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Cassini's Discoveries and Flybys of Phoebe, Hyperion, Dione, and Other Saturn Moons

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

In 1610, fresh off his recent observations of Jupiter, Galileo set his telescope upon Saturn and discovered that it had rings. But to him, they appeared to be something else, like moons in orbit. Like many great scientists of antiquity, he made a mistake, but it was rectified in 1656 when Christiaan Huygens not only discovered Titan but also figured out its ring nature (Douthitt). Despite this mistake, Galileo was right about there being satellites around Saturn. And, oh, how wonderful they are.



Findings: Phoebe

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. Another piece of evidence came from water readings taken of the whole Saturn system. It was found that the entire system shares a water signature, but not Phoebe. It must have formed elsewhere (Weinstock Sept. 2004, Svital Aug 2005, Douthitt 51, Klesman).

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 before temperatures warmed materials 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).



Findings: Hyperion

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. 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").



Findings: Dione

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. Evidence for the subsurface ocean grew as the years went by. More and more gravity readings indicated that liquid water is likely present some 20 miles below the surface of the moon (Lewis, Scharping).

The stripes of Dione.

The stripes of Dione.

Like many solar system objects, Dione also has mysterious stripes on its surface. After looking at Cassini data, scientists discovered that may different groupings of the stripes had linear, parallel features to them. In fact, most are parallel to the equator and are 10 to 100 km in length and are at most 5 km in width. Plate techonics was ruled out, so what could it be? Most seem to be on top of established terrain, indicating a soft, steady supply that layers on top of the surface. Perhaps material from the rings of Saturn slowly grace the surface as they break up (Gohd).



Findings: Mimas

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. That is, until research from Alyssa Rose Rhoden (Arizona State) showed that if a subsurface ocean exists then the surface of the moon would have to be cracked like Europa (Mazza, Ferron "Mimas," JPL "Saturn Moon," Wenz).

A close-up of the ridge.

A close-up of the ridge.

Findings: Iapetus

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 for developing this ridge. Simulations on a small scale done by Angela Stickle and James Roberts (John Hopkins University) showed that so long as the material hit Iapetus at a shallow enough angle, it would create a crater that would be filled by infilling surface material that was kicked up in the collision. This decapitation would take a long time but the study showed a buildup of the smaller and smaller material would eventually create the ridge seen (Douthitt 51, Kruesi).

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The mysterious red streaks.

The mysterious red streaks.

Findings: Tethys

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," CICL).



Findings: Pandora

At 52 miles by 18 miles in size, this moon could have easily gotten lost in the vastness of the Saturn system. But as Cassini finished its mission at Saturn, it got a close up look at the moon which completes an orbit ever 15 hours at a distance of 88,000 miles from Saturn. Density measurements combined with a high-albedo surface have led scientists to theorize that the moon is mainly made of water-ice. And because of the small size of the moon, its compadres pull and tug on it, causing fluctuations in its motion which impact the F-ring where it resides (O'Neill).

Findings: Pan

This little moon, at 35 km across, wouldn't seem like much to talk about. But look at the shape of it: Its like two spheres pushed together and bulged at the contact point! It is one of the closest moons to Saturn and resides in the Encke Gap of Saturn's rings. It is thought that Pan was a leftover from a collision and has slowly gathered material from the ring it lives in, with the material collecting around the rotation point of Pan (Berger).

Findings: Prometheus

For more, look below:

Prometheus pulling on the F ring.

Prometheus pulling on the F ring.

Taken from 23,000 miles away at an 87 degree angle to the sun.

Taken from 23,000 miles away at an 87 degree angle to the sun.

Even though these findings are amazing all by themselves, Cassini is working on the gas giant itself is a revealing portrait of a complex system. And the big moon of Saturn, Titan, has surprised us again and again. Read on about them here and here.

Works Cited

Berger, Eric. "New Images of Saturn's Walnut-Shaped Moon Dazzle Scientists." Conte Nast., 09 Mar. 2017. Web. 01 Nov. 2017.

Betz, Eric. "Couldn't Hyperion's Low Density and Spongy Texture Be Better Explained by It Being a Captured Comet?" Astronomy Mar. 2016. Print.

---. "Moon Beams." Astronomy Feb. 2015: 13. Print.

Carrol, Michael. "The Weird World of Phoebe." Astronomy Mar. 2014: 30-1. Print.

CICL. "Unusual red arcs spotted on Saturn’s icy moon Tethys." Kalmbach Publishing Co., 30 Jul. 2015. Web. 20 Jun. 2017.

Douthitt, Bill. "Beautiful Stranger." National Geographic Dec. 2006: 51, 56. Print.

Ferron, Karri. "Mimas Might Have Subsurface Ocean." Astronomy Feb. 2015: 12. Print.

---. "Tethys Sports Red Streaks." Astronomy Nov. 2015: 16. Print.

Gohd, Chelsea. "Saturn's moon Dione has mysterious stripes all across its surface." Kalmbach Publishing Co., 02 Nov. 2018. Web. 06 Dec. 2018.

JPL. "Saturn Moon May Hide a Fossil Core or an Ocean." Kalmbach Publishing Co., 20 Oct. 2014. Web. 25 Jul. 2016.

Klesman, Allison. "The Saturn system has water just like Earth's...except for Phoebe." Kalmbach Publishing Co., 06 Dec. 2018. Web. 14 Jan. 2019.

Kruesi, Liz. "How Meteorites Built Up Iapetus' Mountain Ridge." Kalmbach Publishing Co., 03 Apr. 2017. Web. 03 Nov. 2017.

Lewis, Tanya. "Saturn's Moon Dione May Have Had Active Subsurface Ocean, Cassini Photos Suggest." Huffington Post, 10 Jun. 2013. Web. 27 Dec. 2014.

Mazza, Ed. "Mimas, One of Saturn's Moons, May Have An Underground 'Life-Friendly' Ocean." Huffington Post: 17 Oct. 2014. Web. 04 Feb. 2015.

NASA/JPL. "Cassini Finds Saturn moon Has Planet-Like Qualities." Kalmbach Publishing Co., 30 Apr. 2012. Web. 26 Dec. 2014.

O'Neill, Ian. "Cassini Buzzes Pandora, Saturn's Moon of Chaos." Discovery Communications, 28 Dec. 2016. Web. 26 Jan. 2017.

Ruvinsky, Jessica. "A Strange Lump in Space." Discover Dec. 2005: 10. Print.

Scharping, Nathaniel. "Dione may be Saturn's third moon hiding an ocean." Kalmbach Publishing Co., 04 Oct. 2016. Web. 17 Jan. 2017.

Svital, Kathy A.“New Moons. Discover Aug. 2005: 10. Print.

Weinstock, Maia. "Cassini Watch.” Discover Sept. 2004: 9. Print.

Wenz, John. "There's Now One Less Ocean World in the Solar System." Kalmbach Publishing Co., 01 Mar. 2017. Web. 30 Oct. 2017.

© 2015 Leonard Kelley

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