APOD: Ganymede from Juno (2023 Nov 28)

[APOD Robot]

Ganymede from Juno

Explanation: What does the largest moon in the Solar System look like? Jupiter's moon Ganymede, larger than even Mercury and Pluto, has an icy surface speckled with bright young craters overlying a mixture of older, darker, more cratered terrain laced with grooves and ridges. The cause of the grooved terrain remains a topic of research, with a leading hypothesis relating it to shifting ice plates. Ganymede is thought to have an ocean layer that contains more water than Earth -- and might contain life. Like Earth's Moon, Ganymede keeps the same face towards its central planet, in this case Jupiter. The featured image was captured in 2021 by NASA's robotic Juno spacecraft when it passed by the immense moon. The close pass reduced Juno's orbital period around Jupiter from 53 days to 43 days. Juno continues to study the giant planet's high gravity, unusual magnetic field, and complex cloud structures.

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[Ann]

Ganymede from Juno. Image Credit & Copyright: NASA/JPL-Caltech/SwRI/MSSS; Processing & License: Kevin M. Gill

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The Moon. Illustration by David A. Aguilar

The Moon and Ganymede may be very different objects - the Moon is an almost entirely rocky body while considerably larger Ganymede is made primarily of water and ice - but they do look at least somewhat similar. Both have bright and dark areas and a magnificent "Tycho crater" bullseye in their, well, nether regions.

Grooved terrain on Ganymede. Image by Galileo.

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The grooved terrain on Ganymede is very interesting, and judging from the Galileo image I just posted, it appears to be mostly confined to the light-colored areas.

APOD Robot wrote:

The close pass reduced Juno's orbital period around Jupiter from 53 days to 43 days.

Really? How is that possible?

Ann

[daddyo]

I noticed some of the larger craters, particularly those on the left in the maria, have a crater within a crater, instead of a peak. Made me wonder if the event that caused the maria “melted” the base of what may have been peaks, causing them to collapse.

[gmPhil]

The featured image was captured in 2021 by NASA's robotic Juno spacecraft when it passed by the immense moon. The close pass reduced Juno's orbital period around Jupiter from 53 days to 43 days.

Have I understood this correctly? That Juno's close pass has affected Ganymede's orbit by 10 days?! Surely not....

[edit] Just seen Ann has queried the same! Glad it's not just me - thought I was losing it for a moment

[edit 2] well, don't I feel silly now! Just glad I wasn't the only one who who misread that... (Apart from @Ann, I also know someone else who read it as I did. Strange really, because it is written perfectly ccorectly and clear. Ho-hum.)

[Whiskybreath]

An interesting detail:
Clipboard01.jpg

[76AD]

Ann & Phil

I'm sure the APOD robot is referring to the orbit time of the spacecraft and not of Ganymede itself. I think Ganymede's orbit is safe for the time being

[richard_schumacher]

Is it thought that Ganymede's ice shell is now too thick for tectonics and moving plates? It appears to have not resurfaced itself for some time.

The double-walled crater thingy is interesting. I'll guess that this happens when an impactor is energetic enough to punch completely through the ice layer. (Do I win the powder-blue Buick?)

[jimbo48]

APOD Robot wrote:
The close pass reduced Juno's orbital period around Jupiter from 53 days to 43 days.

Really? How is that possible?
Ann
[/quote]

Ann,
The OP says Juno's orbital period was reduced, not Gany's. For some reason I had to read it twice before I understood it correctly as well.
Jim

[johnnydeep]

APOD Robot wrote:
The close pass reduced Juno's orbital period around Jupiter from 53 days to 43 days.

Really? How is that possible?
Ann

Ann,
The OP says Juno's orbital period was reduced, not Gany's. For some reason I had to read it twice before I understood it correctly as well.
Jim

Right you are. Fortunately I read it the right way the first time...for a change. But I still wonder if Ann was perhaps questioning how the close pass could have affected Juno's orbit so much? Presumably the speed up is because Juno was sped up, making the orbit more elliptical and with a periapsis (or perijove) closer to Jupiter. But that also means that the apoapsis (or apojove) is farther away and Juno would travel more slowly there. But I suppose the speed up at periapsis is greater. I'm sure there's a precise mathematical equation to tell us...

