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Meet Mimas, the bullseye moon

Mimas showing Herschel Crater

NASA's Cassini spacecraft took this image of Mimas—the most heavily cratered body in the Solar System—from a distance of 103,000 kilometres. The huge Herschel Crater is prominent on the right, spanning a third of the moon's diameter.

  • Mimas, innermost of Saturn’s major moons
  • Most heavily cratered body in the Solar System
  • Herschel Crater is one-third the moon’s width

SATURN’S MOON Mimas (pronounced MY-muss or MEE-muss) looks somewhat like a bullseye when viewed from a certain angle. The feature responsible for this is the huge, 130-kilometre-wide Herschel Crater, which is a third of the diameter of the tiny moon.

If the object that struck Mimas and formed the crater had been larger or moving faster, the moon would probably have been shattered into pieces. Those pieces might have collapsed back to form a new moon or could have scattered to become another ring around Saturn.

Mimas back-dropped by Saturn

An amazing Cassini view of Mimas against the hazy limb of Saturn.

Mimas is the innermost of Saturn’s major moons, averaging 396 kilometres in diameter…not quite big enough to hold a perfectly round shape. It orbits at a distance of 185,520 kilometres from Saturn in a time of 22 hours and 37 minutes. It is also ‘tidally locked’, which means that one side always faces in toward Saturn.

Along with another of Saturn’s moons, Rhea, Mimas has been called ‘the most heavily cratered body in the Solar System‘. It’s close-in orbit means that it probably receives several times the rate of collisions with meteoroids as do the other moons of Saturn.

That it isn’t even more heavily cratered is probably because, being closer to Saturn, it was warmer (and consequently ‘softer’) for a longer time, so early features have softened or eroded away.

However, with so many impacts the youngest craters have tended to obliterate the older ones and, like Rhea, it is cratered about as much as it can get.

The craters in the southern polar region are generally 20 kilometres in diameter or less—this suggests that some melting or other resurfacing processes occurred there later than on the rest of the moon, removing any traces of larger craters. (Interestingly, the south polar region of another of Saturn’s moons, Enceladus, is the source of that moon’s geysers.)

The bullseye crater

The walls of Herschel Crater are approximately five kilometres high, parts of the floor are approximately 10 kilometres deep, and the central peaks are almost six kilometres above the floor of the crater.

A comparable crater on Earth would be 4,000 kilometres in diameter.

Shock waves from the Herschel impact might have caused the fractures—also called chasmata—that appear on the opposite side of Mimas.

Mimas showing Herschel Crater

A mosaic of Cassini images put together to give us a birds-eye view of Herschel Crater.

See the amazing full-size, high-resolution image here (will open in a new window or tab).

Mimas’ low density (1.17 times that of liquid water) indicates that it is composed mostly of water ice with only a small amount of rock. It seems to be solidly frozen at a temperature of -209 degrees Celsius.

This is puzzling because Mimas is closer to Saturn than Enceladus, and its orbit is much more eccentric (out of round) than Enceladus’ orbit. Thus, Mimas should have much more tidal heating than Enceladus. (Tidal heating occurs when the gravity of another body, in this case Saturn, pulls and compresses a moon’s solid body, creating heat.)

Yet, Enceladus has geysers of water, while Mimas has one of the most heavily cratered surfaces in the Solar System. This suggests that Mimas’ frozen surface has persisted for a very long time.

The paradox has led astronomers to use the ‘Mimas test’, by which a hypothesis that explains the partially thawed water of Enceladus must also explain the entirely frozen water of Mimas.

Mimas apparently sweeps out the 4,800-kilometre-wide gap—called the Cassini Division—between Saturn’s two widest rings, the A and B rings. Observations from NASA’s Cassini spacecraft have revealed that there is still some ring material in the Cassini Division, although it is sparse enough that the area appears empty from a distance.

Mimas and Saturn and its rings

Tiny Mimas is dwarfed by Saturn and its rings.

Moons that stick together

Mimas is in orbital ‘resonance’ with two nearby moons, Dione and Enceladus. That is, these moons speed up slightly as they approach each other and slow down as they draw away, causing their orbits to vary slightly in a long series of complex changes, which help keep them locked in their positions.

