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Which moon is this?

The surface of Saturn's moon Dione

Craters and fracture lines cover the surface of Saturn's moon Dione in this image taken by NASA's Cassini spacecraft.

  • Saturn’s heavily cratered moon Dione
  • Always keeps the same face toward Saturn
  • Seems to have been spun around in the past

At first glance it looks a bit like Earth’s Moon, but it’s actually Saturn’s moon Dione.

NASA’s Cassini spacecraft took this close-up view of the cratered, fractured surface on January 27, 2010. Cassini came within about 45,000 kilometres (28,000 miles) of the moon during the flyby, and this image was acquired at a distance of approximately 46,000 kilometres (29,000 miles). It shows detail down to about 270 metres (886 feet) per pixel.

Dione (pronounced Die-OH-nee) is a small moon of 1,118 kilometres (695 miles) diameter that orbits Saturn every 2.7 days at a distance of 377,400 kilometres (234,000 miles), which is roughly the same distance that the Moon orbits around the Earth.

Its features include heavily cratered terrain with craters as large as 100 kilometres (62 miles) across, plus other moderately cratered plains, lightly cratered plains, and fractured areas.

The heavily cratered areas are most common on the trailing hemisphere. Logically, a moon’s leading hemisphere should be the more heavily cratered—just like your car’s windscreen collides with more insects that its back window—so it has been suggested that an impact with another body spun Dione around. It has been calculated that bodies as small as those that made 35-kilometre (22-mile) craters could have spun Dione on its axis.

However, the fact that Dione seems to have spun exactly 180 degrees is a mystery.

A wispy, icy moon

Fractured areas, seen in Voyager spacecraft images as bright thin wispy lines, have lengths of tens to hundreds of kilometres, often cutting through plains and craters. Cassini fly-bys starting in 2005 showed “the wisps” as bright canyon ice walls (some of them several hundred metres high), probably caused by subsidence cracking. The walls are bright because darker material falls off them, exposing the bright water ice underneath. These fracture cliffs suggest Dione experienced tectonic activity in its past. They could be a mature phase of the so-called “tiger stripes” on one of Saturn’s other moons, Enceladus.

Wispy terrain on Dione

Wispy terrain stretches across the trailing hemisphere of Saturn's moon Dione. The wisps are caused by bright ice lining canyon walls.

Very fine ice powder (equivalent to cigarette smoke) from Saturn’s E-ring constantly bombards Dione. The dust in the E-ring originally comes from Enceladus, which has prominent geyser activity.

Dione’s density is 1.48 times that of liquid water, suggesting that about a third of the moon is a dense core (probably silicate rock) and the rest is ice. At Dione’s extremely cold average temperature, ice is very hard and behaves like rock.

As with Earth’s Moon, Dione is “phase locked” with its parent, which means the same side always faces toward Saturn. Likewise, Dione has gravitationally locked two much smaller moons—Helene orbits Saturn 60 degrees ahead of Dione, and Polydeuces orbits Saturn 60 degrees behind Dione.

Dione is in “resonance” with two nearby moons, Mimas 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 helps keep them locked in their positions. Dione keeps Enceladus locked at a period exactly one half of the Dione orbit.

Dione’s discovery

The Italian astronomer Giovanni Cassini discovered Dione in 1684. The English astronomer John Herschel suggested that the moons of Saturn be associated with Greek mythical brothers and sisters of Kronus, known to the Romans as Saturn.

The name Dione comes from the Greek goddess (or titan) Dione, who by some accounts was the daughter of Tethys and Oceanus and who Homer described as the mother of Aphrodite.

Cassini referred to Dione as one of the Sidera Lodoicea (Stars of Louis) after King Louis XIV (the other three were Iapetus, Tethys, and Rhea). Other astronomers named the moons of Saturn by number in terms of distance from the planet. Thus, Dione was Saturn IV.

The International Astronomical Union controls naming of astronomical bodies. Geological features on Dione generally are given names from people and places in Virgil’s Aeneid.

