Mercury (planet): Location, History, Characteristics, Composition

Mercury is the closest planet to the Sun and the least massive in the Solar System. Its distance from the Sun is between 0.31 and 0.47 units (or 46 and 70 million kilometers), which corresponds to an orbital eccentricity of 0.2 — more than twelve times that of Earth, and by far the highest for a planet in the Solar System. It is visible to the naked eye from Earth with an apparent diameter of 4.5 to 13 arcseconds, and an apparent magnitude of 5.7 to −2.3; its observation is however made difficult by its elongation always less than 28.3 ° which drowns it most often in the brightness of the sun.

In practice, this proximity to the sun implies that it can only be seen near the western horizon after sunset or near the eastern horizon before sunrise, usually at dusk.

Mercury has the particularity of being in 3:2 spin-orbit resonance, its period of revolution (~88 days) being worth exactly 1.5 times its rotation period (~59 days), and therefore half of a solar day (~176 days). Thus, relative to fixed stars, it rotates on its axis exactly three times every two revolutions around the Sun.

Orbital characteristics
Semi-major axis	57,909,050 km (0.387,098 au)
Aphelion	69,816,900 km (0.466,701 au)
Perihelion	46,001,200 km (0.307,499 au)
Orbital circumference	359,966,400 km (2,406,226 au)
Eccentricity	0.205 6
Period of revolution	87.969 days
Synodic period	115.88 days
Average orbital speed	47.362 km/s
Maximum orbital speed	58.98 km/s
Minimum orbital speed	38.86 km/s
Tilt on the ecliptic	7,00°
Ascending node	48,33°
Perihelion argument	29,12°
Known satellites	0
Physical characteristics
Equatorial radius	2,439.7 km (0.383 Earth)
Polar radius	2,439.7 km (0.384 Earth)
Volumetric mean radius	2,439.7 km (0.383 Earth)
Flattening	0
Equatorial perimeter	15,329 km (0.383 Earth)
Area	7.48 × 10⁷ km² (0.147 Earth)
Volume	6.083 × 10¹⁰ km³ (0.056 Earth)
Mass	3.301 1 × 10²³ kg (0.055 Earth)
Global density	5,427 kg/m³
Surface gravity	3.70 m/s² (0.378 g)
Release speed	4.25 km/s
Rotation period (sidereal day)	58.645 8 days
Rotational speed (at the equator)	10.892 kph
Axis tilt	0.035 2 ± 0.001 7°
Right ascension of the North Pole	281,01°
Declination of the North Pole	61,45°
Visual geometric albedo	0,142
Bond’s Albedo	0,088
Solar irradiance	9,126.6 W/m² (6,673 Earths)
Blackbody equilibrium temperature	433.9 K (160.9 °C)
Temperature Maximum	700 K (427 °C)
Temperature Average	440 K (167 °C)
Temperature Minimum	90 K (−183 °C)
Characteristics of the atmosphere
Atmospheric pressure	5 × 10⁻¹⁰ Pa
Total mass	Less than 10,000 kg
History
Babylonian deity	Nabu
Greek deity	Stilbôn and Ἑρμῆς
Chinese Name (Related Item)	Shuǐxīng 水星 (water)

Mercury is a terrestrial planet, as are Venus, Earth and Mars. It is nearly three times smaller and almost twenty times less massive than Earth but almost as dense as it. Its remarkable density — surpassed only by that of the Earth, which would be lower without the effect of gravitational compression — is due to the size of its metallic core, which would represent 85% of its radius, against about 55% for the Earth.

Like Venus, Mercury is almost spherical—its flattening can be considered zero—due to its very slow rotation. Lacking a true atmosphere that can protect it from meteorites (there is only one exosphere with a ground pressure of less than 1 nPa or 10⁻¹⁴ atm), its surface is very heavily cratered and broadly similar to the far side of the Moon, indicating that it has been geologically inactive for billions of years. This absence of atmosphere combined with the proximity of the Sun generates surface temperatures ranging from 90 K (−183°C) at the bottom of polar craters (where the Sun’s rays never reach) to 700K (427°C) at the subsolar point at perihelion. The planet is also devoid of natural satellites.

Only two space probes have studied Mercury. Mariner 10, which flew by the planet three times in 1974–1975, mapped 45% of its surface and discovered the existence of its magnetic field. The MESSENGER probe, after three flybys in 2008-2009, goes into orbit around Mercury in March 2011 and carries out a detailed study including its topography, geological history, magnetic field and exosphere. The BepiColombo spacecraft aims to orbit Mercury in December 2025.

The planet Mercury owes its name to the messenger of the gods in Roman mythology, Mercury. The planet is so named by the Romans because of the speed with which it moves in the sky. The astronomical symbol of Mercury is a circle placed on a cross and bearing a semicircle in the shape of horns (Unicode: ☿ ). It is a representation of the caduceus of the god Hermes, equivalent to Mercury in Greek mythology. Mercury also gave its name to the third day of the week, Wednesday (“Mercurii dies“).

Orbit and rotation of Mercury

Eccentricity

Mercury has the highest orbital eccentricity of the planets in the Solar System, with a value of about 0.21. This implies that its distance from the Sun varies from 46 to 70 million kilometers during its revolution. The diagram on the left illustrates the effects of eccentricity, showing Mercury’s orbit superimposed on a circular orbit with the same semi-major axis. This variation in distance from the Sun causes Mercury’s surface to be subject to a tidal force exerted by the Sun that is about 17 times stronger than that of the Moon on Earth. Combined with its 3:2 resonance of the planet’s rotation around its axis, this also results in complex variations in surface temperature.

The eccentricity of Mercury’s orbit varies chaotically from 0 (circular orbit) to a very high value of more than 0.45 over several million years due to the influence of other planets. In 1989, Jacques Laskar of the Bureau of Longitude demonstrated that the inner planets of the Solar System all had chaotic races. However, Mercury is the one whose movement is the most chaotic.

Orbit

Mercury’s orbit is inclined 7 degrees to the plane of Earth’s orbit (ecliptic), as shown in the diagram on the right. Therefore, transits of Mercury in front of the Sun can only take place when the planet crosses the plane of the ecliptic, at the moment when it is between the Earth and the Sun, that is, in May or November. This happens about every seven years on average.

The inclination of Mercury’s axis of rotation on its orbital plane is the lowest in the Solar System, barely 2 arc minutes, or about 0.03 degrees. This is significantly weaker than that of Jupiter, which has the second smallest axial tilt of any planet, at 3.1 degrees. This means that for an observer at the poles of Mercury, the center of the sun never rises more than 2 arc minutes above the horizon.

At some points on Mercury’s surface, an observer could see the sunrise at just over two-thirds of the horizon, then set before rising again, all on the same mercurial day. Indeed, four Earth days before perihelion, the angular orbital velocity of Mercury is equal to its angular rotational speed, so that the apparent motion of the sun ceases; closer to perihelion, Mercury’s angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer of Mercury, the sun seems to be moving in a retrograde direction. Four Earth days after perihelion, the normal apparent movement of the sun resumes and it rises again in the east to set in the west.

For the same reason, there are a couple of points on the equator of Mercury (one of them being located in the Caloris basin), distant 180 degrees in longitude, where at each of which, every other Mercurian year (which is equivalent to once a Mercurian day), the sun passes over it from east to west, then reverses its apparent movement and passes again over from west to east (during the retrograde movement), then reverses its movement a second time and passes over it a third time from east to west.

During the alternating Mercurian year, it is at the other point of this couple that this phenomenon occurs. Since the amplitude of retrograde motion is low at these points, the overall effect is that, for two or three weeks, the sun is almost stationary above the point, and is at its highest level of brightness because Mercury is at perihelion. This prolonged exposure at the time when the planet is closest to the Sun makes these two points the hottest places on Mercury (hence the name Caloris, meaning “heat” in Latin). One of these points served as a reference for the meridian 0°.

Conversely, there are two other points on the equator, 90 degrees longitude away from the first, where the sun passes over it only when the planet is at aphelion, every other Mercurian year, at a time when the apparent motion of the sun in Mercury’s sky is relatively fast. These points thus receive much less solar heat than those of the couple described above.

