-align: center\"><strong>Doppler Radar Measures Rotation.<\/strong><\/div>\n<figure style=\"width: 669px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astrojbrewer\/wp-content\/uploads\/sites\/468\/2017\/05\/OSC_Astro_09_05_Doppler-1.jpg\" alt=\"Illustration of How Doppler Radar Measures Rotation. At left is a drawing of a portion of the Earth, with an oversized radar dish on the surface pointing upward toward a target planet to the right. A transmitted signal is drawn leaving the dish toward the planet. The reflected signal from the planet is shown as five arrows pointing back in the direction of Earth. At far right are four panels plotting radar intensity versus frequency, with frequency increasing toward the right. The upper panel, labeled \u201cTransmitted signal\u201d, shows the transmitted signal as a tall, narrow spike. The panel below, labeled \u201cReturn with no rotation\u201d, plots the return signal if the target planet did not rotate: it is a tall, narrow spike just like the transmitted signal. The next panel is labeled \u201cReturn with slow rotation\u201d. This curve is wider at the base and not as tall as the previous curves. The bottom panel is labeled \u201cReturn with faster rotation\u201d. This curve is very wide at the base and much shorter than the previous plots.\" width=\"669\" height=\"444\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 2.<\/strong> When a radar beam is reflected from a rotating planet, the motion of one side of the planet\u2019s disk toward us and the other side away from us causes Doppler shifts in the reflected signal. The effect is to cause both a redshift and a blueshift, widening the spread of frequencies in the radio beam.<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1170326245084\">Mercury\u2019s period of rotation (how long it takes to turn with respect to the distant stars) is 59 days, which is just two-thirds of the planet\u2019s period of revolution. Subsequently, astronomers found that a situation where the spin and the orbit of a planet (its year) are in a 2:3 ratio turns out to be stable. (See <a href=\"#fs-id1170326024292\" class=\"autogenerated-content\">[link]<\/a> for more on the effects of having such a long day on Mercury.)<\/p>\n<p id=\"fs-id1170324155047\">Mercury, being close to the Sun, is very hot on its daylight side; but because it has no appreciable atmosphere, it gets surprisingly cold during the long nights. The temperature on the surface climbs to 700 K (430 \u00b0C) at noontime. After sunset, however, the temperature drops, reaching 100 K (\u2013170 \u00b0C) just before dawn. (It is even colder in craters near the poles that receive no sunlight at all.) The range in temperature on Mercury is thus 600 K (or 600 \u00b0C), a greater difference than on any other planet.<\/p>\n<div id=\"fs-id1170326024292\" class=\"note astronomy making-connections\">\n<div class=\"textbox shaded\">\n<div class=\"title\"><strong>What a Difference a Day Makes<\/strong><\/div>\n<p id=\"fs-id1170326137292\">Mercury rotates three times for each two orbits around the Sun. It is the only planet that exhibits this relationship between its spin and its orbit, and there are some interesting consequences for any observers who might someday be stationed on the surface of Mercury.<\/p>\n<p id=\"fs-id1170326137361\">Here on Earth, we take for granted that days are much shorter than years. Therefore, the two astronomical ways of defining the local \u201cday\u201d\u2014how long the planet takes to rotate and how long the Sun takes to return to the same position in the sky\u2014are the same on Earth for most practical purposes. But this is not the case on Mercury. While Mercury rotates (spins once) in 59 Earth days, the time for the Sun to return to the same place in Mercury\u2019s sky turns out to be two Mercury years, or 176 Earth days. (Note that this result is not intuitively obvious, so don\u2019t be upset if you didn\u2019t come up with it.) Thus, if one day at noon a Mercury explorer suggests to her companion that they should meet at noon the next day, this could mean a very long time apart!