The gravity of Mimas strongly affects the tiny 3km-diameter moon Methone, the 4km-diameter moon Pallene, and the 2km-diameter moon Anthe, all of which orbit between Mimas and the next major moon going out from Saturn, Enceladus.

The vastly more massive Mimas causes Methone’s orbit to vary by as much as 20 kilometres. The affect is larger for tiny Anthe, and slightly smaller for Pallene.

English astronomer William Herschel discovered Mimas in 1789. His son, John Herschel, suggested that the names of the moons of Saturn be associated with Greek mythical brothers and sisters of Kronus (known to the Romans as Saturn).

The name Mimas comes from the god (or Titan) Mimas in Greek mythology, who was slain by one of the gods of Olympus in the war between the Olympians and the Titans. Different accounts have Mimas dispatched by Hercules, by Ares (the god of war), or by Zeus himself using a thunderbolt. Legend has it that the island of Prochyte near Sicily rests on his body.

For years, ground-based astronomers could only see Mimas as little more than a dot, until the Voyagers 1 and 2 spacecraft flew past and imaged it in 1980. The Cassini spacecraft has made several close approaches and provided detailed images of Mimas since it achieved orbit around Saturn in 2004.

Adapted from information issued by NASA / JPL / Space Science Institute.

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Saturn’s tiny Trojan moon

Saturns' moon Helene

Saturn's 33-kilometre-wide moon Helene, imaged by NASA's Cassini spacecraft from a distance of around 19,000 kilometres.

THIS BLACK AND WHITE image shows Helene, one of the smaller of Saturn’s 62 confirmed moons, seen during a relatively close encounter by NASA’s Cassini spacecraft on March 3, 2010. The grey background is the atmosphere of Saturn.

Helene circles Saturn in the same orbit as the much larger moon, Dione, but ahead of it, making it a “Trojan” moon. A Trojan moon is one that is a location where the gravitational pull of the parent planet and another body (Dione, in this case) balance out, so it always stays in the same relative position. That position is known as a Lagrangian point.

The tiny moon was discovered by astronomers Pierre Laques and Jean Lecacheux in 1980 from the Pic du Midi Observatory in France. It was provisionally designated S/1980 S 6 (meaning it was the sixth new moon of Saturn to be discovered that year), and in 1988 was officially named after Helen of Troy, who in Greek mythology was the granddaughter of Cronus (Saturn).

Helene is just 33 kilometres across at its widest point. It orbits Saturn at a distance of 377,000 kilometres (roughly the same average distance between the Earth and the Moon) and takes about 2.74 Earth days to complete one revolution.

At the time Cassini snapped the image, the spacecraft was about 19,000 kilometres from Helene, which means we can see detail down to about 113 metres.

Images courtesy NASA / JPL / Space Science Institute.

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Life on Titan: up in the air?

Titan, Epimetheus and Saturn's rings

Saturn's moon Titan looms large behind the planet's rings. (A smaller moon, Epimetheus, is in the foreground.) Chemical reactions in Titan's upper atmosphere could form molecules that are the precursor of life.

  • Titan’s atmosphere simulated in the lab
  • Chemical reactions produce amino acids
  • Key ingredients for life as we know it

While simulating possible chemical processes that could occur in the hazy atmosphere of Titan, Saturn’s largest moon, a University of Arizona-led planetary research team found amino acids and nucleotide bases in the mix—the most important ingredients of life on Earth.

“Our team is the first to be able to do this in an atmosphere without liquid water. Our results show that it is possible to make very complex molecules in the outer parts of an atmosphere,” said Sarah Hörst, a graduate student in the University of Arizona’s (UA) Lunar and Planetary Lab, who led the international research effort together with her adviser, planetary science professor Roger Yelle.

The molecules discovered include the five nucleotide bases used by life on Earth to build the genetic materials DNA and RNA: cytosine, adenine, thymine, guanine and uracil, and the two smallest amino acids, glycine and alanine. Amino acids are the building blocks of proteins.

