http://phys.org/news/2016-10-saturn-moon-dione-harbors-subsurface.html

A subsurface ocean lies deep within Saturn’s moon Dione, according to new data from the Cassini mission to Saturn. Two other moons of Saturn, Titan and Enceladus, are already known to hide global oceans beneath their icy crusts, but a new study suggests an ocean exists on Dione as well.

In this study, researchers of the Royal Observatory of Belgium show gravity data from recent Cassini flybys can be explained if Dione’s crust floats on an ocean located 100 kilometers below the surface. The ocean is several tens of kilometers deep and surrounds a large rocky core. Seen from within, Dione is very similar to its smaller but more famous neighbor Enceladus, whose south polar region spurts huge jets of water vapor into space. Dione seems to be quiet now, but its broken surface bears witness of a more tumultuous past. The study is published online this week in Geophysical Research Letters.
The authors modeled the icy shells of Enceladus and Dione as global icebergs immersed in water, where each surface ice peak is supported by a large underwater keel. Scientists have used this approach in the past but previous results have predicted a very thick crust for Enceladus and no ocean at all for Dione. “As an additional principle, we assumed that the icy crust can stand only the minimum amount of tension or compression necessary to maintain surface landforms”, said Mikael Beuthe, lead author of the new study. “More stress would break the crust down to pieces.”

dv said:

http://phys.org/news/2016-10-saturn-moon-dione-harbors-subsurface.html

A subsurface ocean lies deep within Saturn’s moon Dione, according to new data from the Cassini mission to Saturn. Two other moons of Saturn, Titan and Enceladus, are already known to hide global oceans beneath their icy crusts, but a new study suggests an ocean exists on Dione as well.

In this study, researchers of the Royal Observatory of Belgium show gravity data from recent Cassini flybys can be explained if Dione’s crust floats on an ocean located 100 kilometers below the surface. The ocean is several tens of kilometers deep and surrounds a large rocky core. Seen from within, Dione is very similar to its smaller but more famous neighbor Enceladus, whose south polar region spurts huge jets of water vapor into space. Dione seems to be quiet now, but its broken surface bears witness of a more tumultuous past. The study is published online this week in Geophysical Research Letters.
The authors modeled the icy shells of Enceladus and Dione as global icebergs immersed in water, where each surface ice peak is supported by a large underwater keel. Scientists have used this approach in the past but previous results have predicted a very thick crust for Enceladus and no ocean at all for Dione. “As an additional principle, we assumed that the icy crust can stand only the minimum amount of tension or compression necessary to maintain surface landforms”, said Mikael Beuthe, lead author of the new study. “More stress would break the crust down to pieces.”

As a person with a bias towards subsurface liquid water oceans, definitely on Pluto and Uranus, possibly on Charon, I feel the need to take a closer look at this claim.

Dione’s diameter is 1122 km. Compare Enceladus 504 km, Pluto 2374 km, Charon 1212 km. The diameter of Dione could be in the right ballpark, Enceladus is a special case because it gets tidal heating from Saturn, like Io gets tidal heating from Jupiter. Dione is about 1.5 times as far from the centre of Saturn as Enceladus, so experiences much less tidal heating, that’s about the same distance ratio as Europa to Io. The distance from Dione to Saturn is actually less than that of Io from Jupiter, so we can’t rule out tidal heating. The claim seems possible.

“In this study, researchers of the Royal Observatory of Belgium show gravity data from recent Cassini flybys can be explained if Dione’s crust floats on an ocean located 100 kilometers below the surface. The ocean is several tens of kilometers deep and surrounds a large rocky core. Seen from within, Dione is very similar to its smaller but more famous neighbor Enceladus, whose south polar region spurts huge jets of water vapor into space. Dione seems to be quiet now, but its broken surface bears witness of a more tumultuous past. The study is published online this week in Geophysical Research Letters.”

