MERCURY (PLANET): DISCOVERY, CHARACTERISTICS, COMPOSITION, ORBIT, MOVEMENT - ARTE - 2025

Mercury is the closest planet to the Sun and also the smallest of the 8 major planets in the solar system. It can be seen with the naked eye, although it is not easy to find. Despite this, this small planet has been known since ancient times.

Sumerian astronomers recorded their existence around the fourteenth century BC, in the Mul-Apin, a treatise on astronomy. There they gave it the name of Udu-Idim-Gu or "planet of the jump", while the Babylonians called it Nabu, messenger of the gods, the same meaning that the name of Mercury had for the ancient Romans.

Figure 1. The planet Mercury. Source: Pixabay.

As Mercury is visible (with difficulty) at dawn or dusk, the ancient Greeks were slow to realize that it was the same celestial object, so they called the Mercury at dawn Apollo and the one at dusk Hermes, the mail of the gods.

The great mathematician Pythagoras was sure that it was the same star and proposed that Mercury could pass in front of the solar disk seen from Earth, as it does.

This phenomenon is known as transit and it occurs on average 13 times every century. The last transit of Mercury took place in November 2019 and the next will be in November 2032.

Other astronomers of ancient cultures such as the Mayans, Chinese and Hindus also collected impressions of Mercury and the other luminous points that moved in the sky faster than the stars in the background: the planets.

The invention of the telescope prompted the study of the elusive object. Galileo was the first to see Mercury with optical instruments, although the celestial messenger kept many of its secrets hidden until the arrival of the space age.

General characteristics

Inner planet

Mercury is one of the 8 major planets in the solar system and together with Earth, Venus and Mars make up the 4 inner planets, the closest to the Sun and characterized by being rocky. It is the smallest among all and the one with the lowest mass, but on the other hand it is the most dense after Earth.

Data obtained

Much of the data about Mercury comes from the Mariner 10 probe, launched by NASA in 1973, whose purpose was to collect data from neighboring Venus and Mercury. Until then, many characteristics of the small planet were unknown.

It should be noted that it is not possible to point telescopes like Hubble towards Mercury, given the sensitivity of the equipment to solar radiation. For this reason, in addition to the probes, a good part of the data on the planet comes from observations made using radar.

Atmosphere

The Mercurian atmosphere is very thin and the atmospheric pressure there is one trillionth of that of the Earth's. The thin gaseous layer consists of hydrogen, helium, oxygen, and sodium.

Mercury also has its own magnetic field, almost as old as the planet itself, similar in shape to the Earth's magnetic field, but much less intense: barely 1%.

Temperatures

As for the temperatures on Mercury, they are the most extreme of all the planets: during the day they reach scorching 430ºC in some places, enough to melt lead. But at night the temperatures drop to -180 ºC.

However, the day and night of Mercury differ greatly from what we experience on Earth, so later on it is explained how a hypothetical traveler who reaches the surface would see them.

Summary of the main physical characteristics of the planet

-Mass: 3.3 × 10 ²³ kg

-Equatorial radius : 2440 km or 0.38 times the radius of the Earth.

-Shape: the planet Mercury is an almost perfect sphere.

-Average distance to the Sun: 58,000,000 km

-Temperature: on average 167 ºC

-Gravity: 3.70 m / s ²

-Own magnetic field: yes, about 220 nT intensity.

-Atmosphere: dim

-Density: 5430 kg / m ³

-Satellites: 0

-Rings: does not have.

Translation movement

Mercury executes a translational movement around the Sun according to Kepler's laws, which indicates that the orbits of the planets are elliptical. Mercury follows the most elliptical - or elongated - orbit of all the planets and therefore has the highest eccentricity: 0.2056.

The maximum Mercury-Sun distance is 70 million kilometers and the minimum 46 million. The planet takes about 88 days to complete one revolution around the Sun, with an average speed of 48 km / s.

This makes it the fastest of the planets to orbit the Sun, living up to its winged messenger name, however the speed of rotation around its axis is considerably slower.

