Mercury is the closest planet to the Sun. It is somewhat larger than the Moon. Temperatures at noon rise to 370º C. But since it barely has an atmosphere that traps heat, at night temperatures drop to almost 185 degrees below zero. The surface of Mercury is covered with craters, canyons, and high escarpments.
In 1991, astronomers discovered that, despite the heat, Mercury can have small regions of ice at its north and south poles. The ice is inside deep craters, whose ground is always in the shade, so the sun cannot melt the ice. Mercury is a dense and rocky planet. It is likely that its core is composed of iron and nickel. Turning on its axis, the iron in the core may generate a weak magnetic field around Mercury.
For many years it was believed that there was a side of Mercury that always faced the Sun. But in 1965, astronomers discovered that Mercury made three complete turns on its axis for every two orbits around the Sun. Due to this movement, we see the same side of Mercury every time the planet is a minimum distance from Earth. Only one space vehicle, the Mariner 10, has visited Mercury. He passed three times near the planet in 1974 and 1975. He took photos of half the planet's surface, measured temperatures, and discovered a weak magnetic field.
If Mercury ever had an atmosphere, it has long since escaped into space due to the heat of the nearby Sun. There is no air, no water and nothing can grow on the surface of the planet. Astronauts would need very strong protection against the heat and strong radiation of the Sun. But the bulky suits would feel very light because Mercury's surface gravity is only one third of Earth's.
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