Of all the planets of our solar system, Mercury remains the most mysterious of the nine due to little space exploration, with only one satellite has ever flown by it. However, knowledge of this planet has greatly increased since the time of the ancient Romans when they peered into the sky and named the planet after their messenger god, Mercury, as it appeared to travel swiftly through the dawn and dusk sky.
Mercury is the planet closest to the sun and is also the second smallest planet in our solar system with a diameter of 40% smaller than Earth’s but 40% larger than the Moon’s. The rocky terrain on Mercury appears to resemble that of our crater-filled Moon. The planet’s craters are a result of the early formation of Mercury when the planet was under heavy bombardment. Mercury has no natural satellites. If a space explorer were to ever set foot on Mercury, he/she would observe a sun that is two and a half times larger than that on Earth and a black sky as there is almost no atmosphere on Mercury to scatter the different wavelengths of light (blue light scatters the least in Earth’s atmosphere, which results in our blue sky). The atmosphere on Mercury is continually being replenished, however, solar winds from the sun keep blasting the atmosphere into space.
The temperature range on Mercury is the most extreme of all the planets in our solar system. With no atmosphere, almost all heat from the sun is lost during the “nighttime”. Temperature on Mercury can range from 90 K (-183.15 C or -297.67 F) to 700 K (426.85 C or 800.33 F). Quite similar to Earth, the warmest areas are around the equator and the coldest areas are around the poles.
Mercury is one of the four terrestrial planets, meaning it has a rocky surface similar to Earth’s. The density of Mercury is the second highest in the solar system, with Earth being the densest. If it wasn’t for the gravitational compression on Earth, Mars would be the densest planet in the solar system. For Mars to have such a high density with little gravitational compression, geologists predict Mercury to have a large core composed mostly of iron that takes up about 42% of the planet’s volume in compared to Earth’s 17%. The mantle in Mercury is relatively small; only 600 kilometers thick when compared to its core being 1,800 kilometers. One theory of the mantle being so thin suggests a giant impact of a celestial body being several hundred kilometers long stripped away most of the planet’s crust and mantle.
Mercury’s crust is thought to range from 100-200 kilometers thick. The surface Geology of the crust is very similar to that of the Moon. The planet’s crust consists of craters, ridges, mountains, escarpments, plains, and valleys. The planet was once volcanically active during its period of heavy bombardment, but it does not appear to be volcanically active anymore. Mercury’s plains may have been formed during volcanic activity when lava covered a crater-filled area. Some of these plains appear to be wrinkled, which may be a result of the planet shrinking in size after becoming volcanically inactive.
Mercury’s orbit is highly eccentric. The planet is 46 million kilometers at its point closest to the sun (perihelion) and 70 million kilometers at its farthest point (aphelion). Mercury’s period of revolution is determined to be roughly 58.646 earth days long. Until 1962, one “day” of Mercury was thought to equal one “year”, meaning the planet rotates once every revolution around the sun. Now it is determined Mercury rotates three times every two of its years; the only planet in our solar system to have a orbital/rotational resonance with a ratio other than 1:1. Because of Mercury’s eccentric orbit, the sun would appear to take a very strange course through the sky, with some stops and backtracking as well as the sun’s appearance to shrink or expand.
At first it appears that water cannot exist on the planet. However, it does appear to be brighter near the north pole on Mercury, in which scientists are currently contemplating weather or not this glimmer is really ice. Here, the sun barely rises above the horizon, resulting in cooler temperatures. There are also places inside the craters where the surface may not receive any sunlight at all. Scientists think the insides of these craters may remain -161 C. This ice may be coated with dust from the planet, which would result in the bright returns in pictures of the planet’s surface.