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

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The twist in Saturn’s rings

A propeller-shaped disturbance in Saturn's rings

A propeller-shaped structure formed by the influence of an unseen moon on the particles in Saturn's rings, is brightly illuminated by sunlight in this image obtained by NASA’s Cassini spacecraft.

  • Dozens of unseen moons within Saturn’s rings
  • Cause surrounding ring material to twist and warp
  • Twists look like giant propellers

Scientists using NASA’s Cassini spacecraft—currently orbiting through the Saturnian system—are stalking a new class of moons that create distinctive “propeller-shaped” gaps in Saturn’s rings.

Scientists first discovered double-armed propeller features in 2006 in an area now known as the “propeller belts” in the middle of Saturn’s outermost dense ring, the A ring.

The gaps in the rings are created by a new class of moonlets—smaller than known moons, but larger than the particles in the rings—that could clear the space immediately around them.

Those moonlets, estimated to number in the millions, are not large enough to have cleared out their entire path around Saturn, as do the moons Pan and Daphnis.

The new research, led by Matthew Tiscareno, a Cassini imaging team associate based at Cornell University, reveals a new cohort of larger and rarer moons in another part of the A ring farther out from Saturn.

A propeller-shaped disturbance in Saturn's rings

A "propeller" is brightly illuminated on the sunlit side of Saturn's rings, where the ring has been disturbed and ring material boosted above the ringplane.

With “propellers” as much as hundreds of times as large as those previously described, the scientists have been able to track them for as long as four years.

The features are up to several thousand kilometres long and several kilometres wide. The gravitational effect of the moons embedded in the ring appears to kick up ring material as high as 500 metres above and below the flat ring plane, which is well beyond the typical ring thickness of about 10 metres.

Dozens of giant “propellers”

Cassini is too far away to see the moons amid the swirling ring material around them, but scientists estimate that they are about a kilometre in diameter because of the size of the propellers.

Tiscareno and colleagues estimate that there are dozens of these giant propellers; 11 of them were imaged multiple times between 2005 and 2009.

One of them, nicknamed Bleriot after the famous aviator Louis Bleriot, has been a veritable Forrest Gump, showing up in more than 100 separate Cassini images and one ultraviolet imaging spectrograph observation over this time.

“Scientists have never tracked disc-embedded objects anywhere in the universe before now,” Tiscareno said. “All the moons and planets we knew about before orbit in empty space.”

“In the propeller belts, we saw a swarm in one image and then had no idea later on if we were seeing the same individual objects,” added Tiscareno. “With this new discovery, we can now track disc-embedded moons individually over many years.”

“Propellers give us unexpected insight into the larger objects in the rings,” said Linda Spilker, Cassini project scientist based at NASA’s Jet Propulsion Laboratory. “Over the next seven years, Cassini will have the opportunity to watch the evolution of these objects and to figure out why their orbits are changing.”

A short movie of one of Saturn's propellers

A short movie of one of Saturn's propellers

A glimpse into the Solar System’s past

The observations also mark the first time scientists have been able to track the orbits of individual objects in a “debris disc”, which is what the rings are—billions upon billions of chunks of ice of all sizes, encircling the planet.

This gives scientists an opportunity to “time-travel” back into the history of our Solar System, when the planets were forming within a much bigger version of Saturn’s rings, circling the youthful Sun.

“Observing the motions of these disc-embedded objects provides a rare opportunity to gauge how the planets grew from, and interacted with, the disc of material surrounding the early Sun,” said Carolyn Porco, Cassini imaging team lead based at the Space Science Institute.

“It allows us a glimpse into how the Solar System ended up looking the way it does.” And, by extension, what by might be happening in other star systems.

Adapted from information issued by NASA / JPL / SSI.

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Titan gets a visitor

Artist's impression of Cassini passing Titan

NASA's Cassini probe will conduct a close fly-by of Titan on July 7, swooping to within about 1,000 kilometres of its surface.

NASA’s Cassini spacecraft is to conduct a close fly-by of Titan, Saturn’s largest moon, on July 7. The craft will swoop to within 1,005 kilometres of the cloud-covered world, shooting past at a speed of 5.9 km per second (21,240km/h or 13,000mph).