Figure 2. Animation of Mercury's orbit around the Sun (yellow), next to that of the Earth (blue). Source: Wikimedia Commons.

But the funny thing is that Mercury does not follow the same trajectory of the previous orbit, in other words, it does not return to the same starting point of the previous time, but experiences a small displacement, called precession.

That is why it was believed for a time that there was an asteroid cloud or perhaps an unknown planet that disturbed the orbit, which was called Vulcan.

However, the theory of general relativity could satisfactorily explain the measured data, since the space-time curvature is capable of displacing the orbit.

In the case of Mercury, the orbit undergoes a displacement of 43 arc seconds per century, a value that can be calculated precisely from Einstein's relativity. The other planets have very small displacements of their own, which until now have not been measured.

Mercury motion data

The following are the numbers that are known about the motion of Mercury:

-Mean radius of the orbit: 58,000,000 km.

- Inclination of the orbit: 7º with respect to the orbital plane of the Earth.

-Eccentricity: 0.2056.

- Average orbital speed: 48 km / h

- Transfer period: 88 days

- Rotation period: 58 days

- Solar day: 176 Earth days

When and how to observe Mercury

Of the five planets visible to the naked eye, Mercury is the most difficult to detect, because it always appears very close to the horizon, obscured by the sun's glare, and disappears after a short time. Besides that its orbit is the most eccentric (oval) of all.

But there are more appropriate times of the year to scan the sky in your search:

- In the northern hemisphere: from March to April during twilight, and from September to October before dawn.

-In the tropics: throughout the year, under favorable conditions: clear skies and far from artificial lights.

- In the southern hemisphere: during September and October before sunrise, and from March to April after sunset. It is generally easier to see from these latitudes because the planet remains above the horizon longer.

Figure 3. Mercury is visible very low on the horizon. Source: Pixabay.

Mercury looks like a slightly yellowish white point of light that does not flicker, unlike stars. It is best to have binoculars or a telescope with which you can see its phases.

Mercury sometimes remains visible on the horizon for a longer time, depending on where it is in its orbit. And although it is brighter in full phase, paradoxically it looks better in waxing or waning. To know the phases of Mercury, it is advisable to visit websites specialized in astronomy.

In any case, the best opportunities are when it is at its maximum elongation: as far as possible from the Sun, so the darkest sky facilitates its observation.

Another good time to observe this and the other planets is during a total solar eclipse, for the same reason: the sky is darker.

Rotatory motion

In contrast to its fast orbital motion, Mercury rotates slowly: it takes almost 59 Earth days to make one revolution around its axis, which is known as a sidereal day. Therefore a sidereal day on Mercury lasts almost as long as the year: in fact for every 2 "years" 3 "days" pass.

The tidal forces that arise between two bodies under gravitational attraction, are responsible for slowing down the rotation speed of one of them or both. When that happens, tidal coupling is said to exist.

Tidal coupling is very frequent between planets and their satellites, although it can occur between other celestial bodies.

Figure 4. Tidal coupling between the Earth and the Moon. The case of Mercury and the Sun is more complex. Source: Wikimedia Commons. Stigmatella aurantiaca

A special case of coupling occurs when the period of rotation of one of them equals the period of translation, like the Moon. It always shows us the same face, therefore it is in synchronous rotation.

However, with Mercury and the Sun it does not happen exactly this way, since the periods of rotation and translation of the planet are not equal, but in a 3: 2 ratio. This phenomenon is known as spin-orbit resonance and it is also common in the solar system.

Thanks to this, peculiar things can happen on Mercury, let's see:

Day and night on Mercury

If a solar day is the time it takes for the Sun to appear at one point and then reappear in the same place, then on Mercury the Sun rises twice in the same day (solar), which takes 176 Earth days there (see figure 5)

It turns out that there are times when the orbital speed and the rotational speed are equal, so it seems that the Sun recedes in the sky and returns to the same point from which it left, and then moves forward again.

