Category Archives: Cassini

Summer Skies on Saturn


Be sure the click the image to get the larger version.

The original caption:

These two natural color images from NASA’s Cassini spacecraft show the changing appearance of Saturn’s north polar region between 2012 and 2016.

Scientists are investigating potential causes for the change in color of the region inside the north-polar hexagon on Saturn. The color change is thought to be an effect of Saturn’s seasons. In particular, the change from a bluish color to a more golden hue may be due to the increased production of photochemical hazes in the atmosphere as the north pole approaches summer solstice in May 2017.

Researchers think the hexagon, which is a six-sided jetstream, might act as a barrier that prevents haze particles produced outside it from entering. During the seven-year-long Saturnian winter, the polar atmosphere became clear of aerosols produced by photochemical reactions — reactions involving sunlight and the atmosphere. Since the planet experienced equinox in August 2009, the polar atmosphere has been basking in continuous sunshine, and aerosols are being produced inside of the hexagon, around the north pole, making the polar atmosphere appear hazy today.

Other effects, including changes in atmospheric circulation, could also be playing a role. Scientists think seasonally shifting patterns of solar heating probably influence the winds in the polar regions.

Both images were taken by the Cassini wide-angle camera.

Image Credit: NASA/JPL-Caltech/Space Science Institute/Hampton University

Light on the Nightside


The shadow is getting shorter as Saturn nears the northern solstice. The moon is pretty easily seen at about 11:00.

NASA’s description:

NASA’s Cassini spacecraft looks down at the rings of Saturn from above the planet’s nightside. The darkened globe of Saturn is seen here at lower right, along with the shadow it casts across the rings.

The image shows that even on the planet’s night side, the rings remain in sunlight, apart from the portion that lies within Saturn’s shadow. The rings also reflect sunlight back onto the night side of the planet, making it appear brighter than it would otherwise appear.

Saturn’s small moon Prometheus (53 miles or 86 kilometers across) is faintly visible as a speck near upper left. The shadow of Saturn was once long enough to stretch to the orbit of Prometheus. But as northern summer solstice approaches, Saturn’s shadow no longer reaches that far (see PIA20498). So Prometheus will not move into the darkness of the planet’s shadow until the march of the seasons again causes the shadow to lengthen.

This view looks toward the sunlit side of the rings from about 41 degrees above the ring plane. The image was taken in visible light with the Cassini spacecraft wide-angle camera on Aug. 14, 2016.

The view was obtained at a distance of approximately 870,000 miles (1.4 million kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 87 degrees. Image scale is 53 miles (86 kilometers) per pixel. Prometheus has been brightened by a factor of two to enhance its visibility.

Nearing the Summer Solstice


The summer solstice on Saturn that is.  Superb!

The original caption:

Since NASA’s Cassini spacecraft arrived at Saturn in mid-2004, the planet’s appearance has changed greatly. The shifting angle of sunlight as the seasons march forward has illuminated the giant hexagon-shaped jet stream around the north polar region, and the subtle bluish hues seen earlier in the mission have continued to fade. Earlier views obtained in 2004 and 2009 (seePIA06077 and PIA11667) demonstrate how drastically the illumination has changed.

This view shows Saturn’s northern hemisphere in 2016, as that part of the planet nears its northern hemisphere summer solstice in May 2017. Saturn’s year is nearly 30 Earth years long, and during its long time there, Cassini has observed winter and spring in the north, and summer and fall in the south. The spacecraft will complete its mission just after northern summer solstice, having observed long-term changes in the planet’s winds, temperatures, clouds and chemistry.

Cassini scanned across the planet and its rings on April 25, 2016, capturing three sets of red, green and blue images to cover this entire scene showing the planet and the main rings. The images were obtained using Cassini’s wide-angle camera at a distance of approximately 1.9 million miles (3 million kilometers) from Saturn and at an elevation of about 30 degrees above the ring plane. The view looks toward the sunlit side of the rings from a sun-Saturn-spacecraft angle, or phase angle, of 55 degrees. Image scale on Saturn is about 111 miles (178 kilometers) per pixel.

The exposures used to make this mosaic were obtained just prior to the beginning of a 44-hour movie sequence (see PIA21047).

Image: NASA/JPL-Caltech/Space Science Institute


Shadowed Saturn


Saturn’s shadow stretched beyond the edge of its rings for many years after Cassini first arrived at Saturn, casting an ever-lengthening shadow that reached its maximum extent at the planet’s 2009 equinox. This image captured the moment in 2015 when the shrinking shadow just barely reached across the entire main ring system. The shadow will continue to shrink until the planet’s northern summer solstice, at which point it will once again start lengthening across the rings, reaching across them in 2019.

Like Earth, Saturn is tilted on its axis. And, just as on Earth, as the sun climbs higher in the sky, shadows get shorter. The projection of the planet’s shadow onto the rings shrinks and grows over the course of its 29-year-long orbit, as the angle of the sun changes with respect to Saturn’s equator.

This view looks toward the sunlit side of the rings from about 11 degrees above the ring plane. The image was taken in visible light with the Cassini spacecraft wide-angle camera on Jan. 16, 2015.

The view was obtained at a distance of approximately 1.6 million miles (2.5 million kilometers) from Saturn. Image scale is about 90 miles (150 kilometers) per pixel.

Credit: NASA/JPL-Caltech/Space Science Institute

More Refracted Rings


The citizens of Earth have spacecraft orbiting the two largest planets in our solar system. Juno of course is just getting going and has entered its second of 37 orbits – plenty to come, Cassini on the other hand has been at Saturn since 2004 and is still going strong.