Reaction chamber

A window into Titan’s atmosphere: Energised by microwaves, the gas mix inside the reaction chamber lights up like a pink neon sign. Thousands of complex organic molecules accumulated on the bottom of the chamber during this experiment.

The results suggest not only that Titan’s atmosphere could be a reservoir of pre-biotic molecules that serve as the springboard to life, but they offer a new perspective on the emergence of terrestrial life as well: Instead of coalescing in a primordial soup, the first ingredients of life on our planet may have rained down from a primordial haze high in the atmosphere.

Oddball of the Solar System

Titan has fascinated—and puzzled—scientists for a long time.

“It’s is the only moon in our Solar System that has a substantial atmosphere,” Hörst said. “Its atmosphere stretches out much further into space than Earth’s. The moon is smaller so it has less gravity pulling it back down.”

Titan’s atmosphere is much denser, too—on the surface, atmospheric pressure equals that at the bottom of a 5-metre-deep pool on Earth.

“At the same time, Titan’s atmosphere is more similar to ours than any other atmosphere in the Solar System,” Hörst said. “In fact, Titan has been called ‘Earth frozen in time’ because some believe this is what Earth could have looked like early in time.”

Saturn's moon Titan

Saturn's moon Titan has a thick, hazy atmosphere.

When the Voyager I spacecraft flew by Titan in the 1970s, the pictures transmitted back to Earth showed a blurry, orange ball.

“For a long time, that was all we knew about Titan,” Hörst said. “All it saw were the outer reaches of the atmosphere, not the moon’s body itself. We knew it has a an atmosphere and that it contains methane and other small organic molecules, but that was it.”

In the meantime, scientists learned that Titan’s haze consists of aerosols, just like the smog that cloaks many metropolitan areas on Earth. Aerosols, tiny particles about a quarter millionth of an inch across, resemble little snowballs when viewed with a high-powered electron microscope.

The exact nature of Titan’s aerosols remains a mystery. What makes them so interesting to planetary scientists is that they consist of organic molecules—potential ingredients for life.

“We want to know what kinds of chemistry can happen in the atmosphere and how far it can go.” Hörst said. “Are we talking small molecules that can go on to becoming more interesting things? Could proteins form in that atmosphere?”

What it takes to make life’s molecules

For that to happen, though, energy is needed to break apart the simple atmospheric molecules—nitrogen, methane and carbon monoxide—and rearrange the fragments into more complex compounds such as pre-biotic molecules.

“There is no way this could happen on Titan’s surface,” Hörst said. “The haze is so thick that the moon is shrouded in a perpetual dusky twilight. Plus, at -124 degrees Celsius, the water ice that we think covers the moon’s surface is as hard as granite.”

However, the atmosphere’s upper reaches are exposed to a constant bombardment of ultraviolet radiation and charged particles coming from the sun and deflected by Saturn’s magnetic field, which could spark the necessary chemical reactions.

Smog-like particles

Tiny particles are thought to create the smog-like haze that enshrouds Saturn's moon Titan.

To study Titan’s atmosphere, scientists have to rely on data collected by the spacecraft Cassini, which has been exploring the Saturn system since 2004 and flies by Titan every few weeks on average.

“With Voyager, we only got to look,” says Hörst. “With Cassini, we get to touch the moon a little bit.”

During fly-by manoeuvres, Cassini has gobbled up some of the molecules in the outermost stretches of Titan’s atmosphere and analysed them with its on-board mass spectrometer. Unfortunately, the instrument was not designed to unravel the identity of larger molecules—precisely the kind that were found floating in great numbers in Titan’s mysterious haze.

“Cassini can’t get very close to the surface because the atmosphere gets in the way and causes drag on the spacecraft,” Hörst said. “The deepest it went was 900 kilometres (560 miles) from the surface. It can’t go any closer than that.”

To find answers, Hörst and her co-workers had to recreate Titan’s atmosphere here on Earth. More precisely, in a lab in Paris, France.

“Fundamentally, we cannot reproduce Titan’s atmosphere in the lab, but our hope was that by doing these simulations, we can start to understand the chemistry that leads to aerosol formation,” Hörst said. “We can then use what we learn in the lab and apply it to what we already know about Titan.”

