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第1段
1 .listen to part of a lecture in an astronomy class,
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2 .the professor has been discussing the planet Mercury.
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3 .I just want to clarify one thing before we go on.
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4 .There\'s a common misconception that mercury doesn\'t have an atmosphere.
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5 .All the planets in our solar system have gaseous atmospheres.
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6 .Mercury, because of its size,
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7 .can\'t exert enough gravitational force to hold on to anything but a minimal atmosphere.
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8 .So Mercury is almost airless,
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9 .but not quite. The same is true for Earth\'s moon.
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10 .It does have a rudimentary atmosphere.
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11 .despite what most people think.
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12 .What else do Mercury and the moon have in common? Based on what you\'ve read so far.
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13 .Nancy.
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14 .impact craters, right?
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15 .So is there a connection between impact craters and atmospheres? Yeah,
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16 .meteorites usually burn up in the atmosphere before they reach the ground, but there\'s no atmosphere on Mercury.
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17 .no atmosphere.
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18 .Oh, there\'s not much of an atmosphere on Mercury.
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19 .So it gets pounded with space rocks,
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20 .and of course, that goes for the moon as well.
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21 .Okay, but let\'s talk about a difference between Mercury and the moon.
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22 .And there\'s a connection here to meteorite bombardment.
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23 .When you look at the moon, it seems to shine, right
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24 .because its surface reflects light from the sun.
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25 .yes, but Mercury\'s surface appears darker because
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26 .it reflects less sunlight.
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27 .good, and why is that?
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28 .Remember, last week, we were discussing the moon\'s regolith.
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29 .A regolith is that layer of loose material,
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30 .the top layer on the surface of rocky planets like Earth and Mars and Mercury
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31 .and on the moon and asteroids too.
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32 .We think the reason Mercury reflects less sunlight involves its regolith.
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33 .Originally, we thought that iron particles in Mercury\'s regolith might be the reason behind its lower reflectivity.
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34 .Iron is one of the primary known darkening agents on other basically airless bodies.
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35 .But in fact, Mercury\'s regolith contains very little iron.
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36 .We know this from analyzing the dim light that Mercury does reflect.
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37 .A more plausible candidate is carbon, the element
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38 .that makes coal black.
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39 .Carbon\'s the fourth most common element in the universe,
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40 .but unlike iron, it\'s very hard to detect remotely.
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41 .So while we\'re pretty sure there\'s very little iron on Mercury surface,
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42 .it\'s hard to prove the presence of carbon
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43 .one way or the other. Nevertheless, some astronomers do think that carbon is the reason,
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44 .and that mercury is acquiring carbon from comets orbiting the sun.
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45 .Comets, as we know, are big chunks of ice and dust,
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46 .and one of the main components of cometary dust is carbon.
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47 .How does it get to Mercury?
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48 .Micro meteorites,
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49 .you see, comets have this particular type of orbit
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50 .which brings them periodically quite close to the sun and therefore to Mercury, the sun\'s nearest neighbor.
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51 .And as comets swing by the sun, they become unstable and send micrometeorites full of carbon crashing into mercury.
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52 .According to researcher’s calculations,
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53 .mercury gets hit by 50 times more cometary debris than the moon does.
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54 .So on paper, at least, there should be a substantial amount of carbon in Mercury\'s regolith.
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55 .And in fact, laboratory research has strengthened the case for carbon.
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56 .This research two laboratory experiments
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57 .were done at a facility that specializes in simulating collisions between objects in space.
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58 .The experiments involved sending a projectile at high speed
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59 .into a target chamber filled with a powder.
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60 .made of crushed rock.
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61 .This powder had been developed previously to approximate the moon\'s regolith.
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62 .which is low in carbon.
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63 .In the first experiment, researchers mixed sugar into the powder in order to imitate the carbon content of comet dust.
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64 .Why would they use sugar?
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65 .Because sugar comes from plants, and plants contain carbon compounds, right?
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66 .And the force of the impact produced heat,
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67 .which melted the powder and vaporized the sugar,
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68 .leaving tiny black. Carbon particles embedded in the melted material,
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69 .enough particles to darken the powder significantly.
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70 .It reflected less than 5% of the light that hit it.
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71 .Now for the second experiment, the researchers did something different.
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72 .Instead of adding sugar to the powder,
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73 .they used quartz sand.
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74 .There\'s no carbon in quartz sand
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75 .and well, when the researchers fired a projectile into this new mixture.
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76 .the material melted,
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77 .but it did not get darker.
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78 .But there are still unanswered questions.
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79 .One is whether enough carbon has, in fact, accumulated in Mercury\'s regolith.
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80 .over the course of billions of years to.
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81 .so to speak, Paint It Black.
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