The result is a Mercurian day that is also “strange” for an observer located there. This one will see the sunrise and then set, then rise to the eastern horizon; and at the end of the day in the West, the sun will set and then rise, to set again. This phenomenon is also explained by the variation in the orbital speed of Mercury: four days before perihelion, the orbital (angular) velocity of Mercury being exactly equal to its (angular) speed of rotation, the movement of the sun seems to stop.

Mercury reaches its inferior conjunction (the point where it is closest to Earth) every 116 Earth days on average (the so-called synodic period), but this interval can range from 105 days to 129 days, due to the eccentric orbit of the planet. Between 1900 and 2100, Mercury came at least (and therefore will not come closer) to Earth by about 82.1 × 10⁶ kilometers (or 0.55 astronomical units), on May 31, 2015. Its period of retrograde movement can vary from 8 to 15 Earth days on either side of the inferior conjunction. This large amplitude is also due to the high orbital eccentricity of the planet.

Due to its proximity to the Sun, it is Mercury and not Venus that is the closest planet to Earth on average, even if the orbit of Venus is the closest to that of Earth. This reasoning can even be extended, and Mercury is actually the closest planet on average to each of the other planets in the Solar System, including Uranus and Neptune (orbiting at 19 and 30 AU, respectively).

Rotation

While studying Mercury in order to draw a first map, Schiaparelli noticed after several years of observation that the planet always presents the same face to the Sun, as the Moon does with the Earth. He then concluded in 1889 that Mercury is synchronized by tidal effect with the Sun and that its rotation period is equivalent to a Mercurian year, or 88 Earth days. However, this duration is erroneous and it was not until the 1960s that astronomers revised it downwards.

Thus, in 1962, Doppler radar observations were made by the Arecibo radio telescope on Mercury to learn more about the planet and to verify if the rotation period is equal to the period of revolution. The temperatures recorded on the side of the planet supposed to be always exposed to the shade are then too important, which suggests that this dark side is actually sometimes exposed to the Sun. In 1965, the results obtained by Gordon H. Pettengill and Rolf B. Dyce reveal that Mercury’s rotation period is actually 59 Earth days, with an uncertainty of 5 days.

This period was later adjusted in 1971 to 58.65 days to ± 0.25 days thanks to more precise measurements — again by radar — made by R.M. Goldstein. Three years later, the Mariner 10 probe brought better accuracy, measuring the rotation period at 58.646 ± 0.005 days. It turns out that this period is exactly equal to 2/3 of the revolution of Mercury around the Sun; This is called a 3:2 spin-orbit resonance.

This 3:2 resonance, a specificity of Mercury, is stabilized by the variance of the tidal force along Mercury’s eccentric orbit, acting on a permanent dipole component of Mercury’s mass distribution and by the chaotic motion of its orbit. In a circular orbit, there is no such variance, so the only stabilized resonance for such an orbit is 1:1 (e.g., Earth-Moon). At perihelion, where the tidal force reaches its maximum, it stabilizes the resonances, such as 3:2, by forcing the planet to point its axis of least inertia (where the diameter of the planet is largest) approximately towards the Sun.

The reason astronomers thought Mercury was locked with the Sun is that, whenever Mercury was best placed to be observed, it was always at the same point in its orbit (in 3:2 resonance), thus presenting the same face to Earth; which would also be the case if it were totally synchronized with the Sun. This is because Mercury’s actual rotation period of 58.6 days is almost exactly half of Mercury’s synodic period of 115.9 days (i.e. the time taken by Mercury to return to the same Earth–Mercury–Sun configuration) relative to Earth. Schiaparelli’s error can also be attributed to the difficulty of observing the planet with the means of time.

Because of its 3:2 resonance, although a sidereal day (the rotation period) lasts about 58.7 Earth days, the solar day (the duration between two successive returns of the Sun to the local meridian) lasts 176 Earth days, i.e. two Mercurian years. This implies that a day and a night each last exactly one year on Mercury, or 88 Earth days (almost a quarter).

Accurate modeling based on a tidal model demonstrated that Mercury was captured in the 3:2 spin-orbit state at a very early stage of its history, between 10 and 20 million years after its formation. In addition, numerical simulations have shown that a future secular resonance with Jupiter could increase Mercury’s eccentricity to a point where there would be a 1% chance that the planet would collide with Venus within 5 billion years. The long-term prediction of Mercury’s orbit is part of the mechanics of chaos: some simulations even show that the planet could be ejected from the Solar System.

Precession of perihelion

As with all planets in the Solar System, Mercury’s orbit experiences a very slow perihelion precession around the Sun, that is, its orbit is itself rotating around the Sun. However, unlike the other planets, Mercury’s perihelion precession period does not agree with predictions made using Newtonian mechanics.

Indeed, Mercury is experiencing a slightly faster precession than can be expected by applying the laws of celestial mechanics, and is about 43 arc seconds ahead per century.

Search for a third planet

Astronomers therefore first thought of the presence of one or more bodies between the Sun and Mercury’s orbit whose gravitational interaction would disrupt the motion of the latter. The French astronomer Urbain Le Verrier, who had discovered the planet Neptune in 1846 from anomalies in the orbit of Uranus, looked into the problem and suggested the presence of an unknown planet or a second asteroid belt between the Sun and Mercury. Calculations made, taking into account the gravitational influence of these bodies, then had to agree with the observed precession.

On March 28, 1859, Le Verrier is contacted by the French doctor Edmond Lescarbault about a black spot that he would have seen pass in front of the Sun two days before and which was probably, according to him, an intramercurian planet. Le Verrier then postulates that this planet – which he names Vulcan – is responsible for the anomalies of Mercury’s motion and sets out to discover it. From Lescarbault’s information, he concluded that Vulcan would orbit the Sun in 19 days and 7 hours at an average distance of 0.14 AU. He also deduces a diameter of about 2,000 km and a mass of 1/17^th of that of Mercury. This mass is however far too small to explain the anomalies, but Vulcan remains a good candidate for the largest body of a hypothetical asteroid belt internal to the orbit of Mercury.

Le Verrier then took advantage of the solar eclipse of 1860 to mobilize all French astronomers to locate Vulcan, but no one could find it. The planet was then searched for decades, without success although some astronomers thought they had seen it until a relativistic explanation was proposed.

Explanation by general relativity

In 1916, Albert Einstein advanced the theory of general relativity. By applying the so-called post-Keplerian parameters of his theory to the motion of Mercury, Einstein provided the explanation for the observed precession by formalizing gravitation as being affected by the curvature of space-time. The precession formula for the orbit obtained by Einstein is:

Where is the semi-major axis of the ellipse, its eccentricity, the gravitational constant, the mass of the Sun, and the period of revolution on the ellipse.

With numerical values:

and,

we find 0.1038 arcseconds per revolution, which corresponds, with the 415 revolutions of Mercury per century, to:

$Einstein formule Mercury$ arc seconds per century.

The effect is small: only 43 arcseconds per century for Mercury, so it takes about 2.8 million years for a complete turn in excess (or twelve million revolutions), but coincides well with the advance of perihelion previously measured. This validated prediction was one of the first great successes of nascent general relativity.

Physical characteristics of Mercury

Internal structure

Mercury is one of the four terrestrial planets in the Solar System, and has a rocky body like Earth. It is also the smallest, with an equatorial radius of 2,439.7 km. Mercury is also smaller — though more massive — than two natural satellites of the Solar System, Ganymede and Titan. Mercury is composed of about 70% metals (mainly in the core) and 30% silicate (mainly in its mantle). 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 ignored, it is Mercury that would be denser with 5.3 g/cm 3 against 4.4 g/cm ³ for the Earth, due to a composition with denser materials.

Mercury’s density can be used to infer details about its internal structure. Although Earth’s high density is a significant result of gravitational compression, especially at the Earth’s 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 bulky and rich in iron.

Geologists estimate that Mercury’s core occupies about 85% of its radius. would represent about 61.4% of its volume compared to 17% for Earth, for example. Research published in 2007 once suggested that Mercury’s core was totally liquid (nickel and iron). More recently, however, other studies using data from the MESSENGER mission, completed in 2015, lead astronomers to believe that the planet’s inner core is actually solid. Around the core is a solid outer central layer of iron sulfide and a mantle composed of silicates. According to data from the Mariner 10 mission and terrestrial observations, Mercury’s crust would be between 35 and 54 km thick. A distinctive feature of Mercury’s surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. They are thought to have formed when Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.

