Cassini's Discoveries on Enceladus, Phoebe, Hyperion, and Other Saturn Moons
In 1610, fresh off his recent observations of Jupiter, Galileo set his telescope upon Saturn and discovers that it had rings. But to him they had to be something else like moons that orbited closely to it. Like many great scientists of antiquity he made a mistake but it was rectified in 1656 when Christiaan Huygens not only discovers Titan but also figures out the ring nature of Saturn (Douthitt). Despite this mistake Galileo was right about there being satellites on Saturn. And oh how wonderful they are...
Enceladus not only has the highest albedo of the solar system but it also has a rather interesting property that is truly unique: it emits huge plumes. And as it turns out those plumes may be exciting for the possibility of life on Enceladus. In June of 2009 German and UK scientists found that table salt could be up to 2 percent of the material that are in the plumes, nearly the same concentration as that found on Earth. This is encouraging because salt in water usually means that erosion is occurring and hence a good source of minerals. And in July 2009 the mass spectrometer on Cassini found ammonia in the debris. This means that liquid water could exist despite the -136 degrees F conditions it would be under. And later observations showed a ph level between 11 and 12, further indicating the salty and acidic nature of Enceladus. Other chemical signatures detected include propane, methane, and formaldehyde, with sodium carbonate levels comparable to those on Earth's Mono Lake (Grant 12, Johnson "Enceladus", Douthitt 56, Betz "Curtains" 13).
The plumes that leave the moon near its south pole becomes plasmic in nature, or that it exits as a highly ionized gas, as it interacts with Saturn's magnetic field. Scientists can learn about plasma behavior and Saturn's magnetic field based on how the plasma acts after leaving the moon. Cassini's plasma spectrometer, magnetometer, magnetosphere imaging, and the radio and plasma science instruments were key in the finding that the plasma mix is made of particles from a few molecules to almost a thousandth of an inch. They also found that almost 90% of the electrons in the plasma tended to be near the larger particles, causing the larger particles to be negative and the smaller one positive. This is the opposite of normal plasma behavior (JPL "Enceladus").
So, what type of particles are the electrons clinging to? The plasma mix is mainly water vapor and dust and thus has different characteristics. After looking at the data scientists concluded that the water molecules mainly stuck together while dust between a nanometer and a micrometer held a majority of the electrons. Not in any other place in the solar system has this type of plasma interaction been recorded and it is sure to reveal many surprising properties in the field of plasma mechanics (Ibid).
This stream does fluctuate, for Enceldaus orbits Saturn in 33 hours. Because of the elliptical orbit, Enceladus goes through tidal forces, or gravitational pull, that heats up the subsurface water. In fact, as Enceladus gets closer to Saturn the fissures from which the water vapor escapes close up and as Enceladus gets further from Saturn the fissures open up. Infrared observations gathered by the Visual and Infrared Mapping Spectrometer from 2005 to 2012 show that the plumes can increase in size by as much as 3 times their minimum and also escape at a faster velocity. Scientists suspect that the pull of gravity closes the fissures but that once the gravity is less the fissures open back up. This may also explain why the peak for emissions is 5 hours after the moon's perihelion with Saturn (Johnson "Enceladus", NASA "Cassini Spacecraft," Haynes "Saturn's").
After nearly a decade of observations, in mid-2014 scientists announced that 101 separate geysers had been located on Enceladus. They are scattered amongst the cracks at the south pole and correlate to hot spots on the moon, with higher temperatures corresponding to higher emissions. As it turns out, the friction the water vapor produces by leaving the fissure creates the heat that Cassini measured. Most significant, the size of the openings of the geysers were only 20-40 feet in size, too small to be a result of surface friction. They must have a source deep down to allow such small openings to dispel material, giving further evidence for a subsurface ocean (JPL "Cassini Spacecraft", Wall "101").
And do those jets impact Saturn's rings? You betcha. Recent observations and computer analysis from Colin Mitchell of the Space Science Institute in Boulder have shown that each geyser flow and its materials manage to escape the pull of the moon and leave behind a wake that eventually gets stretched out into the E ring. It wasn't easy to spot them, however. Certain lighting conditions were needed to get the material to reflect enough light to be captured on camera. In fact, the size of the particles was found to be 1/100,000 of an inch in diameter which matches the size of the material in the E ring. But it gets even better: By knowing how much mass is leaving the moon, scientists can possibly forecast the future date when all the water will be gone from Enceladus (Cassini Imaging Central Lab "Icy tendrils").
