Gravity measurements confirm subsurface ocean on Enceladus

Apr 03, 2014 by Kimm Fesenmaier
Gravity measurements confirm subsurface ocean on Enceladus
This cartoon depicts the possible interior of Enceladus based on Cassini's gravity investigation. At high southern latitudes, a regional water ocean is shown sandwiched between an icy outer shell and a low density, rocky core. Cassini Imaging Science Subsystem images were used to depict the surface geology and the plumes. Credit: NASA/JPL-Caltech

In 2005, NASA's Cassini spacecraft sent pictures back to Earth depicting an icy Saturnian moon spewing water vapor and ice from fractures, known as "tiger stripes," in its frozen surface. It was big news that tiny Enceladus—a mere 500 kilometers in diameter—was such an active place. Since then, scientists have hypothesized that a large reservoir of water lies beneath that icy surface, possibly fueling the plumes. Now, using gravity measurements collected by Cassini, scientists have confirmed that Enceladus does in fact harbor a large subsurface ocean near its south pole, beneath those tiger stripes.

"For the first time, we have used a geophysical method to determine the of Enceladus, and the data suggest that indeed there is a large, possibly regional ocean about 50 kilometers below the surface of the south pole," says David Stevenson, the Marvin L. Goldberger Professor of Planetary Science at Caltech and an expert in studies of the interior of . "This then provides one possible story to explain why water is gushing out of these fractures we see at the south pole."

Stevenson is one of the authors on a paper that describes the finding in the current issue of the journal Science. Luciano Iess of Sapienza University of Rome is the paper's lead author.

During three flybys of Enceladus, between April 2010 and May 2012, the scientists collected extremely of Cassini's trajectory by tracking the spacecraft's microwave carrier signal with NASA's Deep Space Network. The gravitational tug of a planetary body, such as Enceladus, alters a spacecraft's flight path ever so slightly. By measuring the effect of such deflections on the frequency of Cassini's signal as the orbiter traveled past Enceladus, the scientists were able to learn about the moon's gravitational field. This, in turn, revealed details about the distribution of mass within the moon.

"This is really the only way to learn about internal structure from remote sensing," Stevenson says. In fact, more precise measurements would require the placement of seismometers on Enceladus's surface—something that is certainly not going to happen anytime soon.

The key feature in the gravity data was a so-called negative mass anomaly at Enceladus's south pole. Put simply, such an anomaly exists when there is less mass in a particular location than would be expected in the case of a uniform spherical body. Since there is a known depression in the surface of Enceladus's south pole, the scientists expected to find a negative mass anomaly. However, the anomaly was quite a bit smaller than would be predicted by the depression alone.

"So, you say, 'Aha! This is compensated at depth,'" Stevenson says.

Such compensation for mass is commonly found on planetary bodies, including on Earth. In some cases, the absence of material at the surface is compensated at depth by the presence of denser material. In other cases, the presence of extra material at the surface is compensated by the existence of less dense material at depth. In fact, when the first were made in India, people were struck by the fact that Mount Everest did not seem to produce much of an effect. Today we know that, like most mountains on Earth, Mount Everest is compensated by a low-density root that extends many tens of kilometers below the surface. In other words, the material protruding above the surface is compensated by a reduction of density at depth.

In the case of Enceladus, the opposite is true. The absence of material at the surface is compensated at depth by the presence of material that is denser than ice. "The only sensible candidate for that material is water," Stevenson says. "So if I have this depression at the , and I have beneath the surface 50 kilometers down a layer of water or an ocean, that layer of water at depth is a positive mass anomaly. Together the two anomalies account for our measurements."

Although no one can say for certain whether the subsurface ocean supplies the water that has been seen spraying out of the tiger stripes on Enceladus's surface, the scientists say that it is possible. The suspicion is that the fractures—in some way that is not yet fully understood—connect down to a part of the moon that is being tidally heated by the globe's repeated flexing as it traces its eccentric orbit. "Presumably the tidal heating is also replenishing the ocean," Stevenson says, "so it is possible that some of that water is making its way up through the tiger stripes."

Explore further: Researchers use NASA and other data to look into the heart of a solar storm

More information: The paper in Science is titled "The Gravity Field and Interior Structure of Enceladus." www.sciencemag.org/content/344/6179/78

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Torbjorn_Larsson_OM
4.3 / 5 (6) Apr 03, 2014
"there is a large, possibly regional ocean".

Interestingly there are two global surface features, fractures and pit chains from infallen fractures, that point to a global ocean. Especially the last features reported recently, as they predict a surface ice layer rotating differently than the core (like Titan). And when they measures the tidal dissipation of Saturn, it is enough to heat a global ocean, if wobble of Enceladus is included.

