Universe Today

If you search around Universe Today, you'll find more than just a couple of articles here referring to Mimas as the Death Star moon. That's because it does look like the Galactic Empire's planet-blasting Death Star – round and with a large circular depression at the side. If you're from one of the younger generations, we're referring to the first Star Wars trilogy.

Mimas, which is actually one of Saturn's natural satellite, also has a large circular depression which is actually a very large crater. The crater is named Herschel, after the astronomer who discovered Mimas – William Herschel. Mimas, along with most of Saturn's moons, are named after the Titans of Greek mythology.

Herschel, the crater, is so large that at 139-km wide, it easily covers almost a third of the entire diameter of Mimas. If you want to picture that with respect to the Earth's dimensions, imagine a crater slightly wider than the area occupied by Canada. The impact of the asteroid that may have caused the crater could have been enough to shatter the moon.

Scientists believe that fractures located in the opposite side of the moon may have been caused by shock waves emanating from the point of impact. The walls of Herschel are estimated to be about 5 km high. At the center of the crater, a structure protrudes conspicuously from the the crater's floor to a height of about 6 km.

Mimas' mean radius is 198 km. For comparison, our Moon has a mean radius of 1,737 km.

Saturn has 62 moons all in all and Mimas is one of those considered as its major satellites. However, just like all the other major satellites, the size of Mimas is nothing compared to Saturn's largest satellite – Titan. Titan comprises 96% of the total mass orbiting around Saturn (and that already includes the rings).

The other major satellites, Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus , combine to constitute about 4%. The rest, about 50 plus moons all in all, are so tiny that they only make up around 0.04%.

Many of the images we've seen of Mimas were taken by the Cassini orbiter, with the closest shots captured very recently when the orbiter was only 9,500 km away. This was on February 13, 2010.

Here are some related articles from Universe Today:

Saturn's Moon Mimas
Cassini Survives Close Encounter of the Death Star Kind!

Want some more? Check out a couple of pages from NASA:
Up Close to Mimas
Cassini Set to Do Retinal Scan of Saturnian Eyeball

Tired eyes? Listen to some episodes at Astronomy Cast. Here are two that might interest you:

Stellar Roche Limits, Seeing Black Holes, and Water on Mars
The Search for Extraterrestrial Intelligence

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The small moon Janus is almost hidden between the planet's rings and the larger moon Rhea.Credit: NASA/JPL/Space Science Institute

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Newly released images from last November's close flyby over Saturn's icy moon Enceladus the Cassini spacecraft reveal geyser jets spraying all along the prominent fractures, or "tiger stripes" that cross the moon's south polar region. Additionally, a new detailed temperature map of one fracture reveals warmer temperatures than what was expected. "Enceladus continues to astound," said Bob Pappalardo, Cassini project scientist at the Jet Propulsion Laboratory. "With each Cassini flyby, we learn more about its extreme activity and what makes this strange moon tick."
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Janus, also known as Saturn X, is one of Saturn's natural satellites.

Images of Janus reveal it as a highly-cratered celestial body, with many of its craters even exceeding 30 km. Scientists believe it to be a highly porous and icy rubble pile.

A rubble pile is a bunch of rocks that have merged over time due to the attractive forces of gravity. Since cavities are expected in places where the rocks have not joined perfectly, it is expected that these bodies have lower densities compared to monolithic rocks, i.e., those that are made up of a single piece of rock.

Rubble piles are formed when something collides with an asteroid (whether monolithic or rubble pile) and breaks it into smaller pieces. Interestingly, if the smaller pieces are close enough, they eventually join due to gravitational attraction. This process can be very short, with the longest durations lasting for weeks and the shortest, just within hours.

At only 181 km, Janus is really so tiny. In the article entitled Janus and Saturn here in Universe Today, you'll see it as even smaller than the shadows cast by Saturn's rings on the planet's northern hemisphere. The image even had to zoom in to a specific space on the planet to make Janus distinguishable. And even after that, you can see it as nothing more than a mere dot on the image.

