{"id":295,"date":"2016-11-15T21:36:43","date_gmt":"2016-11-15T21:36:43","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/astronomy\/?post_type=chapter&#038;p=295"},"modified":"2018-01-22T15:50:25","modified_gmt":"2018-01-22T15:50:25","slug":"mercury","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/chapter\/mercury\/","title":{"raw":"9.5 Mercury","rendered":"9.5 Mercury"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\nBy the end of this section, you will be able to:\r\n<ul>\r\n \t<li>Characterize the orbit of Mercury around the Sun<\/li>\r\n \t<li>Describe Mercury\u2019s structure and composition<\/li>\r\n \t<li>Explain the relationship between Mercury\u2019s orbit and rotation<\/li>\r\n \t<li>Describe the topography and features of Mercury\u2019s surface<\/li>\r\n \t<li>Summarize our ideas about the origin and evolution of Mercury<\/li>\r\n<\/ul>\r\n<\/div>\r\nThe planet <strong>Mercury<\/strong> 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.\r\n<h2>Mercury\u2019s Orbit<\/h2>\r\nMercury 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.\r\n\r\nThe semimajor axis of <strong>Mercury\u2019s orbit<\/strong>\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=\".\/chapter\/introduction-to-orbits-and-gravity\/\" target=\"_blank\" rel=\"noopener\">Orbits and Gravity<\/a>).\r\n<h2>Composition and Structure<\/h2>\r\n[caption id=\"\" align=\"alignright\" width=\"350\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03155446\/OSC_Astro_09_05_Mercury.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=\"350\" height=\"350\" \/> <strong>Figure 1. Mercury\u2019s Internal Structure:<\/strong> The interior of Mercury is dominated by a metallic core about the same size as our Moon.[\/caption]\r\n\r\nMercury\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.\r\n\r\n<strong>Mercury\u2019s composition<\/strong> 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 (Figure 1). 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.\r\n<div class=\"textbox examples\">\r\n<h3>Example 1: Densities of Worlds<\/h3>\r\nThe average density of a body equals its mass divided by its volume. For a sphere, density is:\r\n<p style=\"text-align: center\">[latex]\\displaystyle\\text{density}=\\frac{\\text{mass}}{\\frac{4}{3}{\\pi}{R}^{3}}[\/latex]<\/p>\r\nAstronomers can measure both mass and radius accurately when a spacecraft flies by a body.\r\n\r\nUsing the information in this chapter, we can calculate the approximate average density of the Moon.\r\n\r\n[reveal-answer q=\"593698\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"593698\"]For a sphere,\r\n<p style=\"text-align: center\">[latex]\\displaystyle\\text{density}=\\frac{\\text{mass}}{\\frac{4}{3}{\\pi}{R}^{3}}=\\frac{7.35\\times {10}^{22}\\text{kg}}{4.2\\times 5.2\\times {10}^{18}{\\text{m}}^{3}}=3.4\\times {10}^{3}{\\text{kg\/m}}^{3}[\/latex]<\/p>\r\ngives a value of 3.3 g\/cm3, which is 3.3 \u00d7 10<sup>3<\/sup> kg\/m<sup>3<\/sup>.[\/hidden-answer]\r\n<h4>Check Your Learning<\/h4>\r\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?\r\n\r\n[reveal-answer q=\"80470\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"80470\"]\r\n<p style=\"text-align: center\">[latex]\\displaystyle\\text{density}=\\frac{\\text{mass}}{\\frac{4}{3}{\\pi}{R}^{3}}=\\frac{3.3\\times {10}^{23}\\text{kg}}{4.2\\times 1.45\\times {10}^{19}{\\text{m}}^{3}}=5.4\\times {10}^{3}{\\text{kg\/m}}^{3}[\/latex]<\/p>\r\nThat matches the value given when g\/cm<sup>3<\/sup> is converted into kg\/m<sup>3<\/sup>.[\/hidden-answer]\r\n\r\n<\/div>\r\n<h2>Mercury\u2019s Strange Rotation<\/h2>\r\nVisual studies of Mercury\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 <strong>Mercury\u2019s rotation<\/strong> period was equal to its revolution period of 88 days, making one side perpetually hot while the other was always cold.\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"500\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03155449\/OSC_Astro_09_05_Doppler.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 \" width=\"500\" height=\"332\" \/> <strong>Figure 2. Doppler Radar Measures Rotation: <\/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]\r\n\r\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 (Figure 2). The degree of broadening provides an exact measurement of the rotation rate of the planet.\r\n\r\nMercury\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\u00a0<strong>What Difference a Day Makes<\/strong>\u00a0for more on the effects of having such a long day on Mercury.)\r\n\r\nMercury, 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.\r\n<div class=\"textbox shaded\">\r\n<h3>What a Difference a Day Makes<\/h3>\r\nMercury 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.\r\n\r\nHere on Earth, we take for granted that days are much shorter than years. Therefore, the two astronomical ways of defining the local \"day\"\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!\r\n\r\nTo 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 (Figure 4).\r\n\r\nAt 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.\r\n\r\nAt 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.)\r\n\r\nAstronomers call locations like the Caloris Basin the \"hot longitudes\" 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.\r\n\r\nWe 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 \"Earth chauvinism\" you might have. The way things are on our planet is just one of the many ways nature can arrange reality.\r\n\r\n<\/div>\r\n<h2>The Surface of Mercury<\/h2>\r\n[caption id=\"\" align=\"alignright\" width=\"351\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03155454\/OSC_Astro_09_05_Basin.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=\"351\" height=\"351\" \/> <strong>Figure 3. Caloris Basin: <\/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]The first close-up look at <strong>Mercury<\/strong> 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.