Like a spy in a movie

Hörst and her collaborators mixed the gases found in Titan’s atmosphere in a stainless-steel reaction chamber and subjected the mixture to microwaves causing a gas discharge—the same process that makes neon signs glow—to simulate the energy hitting the outer fringes of the moon’s atmosphere.

The electrical discharge caused some of the gaseous raw materials to bond together into solid matter, similar to the way UV sunlight creates haze on Titan. The synthesis chamber, constructed by a collaborating group in Paris, is unique because it uses electrical fields to keep the aerosols in a levitated state.

“The aerosols form while they’re floating there,” Hörst explains. “As soon as they grow heavy enough, they fall onto the bottom of the reaction vessel and we scrape them out.”

“And then,” she added, “the samples went on an adventure.”

To analyse the aerosols, Hörst had to use a high-resolution mass spectrometer in a lab in Grenoble, about a three-hour ride from Paris on the TGV, France’s high-speed train.

“I always joke that I felt like [I was in ] a spy in a movie because I would take our samples, put them into little vials, seal them all up and then I’d get on the TGV, and every 5 minutes I’d open the briefcase, ‘Are they still there? Are they still there?’ Those samples were really, really precious.”

Analysing the reaction products with a mass spectrometer, the researchers identified about 5,000 different molecular formulas.

Sarah Hörst

“When I came back and looked at the screen, I thought: That can’t be right,” said graduate student Sarah Hörst.

“We really have no idea how many molecules are in these samples other than it’s a lot,” Hörst said. “Assuming there are at least three or four structural variations of each, we are talking up to 20,000 molecules that could be in there. So in some way, we are not surprised that we made the nucleotide bases and the amino acids.”

“The mass spectrometer tells us what atoms the aerosols are made of, but it doesn’t tell us the structure of those molecules,” Hörst said. “What we really wanted to find out was, what are all the formulas in this mass spectrum?”

“On a whim, we said, ‘Hey, it would be really easy to write a list of the molecular formulas of all the amino acids and nucleotide bases used by life on Earth and have the computer go through them.’”

“I was sitting in front of my computer one day—I had just written up the list—and I put the file in, hit ‘Enter’ and went to go do something,” she said. “When I came back and looked at the screen, it was printing a list of all the things it had found and I sat there and stared at it for a while. I thought: That can’t be right.”

“I ran upstairs to find Roger, my adviser, and he wasn’t there,” Hörst said with a laugh. “I went back to my office, and then upstairs again to find him and he wasn’t there. It was very stressful.”

“We never started out saying, ‘we want to make these things,’ it was more like ‘hey, let’s see if they’re there.’ You have all those little pieces flying around in the plasma, and so we would expect them to form all sorts of things.”

In addition to the nucleotides, the elements of the genetic code of all life on Earth, Hörst identified more than half of the molecular formulas for the 22 amino acids that life uses to make proteins.

Titan: A window into Earth’s past?

In some way, Hörst said, the discovery of Earth’s life molecules in an alien atmosphere experiment is ironic.

Here is why: The chemistry occurring on Titan might be similar to that occurring on the young Earth that produced biological material and eventually led to the evolution of life. These processes no longer occur in the Earth’s atmosphere because of the large abundance of oxygen cutting short the chemical cycles before large molecules have a chance to form. On the other hand, some oxygen is needed to create biological molecules. Titan’s atmosphere appears to provide just enough oxygen to supply the raw material for biological molecules, but not enough to quench their formation.

“There are a lot of reasons why life on Titan would probably be based on completely different chemistry than life on Earth,” Hörst added, “one of them being that there is liquid water on Earth. The interesting part for us is that we now know you can make pretty much anything you want in an atmosphere. Who knows this kind of chemistry isn’t happening on planets outside our Solar System?”

Adapted from information issued by UA / S. Hörst / NASA.

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Saturn’s eerie polar light show

False-colour image of Saturn

This false-colour composite of 65 Cassini spacecraft images shows the glow of the aurora over Saturn's north polar region. The colours are: blue is sunlight reflecting from the rings and from one of Saturn's cloud levels; red shows where heat is coming from the interior of the planet; and green shows the aurora.

