January 14, 2019 : A New Study Suggests That The Well-Known Moons, Ganymede, Europa, Titan And Enceladus May Not Be Geologically And Biologically Active

A new study led by Paul Byrne, a planetary geologist at North Carolina State University, and recently presented at the annual conference of the American Geophysical Union in Washington, reveals that the well-known moons Ganymede, Europa, Titan and Enceladus may not be geologically and biologically active due to the relatively high thickness and stiffness of their crust. Let's point out, however, that Enceladus which is a tiny moon of the Gas Giant Saturn appears geologically active today but is it enough to allow the presence of a water-based life ? Ganymede and Europa which are moons of Jupiter as well as Titan and Enceladus which orbit around Saturn are believed to harbor a subsurface ocean of liquid water beneath their icy crust. Thanks to the Galileo spacecraft which had evolved inside the system of Jupiter and thanks to the Cassini spacecraft which had studied the system of the Ringed Planet Saturn, we have gathered clues regarding the potential presence of liquid water beneath the crust of several moons of those Gas Giants.

The gravitational influence of the Gas Giants and of the other moons must play a key role in the dynamics of the internal activity of those moons studied by the team of Paul Byrne. The planets engender tidal forces like the tidal forces between the Earth and the Moon. There must be a lot of heat or energy inside those moons. During the previous century, we have sent robots, drones or submarines to explore the abyss of the ocean and we have realized that microorganisms or more complex animals can thrive in the hot springs or in the active volcanoes. Those hot springs shelter ecosystems that appear in a stable state thanks to chemosynthesis, in darkness, in the absence or quasi absence of solar radiations. As a result, can we find microbes or exotic organisms in the subsurface oceans that are believed to exist in the Outer Solar System ? The new work performed by Paul Byrne explores the dynamics of the outer crust of several moons and leads to the conclusion that those moons may not be active inside due to the relatively high stiffness and due to the relatively high thickness of the crust.

Paul Byrne pointed out :  We were wondering, what would it look like if you were in a submarine and you were able to fly over the surface of the ocean floor on [Jupiter's moon] Europa.  Planetologists have imagined hydrothermal vents and an exotic type of black smokers on other worlds or beneath the external crust of other worlds thanks to the heat generated inside the sphere which is likely to favor interactions between the minerals from the ocean floor and the potential microbes, organisms or exotic crawlies thriving in those particular environments. A continuous mix of molecules between the hot rocks, the minerals, the other chemicals and the organisms is believed to occur inside those presumed Ocean Moons like in the extreme environments of our oceans. Researchers want to know whether ecosystems are systematically found in liquid water, in hot springs or in hydrothermal vents. Paul Byrne advanced :  I was hoping we could characterize what the chain of volcanoes would look like, what the rift zones would look like and then we were like, 'Well, I don't think they're going to be there.' 

The theory or the conclusion of the team of Paul Byrne, advancing that the moons on which the researchers focus their attention are devoid of any significant internal activity, is based on calculations regarding the properties or the behavior of the crust for each world. The researchers studied the behavior of the rock. They tried to determine the level of force or energy necessary to break the rock in two configurations that are generally encountered on our planet. The first configuration is the configuration of normal faults and the second configuration is the configuration of thrust faults. Normal faults can be observed when rock is pulled apart and thrust faults can be observed when rock is pushed together. The development of thrust faults implies more force than normal faults. The likelihood of an active geology beneath the surface is lower if the level of force required to produce the fault is higher. The forces must be likely to break the crust or the rock and if they manage to break the crust or the rock, they will allow interactions between fresh rock and the presumed subsurface ocean of liquid water.

Paul Byrne and his collaborators performed some calculations on the strength of the rock for Ganymede, Europa, Titan and Enceladus. The researchers have to face many questions regarding the nature, the properties or the dynamics of the rocks on those worlds but they are in a position, however, to get a better idea upon the strength of the exotic crust or rock of those worlds. On Earth, geologists or specialists regularly perform some calculations on the strength of the rock in the mining industry. The calculations of the team of Paul Byrne are based on the thickness of the cold, solid rock layer which appears over a warmer, mushy layer that appears relatively soft and that can't break. We can compare it to cooking specialties like a Mars bar for instance. Paul Byrne argued :  Think of like a Milky Way bar or a Mars bar, it's where the chocolate and caramel touch.  He added :  That depth, you can treat that as the thickness of the brittle, rigid layer.  It appears obvious that a thicker crust or layer will be harder to break.