If the red bar in the figure were a mountain, starting at position 1 would be noon at the top. At positions 2 and 3 the Sun illuminates part of the mountain until it sets in the west, at position 4. By then it has traveled half the orbit and 44 Earth days have elapsed.

In positions 5, 6, 7, 8 and 9 it is night in the mountains. By occupying 5 it has already made a complete revolution on its axis, turning ¾ of a turn in its orbit around the Sun. At 7 it is midnight and 88 Earth days have elapsed.

Another orbit is required to return to noon, having to pass through positions 8 to 12, which takes another 88 days, in total 176 Earth days.

The Italian astronomer Giuseppe Colombo (1920-1984) was the first to study and explain the 3: 2 resonance of the motion of Mercury.

Figure 5. Day and night on Mercury: orbital resonance, after ½ orbit, the planet has turned ¾ turn on its axis. Source: Wikimedia Commons.

Composition

The average density of Mercury is 5,430 kg / m ³, slightly less than the Earth's. This value, known thanks to the Mariner 10 probe, is still surprising, taking into account that Mercury is smaller than Earth.

Figure 6. Mercury-Earth Comparison. Source: Wikimedia Commons. NASA Mercury image: NASA / APL (from MESSENGER)

Inside the Earth the pressure is higher, so there is an extra compression on the matter, which decreases the volume and increases the density. If this effect is not taken into account, Mercury turns out to be the planet with the highest density known.

Scientists believe it is due to a high content of heavy elements. And iron is the most common heavy element in the solar system.

In general, the composition of Mercury is estimated to be 70% metallic content and 30% silicates. In its volume are:

-Sodium

-Magnesium

-Potassium

-Calcium

-Iron

And among the gases are:

-Oxygen

-Hydrogen

-Helium

-Traces of other gases.

The iron present in Mercury is in its core, in an amount that far exceeds that estimated on other planets. Also, Mercury's core is comparatively the largest of all in the solar system.

Yet another surprise is the existence of ice at the poles, which is also covered in dark organic matter. It is surprising because the average temperature of the planet is very high.

One explanation is that the poles of Mercury are always in perpetual darkness, protected by high cliffs that prevent the arrival of sunlight and also, because the inclination of the axis of rotation is zero.

Regarding its origin, it is speculated that the water may have reached Mercury brought by comets.

Internal structure

Like all terrestrial planets, there are three characteristic structures on Mercury:

-The metal core in the center, solid inside, melted outside

-An intermediate layer called mantle

-The outer layer or crust.

It is the same structure that the Earth has, with the difference that the nucleus of Mercury is much larger, proportionally speaking: approximately 42% of the planet's volume is occupied by this structure. On the other hand, on Earth, the nucleus occupies only 16%.

Figure 7. The internal structure of Mercury is similar to that of Earth. Source: NASA.

How is it possible to reach this conclusion from Earth?

It was through radio observations made through the MESSENGER probe, which detected gravitational anomalies on Mercury. Since gravity depends on mass, the anomalies provide clues about density.

Mercury's gravity also markedly altered the probe's orbit. Added to this, radar data revealed the planet's precessional movements: the planet's axis of rotation has its own spin, another indication of the presence of a cast iron core.

Summarizing:

-Gravitational anomaly

-Precession movement

-Alterations in the orbit of the MESSENGER.

This set of data, plus all that the probe managed to collect, agrees with the presence of a metal core, large and solid inside, and cast iron outside.

The core of Mercury

There are several theories to explain this curious phenomenon. One of them maintains that Mercury suffered a colossal impact during its youth, which destroyed the crust and part of the mantle of the newly formed planet.

Figure 8. Comparative section of the Earth and Mercury, showing the relative size of the layers. Source: NASA.

The material, lighter than the core, was thrown into space. Later the gravitational pull of the planet pulled back some of the debris and created a new mantle and a thin crust.

If a huge asteroid was the cause of the impact, its material could combine with that of the original core of Mercury, giving it the high iron content that it has today.

