Mercury
The formation of the solar system :
The following description is a generally accepted model, although its details are still the subject of much debate. Initially, about 10 billion years ago, what will one day become the solar system is just a tiny fraction of a gigantic cloud of hydrogen and helium that continues its ballet around the galactic center. As time passes, this cloud slowly contracts and becomes enriched with heavier elements as massive stars explode around it, which is why the current abundance of heavy elements is l around 2 percent. Finally, 4.6 billion years ago, under the effect of its own gravity, this cloud collapsed on itself and fragmented into a series of smaller clouds, one of which would become the system. solar.
The evolution of the solar protosystem :
The now well-defined protosystem continues to contract more and more. But, according to the law of conservation of angular momentum, if the size of a body is reduced, its speed of rotation must increase to compensate. The contraction of the protosystem is therefore accompanied by a sharp increase in the speed of rotation. In addition, since the protosystem is not rigid, a strong flattening occurs in the plane perpendicular to the axis of rotation. So we end up with a concentration of matter in the center, the protostar, surrounded by a disc of matter called the protoplanetary disc. This is where our knowledge of the angular momentum distribution comes in. In the simplest training models, the solar system is the result of a simple contraction of a rotating cloud of gas. But this should translate into a speed of rotation of the Sun incompatible with the fact that it has only 3 percent of the total angular momentum.
In reality, the protostar will be slowed down under the action of magnetic forces. Under the physical conditions prevailing at the time, a change in the magnetic field automatically resulted in a change in the distribution of matter and vice versa - the magnetic field lines are said to be frozen in matter.
However, the magnetic field lines which cross the protosystem are deformable but only to a limited extent. This rigidity is transmitted to matter, which creates a link between the protostar and the protoplanetary disk. It is thanks to this link that the central region is braked and gradually loses its angular momentum in favor of the disc, which turns faster and faster.
Under the effect of the slowing down, the centrifugal force on the protostar drops and finally the ejection of matter stops. From this moment, the two previously linked subsystems have an independent evolution. In the center, the protostar continues to contract and its temperature increases rapidly. Eventually, nuclear fusion reactions set in and the star we know appears.
The formation of the planets :
In the protoplanetary disc, atoms agglomerate as they meet to become dust. These come together on their own to form small bodies called planetesimals. This stage lasts a few million years.
As a result of the turbulence in the disc, fluctuations in density appear and evolve into large bodies, in a process called accretion. These bodies continue to capture the planetesimals they find in their path and eventually reach the planet stage. The main accretion phase ends about 4.4 billion years ago, although heavy bombardment continues for a billion years. The final appearance of the planets depends on the distance from the Sun. Near it, the light elements receive a lot of energy and are too hot to condense. The material that constitutes these planets is therefore rich in heavy elements, such as iron or silicon, which explains their high density. Away from the Sun, the accretion of planetesimals creates a dense nucleus which is the starting point for further growth. Around this nucleus accumulates an envelope of gas and we end up with a very large and massive planet, but mainly made up of hydrogen and therefore not very dense.
Mercury :
The first planet in the solar system is Mercury, which is at an average distance of 0.38 astronomical units from the Sun (or 58 million kilometers). The orbit of the planet is a relatively flattened ellipse, so the distance is actually quite variable, between 0.31 and 0.47 astronomical units. Mercury's proximity to our star explains why, seen from Earth, the planet never moves far from the daylight. The maximum angular separation is only 28 degrees. Mercury is therefore only visible from Earth for a very short period of time, during sunrise or sunset. In addition, Mercury has a very small apparent diameter, which makes it virtually impossible to observe even the smallest detail on its surface.
The period of rotation of Mercury :
It was not until the 1960s and the use of radar to measure the period of rotation of the planet. At that time, astronomers sent radio waves to Mercury and analyzed the returned signal. The reflected waves exhibited a wavelength shift related to the Doppler effect induced by the rotational movement of the planet, which made it possible to measure its speed. The rotation period was thus estimated at about 59 Earth days. The peculiarity of this value is that it corresponds exactly to two-thirds of the period of revolution of Mercury around the Sun, i.e. 88 days. This is not a coincidence, but the result of the Sun's gravitational influence on the rotation of Mercury, a mechanism also at play in the case of the Moon. Note that for hypothetical inhabitants of Mercury, the combination of slow rotation and rapid revolution would have a surprising consequence. In fact, on the planet itself, the interval between two passages of the Sun vertically at a given point is equal to twice the period of revolution around the Sun. In other words, a day lasts two years!
Earth :
Earth with a diameter of 12,800 kilometers, slightly larger than that of Venus, Earth is the largest planet in the inner solar system. It orbits the Sun at an average distance of 150 million kilometers in a year. This distance serves as the definition for astronomical unit, a unit of distance used to measure distances in the solar system. The plane of the Earth's orbit around the Sun is called the plane of the ecliptic and also serves as a reference in the solar system.
Seasons :
The Earth spins around in just under 24 hours, giving rise to the alternation of days and nights. Its axis of rotation is inclined 23 degrees from the direction perpendicular to the plane of the ecliptic. This axis keeps a more or less fixed direction relative to the stars, but during the Earth's orbit, its direction relative to the Sun changes. It is this particularity that gives rise to the seasons.So, at the end of June, the northern hemisphere of our planet is tilted slightly towards the Sun and receives more radiation: the days are longer and the temperatures warmer, summer begins in the northern hemisphere. On the contrary, at the end of December, the southern hemisphere is tilted towards the Sun. In the northern hemisphere, the days are shorter and the temperatures lower, winter is on its way. In times of transition, none of the hemispheres are privileged, temperatures are average, as is the length of the day, it is either spring or autumn.
Earth's atmosphere :
One of the characteristics that sets our planet apart is the makeup of its atmosphere. The latter contains 78 percent nitrogen, 21 percent oxygen, the rest consisting of rare gases such as argon, carbon dioxide, water vapor and traces of other constituents, not to mention many suspended particles. By way of comparison, the planets Venus and Mars have an atmosphere dominated by carbon dioxide, with some nitrogen and virtually no oxygen. The large amount of oxygen present is a direct consequence of the most remarkable phenomenon on earth: life. It is indeed the development of living organisms that slowly transformed our atmosphere by injecting oxygen into it.
The limits of the atmosphere are not well defined. Density decreases with altitude, but the atmosphere is still detectable thousands of kilometers above sea level. Changes in temperature with altitude have defined several layers in the atmosphere.
The magnetism of the Earth :
Another equally important element in the vicinity of the Earth is the magnetic field. As we can check every day with the help of a compass, the Earth has a magnetic field. This probably has its origin in the electric currents which circulate in the liquid part of the iron core of our planet. The axis of the magnetic field is not aligned with the axis of rotation, but tilted about 11 degrees. This explains why the magnetic north pole is in Canada, relatively far from the geographic north pole defined by the axis of rotation. The action of the magnetic field gives rise to a region of space, called the magnetosphere, in which the movement of particles is dictated by the Earth's magnetic field.
Une vue d’artiste de l’interaction entre le vent solaire et la magnétosphère terrestre (échelle non respectée). Crédit : ESA
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