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Big, weird bulge on Ganymede
MarcF
post Mar 29 2015, 05:10 PM
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Hi Paul,
Great discovery, congratulations.

http://news.nationalgeographic.com/2015/03...an-ice-science/

"The size and location of Ganymede’s bulge, which appears to be made of thick ice, suggest that once upon a time, the moon’s icy shell rotated atop the rest of the moon, like an interplanetary Magic 8 Ball."
So this is a new indication that there is an ocean underneath the ice crust.
I did not find any details about the precise location of the bulge. Does it correspond to one of the mass anomalies detected by Galileo ?
Regards,
Marc.
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TheAnt
post Mar 30 2015, 12:04 PM
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I second that congratulation to a great find.

Good thinking Marc, 3 km thick seem quite enough to cause a gravitational 'anomaly' when the surface is thought to be rather smooth and round.

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DrShank
post Mar 30 2015, 05:10 PM
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QUOTE (TheAnt @ Mar 30 2015, 06:04 AM) *
I second that congratulation to a great find.

Good thinking Marc, 3 km thick seem quite enough to cause a gravitational 'anomaly' when the surface is thought to be rather smooth and round.



hi all,
the locations of the gravity anomalies are mostly to the north, where the ground tracks for those data are. so we basically miss them. the dome is located very close to 0N, 0W. subJovian . . . .
p


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MarcF
post Mar 30 2015, 07:46 PM
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Thanks a lot for the answer. Yes, the sub-Jovian point... it seems logical. I've checked in your great Atlas of the Galilean Moons and realized that this region was not much covered by Galileo at high resolution. Anyway there is also the dome of Zakar penepalimpsest not very far. Is this related ?
Regards
Marc.
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DrShank
post Mar 30 2015, 08:33 PM
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QUOTE (MarcF @ Mar 30 2015, 02:46 PM) *
Thanks a lot for the answer. Yes, the sub-Jovian point... it seems logical. I've checked in your great Atlas of the Galilean Moons and realized that this region was not much covered by Galileo at high resolution. Anyway there is also the dome of Zakar penepalimpsest not very far. Is this related ?
Regards
Marc.


likely not. zakar is clearly impact related. this has no geologic feature


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algorimancer
post Mar 31 2015, 03:36 PM
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Reminds me of my (independent) modeling of Europa about 25 years ago, while working on my undergraduate degree. I did things like modeling the ice thickness based upon presumed radioactive decay and tidal dissipation, and eventually began contemplating the large-scale dynamics of the ice shell. I envisioned the ice thickening at the poles and thinning at the equator due to the temperature differential, and a steady glacial-style flow from poles to equator (ice freezing from the bottom at the poles, and melting at the bottom at the equator, with flow driven by the pressure differential per the differing thickness), complicated by tidal tugging on the variably thick ice. I'd figured that this could all be modeled, leading to predictions of the fractures, as well as reverse prediction to derive the history of the surface from existing fractures. I lacked the fluid dynamics expertise to go much further than mental models, but went so far as to write-up a summary of my thoughts and send it to someone who'd just published an article on the topic in Icarus (possibly Larry Soderblom, if I'm recalling correctly), but never received a response. My career took me in other directions, but some years ago I felt validated when I saw the publication of an article modeling the flow of ice together with varying gravitational forcing to predict the cycloid ridges. The notion of the ice shell tipping 90 degrees to leave the thicker polar ice at the equator all at once also occurred to me (envisioning the thicker polar ice as existing in an unstable equilibrium considering the essentially frictionless interface with the ocean), but didn't seem supported by the evidence, so I'd assumed that feedback loops much be in place to make this a rather more gradual process.

At the time, my impression was that the consensus was that Callisto and Ganymede had ice shells so thick that this sort of flow would be inconsequential, and the notion that they might also have liquid interior oceans was very debatable. Now we're seriously discussing deep oceans everywhere from Ceres to Pluto. The world just keeps getting better and better smile.gif
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DrShank
post Mar 31 2015, 03:56 PM
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QUOTE (algorimancer @ Mar 31 2015, 09:36 AM) *
Reminds me of my (independent) modeling of Europa about 25 years ago, while working on my undergraduate degree. I did things like modeling the ice thickness based upon presumed radioactive decay and tidal dissipation, and eventually began contemplating the large-scale dynamics of the ice shell. I envisioned the ice thickening at the poles and thinning at the equator due to the temperature differential, and a steady glacial-style flow from poles to equator (ice freezing from the bottom at the poles, and melting at the bottom at the equator, with flow driven by the pressure differential per the differing thickness), complicated by tidal tugging on the variably thick ice. I'd figured that this could all be modeled, leading to predictions of the fractures, as well as reverse prediction to derive the history of the surface from existing fractures. I lacked the fluid dynamics expertise to go much further than mental models, but went so far as to write-up a summary of my thoughts and send it to someone who'd just published an article on the topic in Icarus (possibly Larry Soderblom, if I'm recalling correctly), but never received a response. My career took me in other directions, but some years ago I felt validated when I saw the publication of an article modeling the flow of ice together with varying gravitational forcing to predict the cycloid ridges. The notion of the ice shell tipping 90 degrees to leave the thicker polar ice at the equator all at once also occurred to me (envisioning the thicker polar ice as existing in an unstable equilibrium considering the essentially frictionless interface with the ocean), but didn't seem supported by the evidence, so I'd assumed that feedback loops much be in place to make this a rather more gradual process.

At the time, my impression was that the consensus was that Callisto and Ganymede had ice shells so thick that this sort of flow would be inconsequential, and the notion that they might also have liquid interior oceans was very debatable. Now we're seriously discussing deep oceans everywhere from Ceres to Pluto. The world just keeps getting better and better smile.gif


Ojakangas and Stevenson wrote it up in 1989. thats where we get the theory from. Europa definitely has strong evidence and now Ganymede. Im not convinced yet for Enceladus.
p


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TheAnt
post Mar 31 2015, 08:45 PM
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QUOTE (DrShank @ Mar 30 2015, 06:10 PM) *
hi all,
the locations of the gravity anomalies are mostly to the north, where the ground tracks for those data are. so we basically miss them. the dome is located very close to 0N, 0W. subJovian . . . .
p


Thank you DrShank for the correction. =) Enceladus: Et tu brutus. wink.gif


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