Enceladus: Adressing some questions.
Published on Mar 11, 2006 at 2:13 pm.
1 Comment.
Filed under planets.
Enceladus. Mmm. Mexican food. Oh, wait. Back on topic. This is the usual response to thinking about this moon of Saturn. I teach evening classes a lot, and we are often covering the Saturn system at 8pm or so, with hardly anyone in the class (myself included) having had supper yet.Â
On a more serious, note, though, Seeking Solace had asked for clarification on a news story that she had heard about Enceladus. I got similar requests from students, and other non-astronomy faculty. I missed the actual TV report (that teaching night classes, again), but from all the questions, they must have pretty well botched it. What usually happens is that the report is filled with partial truths and incomplete facts. Part of this is ignorance of the reporters, and part is producers cutting material in order to have more time for commercials. Sadly, a major part is also the difficulty that research scientists often have communicating their subject to the general public. Hmm. That might be a good blog topic for me to tackle later on. In a field so dependent upon public money, we really should, everyone of us, become experts at communicating to the public.
I hadn’t planned on doing a blog about Enceladus until all the confusion began. So, here goes.
Enceladus is one of the medium sized moons of Saturn. It was discovered by William Herschel in 1789. While medium sized in the Saturnian system, Enceladus quite a bit smaller than our Moon, Titan (Saturn’s largest moon), the four major moons of Jupiter, or many other bodies in the Solar System. Its size, coupled with a fairly close orbit to Saturn, makes Enceladus difficult to study from Earth. We really knew very little about it until Voyager I passed Saturn in 1980 and Voyager II in 1981. These spacecraft showed a rather strange body, portions of which appeared to have grooves or ridges, and other portions seemed to be heavily cratered. The Voyagers also found a very, until then unknown, ring around Saturn that seemed to be associated with Enceladus. In keeping with convention, this ring being the fifth discovered around Saturn was named the E Ring (the previous four rings being named A, B, C, and D Rings). Furthermore, Enceladus is one of the brightest bodies in the Solar System, reflecting most of the sunlight that falls on it. Speculation was that perhaps some sort of ice volcanism may be the source of these strange observations.Â
There the story sat for a long time. Saturn is VERY far from Earth, so observations from here simply could not answer the questions. Now, the Cassini spacecraft has been studying Saturn up close for somewhat over a year now. Several flybys of the moon Enceladus were planned for the mission, and these flybys have not disappointed us. Finally, we caught what appears to be an ice geyser in progress. This spews water out at high velocity, perhaps coating the moon with a fine “snowâ€, and even sending ice particles upwards in excess of Enceladus’ escape velocity, thus providing the material for the E Ring. The problem, though, is now to explain how an ice geyser can exist on Enceladus.Â
The way that a geyser works is for liquid under pressure to suddenly have the pressure released, causing the liquid to boil. The pressure of the gas from the boiling liquid then ejects gas and boiling liquid through whatever orifice allowed the pressure to be released. You get a geyser. Now, to a little physics. The boiling and freezing temperatures of a liquid, such as water, are not set in stone. We are taught that water boils at 100C and freezes at 0C. That is only true, though, for pure water at standard atmospheric pressure. The higher the pressure, the higher the boiling point, and conversely lower pressure results in a lower boiling point. You can raise the boiling point in a pot of boiling water by keeping the lid on the pot. In your car’s radiator, the boiling point is raised by having the coolant system under pressure. If you open the radiator while hot, you suddenly reduce pressure and that can make the radiator suddenly boil over (as a geyser of hot steam and boiling water all over you, if you are not careful). Lowering pressure lowers the boiling temperature. So, for example, in high altitude cities, water boils at a lower temperature. Thus, cooking directions are different at high altitudes than for low altitudes. As the pressure continues to drop, so does the boiling temperature. At low enough pressure, the boiling point of water is below normal human body temperature. Hence, your blood would boil at sufficiently high altitudes. At very low pressure, the boiling point of water is actually as low as the freezing temperature. For this and lower pressures, water is never a liquid — it is only a solid or a gas. Carbon dioxide exists in this way at normal Earth atmospheric pressure — it is only a solid (dry ice) or a gas. To be liquid, carbon dioxide must be placed under higher pressure. Mars has this problem with water. On the surface of Mars, the atmospheric pressure is so low that water is only a solid or a gas, not liquid. Enceladus has no atmospheric pressure, so water on or near its surface can exist only as a solid or a gas. If liquid water were to work its way up to near the surface, then the pressure would be low enough for it to boil. The pressure from the boiling water might then break through the surface, and you get a geyser. The temperature is so cold there, though, that the gas would almost instantly crystallize into tiny ice crystals.
So, that is how you get an ice geyser. But it does not answer a deeper question. How do you get the liquid water in the first place? Saturn and its moons get nearly 1% of the sunlight that Earth gets. This means that it is COLD there. The surface temperature is likely colder than -180C. Water can’t be liquid below 0C can it? Well, pure water at atmospheric pressure can not. However, water expands when it freezes. So, kept under pressure, you can lower the freezing point. One of the reasons that ice skates work is that the blades concentrate your weight on a small area, creating high pressure, melting a tiny layer of ice. So, the skates really are sliding on a layer of water that is lubricating the interface between the ice and the skates. However, at low enough temperatures, this would not work, and the skates would drag on the ice. Could pressure be keeping water liquid on Enceladus? No, not by itself. Without some source of heat other than the Sun, Enceladus would be far too cold.
