Satellites of the terrestrial planets

Having studied the structure of the solar system and dwarf planets in one of the previous ones, this article includes the natural satellites of the solar system. This is one of the most interesting avenues in research astronomy, as there are satellites that are larger than planets and have oceans and possibly life forms below their surface.

Let's start with the satellites of the terrestrial planets. Since Mercury and Venus have no natural satellites, acquaintance with the satellites of the solar system should begin with the Earth.

Terrestrial planets: Mercury, Venus, Earth and Mars.

moon

As you know, our planet has only one satellite - the Moon. This is the most studied cosmic body, as well as the first one that a person managed to visit. The Moon is the fifth largest natural satellite of a planet in the solar system.

Although the Moon is considered a satellite, technically it could be considered a planet if it had an orbit around the Sun. The diameter of the Moon is almost three and a half thousand kilometers (3476), for example, the diameter of Pluto is 2374 km.

The Moon is a full member of the Earth-Moon gravitational system. We have already written about another such tandem in the solar system - about. Although the mass of the Earth's satellite is not large and is a little more than a hundredth of the mass of the Earth, the Moon does not revolve around the Earth - they have a common center of mass.

Can the Earth-Moon system be considered a double planet? The differences between a binary planet and a planet-moon system are thought to lie in the location of the system's center of mass. If the center of mass is not located under the surface of one of the objects of the system, then it can be considered a double planet. It turns out that both bodies revolve around a point in space that is between them. According to this definition, the Earth and the Moon are a planet and a satellite, and Charon and Pluto are a double dwarf planet.

Since the distance between the Earth and the Moon is constantly increasing (the Moon is moving away from the Earth), the center of mass, which is currently under the surface of the Earth, will eventually move and be above the surface of our planet. But this is happening rather slowly, and it will be possible to consider the Earth-Moon system as a double planet only after billions of years.

Earth-Moon system

Among the cosmic bodies, the Moon influences the Earth almost the most, except, perhaps, the Sun. The most obvious phenomena of the impact of the satellite on the Earth are the lunar tides, which regularly change the water level in the oceans.

Earth view from the pole (high tide, low tide)

Why is the surface of the moon covered in craters? First, the Moon does not have an atmosphere that would protect its surface from meteorites. Secondly, there is no water or wind on the Moon, which could smooth out the impact sites of meteorites. Therefore, over four billion years, a large number of craters have accumulated on the surface of the satellite.

The largest crater in the solar system. South Pole - Aitken basin (red - uplands, blue - lowlands).

Lunar crater Daedalus: diameter 93 km, depth 2.8 km (image from Apollo 11).

The moon, as already mentioned, is the only satellite visited by man and the first celestial body, samples of which were brought to Earth. Neil Armstrong became the first person to walk on the moon on July 21, 1969. A total of twelve astronauts have walked on the Moon; The last time humans landed on the moon was in 1972.

The first photograph taken by Neil Armstrong after landing on the moon.

Edwin Aldrin on the Moon, July 1969 (NASA photo).

Before scientists got soil samples from the moon, there were two fundamentally different theories about the origin of the moon. Adherents of the first theory believed that the Earth and the Moon were formed at the same time from a gas and dust cloud. According to another theory, it was believed that the Moon was formed elsewhere, and then captured by the Earth. The study of lunar samples has led to a new theory about the "Giant Impact": almost four and a half (4.36) billion years ago, the protoplanet Earth (Gaia) collided with the protoplanet Theia. The blow fell not in the center, but at an angle (almost tangentially). As a result, most of the matter of the impacted object and part of the matter of the earth's mantle were ejected into near-Earth orbit. From these fragments the Moon was assembled. As a result of the impact, the Earth received a sharp increase in the speed of rotation (one revolution in five hours) and a noticeable tilt of the axis of rotation. Although this theory also has flaws, it is currently considered the main one.

Formation of the Moon: Theia's impact with the Earth, which is believed to have formed the Moon.

Moons of Mars

Mars has two small moons: Phobos and Deimos. They were discovered by Asaph Hall in 1877. It is noteworthy that, having become disillusioned with the search for Martian satellites, he already wanted to abandon observation, but his wife Angelina was able to convince him. The next night he discovered Deimos. Six nights later - Phobos. On Phobos, he discovered a giant crater that reaches ten kilometers wide - almost half the width of the satellite itself! Hall gave him Angelina's maiden name, Stickney.

The image of the satellites of Mars with the observance of scales and distances.

Both satellites have a shape close to a triaxial ellipsoid. Due to their small size, gravity is not enough to compress them into a round shape.

Phobos. On the right you can see the Stickney crater.

Interestingly, the tidal influence of Mars is gradually slowing down the movement of Phobos, thereby reducing its orbit, which, as a result, will lead to its fall to Mars. Every hundred years, Phobos gets closer to Mars by nine centimeters, and in about eleven million years it will fall on its surface if the same forces do not destroy it even earlier. Deimos, on the contrary, is moving away from Mars, and will eventually be captured by the tidal forces of the Sun. As a result, Mars will be left without satellites.

Attraction on the "Martian" side of Phobos is practically absent, or rather, it is almost non-existent. This is caused by the proximity of the satellite to the surface of Mars and the strong gravity from the planet. In other parts of the satellite, the gravitational force is different.

The satellites of Mars are always turned to it on one side, since the period of revolution of each of them coincides with the corresponding period of revolution around Mars. On this basis, they are similar to the Moon, the reverse side of which is also never visible from the surface of the Earth.

Deimos and Phobos are very small. For example, the radius of the Moon is 158 times the radius of Phobos and about 290 times the radius of Deimos.

The distances from satellites to the planet are also insignificant: the Moon is at a distance of 384,000 km from the Earth, and Deimos and Phobos are 23,000 and 9,000 kilometers from Mars, respectively.

The origin of the Martian moons remains controversial. They could be asteroids captured by the gravitational field of Mars, but the difference in their structure from the objects of the group of asteroids, of which they could be a part, speaks against this version. Others believe that they were formed as a result of the collapse of the satellite of Mars into two parts.

The following material will be devoted to the satellites of Jupiter, of which as many as 67 have been registered to date! And, perhaps, on some of them there are life forms.