Metallicity

Mercury’s core has a higher iron content than any other object in the Solar System. This high concentration of iron is the reason why it is sometimes referred to as “the metallic planet” or “the iron planet”. Understanding the origin of this concentration would allow us to learn a lot about the early solar nebula and the conditions under which the solar system formed. Three hypotheses have been proposed to explain Mercury’s high metallicity and its gigantic core.

The most widely accepted theory on this is that Mercury originally had a metal-to-silicate ratio similar to that of common chondrite meteorites, which are thought to be typical of Solar System rock matter, and with a mass about 2.25 times its current mass. Then, at the beginning of the history of the Solar System, Mercury would have been struck by a planetesimal about 1/6th of this mass and several thousand kilometers in diameter. The impact would have removed much of the original crust and mantle, leaving behind the metallic core that would have merged with that of the planetesimal, and a thin mantle. A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of the Moon as a result of Earth’s collision with the Theia impactor.

Alternatively, Mercury could have formed from the solar nebula before the Sun’s energy production stabilized. Initially, its mass would have been double that of today, but when the protostar contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and perhaps even reach 10,000K. Much of Mercury’s surface rock could thus have been vaporized at these temperatures, forming an atmosphere of rock vapor that would then have been carried away by the solar wind.

A third hypothesis assumes that the solar nebula caused a drag on the particles from which Mercury accreted, meaning that lighter particles were lost from the accretion material and were not collected by Mercury. Thus, the rate of heavy elements, such as iron, present in the solar nebula was greater in the vicinity of the Sun, or even these heavy elements were gradually distributed around the Sun (the further away from them, the less heavy elements there were). Mercury, close to the Sun, would therefore have amassed more heavy material than other planets to form its core.

However, each hypothesis predicts a different surface composition. The MESSENGER mission found higher-than-expected levels of potassium and sulfur at the surface, suggesting that the hypothesis of a giant impact and vaporization of the crust and mantle did not occur because potassium and sulfur would have been driven away by the extreme heat of these events. Thus, the results obtained so far seem to favor the third hypothesis, but further analysis of the data is needed. BepiColombo, which will arrive in orbit around Mercury in 2025, will make observations to try to provide an answer.

Geology

Mercury’s surface is covered with dusty carpet, breaks and craters. Mercury’s surface is similar to that of the Moon, showing vast plains of minerals (silicates) resembling lunar seas and numerous craters, indicating that it has been geologically inactive for billions of years. For astronomers, these craters are very old and tell the story of the formation of the solar system, when planetesimals collided with young planets to merge with them. In contrast, portions of Mercury’s surface appear smooth, unimpacted.

These are probably lava flows covering the older ground and more marked by impacts. The lava, once cooled, would give rise to the appearance of a smooth, whitish surface. These plains date from a more recent era, after the period of intense bombardment. The discovery of volcanic plains on the surface makes it possible to question the fall of huge asteroids reaching the mantle, and at the same time can create volcanic eruptions opposite the planet.

Knowledge of Mercury’s geology was based only on the flyby of the Mariner 10 probe in 1975 and on terrestrial observations, it was the least well-known of the terrestrial planets until 2011 and the MESSENGER mission. For example, an unusual crater with radiating hollows was discovered through this mission, which scientists once called the Spider Crater before renaming it Apollodorus.

Mercury has different types of geological formations:

Albedos (regions marked by higher or lower reflectivity);
Catenae (crater chains), named after a radio telescope
craters, named after deceased artists, musicians, painters and writers who made an outstanding or fundamental contribution to their field;
Dorsa (ridge), named after astronomers who contributed to the study of Mercury;
the Rupes (escarpments), named after the ships of scientific expeditions
the Faculae (facules), named after the word “snake” in different languages;
the Fossae (pits ), named after an architectural monument;
the Montes (mountains), named after the word “heat” in different languages;
the Planitiae (plains), named by an association with mythology and Mercury in different languages;
the Valles (valleys), after lost cities.

Mercury was heavily bombarded by comets and asteroids during and shortly after its formation, 4.6 billion years ago, as well as during a later, perhaps separate episode called the Great Late Bombardment, which ended 3.8 billion years ago. During this period of intense crater formation, Mercury undergoes impacts on its entire surface, facilitated by the absence of any atmosphere to slow down the impactors. Also, Mercury is then volcanically active; basins such as the Caloris Basin are filled with magma, producing smooth plains similar to lunar seas. After the great bombardment, Mercury’s volcanic activity would have ceased, about 800 million years after its formation.

Mercury’s surface is more heterogeneous than that of Mars or the Moon, both of which contain significant expanses of similar geology, such as Maria and Planitiae.

Impact basins and craters

The diameter of Mercury’s craters varies from small bowl-shaped cavities to multi-annular impact basins several hundred kilometers in diameter. They appear in all states of degradation, from relatively cool radiated craters to remains of highly degraded craters. Mercury’s craters differ subtly from lunar craters in that the area covered by their ejections is much smaller, a consequence of Mercury’s stronger gravity on its surface. According to IAU rules, each new crater must be named after an artist who had been famous for more than fifty years, and had been dead for more than three years, before the date the crater was named.

The largest known crater is the Caloris Basin, with a diameter of 1,550 km (almost a third of the diameter of the planet), which was formed following the fall of an asteroid of a size of about 150 km, nearly 3.85 billion years ago. Its name (Caloris, “heat” in Latin) comes from the fact that it is located on one of the two “hot poles” of Mercury’s surface, poles directly facing the Sun when the planet is at perihelion. The impact that created the Caloris Basin was so powerful that it caused lava eruptions that left a concentric ring more than 2 km high surrounding the impact crater. It is a large circular depression, with concentric rings. Later, lava certainly flowed into this large crater, and smoothed the surface.

At the antipode of the Caloris Basin is a large area of very hilly and rugged terrain, the size of France and Germany combined, known as the “Weird Terrain“. One hypothesis for its origin is that the shock waves generated during the Caloris impact traveled around Mercury, converging at the antipode of the basin ( at 180 degrees). The resulting high stresses fractured the surface, lifting the ground to a height of 800 to 1,000 m and producing this chaotic region. Another hypothesis is that this terrain was formed as a result of the convergence of volcanic ejecta at the antipode of this basin.

The impact that created the Caloris Basin also contributed to the formation of Mercury’s only mountain range: the Caloris Montes.

In total, about 15 impact basins have been identified on Mercury. A notable basin is the Tolstoy Basin, 400 km wide, with multiple rings and which has a cover of ejecta extending up to 500 km from its perimeter and whose appearance marks the era of the Tolstoy. The Rembrandt and Beethoven basins, having volcanic ejecta cover of similar size, are also among the largest impact craters on the planet with a width of 716 and 625 km respectively.

Like the Moon, Mercury’s surface has likely been affected by space erosion processes, including the solar wind and micrometeorite impacts.

Mercury also exhibits partially tectonic phantom craters formed by graben and dorsum. Unique in the Solar System, they were discovered in 2011.

Plains

There are two geologically distinct lowland regions on Mercury.

First, the slightly hilly plains in the regions between the craters are the oldest visible surfaces of Mercury, prior to the heavily cratered terrain. These plains between craters appear to have erased many older craters, and show a general rarity of small craters less than 30 km in diameter.

Second, smooth plains are large, flat areas that fill depressions of various sizes and closely resemble lunar seas. In particular, they fill a large ring surrounding the Caloris basin. Unlike the lunar seas, Mercury’s smooth plains have the same albedos as the ancient plains between craters. Despite the lack of indisputable volcanic features, the location and rounded and lobed shape of these plains strongly support volcanic origins. All of Mercury’s smooth plains formed much later than the Caloris basin, as indicated by their significantly lower crater density compared to that of the Caloris ejection blanket. The bottom of the Caloris Basin is filled with a geologically distinct flat plain, fragmented by ridges and fractures in a roughly polygonal pattern. It is not clear whether these are impact-induced volcanic lava or impactites.