\r\n\r\nSubsequently, the planet was mapped in great detail by the <strong>MESSENGER<\/strong> 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.\r\n\r\nMercury\u2019s surface strongly resembles the Moon in appearance (Figure 3 and Figure 4). 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.\r\n\r\n<strong>MESSENGER<\/strong> 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.\r\n<div class=\"textbox\">Scientists working with data from the\u00a0MESSENGER mission\u00a0put together a <a href=\"http:\/\/messenger.jhuapl.edu\/the_mission\/movies\/MESSENGER_falsecolor_Mercury_globe_spin_big.mov\" target=\"_blank\" rel=\"noopener\">rotating globe of Mercury<\/a>, in false color, showing some of the variations in the composition of the planet\u2019s surface. You can watch it spin.<\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"751\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03155451\/OSC_Astro_09_05_MercuryTopo.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=\"751\" height=\"388\" \/> <strong>Figure 4. Mercury\u2019s Topography: <\/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]\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"350\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03155456\/OSC_Astro_09_05_Discovery.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=\"350\" height=\"356\" \/> <strong>Figure 5. Discovery Scarp on Mercury: <\/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 \"wrinkles\" 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]\r\n\r\nMost 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.\r\n\r\nThere is no evidence of plate tectonics on Mercury. However, the planet\u2019s distinctive long scarps can sometimes be seen cutting across craters; this means the scarps must have formed later than the craters (Figure 5).\u00a0These 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.\r\n\r\nIf 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.\r\n<h2>The Origin of Mercury<\/h2>\r\nThe problem with understanding how Mercury 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?\r\n\r\nThe 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.\r\n<div class=\"textbox\">You can follow some of <a href=\"https:\/\/www.nasa.gov\/mission_pages\/messenger\/whymessenger\/index.html\" target=\"_blank\" rel=\"noopener\">NASA\u2019s latest research on Mercury<\/a> and see some helpful animations on the MESSENGER web page.<\/div>\r\nToday, 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.\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Key Concepts and Summary<\/h3>\r\nMercury 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.\r\n\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<ul>\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 <strong>Mercury<\/strong> 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<h2>Mercury\u2019s Orbit<\/h2>\n<p>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>The semimajor axis of <strong>Mercury\u2019s orbit<\/strong>\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=\".\/chapter\/introduction-to-orbits-and-gravity\/\" target=\"_blank\" rel=\"noopener\">Orbits and Gravity<\/a>).<\/p>\n<h2>Composition and Structure<\/h2>\n<div style=\"width: 360px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03155446\/OSC_Astro_09_05_Mercury.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=\"350\" height=\"350\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Figure 1. Mercury\u2019s Internal Structure:<\/strong> The interior of Mercury is dominated by a metallic core about the same size as our Moon.<\/p>\n<\/div>\n<p>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><strong>Mercury\u2019s composition<\/strong> 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 (Figure 1). 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.<\/p>\n<div class=\"textbox examples\">\n<h3>Example 1: Densities of Worlds<\/h3>\n<p>The average density of a body equals its mass divided by its volume. For a sphere, density is:<\/p>\n<p style=\"text-align: center\">[latex]\\displaystyle\\text{density}=\\frac{\\text{mass}}{\\frac{4}{3}{\\pi}{R}^{3}}[\/latex]<\/p>\n<p>Astronomers can measure both mass and radius accurately when a spacecraft flies by a body.<\/p>\n<p>Using the information in this chapter, we can calculate the approximate average density of the Moon.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q593698\">Show Answer<\/span><\/p>\n<div id=\"q593698\" class=\"hidden-answer\" style=\"display: none\">For a sphere,<\/p>\n<p style=\"text-align: center\">[latex]\\displaystyle\\text{density}=\\frac{\\text{mass}}{\\frac{4}{3}{\\pi}{R}^{3}}=\\frac{7.35\\times {10}^{22}\\text{kg}}{4.2\\times 5.2\\times {10}^{18}{\\text{m}}^{3}}=3.4\\times {10}^{3}{\\text{kg\/m}}^{3}[\/latex]<\/p>\n<p>gives a value of 3.3 g\/cm3, which is 3.3 \u00d7 10<sup>3<\/sup> kg\/m<sup>3<\/sup>.<\/div>\n<\/div>\n<h4>Check Your Learning<\/h4>\n<p>Using 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=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q80470\">Show Answer<\/span><\/p>\n<div id=\"q80470\" class=\"hidden-answer\" style=\"display: none\">\n<p style=\"text-align: center\">[latex]\\displaystyle\\text{density}=\\frac{\\text{mass}}{\\frac{4}{3}{\\pi}{R}^{3}}=\\frac{3.3\\times {10}^{23}\\text{kg}}{4.2\\times 1.45\\times {10}^{19}{\\text{m}}^{3}}=5.4\\times {10}^{3}{\\text{kg\/m}}^{3}[\/latex]<\/p>\n<p>That matches the value given when g\/cm<sup>3<\/sup> is converted into kg\/m<sup>3<\/sup>.<\/div>\n<\/div>\n<\/div>\n<h2>Mercury\u2019s Strange Rotation<\/h2>\n<p>Visual studies of Mercury\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 <strong>Mercury\u2019s rotation<\/strong> period was equal to its revolution period of 88 days, making one side perpetually hot while the other was always cold.<\/p>\n<div style=\"width: 510px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03155449\/OSC_Astro_09_05_Doppler.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\" width=\"500\" height=\"332\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Figure 2. Doppler Radar Measures Rotation: <\/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.