I didn’t see the article on the website of Geophysical Research Letters. Can you find it?
http://agupubs.onlinelibrary.wiley.com/agu/journal/10.1002/(ISSN)1944-8007/

mollwollfumble said:

dv said:

http://phys.org/news/2016-10-saturn-moon-dione-harbors-subsurface.html

A subsurface ocean lies deep within Saturn’s moon Dione, according to new data from the Cassini mission to Saturn. Two other moons of Saturn, Titan and Enceladus, are already known to hide global oceans beneath their icy crusts, but a new study suggests an ocean exists on Dione as well.

In this study, researchers of the Royal Observatory of Belgium show gravity data from recent Cassini flybys can be explained if Dione’s crust floats on an ocean located 100 kilometers below the surface. The ocean is several tens of kilometers deep and surrounds a large rocky core. Seen from within, Dione is very similar to its smaller but more famous neighbor Enceladus, whose south polar region spurts huge jets of water vapor into space. Dione seems to be quiet now, but its broken surface bears witness of a more tumultuous past. The study is published online this week in Geophysical Research Letters.
The authors modeled the icy shells of Enceladus and Dione as global icebergs immersed in water, where each surface ice peak is supported by a large underwater keel. Scientists have used this approach in the past but previous results have predicted a very thick crust for Enceladus and no ocean at all for Dione. “As an additional principle, we assumed that the icy crust can stand only the minimum amount of tension or compression necessary to maintain surface landforms”, said Mikael Beuthe, lead author of the new study. “More stress would break the crust down to pieces.”

As a person with a bias towards subsurface liquid water oceans, definitely on Pluto and Uranus, possibly on Charon, I feel the need to take a closer look at this claim.

Dione’s diameter is 1122 km. Compare Enceladus 504 km, Pluto 2374 km, Charon 1212 km. The diameter of Dione could be in the right ballpark, Enceladus is a special case because it gets tidal heating from Saturn, like Io gets tidal heating from Jupiter. Dione is about 1.5 times as far from the centre of Saturn as Enceladus, so experiences much less tidal heating, that’s about the same distance ratio as Europa to Io. The distance from Dione to Saturn is actually less than that of Io from Jupiter, so we can’t rule out tidal heating. The claim seems possible.

“In this study, researchers of the Royal Observatory of Belgium show gravity data from recent Cassini flybys can be explained if Dione’s crust floats on an ocean located 100 kilometers below the surface. The ocean is several tens of kilometers deep and surrounds a large rocky core. Seen from within, Dione is very similar to its smaller but more famous neighbor Enceladus, whose south polar region spurts huge jets of water vapor into space. Dione seems to be quiet now, but its broken surface bears witness of a more tumultuous past. The study is published online this week in Geophysical Research Letters.”

I didn’t see the article on the website of Geophysical Research Letters. Can you find it?
http://agupubs.onlinelibrary.wiley.com/agu/journal/10.1002/(ISSN)1944-8007/

I think I’ve found it.

Research Letter
“Enceladus’ and Dione’s floating ice shells supported by minimum stress isostasy”
Authors Mikael Beuthe, Attilio Rivoldini, Antony Trinh
Accepted manuscript online: 28 September 2016

Abstract

Enceladus’ gravity and shape have been explained in terms of a thick isostatic ice shell floating on a global ocean, in contradiction of the thin shell implied by librations. Here we propose a new isostatic model minimizing crustal deviatoric stress, and demonstrate that gravity and shape data predict a 38 ± 4km-thick ocean beneath a 23 ± 4km-thick shell agreeing with – but independent from – libration data. Isostatic and tidal stresses are comparable in magnitude. South polar crust is only 7 ± 4km thick, facilitating the opening of water conduits and enhancing tidal dissipation through stress concentration. Enceladus’ resonant companion, Dione, is in a similar state of minimum stress isostasy. Its gravity and shape can be explained in terms of a 99 ± 23km-thick isostatic shell overlying a 65 ± 30km-thick global ocean, thus providing the first clear evidence for a present-day ocean within Dione.

Hmm, with the ± 30km I’m not going to call the status of this subsurface ocean “certain”, but the results are good enough to call it “probable”.

> As a person with a bias towards subsurface liquid water oceans, definitely on Pluto and Uranus, possibly on Charon, I feel the need to take a closer look at this claim.