Compression characteristics

An unusual feature of Mercury’s surface is the presence of numerous compression folds called escarpments (or Rupes) that crisscross the plains. As a result of the hot phase of its formation, that is, after the end of the Great Late Bombardment that once made all the planets of the solar system incandescent balls, Mercury’s interior contracted and its surface began to warp, creating ridges. These escarpments can reach a length of 1,000 km and a height of 3 km. These compression features can be observed simultaneously with other features, such as craters and smooth plains, indicating that they are more recent.

Mapping of Mercury’s characteristics using photographs taken by Mariner 10 initially suggested a total narrowing of Mercury’s radius of the order of 1 to 2 km due to these compressions, an interval that was later increased from 5 to 7 km, as a result of MESSENGER data. Also, small-scale thrust faults are found, several tens of meters high and a few kilometers long, which seem to be less than 50 million years old. This indicates that interior compression and the resulting surface geological activity are still continuing at this small scale. After this discovery, the supposed geological inactivity of Mercury, and small planets in general, could be questioned.

The Lunar Reconnaissance Orbiter discovered in 2019 the existence of similar small thrust faults on the Moon.

Geological periods

As with Earth, the Moon or Mars, the geological evolution of Mercury can be divided into major periods or epochs. These ages are based on relative dating only, so the dates advanced are only orders of magnitude.

Pre-Tolstoyan

It extends from the very beginning of the history of the solar system to the period of intense bombardment, from -4.5 to -3.9 billion years ago. The primitive solar nebula condensed and began to form solid matter; first of small mass which by dint of accumulating (accretion process) produced bodies larger and larger, having an increasingly important force of attraction, until forming the main mass of Mercury. The homogeneous or heterogeneous nature of this accumulation of matter is still unknown: it is not known whether Mercury formed from a mixture of iron and silicate which then dissociated to form separately a metallic core and a silicate mantle, or if the core formed first from metals, then the mantle and crust came only later, when heavy elements like iron became less abundant around Mercury.

There is little chance that Mercury had an initial atmosphere (just after the accumulation of matter), or it would have evaporated very soon before the appearance of the oldest craters. If Mercury had had an atmosphere, we could have noticed an erosion of the craters by the winds, as on Mars. The escarpments present mainly in the “inter-crater” regions (which are surfaces older than craters) and which sometimes cross some of the oldest craters, show that the cooling of the core and the contraction of the planet occurred between the end of the first period and the beginning of the second.

Tolstoyan

The second period (from -3.9 to -3.85 billion years ago) is characterized by a strong meteorite bombardment by relatively large bodies (residues of the accretion process), covering the surface of Mercury by craters and basins (craters more than 200 km in diameter), and ends at the formation of the Caloris basin. It is not certain that this period is the terminal phase of Mercury’s accretion; It is possible that this is only a second episode of bombing independent of this accumulation. Especially since it was the time of the great late bombardment. It bears this name because it saw the formation of the Tolstoy Basin.

Calorien

The formation of the Caloris Basin marks the separation of this period (from -3.85 to -3.80 billion years ago). The meteorite impact gave rise to strong transformations of Mercury’s surface: the creation of the Caloris Montes mountain ring around the crater produced by the impact and chaotic deformations on the other side of the planet. The asymmetry of the internal distribution of masses that it has caused, on a global scale, has been the pivot on which the synchronization of the rotation/revolution periods is based: the Caloris basin is (with its antipode) one of the “hot equatorial poles”.

Upper calorien

The fourth geological epoch of Mercury extends from -3.80 to -3 billion years and begins after the collision giving rise to the Caloris basin. It covers the period of volcanism that followed. Lava flows formed part of the great smooth plains, roughly similar to the lunar Maria. However, the smooth plains covering the Caloris Basin (Suisei, Odin, and Tir Planitia) are thought to have been formed by ejecta during the Caloris impact.

Mansurien and Kuiperien

Extending respectively from -3 billion years to -1 billion years and from -1 billion years to today, these periods are marked by small meteorite impacts: few major events have occurred on Mercury during these periods. These eras are also called craters: the Mansur and the Kuiper.

Volcanism

The presence of younger plains (the smooth plains) is evidence that Mercury experienced volcanic activity in its past. The origin of these plains was highlighted in the late 1990s by Mark Robinson and Paul Lucey while studying photographs of Mercury. The principle is to compare smooth surfaces — formed from lava flows — with others, non-smooth (and older). If they were indeed volcanic eruptions, these regions must have been of a different composition from the one they covered, since they were composed of material from the interior of the planet.

The images taken by Mariner 10 are first recalibrated from images taken in the laboratory before the launch of the probe, and images taken during the mission of the clouds of Venus (Venus has a rather uniform texture) and deep space. Robinson and Lucey then studied various samples of the Moon — which would have experienced similar volcanic activity — and in particular, the reflection of light in order to draw a parallel between the composition and reflection of these materials.

Using advanced digital image processing techniques (which were not possible at the time of the Mariner 10 mission), they color-code images to differentiate dark mineral materials from metallic materials. Three colors are used: red to characterize opaque, dark minerals (the more pronounced the red, the less dark minerals there are); green to characterize both the concentration of iron oxide (FeO) and the intensity of the bombardment of micrometeorites, also called “maturity” (the presence of FeO is less important, or the region is less mature, on greener portions); blue to characterize the UV/visible light ratio (the intensity of blue increases with the ratio). The combination of the three images gives intermediate colors. For example, an area in yellow may represent a combination of a high concentration of opaque minerals (red) and intermediate maturity (green).

Robinson and Lucey note that the plains are marked with different colors compared to the craters and they can deduce that these plains are of different composition compared to the older surfaces (characterized by the presence of craters). These plains must, like the Moon, have been formed by lava flows. New questions then arise as to the nature of these upwellings of molten rock: they can be simple fluid effusions, or explosive eruptions. However, not all plains may originate from lava flows. It is possible that some were formed from dust fallout and soil fragments, ejected during large meteorite impacts.

Some volcanic eruptions may also have occurred as a result of large collisions. In the case of the Caloris basin, the crater generated by the impact was originally supposed to be 130 km deep; probably reaching the mantle and then partially melting it during the impact (due to very high pressure and temperature). The mantle is then raised during the readjustment of the ground, filling the crater. Thus, knowing that part of Mercury’s surface comes from its interior, scientists can deduce information about the internal composition of the planet.

The images obtained by MESSENGER, meanwhile, reveal evidence of fiery clouds on Mercury from low-lying shield volcanoes. These MESSENGER data identified 51 pyroclastic deposits on the surface, 90% of which are in impact craters. A study of the degradation status of impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury during an extended interval.

A “rimless depression” within the southwestern rim of the Caloris Basin consists of at least nine overlapping volcanic vents, each individually up to 8 km in diameter. It is therefore a stratovolcano. The bottoms of the chimneys lie at least 1 km below their walls and resemble volcanic craters carved by explosive eruptions or modified by collapse into empty spaces created by the retreat of magma in a duct. The age of the complex volcanic system would be of the order of a billion years.

Surface conditions of Mercury

Mercury is a very hot planet. The average surface temperature is about 440K (167°C). This is the stabilization temperature below the regolith, where the subsoil is no longer subjected to the alternation of thermal “waves” of day and night. Also, Mercury’s surface temperature ranges from about 100 to 700K (−173 to 427°C). It never exceeds 180K at the poles due to the absence of an atmosphere and a strong temperature gradient between the equator and the poles. The subsolar point at perihelion, namely (0°N, 0°W) or (0°N, 180°W), reaches 700 K at this time but only 550K at aphelion (90° or 270°W). On the unlit side of the planet, the average temperature is 110K.

From the surface of Mercury, the sun appears, depending on the elliptical orbit, between 2.1 and 3.3 times larger than from Earth, and the intensity of sunlight on the surface of Mercury varies between 4.59 and 10.61 times the solar constant, that is, the amount of energy received by a surface perpendicular to the Sun is on average 7 times higher on Mercury than on Earth.

Ice

Although the daylight temperature on Mercury’s surface is generally extremely high, it is possible that ice is present on Mercury. Indeed, because of the almost zero inclination of its axis of rotation, the polar areas of Mercury receive solar rays only grazing. Also, the bottom of the deep craters of the poles is never exposed to direct sunlight, and temperatures remain below 102 K thanks to this permanent darkness, much lower than on the average temperature of the planet of 452K. At these temperatures, the water ice hardly sublimates anymore (the partial vapor pressure of the ice is very low).