Then, the planetologists incorporated other parameters into the problem. Thus, they incorporated the parameter of the gravity of the planetary body at a set depth as well as the weight of water and ice over the rocky surface of each natural satellite. They noticed that when they incorporated a range of potential values for unknown inputs, the outcome of the calculations was in the same general range for each planetary body. Paul Byrne pointed out that those initial conclusions, revealed at the conference, tend to show that the rock on those particular moons is too strong to be regularly cracked by any environmental force which is likely too weak. The stability of the crust is apparently related to the sheer weight of the water and ice found over the layer of rock. Paul Byrne advanced :  When it actually comes to understanding how strong the rock is, it's pretty strong, and it's pretty strong because even though the gravity's pretty low there's a lot of water on top of it. 

In this research work, the calculated rock strength appeared different from one moon to the other moon but the outcome is not in line with the configuration of a dynamic subsurface environment beneath or inside the external crust of those moons. Therefore, those moons may not represent biospheres or may not produce a complex chemistry involving organics. Paul Byrne advanced :  For Europa, it seems really, really hard to make any fractures or faults, and then once you look at Titan and Ganymede, these numbers are stupid high, really, nothing's happening at all on those worlds.  Regarding the value for the rock strength on the bright and particularly small moon Enceladus, the figure is rather positive, however, since the ball of ice is remarkably small and spherical which implies that the weight of the water and ice over the rocky surface of Enceladus is limited. Moreover, the rocky core of the tiny moon is likely more porous than the core of the other moons studied by the team of researchers. If those pores tend to line up, they have the potential to carry water downward deep inside the natural satellite. Paul Byrne argued :  It's within the realms of plausibility that Enceladus might actually be wet and soggy all the way through. 

During the Cassini mission, planetologists have gathered the evidence that Enceladus is geologically active since they clearly identified plumes of seawater emanating from the Tiger Stripes in the south polar region of the icy moon. Those fountains of icy particles, water and organics imply that there are interactions between water and rock inside the tiny moon. The identification of organic compounds in those plumes has potential implications in terms of exobiology. Paul Byrne pointed out :  That's quite encouraging.  He added :  It's hard to explain that it's not rock and water touching.  Paul Byrne advanced that it is hard to imagine a lot of activity at the level of the seafloor of those worlds if the seafloor is too stiff or too strong to undergo breaking or cracks on a regular basis. On the Blue Planet, two major phenomena or processes covering the seafloor are soil washing off the continents and the remnants of organisms going down to decay. Those phenomena or processes may not take shape or develop on the presumed Ocean Worlds studied by the team of Paul Byrne.

The observations of those worlds from the probes don't reveal any scars related to major impact events which are likely to engender fractures in the crust or on the seafloor. The calculations of Paul Byrne suggest that the rock is too strong to allow those worlds to skrink like the planet Mercury or to engender volcanic chains or rift zones. Any submarine may unveil a boring environment in the presumed subsurface ocean of those worlds. Paul Byrne argued :  Basically, it becomes a list of things there won't be.  He added :  It'll look like a billiard ball, it'll be a weird smooth planet. It'll be a new type of rocky world in our solar system.  He pointed out however that they've not produced the final outcome and the study has not been published yet. Paul Byrne and his collaborators want to know how other scientists react to their calculations. The puzzle is still incomplete with a lot of unknown data or parameters. We'll have to explore more in the future in order to better understand those worlds.

Ideally, we would have to send seismometers to the presumed seafloor of those worlds. A seismometer like the seismometer of the InSight lander on the Red Planet would be great to gather new clues regarding the internal structure of those moons. Paul Byrne has a philosophical approach of his work involving rigorous calculations. He may be wrong and that would be better for scientists and the general public. He pointed out :  If we're wrong, fine, that's how science works, that's grand, we'll take it, we'll be happy about it.  He admits that he has not been enthusiastic about the conclusion of his team. He advanced :  It would be great if we found interesting stuff, because these worlds are cool and maybe there's life there.  He concluded :  But if we're right, it means we do need to reconsider these worlds as habitable destinations or destinations for exploring habitability.  Are there microorganisms in the plumes of water ice on Enceladus ? That's undoubtedly one of the major questions we're asking today !

The image above reveals the Earth, Ganymede and Europa, which are moons of the Gas Giant Jupiter, and Titan and Enceladus, which are moons of the Ringed Planet Saturn, at scale. The external crust of those moons may be too thick or too strong to allow the emergence or the development of an active geology or biology beneath their presumed icy crust. But the debate regarding the internal structure of moons in the Outer Solar System is far from being over. Credit for the original view of the Earth: DSCOVR, December 25, 2018. Source of the original view of Ganymede: Wikipedia. Source of the original view of Europa: Wikipedia. Source of the original view of Titan: Wikipedia. Credit for the original view of Enceladus: NASA/JPL-Caltech/Space Science Institute. Montage credit: Marc Lafferre, 2019.

- To get further information on that news, go to: https://www.space.com/42989-ocean-moons-could-be-geologically-dead-inside.html.


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