From the Cassini team:

Saturn’s rings appear to bend as they pass behind the planet’s darkened limb due to refraction by Saturn’s upper atmosphere.

The effect is the same as that seen in an earlier Cassini view (see PIA20491), except this view looks toward the unlit face of the rings, while the earlier image viewed the rings’ sunlit side.

The difference in illumination brings out some noticeable differences. The A ring is much darker here, on the rings’ unlit face, since its larger particles primarily reflect light back toward the sun (and away from Cassini’s cameras in this view). The narrow F ring (at bottom), which was faint in the earlier image, appears brighter than all of the other rings here, thanks to the microscopic dust that is prevalent within that ring. Small dust tends to scatter light forward (meaning close to its original direction of travel), making it appear bright when backlit. (A similar effect has plagued many a driver with a dusty windshield when driving toward the sun.)

This view looks toward the unilluminated side of the rings from about 19 degrees below the ring plane. The image was taken in red light with the Cassini spacecraft narrow-angle camera on July 24, 2016.

The view was acquired at a distance of approximately 527,000 miles (848,000 kilometers) from Saturn and at a sun-Saturn-spacecraft, or phase, angle of 169 degrees. Image scale is 3 miles (5 kilometers) per pixel.

Images: NASA/JPL-Caltech/Space Science Institute

A Bright Spot On the Rings

Cassini sent back this image pointing out the bright spot on the rings which they call a surge – see below.

I even knew what it was, somehow I remembered a previous post – 10 years ago.


Memory aside, I better let the Cassini folks explain and link to that previous example:

An ethereal, glowing spot appears on Saturn’s B ring in this view from NASA’s Cassini spacecraft. There is nothing particular about that place in the rings that produces the glowing effect — instead, it is an example of an “opposition surge” making that area on the rings appear extra bright.

An opposition surge occurs when the sun is directly behind the observer looking toward the rings. The particular geometry of this observation makes the point in the rings appear much, much brighter than would otherwise be expected.

For more on the surge, see PIA08247.

This view looks toward the sunlit side of the rings from about 28 degrees above the ring plane. The image was taken in visible light with the Cassini wide-angle camera on June 26, 2016.

The view was acquired at a distance of approximately 940,000 miles (1.5 million kilometers) from the rings and at a sun-ring-spacecraft, or phase, angle of 0 degrees. Image scale on the rings at center is 56 miles (90 kilometers) per pixel.

Image and caption: NASA/JPL-Caltech/Space Science Institute

Contrasting Dione


Cassini continues to amaze.  Such a good value IMHO.

From NASA:

Dione reveals its past via contrasts in this view from NASA’s Cassini spacecraft. The features visible here are a mixture of tectonics — the bright, linear features — and impact cratering — the round features, which are spread across the entire surface.

Tectonic features tell the story of how Dione (698 miles or 1,123 kilometers across) has been heated and cooled since its formation, and scientists use those clues to piece together the moon’s past. Impact craters are evidence of external debris striking the surface, and thus they tell about the environment in which the moon has existed over its history.

This view looks toward the trailing hemisphere of Dione. North on Dione is up. The image was taken in visible light with the Cassini narrow-angle camera on April 11, 2015.

The view was obtained at a distance of approximately 68,000 miles (110,000 kilometers) from Dione and at a Sun-Dione-spacecraft, or phase, angle of 28 degrees. Image scale is 2,165 feet (660 meters) per pixel.

The Cassini mission is a cooperative project of NASA, ESA (the European Space Agency) and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.

For more information about the Cassini-Huygens mission visit and The Cassini imaging team homepage is at

Credit: NASA/JPL-Caltech/Space Science Institute

Tethys and Hyperion


Credit: NASA/JPL-Caltech/Space Science Institute

 Original caption released with image:

Saturn’s moons Tethys and Hyperion appear to be near neighbors in this Cassini view, even though they are actually 930,000 miles (1.5 million kilometers) apart here. Tethys is the larger body on the left.

These two icy moons of Saturn are very different worlds. To learn more about Hyperion (170 miles or 270 kilometers across), see Odd Hyperion; to learn more about Tethys (660 miles or 1,062 kilometers across) see Dark Belt of Tethys.

This view looks toward the trailing side of Tethys. North on Tethys is up and rotated 1 degree to the left. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Aug. 15, 2015.

The view was acquired at a distance of approximately 750,000 miles (1.2 million kilometers) from Tethys. Image scale is 4.4 miles (7.0 kilometers) per pixel. The distance to Hyperion was 1.7 million miles (2.7 million kilometers) with an image scale of 10 mile (16 kilometers) per pixel.

Nearing The Solstice


Saturn is nearing the northern-hemisphere solstice.  The shadow on the rings is shortening as the time gets closer in this image taken in May 2016 – compare to this image taken in 2007.

Notice the shadow is just beyond the Cassini Division, by the time the solstice gets here in May 2017 the shadow will be just past the half-way point of the B-ring (the wide light colored ring).  A year to wait for the solstice might seem like a long time, but consider it takes about 29.5 Earth-years for ONE Saturn year.  Visit our Saturn page for a lot more information on the planet and the moons.

Shadows on the Saturn system are not new.  Years before the Cassini spacecraft took these images, a gentleman named Robert Hooke noted shadows in drawings he made of Saturn in 1666.

Cassini took this image from about 3.2 million km / 2 million miles and there are a couple of things other than the shadow to take note of:  the northern polar vortex stands out nicely and the moon Mimas is visible to the lower left of the planet.

Image: NASA/JPL-Caltech/Space Science Institute