<\/p>\n<p id=\"fs-id1170326142449\">To make things even more interesting, recall that Mercury has an eccentric orbit, meaning that its distance from the Sun varies significantly during each mercurian year. By Kepler\u2019s law, the planet moves fastest in its orbit when closest to the Sun. Let\u2019s examine how this affects the way we would see the Sun in the sky during one 176-Earth-day cycle. We\u2019ll look at the situation as if we were standing on the surface of Mercury in the center of a giant basin that astronomers call Caloris (<a href=\"#OSC_Astro_09_05_Basin\" class=\"autogenerated-content\">[link]<\/a>).<\/p>\n<p id=\"fs-id1170323907842\">At the location of Caloris, Mercury is most distant from the Sun at sunrise; this means the rising Sun looks smaller in the sky (although still more than twice the size it appears from Earth). As the Sun rises higher and higher, it looks bigger and bigger; Mercury is now getting closer to the Sun in its eccentric orbit. At the same time, the apparent motion of the Sun slows down as Mercury\u2019s faster motion in orbit begins to catch up with its rotation.<\/p>\n<p id=\"fs-id1170324155275\">At noon, the Sun is now three times larger than it looks from Earth and hangs almost motionless in the sky. As the afternoon wears on, the Sun appears smaller and smaller, and moves faster and faster in the sky. At sunset, a full Mercury year (or 88 Earth days after sunrise), the Sun is back to its smallest apparent size as it dips out of sight. Then it takes another Mercury year before the Sun rises again. (By the way, sunrises and sunsets are much more sudden on Mercury, since there is no atmosphere to bend or scatter the rays of sunlight.)<\/p>\n<p id=\"fs-id1170326314575\">Astronomers call locations like the Caloris Basin the \u201chot longitudes\u201d on Mercury because the Sun is closest to the planet at noon, just when it is lingering overhead for many Earth days. This makes these areas the hottest places on Mercury.<\/p>\n<p id=\"fs-id1170324076403\">We bring all this up not because the exact details of this scenario are so important but to illustrate how many of the things we take for granted on Earth are not the same on other worlds. As we\u2019ve mentioned before, one of the best things about taking an astronomy class should be ridding you forever of any \u201cEarth chauvinism\u201d you might have. The way things are on our planet is just one of the many ways nature can arrange reality.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section id=\"fs-id1170326129853\">\n<h1>The Surface of Mercury<\/h1>\n<p id=\"fs-id1170326274756\">The first close-up look at <span class=\"no-emphasis\">Mercury<\/span> came in 1974, when the US spacecraft Mariner 10 passed 9500 kilometers from the surface of the planet and transmitted more than 2000 photographs to Earth, revealing details with a resolution down to 150 meters. Subsequently, the planet was mapped in great detail by the <span class=\"no-emphasis\">MESSENGER<\/span> spacecraft, which was launched in 2004 and made multiple flybys of Earth, Venus, and Mercury before settling into orbit around Mercury in 2011. It ended its life in 2015, when it was commanded to crash into the surface of the planet.<\/p>\n<p id=\"fs-id1170326524111\"><span class=\"no-emphasis\">Mercury\u2019s surface<\/span> strongly resembles the Moon in appearance (<a href=\"#OSC_Astro_09_05_MercuryTopo\" class=\"autogenerated-content\">[link]<\/a> and <a href=\"#OSC_Astro_09_05_Basin\" class=\"autogenerated-content\">[link]<\/a>). It is covered with thousands of craters and larger basins up to 1300 kilometers in diameter. Some of the brighter craters are rayed, like Tycho and Copernicus on the Moon, and many have central peaks. There are also <em>scarps<\/em> (cliffs) more than a kilometer high and hundreds of kilometers long, as well as ridges and plains.