And those particles which enter the E ring have some interesting implications. They had traces of oxygen, sodium, and magnesium but the majority of them were made of silica (Si02) which is not a very common molecule to find in the sizes seen by Cassini. In fact the most likely way they form is as a result of hydrothermal vents. But to make sure scientists replicated the expected conditions on Enceladus and tried to generate the particles. They had hot water with ammonia, sodium bicarbonate, olivine, and pyroxene. After mixing well, the sample was frozen in a manner consistent with leaving Enceladus through a geyser. Turns out the condensation removes silica well because the water no longer has sufficient energy to trap it. So long as the water is above 90 degrees Celsius and has an acidity of 8.5 to 10.5 on the ph scale, the particles can be generated. And here on Earth, life exists at vents like these. Enceldaus makes the case for life better and better (Johnson "Hints," Betz "Hydrothermal").
But after examining images Cassini has taken of Enceladus over the years, scientists concluded that a majority of the eruptions we see from the moon are more spread out along the fissures on the surface and not as concentrated jets at specific places. The perspective is key, with different points of Cassini's orbit yielding new views on the fissures, according to a May 7 2015 issue of Nature by Joseph Spitale (from the Planetary Science Institute). Yes, those types still occur but a majority of the material that leaves the moon departs in these diffuse curtains after image processing constantly showed a background glow of material along the fractures in the surface (JPL "Saturn moon's," Betz "Curtains" 13).
Well, all the signs pointed to this conclusion, but after many gravity readings Cassini was able to confirm that Enceladus does have a liquid ocean. The moon orbited too much for it to have a solid interior and models based off the Cassini data point to a liquid ocean. How so? Gravity tugs at objects and as Cassini beams radio waves back to Earth, Doppler shifts record the intensity of the gravity. After over 19 flybys of the moon enough data was collected to see how different places tugged at different rates. Also, images from Cassini show that the surface rotates at a slightly different rate than the rest of the moon. The potential ocean may be 6 miles deep and underneath 19-25 miles of ice. Another chance for life in our solar system! (NASA "Cassini," JPL "NASA").
On June 11, 2004 Cassini passed by Phoebe, a 140 mile wide moon of Saturn, at 1,240 miles and raised the possibility of it being a captured comet from the Kuiper Belt rather than the prevailing thought of it being an asteroid. This was due to streaks of material and a thin layer of dust rather than a thick one detected. Not too long after the flyby, it was confirmed that Phoebe is likely a captured Kuiper Belt Object. Using Cassini's Visible & Infrared Spectrometer, it was determined that Phoebe is made up of water-ice, iron-high compounds, organic compounds and possible clay, all found in comets. Phoebe is therefore most likely a captured Kuiper Belt Object, and if so could provide a glimpse into the early solar system. That being said, most data indicates that the moons of Saturn formed with the planet and that Phoebe is a rarity (Weinstock Sept. 2004, Svital Aug 2005, Douthitt 51).
But it has other odd features that further distinguish it. Take for example its craters, which don't look like impacts and are ringed with ice. Instead, they appear to be from internal collapses possible from sublimation of surface materials. Phoebe also orbits in a retrograde motion with a high level of eccentricity and heavily inclined to the orbital plane of Saturn, all hinting at its captured nature (Carrol 30-31).
As more data came in, evidence pointed to Phoebe being more spherical in its past but had a crunch in after temperatures warmed material to the point of gravitational collapse. This could have been because of proximity to the Sun or from radioactive materials that were abundant in the early solar system like aluminum-26. This could mean Phoebe formed near the inner solar system, something similar to Kuiper Belt Objects. Also, Phoebe's density closely matches Pluto, a member of the Kuiper Belt, but because of no close flybys by Cassini, scientists are unable to use gravity tugs to gain insights into the internal layout of the moon (NASA "Cassini Finds," Carroll 30-1).
Hyperion, a 165 mile-long moon with an odd spin courtesy of Titan's gravity, does not have a smooth surface but instead one that has been hit by many meteors. Because of these collisions, we have access to material that can reveal its age and composition. We now know it is one of the oldest moons that Saturn has. It is also low in density. Those collisions have shown it to be "fluffy and porous." It is thought to be icy in nature with a thin, dark coating of dust covering it based on how the layers in the impact craters look like. We still do not know where it formed or how it came to be in Saturn's possession. It certainly could be a remnant of a moon that is no longer there (Ruvinsky 10).
Or is it a captured comet? After all, it looks porous like an object which has been sublimated many times, like a comet, and it has low density which is close to comet values and implies a low rock content value. In fact, the shape of the craters hints at the "bouncy" nature of Hyperion for the craters are not as deep as their size indicates they should be nor do we find as much debris as we would expect from an impactor. But we have never found a comet as big as Hyperion, not even close. So even though it has similar qualities, we will have to vote no on it being a comet but yes to it likely being an icy leftover from the early solar system (Betz "Couldn't).
