Tuesday, February 24, 2015

New Cassini Images

For today's blog posting I checked in with the Cassini spacecraft that is orbiting Saturn.  As many of you know, Saturn is my favorite extraterrestrial planet.

If your eyes could only see the color red, this is how Saturn's rings would look.

Many Cassini color images, like this one, are taken in red light so scientists can study the often subtle color variations of Saturn's rings. These variations may reveal clues about the chemical composition and physical nature of the rings. For example, the longer a surface is exposed to the harsh environment in space, the redder it becomes. Putting together many clues derived from such images, scientists are coming to a deeper understanding of the rings without ever actually visiting a single ring particle.

Saturn’s main rings, seen here on their ''lit'' face, appear much darker than normal. That’s because they tend to scatter light back toward its source — in this case, the Sun.

Usually, when taking images of the rings in geometries like this, exposures times are increased to make the rings more visible. Here, the requirement to not over-expose Saturn's lit crescent reveals just how dark the rings actually become. Scientists are interested in images in this sunward-facing ("high phase") geometry because the way that the rings scatter sunlight can tell us much about the ring particles' physical make-up.

Although solid-looking in many images, Saturn's rings are actually translucent. In this picture, we can glimpse the shadow of the rings on the planet through (and below) the A and C rings themselves, towards the lower right hand corner.

For centuries people have studied Saturn's rings, but questions about the structure and composition of the rings lingered. It was only in 1857 when the physicist James Clerk Maxwell demonstrated that the rings must be composed of many small particles and not solid rings around the planet, and not until the 1970s that spectroscopic evidence definitively showed that the rings are composed mostly of water ice.

Saturn is circled by its rings (seen nearly edge-on in this image), as well as by the moons Tethys (the large bright body near the lower right hand corner of this image) and Mimas (seen as a slight crescent against Saturn’s disk above the rings, at about 4 o’clock). The shadows of the rings, each ringlet delicately recorded across Saturn's face, also circle around Saturn's south pole.

Although the rings and larger moons of Saturn mostly orbit very near the planet's equatorial plane, this image shows that they do not all lie precisely in the orbital plane. Part of the reason that Mimas (246 miles, or 396 kilometers across) and Tethys (660 miles, or 1062 kilometers across) appear above and below the ring plane because their orbits are slightly inclined (about 1 to 1.5 degrees) relative to the rings.

NASA/JPL-Caltech/Space Science Institute 

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