Small, Fast, and Closest to the Sun
Mercury is the smallest planet in the solar system, approximately the width of the United States, and is often compared to earth’s moon because it’s terrestrial surface is characterized by level planes and deep impact craters. But Mercury’s elliptical orbit brings it only 45 million kilometers from the Sun at perihelion, and scientists estimate that its iron core erupted with volcanic activity for over a billion years before its surface cooled.
With its metallic core and thin outer mantle, Mercury resembles the earth rather than the moon. However, the small magnetosphere that surrounds the planet is no match for the heat and velocity of the solar winds, so the atmosphere on Mercury has mostly been swept away and contains only traces of hydrogen, helium, and elements such as oxygen and calcium that dissolve from rocks at the surface.
Temperatures range from 420 C at the equator to -173 C at the poles, where in recent years the Messenger spacecraft has taken images of water ice.
Of all the planets in the solar system, Mercury has the smallest axial tilt, orbiting upright on its axis with very little fluctuation in surface temperatures between the northern and southern hemispheres
Mercury in Action
The relationship between Mercury’s orbit and rotation was misunderstood for centuries. For years, astronomers theorized that Mercury rotated on its axis at the same speed that it orbited the sun, similar to the way the moon is tidally locked with the earth, with one side continuously in daylight and the other in darkness.
In the 1960’s, new technology using electromagnetic waves to explore the solar system became available, leading scientists to find that Mercury was rotating a little slower than they originally thought. What they found was a 3:2 “spin orbital resonance;” Mercury rotates on its axis in 59 “earth” days and orbits the sun in 88, meaning that when it orbits twice around the sun it rotates on its axis three times.