Another possibility is that, since its inception, oxygen has been scarce on the planet, in this way iron is conserved as metallic iron instead of forming oxides. In this case, the thickening of the nucleus has been a gradual process.

geology

Mercury is rocky and desert, with wide plains covered by impact craters. In general terms, its surface is quite similar to that of the Moon.

The number of impacts is indicative of age, since the more craters there are, the older the surface.

Figure 9. Dominici Crater (brightest above) and Homer Crater on the left. Source: NASA.

Most of these craters date from the time of the late heavy bombardment, a period when asteroids and comets frequently struck planets and moons in the solar system. Therefore the planet has been geologically inactive for a long time.

The largest of the craters is the Caloris basin, 1,550 km in diameter. This depression is surrounded by a wall 2 to 3 km high created by the colossal impact that formed the basin.

At the antipodes of the Caloris basin, that is, on the opposite side of the planet, the surface is cracked due to the shock waves produced during the impact traveling inside the planet.

The images reveal that the regions between the craters are flat or gently undulating. At some point during its existence Mercury had volcanic activity, because these plains were probably created by lava flows.

Another distinctive feature of Mercury's surface are numerous long, steep cliffs, called escarpments. These cliffs must have been formed during the cooling of the mantle, which, when shrinking, caused numerous cracks to appear in the crust.

Mercury is shrinking

The smallest of the planets in the solar system is losing size and scientists believe this is because it does not have plate tectonics, unlike Earth.

Tectonic plates are large sections of crust and mantle that float above the asthenosphere, a more fluid layer belonging to the mantle. Such mobility gives the Earth a flexibility that planets lacking tectonism do not have.

In its beginnings, Mercury was much hotter than now, but as it cools, it gradually contracts. Once cooling stops, especially that of the core, the planet will stop shrinking.

But what is striking on this planet is how fast it is happening, for which there is still no consistent explanation.

Missions to Mercury

It was the least explored of the inner planets until the 70s, but since then several unmanned missions have taken place thanks to which much more is known about this surprising little planet:

Mariner 10

Figure 10. Mariner 10. Source: Wikimedia Commons. POT

The last of NASA's Mariner probes flew over Mercury three times, from 1973 to 1975. It managed to map just under half of the surface, only on the side illuminated by the Sun.

With its fuel used up, Mariner 10 is adrift, but it has provided invaluable information about Venus and Mercury: images, data about the magnetic field, spectroscopy and more.

MESSENGER (MErcury, Surface, Space ENvironment, GEochemistry

This probe was launched in 2004 and managed to enter the orbit of Mercury in 2011, the first to do so, as Mariner 10 could only fly over the planet.

Among his contributions are:

-High quality images of the surface, including the non-illuminated side, which was similar to the side already known thanks to the Mariner 10.

-Geochemical measurements with various spectrometry techniques: neutron, gamma ray and X-ray.

-Magnetometry.

-Spectrometry with ultraviolet, visible and infrared light, to characterize the atmosphere and carry out a mineralogical mapping of the surface.

The data collected by MESSENGER show that Mercury's active magnetic field, like the Earth's, is produced by a dynamo effect created by the liquid region of the nucleus.

It also determined the composition of the exosphere, a very thin outer layer of the Mercurian atmosphere, which has a peculiar tail shape 2 million kilometers long, due to the action of the solar wind.

The MESSENGER probe ended its mission in 2015 by crashing into the planet's surface.

BepiColombo

Figure 11. The Italian astronomer Giuseppe (Bepi) Colombo. Source: Wikimedia Commons.

This probe was launched in 2018 by the European Space Agency and the Japan Aerospace Exploration Agency. It was named in honor of Giuseppe Colombo, the Italian astronomer who studied the orbit of Mercury.

It consists of two satellites: MPO: Mercury Planetary Orbiter and MIO: Mercury Magnetospheric Orbiter. It is expected to reach the vicinity of Mercury in 2025 and its objective is to study the main characteristics of the planet.

Some goals are for BepiColombo to bring new information about Mercury's remarkable magnetic field, the planet's center of mass, the relativistic influence of solar gravity on the planet, and the peculiar structure of its interior.

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