  • Mini-movie made of Saturn’s polar aurora
  • Uses data from NASA’s Cassini spacecraft
  • Aurora pulses in time with the Sun and Saturn’s spin

As if its rings weren’t spectacular enough, Saturn also puts on a light show for anyone who can see at the right wavelengths.

In this case, those eyes belong to NASA’s Cassini spacecraft, which has been orbiting the Ringed Planet since 2004.

Scientists using Cassini’s visual and infrared mapping spectrometer (VIMS) instrument have been studying Saturn’s aurora, the equivalent of Earth’s Northern and Southern Lights.

Aurorae occur when particles in the solar wind are directed along magnetic field lines towards a planet’s poles. Funnelling down into the atmosphere, they strike gas molecules and cause an eerie, but very pretty, glow.

“Cassini’s instruments have been imaging the aurora in magnificent detail, but to understand the overall nature of the auroral region we need to make a huge number of observations—which can be difficult because Cassini observation time is in high demand,” says Dr Tom Stallard of the University of Leicester in the UK.

Time-lapse video of Saturn's aurora

This time-lapse video covers 20 Earth hours—just under two whole Saturnian days. Parts of the aurora seem synchronised with the direction of the Sun (left-hand side), while other parts appear orchestrated with Saturn's magnetic field.

So Dr Stallard and his colleagues turned to other Cassini images that weren’t specifically targeted at the aurora—but which nevertheless happened to serendipitously capture it—to compile a short video that shows the aurora’s behaviour as Saturn rotates.

“Sometimes the aurora can be clearly seen, sometimes we have to add multiple images together to produce a signal,” Dr Stallard said.

The video shows the aurora changing considerably during the over the course of Saturn’s day, which is around 10 hours and 47 minutes in Earth time.

On the left-hand side of the video—the direction towards the Sun—the aurora brightens, indicating that it is being influenced by the Sun.

Other parts of the aurora seem more connected with the planet below; specifically, with the orientation of Saturn’s magnetic field as the planet spins.

“Saturn’s aurora are very complex and we are only just beginning to understand all the factors involved,” said Stallard.

Story by Jonathan Nally, editor SpaceInfo.com.au

Images courtesy NASA / JPL / University of Leicester / University of Arizona.

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Saturn’s Siamese twin moons

Saturn's moon Epimetheus

Saturn's moon Epimetheus, as seen by the Cassini spacecraft narrow-angle camera from a distance of approximately 37,400 kilometres. A large crater dominates one half of the 116km-wide moon.

  • Epimetheus and Janus circle Saturn at almost the same distance
  • Every four years they swap orbits with each other
  • Both moons are probably loose piles of rocky, icy rubble

Epimetheus (pronounced ep-ee-MEE-thee-us) and the neighbouring moon Janus have been referred to as the Siamese twins of Saturn because they circle Saturn in nearly the same orbit. This co-orbital condition (also called a 1:1 resonance) confused astronomers, who at first could not believe that two moons could share nearly identical orbits without colliding.

The two moons lie amongst Saturn’s rings at a distance from Saturn of roughly 151,500 kilometres (94,100 miles). One moon orbits 50 kilometres (31 miles) higher (farther away from the planet) and consequently moves slightly slower than the other. The slight velocity difference means the inner satellite catches up to the other in approximately four Earth years.

When this happens, the gravitational interaction between the two pulls the inner moon it to a higher orbit. At the same time, the catching-up inner moon drags the leading outer moon backward so that it drops into a lower orbit. The result is that the two exchange places, and the nearest they approach is within 15,000 kilometres (6,200 miles).

At their most recent trade in early 2010, Epimetheus’ orbital radius dropped by approximately 80 kilometres (50 miles) while Janus’ orbit increased by only approximately 20 kilometres (12.4 miles). Janus’ orbit changed only a quarter of that of Epimetheus because Janus is four times more massive than Epimetheus.

Both of the moons are “phase locked” with Saturn, which means that one side always faces toward the planet. And being so close to Saturn, they orbit around it in less than 17 hours.

Saturn's moon Epimetheus

The Cassini spacecraft snapped this image of Epimetheus from a distance of about 107,000 kilometres.

