Category Archives: Cassini

Moon Duo

satmoonduo

When taking images in directions opposite from the sun, most objects appear dark. Surprisingly, however, some of Saturn’s rings get brighter.

Parts of Saturn’s main rings appear dark in backlit views, particularly the dense B ring (as can been seen in PIA14934). However, some rings are comparatively tenuous and made up of dust particles that tend to scatter light in roughly the original direction it was traveling. This is called “forward scattering.” Because of forward scattering, rings like the F ring, which encircles the outer edge of the main rings, appear to glow brightly at this large viewing angle.

Two moons hover above the rings from this perspective — Enceladus (313 miles or 504 kilometers across), at left, and Janus (111 miles or 179 kilometers across), at right.

This view looks toward the unilluminated side of the rings from about 0.5 degrees below the ring plane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 21, 2015.

The view was acquired at a distance of approximately 750,000 miles (1.2 million kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 136 degrees. Janus’ brightness was enhanced by a factor of two to improve its visibility in this image.

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

Helorus

Helorus

Helorus is a crater on the Saturn moon Dione.

From Cassini:
Cassini captures a crater duo on Saturn’s moon Dione that is superimposed on older, linear features. The upper of the pair, named Italus, is overprinted on a grouping of ancient troughs called Petelia Fossae. The lower crater, Caieta, sits atop a feature named Helorus Fossa.

Scientists are confident that Helorus and features like it are very old, both because there are many old craters on top of it and because of the way that material has apparently filled in the shallow valley, giving its edges a softer appearance. Fossae on Dione (698 miles or 1,123 kilometers across) like Helorus are believed to be tectonic features, formed when the area between tectonic faults drops down into trough-like structures.

This view is centered on terrain at 22 degrees south latitude, 73 degrees west longitude. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Sept. 30, 2015.

The view was obtained at a distance of approximately 25,000 miles (41,000 kilometers) from Dione and at a Sun-Dione-spacecraft, or phase, angle of 64 degrees. Image scale is 804 feet (245 meters) per pixel.

Credit: NASA/JPL-Caltech/Space Science Institute

Saturnian Sisters

saturnsisers

I liked the NASA title so I kept it. For comparison to the numbers below our moon averages 384,400 km / 238,855 miles from Earth.

From NASA:
Similar in many ways, Saturn’s moons Tethys and Rhea (left and right, respectively) even share a discoverer: Giovanni Cassini, namesake of the NASA spacecraft that captured this view.

The moons are named for sisters — two Titans of Greek mythology. Although somewhat different in size, Rhea (949 miles or 1,527 kilometers across) and Tethys (660 miles or 1,062 kilometers across) are medium-sized moons that are large enough to have pulled themselves into round shapes. They are both composed largely of ices and are generally thought to be geologically inactive today.

The view looks toward the anti-Saturn sides of Tethys and Rhea. North on both moons is up. The image was taken in visible red light with the Cassini spacecraft narrow-angle camera on Oct. 11, 2015.

The two moons appear close together here, but Tethys was about 220,000 miles (360,000 kilometers) farther away from Cassini when the image was captured — nearly the distance from Earth to our moon. Thus, the view does not accurately reflect the bodies’ relative sizes.

The image was obtained at a distance of approximately 708,000 miles (1.14 million kilometers) from Rhea. Image scale on Rhea is 4 miles (7 kilometers) per pixel. Tethys was 930,000 miles (1.5 million kilometers) away during this observation and has a pixel scale of 6 miles (9 kilometers) per pixel.

Credit: NASA/JPL-Caltech/Space Science Institute

Three Moons

threemoonssaturn

Three of Saturn’s moons — Tethys, Enceladus and Mimas — are captured in this group photo from NASA’s Cassini spacecraft.

Tethys (660 miles or 1,062 kilometers across) appears above the rings, while Enceladus (313 miles or 504 kilometers across) sits just below center. Mimas (246 miles or 396 kilometers across) hangs below and to the left of Enceladus.

This view looks toward the sunlit side of the rings from about 0.4 degrees above the ring plane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 3, 2015.

The view was acquired at a distance of approximately 837,000 miles (1.35 million kilometers) from Enceladus, with an image scale of 5 miles (8 kilometers) per pixel. Tethys was approximately 1.2 million miles (1.9 million kilometers) away with an image scale of 7 miles (11 kilometers) per pixel. Mimas was approximately 1.1 million miles (1.7 million kilometers) away with an image scale of 6 miles (10 kilometers) per pixel.

Image: NASA/JPL-Caltech/Space Science Institute

Saturn’s Moon Dione

dionecassini

More about Dione from TNP.

From Cassini:
Dione appears cut in two by Saturn’s razor-thin rings, seen nearly edge-on in a view from NASA’s Cassini spacecraft. This scene was captured from just 0.02 degrees above the ring plane.

The bright streaks of Dione’s wispy terrain (see PIA12553) are seen near the moon’s limb at right. The medium-sized crater Turnus (63 miles, 101 kilometers, wide) is visible along Dione’s terminator.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 25, 2015. The view was acquired at a distance of approximately 1.4 million miles (2.3 million kilometers) from Dione and at a Sun-Dione-spacecraft, or phase, angle of 115 degrees. Image scale is 8.6 miles (13.8 kilometers) per pixel.