<\/p>\n<\/div>\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 (Figure 2). The degree of broadening provides an exact measurement of the rotation rate of the planet.<\/p>\n<p>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\u00a0<strong>What Difference a Day Makes<\/strong>\u00a0for more on the effects of having such a long day on Mercury.)<\/p>\n<p>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 class=\"textbox shaded\">\n<h3>What a Difference a Day Makes<\/h3>\n<p>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>Here on Earth, we take for granted that days are much shorter than years. Therefore, the two astronomical ways of defining the local &#8220;day&#8221;\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>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 (Figure 4).<\/p>\n<p>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>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>Astronomers call locations like the Caloris Basin the &#8220;hot longitudes&#8221; 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>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 &#8220;Earth chauvinism&#8221; 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<h2>The Surface of Mercury<\/h2>\n<div style=\"width: 361px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03155454\/OSC_Astro_09_05_Basin.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=\"351\" height=\"351\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Figure 3. Caloris Basin: <\/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)<\/p>\n<\/div>\n<p>The first close-up look at <strong>Mercury<\/strong> 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.<\/p>\n<p>Subsequently, the planet was mapped in great detail by the <strong>MESSENGER<\/strong> 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>Mercury\u2019s surface strongly resembles the Moon in appearance (Figure 3 and Figure 4). 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><strong>MESSENGER<\/strong> 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 class=\"textbox\">Scientists working with data from the\u00a0MESSENGER mission\u00a0put together a <a href=\"http:\/\/messenger.jhuapl.edu\/the_mission\/movies\/MESSENGER_falsecolor_Mercury_globe_spin_big.mov\" target=\"_blank\" rel=\"noopener\">rotating globe of Mercury<\/a>, in false color, showing some of the variations in the composition of the planet\u2019s surface. You can watch it spin.<\/div>\n<div style=\"width: 761px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03155451\/OSC_Astro_09_05_MercuryTopo.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=\"751\" height=\"388\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Figure 4. Mercury\u2019s Topography: <\/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)<\/p>\n<\/div>\n<div style=\"width: 360px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1095\/2016\/11\/03155456\/OSC_Astro_09_05_Discovery.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=\"350\" height=\"356\" \/><\/p>\n<p class=\"wp-caption-text\"><strong>Figure 5. Discovery Scarp on Mercury: <\/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 &#8220;wrinkles&#8221; 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)<\/p>\n<\/div>\n<p>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 scarps can sometimes be seen cutting across craters; this means the scarps must have formed later than the craters (Figure 5).\u00a0These 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>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<h2>The Origin of Mercury<\/h2>\n<p>The problem with understanding how Mercury 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>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 class=\"textbox\">You can follow some of <a href=\"https:\/\/www.nasa.gov\/mission_pages\/messenger\/whymessenger\/index.html\" target=\"_blank\" rel=\"noopener\">NASA\u2019s latest research on Mercury<\/a> and see some helpful animations on the MESSENGER web page.<\/div>\n<p>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<div class=\"textbox key-takeaways\">\n<h3>Key Concepts and Summary<\/h3>\n<p>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<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-295\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Astronomy. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/2e737be8-ea65-48c3-aa0a-9f35b4c6a966@10.1\">http:\/\/cnx.org\/contents\/2e737be8-ea65-48c3-aa0a-9f35b4c6a966@10.1<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/2e737be8-ea65-48c3-aa0a-9f35b4c6a966@10.1<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":6,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Astronomy\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/2e737be8-ea65-48c3-aa0a-9f35b4c6a966@10.1\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/2e737be8-ea65-48c3-aa0a-9f35b4c6a966@10.1\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-295","chapter","type-chapter","status-publish","hentry"],"part":265,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/chapters\/295","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":9,"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/chapters\/295\/revisions"}],"predecessor-version":[{"id":2306,"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/chapters\/295\/revisions\/2306"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/parts\/265"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/chapters\/295\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/wp\/v2\/media?parent=295"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/pressbooks\/v2\/chapter-type?post=295"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/wp\/v2\/contributor?post=295"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-geneseo-astronomy\/wp-json\/wp\/v2\/license?post=295"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}