Janus once baffled scientists in the early years after its discovery in 1966. The reasons for its peculiar behaviors were given light, 12 years later, when Stephen M. Larson and John W. Fountain suggested that the only logical explanation was that observers of Janus were actually looking at two distinct objects.

This was later confirmed two years after (in 1980) by the robotic space probe Voyager 1. The other natural satellite that shares practically the same orbit as Janus has been named Epimetheus.

The mean orbital radius of Janus is 50 km less than that of Epimetheus'. Since the dimensions of Epimetheus is 135 x 108 x 105 km and Janus' is 193 x 173 x 137 km, the distance between the two is less than even their smallest dimensions.

Images taken by the Cassini spacecraft in 2006 revealed a dust ring along the orbit of Janus and Epimetheus. The particles found there are believed to have been part of both moons, perhaps blasted away by impacts with asteroids and meteoroids.

Here are two articles from Universe Today that feature Janus:
Spotty Janus
Janus Poses Above Saturn

Here's an article of Janus from NASA. The image from the link entitled "Janus and Saturn" in the article is too small. Here's a larger picture of Janus from NASA.

Tired eyes? Listen to some episodes at Astronomy Cast. Here are two that might interest you:

Stellar Roche Limits, Seeing Black Holes, and Water on Mars
The Search for Extraterrestrial Intelligence

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Saturn's moon Dione. Credit: NASA

Cassini, who along with Robert Hooke is credited for the discovery of the Great Red Spot in Jupiter, named Saturn's moons collectively as Sidera Lodoicea. Dione was discovered on the same day as another moon of Saturn, on March 21, 1684. The two other moons, Iapetus and Rhea, were discovered earlier – on October 25, 1671 and December 23, 1672 respectively.

Sidera Lodoicea, which means Louisian Stars, was chosen by Cassini to honor his benefactor as patron of the Paris Observatory, France's King Louis XIV. Cassini was the director of Paris Observatory during King Louis XIV's reign.

Dione is believed to be made mostly of high density water ice. However, it is also believed that silicate rock may be found underneath. Dione's water ice has a high density because gravity is pulling the massive structure together.

Also known as Saturn IV, Dione has a mean radius of about 561 km. It's much smaller than our own Moon, which has a mean radius of 1,737 km. It's mass is only 1.095 x 10²¹ kg, and has an escape velocity of 0.510 km/s.

It is easy to understand why all of the water on the Dionean surface is frozen. This moon has a temperature of only 87 K or -186°C.

There are three main types of geological features that can be found on Dione: Chasmata or chasms, Lineae or wispy features, and the almost ever-present craters.

Dione's surface reveal the same violent past as the Moon's, wherein these celestial bodies may have been heavily bombarded with asteroids and meteoroids as evidenced by the craters on its surface. Many of Dione's craters have radii over 100 km in diameter.

The craters in Dione have a peculiar concentration, i.e. most of the highly cratered regions can be found in the trailing hemisphere. It is peculiar because tidally locked satellites like Dione are expected to suffer heavier bombardment on their leading hemisphere. Tidally locked satellites are those that have one side always facing the planet they orbit.

Scientists believe that, due to Dione's small size, the moon spun during the period of heavy bombardment. In other words, the current trailing hemisphere may have been the leading hemisphere before.

Want to know what is the largest moon in the Solar System? Find the answer here in Universe Today. If you're more interested in Saturn, you might want to know what Saturn's rings are made of.

Here's a closer look of Dione at NASA. Here's another one of Dione from the same site.

Relax and listen to some interesting episodes at Astronomy Cast. Want to know more about Ultraviolet Astronomy? How different is it from Optical Astronomy?

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Cassini's Mimas, from 70,000 km (unprocessed image; credit: NASA/JPL/Space Science Institute)

It passed directly over Herschel, a giant crater whose creation almost shattered the moon … and which, in its appearance in some earlier images, earned Mimas the nickname "Death Star", after the iconic Star Wars prop.