During the close approach, instruments will study the chemical make-up of its atmosphere, while Cassini’s radar will scan a poorly-covered region of the moon. Other instruments will keep an eye on clouds in Titan’s atmosphere.

A black and white view of Titan

A black and white view of Titan, showing the dark region known as Senkyo.

Ice world with a thick atmosphere

In many respects Saturn’s largest moon is one of the most Earth-like worlds found to date. With its thick atmosphere and organic-rich chemistry, Titan resembles a frozen version of Earth, several billion years ago, before life began pumping oxygen into our atmosphere.

Titan is of great interest to scientists because it has a substantial, active atmosphere and complex, Earth-like processes that shape its surface. The moon is enveloped by an orange haze of naturally produced photochemical smog that frustratingly obscured its surface prior to Cassini’s arrival. Since 2004, the spacecraft’s observations have taken the study of this unique world into a whole new dimension.

Cassini has revealed that Titan’s surface is shaped by rivers and lakes of liquid ethane and methane (the main component of natural gas), which forms clouds and occasionally rains from the sky as water does on Earth. Winds sculpt vast regions of dark, hydrocarbon-rich dunes that girdle the moon’s equator and low latitudes. Volcanism may occur as well, but with liquid water as the lava.

First landing in the outer Solar System

Rounded river rocks on Titan and Earth.

Rounded river rocks on Titan (left) and Earth.

On its journey to Saturn, Cassini carried the European-built Huygens probe. On January 14, 2005, Huygens achieved humankind’s first landing on a body in the Outer Solar System when it parachuted through Titan’s murky skies. Huygens took measurements of atmospheric composition and wind speeds during its decent, along with an incredible series of images showing telltale patterns of erosion by flowing liquid. The probe came to rest on what appeared to be a floodplain, surrounded by rounded cobbles of water ice.

As the now-renamed Cassini Equinox Mission progresses, the spacecraft will monitor Titan’s atmosphere and surface for signs of seasonal change. The spacecraft’s radar and camera systems will continue to peer through the haze, expanding high-resolution maps of the surface. And scientists will eagerly await new data that could confirm the presence of a liquid ocean beneath the giant moon’s surface.

Adapted from information issued by NASA / JPL / Space Science Institute / ESA / University of Arizona / S.M. Matheson.

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Shadows on Saturn

The shadows of Enceladus (left) and Titan (right) on Saturn's cloud tops.

The shadows of Enceladus (left) and Titan (right) on Saturn's cloud tops.

These two views from NASA’s Cassini spacecraft, currently in orbit around Saturn, show the huge difference in scale between it’s largest Moon, Titan, and a smaller one, Enceladus—even though the moons themselves are not in view.

On the left is a view taken with Saturn”s rings almost edge-on. On the planet’s clouds, just below the rings, can be seen a dark spot—this is the shadow being cast by Enceladus. The moon itself is a long way off to the left and not visible in this frame. Enceladus is about 500 kilometres in diameter.

On the right is another view with almost the same geometry, but this time there is a huge shadow on Saturn’s clouds, stretched out by the curve of the planet. This is the shadow of Titan, Saturn’s largest planet and one that is currently the target of many investigations.

Titan has a thick, nitrogen atmosphere, similar to what Earth’s atmosphere is thought to have been like billions of years ago. Titan is 10 times bigger than Enceladus, with an average diameter of 2,576 kilometres.

A view of the surface of Titan, taken by the Huygens probe

A view of the surface of Titan, taken by the Huygens probe after it landed on January 14, 2005

On January 14, 2005, the Huygens probe—which had been carried by Cassini all the way from Earth—descended through Titan’s clouds and landed safely on its surface. It found a frozen world, but one that sometimes experiences rain and rivers of methane and ethane at super-cold temperatures.

In just a couple of days from now, July 7, Cassini will make another close fly-by of Titan—swooping over the moon at a distance of only 1,005 kilometres—and will train its suite of instruments on the thick clouds and frozen surface.

