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Mercury is the first and smallest planet in the Solar System, the closest to the Sun, at a distance of 0.4 Astronomical units (AU) from it's host star and even though it is in this position, it is not the hottest of the planets. This is thanks to Venus's atmosphere, which uses the greenhouse effect to trap heat, increasing the temperature significantly, making Venus the hottest planet and Mercury the second hottest.[1]

This is juxtaposed to Mercury, as it has a very weak atmosphere, being too hot and light to carry one properly. Because of this, it only has an exosphere, containing hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor.[2]The atmosphere of Mercury, which consists only of an exosphere, is not thick and heavy enough to trap heat. Because of this, the small planet gets very cold at night-time, at extremely low temperatures from -180 to -193 degrees Celsius, while simultaneously, it is the closest planet to the Sun, about 50 million km closer to the Sun, than the actual hottest planet, Venus.

Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; and MESSENGER, launched in 2004, orbited Mercury over 4,000 times in four years before exhausting its fuel and crashing into the planet's surface on April 30, 2015. The BepiColombo spacecraft is planned to arrive at Mercury in 2025. Mercury is largely studied by the spacecraft MESSENGER and many data about the planet's origins, composition, surface etc. have been gathered by this spacecraft telescope. While Mercury is the smallest and least massive planet, it's also the second densest planet, with many characteristics and also, mysteries.

Origins

Mercury's origins are so far only theoretical, but research has led to the following conclusions.

Chemicals (example: Sulphur) were found on Mercury that indicate it's origins came from, not within a close proximity to the Sun, but as far out as Mars, or maybe even farther, in the asteroid belt.[3]

Mercury formed about 4.54 billion years ago, when gravity pulled materials and dust together. Then, a possible, strong planetary collision occurred, causing the planet to move closer and lose some of it's crust and mantle. However, the gravity of Mercury at 3.7 m/s², was strong enough to keep the planet together, causing the crust and the mantle to reform.[4]

Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. It would initially have had twice its present mass, but as the Proto-Sun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and possibly even as high as 10,000 K. Much of Mercury's surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" that could have been carried away by a very strong Solar Wind.

A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material and not gathered by Mercury. Each hypothesis predicts a different surface composition, and there are two space missions set to make observations. MESSENGER, which ended in 2015, found higher-than-expected potassium and sulfur levels on the surface, suggesting that the giant impact hypothesis and vaporization of the crust and mantle did not occur because potassium and sulfur would have been driven off by the extreme heat of these events. BepiColombo, which will arrive at Mercury in 2025, will make observations to test these hypotheses. The findings so far would seem to favor the third hypothesis; however, further, more detailed analysis of the data is needed.

Magnetic field and Magnetosphere

Diagram showing the weak magnetic lines of Mercury. Because Mercury doesn't have a strong magnetic field, it's atmosphere has been blown away, leaving only a very thin exosphere.

Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by Mariner 10, it is about 1.1% the strength of Earth's. The magnetic-field strength at Mercury's equator is about 300 nT. Like that of Earth, Mercury's magnetic field is dipolar. Unlike Earth's, Mercury's poles are nearly aligned with the planet's spin axis. Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.

It is likely that this magnetic field is generated by a dynamo effect, in a manner similar to the magnetic field of Earth. This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong tidal effects caused by the planet's high orbital eccentricity would serve to keep the core in the liquid state necessary for this dynamo effect.

Mercury's magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within Earth, is strong enough to trap solar wind plasma. This contributes to the space weathering of the planet's surface. Observations taken by the Mariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles in the planet's magnetotail indicate a dynamic quality to the planet's magnetosphere.

Strength of Mercury's magnetic field.

During its second flyby of the planet on October 6, 2008, MESSENGER discovered that Mercury's magnetic field can be extremely "leaky". The spacecraft encountered magnetic "tornadoes" – twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space – that were up to 800 km wide or a third of the radius of the planet. These twisted magnetic flux tubes, technically known as flux transfer events, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface via magnetic reconnection This also occurs in Earth's magnetic field. The MESSENGER observations showed the reconnection rate is ten times higher at Mercury, but its proximity to the Sun only accounts for about a third of the reconnection rate observed by MESSENGER.