No doubt the south pole is active. But perhaps Enceladus is a lot more like Europa than that activity suggests.
Surly
4.7 / 5 (3) Apr 03, 2014
Out of curiosity, could someone explain why liquid water is the only sensible candidate for the denser-than-ice material?
Captain Stumpy
4.2 / 5 (6) Apr 04, 2014
Out of curiosity, could someone explain why liquid water is the only sensible candidate for the denser-than-ice material?
@Surly
from what I can tell, the statement comes from a few points:
1- the comment in the article
This then provides one possible story to explain why water is gushing out of these fractures we see at the south pole
for water to gush out of the fractures, it has to be present in enough volume to do so
2- from the paper in sciencemag
...vapor and ice emerge as discrete jets. Concurrent observations of sodium and potassium salts in the plume, and the measured temperatures within the fractures, strongly argue for the presence of liquid water in the subsurface source region. The plume exhibits a time variability well correlated to the predicted tidal stresses of the body
shows there is a strong possibility as well

does that help answer your question?
the paper might be paywalled. there is quite a bit to it.
cantdrive85
1 / 5 (7) Apr 04, 2014
This electric current;
http://solarsyste...ID=37102
flowing through the surface is likely responsible for the "jets" and the "hot" tiger stripes on this ice cube;
https://solarsyst...ipes.jpg
Captain Stumpy
4.2 / 5 (6) Apr 04, 2014
that point to a global ocean
@Torbjorn_Larsson_OM
can you view the whole paper?
Although the gravity data cannot rule out a global ocean, a regional sea is consistent with the gravity, topography, and high local heat fluxes and does not suffer from the thermal problems that a global ocean encounters. A global ocean would yield larger and potentially detectable longitudinal librations than are predicted for a solid body. The gravity coefficients and inferred MOI of Enceladus are not consistent with a forced 4:1 secondary libration at the 2σ level
this was the last paragraph. I don't know if it is paywalled or not. thought I would share for you. interesting paper.
dan42day
3 / 5 (6) Apr 04, 2014
In fact, more precise measurements would require the placement of seismometers on Enceladus's surface—something that is certainly not going to happen anytime soon.


This was the most interesting statement in the entire article for me. We as a civilization spend way too much on social programs trying to negate the effects of the natural selection that got us to where we are, and not nearly enough on extending our reach into the solar system and beyond.
alfie_null
3.7 / 5 (3) Apr 04, 2014
We as a civilization spend way too much on social programs trying to negate the effects of the natural selection that got us to where we are, and not nearly enough on extending our reach into the solar system and beyond.

As a civilization, we hardly have a consensus on how we spend our money. However, as individual nations - well, why don't you try to find some correlation between nations that spend little on social programs and nations that funded the Cassini mission? Something to support your contention?

As you're being so outspoken, I'm curious as to your views on eugenics? How far would you go? What traits would you wisely select for? Future generations of people just like you?
Zwentoo
Apr 04, 2014
This comment has been removed by a moderator.
GSwift7
5 / 5 (3) Apr 04, 2014
could someone explain why liquid water is the only sensible candidate for the denser-than-ice material?


The article is worded a little funny, but what they were trying to say is that they extected to see a drop in gravity in that region due to the depression at the surface. When they measured it, the drop in gravity wasn't quite as much as it should have been, but not too much more either. Solid rock would be too dense to account for the difference though, so they're looking for something less dense than rock, but more dense than ice. Since we know the surface ice is water, liquid water is a sensible choice to account for the difference in gravity. This is especially sensible when you consider that the Antarctic ice sheet has liquid water lakes beneath it, and the ice sheets on Enceladus are much thicker.
cantdrive85
1 / 5 (6) Apr 05, 2014
Similar electric currents are likely what is causing "volcanic" activity on Jupiter's moon Io.

https://solarsyst...ckground

Being the hot spots on Io aren't in the "right place"

http://phys.org/n...tml#nRlv

The notion these electric currents are actually doing something should not be a stretch.

There are even photos of the electric discharge machining taking place, carving the cliff edges of vast mesas just as is postulated by EUT.

https://solarsyst...2550.jpg

The upper left there are two bright arc discharges on the edges of the mesas, said geological processes in action. The blacked out areas are likely caused by intensive arc discharges shinning too hot for the cameras to record.

"The apparent filamentary penumbra on Io may be the first direct verification of the plasma gun mechanism at work in the solar system." Anthony Peratt

barakn
5 / 5 (3) Apr 05, 2014
Cantdrive85, I realize you have a poor memory, so I'd like to help. Here's a link to discussions where the "bright arc discharges" were robustly debunked:
http://phys.org/n...rth.html and http://phys.org/n...ars.html .
cantdrive85
1 / 5 (6) Apr 05, 2014
Debunked only in your mind...The idea that these finding may be debunked on a comment thread is laughable, especially when the mission scientists acknowledge the discrepancies.
barakn
5 / 5 (3) Apr 05, 2014
Actually it was quite easy to show that you and Hannes were ignorant of the most basic of facts regarding the data collection, such as what wavelength of light each image was taken in. When Hannes claimed that lava doesn't show up saturated in images, debunking is as simple as posting links to images with saturated lava. I think the real problem with both you and Hannes is that you are just a couple of cop-and-paste trolls. You are ignorant of the fundamentals of physics and math, and are therefore oblivious to the fact that your arguments have been debunked. You are blind to that moment when the veil has been lifted and everyone else can see the naked fallacy behind something you've said.
cantdrive85
1 / 5 (5) Apr 06, 2014
Here is Io's plasma gun;
https://www.youtu...7w801UYA

Here is a laboratory plasma gun;

http://focusfusio...eleased/

The identical morphology along with other evidence led to Peratt saying;

"The apparent filamentary penumbra on Io may be the first direct verification of the plasma gun mechanism at work in the solar system."
barakn
5 / 5 (3) Apr 06, 2014
The fact that they don't look anything like each other apparently doesn't faze you. But thanks for posting that, it's a perfect example of how you and Hannes handle that moment when you should have come up with a clever counter-argument. Instead you try to change the subject.