Mercury

## Atmosphere

### **Composition**

* 70% metallic
* 30% silicate material

## General Information

Mercury is the smallest planet in the Solar System and the first planet out from the Sun It is a terrestrial planet whose surface has been severely damaged by several impact occurrences. Because the planet has little geological activity and a very thin atmosphere known as the exosphere, these characteristics have been well preserved. Mercury has an average diameter of 4,880 km, which is 38% of Earth's, making it the smallest planet in the Solar System. However, Mercury is dense enough to have a surface gravity that is similar to Mars'. Mercury has no natural satellites and an ever-evolving magnetic field that is just 1% as strong as Earth's.

Mercury is thought to have a solid inner core, a deeper liquid core layer, and a solid silicate crust and mantle covering them all, based on current theories. Mercury's surface temperatures vary greatly throughout the day, from 700 K (427 °C; 800 °F) in the equatorial regions to 100 K (−173 °C; −280 °F) at night due to the planet's nearly nonexistent atmosphere. There are huge water ice reservoirs near Mercury's poles that have no exposure to the sun; their mass is estimated to be between 0.025 and 0.25% that of the Antarctic ice sheet. Regarding Mercury's formation and history, there are several conflicting theories, some of which include rock vaporization and collisions with planetesimals.

Mercury is so close to the Sun that sunlight intensity on its surface ranges from 4.59 to 10.61 times that of the solar constant, which is the amount of energy that the Earth typically receives from the Sun. Mercury has a 3:2 spin–orbit resonance, which means that for every two rotations around the Sun, it turns on its axis precisely three times in relation to the background stars. Mercury has the highest orbital eccentricity of any known planets within the Solar System and the lowest tilt of any planet's axis, measuring around 1⁄30 of a degree.

It is named after the Roman god Mercurius, also known as Mercury, who is the god of trade, communication, and divine messenger. Mercury is the planet that requires the most shift in a spacecraft's velocity to approach from Earth, making it the most challenging to reach. As of 2023, just three spacecraft had made a visit to Mercury: Launched in 2004, Mariner 10 made its first flyby of Mercury in 2008 and completed nearly 4,000 orbits of the planet between 2011 and 2015.

### Properties

Since Mercury is one of the Solar System's four terrestrial planets, it is a rocky world similar to Earth. The planet's equatorial radius is 2,439.7 kilometers, making it the smallest in the Solar System. Moreover, Mercury is more massive than Ganymede and Titan, the Solar System's two biggest natural satellites, while being smaller overall. About 70% of mercury is made up of metallic elements, and the remaining 30% is silicate.

#### Structure

There appears to be a solid silicate crust and mantle covering a solid inner core, a deeper liquid core layer, and a solid, metallic outer core layer on Mercury. Although the iron-rich core's exact composition is unknown, it most likely consists of nickel, silicon, carbon, sulfur, and maybe trace quantities of other elements. At 5.427 g/cm3, the planet's density is just somewhat less than Earth's density of 5.515 g/cm3, making it the second-highest in the Solar System. Mercury's composition would be denser than Earth's if the effects of gravitational compression were taken into account for both planets. Mercury's uncompressed density would be 5.3 g/cm3, compared to Earth's 4.4 g/cm3.

Based on internal models limited to be in accordance with a moment of inertia factor of 0.346±0.014, the estimated radius of Mercury's core is 2,020 ± 30 km. As a result, the core of Mercury makes up around 57% of its volume, compared to 17% for Earth. Mercury's surface is characterized by a large number of small ridges that can reach lengths of hundreds of kilometers.  These are believed to have originated when the crust had already hardened and Mercury's core and mantle cooled and constricted.

Several ideas have been put out to explain why Mercury's core has more iron than any of the other planets in the solar system. The most commonly accepted theory states that Mercury had an initial mass of around 2.25 times that of its present mass and a metal-silicate ratio akin to that of common chondrite meteorites, which are believed to be typical of the rocky materials in the Solar System. A planetesimal with a mass of around 1/6 that of Mercury and a diameter of several thousand kilometers may have collided with Mercury early in the history of the Solar System. The initial crust and mantle would have been mostly removed by the collision, leaving the core as a rather important component.