The left-hand view above was taken from a distance of around 1.7 million kilometres from Saturn, while the right-hand view was from around 2.1 million kilometres.

Story by Jonathan Nally, Editor,

Images courtesy NASA / JPL / Space Science Institute.

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Saturn’s tiny moon Helene

Saturn's tiny moon Helene

Saturn's tiny moon Helene is just 33km wide, and shares an orbit with a larger moon.

The Cassini spacecraft snapped this image during the spacecraft’s closest flyby of Saturn’s moon Helene, on March 3, 2010.

Helene—just 33 kilometres, or 21 miles, wide—leads the much larger Dione by 60 degrees in the two moons’ shared orbit around Saturn. This makes Helene a “Trojan” moon of Dione, named for the Trojan asteroids that orbit 60 degrees ahead of and behind Jupiter as the giant planet circles the Sun.

The lit terrain seen here is on the side of Helene that faces away from Saturn. The southern pole of the moon is in the lower right of the image.

The image was taken in visible light with the Cassini spacecraft’s wide-angle camera. The view was obtained at a distance of approximately 1,900 kilometres (1,200 miles). The scale in the original image was 235 metres (771 feet) per pixel, but the image has been magnified by a factor of two and contrast-enhanced to aid visibility.

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

Saturnian moons line up

Image showing the moons Rhea and Epimetheus with Saturn and its rings in the background.

Image showing the moons Rhea and Epimetheus with Saturn and its rings in the background.

This amazing black and white image shows two of Saturn’s moons, Rhea and Epimetheus, against a backdrop of the planet and its rings.

Saturn has more than 60 known moons, each a different size and orbiting at different distances from the planet. They orbit at different speeds, and often overtake each other, leading to views like this when the Cassini spacecraft’s camera is pointed in the right direction.

Although they look close, the two moons are actually far apart. The view was obtained at a distance of approximately 1.2 million kilometres (746,000 miles) from Rhea, while Epimetheus is 400,000 kilometres further away at 1.6 million kilometres (994,000 miles).

The image gives a good indication of the scale of things in the Saturnian system. At 1,528 kilometres (949 miles) diameter, Rhea is by no means Saturn’s largest moon, yet it is more than one-tenth the width of Earth. Compare that with the huge bulk of Saturn in the background.

Epimetheus is tiny, only 113 kilometres (70 miles) wide.

At Cassini’s huge distance when it took this image, detail as small as 7 kilometres (4 miles) per pixel can be seen on Rhea, and 10 kilometres (6 miles) per pixel on Epimetheus.

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

Life on Titan could eat acetylene

Artist's concept of a lake on the surface of the moon Titan

This artist concept shows a mirror-smooth lake on the surface of the smoggy moon Titan. Cassini scientists have concluded that at least one of the large lakes observed on Saturn's moon Titan contains liquid hydrocarbons, and have positively identified ethane. This result makes Titan the only place in our Solar System beyond Earth known to have liquid on its surface.

  • Chemicals are disappearing on Titan
  • Could be food for primitive life

Strange chemistry on Saturn’s moon Titan could indicate the presence of primitive life, say scientists.

While non-biological chemistry offers one possible explanation, some scientists believe the chemical indications bolster the argument for a primitive, exotic form of life or precursor to life on Titan’s surface.

One key finding shows hydrogen molecules flowing down through Titan’s atmosphere and disappearing at the surface. Another is that maps of hydrocarbons on the surface show a lack of acetylene, commonly known as welding gas.

The lack of acetylene is important because that chemical would likely be the best energy source for a methane-based life on Titan, said Chris McKay, an astrobiologist at NASA Ames Research Centre, who proposed a set of conditions necessary for this kind of methane-based life on Titan in 2005.

One interpretation is that the acetylene is being consumed as food. But McKay said the flow of hydrogen is even more critical because all of the proposed life mechanisms involved the consumption of hydrogen.

Titan as seen by the Cassini spacecraft

Saturn's moon Titan is very cold and smothered in hydrocarbon smog.