[Pastorian]

Yes, Ganymede's resemblance to the moon is striking.
Could the detail posted by Whiskeybreath be an object that broke up prior to surface impact?

[johnnydeep]

An interesting detail:
Clipboard01.jpg

Yup, you beat me to mentioning this. Clearly that crater chain was caused by a single aggregate impactor event where the impactor was broken up prior to impacting, much like when Comet Shoemaker-Levy 9 impacted Jupiter in July of 1994.

[bls0326]

I noticed Ganymede bulge about the 11 o'clock position. One link I found https://phys.org/news/2015-04-jupiter-m ... bulge.html
includes "As noted by National Geographic, the bulge, or protuberance, is approximately the size of Ecuador and about half the height of Mount Kilimanjaro." It also notes "The researchers proposed at the conference that the bulge came about due to polar wandering—where ice built up at one of the poles and then slid over the top of an ocean below, to the equator"

[Chris Peterson]

Really? How is that possible?
Ann

Ann,
The OP says Juno's orbital period was reduced, not Gany's. For some reason I had to read it twice before I understood it correctly as well.
Jim

Right you are. Fortunately I read it the right way the first time...for a change. But I still wonder if Ann was perhaps questioning how the close pass could have affected Juno's orbit so much? Presumably the speed up is because Juno was sped up, making the orbit more elliptical and with a periapsis (or perijove) closer to Jupiter. But that also means that the apoapsis (or apojove) is farther away and Juno would travel more slowly there. But I suppose the speed up at periapsis is greater. I'm sure there's a precise mathematical equation to tell us...

The flybys reduced the eccentricity and semimajor axis of Juno's orbit. Perijove was largely unchanged, apojove got somewhat closer to the planet.

[Ann]

Ann,
The OP says Juno's orbital period was reduced, not Gany's. For some reason I had to read it twice before I understood it correctly as well.
Jim

Right you are. Fortunately I read it the right way the first time...for a change. But I still wonder if Ann was perhaps questioning how the close pass could have affected Juno's orbit so much? Presumably the speed up is because Juno was sped up, making the orbit more elliptical and with a periapsis (or perijove) closer to Jupiter. But that also means that the apoapsis (or apojove) is farther away and Juno would travel more slowly there. But I suppose the speed up at periapsis is greater. I'm sure there's a precise mathematical equation to tell us...

The flybys reduced the eccentricity and semimajor axis of Juno's orbit. Perijove was largely unchanged, apojove got somewhat closer to the planet.

I like your number of posts, Chris. 17777.

Don't destroy that number by answering me!

Ann

[Eclectic Man]

Confused by the illumination of so much of Ganymede's surface. I was assuming that it was illuminated by the Sun, but that would mean only half the surface would be lit, whereas clearly from the image much more than half appears to be lit. Is Ganymede illuminated by Jupiter, or is there some pother explanation?

[Chris Peterson]

Confused by the illumination of so much of Ganymede's surface. I was assuming that it was illuminated by the Sun, but that would mean only half the surface would be lit, whereas clearly from the image much more than half appears to be lit. Is Ganymede illuminated by Jupiter, or is there some pother explanation?

We don't know how far the camera is from the Moon, so we don't know what percentage of a full hemisphere we're seeing here. But how do you conclude that more than half of the surface is lit? The most we can see is half (and probably much less), and if the spacecraft is between Ganymede and the Sun, all of the visible surface would be lit.

(Sorry, Ann.)

[johnnydeep]

Ann,
The OP says Juno's orbital period was reduced, not Gany's. For some reason I had to read it twice before I understood it correctly as well.
Jim

Right you are. Fortunately I read it the right way the first time...for a change. But I still wonder if Ann was perhaps questioning how the close pass could have affected Juno's orbit so much? Presumably the speed up is because Juno was sped up, making the orbit more elliptical and with a periapsis (or perijove) closer to Jupiter. But that also means that the apoapsis (or apojove) is farther away and Juno would travel more slowly there. But I suppose the speed up at periapsis is greater. I'm sure there's a precise mathematical equation to tell us...

The flybys reduced the eccentricity and semimajor axis of Juno's orbit. Perijove was largely unchanged, apojove got somewhat closer to the planet.

Hmm, so Juno's was made more circular, and I suppose smaller in diameter overall to result in a faster orbital time? Orbits are confusing.