<\/p>\n<p id=\"fs-id1170326449545\">MESSENGER instruments measured the surface composition and mapped past volcanic activity. One of its most important discoveries was the verification of water ice (first detected by radar) in craters near the poles, similar to the situation on the Moon, and the unexpected discovery of organic (carbon-rich) compounds mixed with the water ice.<\/p>\n<div id=\"fs-id1170324139787\" class=\"note astronomy link-to-learning\">\n<div class=\"textbox shaded\">Scientists working with data from the <a href=\"https:\/\/openstax.org\/l\/30MESSmercuryrt\">MESSENGER mission<\/a> put together a rotating globe of Mercury, in false color, showing some of the variations in the composition of the planet\u2019s surface. You can watch it spin.<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<figure id=\"OSC_Astro_09_05_MercuryTopo\">\n<div class=\"title\" style=\"text-align: center\"><strong>Mercury\u2019s Topography.<\/strong><\/div>\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astrojbrewer\/wp-content\/uploads\/sites\/468\/2017\/05\/OSC_Astro_09_05_MercuryTopo-1.jpg\" alt=\"False Color Image of Mercury\u2019s Topography. Data from the MESSENGER spacecraft was used to compile this detailed image of Mercury\u2019s northern hemisphere. The lowest regions are shown in purple and blue, and the highest regions are shown in red. The difference in elevation between the lowest and highest regions shown here is roughly 10 kilometers.\" width=\"975\" height=\"504\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 3.<\/strong> The topography of Mercury\u2019s northern hemisphere is mapped in great detail from MESSENGER data. The lowest regions are shown in purple and blue, and the highest regions are shown in red. The difference in elevation between the lowest and highest regions shown here is roughly 10 kilometers. The permanently shadowed low-lying craters near the north pole contain radar-bright water ice. (credit: modification of work by NASA\/Johns Hopkins University Applied Physics Laboratory\/Carnegie Institution of Washington)<\/figcaption><\/figure>\n<\/figure>\n<figure id=\"OSC_Astro_09_05_Basin\">\n<div class=\"title\" style=\"text-align: center\"><strong>Caloris Basin.<\/strong><\/div><figcaption><\/figcaption><figure style=\"width: 325px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astrojbrewer\/wp-content\/uploads\/sites\/468\/2017\/05\/OSC_Astro_09_05_Basin-1.jpg\" alt=\"Photograph of the Caloris Basin on Mercury. The circular, flat plain of Caloris Basin is surrounded by cratered highlands and rough terrain. A few impact craters are scattered over the smooth surface of the basin.\" width=\"325\" height=\"325\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 4.<\/strong> This partially flooded impact basin is the largest known structural feature on Mercury. The smooth plains in the interior of the basin have an area of almost two million square kilometers. Compare this photo with [link], the Orientale Basin on the Moon. (credit: NASA\/Johns Hopkins University Applied Physics Laboratory\/Carnegie Institution of Washington)<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1170326120983\">Most of the mercurian features have been named in honor of artists, writers, composers, and other contributors to the arts and humanities, in contrast with the scientists commemorated on the Moon. Among the named craters are Bach, Shakespeare, Tolstoy, Van Gogh, and Scott Joplin.<\/p>\n<p>There is no evidence of plate tectonics on Mercury. However, the planet\u2019s distinctive long <span class=\"no-emphasis\">scarps<\/span> can sometimes be seen cutting across craters; this means the scarps must have formed later than the craters (<a href=\"#OSC_Astro_09_05_Discovery\" class=\"autogenerated-content\">[link]<\/a>). These long, curved cliffs appear to have their origin in the slight compression of Mercury\u2019s crust. Apparently, at some point in its history, the planet shrank, wrinkling the crust, and it must have done so after most of the craters on its surface had already formed.