Interestingly, Hyperion may be the only object in the solar system which has an electrostatically-charged surface. Cassini detected electrons coming off the surface of Hyperion during its 2005 pass of the moon. The mechanism for this is unknown at the time but the solar wind or Saturn's magnetic field may play roles (Betz "Moon").
The list of places in the solar system with water increased after Cassini observed the mountain Janiculum Dorsa on Dione. How? The mountain displays evidence of deformation near its base which would suggest that the crust condensed, possible as a result of material leaving the moon. Cassini did observe particles of water vapor and dust emanating from the moon using its magnetometer. This is similar behavior as Enceladus, implying that a subsurface source of water likely exists. And how would it remain liquid? Likely because of tidal forces pulling on Dione, causing the water to heat up (Lewis).
Besides its uncanny resemblance to the Death Star, Mimas has another interesting feature: it may be another place in the solar system with liquid water. A study by Radwan Tajeddine from the University of Cornell using measurements from Cassini shows that the moon moves around its axis of rotation almost twice as much as expected in a manner consistent with a floating crust. The wobble is also consistent with a lopsided, football-shaped core, but it would have to be elongated (in fact, beyond the realm of reasonability based on the surface shape of Mimas). This is all reasonable for Mimas, for it like other moons goes through libation, or differential gravity tugs at certain points in its orbit. More data will be needed before anything can be confirmed, especially because the outer surface betrays nothing unusual about the interior of the moon (Mazza, Ferron "Mimas," JPL "Saturn Moon").
Roughly 905 miles wide, this strange moon has both white and dark sides that deeply contrast it. Ice accounts for the white color most likely while the black material is organic (carbon-based). But it gets stranger. Other data shows that Iapetus has a huge equatorial ridge that runs almost all the way around the moon (over 1000 miles long, and nearly twice as high as the Himalayas). A collision with another celestial object or gravitational forces between the moon and Saturn are the most likely culprits (Douthitt 51).
After examining the northern altitudes of this moon, Cassini spotted some odd patterns that looked like red lines. Each was only a few miles wide but would go on for hundreds of miles! No one is quite sure what to make of them, but some wonder if it is a chemical reaction with something on the surface or it could be deposits from a nearby object (Farron "Tethys").
For more, look below:
More Moon Pics!
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---. "Hydrothermal Vents Brew in Enceladus' Ocean" Astronomy Jul. 2015: 15. Print.
---. "Moon Beams." Astronomy Feb. 2015: 13. Print.
Carrol, Michael. "The Weird World of Phoebe." Astronomy Mar. 2014: 30-1. Print.
Douthitt, Bill. "Beautiful Stranger." National Geographic Dec. 2006: 51, 56. Print.
Ferron, Karri. "Mimas Might Have Subsurface Ocean." Astronomy Feb. 2015: 12. Print.
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Grant, Andrew. "Wonder Worlds." Discover Oct. 2009: 12. Print.
Haynes, Korey. "Saturn's Moons are Young and Active." Astronomy Jul. 2016: 9. Print.
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Lewis, Tanya. "Saturn's Moon Dione May Have Had Active Subsurface Ocean, Cassini Photos Suggest." HuffingtonPost.com. Huffington Post, 10 Jun. 2013. Web. 27 Dec. 2014.
Mazza, Ed. "Mimas, One of Saturn's Moons, May Have An Underground 'Life-Friendly' Ocean." HuffingtonPost.com Huffington Post: 17 Oct. 2014. Web. 04 Feb. 2015.
NASA/JPL. "Cassini Finds Saturn moon Has Planet-Like Qualities." Astronomy.com. Kalmbach Publishing Co., 30 Apr. 2012. Web. 26 Dec. 2014.
---. "Cassini Spacecraft Reveals Forces Controlling Saturn Moon Jets." Astronomy.com. Kalmbach Publishing Co., 02 Aug. 2013. Web. 27 Dec. 2014.
NASA. "Cassini finds global ocean in Saturn’s moon Enceladus." Astronomy.com. Kalmbach Publishing Co., 15 Sept. 2015. Web. 22 Oct. 2015.
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Svital, Kathy A.“New Moons. Discover Aug. 2005: 10. Print.
Wall, Mike. "101 Geysers Discovered On Saturn's Icy Moon Enceladus." HuffingtonPost.com. Huffington Post, 30 Jul. 2014. Web. 29 Dec. 2014.
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© 2015 Leonard Kelley
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