Rubble piles

Epimetheus and Janus may have formed by the break-up of one moon. If so, it would have happened early in the life of the Saturnian system because both moons have ancient cratered surfaces, many with soft edges because of dust. They also have some grooves (similar to grooves on the Martian moon Phobos) suggesting some glancing blows from other bodies.

They are both thought to be composed of largely of water ice, but their density of less than 0.7 is much less than that of water. Thus, they are probably “rubble piles”—each a collection of numerous pieces held together loosely by gravity.

Each moon has dark, smoother areas, along with brighter areas of terrain. One interpretation of this is that the darker material evidently moves down slopes, leaving shinier material such as water ice on the walls of fractures.

Their temperature is approximately -195 degrees Celsius (-319 degrees Fahrenheit). Their reflectivity (or albedo) of 0.7 to 0.8 in the visual range again suggests a composition largely of water ice.

Epimetheus has several craters larger than 30 kilometres, including Hilaeira and Pollux.

Saturn's moon Janus

Janus, seen here, swaps orbits with Epimetheus every four Earth years.

Discovery

French astronomer Audouin Dollfus spotted a moon of Saturn on December 15, 1966, for which he proposed the name “Janus.” On December 18 of the same year, Richard Walker made a similar observation, now credited as the discovery of Epimetheus.

At the time, astronomers believed that there was only one moon, unofficially known as “Janus,” in the given orbit.

Twelve years later, in October 1978, Stephen M. Larson and John W. Fountain realised that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits. Observations by the Voyager I spacecraft confirmed this in 1980, and so Larson and Fountain officially share the discovery of Epimetheus with Walker.

The Cassini spacecraft has made several close approaches and provided detailed images of the moon since it achieved orbit around Saturn in 2004.

Nineteenth-century English astronomer John Herschel suggested that the moons of Saturn be associated with mythical brothers and sisters of Kronus, known to the Romans as Saturn. (The International Astronomical Union now controls the official naming of astronomical bodies.)

The name Epimetheus comes from the Greek god (or titan) Epimetheus (or hindsight) who was the brother of Prometheus (foresight). Together, they represented humanity. The craters on Epimetheus include Hilaeira (who was a priestess of Artemis and Athena) and Pollux (who was a warrior in The Illiad and who carried off Hilaeira).

Astronomers also refer to Epimetheus as Saturn XI and as S/1980 S3, and they refer to Janus as Saturn X and as S/1980 S1.

Epimetheus data:

  • Discovered: 1966 by R. Walker
  • Distance from Saturn: 151,422 km
  • Period of orbit around Saturn: 16.7 hours
  • Diameter: 138 x 110 x 110 km
  • Mass: 5.3 x 10^17 kg

Adapted from information issued by NASA / JPL / Space Science Institute.

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The eruptions of Enceladus

Enceladus

Ice plumes erupting from the near the south pole of Saturn's moon Enceladus.

Barely 500 kilometres wide, Saturn’s moon Enceladus has attracted a lot of attention in recent years since the discovery of geysers shooting out from near its south pole.

NASA’s Cassini spacecraft spotted plumes, large and small, spraying water ice out from many locations along what have been dubbed “tiger stripes”…fissures in the crust that spray icy particles, water vapour and organic compounds.

The image above was taken from a distance of approximately 431,000 kilometres (268,000 miles).

In the Cassini image below, over 30 individual jets of different sizes can be seen, more than 20 of which had not been seen before the image was taken.

Enceladus

Scientists have counted over 30 individual plumes shooting out from a region of fissures on Enceladus known as the "tiger stripes".

This mosaic was created from two high-resolution images that were captured by the narrow-angle camera when NASA’s Cassini spacecraft flew past Enceladus and through the jets on November 21, 2009. Imaging the jets over time will allow Cassini scientists to study the consistency of their activity.

The view was obtained at a distance of approximately 14,000 kilometres (9,000 miles), giving a resolution of 81 metres (267 feet) per pixel.

Here’s a short video on some of the recent investigations of Enceladus.

Adapted from information issued by NASA / JPL / Space Science Institute / ESA.

