by neufer » Mon Jun 14, 2021 1:53 pm
https://en.wikipedia.org/wiki/Ganymede_(moon) wrote:
<<Ganymede is composed of approximately equal amounts of silicate rock and water. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans combined. Its surface is composed of two main types of terrain.
Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of it. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity due to tidal heating. The dark terrain, which comprises about one-third of the surface, contains clays and organic materials that could indicate the composition of the impactors from which Jovian satellites accreted.
Cratering is seen on both types of terrain, but is especially extensive on the dark terrain: it appears to be saturated with impact craters and has evolved largely through impact events. The brighter, grooved terrain contains many fewer impact features, which have been only of a minor importance to its tectonic evolution. The density of cratering indicates an age of 4 billion years for the dark terrain, similar to the highlands of the Moon, and a somewhat younger age for the grooved terrain (but how much younger is uncertain). Ganymede may have experienced a period of heavy cratering 3.5 to 4 billion years ago similar to that of the Moon. If true, the vast majority of impacts happened in that epoch, whereas the cratering rate has been much smaller since. Craters both overlay and are crosscut by the groove systems, indicating that some of the grooves are quite ancient. Relatively young craters with rays of ejecta are also visible. Ganymedian craters are flatter than those on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust, which can (or could) flow and thereby soften the relief. Ancient craters whose relief has disappeared leave only a "ghost" of a crater known as a palimpsest.
The heating mechanism required for the formation of the grooved terrain on Ganymede is an unsolved problem in the planetary sciences. The modern view is that the grooved terrain is mainly tectonic in nature. Cryovolcanism is thought to have played only a minor role, if any. The forces that caused the strong stresses in the Ganymedian ice lithosphere necessary to initiate the tectonic activity may be connected to the tidal heating events in the past, possibly caused when the satellite passed through unstable orbital resonances. The tidal flexing of the ice may have heated the interior and strained the lithosphere, leading to the development of cracks and horst and graben faulting, which erased the old, dark terrain on 70% of the surface. The formation of the grooved terrain may also be connected with the early core formation and subsequent tidal heating of Ganymede's interior, which may have caused a slight expansion of Ganymede by 1–6% due to phase transitions in ice and thermal expansion. During subsequent evolution deep, hot water plumes may have risen from the core to the surface, leading to the tectonic deformation of the lithosphere. Radiogenic heating within the satellite is the most relevant current heat source, contributing, for instance, to ocean depth. Research models have found that if the orbital eccentricity were an order of magnitude greater than currently (as it may have been in the past), tidal heating would be a more substantial heat source than radiogenic heating.>>
[quote=https://en.wikipedia.org/wiki/Ganymede_(moon)]
[float=left][img3=Enhanced-color Galileo spacecraft image of Ganymede's trailing hemisphere. The crater Tashmelefttum's prominent rays are at lower right, and the large ejecta field of Hershef at upper right. Part of dark Nicholson Regio is at lower left, bounded on its upper right by Harpagia Sulcus.]https://upload.wikimedia.org/wikipedia/commons/7/7f/Ganymede-moon.jpg[/img3][/float]
<<Ganymede is composed of approximately equal amounts of silicate rock and water. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans combined. Its surface is composed of two main types of terrain. [b][color=#0000FF]Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of it.[/color] [color=#FF0000]Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity due to tidal heating. [/color][color=#0000FF]The dark terrain, which comprises about one-third of the surface, contains clays and organic materials that could indicate the composition of the impactors from which Jovian satellites accreted.[/color][/b]
Cratering is seen on both types of terrain, but is especially extensive on the dark terrain: it appears to be saturated with impact craters and has evolved largely through impact events. The brighter, grooved terrain contains many fewer impact features, which have been only of a minor importance to its tectonic evolution. The density of cratering indicates an age of 4 billion years for the dark terrain, similar to the highlands of the Moon, and a somewhat younger age for the grooved terrain (but how much younger is uncertain). Ganymede may have experienced a period of heavy cratering 3.5 to 4 billion years ago similar to that of the Moon. If true, the vast majority of impacts happened in that epoch, whereas the cratering rate has been much smaller since. Craters both overlay and are crosscut by the groove systems, indicating that some of the grooves are quite ancient. Relatively young craters with rays of ejecta are also visible. Ganymedian craters are flatter than those on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust, which can (or could) flow and thereby soften the relief. Ancient craters whose relief has disappeared leave only a "ghost" of a crater known as a palimpsest.
The heating mechanism required for the formation of the grooved terrain on Ganymede is an unsolved problem in the planetary sciences. The modern view is that the grooved terrain is mainly tectonic in nature. Cryovolcanism is thought to have played only a minor role, if any. The forces that caused the strong stresses in the Ganymedian ice lithosphere necessary to initiate the tectonic activity may be connected to the tidal heating events in the past, possibly caused when the satellite passed through unstable orbital resonances. The tidal flexing of the ice may have heated the interior and strained the lithosphere, leading to the development of cracks and horst and graben faulting, which erased the old, dark terrain on 70% of the surface. The formation of the grooved terrain may also be connected with the early core formation and subsequent tidal heating of Ganymede's interior, which may have caused a slight expansion of Ganymede by 1–6% due to phase transitions in ice and thermal expansion. During subsequent evolution deep, hot water plumes may have risen from the core to the surface, leading to the tectonic deformation of the lithosphere. Radiogenic heating within the satellite is the most relevant current heat source, contributing, for instance, to ocean depth. Research models have found that if the orbital eccentricity were an order of magnitude greater than currently (as it may have been in the past), tidal heating would be a more substantial heat source than radiogenic heating.>>[/quote]