I’m having a rethink of Pluto. Pluto has water-ice on its surface, yes. Pluto has a subsurface ocean, yes. Pluto has a subsurface ocean of liquid water, that doesn’t automatically follow. It makes more sense if Pluto has a subsurface ocean of liquid nitrogen above a core made of solid ices.

mollwollfumble said:

> As a person with a bias towards subsurface liquid water oceans, definitely on Pluto and Uranus, possibly on Charon, I feel the need to take a closer look at this claim.

I’m having a rethink of Pluto. Pluto has water-ice on its surface, yes. Pluto has a subsurface ocean, yes. Pluto has a subsurface ocean of liquid water, that doesn’t automatically follow. It makes more sense if Pluto has a subsurface ocean of liquid nitrogen above a core made of solid ices.

Mmm, Pluto having a liquid water ocean would be like, say, Ceres having a liquid nitrogen ocean. How?

> As a person with a bias towards subsurface liquid water oceans, definitely on Pluto and Uranus, possibly on Charon, I feel the need to take a closer look at this claim.

I’m having a rethink of Pluto. Pluto has water-ice on its surface, yes. Pluto has a subsurface ocean, yes. Pluto has a subsurface ocean of liquid water, that doesn’t automatically follow. It makes more sense if Pluto has a subsurface ocean of liquid nitrogen above a core made of solid ices.

Pluto’s surface temperature is 33 to 55 Kelvin.
Nitrogen melts at 63 Kelvin.
Nitrogen is a very common element and is plentiful on Pluto.
Temperature increases with depth at an indeterminate rate.
Whatever the rate of temperature increase with depth, Pluto ought to have a subsurface ocean of nitrogen at a quite shallow depth.
But Pluto is unlikely to have a subsurface ocean of liquid water because 273 Kelvin is very much greater than 63 Kelvin. Ditto for Charon.

Moving a bit closer to the Sun, Neptune’s moon Triton has an active geyser of nitrogen. Triton’s surface temperature is 34.5 Kelvin, as cold as Pluto. The web doesn’t say what the temperature range of Triton’s surface is. This strongly suggests that Triton also has a subsurface ocean of liquid nitrogen.

Saturn’s moon Tethys lies between Enceladus and Dione. It’s only 5.5% smaller than Dione. If Dione and Enceladus both have subsurface oceans of liquid water then Tethys should, too.

dv said:

mollwollfumble said:

> As a person with a bias towards subsurface liquid water oceans, definitely on Pluto and Uranus, possibly on Charon, I feel the need to take a closer look at this claim.

I’m having a rethink of Pluto. Pluto has water-ice on its surface, yes. Pluto has a subsurface ocean, yes. Pluto has a subsurface ocean of liquid water, that doesn’t automatically follow. It makes more sense if Pluto has a subsurface ocean of liquid nitrogen above a core made of solid ices.

Mmm, Pluto having a liquid water ocean would be like, say, Ceres having a liquid nitrogen ocean. How?

One of the great unknowns of planetary science is:
“What is the core temperature of a solid body?”

Last time I checked, there were still debates over what the core temperature of the Earth is, and for Earth the location of the boundary between the outer and inner core ought to be a dead give-away.

For gaseous bodies, the situation isn’t too bad, because adiabatic equations approximate the rate of temperature increase with depth, and because we can directly measure the amount of internal heat radiated from the surface.

For very large bodies, again the situation isn’t too bad because the gravitational energy retained during formation sl;owly leaks out and forms a large fraction of the total heat within the body.

When there is heating from tidal influences (eg. Io, Enceladus and to a lesser extent Dione) the internal heat generated by the tides adds to the object’s core temperature and this can be calculated.

Beyond those influences, though, we have solid objects such as Mars, Mercury, our Moon, Ceres, Pluto, where the primary source of internal heat is radioactive elements. This heat can not be estimated in any reliable way and so the core temperature cannot be reliably determined. This allows the possibility of core temperatures for bodies such as Mercury, Mars and the Moon to be high enough for the core to be made of liquid rock. And it allows the possibility of the cores of Ceres and Pluto and large moons of Jupiter and Saturn having a temperature high enough to generate a subsurface ocean of liquid water.