Radar observations made in the early 1990s from the Arecibo radio telescope and the Goldstone antenna indicate the presence of water ice at Mercury’s north and south poles. This is because water ice is characterized by areas of high radar reflection and a highly depolarized signature, unlike the typical radar reflection of silicate, which makes up most of Mercury’s surface. Also, there are areas of high radar reflection near the poles. The results obtained with the Arecibo radio telescope show that these radar reflections are concentrated in circular spots the size of a crater. According to images taken by Mariner 10, the largest of them, at the South Pole, seems to coincide with the crater Chao Meng-Fu. Others, smaller, also correspond to well-identified craters.

It is estimated that icy regions contain approximately 10¹⁴ to 10¹⁵kg of ice. These are potentially covered with regolith preventing sublimation. In comparison, the Antarctic ice sheet on Earth has a mass of about 4 × 10 18 kg and the southern polar cap of Mars contains about 10¹⁶kg of water. Two probable sources for the origin of this ice are envisaged: meteorite bombardment or the degassing of water from the interior of the planet. Meteorites hitting the planet may have brought water that would have remained trapped (frozen by low temperatures at the poles) to the places where the impacts occurred. Similarly, for outgassing, some molecules may have migrated to the poles and become trapped there.

Although ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely. The BepiColombo probe, which will orbit the planet around 2025, will have among its tasks to identify the presence or absence of ice on Mercury.

Exosphere

Mercury is too small and hot for its gravity to hold a significant atmosphere over long periods of time. Thus, it is almost non-existent to such an extent that gas molecules in the “atmosphere” collide more often with the surface of the planet than with other gas molecules. It is thus more appropriate to speak of its exosphere, starting from the surface of Mercury, directly “open” to space. It is tenuous and limited in surface, mainly composed of potassium, sodium and oxygen (9.5%). There are also traces of argon, neon, hydrogen and helium. The surface pressure exerted is less than 0.5 nPa (0.005 picobar).

This exosphere is not stable and is actually transient: the atoms composing mainly the exosphere of Mercury (potassium and sodium) have an estimated lifetime of three hours before being released into space and an hour and a half when the planet is at perihelion. Thus, atoms are continuously lost and replenished from various sources.

The hydrogen and helium atoms probably come from capturing ions from the solar wind, diffusing into Mercury’s magnetosphere before escaping back into space. The radioactive decay of elements in Mercury’s crust is another source of helium, as well as sodium and potassium. Water vapor is present, released by a combination of processes such as comets hitting its surface, sputtering (creating water from hydrogen from the solar wind and oxygen from the rock) and sublimation from water ice reservoirs in permanently shaded polar craters. The MESSENGER probe also detected large amounts of water-bound ions such as O+, OH^–, and H3 O⁺. Because of the amounts of these ions that have been detected in Mercury’s space environment, astronomers assume that these molecules were blown from the surface or exosphere by the solar wind.

Sodium, potassium and calcium were discovered in the atmosphere during the 1980s and 1990s, the consensus being that they resulted mainly from the vaporization of surface rock struck by micrometeorite impacts, including that of Comet Encke, which created a zodiac cloud. However, another hypothesis is that sunlight also contributes to the release of sodium from the planet’s surface. In 2008, magnesium was discovered by MESSENGER. Studies indicate that, sometimes, sodium emissions are localized at points that correspond to the magnetic poles of the planet. This would indicate an interaction between the magnetosphere and the surface of the planet.

Magnetic field

Despite its small size and slow rotation period of 59 days, Mercury has a notable magnetic field. Revealed by the magnetometers of Mariner 10, in March 1974, it surprised astronomers who thought until then that Mercury was devoid of any magnetosphere because its slow rotation speed decreases the dynamo effect. In addition, it was assumed at the time that the planet’s core had already solidified due to its small size. The magnetic field strength at Mercury’s equator is about 200 nT, or 0.65% of the Earth’s magnetic field of 31 μT. Like Earth’s, Mercury’s magnetic field is dipole. However, unlike Earth, Mercury’s poles are aligned with the planet’s axis of rotation. Measurements from the Mariner 10 and MESSENGER space probes indicate 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 Earth’s magnetic field. This dynamo effect would result from the circulation of the planet’s iron-rich liquid outer core. Particularly strong tidal effects, caused by the high orbital eccentricity of the planet, would 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 located between two arc shocks (or “bow shock“). The planet’s magnetosphere, although small enough to be contained in Earth’s volume, is strong enough to trap plasma from the solar wind. This contributes to the spatial erosion of the planet’s surface. Mariner 10 observations have detected this low-energy plasma in the magnetosphere on the dark side of the planet. The bursts of energetic particles in the tail of the planet’s magnetosphere indicate that it is dynamic. In addition, experiments conducted by the probe have shown that, like that of the Earth, Mercury’s magnetosphere has a tail separated in two by a neutral layer.

During its second flyby of the planet on October 6, 2008, MESSENGER discovers that Mercury’s magnetic field can be extremely permeable. The spacecraft encounters magnetic “tornadoes” twisted beams of magnetic fields connecting the planetary magnetic field to interplanetary space) measuring up to 800 km wide, or a third of the radius of the planet. These twisted magnetic flux tubes form open windows in the planet’s magnetic shield through which the solar wind can enter and directly impact Mercury’s surface by magnetic reconnection. This also occurs in the Earth’s magnetic field, however, the reconnection rate is ten times higher on Mercury.

Mercury observation

Visibility

Mercury’s apparent magnitude can vary between -2.48 (then brighter than Sirius) at its upper conjunction and +7.25 (then exceeding the limit of visibility to the naked eye located at +6 and thus making it invisible) around the lower conjunction. The mean apparent magnitude is 0.23 with a standard deviation of 1.78, the largest of all planets, due to the orbital eccentricity shape of the planet. The mean apparent magnitude at the upper conjunction is -1.89 while that at the lower conjunction is +5.93. The observation of Mercury is complicated because of its proximity to the sun in the sky, because it is then lost in the glare of the star. Mercury can only be observed for a short period of time at dawn or dusk.

Like many other planets and brightest stars, Mercury can be observed during a total solar eclipse. In addition, like the Moon and Venus, Mercury has phases seen from Earth. It is said to be “new” at the inferior conjunction and “full” at the superior conjunction. However, the planet is rendered invisible from Earth on both occasions because it is obscured by the Sun (except during a transit). Also, technically, Mercury is brightest when it is full. Thus, although Mercury is farthest from Earth when it is full, it has a larger visible illuminated surface and the opposition effect compensates for the distance. The reverse is true for Venus, which appears brighter when it is crescent because it is much closer to Earth.

Nevertheless, the brightest (full-phase) appearance of Mercury is actually incompatible with practical observation, due to its extreme proximity of the planet to the sun at this time. The best time to observe Mercury is during its first or last quarter, although these are phases of lower luminosity. The first and last quarter phases occur during the greatest elongation to the east (around September/October), and west (around March/April) of the sun, respectively. At these two times, Mercury’s separation from the sun varies between 17.9° at perihelion and 27.8° at aphelion. At its maximum elongation in the west, Mercury rises before sunrise, and at its maximum elongation in the east, it sets after sunset, making it more easily observable.

Mercury is more easily visible from tropical and subtropical regions than from higher latitudes. Seen from low latitudes and at the right times of the year, the ecliptic cuts the horizon at an acute angle. At this time, Mercury is directly above the sun (i.e. its orbit appears vertical from Earth) and it is at the maximum of its elongation relative to the sun (28°). When the time of day arrives when the sun is 18° below the horizon so that the sky is completely dark (astronomical twilight), Mercury is at an angle of 28-18=10° above the horizon in a completely dark sky: it is then at its maximum visibility for a terrestrial observer.

In addition, observers in the southern hemisphere have an advantage over those in the north, with an equal absolute latitude. Indeed, in this hemisphere, the maximum elongation of Mercury in the west (morning) occurs only in early autumn (March/April) and its maximum elongation in the east (evening) occurs only in late winter (September/October).

