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listen to part of a lecture in an astronomy class, 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, can\'t exert enough gravitational force to hold on to anything but a minimal atmosphere.

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So Mercury is almost airless, but not quite. The same is true for Earth\'s moon.

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It does have a rudimentary atmosphere, 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, Nancy？

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They\'re both covered with craters where meteorites have hit them, impact craters.

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Right. So is there a connection between impact craters and atmospheres?

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Yeah, 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? because...

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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, the top layer on the surface of rocky planets like Earth and Mars and Mercury 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 that makes coal black.

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

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it\'s hard to prove the presence of carbon one way or the other.

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Nevertheless, some astronomers do think that carbon is the reason, 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 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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Micrometeorites, 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 researchers\' calculations, 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, was 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 into a target chamber filled with a powder made of crushed rock.

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

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Right. And the force of the impact produced heat, which melted the powder and vaporized the sugar

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leaving tiny black carbon particles embedded in the melted material, 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, they used quartz sand.

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There\'s no carbon in quartz sand, and well, when the researchers fired a projectile into this new mixture, the material melted, 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 over the course of billions of years to, so to speak, paint it black.

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