Starship Asterisk*

Ann wrote: ↑Mon Jun 14, 2021 4:46 am

Ganymede with bright impact craters and large dark angular features from Juno. Image Credit: NASA/JPL-Caltech/SwRI/MSSS; Processing & License: Kevin M. Gill

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Earth's Moon with bright impact crater Tycho Brahe (bottom) and large dark roundish features. Image courtesy of the Lunar and Planetary Institute.

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It is interesting to contemplate the similarities and differences of our own Moon and Ganymede.

It's easy to see how the impact craters got there, but what about the general surface markings?

Chris Peterson wrote: ↑Mon Jun 14, 2021 5:18 am The difference between being made of rock and being made of ice?

VictorBorun wrote: ↑Mon Jun 14, 2021 6:16 am

Chris Peterson wrote: ↑Mon Jun 14, 2021 5:18 am The difference between being made of rock and being made of ice?

"underground saltwater ocean on Ganymede" does not imply an icy surface. It can all be proper rock.

By the way, how has smaller Titan managed to grow a thicker atmosphere?

JohnD wrote: ↑Mon Jun 14, 2021 9:06 am AHEM! "no known geological mechanism"???

Chris, see: https://www.theguardian.com/environment ... rrier-reef Underwater eruptions produce pumice, lava so full of gas that when solidified, it floats.

How long it can go on flaoting is the subject of research: https://www.universityofcalifornia.edu/ ... cks-pumice

And thank you for a thoroughly 'scientific' APOD - and beautiful as well!

JOhn

Chris Peterson wrote: ↑Mon Jun 14, 2021 5:18 am

Ann wrote: ↑Mon Jun 14, 2021 4:46 am

Ganymede with bright impact craters and large dark angular features from Juno. Image Credit: NASA/JPL-Caltech/SwRI/MSSS; Processing & License: Kevin M. Gill

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Earth's Moon with bright impact crater Tycho Brahe (bottom) and large dark roundish features. Image courtesy of the Lunar and Planetary Institute.

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It is interesting to contemplate the similarities and differences of our own Moon and Ganymede.

It's easy to see how the impact craters got there, but what about the general surface markings?

The difference between being made of rock and being made of ice?

Chris Peterson wrote: ↑Mon Jun 14, 2021 9:13 am

JohnD wrote: ↑Mon Jun 14, 2021 9:06 am AHEM! "no known geological mechanism"???

Chris, see: https://www.theguardian.com/environment ... rrier-reef Underwater eruptions produce pumice, lava so full of gas that when solidified, it floats.

How long it can go on flaoting is the subject of research: https://www.universityofcalifornia.edu/ ... cks-pumice

And thank you for a thoroughly 'scientific' APOD - and beautiful as well!

JOhn

Again, there is no known geological mechanism for a rocky crust floating on a thick water layer. We're not talking about some bits of rock that have a density less than water due to trapped gas, but the entire crust of a planetary body! Ganymede is a fairly conventional terrestrial body- an iron core, surrounded by a rocky mantle, with an outer ocean, the uppermost part of which is frozen.

Chris Peterson wrote: ↑Mon Jun 14, 2021 7:41 am In any case, spectroscopic data reveal that the surface of Ganymede is made of ice, primarily water ice, but also various other ices as well.

https://en.wikipedia.org/wiki/Ganymede_(moon) wrote:

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.

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<<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.>>

Alex_515 wrote: ↑Mon Jun 14, 2021 7:36 am On the left part of the image, middle, between the dark rectangular area and a bright area, there is a strange alignment of small craters.

johnnydeep wrote: ↑Mon Jun 14, 2021 7:44 pm

Alex_515 wrote: ↑Mon Jun 14, 2021 7:36 am
On the left part of the image, middle, between the dark rectangular area and a bright area, there is a strange alignment of small craters.

