Image by NASA.
Mercury is named for the Roman deity who was the speedy messenger for the Roman gods. The name was inspired by the fact that planet Mercury changes its position in the sky faster than any planet in the Solar System. Mercury is difficult to observe from Earth because it orbits the Sun at just 0.39 AU and usually lies in the Sun’s glare. It is considered to be the smallest of the eight planets, with a radius about 1/3 of Earth and a mass of 18 times smaller –not pretty much bigger than the Moon. It has been visited by two space probes, Mariner 10 in 1974-75 and Messenger, which began a long term rendezvous in 2008. Before Messenger arrived, less than half of Mercury’s surface had been seen. Images seen by the former spacecraft show that although Mercury’s surface resembles the Moon in many ways, there are hints of unique geological processes on this airless planet.
At first glance, Mercury’s landscape is difficult to distinguish from the Moon’s; but careful examinations reveal differences. First, Mercury’s impact craters generally overlap less than the Moon’s do in lunar highlands. In addition, the crater walls tend to be less steep, most likely because its surface gravity is more than twice as strong as the Moon’s, making steep hills less stable on Mercury. Congealed lava flows flood not only many of its old craters but much of its surface. Finally, the lava flows do not appear as dark as the lunar maria, and probably have a different composition from the Moon’s basalt. Also, Mercury’s surface contains some features quite different from the Moon’s. Enormous “Scarps” –cliffs formed where the crust has shifted –covers its surface. Some run for hundreds of kilometers and range up to 3 kilometers high. In addition to the large lava flows, there are also indications of volcanic activity dating after most of the craters formed. Several locations features have been found that appear to be volcanic vents that have expelled material that coats the surrounding area.
By far, the largest of the impact features on the planet is the vast “Caloris Basin” (a mountain-ringed depression), only the edge of which was seen by Mariner 10. Astronomers have waited more than three decades to see the rest of this Basin, which has now been detailed by Messenger. Also, the latter’s imaging system detected subtle color differences between different portions of Mercury’s surface. Lava flows in Caloris Basin appear orange, old impact craters dark blue, and young ones white. Near the center of Caloris Basin is an unusual an poorly understood feature, a spider-shaped of troughs radiating away from a small crater. The Caloris impact had global effects on Mercury. On the side of the planet opposite the Caloris Basin, the terrain has a hilly, jumbled appearance.
Mercury’s surface is one of the hottest places in the Solar System, and it undergoes some of the most extreme changes of temperature. At its equator, noon temperatures can reach about 710 K (about 820 F). Nighttime temperatures are among the coldest, dropping about 80 K (about -320 F). This is due to its closeness to the Sun, and its lack of atmosphere. It gets colder because its orbit takes it away about 50% from the Sun (43 million miles). However, based on affected radar waves, an existence of ice on the planet have been reflected (in perpetual shadows within craters near the planet’s poles). A discovery that is surely surprising.
- Works were heavily cited from: “Pathways of Astronomy”, 4th By, Stephen E. Schneider & Thomas T. Arny.