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Saturn’s smooth operator

Saturn's moon Telesto

The smooth surface of Saturn's moon Telesto is shown to good effect in this image captured during the Cassini spacecraft's August 27, 2009, fly-by.

Most small, rocky moons are covered in craters, having suffered millions of years of bombardment by meteoroids that happened to come along and find themselves in the wrong place at the wrong time.

Those small moons do not have volcanic, tectonic or other processes to wipe out the traces of those impacts, the craters. (By comparison, on a large world like Earth, there are lots of erosion processes—eg. wind, rain, volcanism—that have obliterated almost all traces of impact craters.)

But in the small moon stakes, Saturn’s Telesto is an exception.

Very small, only 24 kilometres long at its widest point, this moon doesn’t appear to have much in the way of craters. Rather, it seems to be covered by a layer of fine, dust-sized particles of (probably) ice, giving it a smooth appearance.

NASA’s Cassini spacecraft—which has been reconnoitering the Saturnian system since 2004—has taken several images of this mysterious moon. None of the images are very large, as Telesto is tiny and the images were taken from quite large distances.

Cassini’s closest approach to Telesto occurred in October, 2005, during which it managed to capture an image from a distance of approximately 14,500 kilometres (9,000 miles) showing detail down to 86 metres (283 feet) per pixel.

False colour view of Saturn's moon Telesto

A false colour view of Telesto reveals surface variations.

Cassini’s images reveal Telesto’s smooth nature, and show a handful of broad features such as large craters, depressions and mounds.

A special false-colour image shows subtle variations in surface texture across the face of the moon. No one’s really sure what causes the variations, but it’s probably due to small differences in the chemical composition of the ice particles, or maybe their sizes.

Another of Saturn’s moons, Pandora, has a similar dusty surface (see our earlier story on Pandora).

Telesto (pronounced teh-LESS-toh) is also a “Trojan” moon, meaning that it has an orbit that keeps it at the same distance from another body in the same orbit. In this case, Telesto always orbits 60 degrees behind the larger moon Tethys. Another moon, Calypso, orbits 60 degrees in front of Tethys. Telesto is the “leading Trojan,” Calypso is the “trailing Trojan.”

Telesto facts:

  • Discovered: in 1980 in ground-based observations by Brad Smith, Harold Reitsema, Stephen Larson and John Fountain
  • Distance from Saturn: 294,660 km (about 183,090 miles)
  • Equatorial diameter: 30 x 25 x 15 km (19 x 15.5 x 9 miles)
  • Mass: 8 x 10^17 kg

Story by Jonathan Nally, editor SpaceInfo.com.au

Images courtesy NASA / JPL / Space Science Institute.

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Moody Saturn

Saturn as seen from the Cassini spacecraft

Saturn as seen from the Cassini spacecraft during the planet's equinox in July 2009. The rings are edge-on to the Sun.

The shadows of Saturn’s rings are cast onto the planet and appear as a thin band at the equator in this image taken as the planet approached its August 2009 equinox.

Approximately every 15 years, Saturn’s experiences equinox. Just like on Earth, the equinox occurs when the Sun is directly over the equator. And because Saturn’s rings orbit around its equator, the Saturnian equinox also means that the rings are exactly edge on to the Sun.

This angle makes the rings appear significantly darken than normal, and causes anything sticking up out of the plane of the rings to look anomalously bright and to cast shadows across the rings.

These sorts of scenes are possible only during the few months before and after Saturn’s equinox, at which times Cassini’s cameras have spotted not only the predictable shadows of some of Saturn’s moons, but also the shadows of newly revealed vertical structures in the rings themselves.

The planet’s southern hemisphere can be seen through the transparent D ring in the lower right of the image. The rings have been brightened by a factor of 9.5 relative to the planet to enhance visibility.

The view overall looks toward the northern, unilluminated side of the rings from about 30 degrees above the ringplane.

See a larger version of the image here.

Images taken using red, green and blue spectral filters were combined to create this natural colour view. The images were obtained with the Cassini spacecraft wide-angle camera on July 18, 2009 at a distance of approximately 2.1 million kilometres from Saturn. Image scale is 122 kilometres (76 miles) per pixel.