Alternatively, before the Sun's energy production stabilized, Mercury may have evolved from the solar nebula. The protosun would have started off with double the mass it has now, but as it contracted, temperatures close to Mercury may have reached as high as 10,000 K. At such temperatures, a large portion of Mercury's surface rock may have evaporated, creating an atmosphere of "rock vapor" that might have been removed by the solar wind. According to a different theory, lighter elements were lost from the expanding material rather than being captured by Mercury because of drag from the solar nebula on the particles that Mercury was accreting.

Two satellite missions have been entrusted with observing the surface composition that each theory expects to be different. To verify these theories, BepiColombo, an orbiting spacecraft that will reach Mercury in 2025, will conduct observations. Thus far, the results appear to support the third hypothesis; nevertheless, further data analysis is required.

#### Surface

With its vast plains like mares and significant cratering, Mercury's surface resembles that of the Moon and suggests that it has been geologically dormant for billions of years. Compared to the surfaces of Mars and the Moon, which both include substantial areas of comparable geology, such as maria and plateaus, it is more diverse. Impact craters, the ejecta that follow from them, and ray systems are examples of places with noticeably varied reflectivity that are known as albedo features. Higher reflectivity plains are correlated with larger albedo characteristics. Mercury features plains, escarpments, mountains, valleys, and "wrinkle-ridges" resembling the highlands of the Moon.

The chemical heterogeneity of the planet's mantle indicates that the planet had a magma ocean phase early in its existence. Large-scale differences in the chemical composition of the crust were detected on its surface as a result of mineral crystallization and convective overturn, resulting in a layered, chemically heterogeneous crust. Because of the greater early chemically reducing circumstances than on other terrestrial planets, the crust has a high sulfur content but a low iron content. Enstatite and forsterite, which are iron-poor pyroxene and olivine, respectively, predominate on the surface, accompanied by sodium-rich plagioclase and mixed magnesium, calcium, and iron-sulfide minerals. There is a lot of carbon in the less reflecting parts of the crust, likely in the form of of graphite.

Comets and asteroids strongly bombarded Mercury during and shortly after its creation 4.6 billion years ago. This bombardment also occurred during a potentially distinct later period known as the Late Heavy Bombardment, which ended 3.8 billion years ago. During this time of rapid crater building, impacts occurred everywhere across Mercury's surface, made possible by the absence of an atmosphere to slow down impactors. Mercury was experiencing volcanic activity at this period; magma filled basins to create flat plains like the maria on the Moon. Apollodorus (also known as "the Spider") is one of the most peculiar craters; it has a number of radiating troughs that stretch from the impact site.

The size of craters on Mercury varies, ranging from tiny, bowl-shaped hollows to multi-ringed impact basins spanning hundreds of kilometers. They can be found in crater remains that are severely deteriorated or rather new rayed craters. Because of Mercury's higher surface gravity, the area covered by the ejecta from Mercurian craters is significantly smaller than that of Lunar craters. As to the regulations set forth by the International Astronomical Union, every newly formed crater has to be named after an artist who achieved fame for over fifty years and passed away more than three years before to the crater's official designation date.

Caloris Planitia, also known as Caloris Basin, has a diameter of 1,550 km, making it the biggest known crater. The impact that formed the Caloris Basin was so strong that it erupted lava and left behind a ring of mountains that around the impact crater, measuring about 2 km (1.2 mi) in height. A geologically unique flat plain filled with ridges and fractures arranged roughly in a polygonal pattern makes up the floor of the Caloris Basin. It's unclear if the collision caused massive sheets of impact melt or volcanic lava flows.

The "Weird Terrain" is a broad area of peculiar, steep terrain located at the antipode of the Caloris Basin. One theory about its genesis is that shock waves from the collision of Caloris circled Mercury and converged in the antipode of the basin. The surface was broken by the enormous strains that followed. On the other hand, it has been proposed that this topography originated from the convergence of ejecta near the antipode of this basin.