<\/p>\n<p id=\"fs-id1170323797705\">If the standard cratering chronology applies to Mercury, this shrinkage must have taken place during the last 4 billion years and not during the solar system\u2019s early period of heavy bombardment.<\/p>\n<figure id=\"OSC_Astro_09_05_Discovery\">\n<div class=\"title\" style=\"text-align: center\"><strong>Discovery Scarp on Mercury.<\/strong><\/div>\n<figure style=\"width: 487px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astrojbrewer\/wp-content\/uploads\/sites\/468\/2017\/05\/OSC_Astro_09_05_Discovery-1.jpg\" alt=\"Large Scarp on Mercury. This long cliff structure crosses both flat lands and craters as it extends down the right hand side of this image.\" width=\"487\" height=\"495\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 5.<\/strong> This long cliff, nearly 1 kilometer high and more than 100 kilometers long, cuts across several craters. Astronomers conclude that the compression that made \u201cwrinkles\u201d like this in the plank\u2019s surface must have taken place after the craters were formed. (credit: modification of work by NASA\/JPL\/Northwestern University)<\/figcaption><\/figure>\n<\/figure>\n<\/section>\n<section id=\"fs-id1170326036946\">\n<h1>The Origin of Mercury<\/h1>\n<p id=\"fs-id1170326273827\">The problem with understanding how <span class=\"no-emphasis\">Mercury<\/span> formed is the reverse of the problem posed by the composition of the Moon. We have seen that, unlike the Moon, Mercury is composed mostly of metal. However, astronomers think that Mercury should have formed with roughly the same ratio of metal to silicate as that found on Earth or Venus. How did it lose so much of its rocky material?<\/p>\n<p id=\"fs-id1170326036144\">The most probable explanation for Mercury\u2019s silicate loss may be similar to the explanation for the Moon\u2019s lack of a metal core. Mercury is likely to have experienced several giant impacts very early in its youth, and one or more of these may have torn away a fraction of its mantle and crust, leaving a body dominated by its iron core.<\/p>\n<div id=\"fs-id1170326088167\" class=\"note astronomy link-to-learning\">\n<div class=\"textbox shaded\">You can follow some of <a href=\"https:\/\/openstax.org\/l\/30NASAresmercu\">NASA\u2019s latest research on Mercury<\/a> and see some helpful animations on the MESSENGER web page.<\/div>\n<\/div>\n<p id=\"fs-id1170326283206\">Today, astronomers recognize that the early solar system was a chaotic place, with the final stages of planet formation characterized by impacts of great violence. Some objects of planetary mass have been destroyed, whereas others could have fragmented and then re-formed, perhaps more than once. Both the Moon and Mercury, with their strange compositions, bear testimony to the catastrophes that must have characterized the solar system during its youth.<\/p>\n<\/section>\n<section id=\"fs-id1170323910953\" class=\"summary\">\n<h1>Key Concepts and Summary<\/h1>\n<p id=\"fs-id1170326114606\">Mercury is the nearest planet to the Sun and the fastest moving. Mercury is similar to the Moon in having a heavily cratered surface and no atmosphere, but it differs in having a very large metal core. Early in its evolution, it apparently lost part of its silicate mantle, probably due to one or more giant impacts. Long scarps on its surface testify to a global compression of Mercury\u2019s crust during the past 4 billion years.<\/p>\n<\/section>\n<section id=\"fs-id1170324108804\" class=\"further-exploration\">\n<h1>For Further Exploration<\/h1>\n<section id=\"fs-id1170326091444\">\n<h2>Articles<\/h2>\n<section id=\"fs-id1170326023791\">\n<h3><em>The Moon<\/em><\/h3>\n<p id=\"fs-id1170324010712\">Bakich, Michael. \u201cAsia\u2019s New Assault on the Moon.\u201d <em>Astronomy<\/em> (August 2009): 50. The Japanese Selene and Chinese Chang\u2019e 1 missions.