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Views of Moons

NASA’S CASSINI SPACECRAFT, in orbit around the planet Saturn, has been sending back some wonderful views of its moons. In particular, it has captured images where one moon seems to float in front of the other. Here we present a selection of recent images.

Cassini image of Titan and Tethys

Can you tell which of these moons in the foreground? It's Titan, the large one (diameter 5,150 kilometres; bigger than our Moon) with the orange atmosphere, with smaller, shiny, icy Tethys in the background. Titan was 2.3 million kilometres from Titan, and 3.4 million from Tethys when it took this image. Saturn's rings can be seen edge-on in the distance.

Cassini image of Rhea and Titan

This black-and-white image shows the moon Rhea (1,528 km diameter) in front of Titan. Cassini was 2 million kilometres from Titan and 1.3 million kilometres from Rhea when it took this image.

Cassini image of Titan and Dione

This view shows Titan again, this time with the much smaller moon Dione (1,123 km diameter) peering around from behind, with Saturn and its rings (edge-on) in the background. Cassini was 2.3 million kilometres from Titan and 3.2 million kilometres from Dione when it took the image. The haze that surrounds Titan can clearly be seen. Titan has a mostly nitrogen atmosphere that extends far from the surface. The surface pressure is about 1.5 times that on Earth.

Cassini image of Titan

In this view, Titan appears to float in front of Saturn and its rings. Titan is not only the second-largest moon in the Solar System; it's also about 300 kilometres wider than the planet Mercury!

More information: Cassini mission

Story by Jonathan Nally. Images courtesy NASA / JPL-Caltech / Space Science Institute.

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Gallery – Dione and friends

Saturn's moon Dione

Saturn's moon Dione, seen along with half of Saturn and the planets rings.

SATURN’S MOON DIONE coasts along in its orbit appearing in front of its parent planet in this Cassini spacecraft view.

The wispy terrain on the trailing hemisphere of Dione (1,123 kilometres wide) can be seen on the left of the moon here.

Dione (pronounced dy-OH-nee) is the second densest moon of Saturn, after Titan. Dione is probably composed of a rocky core making up one-third of the moon’s mass, and the rest is composed of water ice. It is similar to two other Saturnian moons, Tethys and Rhea.

Dione’s icy surface includes heavily cratered terrain, with moderately and lightly cratered plains, as well as some severely cracked areas, with very bright material on the walls of the fractures. The heavily cratered terrain has numerous craters greater than 100 kilometres in diameter. The plains area tends to have craters less than 30 kilometres in diameter.

Telesto and Epimetheus

The moon Telesto can be seen above the rings on the left, and Epimetheus is just on the bottom edge of the rings.

The tiny moon Telesto (25 kilometres wide) is visible as a white speck above and to the left of the rings in this view. Epimetheus (113 kilometres) appears just below the rings near the centre of the image. This view looks toward the northern, sunlit side of the rings from just above the ringplane.

The image was taken with the Cassini spacecraft narrow-angle camera on July 18, 2011. The view was acquired at a distance of approximately 2.2 million kilometres from Dione.

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

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Gallery – Saturn’s four moon shuffle

Cassini image of four Saturnian moons

Four of Saturn's moons are visible in the image taken by NASA's Cassini spacecraft.

A QUARTET OF SATURN’S MOONS, from tiny to huge, surround and are embedded within the planet’s rings in this Cassini image. Saturn itself is out of frame to the left.

Saturn’s largest moon, Titan (5,150 kilometres wide), is in the background of the image.

Next, in the foreground is Dione (1,123 kilometres wide), with the wispy terrain on its trailing hemisphere easily visible.

The third moon is Pandora(81 kilometres wide), which orbits just beyond the rings on the right of the image.

Saturn's rings with Pan in the  Encke gap

The tiny moon Pan appears as a speck in the gap in the rings.

Finally, tiny Pan (28 kilometres wide) can just be seen as a tiny speck in the ‘Encke Gap’ of the A ring on the left of the image.

Saturn has 62 known moons, with the vast majority of them being 50 kilometres or less in diameter.

The image was taken with the Cassini spacecraft narrow-angle camera on September 17, 2011, at a distance of approximately 2.1 million kilometres from Dione.

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

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Trio of Saturnian moons

Three of Saturn's moons

Three of Saturn's moons captured by NASA's Cassini spacecraft. From left: Dione, Rhea and Enceladus.

NASA’S CASSINI SPACECRAFT snapped this image of three of Saturn’s moons and part of the planet’s rings.

Saturn is not illuminated in this image, but it can be detected as the dark patch on the left that lies behind the foreground rings but in front of the background rings. It also partially obscures the moon on the left.

That moon is Dione (1,123 kilometres wide), around 3.1 million kilometres from Cassini when this image was taken.

In the foreground is Rhea (1,528 km wide). It is closest to the camera, at a distance of about 2.2 million kilometres.

