Cassini - is it just me who's excited?

[Astronuc]

http://photojournal.jpl.nasa.gov/catalog/PIA07744

Now this is a cool shot.    Just imagine coming into orbit and passing this on the way.

[Astronuc]

Enceladus Top and Bottom

[Astronuc]

The top image shows an excellent grouping of three moons --Dione, Tethys and Pandora-- near the rings provides a sampling of the diversity of worlds that exists in Saturn's realm.
A 330-kilometer-wide (205 mile) impact basin can be seen near the bottom right on Dione (at left). Ithaca Chasma and the region imaged during the Cassini spacecraft's Sept. 24, 2005, flyby can be seen on Tethys (middle). Little Pandora makes a good showing here as well, displaying a hint of surface detail.

Tethys is on the far side of the rings in this view; Dione and Pandora are much nearer to the Cassini spacecraft.

Dione is 1,126 kilometers (700 miles) across. Tethys is 1,071 kilometers (665 miles) across and Pandora is 84 kilometers (52 miles) across.

The top image was taken in visible blue light with the Cassini spacecraft narrow-angle camera on Sept. 22, 2005, at a distance of approximately 1.2 million kilometers (800,000 miles) from Saturn. The image scale is about 5 kilometers (3 miles) per pixel on Dione and Pandora and 9 kilometers (6 miles) per pixel on Tethys.


In the bottom image, the moons Dione and Tethys face each other across the gulf of Saturn's rings. Here, the Cassini spacecraft looks on the Saturn-facing hemisphere of Tethys below and the anti-Saturn side of Dione above. The dark groove in the rings is the Cassini Division.   The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Sept. 22, 2005, at a distance of approximately 860,000 kilometers (530,000 miles) from Dione. Tethys was on the far side of the rings, 1.5 million kilometers (900,000 miles) from Cassini. The image scale is 5 kilometers (3 miles) per pixel on Dione and 9 kilometers (6 miles) per pixel on Tethys.

[Astronuc]

Great White Splat
May 13, 2005

Top image - Looking closely at Saturn's moon Rhea during a somewhat distant flyby, Cassini provides this view of what appears to be a bright, rayed and therefore relatively young crater. This crater was also observed by Cassini at much lower resolution in the fall of 2004 and in spring of 2005.  For comparison, viewing the same crater near the terminator (the line between day and night) would highlight the crater's topography (vertical relief), compared to its brightness, which is highlighted in this view where the Sun is at a higher angle.

North on Rhea is up and rotated about 15 degrees to the left. This view shows principally the leading hemisphere on Rhea.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 14, 2005, at a distance of approximately 247,000 kilometers (153,000 miles) from Rhea and at a Sun-Rhea-spacecraft, or phase angle of 70 degrees. Resolution in the image is 1 kilometer (0.6 mile) per pixel.


The bottom image shows a view of the tremendous bright splat that coats much of Rhea's leading hemisphere. The bright feature may be impact-related and is visible in other Cassini images of Rhea (see Diversity of Impacts ).  North on Rhea is up in this view.

The image was taken in visible green light with the Cassini spacecraft narrow-angle camera on June 25, 2005, at a distance of approximately 1.1 million kilometers (700,000 miles) from Rhea and at a Sun-Rhea-spacecraft, or phase, angle of less than one degree. Resolution in the original image was 7 kilometers (4 miles) per pixel. The image has been contrast-enhanced and magnified by a factor of two to aid visibility.

Rhea is 1,528 kilometers (949 miles) across. 

Next Rhea Flyby
 500 km (300 mi)
 Nov. 26, 2005 - 10 days, 15 hrs

[Astronuc]

The dark Cassini Division, within Saturn's rings, contains a great deal of structure, as seen in this color image. The sharp inner boundary of the division (left of center) is the outer edge of the massive B ring and is maintained by the gravitational influence of the moon Mimas. 

Spectroscopic observations by Cassini indicate that the Cassini Division, similar to the C ring, contains more contaminated ice than do the B and A rings on either side.

This view is centered on a region approximately 118,500 kilometers (73,600 miles) from Saturn's center. (Saturn is 120,500-kilometers-wide (74,900 miles) at its equator.) From left to right, the image spans approximately 11,000 kilometers (6,800 miles) across the ringplane.

A closer view of the outer edge of the Cassini Division can be seen in The Cassini Division's Edge.