There are 46 impact basins found in all. The 400 km (250 mi) wide, multi-ring Tolstoj Basin is a noteworthy basin with a floor filled with smooth plains materials and an ejecta blanket that stretches up to 500 km (310 mi) beyond its rim. Beethoven Basin has a rim that is 625 km (388 mi) in diameter and an ejecta blanket of comparable magnitude. Similar to the Moon, Mercury's surface has probably been impacted by micrometeorite strikes and solar wind as a result of space weathering processes.

On Mercury, there are two plains zones that differ geologically. Mercury's first observable surfaces are the gently undulating, highland plains that exist between its highly cratered areas. There is a general dearth of smaller craters less than 30 km (19 mi) in diameter in these inter-crater plains, which appear to have destroyed many previous craters.

Smooth plains are broad, level regions that occupy different-sized depressions and resemble lunar maria. Mercury's smooth plains share the same albedo as the earlier inter-crater plains, in contrast to lunar maria. Although these plains lack clearly volcanic features, their localization and lobate, rounded form strongly suggest volcanic origins.In somewhat lower crater concentrations than on the Caloris ejecta blanket indicate that all of Mercury's flat plains developed somewhat later than the Caloris basin.

There is proof that low-profile shield volcanoes on Mercury have produced pyroclastic flows. Ninety percent of the 51 pyroclastic deposits that have been observed are located inside impact craters. According to an analysis of the impact craters' states of degradation, which support pyroclastic deposits, Mercury had pyroclastic activity over an extended period of time.

A "rimless depression" with a diameter of up to 8 km (5.0 mi) is located inside the Caloris Basin's southwest rim and is made up of at least nine overlapping volcanic vents. As a result, it is a "compound volcano". The vent floors resemble volcanic craters that have been altered by collapse into void spaces generated by magma withdrawal back down into a conduit, or they are at least 1 km (0.62 mi) below their brinks. Although scientists were unable to determine the exact age of the volcanic complex system, they estimated that it may be billions of years old.

#### Magnetosphere

Mercury is a tiny planet with a 59-day sluggish revolution, yet it possesses a strong magnetic field that appears to be widespread. Mariner 10's readings indicate that it is around 1.1% the strength of Earth's. At Mercury's equator, the magnetic field strength is roughly 300 nT. Mercury has a dipolar magnetic field, much like Earth. Mercury's poles are almost in line with the planet's spin axis, in contrast to Earth's. The magnetic field's intensity and form remain steady, according to measurements made by both of the Mariner 10 space missions.

This magnetic field is most likely produced by a dynamo mechanism, much like the magnetic field on Earth. The planet's rich in iron liquid core would circulate, creating this dynamo effect. Due to the planet's high orbital eccentricity, there would be particularly powerful tidal heating effects that would maintain a portion of the core in the liquid condition required for this dynamo effect.

Mercury has a magnetosphere formed by the solar wind being deflected by the planet's powerful magnetic field. Despite being tiny enough to fit within Earth, the planet's magnetosphere is powerful enough to capture solar wind plasma. This adds to the planet's surface weathering in space. This low energy plasma was discovered by the Mariner 10 spacecraft during observations made in the planet's nightside magnetosphere. The planet's magnetosphere appears active, as evidenced by outbursts of high-energy particles in the planet's magnetotail.

On October 6, 2008, MESSENGER made the discovery that Mercury's magnetic field may be exceedingly "leaky" on its second visit of the planet. Up to 800 km across, or one-third the planet's radius, the spacecraft discovered magnetic "tornadoes"—twisted bundles of magnetic fields linking the planetary magnetic field to interplanetary space. Technically referred to as flux transfer events, these twisted magnetic flux tubes create gaps in Mercury's magnetic shield that allow solar wind to penetrate and have a direct influence on the planet's surface through magnetic reconnection. The magnetic field of Earth experiences this as well.

