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The steep steps resulted from a global reduction of the planet. Its radius decreased to seven kilometers. On no other planet in the solar system does the crust’s movements result in such a large contraction. The most likely reason is that Mercury’s interior has cooled considerably since its formation and contraction in the process.
Exosphere instead of atmosphere
Unlike the other three Earth-like planets in the solar system, Mercury does not have a remarkable atmosphere. Mariner 10 discovered only an extremely thin one, called an exosphere in 1974. It consists of a gas envelope in which the particle density is so low that the gas particles practically do not collide with each other. The pressure on Mercury’s surface is only one billionth of the value of the earth. The exosphere is composed primarily of the elements hydrogen and oxygen, as well as water vapor, helium, and sodium with traces of magnesium, potassium, and argon.
The exosphere is not stable for long periods of time. If the planet’s surface did not continuously supply gas particles, it would evaporate into space in two to three days. The exosphere particles are the planet’s surface material, particles from the solar wind, and dust particles that come mainly from comets and deposit on Mercury. Later, these particles can be released again from the surface by chemical and physical processes, which are activated by solar radiation or by the impact of the particles.
The density of the exosphere varies with the distance from Mercury to the Sun. Presumably, this is a consequence of the change in solar radiation and the entry of particles from the sun during orbit, as well as temporal fluctuations in the input of material from the comet. There are also differences in the density and composition of particles on the day and night sides of the planet.
A small magnetosphere
Besides Earth, Mercury is the only other terrestrial planet with an internal magnetic field. It was discovered by Mariner 10 and then measured with Messenger. Similar to the magnetic field of the earth, it is a dipole field that is similar to that of a bar magnet. However, it is a factor of 130-340 weaker than its earthly counterpart. The axis of the dipole is inclined less than 0.8 degrees against the axis of rotation Mercury; for Earth, this variable angle is currently about eleven degrees. The dipole is offset by 480 kilometers north of the planet’s center, which corresponds to approximately 20 percent of the planet’s radius, compared to just eight percent of Earth. Mercury’s magnetic field probably emerges in a similar way to Earth’s through movements of electrically conductive liquid material within the planet.
BepiColombo is one of the most complex planetary missions in European space travel to date
As Mercury’s magnetic field interacts with the solar wind, a magnetosphere forms around the planet. Contains electrically charged particles, a plasma. The particles are released from the surface of Mercury by the impact of particles and solar radiation, or are captured directly from the solar wind. The magnetosphere has a structure similar to Earth’s magnetosphere, but its volume is only one-twentieth due to the much weaker magnetic field. It is much more dynamic than Earth’s magnetosphere, making it change much faster and on shorter spatial scales.
On the side of Mercury, which faces the sun, is the magnetopause, the edge of the magnetosphere, only about 1,000 kilometers above the surface. Mercury’s magnetosphere is about the size of Jupiter Ganymede’s moon’s magnetic field, the only moon in our solar system with such a field.
In Mercury’s polar regions, as on Earth, there are areas where the solar wind can penetrate deep into the magnetosphere. Since the planet has no atmosphere, the solar wind reaches the surface unhindered. In times of strong solar activity, magnetopause is even pressed almost to the surface, meaning more particles are released there. Since the dipole field is strongly displaced from the center of the planet, the surface exposed to the solar wind at the south pole extends over an area four times larger than at the north pole.
Loading …© Spectrum of science / SuW graphics (detail)
Measuring instruments of the orbiting planet
Particularly strong solar activity temporarily compresses the magnetosphere so that large parts of the planet’s surface are directly exposed to the solar wind, something that does not happen on Earth. On the side of Mercury facing the sun, a long tail is created, analogous to Earth, in which electrically charged particles from the magnetosphere flow off the planet. Magnetospheric plasma is mainly made up of hydrogen and helium ions from the solar wind, as well as oxygen, sodium, magnesium and calcium ions, which come mainly from the surface of Mercury.
How did the iron core of Mercury come about?
One of the main questions in Mercury’s research is how it could form such a large iron core compared to other Earth-like planets.
Several hypotheses are currently being discussed: the first assumes an accumulation of iron in the inner area of the primary solar nebula from which our solar system formed 4.56 billion years ago. Alternatively, a large part of its shell could have been removed shortly after the planet formed. Either Mercury was hit by a large celestial body, which destroyed a large part of the still young crust and mantle, or the then young, hot sun bombarded the future planet with strong electromagnetic and particle radiation. In another model, metallic iron is chemically enriched by reactions with carbon-rich dust in the internal solar system.
Although all of these explanatory approaches make different predictions for the composition of the planet’s silicon-rich crust, none of Messenger’s results has been clearly ruled out. The latter model seems to better describe the measurements at this time. With the BepiColombo space mission, planetary researchers await new ideas about the planet’s formation.