How else can you make water liquid at low temperatures? Well, you can add impurities. You do this with the coolant of your car. Add antifreeze, and the freezing temperature of the water is lowered (also the boiling temperature is raised, but that is a different story). What could act as antifreeze on Enceladus, though? There has been speculation that ammonia might do the job. Ammonia is known to be fairly common in the outer Solar System. The clouds on Saturn, for example, are ammonia clouds. The bright zones on Jupiter are ammonia clouds. In sufficient concentrations, ammonia can act as an antifreeze, lowering the freezing temperature. However, ammonia can only lower the freezing temperature to at best to a bit over -100C. This is still too warm of a temperature for Enceladus, without some other source of heat. Furthermore, a boiling mixture of ammonia and water would leave ammonia gas. Analysis of the geyser plumes shows no ammonia so far, within detection limits of the Cassini spacecraft. Thus, ammonia must be in such low concentration that at best it could only lower the freezing temperature by a few degrees. So, the water must be at only slightly below 0C temperature. This can only be accomplished by some heat source. So, where does Enceladus get its heat?
Following the Voyager images of Enceladus, scientists began to suspect that perhaps tidal heating might be the heat source for the moon. Many astronomers and planetary scientists were shocked to find very active volcanoes on Io, one of Jupiter’s moons. Io is far too small to be geologically active, they thought. However, Io turns out to be the most volcanically active body that we have observed in the Solar System. It achieves this activity by tidal heating. Orbital resonances between Io and the other major Jovian moons (Europa, Ganymende, and Callisto) regularly stretch and deform Io. This stretching, then relaxing, then stretching, then relaxing has the effect of kneading the interior of the moon. This produces the heat energy to keep Io’s interior molten. Thus, Io is volcanically active. Could something like this be at work with Enceladus? Well, the same mechanism can not be at work. Saturn really only has one moon of sufficient mass to do the job: Titan. However, Titan it too far away from Enceladus, and the two moons do not have an orbital resonance. So, interactions with the other moons can’t be a source of heating. However, Enceladus has an interesting relationship between its orbital period and its rotational period. Enceladus has a 1:4 spin-orbit coupling. That means that the moon rotates once per four orbits. Spin-orbit coupling is nothing new. Our own moon has a 1:1 spin-orbit coupling, so that it rotates one per orbit, thus keeping the same side towards Earth. The planet Mercury has a 3:2 spin-orbit coupling, meaning that it rotates exactly 3 times per 2 orbits. Thus, every orbit, it appears to have the opposite side towards the Sun. Enceladus’ spin-orbit coupling, together with its fairly close orbit to Saturn, would result in varying tidal stresses. These stresses would heat the interior of the moon, just as similar stresses heat Io’s interior. However, computations show that the tidal stresses are simply not enough to warm the interior of the moon to the point that it would melt the ice.
So, what else can heat the moon? The only way to determine the mass of a body in space is to see how gravity affects its motion, or how its gravity affects the motion of something else. So, we have until now just guessed at the composition of Enceladus. We know that the moons of the outer solar system are icy in nature, so we assumed a standard ratio of ice to rock for Enceladus. However, the Cassini spacecraft’s orbit is slightly altered passing by Enceladus. Analysis of Enceladus’ effect on the spacecraft allows for the determination of the moon’s mass. We find that Enceladus is somewhat denser than we though. That means that a larger portion of the moon is rock than we had assumed. More rock also means more radioactive elements. We know that energy released through radioactive decay is a significant source of heat in bodies such as Earth and the other rocky planets, asteroids, etc. So, could the increased rock content of Enceladus result in more radioactive heating. Yes! However, calculations seem to indicate that it is not enough radioactive heating to account for a temperature sufficient to melt ice.
A further mystery is that only the southern portion of Enceladus seems to be warm. These mechanisms would heat the moon more or less uniformly. Why would only the southern portion of the moon be warm? One speculation is that perhaps the ammonia antifreeze model might actually be at work in deeper layers of the moon, somehow carrying heat from the deeper interior to nearer the surface, but only in one area of the moon. Again, we look to Mars to see hot spot plumes that create volcanic regions on the planet, leaving other portions of the planet almost completely untouched by tectonic activity. Could a deep ammonia rich mantle be carrying heat to the south polar region?
Well, the answer might be a combination of all these factors. Radioactive heating might add to the heat produced by tidal stresses. Asymmetric transport of heat might concentrate it in the southern polar regions. There is speculation that this might provide just enough thermal energy to keep water liquid several kilometers below the surface of the moon without adding ammonia to the water. This water, under pressure, works its way to the surface, and erupts as ice geysers. The only real catch seems to be that there is enough heat, perhaps, to keep liquid water liquid, but not enough to melt ice into liquid. So, we still have a mystery as to how it got to be liquid in the first place.Â
So, there you have the mystery of Enceladus. Hopefully this answers some of the questions that people may have. Of course, it may raise others …
-Astroprof
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Astroprof’s Page » Diving past Enceladus on March 11, 2008 at 3:00 pm: 1
[…] But, the Cassini spacecraft has shown that Enceladus is even more baffling than had been imagined. Enceladus appears to have plumes of water and ice spewing up form its south pole. I wrote about those plumes and some of the ideas surrounding them two years ago. Later studies have ruled out some ideas, and the plumes are still not completely understood. One of the problems is that these plumes seem to result from high velocity geysers. That would require liquid water under pressure. The problem is that Enceladus should be too cold for liquid water. Early ideas suggested that a combination of factors could account for this. Radioactive heating could be providing some heat to Enceladus’ interior. Tidal stresses between Enceladus, Saturn, and Saturn’s other moons might also heat Enceladus. And, if the water had salt or ammonia in it, then the melting temperature would be lower, and these heat sources may be enough to melt the water. Unfortunately, spectral analysis of the plumes does not see enough ammonia or mineral to account for much of a decrease in the freezing temperature of the water. That may mean that there is a pool of nearly pure water beneath the surface ice of Enceladus’ south pole. […]