Images taken using red, green and blue spectral filters were combined to create the view, of the rings which approximates what the human eye might see. The image was taken with the Cassini spacecraft narrow-angle camera on May 18, 2005, at a distance of approximately 1.6 million kilometers (1 million miles) from Saturn. The image scale is 9 kilometers (6 miles) per pixel.

[Astronuc]

A Privileged View  -  PIA07629
November 11, 2005

From Saturn orbit, the Cassini spacecraft provides a perspective on the ringed planet that is never seen from Earth.

In our skies, Saturn's disk is always nearly fully illuminated by the sun. From this vantage point -- nearly in the ringplane, with the sun over to the right -- the Cassini spacecraft can see both lit and dark hemispheres, with the shadow of the rings on the northern hemisphere.

Saturn's low density and fast rotation cause its shape to deviate from spherical to a pronounced oblateness, very apparent here.

The image was taken using the Cassini spacecraft wide-angle camera and a filter sensitive to wavelengths of infrared light centered at 728 nanometers. The image was acquired on Sept. 30, 2005, at a distance of approximately 2.4 million kilometers (1.5 million miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 79 degrees. The mage scale is 139 kilometers (86 miles) per pixel.

[Retrospector]

Astronuc, thanks for posting all of the stuff on the moons. It made me think again of Iapetus, which Cassini encountered about a year ago (discussion back on about p. 12 of this thread). Here's an article from January about the huge ridge and the funny and yet unexplained difference in brightness between the two hemispheres of the moon.

http://www.astronomy.com/asy/default.aspx?c=a&id=2703

The most remarkable feature seen in Cassini's Iapetus images is a topographic ridge that coincides almost exactly with the satellite's geographic equator. The ridge is conspicuous in the picture above as an approximately 12-mile wide (20 kilometer) band that extends from the western limb of the disk (left) almost to the terminator, the boundary between day and night on Iapetus (right).

The moon was discovered by French astronomer Jean-Dominique Cassini in 1672. It was Cassini, for whom the Cassini-Huygens mission is named, who correctly deduced that one side of Iapetus is dark, while the other is white. Iapetus' leading hemisphere is one of the darkest surfaces in the solar system. By contrast, its trailing hemisphere is as white as freshly fallen snow. The question astronomers want to answer is: Did the dark material originate from an outside source, or was it created within Iapetus?"

[remcook]

http://www.planetary.org/news/2005/1207_Cassini_Completes_Initial.html

cool summary of observations of saturn's moons

[Astronuc]

Accented Vortex
March 10, 2006

http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2022

Pretty cool - it must awe inspriring to see one of those from orbit.

 

[Astronuc]

More on Enceladus

http://saturn.jpl.nasa.gov/science/moons/moonDetails.cfm?pageID=5

 

[Astronuc]

OK - we missed this last year. 

Iapetus bulges at equator

The Cassini spacecraft's flyby of Saturn's moon Iapetus has revealed a bizarre geological feature in its images: a bulging ridge at its equator.
Mission scientists have started to release detailed images of the moon's surface, which is sharply divided into a bright half and a dark half.

The ridge is around 13km (8 miles) high in some places - taller than Mount Everest, the tallest peak on Earth.

http://news.bbc.co.uk/2/hi/science/nature/4157689.stm

The surface of Iapetus changes to a much brighter, icy terrain near the pole. A topographic ridge can also be clearly seen that coincides almost exactly with the moon's geographic equator. NASA/JPL/Space Science Institute

[remcook]

the ridge is thought to be caused by a rapid despinning of the moon. That's one theory anyway...

[Astronuc]

How would rapid "despinning" cause the ridge?

If anything, would not despinning reduce the centripetal force on the equator?

It looks like two hemispherical halves were just pushed together and the seam is bulging.

[Orstio]

Perhaps the despinning and the ridge have the same cause, and the wording is just confused.  If Iapetus is a captured moon, and it was spinning rapidly before being captured, it would bulge at the equator and flatten at the poles as it came near the massive Saturn.  The tidal forces would also slow the moon's spinning, which would then reduce the size of the massive bulge, and probably leaving a remnant in the form of a geological anomaly at the equator.  The ridge could be the result of tectonic flexing, or even lava flows if the pressure of the tidal forces was great enough.

[Astronuc]

That sounds better!  Certainly with a larger diameter, the moon would despin trying to conserve angular momentum.