Secrets from sun-scorched mercury: the planet's "extreme and dramatic orbital eccentricity, in combination with its rotation rate of three times in two of its years, causes some strange things to occur.".

AuthorBraffman-Miller, Judith
PositionScience & Technology

SUN-SCORCHED Mercury is the smallest major planet in our solar system. Only a bit larger than Earth's moon, it basks and broils in the hot glow of our star, and shows a surface that is splotched by cratered plains of uncertain origin. In marked contrast, this innermost planet inhabiting the sun's family also displays flat, undisturbed, smooth plains. Although Mercury has been known since the time of the ancient Sumerians, whose civilization flourished in the third millennium B.C., it has managed to keep some of its secrets well-hidden from the prying eyes of curious planetary scientists.

Like Earth's moon, Mercury has very little atmosphere to prevent impacts from crashing space objects, and it is pockmarked by a multitude of craters. Mercury's day-side is sun baked and searing hot. At night, though, temperatures plummet down to hundreds of degrees below freezing and water-ice is thought to exist within the cold shadows of its many craters, where the light and heat of the sun never reach. Mercury's egg-shaped (elliptical) orbit takes it around the sun every 88 Earth days.

At Mercury's closest approach to the sun (perihelion), during the course of its out-of-round orbit, it is a mere 46,000,000 kilometers away from it, while at its aphelion--when it is most distant from the sun--it is 70,000,000 kilometers from the fiery face of our stellar parent. Mercury's extreme and dramatic orbital eccentricity, in combination with its rotation rate of three times in two of its years, causes some strange things to occur. For example, at certain longitudes, an observer standing on its surface would see the sun rise and then slowly grow increasingly larger and larger, as it lazily makes its long journey towards its highest point in the sky. Simultaneously, all of the sparkling host of stars would zip three times faster in their flight across Mercury's sky. Standing on Mercury's surface at perihelion, the sun would appear more than three times larger than it does on Earth.

One day on Mercury (the time it takes for Mercury to rotate, or spin, once) takes 59 Earth days. It is a rocky planet, also known as a terrestrial planet, just like Earth, Venus, and Mars. Mercury's extremely thin atmosphere, or exosphere, primarily is composed of oxygen, sodium, hydrogen, helium, and potassium. The atoms that are shot off the surface of this planet by solar wind and micrometeoroid impacts create Mercury's exosphere. Only two spacecraft have visited moonless Mercury...

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