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Enceladus and Mimas: A Tale of Two Worlds
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Enceladus and Mimas: A Tale of Two Worlds Enceladus may be of the most unique and interesting places in the solar system. It is a small moon of Saturn, small enough that you could place it inside of the borders of the state of New Mexico. It has a diameter of about 310 miles. But it has unique characteristics that make it stand out, not just among the moons of Saturn, but even among all the moons of the solar system. The fact that it is unique in the Saturn system is the basis of today’s story. The first thing tends notice about Enceladus is the relative lack of impact cratering. There are some, but large areas of the surface are free of evidence of impact events, indicting a young surface. The enormous surface cracks called “tiger stripes” are also very prominent. Perhaps the most striking feature of the small world is its surface albedo, the amount of light it reflects back into space. Enceladus has a surface albedo of nearly 100%. In other words, nearly all the light it receives from the sun is reflected back into space. All of these characteristics are related. The albedo of Enceladus is due the fact that its surface is water ice. But even more interesting is what lies that crust of ice. At the core of Enceladus, there is likely a small core of rock, or at least silicate materials. Enceladus orbits Saturn at an average distance of 238,040 miles, roughly the distance as our moon is from the Earth (238,900 miles). But keep in mind that Saturn is ninety- times more massive than the Earth. The orbital eccentricity of Enceladus, the degree which its orbit varies away from circular towards elliptical, is 0.0047. That is a pretty round orbit by solar system standards, but it still brings the little world a little over hundred miles closer Saturn at closest approach or the distance further at its most distant. This variance creates a huge amount of tidal friction, heating the solid core at the center of the little moon. As it orbits Saturn, it is constantly stretched and crushed, thus generating heat in the interior. The interesting consequence of this enormous tidal heating is that between the surface crust of ice and the rocky interior, there appears be a global ocean of briny liquid water. In fact, even factoring in the small size of Enceladus, there may be as much water in the interior of this small moon as there is in all the oceans of Earth. It is the presence of this vast reservoir of liquid water that explains the lack of craters on the surface and those enormous cracks. The tiger stripe cracks on the surface of Enceladus are the conduits for the Saturn system’s answer to Yellowstone Park. Through these enormous surface cracks, water from the interior constantly erupts toward the surface/ Some of this erupting water rains back upon Enceladus, constantly resurfacing the small world. The remainder escapes and constantly replenishes Saturn’s E-Ring. The Cassini spacecraft flew through the water plumes of Enceladus on 2 occasions during its mission. The spacecraft was able to analyze the content of the plumes, which demonstrated conclusively that they are water, infused with various salts including Sodium Chloride (NaCl), Sodium Bicarbonate (NaHCo3) and traces of other salts. But more interesting was the finding that the plumes also contained about 1.5% methane as well as ammonia. Methane is a possible indication that something could be alive in the underground ocean of Enceladus. Methane on Earth is almost entirely created by biological process. There are other chemical processes that can create methane, so those must be eliminated before the biology conclusion can be considered as primary. We will consider the implications of the presence of ammonia later. When the Voyagers flew past Saturn in 1.9.8.0 and 1.9.8.1 respectively, we had tantalizing images of Enceladus that showed us the young surface, an indication of some yet mysterious recently active processes that were resurfacing the little moon. It wasn’t the Cassini mission that we were able determine that not was Enceladus geologically active in its recent history, it remains an active and dynamic world worthy of future exploration. But the dynamic nature of Enceladus opens the door to another mystery, closer in to Saturn itself. Closer in to Saturn there is another small moon named Mimas. It has an equatorial diameter of about 8 miles, less than half the size of Enceladus but it has a similar composition and surface reflectivity at just over 96%. Mimas orbits Saturn at a distance of 1,289 miles, a little less than half orbital distance of Enceladus. It’s orbital eccentricity is substantially higher than that of Enceladus at 0.0196. Curiously, Mimas has a battered surface, the most striking surface feature being Herschel Crater, a huge impact scar 81 miles across that gives the moon an appearance which reminds many viewers of the Star Wars “Death Star.” We live in a golden age of solar system exploration A generation ago, worlds like Enceladus and Mimas were points of light in the best telescopes we had on Earth. But the 2 Voyagers in the 80’s gave us the first glimpses of these worlds up close, then Cassini provided us the opportunity take an even closer and longer . of the greatest rewards of planetary astronomy in recent years has been the opportunity lift the veil on these worlds and pursuing the mysteries they present. It seems that nothing is ever quite what you expect when we have ventured into the solar system with our robotic emissaries, and every answer leads to dozens of new questions. Mimas and Enceladus present a case in point. Here we have small worlds in orbit around Saturn. The closer in of the is frozen and battered. It has an ancient, cratered surface and is almost exactly the model we would expect for the icy, close in moons of Saturn. Yet just a small distance further , we have another small, icy world that is alive with cryovolcanism and just maybe, another opportunity find life in the solar system. Mimas is closer in and has a significantly greater orbital eccentricity. Tidal friction in the interior makes it a much better candidate for dynamic geologic activity. Yet it is Enceladus, not Mimas, that is the dynamic, active world. Why? Even after a decade and a half of exploration in the Saturn system, data from Cassini spacecraft hasn’t provided us with entirely satisfactory answers. We have some suspicions, based upon Cassini’s analysis of the geyser plumes of Enceladus. As we noted above, of the things Cassini found in the plumes was NH3, or as we it on Earth, ammonia. Most of us are familar with ammonia for its practical uses here on Earth, we use it for lots of household cleaning and it has an important role in helping transport us in our automobiles in cold weather months, as it goes by another called, “anti-freeze.” It may be possible that the presence of ammonia in the underground ocean of Enceladus is just sufficient keep the briny waters just enough remain liquid, while a dearth of ammonia on Mimas allowed the little moon freeze solid. But I find this hypothesis unsatisfactory. It seems unlikely to that the availability of ammonia in the vicinity of Enceladus early in the body’s history would have been greater than for Mimas. So another possibility exists, that has been suggested by evidence from Cassini’s observations. The interior of Mimas may not be frozen. It too, may contain an ocean of liquid water in its interior. This is suggested by observations of the moon’s orbital libration patterns, which suggests the possibility of a liquid interior. The fact that the impact event that created the Herschel Crater didn’t shatter the moon entirely also suggests that an interior ocean may have allowed the moon sufficient “give” at the surface at the time of collision to keep it intact. But then the question arises, why do we see the cryovolcanism on Enceladus, but not on Mimas? At this point, we not run of answers, we get pretty short on reasonable conjecture. We still have a lot learn about our solar system, and the way gain answers is continue venture outward with our robotic explorers and perhaps, eventually, with humans as well. There are worse ways spend our and we do so with stunning regularity. |
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