While both are terrestrial in character, Mercury is much smaller and less massive than Earth, while having a similar density. Mercury's composition is likewise far more metallic than that of Earth, and its 3:12 orbital resonance causes a single day to be twice as long as a year.
These differences result in very different environments for these two planets. On Earth, our atmosphere protects us from most of the physical damage caused by solar radiation, but on Mercury there is very little air to reflect light waves back into space, so it receives much more ultraviolet radiation than Earth does. This high energy environment results in very acidic water and a surface that is eroding away quickly.
Like Earth, however, Mercury has regions where ice may be found. The only difference is that because there is no magnetic field, all the ice would have to form in a single place, which would need to be extremely cold. Otherwise, it would be destroyed by the intensity of the sun's heat.
So far, data from NASA's Messenger spacecraft indicate that there is no global ice layer overlying a liquid metal ocean, but rather the opposite - a dry crust about 50 miles deep, with small regions of relatively rich organic matter near the surface.
Messenger arrived at Mercury in 2011 after being sent there by NASA's Office of Science.
The diameter of Mercury is 3,030 miles (4,878 km), which is equivalent to the size of the continental United States. It is thus around two-fifths the size of the Earth. It is smaller than Ganymede, Jupiter's moon, and Titan, Saturn's moon. Mercury has a dense iron core surrounded by a crust of silicate rock.
The distance from the center of Mercury to the surface is 485 miles (802 km). A spacecraft currently in orbit around Mercury could travel over 100 miles per hour, which would put it into orbit around the Sun before returning home to tell others about its small but very dynamic planet.
It is important to note that distances are relative and depend on where you are on Mercury. The closest point to the sun is called "the point of first appearance" or "the sub-solar point." From this point, shadows will fall across the planet for the first time each day. As the day progresses, the point of first darkness will move away from the sub-solar point until finally, at quadrature, it reaches a distance of 150 miles from the center of the planet. At this point, shadows remain throughout the night and into the next day.
As far as we know, no one has ever traveled to Mercury, so there are no measurements to compare its size to that of other planets.
According to PCM, Mercury is closest to Earth over half of the time, with the rest divided between Mars and Venus. As a result, Mercury is closer. Even Neptune, which orbits the sun at 30 AU, is closer to Mercury on average than Uranus, which revolves at 19 AU.
However, because of their sizes, only Mercury and Venus can be seen from the Earth with the naked eye. Mars is too small and approaches the Earth from behind the Sun. From our planet, it takes 6 months for messages to reach Mars but only 35 minutes for them to reach Venus.
Mercury is always visible in the night sky as a bright star-like object due to its proximity to the Earth. It can be found just north of the constellations Virgo and Leo. Visually, Venus also appears as a star because it's brightness easily outshines all other objects in the evening sky. It can be found just south of Virgo near Zubenel Gabbana, one of the constellation's stars. From Earth, Mars is always on the opposite side of the Sun when viewed from our planet, so it never gets morning or sunset. It can only be seen in the daytime from Earth.
Mercury is the solar system's smallest terrestrial planet, measuring around one-third the size of Earth. It has a thin atmosphere, which allows temperatures to fluctuate between scorching and freezing. Mercury, like Earth, is a dense planet made largely of iron and nickel, with an iron core. However it has no ocean because of its small size; instead, it has a large magnetic field.
These characteristics indicate that Mercury is a "inner" planet: it lies closer to the sun than any other planet except for Venus, which orbits inside Earth's orbit. This location brings heat to the planet's surface, but also causes it to be shrouded in darkness most of the time.
Also like Earth, Mercury goes through major changes as it ages. Its crust gets thinner, its core grows larger, and its magnetic field weakens. These processes cause periods of intense heat and pressure, which may have been responsible for creating some of the planet's more unusual features.
Finally, a spacecraft has visited Mercury! The Mariner 10 mission flew by the planet in 1974-1975 and provided the first detailed images and measurements of it. Since then, several other missions have passed by or flown directly over the planet, providing more information about its history and future.
Mercury has a volume of 6.1 x 1010 km3, which is equal to 5.4 percent of the volume of the Earth. In other words, you could fit Mercury inside the Earth 18 times and still have some space left over. Mercury and the Earth's orbits are most likely diametrically opposed. This means that they don't pass directly overhead but instead travel around each other in opposite directions at almost the same speed. Because both planets orbit the Sun in approximately the same time period, this arrangement continues for about 90% of each planet's year.
This situation can only last for so long before one planet decides it's had enough and moves away from the Sun. Astronomers call this kind of migration a "tidal force." The gravitational pull of the Sun causes Mercury's orbit to expand or contract, depending on whether Mercury is moving closer or farther from the Sun at any given moment. If this expansion or contraction becomes large enough, then Mercury leaves the Sun's vicinity and begins orbiting the Sun in its own path.
It is believed that early in Mercury's history, when it was more volatile (i.e., more able to melt), it may have been covered in ice. The presence of water molecules under these conditions would not change the fact that Mercury is mostly made of iron, but it would make the planet more volatilized (or fiery). As time passed, this ice melted, leaving behind a network of channels in which the remaining water collected.