Chapter 1826 Ganymede Terrain

Chapter 1826 Ganymede Terrain

Ganymede is the largest moon in the solar system. The diameter is larger than that of Mercury, and its mass is about half that of Mercury.

Composed mostly of silicate rock and ice, Ganymede is clearly layered and has an iron-rich, fluid core. Larger than Mercury, it is the only known satellite in the solar system to have a magnetosphere.

Ganymede was not first discovered by Galileo. Between 400 BC and 360 BC (most likely in the summer of 364 BC), according to the "Tang Kaiyuan Zhanjing" quoting Gander's words about Jupiter: "If there is a small red star around On its side”, Mr. Xi Zezong, a famous historian of astronomy, pointed out: Gander observed Jupiter’s last satellite Europa in the middle of the 4th century BC.

As for the discovery of Jupiter's satellites, it was discovered in modern times after the invention of the telescope in the early 17th century, when the great Italian scientist Galileo used it to observe Jupiter in 1610. But Gander was nearly two thousand years earlier than Galileo, and it was a miracle that he discovered the satellites of Jupiter with the naked eye without a telescope.

Later, the astronomer Simon Marius named it after Ganymedes, the lover of Zeus in Greek mythology. The Voyager spacecraft precisely measured the moon's size, and the Galileo probe discovered its subsurface ocean and magnetic field.

Ganymede is the only known moon in the solar system to have a magnetosphere, probably created by convective motion in a flowing iron-rich core. A small number of these magnetospheres overlap Jupiter's larger magnetic field, creating field lines that radiate outward. Ganymede has a thin oxygen-containing atmosphere containing atomic oxygen, oxygen and ozone, and atomic hydrogen is also a component of the atmosphere. Whether Ganymede has an ionosphere has not yet been determined.

Ganymede is mainly composed of silicate rocks and ice bodies, and the star is clearly layered, with an iron-rich, liquid core. Scientists speculate that 200 kilometers below Ganymede's surface lies a saltwater ocean sandwiched between two layers of ice. There are two main terrains on the surface of Ganymede: the darker area accounts for about one-third of the total area of ​​the star, and it is densely covered with impact craters. The geological age is estimated to be 4 billion years old; The grooves and ridges are slightly younger in geological age than the former. The reason for the fractured geological structure in the bright area is still a mystery, but it may be caused by tectonic activity caused by tidal heat.

Ganymede is the only moon in the solar system known to have a magnetosphere. It has a thin, oxygen-containing atmosphere containing atomic oxygen, oxygen, and ozone. Whether Ganymede has an ionosphere has yet to be determined.

There are two main types of terrain on the surface of Ganymede: one is very old, dark areas densely covered with impact craters, and the other is slightly younger (but still very old in geological age) with a large number of grooves and Bright area of ​​the ridge. The dark zone, which covers about a third of the sphere's total area, contains clay and organic matter, probably brought by meteorites that struck Ganymede.

The heating mechanism that produces the grooved terrain remains a major puzzle in planetary science. The current view is that the trough topography was formed primarily by tectonic activity in nature; and that cryovolcanoes played a minor role, if any.

In order to cause this tectonic activity, Ganymede's lithosphere must be under sufficiently strong pressure, and the force that causes this pressure may be related to tidal heating that has occurred in the past-this effect may be in Ganymede. Stable orbital resonance occurs when gravitational tides deflect the icy body, heating the interior of the star, putting pressure on the lithosphere, and further leading to the formation of fissures, horsts, and grabens, which replace 70% of Ganymede's surface area. % of the Ancient Dark Zone.

The formation of the trench topography may also be related to the formation process of the early core and the subsequent tidal heat in the interior of the star. The phase transition and thermal expansion and contraction of the ice body caused by them may cause Ganymede to undergo a slight expansion, with a magnitude of 1-6%. With the further development of the star, hot water jets are squeezed from the core to the surface of the star, causing structural deformation of the lithosphere. Heat from radioactive decay in the star's interior is the most likely source of heat, and the formation of Ganymede's subsurface ocean may depend on it. Through the research model, people found that if Ganymede's orbital eccentricity value in the past was much higher than today's (in fact, it may be the case), then tidal heat may replace radioactive decay heat source and become Ganymede's most important heat source.

Visible in both terrains, but more densely distributed in the dark zone: this area has been bombarded by large-scale meteorites, so the distribution of impact craters is saturated. There are fewer impact craters in the brighter trench topography, where the topography developed due to structural deformation becomes the main geological feature. The density of impact craters suggests that the dark zone is geologically old to 4 billion years, which is close to that of the highland terrain on the Moon; the trough terrain is slightly younger (but its exact age cannot be determined).

Similar to the moon, Ganymede experienced a period of violent meteorite bombardment 3.5-4 billion years ago. If this is true, then a large-scale bombardment event occurred in the solar system during this period, and the bombardment rate after this period In the bright area, there are impact craters covering trenches and trenches cutting impact craters, which shows that the geological age of some trenches is also very old.

There are also relatively young impact craters on Ganymede, and their outward radiation is still clearly visible. Ganymede's impact craters are not as deep as those on the moon and Mercury. This may be due to the weak texture of Ganymede's icy layers and displacement, which can transfer part of the impact force. The crater structure of many geologically old impact craters has disappeared, leaving only a remnant of what is known as a residual structure.

Ganymede's Salient Features Package

It includes a darker plain known as the Galilean zone, a region of concentric rings of grooves that may have formed during a period of geological activity. Another notable feature is Ganymede's two polar caps, which may be composed of frost bodies.

This layer of frost extends to a latitude of 40°. Voyager first spotted Ganymede's polar caps. There are currently two theories to explain the formation of the polar cap, one is considered to be caused by the diffusion of ice bodies at high latitudes, and the other is believed to be caused by the bombardment of plasma ice bodies in outer space. Galileo's observations favor the latter theory.