<\/p>\n<p id=\"fs-id1170326204621\">Beatty, J. \u201cNASA Slams the Moon.\u201d <em>Sky &amp; Telescope<\/em> (February 2010): 28. The impact of the LCROSS mission on the Moon and what we learned from it.<\/p>\n<p id=\"fs-id1170326169395\">Bell, T. \u201cWarning: Dust Ahead.\u201d <em>Astronomy<\/em> (March 2006): 46. What we know about lunar dust and the problems it can cause.<\/p>\n<p>Dorminey, B. \u201cSecrets beneath the Moon\u2019s Surface.\u201d <em>Astronomy<\/em> (March 2011): 24. A nice timeline of the Moon\u2019s evolution and the story of how we are finding out more about its internal structure.<\/p>\n<p id=\"fs-id1170326274468\">Jayawardhana, R. \u201cDeconstructing the Moon.\u201d <em>Astronomy<\/em> (September 1998): 40. An update on the giant impact hypothesis for forming the Moon.<\/p>\n<p id=\"fs-id1170326450526\">Register, B. \u201cThe Fate of the Moon Rocks.\u201d <em>Astronomy<\/em> (December 1985): 15. What was done with the rocks the astronauts brought back from the Moon.<\/p>\n<p id=\"fs-id1170326024132\">Schmitt, H. \u201cExploring Taurus\u2013Littrow: Apollo 17.\u201d <em>National Geographic<\/em> (September 1973). First-person account given by the only scientist to walk on the Moon.<\/p>\n<p id=\"fs-id1170326056054\">Schmitt, H. \u201cFrom the Moon to Mars.\u201d <em>Scientific American<\/em> (July 2009): 36. The only scientist to walk on the Moon reflects on the science from Apollo and future missions to Mars.<\/p>\n<p id=\"fs-id1170326118921\">Schultz, P. \u201cNew Clues to the Moon\u2019s Distant Past.\u201d <em>Astronomy<\/em> (December 2011): 34. Summary of results and ideas from the LCROSS and LRO missions.<\/p>\n<p id=\"fs-id1170323821560\">Shirao, M. \u201cKayuga\u2019s High Def Highlights.\u201d <em>Sky &amp; Telescope<\/em> (February 2010): 20. Results from the Japanese mission to the Moon, with high definition TV cameras.<\/p>\n<p id=\"fs-id1170326530121\">Wadhwa, M. \u201cWhat Are We Learning from the Moon Rocks?\u201d <em>Astronomy<\/em> (June 2013): 54. Very nice discussion of how the rocks tell us about Moon\u2019s composition, age, and origin.<\/p>\n<p id=\"fs-id1170326035446\">Wood, Charles. \u201cThe Moon\u2019s Far Side: Nearly a New World.\u201d <em>Sky &amp; Telescope<\/em> (January 2007): 48. This article compares what we know about the two sides and why they are different.<\/p>\n<p id=\"fs-id1170326093528\">Zimmerman, R. \u201cHow Much Water is on the Moon?\u201d <em>Astronomy<\/em> (January 2014): 50. Results from the LRO\u2019s instruments and good overview of issue.<\/p>\n<\/section>\n<section id=\"fs-id1170324015707\">\n<h3><em>Mercury<\/em><\/h3>\n<p id=\"fs-id1170326419815\">Beatty, J. \u201cMercury Gets a Second Look.\u201d <em>Sky &amp; Telescope<\/em> (March 2009): 26. The October 2008 MESSENGER mission flyby.<\/p>\n<p id=\"fs-id1170326287953\">Beatty, J. \u201cReunion with Mercury.\u201d <em>Sky &amp; Telescope<\/em> (May 2008): 24. The January 2008 MESSENGER encounter with Mercury.<\/p>\n<p id=\"fs-id1170321783964\">\u201cMercury: Meet the Planet Nearest the Sun.\u201d <em>Sky &amp; Telescope<\/em> (March 2014): 39. Four-page pictorial introduction, including the new MESSENGER probe full map of the planet provided.<\/p>\n<p id=\"fs-id1170326254441\">Oberg, J. \u201cTorrid Mercury\u2019s Icy Poles.\u201d <em>Astronomy<\/em> (December 2013): 30. A nice overview of results from MESSENGER mission, including the ice in polar craters.<\/p>\n<p id=\"fs-id1170324016600\">Sheehan, W., and Dobbins, T. \u201cMesmerized by Mercury.\u201d <em>Sky &amp; Telescope<\/em> (June 2000): 109. History of Mercury observations and how amateur astronomers can contribute.<\/p>\n<p id=\"fs-id1170323803403\">Talcott, R. \u201cSurprises from MESSENGER\u2019s Historic Mercury Fly-by.\u201d <em>Astronomy<\/em> (March 2009): 28.<\/p>\n<p id=\"fs-id1170323845110\">Talcott, R. \u201cMercury Reveals its Hidden Side.\u201d <em>Astronomy<\/em> (May 2008): 26. Results and image from the MESSENGER mission flyby of January 2008.