My guess is it was caused by an asteroid that broke apart on it's way in (like that string of comet fragments that hit Jupiter in 1994 - see https://solarsystem.nasa.gov/resources/ ... iter-1994/), or perhaps this was a case of a "stone skipping on water" effect?

https://en.wikipedia.org/wiki/Crater_chain wrote:

Enki Catena is a chain of impact craters on Ganymede, caused by a fragmented space body (probably a comet). The picture covers an area about 193 kilometers wide.

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<<A crater chain is a line of craters along the surface of an astronomical body. The descriptor term for crater chains is catena, plural catenae (Latin for "chain"), as specified by the International Astronomical Union's rules on planetary nomenclature.

Many examples of such chains are thought to have been formed by the impact of a body that was broken up by tidal forces into a string of smaller objects following roughly the same orbit. An example of such a tidally disrupted body that was observed prior to its impact on Jupiter is Comet Shoemaker-Levy 9. During the Voyager observations of the Jupiter system, planetary scientists identified 13 crater chains on Callisto and three on Ganymede (except those formed by secondary craters). Later some of these chains turned out to be secondary or tectonic features, but some other chains were discovered. As of 1996, 8 primary chains on Callisto and 3 on Ganymede were confirmed.

Other cases, such as many of those on Mars, represent chains of collapse pits associated with grabens (see, for example, the Tithoniae Catenae near Tithonium Chasma). Crater chains seen on the Moon often radiate from larger craters, and in such cases are thought to be either caused by secondary impacts of the larger crater's ejecta or by volcanic venting activity along a rift.>>

neufer wrote: ↑Mon Jun 14, 2021 9:04 pm

johnnydeep wrote: ↑Mon Jun 14, 2021 7:44 pm

Alex_515 wrote: ↑Mon Jun 14, 2021 7:36 am
On the left part of the image, middle, between the dark rectangular area and a bright area, there is a strange alignment of small craters.

My guess is it was caused by an asteroid that broke apart on it's way in (like that string of comet fragments that hit Jupiter in 1994 - see https://solarsystem.nasa.gov/resources/ ... iter-1994/), or perhaps this was a case of a "stone skipping on water" effect?

https://en.wikipedia.org/wiki/Crater_chain wrote:

Enki Catena is a chain of impact craters on Ganymede, caused by a fragmented space body (probably a comet). The picture covers an area about 193 kilometers wide.

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<<A crater chain is a line of craters along the surface of an astronomical body. The descriptor term for crater chains is catena, plural catenae (Latin for "chain"), as specified by the International Astronomical Union's rules on planetary nomenclature.

Many examples of such chains are thought to have been formed by the impact of a body that was broken up by tidal forces into a string of smaller objects following roughly the same orbit. An example of such a tidally disrupted body that was observed prior to its impact on Jupiter is Comet Shoemaker-Levy 9. During the Voyager observations of the Jupiter system, planetary scientists identified 13 crater chains on Callisto and three on Ganymede (except those formed by secondary craters). Later some of these chains turned out to be secondary or tectonic features, but some other chains were discovered. As of 1996, 8 primary chains on Callisto and 3 on Ganymede were confirmed.

Other cases, such as many of those on Mars, represent chains of collapse pits associated with grabens (see, for example, the Tithoniae Catenae near Tithonium Chasma). Crater chains seen on the Moon often radiate from larger craters, and in such cases are thought to be either caused by secondary impacts of the larger crater's ejecta or by volcanic venting activity along a rift.>>

MarkBour wrote: ↑Tue Jun 15, 2021 12:59 am I'm struck by the white areas around the largest impact sites. At least they appear white in this image. I guess that when a large impact occurs on Ganymede, what gets thrown up includes a bunch of the subsurface water (saltwater?) which then falls back and freezes on the surface.

Chris Peterson wrote: ↑Tue Jun 15, 2021 1:18 am As always with these images, we've got to be careful with interpreting color and intensity. Images are likely stretched to cover a range from white to black, which hides the true differences between areas. For that we'd need to look at unprocessed, linear images.

orin stepanek wrote: ↑Tue Jun 15, 2021 12:10 pm What intrigues me more is the dark area that seems to be inlaid like
a piece of tile!

file.jpg