The third moon, on the right, is Enceladus (504 km wide), seen at a distance of about 3 million kilometres.

Enceladus is the source of much interest at the moment, as Cassini’s instruments have detected huge plumes of salty spray shooting up from cracks near it’s south pole, suggesting a liquid ocean lies beneath the frozen surface.

More information:

Cassini-Huygens mission

Cassini imaging team homepage

Story by Jonathan Nally, SpaceInfo.com.au. Images courtesy NASA / JPL / Space Science Institute.

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One image, five moons

Five of Saturn's moons in one image

Five of Saturn's moons appear in this single image taken by NASA's Cassini spacecraft.

NASA’s CASSINI SPACECRAFT snapped this image showing part of Saturn’s rings edge-on, and with five of the giant planet’s moons in the same frame.

The moon Rhea (1,528 kilometres in diameter) dominates the image, and is in the foreground of the tableau. Below it and appearing to sit on the rings, is Dione (1,123km wide). Dione is actually far in the background.

Just to the right of Dione is what looks like a small bump in the rings. This is actually Prometheus (86km wide), a “shepherd moon” that orbits Saturn along the inner edge of the F ring.

The tiny dot off to the right of the rings is Epimetheus (113km wide), and the larger moon right on the edge of the image is Tethys (1,062km wide). Epimetheus is very interesting, as it shares almost exactly the same orbit as another moon, Janus. In fact, their orbits are different by a factor of only 50 kilometres. And every now and then they come close together and swap positions!

Cassini was about 61,000 kilometres from Rhea when it took this image on January 11, 2011. Detail can be seen on Rhea down to about 2km per pixel.

Just so that you know what each of the moons looks like close up, here are images of them, also taken by Cassini.

Saturn's moon Rhea.

Saturn's moon Rhea.

Saturn's moon Dione.

Saturn's moon Dione.

Saturn's moon Prometheus.

Saturn's moon Prometheus.

Saturn's moon Epimetheus.

Saturn's moon Epimetheus.

Saturn's moon Tethys.

Saturn's moon Tethys.

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

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Saturn’s three-in-one

Image showing Rhea, Dione and Saturn's rings

This image shows two of Saturn's moons—Rhea (foreground, top) and Dione (background)—with the planet's famous rings in between.

THIS IMAGE ISN’T made up. It’s a real shot from NASA’s Cassini spacecraft, showing three components of the Saturnian system in the one frame.

In the foreground at top is the south polar area of Saturn’s moon Rhea. In the background is another moon, Dione, with Saturn’s almost edge-on rings in between.

Visible on Dione is its famous light-coloured ‘wispy’ terrain.

Rhea is 1,528 kilometres in diameter, and Dione is 1,123 kilometres wide.

At the moment the image was taken, on January 11, 2011, the Cassini spacecraft was about 61,000 kilometres from Rhea and 924,000 kilometres from Dione.

Detail down to a resolution of 358 is visible on Rhea, and to a resolution of six kilometres on Dione.

Story copyright 2011 Jonathan Nally, SpaceInfo.com.au. Image 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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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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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.

Saturn’s icy moon Dione

One side of Saturn's moon, Dione

Icy terrain with wispy patterns covers one side of Saturn's moon Dione.

Wispy terrain stretches across the trailing hemisphere of Saturn’s moon Dione in this view taken by NASA’s Cassini spacecraft during its January 27, 2010, non-targeted flyby.

Cassini came within about 45,000 kilometres of the moon during this flyby, but this image was acquired at a distance of approximately 137,000 kilometres from Dione. This view looks toward the side of the moon that was facing away from Saturn, and in particular its trailing hemisphere (the half of Dione that faces “backwards” as the moon orbits the planet).

Dione (pronounced dy-OH-nee) is 1,123 kilometres wide, and this image shows detail down to 819 metres per pixel.

Dione is the second densest moon of Saturn, after Titan. Dione is probably composed of a rocky core making up one-third of the moon’s mass, and the rest is composed of water ice. It is similar to two other Saturnian moons, Tethys and Rhea.

Dione’s icy surface includes heavily cratered terrain, with moderately and lightly cratered plains, as well as some severely cracked areas, with very bright material on the walls of the fractures. The heavily cratered terrain has numerous craters greater than 100 kilometres in diameter. The plains area tends to have craters less than 30 kilometres in diameter.

Contrary to what scientists had expected upon studying this fascinating moon, much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains area existing on the leading hemisphere. This anomaly suggests that during the period of heavy meteors bombardment, Dione was ‘tidally locked’ to Saturn in the opposite orientation. (A moon is tidally locked when it keeps the same face to its parent planet at all times.)

Because Dione is relatively small, an impact big enough to cause a 35-kilometre-diameter crater could have spun the moon. Since there are many craters larger than 35-kilometres, Dione could easily have been spun more than once. The moon has probably been tidally locked in its current orientation for the past several billion years.

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