You Can’t See This From Here

The Cassini spacecraft took this wide-angle view of Saturn on 28 October, 2016, when it was about 1.4 million km from the planet. This point of view is from the far side of the planet showing shadows that can’t be seen from Earth. The spacecraft has spent 13 years exploring Saturn and its moons before being de-orbited into the planet’s atmosphere..

Image Credit: NASA

Dione

Dione chasmsSome parts of the surface of Saturn’s moon Dione are covered by linear features, called chasmata, in dramatic contrast to the round impact craters that cover most moons. The bright network of fractures on Dione was seen in poor resolution Voyager images and was called “wispy terrain.” The actual nature of this terrain was unclear until Cassini photos showed we weren’t seeing something like surface deposits of frost but a pattern of bright icy cliffs among myriad fractures. This stress pattern may be related to Dione’s orbital evolution and the effect of tidal stresses over time.

Image Credit: NASA

Sunlight on a Lake

titan_lake_flashTwo bodies in the Solar System have freely flowing liquids on their surface. The Earth has water. Saturn’s largest moon Titan is too cold for liquid water, but it does have liquid methane.

This image shows a flash of sunlight reflected off a lake on Titan. Its northern hemisphere is shrouded in darkness for nearly 15 years, but the sun begins to illuminate the area again as it approaches its spring equinox. The Cassini spacecraft was able to detect the glint at the beginning of Titan’s spring in 2009. The moon’s hazy atmosphere scatters and absorbs many wavelengths of light, including most of the visible spectrum. But an onboard instrument was able to detect the glint in infrared wavelengths that can penetrate through Titan’s atmosphere. This image was created using wavelengths of light in the 5 µm range.

Image Credit: NASA

A Couple of Moons

Dione_MimasBecause of the angle that the Sun’s light is falling on them, Mimas (right) and Dione (left) appear to be staring up at Saturn looming in the background of this image captured by the Cassini spacecraft.

Although certainly large enough to be noticeable, moons like Mimas (396 km across) and Dione (1123 km across) are tiny compared to Saturn (120,700 km across). Even the enormous moon Titan (5,150 kilometers across, larger than the planet Mercury) would be dwarfed by the giant planet in such a picture.

Image Credit: NASA

Titan Over Saturn

The Cassini spacecraft took this picture of Saturn’s Moon Titan looking at the side that always faces away from the planet because the moon’s orbit is tidal locked (like Earth’s Moon’s) Titan is the only moon in the Solar System with a dense atmosphere (visible in this picture), liquid on its surface, and a cycle of evaporation and liquid rain.

Image Credit: NASA

The Storm Around Saturn’s North Pole

Saturn_hurricane1The Cassini spacecraft relayed to Earth thist close-up, visible-light views of a monster hurricane swirling around Saturn’s north pole. It’s 2,000 km wide, 20 times larger than the average hurricane eye on Earth. The clouds at the outer edge of the hurricane are moving at 150 meters per second as the storm swirls inside a large, six-sided weather pattern known as the hexagon. This false-color image highlights the storm at Saturn’s north pole.

Saturn_hurricane3A natural color image is at left.

Hurricanes on Earth feed off warm ocean water, but there is no body of water on Saturn. Learning how these Saturnian storms use water vapor might tell scientists more about how terrestrial hurricanes are generated and sustained. Both terrestrial hurricanes and Saturn’s north polar vortex have a central eye with no clouds or very low clouds. Both have high clouds forming an eye wall, other high clouds spiraling around the eye, and a counter-clockwise spin in the northern hemisphere. But the one on Saturn is much bigger than its counterparts on Earth and spins surprisingly fast. On Saturn, the wind in the eye wall blows more than four times faster than hurricane-force winds on Earth. Terrestrial hurricanes move around, but Saturn’s hurricane is parked over the planet’s north pole. On Earth, hurricanes tend to drift northward because of the forces acting on the winds as the planet rotates. Saturn’s does not drift; it’s already as far north as it can be.

Saturn_hurricane2Scientists believe the massive storm has been churning for years. When Cassini arrived in the Saturn system in 2004, Saturn’s north pole was in winter darkness. Cassini‘s composite infrared spectrometer and visual and infrared mapping spectrometer detected the great vortex, but a visible-light image had to wait for the equinox in 2009 when sunlight begin falling on the higher latitudes of Saturn’s northern hemisphere.

