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Astronomical Symbol of MercuryMercury is the smallest planet in our solar system. It is only a little larger than the Moon. Aside from being the smallest planet, it is also the closest one to the Sun—roughly 38 million miles away. Mercury rotates much more slowly than Earth making a day on Mercury equivalent to around 59 days on Earth. Mercury’s year is 88 days which means it rotates on its axis three times for every two revolutions around the Sun. An observer on Mercury would therefore experience one day every two Mercurian years. Mercury’s axis has the smallest tilt of any other planet. Its orbital eccentricity (orbital deviation from a perfect circle) is the largest of the planets in the Solar System. At perihelion, Mercury’s distance from the Sun is only about 66% of its distance at aphelion.

Unlike most planets in our solar system, Mercury has no rings nor does it have any moons. Since it is so close to the sun, it has an extremely harsh environment, sharing a lot of similarities with our Moon. Mercury is a terrestrial planet with an extemely very thin atmosphere composed of oxygen, helium, potassium, sodium, and hydrogen. Because Mercury has little to no atmosphere, when it is “daylight” on Mercury, the temperature can get above 800 degrees Fahrenheit. “Nighttime” is the extreme opposite with temperatures that can get below -290 degrees Fahrenheit.


Mercury was first discovered by what appears to have been an Assyrian astronomer in the 14th century BC. Back then, it was known by a name that translates to “the jumping planet.” There are also records that come from ancient Babylon from the 1st millennium BC. These records call Mercury Nabu. This name was given because it was also the name of their messenger that spoke for them to their gods. Ancient Greece called the planet by a name that translates to “the gleaming” because of how the Sun causes the planet to appear as though it can shine. Later, the planet received its actual name because the Romans named it after their god who was known as a swift messenger. This name was given because it moves across the sky very quickly as its orbit is very short and close to the Sun.

Galileo was the first to observe the planet with the help of a telescope back in the 17th century. However, his telescope wasn’t strong enough to view different phases of Mercury. In 1639 it was discovered that the planet actually orbited around the Sun, as they were able to see how it would cross the face of the Sun.

On May 28, 1737, it was observed that Mercury and Venus occulted one another. This means that they lined up perfectly with one another and it was able to have been seen from Earth. This only tends to happen every few centuries, and it hasn’t been seen or recorded since. The next time that this is supposed to happen, it should be in the year 2133.

It was discovered in the 1880’s that the orbit of Mercury around the Sun took 88 days, which is correct. It has been very challenging to study this planet because of how close it is to the Sun, which makes it hard to observe when you only have very limited chances to view it. A book was published in 1934 that actually showed different maps of the surface of Mercury. Radar observations began in 1962 as it was shown to be successful to bounce a radar signal off of the planet and then receive it again later.


Mercury shines as an evening star in the western sky setting about an hour after the sun. Mercury’s apparent magnitude varies in between −2.48 at superior conjunction and +7.25 at inferior conjunction. As a morning “star,” Mercury rises about an hour ahead of the Sun in the eastern sky. There must be a clear, unobstructed horizon on these occasions. Mercury usually appears as a bright “star” with a yellowish or ochre hue.

The brightest or full phase appearance of Mercury is difficult for practical observation, because of the extreme proximity of the Sun. Mercury is best observed at quarter phases, although those phases are of lesser brightness. The quarter phases occur at greatest elongation east and west of the Sun. Mercury rises at its earliest before sunrise, and at greatest eastern elongation, it sets at its latest after sunset.

Mercury can best be seen from the tropics and subtropics as opposed to higher latitudes on Earth. This is because the ecliptic (the Sun’s apparent path through the sky) intersects the horizon at a steep angle. At this point, Mercury is 10° above the horizon when the planet appears directly above the Sun and is at maximum elongation from the Sun at about 28°. Mercury remains visible after sunset up to the Sun being 18º, maximum.


It seems as though Mercury has a very unique makeup. The outer crust is made of a silicate-type material. This seems to be covering a second layer that is made up of iron sulfide. Beneath this, there is a liquid core layer. Quite possibly, there could be a solid core at the very center. It is like Earth in the sense that it is one of four planets that are considered terrestrial, and very rocky. It is tiny, but also very dense.

It is just behind Earth in terms of the densest planet in the solar system. Mercury’s crust is thought to be approximately 35 kilometers thick. One of the better features of the earth’s surface is the number of large ridges that are found everywhere. These ridges are known to be up to several hundred kilometers in length.

Surface Exploration

The surface of Mercury looks very similar to the surface of the Earth’s Moon. There are very large plains, and there are also huge craters everywhere. All of these craters show that this planet is geologically inactive, and has been for a very long time. There have only been two flybys conducted on the planet as of now, so this planet is the least understood of any of the four terrestrial planets that are found in our solar system. There are many different features on the surface: there are highlands, mountains, plains, and valleys. Asteroids and comets hit this planet very hard, which can attest to some of the craters that are seen on it. Comets and asteroids have stricken the entire surface of the planet at some time or other. This happened more often than initially thought because the atmosphere of Mercury is so thin, there is very little that can help protect the surface.

Craters on the surface come in all different kinds of shapes and sizes. There are really small ones, and then there are some that are several kilometers across. The largest crater that has been found has a diameter that has been measured as 1,550 kilometers.

Two different plains regions have been located on the planet. These plains are the first things that were observed on Mercury. They are the spaces that are in between all of the different craters. Many different factors help scientists understand that there used to be a lot of volcanic activity on this planet.

All of these different ridges that are unique to Mercury are thought to have occurred after the planet was formed and the insides of had begun to cool. It looks as though the planet itself had begun to shrink, which caused those ridges.

The flybys that have happened over time have shown quite a bit about the surface of Mercury. There have been signs that point to volcanic activity, although none that are active. Many different shield volcanoes are located on this planet. There are also other, very complex compound volcanoes that have been found within craters on the surface.

Daylight temperatures are incredibly high on Mercury. That being said, there have also been indications that there could possibly be frozen water on the planet. There could be such water deep within some of the craters that never become exposed to sunlight, and therefore never exposed to any form of heat. Such ice can be found with the help of radar detection. Such detections have been seen located near the poles of the planet. This is where it is typically much cooler, mostly because of less direct exposure to the Sun.

There isn’t much of an atmosphere on the planet. However, since there are solar winds which stir up many elements on the surface, there is an exosphere. Calcium, helium, hydroxide, magnesium, oxygen, potassium, along with water vapor are those elements that can help make up the exosphere.


There have been some exploration attempts with space probes. This has been difficult to accomplish because the planet is so close to the Sun. In order to reach Mercury from the Earth, the probe needs to travel over 57 million miles, all while being pulled into the Sun’s gravitational field. It would actually take up more fuel for a spacecraft that would want to land there than it would take enough fuel for some spacecraft to try and escape the entire solar system.

The Mariner 10 was the first spacecraft that visited Mercury. NASA launched this between 1974-1975. The gravitational pull of Venus was used so that the correct angle could be achieved to reach Mercury. It was this that gave the first images of the surface of the planet. Mariner 10 was able to get 200 miles away from the surface of Mercury. It also detected a magnetic field.

Messenger is the second probe to be sent to Mercury. Messenger stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. This probe made flybys back in 2008. This spacecraft finally crashed on Mercury in 2015, and it left a crater that was roughly 16 meters in diameter.

There is currently another mission that is on its way to this planet. This one is known as BepiColombo. The European and Japanese Space Agencies have gotten together to make this space exploration attempt occur. This probe has two jobs. To map out the surface and understand why it has a magnetic field. It will reach Mercury in 2025.

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