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Planetary News: Cassini-Huygens (2009)Snapshots from Saturn's EquinoxBy Emily LakdawallaSeptember 21, 2009 It's a once-in-fifteen-year event, and, for the first time, we were able to watch it from up close: on August 11 the Sun passed through the plane of Saturn's rings, darkening them to near-invisibility, making their shadow on the planet virtually vanish, and highlighting tiny, undiscovered out-of-plane features that lurk within the gigantic circumplanetary disk. It will take months or years for scientists to sort out the science from data gathered by the Cassini orbiter as it watched this stately drama unfold. In the meantime, we can appreciate some of the spectacular images that Cassini returned, showing the Saturn system with dramatic side lighting.
Just a day after the equinox, Cassini turned to capture the 75 images necessary to compose this striking panorama across Saturn and its rings. It's a rare and bizarre view of Saturn with the Sun poised almost exactly over the equator. With this lighting geometry, the rings are nearly dark, except where reflected light from Saturn strikes them, or where structures that rise above the plane of the rings catch the sunlight. The rings' shadow has collapsed to a skinny line falling across Saturn's waist. Because of difficult geometric and lighting conditions, extensive processing was required to create this view. Cassini was relatively close to Saturn when it began taking the images for the mosaic and was continuously receding from the planet during the eight hours it took to capture the 200 images contained in the observation. (Photos were captured through infrared, violet, and clear filters in addition to the red, green, and blue-filter images used for the mosaic shown here.) So each of the images had to be reprojected into a common viewing geometry. The photos also had to be digitally processed to remove lens flares (artifacts that result from light scattering within the camera's optics) and to deemphasize seams between images. Additional processing was necessary to make the rings visible on your computer screen. Without enhancement, the rings would have been so dark as to be invisible. The dark half of the rings (right side of the mosaic) has been brightened relative to the bright half, (left side of the mosaic) by a factor of three, and then the entire ring system brightened by a factor of 20. Spokes are visible in the B ring on the right side of the image. Similar brightening was required to make the moons visible. Janus (179 kilometers in diameter) is on the lower left of this image. Epimetheus (113 kilometers) appears near the middle bottom. Pandora (81 kilometers) orbits outside the rings on the right of the image. The small moon Atlas (30 kilometers) orbits inside the thin F ring on the right of the image. Other bright specks are background stars. Cassini had passed through its orbital periapsis at 11:37 UTC on August 11 and almost immediately crossed to the northern face of the rings, at 12:59. This crossing was only 10 hours after the Sun had precisely balanced over the plane (at 02:44). Cassini began capturing the images for this mosaic at about 06:00 on August 12, about 1.25 days after the moment of equinox, and finished taking data at about 14:00. The equinox didn't just affect lighting conditions; it also affected temperatures throughout the Saturn system. This video explains. (If you do not have the bandwidth for video, there are a few stills posted below.) Cassini's deputy project scientist, Linda Spilker, explained in an email how Cassini's Composite Infrared Spectrometer (CIRS) has observed the rings' temperature respond to the changing season. "On the north side of the rings (unlit side before equinox), the B ring is the coldest ring. It is interesting to watch first the A ring, then the other rings, cool to almost match the cold B ring temperature. Most of the cooling happens in the last few degrees of solar elevation before equinox. The B ring is coldest on the north side because it has the most ring particles (optically densest ring) which block the sunlight from the south side. "At equinox, CIRS is seeing the same temperature on the north and south sides of the main rings and the ring temperatures are the coldest temperatures ever seen" on Saturn's rings, Spilker continued. Temperatures there reached a chilly 43 Kelvin (-230 Celsius / -380 Fahrenheit). "At equinox, when the sun is shining on the rings edge-on, Saturn's visible and thermal energy is the main source of heating for the rings. That is why the temperatures are the same on both sides of the rings. As the sun continues to rise in the North, the north side of the rings will start to warm up. In fact, both sides of the rings will warm up!"