Image Credits: NASA

Iapetus

Iapetus is the third-largest satellite of Saturn and the eleventh-largest in the Solar System. Iapetus has a bright hemisphere and a dark hemisphere. Because it is tidally locked, it always keeps the same face towards Saturn. The bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and the dark hemisphere is visible when Iapetus is on the eastern side.

This image was pieced together from data taken by the Cassini spacecraft.

Image Cedit: NASA

Ring Shepherds

moons_at_workSaturns rings are divided into distinct bands. The Saturnian moons Prometheus and Pan are both caught “shepherding” their respective rings in this image (click the image to embiggen it). Through their gravitational effect on nearby ring particles, one moon maintains a gap in the outer A ring, and the other helps keep another ring narrowly confined.

Prometheus (86 km across) and its partner Pandora (not seen here), maintains the narrow F ring seen at the bottom left in this image. Pan (28 km across) clears the Encke gap in as it moves along the gap’s center. You may not be able to see Pan unless you embiggen the image. The other bright dot near the inner edge of the Encke gap is a star in the background.

Image Credit: NASA

A Moon with Weather and Erosion

titan_cratersTitan is the only moon in the solar system with a thick atmosphere, and the only world besides Earth known to have lakes and seas on its surface. However, with a frigid surface temperature of around -290° F (94 K), the rain falling on Titan isn’t water. It’s liquid methane and ethane, compounds that are gases at room temperature on Earth.

Most of Saturn’s moons display their ancient faces pockmarked by thousands of craters. Titan, Saturn’s largest moon, looks younger than it really is because its craters are being eroded. Radar observations by the Cassini spacecraft show that dunes of hydrocarbon sand are filling in the craters.

This image taken with the Cassini radar shows two craters on Titan. On the left is crater Sinlap which is a relatively ‘fresh’ crater, with a depth-to-diameter ratio similar to is found on other large moons in the solar system such as Ganymede. One the right is Soi, an extremely eroded crater with a very small depth compared to similar craters on Ganymede. These craters are both about 80 km (almost 50 miles) in diameter.

Image Credit: NASA

Janus, Rhea, and Rings

This snapshot was taken by the Cassini spacecraft. The small moon that seems to be hovering over Saturn’s rings is Janus. It’s only about 180 km across. The larger moon is Rhea, which is around 1500 km across. Saturn’s thin outer F ring is visible in front of Rhea, and the top of the moon is visible between the larger A and B rings.

Image Credit: NASA

You’re On One of the Pixels in this Image

newrings_cassiniWell, you are if you’re over 15 years old. Back in 2006, the Cassini spacecraft orbiting Saturn drifted in giant planet’s shadow and looked back toward the eclipsed Sun. Saturn’s rings lit up so much that new rings were discovered, although they are hard to see in this image. Saturn’s E ring, the ring created by the newly discovered ice-fountains of the moon Enceladus and the outermost ring visible above, does show up in vivid detail. Far in the distance, at about 10 o’clock on the left, just above the bright main rings, is the almost ignorable pale blue dot of Earth. You may have to click on the image to embiggen it in order to see the Earth.

Image Credit: NASA

Dione

The Cassini spacecraft snapped this picture of the moon Dione orbiting Saturn. At 1122 km in diameter, Dione is the 15th largest moon in the Solar System. Its interior is probably a combination of equal masses of silicate rock and water ice.

Shape and gravity observations collected by Cassini suggest the moon has a  core of around 400 km of rock surrounded by a roughly 160 km envelope of water, probably in the form of ice. However, some models suggest the lowermost part of this layer could be in the form of an internal liquid salt water ocean.

Image Cedit: NASA

Smog on Saturn

north-poleThese two natural color images taken by the Cassini spacecraft show how Saturn’s north polar region has changed between 2012 and 2016. The color change is thought to be an effect of Saturn’s seasons. It’s believed that the change from a bluish color to a more golden hue is caused by the increased production of smog in the atmosphere as the north pole approached the summer solstice due in May, 2017.