“We suggested hydrogen consumption because it’s the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth,” McKay said. “If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth.”

Life in deep-freeze

To date, methane-based life forms are only hypothetical. Scientists have not yet detected this form of life anywhere, though there are liquid-water-based microbes on Earth that thrive on methane or produce it as a waste product.

On Titan, where temperatures are around 90 Kelvin (minus 283 degrees Celsius), a methane-based organism would have to use a substance that is liquid for living processes, but not water itself. Water is frozen solid on Titan’s surface and much too cold to support life as we know it.

“Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed,” said Mark Allen, principal investigator with the NASA Astrobiology Institute Titan team.

“We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process, without biology, can explain these results—for example, reactions involving mineral catalysts.”

Adapted from information issued by NASA / JPL.

Rings and moons

Cassini spacecraft image of Rhea, Prometheus and Saturn's rings.

Cassini spacecraft image of Rhea, Prometheus and Saturn's rings.

  • Rhea & Prometheus
  • Saturn’s rings seen edge-on
  • Images by the Cassini spacecraft

From just below the plane of Saturn’s thin rings, the Cassini spacecraft took this image of the rings edge-on with the planet’s second largest moon, Rhea, beyond.

Although Rhea may appear to be in the foreground of this image, it isn’t. The rings are closer. The small moon Prometheus, orbiting between the A ring and the thin F ring, is also visible within the rings near the upper middle of the image.

This view looks toward the Saturn-facing side of Rhea (1,528 kilometres wide) and the leading hemisphere of Prometheus (86 kilometres wide). This view looks toward the southern, unilluminated side of the rings from just below the ringplane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on January 31, 2010. The view was obtained at a distance of approximately 2.5 million kilometres from Rhea and approximately 2 million kilometres from Prometheus. Image scale is 15 kilometres per pixel on Rhea and 12 kilometres per pixel on Prometheus.

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

Saturn’s super snowstorm

Amateur astronomer Christopher Go took this image of the storm on Saturn on March 13, 2010.

Amateur astronomer Christopher Go took this image of the storm on March 13, 2010. The arrow indicates the location of the storm and the red outlines show which area the Cassini spacecraft's infrared spectrometer studied.

With the help of amateur astronomers, the composite infrared spectrometer instrument aboard NASA’s Cassini spacecraft has taken its first look at a massive blizzard in Saturn’s atmosphere. The instrument collected the most detailed data to date of temperatures and gas distribution in that planet’s storms.

The data showed a large, turbulent storm, dredging up loads of material from the deep atmosphere and covering an area at least five times larger than the biggest blizzard in this year’s Washington, D.C.-area storm front nicknamed “Snowmageddon.”

“We were so excited to get a heads-up from the amateurs,” said Gordon Bjoraker from at NASA’s Goddard Space Flight Centre, and a team member for Cassini’s composite infrared spectrometer. Normally, he said, “Data from the storm cell would have been averaged out.”

Send in the amateurs

Cassini’s radio and plasma wave instrument and imaging cameras have been tracking thunder and lightning storms on Saturn for years in a band around Saturn’s mid-latitudes nicknamed “storm alley.”

But storms can come and go on a time scale of weeks, while Cassini’s imaging and spectrometer observation schedules have to be locked in place months in advance.

The radio and plasma wave instrument regularly picks up electrostatic discharges associated with the storms, so team members have been sending periodic tips to amateur astronomers, who can quickly go to their backyard telescopes and try to see the bright convective storm clouds.

Amateur astronomer Anthony Wesley obtained this image of a storm on Saturn

Amateur astronomer Anthony Wesley obtained this image of a storm on Saturn from his backyard telescope in Murrumbateman, Australia, on March 22, 2010. He sent it to scientists working with NASA's Cassini spacecraft the next day.

Amateur astronomers including Anthony Wesley, Trevor Barry and Christopher Go got one of those notices in February and were able to take dozens of pictures over the next several weeks.

In late March, Wesley, an amateur astronomer from Australia who was actually the first person to detect the new dark spot caused by an impact on Jupiter last summer, sent Cassini scientists an e-mail with a picture of the storm.