But of course images really tell the most spectacular stories about equinox at Saturn. This image contains numerous discoveries revealed by the dramatic side-lighting of equinox season.
Cassini looked down onto the sunlit northern face of the rings about a week after equinox, on August 19, 2009, to capture this unusual view showcasing the vertical structures within Saturn's rings. Sunlight illuminates the rings almost edge-on, so bright features in this image are ones that extend vertically to the north of the ring plane, catching the sideways sunlight (the Sun is coming from the left). The bright features are accompanies by corresponding shadows to the right of each feature. The image includes the F ring (the brightest feature, truncated on the left side of the image) and much of the A ring. The F ring is more vertically extended than the main rings and also composed of sparse, dust-sized particles; it appears bright because the particles are far enough apart that there is little shadowing within the F ring, and the particles scatter light in all directions, including to Cassini's cameras. Moving to the right of the F ring we come first to the outer edge of the A ring and then the skinny Keeler gap. The Keeler gap is carved out by the tiny moon Daphnis (8 kilometers across). Daphnis is within the field of view, but is too small to be resolved at Cassini's distance from it. However, its effects on the rings are quite visible in the form of a set of sawtoothed vertical strictures (bright spikes near the left edge of the rings) with corresponding shadows cast to the right. The shadows are about 450 kilometers long, indicating that the Daphnis ring waves rise about a kilometer above the ring plane. At other longitudes (and other positions on Daphnis' slightly inclined and slightly eccentric orbit), the Daphnis ring waves have been observed to rise as much as 4 kilometers above and below the ring plane. Moving inward from the Keeler gap, the darkest band across the A ring is the Encke gap, in which the moon Pan orbits. Pan is not in the frame. The Encke gap also contains discontinuous ringlets which can be seen here to have vertically extended clumps, each of which casts a shadow to the right. The clump shadows are about 275 kilometers long, so the clump height is about 600 meters above the ring plane. Moving inward again, the brightest "ringlet" crossing the center of the view is a "bending wave" produced by gravitational effects from Mimas. Mimas has an inclined orbit and so tugs ring material from above and below as it circles Saturn. At certain radial positions from Saturn there are resonances, where the relationship between the orbital periods of Mimas and the ring material can be expressed as a ratio of integers. The brightest ringlet is the Mimas 5:3 bending wave, where ring material orbits Saturn five times for every three Mimas orbits. To its immediate right is a dark band, the shadow of the bending wave. Then there is another, more diffuse bright band; this is the Mimas 7:4 bending wave. Another one, the Mimas 8:5 bending wave, is to the left, between the Keeler and Encke gaps, a bit closer to the Keeler gap. Those vertical features, though spectacular, were somewhat expected. The vertical features visible in the image below were not.
The lighting geometry in this image is extreme. It was taken just hours before equinox, mostly of the C ring, one of the dimmest of Saturn's main rings. The B ring's inner boundary is on the right; the C ring's Maxwell gap (demarcated by two bright arcs, which are bright because they rise above the ring plane) is on the left. There is a periodic brightness variation in the image. The side lighting geometry means that brightness variations mostly result from changing slopes of the surface of the ring plane due to out-of-plane disturbances in the rings -- in layman's terms, the rings here are corrugated like a washboard. Previous images had revealed these corrugations in the D ring (located off the view to the left). This image reveals that the corrugation extends beyond their origin in the D ring (which begins not far above Saturn's cloud tops), right through the C ring, to the inner part of the B ring, the densest of Saturn's rings, covering a distance of 17,000 kilometers. This is a wholly unexpected observation that the science team is now working to understand. Based on the earlier information, scientists had speculated that a comet or asteroid may have collided with the D ring in the early 1980s to cause the vertical disturbance. This explanation fails to work now that scientists understand the true extent of the disturbance. All they can say is that something happened in the early 1980s to tilt a vast region of the inner rings. Over the intervening years, the tendency for inclined orbits to systematically wobble (or "precess") at different rates, depending upon their distance from Saturn, has created a tightly wound spiral corrugation in the ring plane from the initial tilt to the ring plane. That close-up view is part of this vast high-resolution panorama across the rings. It must be enlarged to be appreciated.