Composition

Geological map of the planet Mercury. Dark browns and tans - pre-Tolstojan craters, basins, and intercrater plains. Lighter browns and orange – Tolstojan craters and plains, respectively. Blues - units of the Caloris basin and craters of Calorian age. Pink - Calorian smooth plains. Greens and Yellows - Mansurian and Kuiperian age impact craters. [5]

The interior of Mercury and some cool facts about the smallest but second densest planet Mercury.

Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 kilometres (1,516.0 mi). Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material.

The composition of Mercury is unique. Mercury is often called ''the iron planet'' or ''a ball of iron'' because it contains very large amounts of iron. A lot of iron on the surface of Mercury, is covered by rocks, that are burnt away in the form of dust. Mercury is a ball of iron with an exosphere and very high density. In the article about the composition of the little planet, we discuss it's interior, it's exosphere and we also look at data about Mercury's density, and how it is that high, since Mercury is the second densest planet. The composition and the exosphere are much denser than the other planets except Earth.

Mercury appears to have a solid silicate crust and mantle overlying a solid, iron sulfide outer core layer, a deeper liquid core layer, and a solid inner core. Although Mercury is one of the four rocky planets, and even though it is the smallest, even smaller than Ganymede or Titan, it is heavier than both Venus and Mars, due to its highly (70.8%) metallic and silicate (29.2%) make-up. In fact, its density is the second highest in the Solar System, 5.427 g/cm³. Mercury has no real atmosphere, but a layer, the exosphere, which is made up of hydrogen, helium, oxygen, sodium, magnesium, potassium and very small amounts of calcium and water vapor. Because of this, not only is the planet cold at night, but it's surface also has many craters, as well as deep plains of craters inside larger craters, because of asteroids that do not burn up in Mercury's very thin and tenuous exosphere, which consists of atoms blasted off its surface by the Solar Wind.

Mercury's density is the second highest in the Solar System at 5.427 g/cm3, only slightly less than Earth's density of 5.515 g/cm3. If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be even denser than those of Earth, with an uncompressed density of 5.27 g/cm3 versus Earth's 4.4 g/cm3.

Mercury is made primarily out of iron. On top of that, the core makes up 55% of Mercury, compared to the 17% that occupies the Earth.[6]Mercury's crust is rocky and contains frozen water at the poles, since sunlight hits the planet always a little in these regions. It is unknown whether liquid water exists bellow this little water ice, but if there is, maybe there is potential micro-life on the poles of the little planet.

The planet's relatively large iron core and 500-700 km. mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, or that it was prevented from fully accreting by the young stellar energy of the Sun. Mercury's density can be used to infer details of its inner structure. Although Earth's high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.

Mercury's interior and magnetic field.

Geologists estimate that Mercury's core occupies about 55% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has a molten core. Surrounding the core is a 500–700 km (310–430 mi) mantle consisting of silicates. Based on data from the Mariner 10 mission and Earth-based observation, Mercury's crust is estimated to be 35 km (22 mi) thick. However, this model may be an overestimate and the crust could be 26 ± 11 km (16.2 ± 6.8 mi) thick based on an Airy isostacy model. One distinctive feature of Mercury's surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had already solidified.

Mercury's core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal–silicate ratio similar to common chondrite meteorites, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass. Early in the Solar System's history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass and several thousand kilometers across. The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component. A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of the Moon.

Surface and Plains

Map of Mercury's northern hemisphere by the MLA instrument on MESSENGER

The surface of Mercury is the least understood of all planets in the Solar System. All sources and knowledge come from a single flyby of the 1975 Mariner 10. This, however, is not constant, as new data is being analyzed from the 2008 MESSENGER. This spacecraft is also the one that discovered first evidence of amounts of magnesium and water vapor in the upper, outer exosphere of the small planet, and important data about the surface of Mercury have been gathered by MESSENGER.

MASCS spectrum sc an of Mercury's surface by MESSENGER.

Mercury has a surface consisting of many craters and also mountains, ridges, valleys and plains, similar to Earth, except more chaotic, as Earth doesn't regularly feature meteor impacts. 75% of Mercury's surface is covered in craters, similar to the Moon and the planet's surface only known geological features are lobed ridges or rupes that were probably produced by a period of contraction early in its history. Mercury's surface, as mentioned a few times above, in the composition and origins of Mercury, is prone to Solar Wind. That's because Mercury has neither an atmosphere, but only a thin exosphere, nor a very strong magnetosphere, since the gravity of Mercury is the weakest of all the planets.