#### Orbit

Of every planet in the Solar System, Mercury possesses the most eccentric orbit with a range of 46,000,000 to 70,000,000 km from the Sun. An orbit takes 87.969 Earth days to finish. The diagram, which superimposes Mercury's orbit over a circular orbit with the same semi-major axis, demonstrates the implications of the eccentricity. Mercury travels a longer distance in each 5-day interval, which is indicative of its faster velocity when it is close to perihelion. The planet's size in the diagram corresponds inversely to Mercury's distance from the Sun, reflecting the planet's fluctuating distance from the Sun.

Of the eight known solar planets, Mercury has the largest orbit, with an angle of 7 degrees to the ecliptic, the plane of Earth's orbit. Because of this, Mercury may only transit over the face of the Sun when it is in the plane of the ecliptic, which happens in May or November, when the planet is positioned between Earth and the Sun. On average, this happens every seven years.

Mercury has a nearly zero axial tilt; the lowest recorded measurement is 0.027 degrees. Compared to Jupiter, which has the second-smallest axial tilt of any planet at 3.1 degrees, this is noticeably smaller. This indicates that the Sun's center never rises higher than 2.1 arcminutes above the horizon for an observer at Mercury's poles. In contrast, the Sun's angular dimensions as viewed from Mercury vary from 1+1/4 to 2 degrees in diameter.

An observer would be able to witness the Sun rise just beyond the horizon at specific locations on Mercury's surface, then reverse and set before rising once more, all inside the same Mercurian day. This is due to the fact that, around four Earth days before to perihelion, Mercury's angular orbital velocity equals that of its angular rotational velocity, which causes the Sun to appear to stop moving; nevertheless, as perihelion approaches, Mercury's angular orbital velocity increases. Thus, the Sun seems to be moving backwards to a hypothetical observer on Mercury. After perihelion, the Sun resumes its regular apparent velocity four Earth days later.

Either at perihelion in alternating Mercurian years (once a Mercurian day), for the same reason, there are two points on Mercury's equator that are 180 degrees apart in longitude. At either of these points, the Sun passes overhead, then appears to reverse its motion and passes overhead again, then reverses again and passes overhead a third time, a process that takes roughly 16 Earth-days in total. The identical phenomenon occurs at the other of these two periods in the other alternative Mercurian years. Mercury is at perihelion, or its closest point to the Sun, during the two or three weeks that the Sun is nearly motionless above due to the limited amplitude of the retrograde motion. This results in the Sun seeming at its brightest. Due to diurnal temperature lag, the maximum temperature happens around 0.4 Mercury days and 0.8 Mercury years after dawn, when the Sun is at an attitude of roughly 25 degrees beyond midday. On the other hand, the Sun only passes over two additional equator sites, separated by 90 degrees of longitude, during alternating years when the planet is at aphelion and the Sun's apparent velocity in Mercury's sky is rather fast. These equatorial points, where the Sun appears to be moving backward when it crosses the horizon as mentioned in the previous paragraph, get a lot less solar heat than the initial spots mentioned above.

Mercury's eccentric orbit causes it to reach an inferior conjunction, or closest approach to Earth, every 116 Earth days on average, however this period can vary from 105 days to 129 days. Mercury's maximum distance from Earth is 82,200,000 km, and it is gradually decreasing: It won't go any closer to Earth than 80,000,000 km (50 million km) until 28,622, but it will get within 82,100,000 km (51 million mi) in 2679 and 82,000,000 km (51 million mi) in 4487. Between eight and fifteen days can pass between an inferior conjunction and the start of its retrograde motion as observed from Earth. The high orbital eccentricity of the planet is the cause of this wide range.

### History

The MUL.APIN tablets contain the oldest known observations of Mercury. Most likely, an Assyrian astronomer from the 14th century BC made these observations. The name Mercury was given in cuneiform on the MUL.The leaping planet is how APIN tablets are written. Mercury was first mentioned in Babylonian writings from the first millennium BC. The planet Nabu was named by the Babylonians after their mythological messenger to the gods.

