Core

Orbit

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Terrestrial planets have similar structures – a core composed of heavy metals which is usually iron, solid surfaces of craters, canyons, mountains and volcanoes and secondary atmospheres (generated through volcanic activity and comet impacts), as opposed to the gas giants which has primary atmospheres (captured directly from the original nebula). Note that while the moon has similar structure, it doesn't have an iron core. Plutoids are also similar to terrestrial planets in that they have solid surfaces; but instead or rocky surfaces, they are composed of icy materials.

Mercury is one dense planet composed of iron and nickel, with a very thin atmosphere. Venus has a thick atmosphere, mostly of carbon dioxide, which causes a greenhouse effect. Earth has the highest density among these planets with an atmosphere composed of elements that are conducive for human habitation and Mars has a rocky red surface and a thin atmosphere. Note that among these 4, only Earth and Mars has moons; Earth has one and Mars has two). Extra-solar terrestrial planets are the subject of much interest, as people naturally want to know of the existence of other planets that are habitable for humans, or for the theoretical extraterrestrial beings.

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When compared to planet Earth, that density measurement is only a bit lesser. If you would have the chance of standing on planet Mercury’s surface, you will realize that the gravitational force in Mercury is only 38% of what you experience on planet Earth.

Images of planet Mercury taken from a spacecraft that flew within it, one can say that it is hard to differentiate planet Mercury from the Earth’s moon. The two may be described as both airless and rocky worlds that are covered by impact craters. Mercury is said to have no real atmosphere for the heat of the sun to hold just like the moon. Because of this, the planet’s side that faces the sun has its temperature rising to 700 Kelvin, while the temperature of that side that faces away from the sun goes down to only about 80 Kelvin. There is also a huge possibility that craters in the planet Mercury’s southern and northern poles have within them reservoirs of ice bathed in eternal shadows.

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Even with the recent probe, there is no large amount of information on the planet. Therefore, is it safe to claim that there is also no ancient information about this elusive planet? The answer here is actually somewhat surprising. While like the other five planets closest to the sun (excluding Earth, of course), no one really knows the exact date when Mercury was discovered, there are ancient astronomers who have recorded data on ancient Mercury even before Nicolas Copernicus and Galileo Galilee.

Perhaps the best known and the earliest recorded observation of the planet can from the Babylonian tablet called the Mul Apin. According to estimates, these tablets were compiled during 1000 BC or the first millennium. The Babylonians called Mercury on the tablets as Nabu, the god of writing and writing for the Babylonians. Astrologers in ancient Greece and China also have recorded information and observations of the planet. Of course, it is a given fact that Copernicus and Galileo made the all information discoveries regarding the true nature of Mercury, as well as the other four planets.

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The surface of Mercury is very similar to that of Earth’s moon, with closer examination, you can’t tell them apart. Mercury has had its share of outer space objects landing on its surface hence the formation of some of its craters. The other craters on the planet are caused by volcanic eruptions but these eruptions have also caused some of the smooth surfaces on the planet. When a hole or crater is created on the surface or just low lying areas, the magma from the volcanoes flow and fill up those gaps thus creating the smooth areas.

Because Mercury is abundant in craters, two of the biggest craters known on the planet are Carolis Basin which spans an area of 1,550 km in diameter and the largest Skinakas Basin with a diameter of 1,600 km. Because of Mercury’s distance from the Sun, every once in a while, it experiences intense tides that changes the level of its surface. Each of the craters on the planet has certain different features distinguishable from its surroundings. One of Mercury’s craters is said to have been from an impact that was so intense that it caused ripples throughout it surrounding causing small canyon or mountain like features.

Mercury's Craters

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If Venus is comparable to the Earth in appearance, Mercury is comparable to the Earth’s moon. Both are heavily cratered and have plains reminiscent of Lunar maria. Due to the lack of “hands-on” studies made to Mercury, it is the most mysterious of the four terrestrial planets. A great mystery that surrounds Mercury is the absence of a natural satellite.

There has been no explanation regarding Mercury’s lack of a natural satellite. However, there are several theories and assumptions circulation that could answer this mystery:

• The proximity of Mercury to the Sun would make it hard to maintain a natural satellite. Earth’s moon was caused by a large impact between the planet and a large object with a mass comparable to Mars, which resulted to the appearance of the moon. Even if the same thing happened to Mercury, the Suns powerful gravitational force would pull it away from Mercury.
• Mercury will never have the opportunity to capture smaller and less massive celestial bodies. Several planets obtained satellites by capturing asteroids and smaller celestial bodies that strayed too close. Mars obtained its two moons, Phobos and Deimos in this manner because of the asteroid belt near the red planet. Mercury is too far away from the asteroid belt for this to occur.
• Pluto’s Nix and Hydra became natural satellites similar to how Mars obtained Phobos and Deimos. The only difference is that Nix and Hydra were most likely comets instead of asteroids. Since comets are icy celestial bodies traveling in space, it would melt because of its proximity to the sun.