The Cassini team has just released some "Raw Previews" of Cassini's close encounter; time to feast your eyes.
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Saturn's plaid rings. Credit: NASA/JPL/Space Science Institute

And if you don't get the "plaid" reference, go watch Spaceballs.
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Saturn from Cassini. Image credit: NASA/JPL/SSI

With the new 2011 NASA budget allowing for more space science activities, the space agency has extended the Cassini mission to explore Saturn and its moons to 2017. "This is a mission that never stops providing us surprising scientific results and showing us eye popping new vistas," said Jim Green, director of NASA's planetary science division. "The historic traveler's stunning discoveries and images have revolutionized our knowledge of Saturn and its moons." This is the second mission extension for Cassini, and the new "Solstice Mission" will allow scientists to study seasonal and other long-term weather changes on the planet and its moons.
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Here are some amazing Cassini images. There are some pictures of the spacecraft, and some pictures taken by the spacecraft.

Launch of Cassini Orbiter and Huygens Probe on Titan IV

Here's a picture of the launch of Cassini and Huygens atop a Titan IV rocket. This was the beginning of a 7-year journey to travel from Earth all the way to Saturn, with a few detours along the way.

Cassini Begins Transmitting Data From Enceladus Flyby

This is an artist's impression of Cassini flying past Saturn's moon Enceladus. Cassini discovered strange geysers on Enceladus, blasting water ice into space. These geysers are coming from cracks at Saturn's south pole, and scientists think it could mean there's an ocean of liquid water underneath the ice.

Cassini

This is an artist's impression of Cassini approaching Saturn. It arrived at Saturn in 2004, beginning many years of incredible research about the ringed planets and its intriguing moons.

Enceladus Near Saturn

Here's another view of Saturn's moon Enceladus, with Saturn itself as a backdrop. You can see Saturn's rings at the bottom of the image, cutting a line across the photo.

Crescent Rhea Occults Crescent Saturn

You're looking at a photo of Saturn's moon Rhea passing in front of Saturn. Both Rhea and Saturn are cloaked in shadow.

We've written many articles about Cassini for Universe Today. Here's an article about the sunlight glinting off a lake on Titan, and here are some amazing pictures of Saturn during its equinox.

If you'd like more info on Saturn, check out NASA's World Book on Saturn. And here's a link to the homepage of NASA's Cassini spacecraft, which is orbiting Saturn.

We've also recorded an episode of Astronomy Cast all about Saturn. Listen here, Episode 59: Saturn.

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Spokes visible in Saturn's B ring. Credit: NASA/JPL/Space Science Institute

The mysterious spokes that sometimes appear in Saturn’s largest ring, the B ring, have been unexplained. But new measurements from Cassini's Visual Infrared Mapping spectrometer (VIMS) suggests the radial spokes that sometimes form across the ring are entirely composed of water ice. The existence of the spokes were unexpected and were first observed when the Voyager spacecraft flew by Saturn in 1980. When Cassini arrived at Saturn in 2004, the spokes were not visible, but later appeared in 2005; the VIMS instrument was not able to observe the spokes until 2008. Even with this new information, the spokes are still mysterious, as they appear to be a seasonal phenomenon and can become visible and then fade within a few hours. The process that creates and dissipates the spokes is unknown.
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The Cassini mission is just a non-stop faucet of fantastic images! Here are two that were released today, for your viewing pleasure. The first image, above, is an eclipse of Saturn's moon Tethys, which lies in the background, by Dione. The three images were each taken one minute apart.

