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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 – Geyser moon seen in silhouette

Saturn's moon Enceladus

Saturn's moon Enceladus appears in silhouette in this image taken by NASA's Cassini spacecraft. The bulk of Saturn is in the background, the planet's rings seen edge-on appear as the dark horizontal line.

NASA’S CASSINI SPACECRAFT took this image of Saturn’s moon Enceladus on October 19, 2011. As the spacecraft passed Enceladus, its infrared instruments, cameras and other instruments monitored activity on the moon, in particular the famed jets erupting from the its south pole. The orbiter flew within about 1,230 kilometres of Enceladus’ surface.

Although it appears dark in the silhouetted view, Enceladus, 504 kilometres wide, is actually one of the most reflective bodies in the Solar System because it is constantly coated by fresh, white particles of ice.

Also visible are Saturn’s rings, seen edge on.

More information:

Enceladus – Saturn’s shiny moon

The eruptions of Enceladus

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

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Gallery – Quintet of Saturnian moons

Five of Saturn's moons

Five of Saturn's moons can be seen in this image taken by NASA's Cassini spacecraft.

A QUINTET OF SATURN’S MOONS can be seen in this view taken by the Cassini spacecraft.

Janus (179 kilometres wide) is on the far left. Pandora (81 kilometres) orbits between the A ring and the thin F ring near the middle of the image. Brightly reflective Enceladus (504 kilometres) appears above the centre of the image. Saturn’s second largest moon, Rhea (1,528 kilometres), is bisected by the right edge of the image. The smaller moon Mimas (396 kilometres) can be seen beyond Rhea also on the right side of the image.

This view looks toward the northern, sunlit side of the rings from just above the ringplane. Rhea is closest to the camera. Saturn’s rings are beyond Rhea and Mimas. Enceladus also is beyond the rings.

The image was taken with the Cassini spacecraft narrow-angle camera on July 29, 2011. The view was acquired at a distance of approximately 1.1 million kilometres from Rhea and 1.8 million kilometres from Enceladus.

Adapted from information issued by 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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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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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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Saturn’s shepherd moon

Saturn's moon Daphnis in the Keeler Gap in Saturn's rings

One of Saturn's two shepherd moons, Daphnis (upper left), inhabits the Keeler Gap in the planet's famous ring system. The moon's gravity causes a wave-like effect on the inner and outer edge of the Gap.

Saturn has more than 60 moons of many different sizes and shapes. Most of them orbit well outside the realm of the planet’s rings, but some live within the rings.

One such is Daphnis, a tiny 7-kilometre-diameter body that circles Saturn within a gap in the rings known as the Keeler Gap, which itself is only 42 kilometres wide.

The moon is named after a figure from Greek mythology. Daphnis was a shepherd, the some of Hermes and brother of Pan. (Saturn’s other shepherd moon is named Pan.)

Daphnis orbit is not perfectly circular; it is ever so slightly elliptical. Plus, it doesn’t orbit cleanly in the same plane as the rings, but has an inclined orbit that makes it range up to about 8 kilometres above and about 8 kilometres below the ring plane.

As it zips along through the Keeler Gap, Daphnis’ gravity disturbs the material in the rings on each edge of the Gap, resulting in the edges forming a “wavy” appearance. The wave on the inside edge of the Gap moves ahead of Daphnis, while the wave on the outer edge of the Gap lags behind, due to the different speeds at which Daphnis and material in the inner and outer edges circle Saturn.

Scientists had suspected that an undiscovered moon was causing the wavy edges of the Keeler Gap, but it wasn’t until May 2005 that it was spotted. The Cassini Imaging Science Team made the discovery on May 6, 2005 from images obtained five days earlier. It was subsequently spotted in other images taken on May 2, and earlier images taken in April 2005.

The image shown here was taken on July 5, 2010 by NASA’s Cassini spacecraft, and transmitted to Earth the following day.

Story by Jonathan Nally, Editor, SpaceInfo.com.au

Image courtesy NASA / JPL / Space Science Institute.

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