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listen to part of a lecture in an astronomy class,

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the professor has been discussing the planet Mercury.

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I just want to clarify one thing before we go on.

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There\'s a common misconception that mercury doesn\'t have an atmosphere.

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All the planets in our solar system have gaseous atmospheres.

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Mercury, because of its size,

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can\'t exert enough gravitational force to hold on to anything but a minimal atmosphere.

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So Mercury is almost airless,

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but not quite. The same is true for Earth\'s moon.

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It does have a rudimentary atmosphere.

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despite what most people think.

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What else do Mercury and the moon have in common? Based on what you\'ve read so far.

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Nancy.

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impact craters, right?

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So is there a connection between impact craters and atmospheres? Yeah,

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meteorites usually burn up in the atmosphere before they reach the ground, but there\'s no atmosphere on Mercury.

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no atmosphere.

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Oh, there\'s not much of an atmosphere on Mercury.

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So it gets pounded with space rocks,

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and of course, that goes for the moon as well.

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Okay, but let\'s talk about a difference between Mercury and the moon.

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And there\'s a connection here to meteorite bombardment.

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When you look at the moon, it seems to shine, right

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because its surface reflects light from the sun.

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yes, but Mercury\'s surface appears darker because

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it reflects less sunlight.

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good, and why is that?

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Remember, last week, we were discussing the moon\'s regolith.

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A regolith is that layer of loose material,

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the top layer on the surface of rocky planets like Earth and Mars and Mercury

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and on the moon and asteroids too.

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We think the reason Mercury reflects less sunlight involves its regolith.

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Originally, we thought that iron particles in Mercury\'s regolith might be the reason behind its lower reflectivity.

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Iron is one of the primary known darkening agents on other basically airless bodies.

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But in fact, Mercury\'s regolith contains very little iron.

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We know this from analyzing the dim light that Mercury does reflect.

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A more plausible candidate is carbon, the element

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that makes coal black.

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Carbon\'s the fourth most common element in the universe,

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but unlike iron, it\'s very hard to detect remotely.

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So while we\'re pretty sure there\'s very little iron on Mercury surface,

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it\'s hard to prove the presence of carbon

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one way or the other. Nevertheless, some astronomers do think that carbon is the reason,

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and that mercury is acquiring carbon from comets orbiting the sun.

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Comets, as we know, are big chunks of ice and dust,

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and one of the main components of cometary dust is carbon.

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How does it get to Mercury?

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Micro meteorites,

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you see, comets have this particular type of orbit

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which brings them periodically quite close to the sun and therefore to Mercury, the sun\'s nearest neighbor.

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And as comets swing by the sun, they become unstable and send micrometeorites full of carbon crashing into mercury.

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According to researcher’s calculations,

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mercury gets hit by 50 times more cometary debris than the moon does.

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So on paper, at least, there should be a substantial amount of carbon in Mercury\'s regolith.

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And in fact, laboratory research has strengthened the case for carbon.

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This research two laboratory experiments

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were done at a facility that specializes in simulating collisions between objects in space.

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The experiments involved sending a projectile at high speed

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into a target chamber filled with a powder.

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made of crushed rock.

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This powder had been developed previously to approximate the moon\'s regolith.

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which is low in carbon.

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In the first experiment, researchers mixed sugar into the powder in order to imitate the carbon content of comet dust.

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Why would they use sugar?

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Because sugar comes from plants, and plants contain carbon compounds, right?

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And the force of the impact produced heat,

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which melted the powder and vaporized the sugar,

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leaving tiny black. Carbon particles embedded in the melted material,

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enough particles to darken the powder significantly.

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It reflected less than 5% of the light that hit it.

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Now for the second experiment, the researchers did something different.

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Instead of adding sugar to the powder,

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they used quartz sand.

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There\'s no carbon in quartz sand

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and well, when the researchers fired a projectile into this new mixture.

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the material melted,

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but it did not get darker.

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But there are still unanswered questions.

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One is whether enough carbon has, in fact, accumulated in Mercury\'s regolith.

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over the course of billions of years to.

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so to speak, Paint It Black.

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