In these two cases, the angle of intersection of the planet’s orbit with the ecliptic (and therefore the horizon) is then at its maximum during these seasons, which allows Mercury to rise several hours before sunrise in the first case and to set only several hours after sunset in the second, from southern mid-latitudes such as Argentina and South Africa. Conversely, in the northern hemisphere, the ecliptic is much less inclined in the morning in March/April and in the evening in September/October, so Mercury is very close to the horizon even during its maximum elongation even if it happens that it is clearly visible, near Venus, in the sky.

Another method of observing Mercury is to observe the planet during daylight hours when conditions are clear, ideally when it is at its greatest elongation. This makes it easy to find the planet, even using telescopes with small apertures. However, great care must be taken to ensure that the instrument is not pointed directly at the Sun because of the risk of eye damage. This method circumvents the limitation of observation at dusk when the ecliptic is located at low altitude (e.g. autumn evenings).

In general, however, observations of Mercury through a ground-based telescope reveal only a partial disk of orange color illuminated with little detail. The proximity of the horizon makes it difficult to observe with telescopes, as its light has to travel a greater distance through the Earth’s atmosphere and is disturbed by turbulence, such as refraction and absorption that blur the image. The planet usually appears in the telescope as a crescent-shaped disk. Even with powerful telescopes, there are practically no distinctive features on its surface. On the other hand, the Hubble Space Telescope cannot observe Mercury at all, due to safety procedures that prevent it from pointing too close to the Sun.

Transit of Mercury

A transit of Mercury occurs when the planet is between the observer and the Sun. It is then visible as a very small black dot crossing the solar disk. It would also be possible for an observer on another planet to see a transit, such as the transit of Mercury from Venus. Transits of Mercury seen from Earth occur with a relatively regular frequency on an astronomical scale of about 13 or 14 per century, due to the planet’s proximity to the Sun.

The first transit of Mercury is observed on November 7, 1631, by Pierre Gassendi, although its existence was foreseen by Johannes Kepler before his death in 1630. In 1677, the observation of the transit of Mercury made it possible for the first time to highlight the phenomenon of the black drop, an effect of the diffraction of optical instruments.

The transit of Mercury has also made it possible to make various measurements, including that of the size of the universe or long-term variations in the Sun’s radius.

Transits can occur in May at intervals of 13 or 33 years, or in November every 7, 13 or 33 years. The last four transits of Mercury date from May 7, 2003, November 8, 2006, May 9, 2016, and November 11, 2019; The next four will take place on November 13, 2032, November 7, 2039, May 7, 2049, and on November 9, 2052.

History of its observation

Earth observation

Before telescopes

Mercury has been known since humans became interested in the night sky; the first civilization to have left written traces of it is the Sumerian civilization (third millennium BC) who named it “Ubu-idim-gud-ud“(meaning the “jumping planet”).

The first writings of detailed observations of Mercury come to us from the Babylonians with the tablets of Mul Apin. The Babylonians call this star Nabu in reference to the god of knowledge in Mesopotamian mythology. They were also the first to study the apparent motion of Mercury, which is different from that of other planets.

Later, in antiquity, the Greeks, heirs of Indo-European conceptions (paleoastronomy) consider until the fourth century BC. that Mercury visible before sunrise on the one hand and Mercury visible after sunset on the other hand belonged to two separate celestial bodies. These are called respectively Στίλβων (Stilbōn), meaning “the one who shines” and Ἑρμῆς (Hermes) because of its rapid movement. The latter is still the name of the planet in modern Greek. The morning star would also have been called Ἀπόλλων (Apollo). The Egyptians did the same by naming the morning star Seth and Horus after the evening star.

The Romans named the planet after the messenger of the god Mercury (in Latin Mercurius), the equivalent of Hermes in Roman mythology, because it moves in the sky faster than all other planets. Also, protector god of traders, doctors and thieves, the astronomical symbol of Mercury is a stylized version of the caduceus of Hermes. It is also assumed that the symbol comes from a derivation of the first letter of its ancient Greek name Στίλβων (Stilbōn).

Ferry, a contributor to Wahlen’s Dictionary, writes:

“Why then does such a small planet in the system of which it is part bear the name of the messenger of the gods in mythological Olympus? This is because it is quite frequently found in conjunction with other planets between which these connections are much rarer. As the duration of its revolution around the Sun or its year is only a quarter of the Earth year, in this short space of time it is seen heading towards a planet and after approaching it move away to make another visit as quickly completed. The frequent repetition of this kind of travel may have made the idea of another messenger conceive.

The Greco-Egyptian astronomer Ptolemy evokes the possibility of planetary transits in front of the Sun in his book Planetary Hypotheses. He suggests that if no transit had ever been observed, it was either because planets such as Mercury were too small to be seen, or because the transits were too infrequent.

In ancient China, Mercury is known as the “pressed star” (Chen-xing 辰星). It is associated with the northern direction and the water phase in the Five Phase (Wuxing) cosmology system. Modern Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet literally as the “water star” (水星), based on the Five Elements. Hindu mythology uses the name Buddha for Mercury, and it is believed that this god presided over Wednesday. The god Odin of Norse mythology is also associated with the planet Mercury and Wednesday. This link with the third day of the week is also found among the Romans, which later gave in French the name Wednesday (for “Mercurii dies“, the day of Mercury).

The Mayan civilization would have represented Mercury as an owl (or potentially four, two representing his morning appearance and two that of the evening) serving as a messenger to the underworld.

In Arabic astronomy, the astronomer Al-Zarqali described in the eleventh century the geocentric orbit of Mercury as an ellipse, although this intuition did not influence his astronomical theory or astronomical calculations. In the twelfth century, Ibn Bajja observed “two planets like black spots on the face of the Sun”, which was later suggested as the transit of Mercury and/or Venus by the Maragha astronomer Qutb al-Din Shirazi in the thirteenth century. However, doubts have been raised by more recent astronomers about the observation of transits by medieval Arab astronomers, as these have potentially been confused with sunspots. Thus, any observation of a transit of Mercury before telescopes remains speculative.

In India, the astronomer Nilakantha Somayaji of the Kerala school developed in the fifteenth century a partially heliocentric model in which Mercury orbits the Sun, which in turn orbits the Earth, similar to the Tychonic system of Tycho Brahe then proposed in the sixteenth century.

Telescope research from Earth

The first telescopic observations of Mercury were made by Galileo in the early sixteenth century. Although he observed phases when he looked at Venus, his telescope is not powerful enough to see the phases of Mercury. In 1631, Pierre Gassendi made the first telescopic observations of the transit of a planet through the Sun when he saw a transit of Mercury predicted by Johannes Kepler. In 1639, Giovanni Zupi used a telescope to discover that the planet has phases similar to those of Venus and the Moon. The observation conclusively demonstrates that Mercury orbits the Sun.

A rare event in astronomy is the passage of one planet in front of another seen from Earth (occultation). Mercury and Venus occult each other every few centuries and the event of May 28, 1737, is the only one to have been observed historically, having been seen by John Bevis at the Royal Observatory of Greenwich. The next occultation of Mercury by Venus will take place on December 3, 2133.

The difficulties inherent in observing Mercury mean that it has been much less studied than other planets. In 1800, Johann Schröter made observations of its surface, claiming to have observed mountains 20 kilometers high. Friedrich Bessel uses Schröter’s drawings to wrongly estimate the rotation period as 24 hours and an axial inclination of 70°. In the 1880s, Giovanni Schiaparelli mapped the planet more accurately and suggested that Mercury’s rotation period is 88 days, the same as its orbital period due to synchronous rotation. The effort to map Mercury’s surface was continued by Eugene Antoniadi, who published a book in 1934 that included both maps and his own observations. Many features of the planet’s surface, especially albedo formations, take their name from Antoniadi’s map.

In June 1962, Soviet scientists at the Institute of Radioengineering and Electronics of the USSR Academy of Sciences, headed by Vladimir Kotelnikov, were the first to bounce a radar signal off Mercury and receive it, allowing radar observations of the planet to begin. Three years later, the radar observations of Americans Gordon H. Pettengill and Rolf B. Dyce, using the 300-meter radio telescope at the Arecibo Observatory in Puerto Rico, conclusively show that the planet’s rotation period is about 59 days.