In his book Planetary Hypotheses, the Greco-Egyptian astronomer Ptolemy discussed the potential for planetary transits over the Sun's surface. He proposed that either transits were too rare or planets like Mercury were too tiny to witness transits. This would explain why no transits had been seen.

Thomas Harriot and Galileo made the first telescopic observations of Mercury in 1610. Simon Marius deduced that Mercury had phases in 1612 after seeing that the planet's brightness changed with its orbital position. The first telescopic observations of a planet transiting the Sun were made in 1631 by Pierre Gassendi, who witnessed Mercury's transit, which Johannes Kepler had predicted. Giovanni Zupi discovered in 1639 that the planet's orbital phases resembled those of Venus and the Moon through the use of a telescope. The sighting proved without doubt that Mercury was in an orbit around the Sun.

The transit of one planet ahead of another, or occultation, as viewed from Earth, is an uncommon occurrence in astronomy. Every few centuries, Mercury and Venus occult each other; the sole known occultation in history is that on May 28, 1737, which was witnessed by John Bevis at the Royal Greenwich Observatory. On December 3, 2133, Venus will occultate Mercury once more.

Compared to the other planets, Mercury has received significantly less research due to the challenges of its observation. Johann Schröter recorded surface characteristics in 1800 and claimed to have seen mountains rising to a height of 20 kilometers. Friedrich Bessel incorrectly estimated the rotation time to be 24 hours and the axial tilt to be 70° using Schröter's illustrations. Giovanni Schiaparelli proposed that Mercury's orbital period, which is determined by tidal locking, is 88 days, based on his more precise mapping of the planet in the 1880s. Synchronous rotation is the name given to this event. Eugenios Antoniadi, who released a book in 1934 with maps and his personal observations, carried on the task of mapping Mercury's surface.

Radar observations of Mercury began in June 1962 when Soviet scientists under the direction of Vladimir Kotelnikov at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences became the first to bounce a radar signal off the planet and receive it. Three years later, US scientists Gordon H. Pettengill and Rolf B. Dyce's radar measurements using Puerto Rico's 300-meter-wide (330-yard) Arecibo radio telescope proved without a shadow of a doubt that the globe rotates every 59 days. Astronomers were surprised to learn of these radio findings since they had come to support the generally accepted assumption that Mercury's rotation was synchronous.

The planet's orbital and rotational periods may be locked into a 3:2 resonance rather than a 1:1 one, according to a 1965 observation by Italian astronomer Giuseppe Colombo, who also pointed out that the rotation value was around two-thirds of Mercury's orbital period. This interpretation was subsequently verified by data from Mariner 10. This indicates that the maps created by Schiaparelli and Antoniadi were not "wrong". Rather, because of poor viewing circumstances caused by the orbital geometry, the astronomers only documented characteristics they saw during Mercury's second orbit and ignored those saw with Mercury's other face being toward the Sun.

While ground-based optical observations of Mercury did not reveal much more, radio astronomers were able to determine the physical and chemical properties of the subsurface layers down to several meters by employing a technique called microwave interferometry, which allows for the removal of solar radiation. Numerous key morphological features of Mercury were unknown until the first space probe passed by it. Furthermore, improvements in ground-based observations have resulted from technology breakthroughs. In 2000, the 1.5-meter (4.9-foot) Hale telescope at Mount Wilson Observatory carried out high-resolution lucky imaging studies. They offered the first perspectives that clarified surface characteristics on the regions of Mercury that the Mariner 10 mission did not capture.

#### Research

Because Mercury orbits so far closer to the Sun than Earth does, getting to Mercury from Earth presents considerable technological obstacles. Launched from Earth, a spacecraft destined for Mercury must travel more than 91 million kilometers into the gravitational potential well of the Sun. Earth orbits at a speed of 29.8 km/s (18.5 mi/s), while Mercury orbits at 47.4 km/s (29.5 mi/s). Therefore, in comparison to the delta-v needed for, example, Mars planetary missions, the spacecraft must make a higher change in velocity (delta-v) in order to reach Mercury and subsequently enter orbit.