Fifteen narrow-angle camera images comprise this panoramic mosaic. At extreme left, ringlets within the D ring appear bright, indicating that they extend vertically above and below the ring plane. Next is the C ring. The fact that the edges of the C ring's gaps, and some of the ringlets within it, are bright indicate that they extend vertically above and below the plane of the rings. Some of the C ringlets also contain clumps, which is a surprise. Next comes the B ring. We haven't had such a fine view of its intricate corrugations and waves since Cassini entered Saturn orbit more than five years ago. In the outer B ring, bright spokes cross the rings. These consist of tiny particles electrostatically levitated above the ring plane. Ringlets within the Cassini division are bright just like those in the C ring. Just beyond the Cassini division, in the A ring, is a beautiful corrugation, a bending wave (vertical disturbance of the ring plane) that originates from an orbital resonance with Iapetus. Iapetus is the most distant of Saturn's major satellites but also has a markedly inclined orbit, helping it to create bending waves in the rings. In the outer A ring, the shadow of Dione shows up several times, as Dione was moving during the time that separated each frame of the mosaic. Apart from Iapetus, the major moons all orbit within the plane of Saturn's rings, so their shadows only cross the rings near equinox. The ringlets within the Encke gap are very bright, again indicating that they are vertical disturbances. At the very outer edge of the A ring, between the Encke and Keeler gaps, are several spiral density waves. They are, again, bright because they extend above the ring plane, but not because of motion excited by inclined moons. Instead, they indicate places where the ring particles jostle so closely together that, squeezed from the sides, they have nowhere to go but up (and down) -- analagous to the reason that some mountains (like the Himalayas) exist on Earth. In many places, stars are visible shining through the rings. There are also many blemishes from cosmic rays that hit the camera's detector during the long exposures that were necessary to capture so much detail under such dim lighting conditions. From the vastness of the entire ring system, we now focus in on some of the tiniest features within it. "Propellers" were first spotted in close-up images of the ring taken by Cassini during its orbit insertion. Scientists determined that they indicate the presence of isolated 100-meter-size objects within the rings -- bodies intermediate in size between things that are named as moons and the innumerable individual particles that make up the rings themselves. Cassini spotted many more of them under the extreme equinox lighting conditions, and it now seems that there is a full spectrum of particle sizes from the biggest, Titan, thousands of kilometers in diameter, to the medium-sized moons, hundreds of kilometers in diameter, to the smaller rocks, tens of kilometers in diameter, to the ring-embedded moons, a few kilometers in diameter, to the propellers, hundreds to tens of meters in diameter, to ring particles, on down to dust.
This propeller, spotted just after equinox on August 13, 2009, is very bright because it sticks up above the ring plane at a time when the sunlight was coming almost directly from the side. It's the largest propeller yet observed, sticking up out of the ring plane by 200 meters in each direction, indicating that the body is about 400 meters in diameter. Clumps of ring material surround it ahead of it and behind it in its orbit, extending the whole feature to a length of 130 kilometers. These are just a few of the discoveries made by two of the instruments on Cassini throughout the equinox season. That season continues for a few more months, as the Sun's elevation is still quite low. Now Cassini will enjoy watching sunlight warm Saturn's north pole and illuminate the northern polar regions of Saturn's moons for the first time, at the same time as it will see darkness come to the south poles. There'll be no more optical images of the south poles of the moons for Cassini, but there's one moon whose south pole it'll be watching closely: Enceladus, whose south pole emanates heat and blasts water ice crystals from numerous geysers. In fact, Cassini's next big event will be two close flybys of Enceladus, on November 2 and 21, 2009. Even without sunlight, Cassini will be able to see the warm glow of Enceladus' vents with CIRS. |
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