The BepiColombo mission
The BepiColombo project is named after the Italian mathematician Giuseppe (Bepi) Colombo (1920–1984), a space pioneer who made the flight from Mariner 10 to Mercury possible in the first place thanks to his orbit calculations. BepiColombo consists of two independent space probes and a transfer module. The mission is a joint project between ESA and the Japanese space agency JAXA.
BepiColombo was originally supposed to start in 2013. Technical problems, especially due to high thermal loads in the Mercury environment, delayed startup over and over again. On the outer sides of the two probes, temperatures will be over 360 degrees Celsius, while inside, for the operation of scientific instruments, 40 degrees Celsius should not be exceeded. The probes will not only heat direct sunlight, but also heat radiation from Mercury’s surface, which is up to 470 degrees Celsius.
The larger of the two probes, the “Mercury Planetary Orbiter” (MPO), was developed and built by ESA. It is equipped with a total of eleven scientific instruments (see table). The instruments in MPO are designed to examine the planet globally and are dedicated to its interior, its surface, as well as its exosphere and magnetosphere. Furthermore, the MPO should verify Einstein’s theory of relativity by measuring the perihelion rotation of Mercury’s orbit very precisely with the help of its radio signals.
Mercury – last rock before the sun
Average distance from the sun 57.9 million km = 0.387 astronomical units
Orbital eccentricity 0.21
Orbital period around the sun 88 days = 0.241 years
Inclination of the path against the ecliptic 7 degrees
Equatorial diameter 4880 kilometers = 0.38 diameter of the earth
Inclination of the axis of rotation against the path plane 0.03 degrees
Rotation period 58.65 days = 0.16 years
Mass 0.055 masses of land
Average density 5.44 grams per cubic centimeter
Maximum surface temperature near the sun (perihelion) +467 degrees Celsius
Minimum surface temperature near the sun in the shade −183 degrees Celsius
The second probe, the Mercury Magnetospheric Orbiter (MMO), is contributed by JAXA. Its scientific payload consists of five instruments. In Mercury’s orbit, among other things, both probes perform coordinated measurements from different positions in space. In this way, they can investigate the exosphere and magnetosphere and their interactions with the solar wind and the planet, a huge advantage over individual measurements from the Messenger space probe.
During the flight to Mercury, both space probes are permanently connected to each other and are located in the transfer module. In addition, a sun protection shield surrounds the MO space probe and protects it from excessive temperatures.
The transfer module is equipped with two different units: for interplanetary flight there are four ion engines, which receive the necessary electricity for their operation by solar cells with a total area of 40 square meters. Xenon gas is used as fuel.
To save fuel, Swingby maneuvers are planned on Earth, Venus and Mercury. They allow you to reach the planet closest to the sun and adapt to its orbit speed. Without these swingbys, many tons of fuel would be needed for the push maneuvers, which would raise the total mass of BepiColombo so that the probe could no longer be started with an Ariane-5. During long intermediate flight phases, the ion engine is active for longer periods. Additional chemical motors are used for position and orbit control and are used during Swingby maneuvers.
In 2021, BepiColombo will fly past Mercury for the first time and repeat this five times before the two probes enter different orbits around the planet closest to the sun in December 2025. Both probes will initially surround Mercury for a year. As a result, the mission could be extended for another year.
Both probes will orbit the planet in polar orbits. These are designed in such a way that the orbital times of the probes are in a ratio of approximately 1: 4, which means that when MMO makes an orbit, MPO surrounds Mercury four times. The path planes of the probes are identical. During the first few months, both space probes will repeatedly zoom up to about 100 kilometers, which should primarily serve to calibrate your instruments.
The scientific objectives
NASA’s Messenger spacecraft has already provided many new ideas about Mercury, which are the basis for a more detailed exploration of the planet with BepiColombo. Among other things, MPO is supposed to explore the planet’s south pole region, which Messenger does little research on.
For the first time, two space probes will carry out measurements in the vicinity of the planet at the same time to investigate temporal and spatial changes, particularly in the magnetosphere and exosphere. MPO’s nearly circular orbit allows an accurate measurement of the planet’s gravitational field. Another approach is studies of its surface composition and topography.
Open scientific questions essentially refer to the origin and development of the planet so close to its central star, its internal structure and composition. BepiColombo will also investigate the dynamics within Mercury and the magnetic field generated there. Other objectives refer to changes in the planet’s surface due to geological processes such as crater formation, tectonics and volcanism. In addition, the structure and dynamics of the exosphere and magnetosphere are examined, as well as the composition and origin of possible ice deposits in the planet’s polar regions. Also interesting is the question of whether Mercury is still volcanically active, about which Messenger was unable to provide any information.
BepiColombo is one of ESA’s most complex interplanetary space missions to date. If successful, the two space probes will provide a wealth of new insight into the innermost planet in our solar system. Hopefully, they will also provide new insights into the conditions under which Mercury formed 4.56 billion years ago.