Saturn facts:

  • Saturn is a gas giant planet, composed mainly of hydrogen and helium.
  • There are trace amounts of ammonia, phosphine, methane and other gases.
  • We see only the tops of Saturn’s clouds and outer atmospheric layers.
  • Underneath the clouds is thought to be a thick layer of liquid hydrogen, under which is a layer of metallic hydrogen.
  • Deep inside is thought to be a core about 12,000km wide, made of rock plus water and other gases solidified under extreme pressure.
  • The core temperature is probably 10,000 to 15,000 degrees Celsius.
  • Saturn has clouds, winds, rain, snow, storms and lightning.
  • Saturn’s overall density is less than that of water…so if you could find a lake large and deep enough, Saturn would float in it!

Adapted from information issued by NASA / JPL / Space Science Institute.

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Meet the real Pandora

Close-up view of the moon Pandora

The Cassini spacecraft's best close-up view of Saturn's F ring shepherd moon, Pandora, shows that it is coated in fine dust-sized icy material. Cassini took this image from a distance of 52,000 kilometres.

Pandora is one of over 60 moons that orbit Saturn. Discovered in 1980 in images taken by the Voyager 1 spacecraft, the tiny potato-shaped world is just 110 x 88 x 62 kilometres.

Circling the ringed planet at a distance of just under 142,000 kilometres, it takes only 15.1 hours to complete one orbit.

Pandora, along with its sibling Prometheus, is a shepherd moon, orbiting just outside Saturn’s thin F ring. The gravitational influence of the two moons helps keep the material in the F ring in check.

The small moon’s surface appears to be covered in a layer of dust-sized particles of ice. There also are a number of craters, a couple of them being around 30 kilometres wide.

Two of Saturn's moons and the F ring

Two of Saturn's small moons orbiting beyond the planet's thin F ring—Pandora on the left and Epimetheus on the right.

Pandora and Saturn's rings

An almost edge-on view of Saturn's rings, also showing Pandora and the giant ringed planet in the background.

Pandora casts its shadow upon the F ring.

Pandora casts its shadow upon the F ring.

Images courtesy NASA / JPL / Space Science Institute.

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Moons with a view

NASA’s Cassini spacecraft has been orbiting Saturn since July 2004. The ringed planet has more than 60 moons, and Cassini has taken numerous images of them.

Sometimes, when the angles are just right, Cassini’s camera can fit more than one moon into its field of view—with one moon in the background and one in the foreground.

Many of the moons orbit near or within the planet’s famous rings, so the rings often appear in the images too.

Here’s a selection of recent shots showing some of Saturn’s natural satellites, large and small.

Rhea, Prometheus and Saturn's rings

In this view, the moon Rhea (1,530km wide) is on the far side of the rings. Much smaller Prometheus (86km wide) is on the nearside, orbiting between the main portion of the rings and the thin outer F ring. Camera distance to Rhea: approx. 1.6 million km. Camera distance to Prometheus: approx. 1 million km.

Dione and Titan

The cratered and cracked moon Dione (1,120km wide) seems to hang suspended in place in front of Titan (5,150km wide) in the background. Camera distance to Dione: approx 1.8 million km. Camera distance to Titan: approx. 2.7 million km.

Tethys and Dione

Dione, in the foreground of this image, appears darker than the moon Tethys (1,070km wide). Tethys appears brighter because it has a higher albedo than Dione, meaning Tethys reflects more sunlight. Camera distance to Dione: approx. 1.2 million km. Camera distance to Tethys: 1.8 million km.

Epimetheus and Janus

Saturn's moon Epimetheus (86km wide) moves in front of the larger moon Janus (179km wide) as seen by the Cassini spacecraft. Camera distance to Epimetheus: approx. 2.1 million km. Camera distance to Janus: 2.2 million km.

Janus and Prometheus

In this image, Janus is on the far side of Saturn's rings. Prometheus is on the nearside, orbiting in the gap between the main rings and the outer, thin F ring. Camera distance to Janus: approx. 1.1 million km. Camera distance to Prometheus: 1 million km.

Images courtesy of NASA / JPL / Space Science Institute.

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