Mercury
How is planet Mercury like Earth's Moon? What is the structure of Mercury? What are scarps? What is orbital resonance? Why does Mercury exhibit orbital resonance?
Mercury is the smallest of eight planets in our solar system, and the closest to the Sun. It has the fastest orbit of any of the planets. This image of Mercury is probably more colorful than you would expect. The colors are real, but more enhanced than your eye would see. The differences in color indicate that there are distinct regions with varying chemistry.
There are some obvious similarities between Mercury and Earth's Moon. Both are covered with craters. This is an indication that resurfacing mechanisms like erosion and plate tectonics are not at work on Mercury. The craters have continued to collect on its surface for a very long time.
The large brown plain in the upper right of this photo was most likely caused by the impact of a comet or asteroid very early in Mercury's history, flooding like the Maria on the Moon's surface.
Mercury is not much bigger than Earth's Moon, but it has considerably more mass than the Moon, so it is much denser. It is mostly made of heavy metals like iron. Still, compared to other terrestrial planets in our Solar system, Venus, Earth and Mars, Mercury has a relatively small mass. The small mass and close proximity to the Sun mean it is very easy for gases to reach escape velocity on the small planet. Mercury's atmosphere is very transitory, and is called an exosphere. The intense heat of the Sun continually atomizes material, creating gases like an atmosphere, but these gases are stripped away very rapidly from the Sun and replenished.
The lack of a stable atmosphere on Mercury means that the surface temperature is not regulated. It gets very hot in the day, about 800 degrees Fahrenheit (450 degrees Celsius) and very cold at night, about -280 degrees Fahrenheit (-175 degrees Celsius).
Mercury does have a magnetosphere, probably due to a dynamo mechanism, similar to that on Earth. Mercury's magnetic field is about a hundred times weaker than that of Earth, and varies over time. We believe Mercury has relatively a very large metallic core.
Mercury has giant cracks on its surface, called scarps. The scarp in the photograph above is about 350 km long. These cracks can be seen to run right through craters, which is evidence that they formed after the heavy bombardment period. We believe that scarps are formed from contraction due to cooling of the planet.
This image shows multiple photos of Mercury superimposed over one evening background, taken over the span of one month. Many people have never seen Mercury; because it is so small and lies so close to the sun, it is never high in the sky.
At most, Mercury can only be seen as much as 28 degrees from the Sun from Earth. Also, we never see it in a "full" phase, since it would be behind the Sun from our viewpoint.
Mercury is very close to the Sun, and we would expect it to become tidally locked to the sun, such that one side of Mercury would continually face the Sun. However, this is not the case. Mercury also has a very elliptical orbit. By Kepler's law, Mercury moves faster when it is closer to the Sun than when it is farther away. To be tidally locked, Mercury would have to change its rate of rotation on its axis.
With the combination of these two factors, Mercury evolved into a 3:2 orbital resonance with Mercury rotating three times for every two orbits, as shown in this video.
This video shows what it would look like from Mercury. The eccentricity of the orbit would make the Sun appear closer twice over the course of the three-orbit cycle.
This video is also from the viewpoint of Mercury, but this time, you are staying fixed on Mercury's surface. It appears as though the Sun rises twice. This happens because of the change in the orbital speed of Mercury, as its rotation stays constant. At the farthest point in the orbit (apogee) Mercury has slowed down to a point where the orbital velocity is faster than the rotational velocity, so the Sun appears to move "backwards."
There is another aspect of Mercury's orbit that can only be explained using Einstein's theory of General Relativity. This theory predicts that time passes more slowly near a dense massive body. Mercury's orbit precesses, or changes the direction if its semimajor axis, because time passes more slowly when Mercury is close to the sun. The effect is greatly exaggerated in this diagram. Mercury's precession is measured to be about 574 arcseconds per century, in good agreement with Einstein's theory.
Check out this great video where Neil deGrasse Tyson explains about the precession of Mercury's orbit.
Check out this NASA video of Mercury, made from images and data from the Messenger spacecraft orbiting Mercury from 2011 to 2015.