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Since Mercury is a classical planet, it isn’t possible to know who discovered the planet. There are people, of course, who made serious probes and discoveries regarding the planet. The earliest recorded document was made by a Babylonian astrologer and possibly written by an Assyrian astronomer. The Greeks, seeing the planet during the morning and evening sky, had a name for it. It was called Apollo during the morning and Hermes at night. However, they did not know the two stars were actually just one object. It was later during the 4th century that the Greeks discovered that Apollo and Hermes were the same.

The name could be attributed to the Romans, who named the planet after the messenger god due to its visibility (it can only be seen during very short periods). Of course, the penultimate observation was that of Galileo Galilee, who proved the claim of Nicolas Copernicus regarding the classical planets. It was actually the French philosopher Pierre Gassendi who saw the planet in transit, as Galileo’s telescope wasn’t strong enough to see as far as Mercury (he only saw Mercury, in part, when he observed Venus).

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Before 1974, the existence of an atmosphere in Mercury was a topic that produced heated debates. Because Mercury is too close to the Sun, it was very hard to study. It wasn’t until the Mariner 10 spacecraft discovered the exosphere of Mercury's atmosphere. The data gathered by the Mariner 10 spacecraft confirmed that only an exosphere existed on Mercury and is similar to the Moon in having no atmosphere. On 2008 the Messenger spacecraft found magnesium in its exosphere.

Mercury's exosphere varies in temperature. It all depends on the location and the type of matter on the surface. The calcium atoms are some of the hottest elements on this planet, with temperatures reaching 12,000 k – 20,000 K. Sodium atoms are also very hot with the ones located on the equator having temperatures reaching 750 – 1500 K. The ones on the poles vary from 1500-3500 K. Atomic hydrogen is relatively lower than these elements with temperatures around 420 K.

Most of the particles in the exosphere escape into space. This implies that the elements in the exosphere are constantly being supplied by its constituents. The source of the hydrogen and helium in the exosphere is the Solar Wind. Other atoms and molecules come from the crust of Mercury itself. The main supply of the elements found in Mercury’s exosphere are from vaporized surface material caused by meteors hitting its surface, energetic charged particles that are emitted by the Solar Wind and the desorption of atoms of alkali metals.

Mercury – 2nd Messenger Flyby

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The planet still get hot on the side that faces the Sun, it experiences the most extreme temperatures, which can reach as high as 700° Kelvin (427°C). In fact, Mercury receives 6.5 times the amount of sunlight that the Earth does and every part of the surface of the planet receives direct sunlight for 59 days at a time.

On the dark side of the planet, on the other hand, temperatures can fall to as low as 100°K (-173°C) , making it the coldest of all the inner planets (which also include Earth and Mars). There are also believed to be craters dotting the north and south poles, and temperatures in them might even reach temperatures as low as 90°K. The mean temperature of Mercury averages to around 452°K.

Between the extreme temperatures and the thin atmosphere, there is almost no possibility of plant or animal life on Mercury as we know it. This is because liquid water on Mercury is impossible, as it would be boiled away by the heat of the Sun.

Mercury also has no weather due to the lack of an atmosphere. Its surface strongly resembles that of the Moon, as small comets and meteors freely crash into the planet without burning up. Just like Earth, however, Mercury is believed to have a core of solid iron which makes up some three-fourths of its radius and is surrounded by a layer of liquid iron.

The Planet Mercury

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A day in Mercury is twice as long as a year in Mercury. One solar day lasts 176 earth days, which is almost twice its orbital period, which means that one year is half a day. The axial tilt of Mercury is 0.027 degrees. This is the smallest axial tilt in the solar system with Jupiter coming in at second.

In some parts of the surface of Mercury, the sun rises halfway then sets again and then rises again. This happens is equivalent to one day in Mercury. The reason for this is that four days before perihelion, the angular orbital velocity of Mercury is equivalent to its angular rotational velocity. The apparent motion of the sun stops. When perihelion occurs the rotational velocity is less than its angular orbital velocity. After four days, the sun’s apparent motion will return back to normal.

It was once believed that only one side of Mercury faced the sun. It wasn’t until 1965 that it was discovered that the planet has a 3:2 spin-orbit resonance. It rotates three times every two revolutions; a day on Mercury is equivalent to 58.66 (88×2/3) Earth days. The reason why they thought that only one side of Mercury faced the Sun was a mere coincidence as every time Mercury was observed, the same side is seen by astronomers.

The Inner Planet

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