The hexagon, Saturn’s six-sided jetstream, seems to act as a barrier preventing haze particles produced outside it from entering. If that’s the case, the polar atmosphere becomes clear of aerosols produced by photochemical reactions, reactions caused by sunlight, during the winter darkness. After Saturn’s northern spring equinox, the north pole polar is in continuous sunshine, and smog aerosols can be produced inside the hexagon, making the polar atmosphere appear hazy.

Image Credit: NASA

You Are Here

earthmoon_cassini_960This is the Earth-Moon system as seen by the Cassini spacecraft orbiting Saturn in the outer Solar System. Earth is the larger of the two spots near the center; the Moon is to its lower left. This raw, unprocessed image shows several streaks that are not stars. They are cosmic rays that struck the digital camera while it was taking the picture.

Image Credit: NASA

Tethys Takes a Look at Saturn

Tethys and SaturnIn this picture taken by the Cassini spacecraft the two large craters on Tethys near the line where day fades to night seem to be looking at Saturn. (Click the image to embiggen it.)

The shadowing on the craters caused by being near Tethys’ terminator throws their topography into sharp relief. The larger, southernmost of the two shows a more complex structure. Its central peak is  probably the result of the surface reacting to the violent post-impact excavation of the crater. The northern crater doesn’t have a similar feature. The impact was likely too small to form a central peak, or the composition of the material in the immediate vicinity couldn’t support the formation of a central peak.

Image Credit: NASA

Mimas

Mimas is one of Saturns moons. It’s about 130 km in diameter, one of the smallest bodies in the Solar System with sufficient gravity to pull itself into a spherical shape. This picture was taken in 2010 by the Cassini spacecraft.

The Gentle Reader may make his own moon-not-a-space-station or AT&T-naming-rights jokes.

Image Credit: NASA

Hyperion

HyperionHyperion is one of Saturn’s moons. It is named for one of the Titans who was the Greek god of watchfulness and observation and the older brother of Cronus. Saturn was the analog of Cronus in Roman mythology.

Hyperion is one of the largest irregularly shaped bodies in the Solar System, and it rotates chaotically, tumbling unpredictably as it orbits Saturn. That made it  challenging to target a specific region of the moon’s surface for observation by the Cassini spacecraft, and most of Cassini‘s approaches saw the same side of the craggy moon. The view above is from a closest encounter in 2005.

BTW, the first time I saw this picture, I was reminded of a wasps’ nest.

Image Credit: NASA

B and C in False Color UV

RingsInUVOn 1 July, 2004, the Cassini spacecraft arrived at Saturn, marking the end of the spacecraft’s nearly seven-year journey through the solar system and the beginning of its tour of Saturn and the planet’s rings and moons.

This picture was taken in ultraviolet on 30 June, 2004 during Cassini’s orbital insertion maneuver. It shows, from left to right, the outer portion of the C ring and inner portion of the B ring which begins a little more than halfway across the image. The “dirty” particles are indicated by red, and “cleaner: ice particles shown in turquoise.

Saturn’s ring system is labeled from the inside out with the D, C, B and A rings followed by the F, G and E rings.

Image Credit: NASA

That’s No Space Station. It’s a Moon.

TethysSaturn’s moon Tethys’s trailing side shows two terrains that tell a story of a rough past. To the north (up in this picture) is older, rougher terrain, while to the south is new material dubbed “smooth plains” by scientists. The smooth plains are roughly antipodal to the large impact crater Odysseus. Odysseus, which is on the far side of Tethys, is out of view. The leading theory is that the impact that created Odysseus also created the smooth plains, although exactly how this happened is not yet clear.

Image Credit: NASA

A Speck in the Corner

Saturn & TethysAt 116,500 km across, Saturn is roughly 10 times the diameter of Earth. The planet is much larger in relation to its moons than our Earth to its Moon. Saturn’s moon Tethys (which is a bit more than 1,000 km in diameter and could be counted as a dwarf planet it orbited the Sun by itself) can be seen as a speck in the lower right of the picture.

Image Credit: NASA

A Lunar Lineup

Enceladus_Tethys_bullseyEnceladus and Tethys line up almost perfectly in this shot from the Cassini spacecraft. Since the two moons are not only aligned, but also at nearly the same distance from Cassini, their apparent sizes are a reasonable approximation of their relative sizes. Enceladus is 504 km across, and Tethys is 1,062 km in diameter.

Image Credit: NASA