Image: NASA/JPL-Caltech/Space Science Institute

Tethys

tethys

A new look at Tethys from Cassini. We get a nice look at the 400 km / 249 mile wide crater called Odysseus.

Saturn’s moon Tethys appears to float between two sets of rings in this view from NASA’s Cassini spacecraft, but it’s just a trick of geometry. The rings, which are seen nearly edge-on, are the dark bands above Tethys, while their curving shadows paint the planet at the bottom of the image.

Tethys (660 miles or 1,062 kilometers across) has a surface composed mostly of water ice, much like Saturn’s rings. Water ice dominates the icy surfaces in the the far reaches of our solar system, but ammonia and methane ices also can be found.

The image was taken in visible light with the Cassini spacecraft wide-angle camera on Nov. 23, 2015. North on Tethys is up. The view was obtained at a distance of approximately 40,000 miles (65,000 kilometers) from Tethys. Image scale is 2.4 miles (4 kilometers) per pixel.

Image: NASA/JPL-Caltech/Space Science Institute

Saturn in Methane

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When viewed through a methane filter the banding on Saturn really stands out.

The Cassini caption:
The soft, bright-and-dark bands displayed by Saturn in this view from NASA’s Cassini spacecraft are the signature of methane in the planet’s atmosphere.

This image was taken in wavelengths of light that are absorbed by methane on Saturn. Dark areas are regions where light travels deeper into the atmosphere (passing through more methane) before reflecting and scattering off of clouds and then heading back out of the atmosphere. In such images, the deeper the light goes, the more of it gets absorbed by methane, and the darker that part of Saturn appears.

The moon Dione (698 miles or 1,123 kilometers across) hangs below the rings at right. Shadows of the rings are also visible here, cast onto the planet’s southern hemisphere, in an inverse view compared to early in Cassini’s mission at Saturn (see PIA08168).

This view looks toward the unilluminated side of the rings from about 0.3 degrees below the ringplane. The image was taken with the Cassini spacecraft wide-angle camera on Sept. 6, 2015, using a spectral filter which preferentially admits wavelengths of near-infrared light centered at 728 nanometers.

The view was acquired at a distance of approximately 819,000 miles (1.32 million kilometers) from Saturn. Image scale is 49 miles (79 kilometers) per pixel. Dione has been brightened by a factor of two to enhance its visibility.

Rings!

wegotrings

This is a raw image from Cassini.  The bright white specks you see are artifact that can be eliminated with processing.

This reminds me of a record album. A what? If you don’t know what that is ask your Grand Parents. 🙂

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

Enceladus Surface

enceladussurface

One of the things that always strikes me about Enceladus is the differences in terrain.  Look at the terrain in the lower right side of the picture and how the crater is cut cleanly in half as is the whole side of the moon.

Here’s the Cassini description:

This half-lit view of Enceladus bears a passing resemblance to similar views of Earth’s own natural satellite, but the similarities end there. Earth’s rocky moon is covered in dark, volcanic basins and brighter, mountainous highlands — both exceedingly ancient. The surface of icy Enceladus is uniformly bright, far brighter than Earth’s moon. Large areas of Enceladus’ surface are characterized by youthful (on geologic timescales), wrinkled terrains.

Although the north pole of Enceladus (313 miles or 504 kilometers across) was dark when Cassini arrived at Saturn, the march of the seasons at Saturn have brought sunlight to the north and taken it from the south.

This view looks toward the leading hemisphere of Enceladus. North on Enceladus is up. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Sept. 8, 2015.

The view was acquired at a distance of approximately 80,000 miles (129,000 kilometers) from Enceladus. Image scale is 2,530 feet (772 meters) per pixel.

Image: NASA/JPL-Caltech/Space Science Institute

Tethys and Janus

tethysandjanus

Click the image for a much larger version.

From Cassini:
Janus and Tethys demonstrate the main difference between small moons and large ones. It’s all about the moon’s shape.

Moons like Tethys (660 miles or 1,062 kilometers across) are large enough that their own gravity is sufficient to overcome the material strength of the substances they are made of (mostly ice in the case of Tethys) and mold them into spherical shapes. But small moons like Janus (111 miles or 179 kilometers across) are not massive enough for their gravity to form them into a sphere. Janus and its like are left as irregularly shaped bodies.

Saturn’s narrow F ring and the outer edge of its A ring slice across the scene.

This view looks toward the unilluminated side of the rings from about 0.23 degrees below the ring plane. The image was taken in visible green light with the Cassini spacecraft narrow-angle camera on Oct. 27, 2015.

The view was obtained at a distance of approximately 593,000 miles (955,000 kilometers) from Janus. Image scale at Janus is 3.7 miles (6 kilometers) per pixel. Tethys was at a distance of 810,000 miles (1.3 million kilometers) for an image scale of 5 miles (8 kilometers) per pixel.

Image: NASA/JPL-Caltech/Space Science Institute