The surface of Mercury also has these basic features, it contains 9 overlapping volcanic vents on the southwest rim of the Caloris Planitia, with each one of them being up to 8-9 km in diameter and they are predicted to be billions of years old. It is not clear whether there is volcanic lava or a large sheet of impact melt. Another distinctive feature of Mercury's surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when Mercury's crust had already solidified.[7] The surface of Mercury also has a lot of iron. As mentioned above, in the composition of Mercury, there are large amounts of iron on it's surface, covered by rocks that are burnt away during Mercury's daytime, because of extreme temperatures, from 426 to 434 °C. These rocks leave iron on Mercury's surface, while they are burnt away in the form of planetary dust.

Mercury's surface

The planet's mantle is chemically heterogeneous, suggesting the planet went through a magma ocean phase early in its history. Crystallization of minerals and convective overturn resulted in layered, chemically heterogeneous crust with large-scale variations in chemical composition observed on the surface. The crust is low in iron but high in sulfur, resulting from the stronger early chemically reducing conditions than is found in the other terrestrial planets. The surface is dominated by iron-poor pyroxene and olivine, as represented by enstatite and forsterite, respectively, along with sodium-rich plagioclase and minerals of mixed magnesium, calcium, and iron-sulfide. The less reflective regions of the crust are high in carbon, most likely in the form of graphite. Names for features on Mercury come from a variety of sources. Names coming from people are limited to the deceased. Craters are named for artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field. Ridges, or dorsa, are named for scientists who have contributed to the study of Mercury. Depressions or fossae are named for works of architecture. Montes are named for the word "hot" in a variety of languages. Plains or planitiae are named for Mercury in various languages. Escarpments or rupēs are named for ships of scientific expeditions. Valleys or valles are named for abandoned cities, towns, or settlements of antiquity.

100 mile scarp on Mercury and craters.

There are two geologically distinct plains regions on Mercury. Gently rolling, hilly plains in the regions between craters are Mercury's oldest visible surfaces, predating the heavily cratered terrain. These inter-crater plains appear to have obliterated many earlier craters, and show a general paucity of smaller craters below about 30 km in diameter. Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to the lunar maria. Notably, they fill a wide ring surrounding the Caloris Basin. Unlike lunar maria, the smooth plains of Mercury have the same albedo as the older inter-crater plains. Despite a lack of unequivocally volcanic characteristics, the localisation and rounded, lobate shape of these plains strongly support volcanic origins. All the smooth plains of Mercury formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket. The floor of the Caloris Basin is filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they are volcanic lavas induced by the impact, or a large sheet of impact melt.

Estimated details of the impact of MESSENGER on April 30, 2015.

One unusual feature of Mercury's surface is the numerous compression folds, or rupes, that crisscross the plains. As Mercury's interior cooled, it contracted and its surface began to deform, creating wrinkle ridges and lobate scarps associated with thrust faults. The scarps can reach lengths of 1000 km and heights of 3 km. These compressional features can be seen on top of other features, such as craters and smooth plains, indicating they are more recent. Mapping of the features has suggested a total shrinkage of Mercury's radius in the range of ~1 to 7 km. Most activity along the major thrust systems probably ended about 3.6–3.7 billion years ago. Small-scale thrust fault scarps have been found, tens of meters in height and with lengths in the range of a few km, that appear to be less than 50 million years old, indicating that compression of the interior and consequent surface geological activity continue to the present. The Lunar Reconnaissance Orbiter discovered that similar but smaller thrust faults exist on the Moon.

Volcanism

Picasso crater — the large arc-shaped pit located on the eastern side of its floor are postulated to have formed when subsurface magma subsided or drained, causing the surface to collapse into the resulting void.

Images obtained by MESSENGER have revealed evidence for pyroclastic flows on Mercury from low-profile shield volcanoes. MESSENGER data has helped identify 51 pyroclastic deposits on the surface, where 90% of them are found within impact craters. A study of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval.

A "rimless depression" inside the southwest rim of the Caloris Basin consists of at least nine overlapping volcanic vents, each individually up to 8 km in diameter. It is thus a "compound volcano". The vent floors are at a least 1 km below their brinks and they bear a closer resemblance to volcanic craters sculpted by explosive eruptions or modified by collapse into void spaces created by magma withdrawal back down into a conduit. Scientists could not quantify the age of the volcanic complex system, but reported that it could be of the order of a billion years.

References

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