The theory that Mercury’s rotation is synchronous was widespread at that time and so it came as a surprise to astronomers when these radio observations were announced. If Mercury were actually locked as previously thought, its dark side would have been extremely cold, but measurements of radio emissions reveal that it is much hotter than previously thought. Astronomers were reluctant for a time to abandon the theory of synchronous rotation and proposed alternative mechanisms such as powerful heat distribution winds to explain the observations.

The Italian astronomer Giuseppe Colombo notes that the rotation period is about two-thirds of Mercury’s orbital period, and he is the first to propose that the planet’s orbital and rotation periods be locked in a resonance of 3:2 rather than 1:1, as is the case between the Earth and the Moon for example. Data from Mariner 10 subsequently confirmed this.

Optical observations on the ground have not revealed much more about Mercury, but radio astronomers using microwave interferometry, a technique that eliminates solar radiation, have been able to discern the physical and chemical characteristics of underground layers at a depth of several meters. In 2000, high-resolution observations called lucky imaging were made by a telescope at Mount Wilson Observatory. They provide the first views to know the surface characteristics of parts of Mercury that had not been imaged during the Mariner 10 mission. Most of the planet is mapped by the Arecibo Radar Telescope, including deposits of what may be water ice in shaded polar craters.

Cartography of Mercury

From Earth observations

The first astronomer to have discerned geological features of Mercury is Johann Hieronymus Schröter who, towards the end of the eighteenth century, draws in detail what he had been able to observe, including very high mountains. His observations were however refuted by William Herschel who could not see any of these characteristics.

Subsequently, other astronomers drew maps of Mercury, including the Italian Giovanni Schiaparelli and the American Percival Lowell (in 1896). They see dark areas in the form of lines, similar to the canals of Mars that they had also drawn and that they thought were artificial. The best map before Mariner 10 comes from the Franco-Greek Eugène Antoniadi, in the early 1930s. It was used for nearly 50 years until Mariner 10 returned the first photos of the planet. Antoniadi shows that the canals were only an optical illusion.

He acknowledges that the development of an accurate map of Mercury is impossible from observations made at dawn or dusk because of atmospheric disturbances (the thickness of Earth’s atmosphere that light must pass through when Mercury is on the horizon is large and creates distortions of the image). He then began to make dangerous observations in broad daylight when the sun was well above the horizon. It gains sharpness, but loses contrast because of sunlight. Antoniadi still managed to complete his map in 1934, composed of plains and mountains.

The coordinates used on these maps are of little importance since they were established when it was thought, as Schiaparelli had claimed, that the period of rotation of Mercury on itself was the same as the period of revolution around the Sun. It is therefore the supposedly always illuminated face that has been mapped. Only Lowell and Antoniadi had annotated their maps.

From Mariner 10

In 1974–75, Mariner 10 reported high-resolution photographs to map about 45% of its surface, revealing topographical details never seen before: a crater-covered surface with mountains and plains, very similar to that of the Moon. It is quite difficult to make a correlation between the characteristics photographed by the probe and the maps established by a telescope. Some of the geological manifestations of Antoniadi’s map have proved to be non-existent. Also, these photographs allow the publication in 1976 of the first atlas of the planet by NASA (Atlas of Mercury), revealing for the first time the geological formations of the planet including, for example, its only mountain range: Caloris Montes.

The International Astronomical Union defined in 1970 the meridian 0° as the solar meridian at the first perihelion after January 1, 1950, that is to say at one of the two hot spots. However, the coordinate system used by Mariner 10 is based on the 20° meridian that intersects the crater Hun Kal (meaning “20” in Maya) — which gives a slight error of less than 0.5° compared to the 0° meridian defined by the IAU — because the 0 meridian was in darkness during its flybys.

The crater Hun Kal is in a way the Greenwich of Mercury. The equator lies in the plane of Mercury’s orbit and longitudes are measured from 0° to 360° going west. Thus, the two hottest points on the equator are at longitudes 0°W and 180°W, and the coldest points on the equator are at longitudes 90°W and 270°W. Conversely, the MESSENGER project uses a positive convention towards the east.

Mercury is divided into 15 quadrangles. Several projection methods are used to map Mercury’s surface, depending on the position of the quadrangle on the globe. Five Mercator projections (cylindrical projection tangent to the equator) surround the planet at the equator, between latitudes 25° north and 25° south; four Lambert projections (conical projection) between 20° and 70° latitude for each hemisphere; and two stereographic projections to map the poles (up to 65° latitude).

Each quadrangle begins with the letter H (for “Hermes”), followed by its number (from 1, North Pole, to 15, South Pole). Their name comes from an important feature present in their region (basin, crater, etc.) and an albedo name (in parentheses) is attributed to them. The albedo names assigned for this new map come from Antoniadi’s, since it was the one used until then by all observers for several decades. They are used to locate quadrangles during telescope observations from Earth, where only variations in light intensity are distinguished.

Quadrangles of Mercury
Quadrangle	Name	Latitude	Longitude	Projection
H-1	Borealis (Borea)	65º – 90° N	0º – 360° W	Stereographic
H-2	Victoria (Aurora)	21º – 66° N	0° – 90° W	Lambert
H-3	Shakespeare (Caduceata)	21º – 66° N	90° – 180° W	Lambert
H-4	Raditladi (Liguria)	21º – 66° N	180° – 270° W	Lambert
H-5	Hokusai (Apollonia)	21º – 66° N	270° – 360° W	Lambert
H-6	Kuiper (Tricrena)	22º N – 22° S	0° – 72° W	Mercator
H-7	Beethoven (Solitudo Lycaonis)	22º N – 22° S	72º – 144° W	Mercator
H-8	Tolstoj (Phaethontias)	22º N – 22° S	144º – 216° W	Mercator
H-9	Eminescu (Solitudo Criophori)	22º N – 22° S	216º – 288° W	Mercator
H-10	Derain (Pieria)	22º N – 22° S	288º – 360° W	Mercator
H-11	Discovery (Solitudo Hermae Trismegisti)	21º – 66° S	0º – 90° W	Lambert
H-12	Michelangelo (Solitudo Promethei)	21º – 66° S	90° – 180° W	Lambert
H-13	Neruda (Solitudo Persephones)	21º – 66° S	180° – 270° W	Lambert
H-14	Debussy (Cyllene)	21º – 66° S	270° – 360° W	Lambert
H-15	Bach (Australia)	65º – 90° S	0° – 360° W	Stereographic

In 2016, thanks to more than 100,000 images taken by the MESSENGER probe, NASA provided the first topographic model of Mercury. This gives the maximum and minimum elevation points of the planet, respectively 4.48 km above the average elevation located on one of the oldest terrains of the planet near the equator and 5.38 km below the average elevation of the planet, at the bottom of the Rachmaninoff Basin.

Robotic exploration

Reaching Mercury from Earth poses significant technical challenges, as it orbits much closer to the Sun than Earth. This implies that a probe going to Mercury must expend more energy than to go to Pluto.

Mercury has an orbital speed of 48 km/s, while the orbital speed of the Earth is 30 km/s. Therefore, the spacecraft must perform a large Delta-v speed shift to enter a Hohmann transfer orbit that passes near Mercury, compared to the Delta-v required for other planetary missions. In addition, it is necessary to place oneself in the orbital plane of Mercury, which is inclined by 7° with respect to the ecliptic, which also requires energy.

The potential energy released by descending the Sun’s potential well becomes kinetic energy: a large negative variation in velocity then becomes necessary to slow down and enter stable orbit. Because of Mercury’s negligible atmosphere, a spacecraft relies entirely on its jet engines, with aerobraking excluded. For these reasons, a mission involving a landing on Mercury is very difficult, which is why it has never been done before.

However, advances in space mechanics make this type of mission feasible at a reasonable cost through a sequence of gravitational assist maneuvers.

Also, Mercury’s proximity to the Sun implies that a probe orbiting the planet receives about ten times more energy from the Sun than when it is in an Earth orbit and Mercury’s soil on its illuminated face reflects much of the heat it receives from the Sun increasing the thermal stress experienced by a craft at low altitude (temperatures can exceed 400 °C on the surface of the probe).