“I wanted to be sure that images like these were being seen by the Cassini team just in case this was something of interest to be imaged directly by Cassini or the Hubble Space Telescope,” Wesley wrote.

Lightning down below

Cassini scientists eagerly pored through the images, including a picture of the storm at its peak on March 13 by Go, who lives in the Philippines.

By a stroke of luck, the composite infrared spectrometer happened to be targeting the latitude of the storms. The instrument’s scientists knew there could be storms there, but didn’t know when they might be active.

Data obtained by the spectrometer on March 25 and 26 showed larger than expected amounts of phosphine, a gas typically found in Saturn’s deep atmosphere and an indicator that powerful currents were dredging material upward into the upper troposphere.

The spectrometer data also showed another signature of the storm — the tropopause, the dividing line between the serene stratosphere and the lower, churning troposphere, was about 0.5 degree Celsius colder in the storm cell than in neighbouring areas.

“A balloonist floating about 100 kilometres down from the bottom of Saturn’s calm stratosphere would experience an ammonia-ice blizzard with the intensity of Snowmageddon,” said Brigette Hesman, a composite infrared spectrometer team member who is an assistant research scientist at the University of Maryland.

“These blizzards appear to be powered by violent storms deeper down — perhaps another 100 to 200 kilometers down — where lightning has been observed and the clouds are made of water and ammonia.”

Adapted from information issued by JPL. Image credits: Christopher Go / Anthony Wesley / NASA / JPL-Caltech / GSFC / Mattias Malmer and Cassini Imaging Team.

Double-moon fly-by at Saturn

Artist's impression of Cassini in orbit around Saturn.

Artist's impression of the Cassini spacecraft in orbit around Saturn.

In a special double flyby this week, NASA’s Cassini spacecraft will visit Saturn’s moons Titan and Dione within a period of about a day and a half, with no manoeuvres in between.

A fortuitous orbital alignment allows Cassini to attempt this doubleheader, and the interest in swinging by Dione influenced the design of its extended mission.

The Titan flyby, planned for Monday, April 5, will take Cassini to within about 7,500 kilometres (4,700 miles) of the moon’s surface. The distance is relatively long as far as encounters go, but it works to the advantage of Cassini’s imaging science subsystem.

Cassini’s cameras will be able to stare at Titan’s haze-shrouded surface for a longer time and capture high-resolution pictures of the Belet and Senkyo areas, dark regions around the equator that ripple with dunes.

In the early morning of Wednesday, April 7 in UTC time zones, which is around 9 pm on Tuesday, April 6 in California, Cassini will make its closest approach to the medium-sized icy moon Dione. Cassini will plunge to within about 500 kilometres (300 miles) of Dione’s surface.

Saturn's moon Dione

A detailed image of Saturn's moon Dione taken during Cassini's close approach on December 14, 2004. To the surprise of scientists, the wispy terrain is not thick ice deposits, but rather bright ice cliffs created by tectonic fractures.

This is only Cassini’s second close encounter with Dione. The first flyby in October 2005, and findings from the Voyager spacecraft in the 1990s, hinted that the moon could be sending out a wisp of charged particles into the magnetic field around Saturn and potentially exhaling a diffuse plume that contributes material to one of the planet’s rings.

Like Enceladus, Saturn’s more famous moon with a plume, Dione features bright, fresh fractures. But if there were a plume on Dione, it would certainly be subtler and produce less material.

Cassini plans to use its magnetometer and fields-and-particles instruments to see if it can find evidence of activity at Dione. Thermal mapping by the composite infrared spectrometer will also help in that search. In addition, the visual and infrared mapping spectrometer will examine dark material found on Dione. Scientists would like to understand the source of this dark material.

Cassini has made three previous double flybys and another two are planned in the years ahead. The mission is nearing the end of its first extension, known as the Equinox mission. It will begin its second mission extension, known as the Solstice Mission, in October 2010.

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

Image credit: NASA / JPL / Space Science Institute