As you can see, from Cassini's perspective Dione passes right in front of Tethys. Make no mistake in thinking that these two Saturnian companions are close together in this shot, however; Dione, the moon in the foreground, is 2.2 million kilometers (1.4 million miles) from the Cassini spacecraft, while Tethys is 2.6 million kilometers (1.6 million miles) away. Read more…

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Caption: Geysers on Enceladus. Credit: NASA, JPL, Space Science Institute

One of the most exciting but unexpected discoveries of the Cassini mission is seeing the activity taking place on Saturn's small moon Enceladus. Between the active geysers, the unusual "tiger stripes" and the surprisingly young surface near the moon's south pole, Enceladus has surprised scientists with almost all the images and data the gathered by the spacecraft. But is the moon always active, or are we just in the right place at the right time, lucky to be catching it during an active phase? A recent paper outlines a model in which the kind of geologic eruptions now visible on Enceladus only occur every billion years or so.
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Caption: Cassini captures Rhea and Saturn's rings. Credit: NASA/JPL/Space Science Institute

The stunning images just keep coming from Cassini. Here's a collection of three recent images. Above, Saturn's moon Rhea teams up with the planet's rings, creating an image that could only come from the Saturn system. Taken on Nov. 4, 2009, Rhea's trailing hemisphere shows off its wispy terrain. The view was acquired at a distance of approximately 762,000 kilometers (473,000 miles) from Rhea, with the rings farther away, off in the distance.
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Saturn from Cassini. Image credit: NASA/JPL/SSI

The Cassini probe was the part of the space craft fabricated by NASA. It along with is sister probe had 7 main goals to achieve. First, the probe was to examine Saturn's rings, documenting their structure and composition. Second, the Cassini mission was to determine the composition and geological history of each of Saturn's moons. Third was to look into certain unusual characteristics of the Iapetus moon. Fourth, the probe was to measure the structure of Saturn's magnetosphere. Fifth, the Cassini probe was to measure and study Saturn's atmosphere. The Sixth and Seventh objectives of the mission were exclusively done by the Huygens probe which was to examine Titan moon.

The mission launched on October 13, 1997 and took around 7 years to reach Saturn. The mission is still ongoing as Cassini had it mission renewed recently. This is interesting since the NASA had to go undergo some major budget cuts. This emphasizes the importance of the project.

Another interesting fact about the mission was the power source used to power its systems. The Cassini probe used a radioisotope thermoelectric generator. This is a special type of generator powered by the heat that is emitted by decaying radioactive isotopes. In the case of the Cassini probe, the RTG is being used to power all its systems. This is considered one of the best power sources since it can consistently produce a few watts of power over a long period of time.

The Cassini probe has yielded many significant discoveries about Saturn and space. For example scientists tested out Einsteins theory by sending radiowaves to and from the probe. The theoretical calculations made using relativity bore out when compared to the real life measurements. Another interesting discovery was the existence of three new moons. They are now officially named Methone, Polydeuces, and Pallene.

The Cassini Probe has brought a lot more knowledge about Saturn and space than ever before. The probe is the fourth sent to Saturn and who knows how many more may be sent in the future. There may even be manned missions in the future if the technology is advanced enough.

If you enjoyed this article there are several others on Universe Today that you will like. There is an interesting article called Naked Saturn that has pictures of the planet taken by Cassini. There is also a great article about the rotation of Saturn.

There are also some great resources online. You can check out the Cassini Mission web page on the NASA web site. You can also check out the ESA's Cassini Mission web page

You can also listen to shows on Astronomy Cast. Episode 96 is part 3 of the Mars series and talks about terraforming Mars.

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Moons dancing around Saturn. Credit: NASA/JPL/Space Science Institute

The Cassini CICLOPS imaging team has released some new movies of several moons orbiting Saturn as if in a cosmic ballet around the ringed planet. In one scene that blends 12 images taken over the span of 19 minutes, Rhea skates in front of Janus, as Mimas and Pandora slide across the screen in the opposite direction.

"As yet another year in Saturn orbit draws to a close, these wondrous movies of an alien place clear across the solar system remind us how fortunate we are to be engaged in this magnificent exploratory expedition," said Carolyn Porco, Cassini imaging team leader.

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