These difficulties imply that a journey to Mercury requires more fuel than is needed to escape completely from the Solar System. Therefore, its exploration was later than planets such as Venus or Mars and only two space probes visited it before the arrival of BepiColombo scheduled for 2025.

Summary table of missions to Mercury
Probe	Status	Event	Date	Space Agency	Main achievements
Marinate 10	Mission completed	Launch	November 1973	Nasa	First successful flyby of Mercury. First use of a planet’s gravitational assist to change the speed and trajectory of a space probe.
		First overview	March 1974
		Second overview	September 1974
		Third overview	March 1975
MESSENGER	Mission completed	Launch	August 2004	Nasa	First orbit around Mercury.
		First overview	January 14, 2008
		Second overview	October 6, 2008
		Third overview	September 30, 2009
		Putting into orbit	March 18, 2011 at 01:00 UTC
BepiColombo	Current mission	Launch	October 19, 2018	ESA/JAXA
BepiColombo	Current mission	Putting into orbit	planned for 2025	ESA/JAXA

Marinate 10

Mariner 10 is the first probe to study Mercury closely. Developed by the US space agency, NASA, and launched on November 3, 1973, it flew by the planet three times, in March and September 1974 and in March 1975. Originally, it was intended to fly by and study Venus, but astronomers believe they could also use it to study Mercury, of which little was known. Mariner 10 is thus the first probe to have used the gravitational assistance of one planet — Venus — to reach another.

Equipped with a camera, a magnetometer and several spectrometers, Mariner 10 allows the discovery of a significant magnetic field and the high density of the planet, revealing a large ferrous core. The most powerful ground-based telescopes had not been able to obtain quality images of the surface, due to the proximity of the alignment with the Sun. During these three passes, the probe took more than 2,000 photographs of Mercury. The photos taken by Mariner 10, however, only allow to map nearly 45% of the surface of the planet, because, during the three passages, Mercury presented the same face to the Sun; The regions in the shade were therefore impossible to map. These images reveal a surface covered with craters, very similar in appearance to that of the Moon.

Mariner 10 allows to discover the presence of a very thin atmosphere, as well as a magnetosphere. The latter was a surprise to astronomers. It also provides details on its rotational speed. The mission ends on March 24, 1975, when the probe ran out of fuel. As its orbit could no longer be precisely controlled, mission controllers ordered the probe to shut down. Mariner 10 would thus always orbit the Sun, passing close to Mercury every few months.

MESSENGER

MESSENGER (for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is the seventh mission of the Discovery program, which brings together projects to explore the Solar System at moderate cost and short development time. The probe, whose mass, including propellants, is 1.1 tons, carries seven scientific instruments, including several spectrometers, a laser altimeter, a magnetometer and cameras. It was launched on August 3, 2004 at Cape Canaveral, aboard a Delta II launcher, the launch having been postponed by one day due to bad weather.

It takes about six and a half years for the probe to enter orbit around Mercury. To achieve this, it carried out six close flybys of the inner planets during its transit (Earth in February 2005, Venus twice in October 2006 and 2007 and Mercury three times, in January and October 2008 and in September 2009), with some intermediate trajectory corrections. During these flybys of Mercury, enough data is collected to produce images of more than 95% of its surface. MESSENGER also observes the solar maximum of 2012.

The objective of the mission is to carry out a complete mapping of the planet, to study the chemical composition of its surface and exosphere, its geological history, its magnetosphere, the size and characteristics of its core and the origin of its magnetic field.

The end of the mission, initially set at March 2011, is pushed back twice until April 2015, and in the final phase, the spacecraft is placed in a closer orbit, extending the observation time of its instruments and increasing the resolution of the data. MESSENGER, after exhausting the propellants used to maintain its orbit, crashes to the ground of Mercury on April 30, 2015.

During its mission, MESSENGER took more than 277,000 photos, some with a resolution of 250 meters per pixel, and produced maps of its global composition, a three-dimensional model of the magnetosphere, the topography of the northern hemisphere, and characterized the volatile elements present in the constantly shaded craters of the poles.

BepiColombo

In the 2000s, the European Space Agency planned in collaboration with the Japanese Space Agency a mission called BepiColombo. It plans to place two probes in orbit around Mercury: one for the study of the interior and surface of the planet (Mercury Planetary Orbiter), developed by ESA, and the other to study its magnetosphere (Mercury Magnetospheric Orbiter), developed by JAXA. A project to send a lander on board with the mission is planned and then abandoned, for budgetary reasons. These two probes are sent by an Ariane 5 launcher on October 20, 2018. They should reach Mercury about eight years later, at the end of 2025, using, like previous probes, gravitational assistance. Its main mission will last until May 2027, with a possible extension until May 2028.

The BepiColombo program aims to answer a dozen questions that astronomers ask, including about the magnetosphere and the nature of Mercury’s core (liquid or solid), the possible presence of ice at the bottom of craters constantly in the shade, the formation of the Solar System and the evolution in general of a planet in the vicinity of its star. Very precise measurements of Mercury’s motion will also be made, in order to verify the theory of general relativity, the current explanation of the perihelion precession observed in its orbit.

Habitability of Mercury

The planet Mercury is a recurring location in science fiction. Common themes related to this planet include the dangers of being exposed to solar radiation and the possibility of escaping excessive radiation by staying in the planet’s slow terminator (the boundary between day and night), especially for works written before 1965, when Mercury was still thought to have a 1:1 synchronous rotation with the Sun (and therefore had a permanent face). to the Sun), as in Isaac Asimov’s Vicious Circle, or in Leigh Brackett’s short stories.

Another theme addressed is that of autocratic or violent governments, with, for example, Rendez vous avec Rama by Arthur C. Clarke. Although these accounts are fictitious, according to studies published in March 2020, it is possible to consider that parts of the planet may have been habitable. Thus, real-life forms, although probably primitive microorganisms, may have existed on the planet.

In addition, a crater, at Mercury’s north or south pole, would perhaps be one of the best extraterrestrial places for the establishment of a human colony, where the temperature would remain constant at about −200°C. This is due to an almost zero axial tilt of the planet, and the near-perfect vacuum on its surface, preventing the supply of heat from the portions illuminated by the Sun. In addition, ice is found in these craters, allowing access to water for the colony.

A base anywhere else would be exposed, during the Mercurian day (for about three Earth months), to the intense heat of the Sun, and then during an identical night period, would be deprived of any external heat source: it would then experience daytime temperatures of 430 °C and night temperatures of −180 °C. However, to avoid these thermal variations, the installations could be buried under several meters of regolith which, in a vacuum, would serve as both thermal insulation and radiation shield. Similar approaches have been proposed for the installation of bases on the Moon, which last two weeks, followed by a two-week night as well.

In general, the colonization of Mercury has some similarities with that of the Moon, because of their relatively long period around the Sun, their almost zero inclination and their absence of atmosphere: the colonization of Mercury could be done with almost the same technologies. Mercury would even have an advantage over the Moon: gravity being on the planet 38% of that of the Earth, this is enough to prevent astronauts from the reduction in bone mass occurring in a very low gravity environment.

Moreover, since the planet is close to the Sun, it would be possible to capture large amounts of energy during the day, and then use it at night. On the other hand, protecting robots and vehicles from the heat of the star could pose much more difficulties, resulting in a limitation of surface activities during the day or very important thermal protection.

Another solution is evoked in the novels and short stories of Kim Stanley Robinson, especially in The Mars Trilogy (1996) and 2312 (2012), where Mercury is home to a vast city called the Terminator, populated by a large number of artists and musicians. To avoid dangerous solar radiation, the city circles the planet’s equator on rails at a speed following the planet’s rotation, so that the sun never rises completely above the horizon. A city located on the dark side of the planet, and following the slow rotation of the planet on rails to precede the sun is thus a solution really considered.

Finally, colonization of Mercury would be of economic interest, because there are much higher concentrations of minerals than on all the other planets of the Solar System.

References (sources)

Dr. David R. Williams, « Mercury Fact Sheet », nssdc.gsfc.nasa.gov, 2018;
« MESSENGER Scientists ‘Astonished’ to Find Water in Mercury’s Thin Atmosphere », planetary.org, 2020;
Mercure (planète), Wikipedia;
« Day and Night on Mercury », eso.org;
« BepiColombo », European Space Agency.