[johnnydeep]

I noticed Ganymede bulge about the 11 o'clock position. One link I found https://phys.org/news/2015-04-jupiter-m ... bulge.html
includes "As noted by National Geographic, the bulge, or protuberance, is approximately the size of Ecuador and about half the height of Mount Kilimanjaro." It also notes "The researchers proposed at the conference that the bulge came about due to polar wandering—where ice built up at one of the poles and then slid over the top of an ocean below, to the equator"

Is that really the "polar wandering bulge" you're observing? I barely see anything different there. I'm still trying to find some article or paper that actually points out the bulge on a a real image of Juno!

[Chris Peterson]

Right you are. Fortunately I read it the right way the first time...for a change. But I still wonder if Ann was perhaps questioning how the close pass could have affected Juno's orbit so much? Presumably the speed up is because Juno was sped up, making the orbit more elliptical and with a periapsis (or perijove) closer to Jupiter. But that also means that the apoapsis (or apojove) is farther away and Juno would travel more slowly there. But I suppose the speed up at periapsis is greater. I'm sure there's a precise mathematical equation to tell us...

The flybys reduced the eccentricity and semimajor axis of Juno's orbit. Perijove was largely unchanged, apojove got somewhat closer to the planet.

Hmm, so Juno's was made more circular, and I suppose smaller in diameter overall to result in a faster orbital time? Orbits are confusing.

I'd stick with my terminology. "Reduced eccentricity" as opposed to "more circular", and "reduced semi-major axis" as opposed to "smaller diameter".

Simple Keplerian two-body orbits are pretty easy to understand. But we are dealing here with a three-body problem, and that gets complicated fast. There was a transfer of (conserved) angular momentum between Juno and Ganymede during the flyby, resulting in Juno being in a lower (and therefore faster) orbit around Jupiter. Ganymede's orbit was also changed, of course. Not unlike the way that Earth moves a little bit when you jump off the ground.

[Eclectic Man]

Confused by the illumination of so much of Ganymede's surface. I was assuming that it was illuminated by the Sun, but that would mean only half the surface would be lit, whereas clearly from the image much more than half appears to be lit. Is Ganymede illuminated by Jupiter, or is there some pother explanation?

We don't know how far the camera is from the Moon, so we don't know what percentage of a full hemisphere we're seeing here. But how do you conclude that more than half of the surface is lit? The most we can see is half (and probably much less), and if the spacecraft is between Ganymede and the Sun, all of the visible surface would be lit.

Actually we do know how far the camera is from the moon, according to the data for the three images used to create the composite, the spacecraft altitude was just shy of 1,000,000km, so I would have thought that was far enough away to show the poles, as compared to the view of the Moon from the Earth, however, the terminator in the image does not reach to opposite edges of the disc, so it appears that significantly more than half of Ganymede is illuminated. I guess this is due to the fact that the image is a composite.

See: https://www.flickr.com/photos/kevinmgill/51238659798/
https://www.missionjuno.swri.edu/junoca ... C00001_V01
https://www.missionjuno.swri.edu/junoca ... C00002_V01
https://www.missionjuno.swri.edu/junoca ... C00003_V01

Even in the separate images 34C00001/2/3 the terminator does not reach opposite points of the disc.

[Chris Peterson]

Confused by the illumination of so much of Ganymede's surface. I was assuming that it was illuminated by the Sun, but that would mean only half the surface would be lit, whereas clearly from the image much more than half appears to be lit. Is Ganymede illuminated by Jupiter, or is there some pother explanation?

We don't know how far the camera is from the Moon, so we don't know what percentage of a full hemisphere we're seeing here. But how do you conclude that more than half of the surface is lit? The most we can see is half (and probably much less), and if the spacecraft is between Ganymede and the Sun, all of the visible surface would be lit.

Actually we do know how far the camera is from the moon, according to the data for the three images used to create the composite, the spacecraft altitude was just shy of 1,000,000km, so I would have thought that was far enough away to show the poles, as compared to the view of the Moon from the Earth, however, the terminator in the image does not reach to opposite edges of the disc, so it appears that significantly more than half of Ganymede is illuminated. I guess this is due to the fact that the image is a composite.