<\/p>\n<\/section>\n<\/section>\n<section id=\"fs-id1170326295565\">\n<h2>Websites<\/h2>\n<section id=\"fs-id1170326126801\">\n<h3><em>The Moon<\/em><\/h3>\n<p id=\"fs-id1170326098030\">Apollo Lunar Surface Journal: http:\/\/www.hq.nasa.gov\/office\/pao\/History\/alsj\/. Information, interviews, maps, photos, video and audio clips, and much more on each of the Apollo landing missions.<\/p>\n<p id=\"fs-id1170326256008\">Lunar &amp; Planetary Institute: http:\/\/www.lpi.usra.edu\/lunar\/missions\/. Lunar Science and Exploration web pages.<\/p>\n<p id=\"fs-id1170326119274\">Lunar Reconnaissance Orbiter Mission Page: http:\/\/lro.gsfc.nasa.gov\/.<\/p>\n<p id=\"fs-id1170326326771\">NASA\u2019s Guide to Moon Missions and Information: http:\/\/nssdc.gsfc.nasa.gov\/planetary\/planets\/moonpage.html.<\/p>\n<p id=\"fs-id1170326177526\">Origin of the Moon: http:\/\/www.psi.edu\/projects\/moon\/moon.html. By William Hartmann, who, with a colleague, first suggested the giant impact hypothesis for how the Moon formed, in 1975.<\/p>\n<p id=\"fs-id1170326335326\"><em>Sky &amp; Telescope<\/em> magazine\u2019s observing guides and articles about the Moon: http:\/\/www.skyandtelescope.com\/observing\/celestial-objects-to-watch\/moon\/.<\/p>\n<p id=\"fs-id1170324010150\">To the Moon: http:\/\/www.pbs.org\/wgbh\/nova\/tothemoon\/. PBS program on the Apollo landings.<\/p>\n<p id=\"fs-id1170326448981\">We Choose the Moon: http:\/\/wechoosethemoon.org\/. A recreation of the Apollo 11 mission.<\/p>\n<\/section>\n<section id=\"fs-id1170326275417\">\n<h3><em>Mercury<\/em><\/h3>\n<p id=\"fs-id1170326145193\">Mercury Unveiled by G. Jeffrey Taylor (summarizing the Mariner 10 Mission): http:\/\/www.psrd.hawaii.edu\/Jan97\/MercuryUnveiled.html.<\/p>\n<p id=\"fs-id1170326445532\">MESSENGER Mission Website: http:\/\/messenger.jhuapl.edu\/.<\/p>\n<p id=\"fs-id1170324331311\">NASA Planetary Data Center Mercury Page: http:\/\/nssdc.gsfc.nasa.gov\/planetary\/planets\/mercurypage.html.<\/p>\n<p id=\"fs-id1170326054102\">Views of the Solar System Mercury Page: http:\/\/solarviews.com\/eng\/mercury.htm.<\/p>\n<\/section>\n<\/section>\n<\/section>\n<section id=\"fs-id1170321585226\" class=\"group-activities\"><\/section>\n<div>\n<h2>Footnotes<\/h2>\n<ol>\n<li><a name=\"footnote1\" href=\"#footnote-ref1\" id=\"footnote1\">1<\/a> Recall from the <a href=\"\/contents\/bc5ce180-f2b3-4469-a882-22bf96435c99\" class=\"target-chapter\">Radiation and Spectra<\/a> chapter that magnetism is an effect of moving electric charges. In atoms of metals, the outer electrons are easier to dislodge and they can form a current when the metal is in liquid form and can flow.<\/li>\n<\/ol>\n<\/div>\n","protected":false},"author":395,"menu_order":1,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-365","chapter","type-chapter","status-publish","hentry"],"part":330,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/wp-json\/pressbooks\/v2\/chapters\/365","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/wp-json\/wp\/v2\/users\/395"}],"version-history":[{"count":1,"href":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/wp-json\/pressbooks\/v2\/chapters\/365\/revisions"}],"predecessor-version":[{"id":366,"href":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/wp-json\/pressbooks\/v2\/chapters\/365\/revisions\/366"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/wp-json\/pressbooks\/v2\/parts\/330"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/wp-json\/pressbooks\/v2\/chapters\/365\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/wp-json\/wp\/v2\/media?parent=365"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/wp-json\/pressbooks\/v2\/chapter-type?post=365"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/wp-json\/wp\/v2\/contributor?post=365"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/a7000y2018\/wp-json\/wp\/v2\/license?post=365"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}