See: https://www.flickr.com/photos/kevinmgill/51238659798/
https://www.missionjuno.swri.edu/junoca ... C00001_V01
https://www.missionjuno.swri.edu/junoca ... C00002_V01
https://www.missionjuno.swri.edu/junoca ... C00003_V01

Even in the separate images 34C00001/2/3 the terminator does not reach opposite points of the disc.

That altitude is its distance from Jupiter, not Ganymede. We are seeing less than a full hemisphere, and the fact that it's a composite makes no difference. The three images were taken over just two minutes (2021-06-07T16:57:20.470, 2021-06-07T16:58:21.048, 2021-06-07T16:59:21.723). Not enough to significantly change the view of the moon.

[johnnydeep]

The flybys reduced the eccentricity and semimajor axis of Juno's orbit. Perijove was largely unchanged, apojove got somewhat closer to the planet.

Hmm, so Juno's was made more circular, and I suppose smaller in diameter overall to result in a faster orbital time? Orbits are confusing.

I'd stick with my terminology. "Reduced eccentricity" as opposed to "more circular", and "reduced semi-major axis" as opposed to "smaller diameter".

Simple Keplerian two-body orbits are pretty easy to understand. But we are dealing here with a three-body problem, and that gets complicated fast. There was a transfer of (conserved) angular momentum between Juno and Ganymede during the flyby, resulting in Juno being in a lower (and therefore faster) orbit around Jupiter. Ganymede's orbit was also changed, of course. Not unlike the way that Earth moves a little bit when you jump off the ground.

Thanks. So, "more circular" does not always reflect "reduced eccentricity"?

Click to view full size image

[Chris Peterson]

Hmm, so Juno's was made more circular, and I suppose smaller in diameter overall to result in a faster orbital time? Orbits are confusing.

I'd stick with my terminology. "Reduced eccentricity" as opposed to "more circular", and "reduced semi-major axis" as opposed to "smaller diameter".

Simple Keplerian two-body orbits are pretty easy to understand. But we are dealing here with a three-body problem, and that gets complicated fast. There was a transfer of (conserved) angular momentum between Juno and Ganymede during the flyby, resulting in Juno being in a lower (and therefore faster) orbit around Jupiter. Ganymede's orbit was also changed, of course. Not unlike the way that Earth moves a little bit when you jump off the ground.

Thanks. So, "more circular" does not always reflect "reduced eccentricity"?

Click to view full size image

It's just too colloquial for my taste. Orbital elements are well defined. There is no concept of "circular" or "diameter". Every orbit is elliptical, where that ellipse is defined by an eccentricity and a semimajor axis. It happens that if the eccentricity is zero, the orbit is circular, and the semimajor axis is the same as the radius. But those aren't the best terms.

[johnnydeep]

I'd stick with my terminology. "Reduced eccentricity" as opposed to "more circular", and "reduced semi-major axis" as opposed to "smaller diameter".

Simple Keplerian two-body orbits are pretty easy to understand. But we are dealing here with a three-body problem, and that gets complicated fast. There was a transfer of (conserved) angular momentum between Juno and Ganymede during the flyby, resulting in Juno being in a lower (and therefore faster) orbit around Jupiter. Ganymede's orbit was also changed, of course. Not unlike the way that Earth moves a little bit when you jump off the ground.

Thanks. So, "more circular" does not always reflect "reduced eccentricity"?

Click to view full size image

It's just too colloquial for my taste. Orbital elements are well defined. There is no concept of "circular" or "diameter". Every orbit is elliptical, where that ellipse is defined by an eccentricity and a semimajor axis. It happens that if the eccentricity is zero, the orbit is circular, and the semimajor axis is the same as the radius. But those aren't the best terms.

Ok, but I guess I'll just have to be too colloquial regarding this particular terminology.

[Chris Peterson]

Thanks. So, "more circular" does not always reflect "reduced eccentricity"?

Click to view full size image

It's just too colloquial for my taste. Orbital elements are well defined. There is no concept of "circular" or "diameter". Every orbit is elliptical, where that ellipse is defined by an eccentricity and a semimajor axis. It happens that if the eccentricity is zero, the orbit is circular, and the semimajor axis is the same as the radius. But those aren't the best terms.

Ok, but I guess I'll just have to be too colloquial regarding this particular terminology.

It's clear what you mean, which is what's most important.