Cassini monitors monstor storm on Saturn

Cassini snapped this picture of a monster storm on Saturn on Feb. 25, 2011. This storm started in December 2010, and has since wrapped itself all the way around the planet, spanning an area about 8 times that of the Earth's total surface. The thin dark band at the equator is Saturn's rings seen edge on, and the broader dark bands below are the shadows cast by the rings.

The image below is color coded to indicate the altitude of the various cloud layers in the storm. Blue, white, and yellow indicate clouds at higher altitudes, with green, red, and brown colors showing clouds that are deeper in this gas giant planet's atmosphere. Check the CICLOPS website for more information and Cassini images of Saturn.

Helene at Saturn

The Cassini spacecraft captured this closeup image of Helene on June 18, 2011. Helene is a small satellite of Saturn about 20 miles across. Helene is a coorbital satellite, which means that it shares an orbit with the much larger satellite Dione that is ~30 times larger and ~30,000 times more massive. Helene resides at Dione's L4 Lagrange point, which is a stable niche in Dione's orbit that leads that satellite by 60 degrees. The other stable niche is of course the L5 Lagrange point that trails Dione by 60 degrees. The orbit of a coorbital satellite is analogous to the Trojan asteroids that lead or trail Jupiter by 60 degrees in its orbit about the Sun.

No one knows how a coorbital satellite like Helene came to reside in such a special orbit. But it is conceivable that a coorbital satellite is debris that was excavated when the larger satellite Dione was stuck by a comet long ago. If this scenario is correct, then a lucky fraction of that debris managed to find and settle into one or both Lagrange points where it could have reassembled into a small coorbital satellite like Helene.

To see more images of Helene, as well as the rest of the Saturnian system, visit Cassini's CICLOPS website.

Propellers and wakes in Saturn's rings

This figure shows results from an Nbody simulation of a small patch in Saturn's rings; click figure to zoom in. Small dots represent meter sized ring particles, while the circle at the center is a 150m moonlet that is embedded in the ring. All bodies are travelling to the right as they orbit Saturn, but keep in mind that those nearer Saturn (which is far downwards in this figure) orbit faster, so ring particles in the lower x<0 half of this figure are drifting towards the right side of the moonlight, while those in the upper x>0 half are drifting left of the moonlet. This Nbody simulation was performed by Shugo Michikoshi and Eiichiro Kokubo, and their results are detailed in this preprint.

The upper figure shows what happens in a low mass ring having a surface density of 60 grams/cm^2. As particles drift past the moonlet they receive a kick due to the moonlet's gravity, which in turn opens a propeller-shaped gap in the ring. The Cassini spacecraft has in fact observed many such propellers orbiting in Saturn's A ring, like the one seen below; see the CICLOPS website for more details about this image. Curiously, the model predicts that the propeller should appear as a dark gap in the ring, while the Cassini image below shows that a propeller is bright. The meaning of this is unclear, but it may indicate that the propeller gap is also filled with sunlight-reflecting dust grains that are produced as ring particles collide near the moonlet.

The lower Nbody simulation (in the lower half of the top graphic) shows results for a high mass ring of surface density 400 grams/cm^2. In this case, the higher ring gravity cause the ring particles to condense into ropy or taffy-like structures that are known as wakes. These wakes dominate the ring's appearance and completely wash-out the propeller that the moonlet is trying to form. The fact that propellers are seen in Saturn's A ring, while none have been observed in Saturn's B ring, suggest that the A ring is a relatively low mass ring that allows moonlets to form propellers, while the B ring is massive and full of gravitating wakes that inhibit any such propellers.

Closeup of Helene

Cassini acquired this interesting image of the satellite Helene on March 3. Helene is a small 30km-wide satellite of Saturn. Its orbit is also very curious, since it resides at the L4 Lagrange point of the much larger 1000km-wide satellite Dione. The L4 Lagrange point leads Dione's motion by 60 degrees in longitude. Such satellites are known as coorbitals, since they co-orbit with another larger body, and only a few such coorbital satellites are known. This coorbital motion is analgous to the Trojan asteroids, which lead/trail Jupiter by 60 degrees. Note also Helene's smooth surface. Although the outlines of large craters are clearly evident, their filled-in appearance suggests that this satellite is also being bombarded and coated by dust that might also be present in this part of the Saturnian system. Keep an eye on the CICLOPS website for information about this small but interesting satellite.

Closeup of the Deathstar moon

Cassini's closest ever approach to Saturn's satellite Mimas occurred Saturday Feb. 13. Other pictures of the Deathstar moon are also available at the CICLOPS website.

Iapetus' dichotomy explained

Iapetus is Saturn's icy yin-yang satellite. This Cassini image shows that the side of this satellite that leads in its orbit about Saturn is 10 times darker than its polar regions. This darkening was long thought to be due to contamination from Saturn's even more distant satellite Phoebe. Meteorites crashing into Phoebe will launch small debris that goes into orbit about Saturn, resulting in a vast dust ring that also contaminates Iapetus. This dust ring was only recently discovered by the Spitzer Space Telescope. Some of that dark dust gets deposited at Iapetus' leading face, which is then warmed by sunlight. Recent work by John Spencer and Tilmann Denk show that this can cause surface ice there to evaporate at the equator. That water vapor can migrate towards and then freeze out at the poles, brightening the satellite there, and giving it its yin-yang appearance. Additional images can be found at Cassini's CICLOPS website.

Topography at outer edge of Saturn's B ring?

This very interesting Cassini image shows a close-up of the outer edge of Saturn's B ring. Saturn is far to the left, so the ring's orbital motion here is either up or down (and my best guess says down). The Cassini Division is to the right, just beyond the B ring. The Huygen's ringlet is the prominent gray band that orbits 300km beyond the B ring's edge.

Recall that Saturn is almost at equinox (Tuesday August 11!), so the Sun is just above Saturn's equator. Consequently, sunlight is streaming almost parallel to the ring plane, which allows even very modest vertical structures in the ring to cast long shadows across the ring plane. Evidently, the B ring's outer edge has topography, since it cast shadows that are hundreds of kilometers long! Judging by how ragged the shadows are, this ring-edge seems to resemble a mountain range, which is quite a surprise since the rest of the ring-plane is extremely flat. Note also the bright diagonals, one of which is clearly casting a shadow. These streaks might be due to ring material moving radially, perhaps due to avalanches of ring-matter tumbling down the supposed mountainside? Such radial motion would then get dragged along a diagonal due to the ring's faster orbital speed in regions closer to Saturn. But at this stage, this is all just speculation...

This image was acquired on July 26, 2009, and can be found at the Cassini Equinox Mission's raw image archive. A followup comment will describe how to use this archive effectively to search for other interesting Cassini images.

Vertical ripples in Daphnis' edge-waves

The small speck casting a shadow here is Daphnis, which is an 8km satellite that orbits within the narrow Keeler gap that lies near the outer edge of Saturn's rings. Saturn is also very near equinox, so the sunlight that is illuminating this scene travels nearly parallel to the ring-plane. Consequently, this small satellite casts a rather prominant shadow across the ring. Daphnis also has a small inclination, so its motion carries it a bit above and then below the ring plane during each orbit of Saturn. Daphnis' gravity then tugs the nearby ring material up and down, too, which results in the vertical ripples that are seen at the Keeler gap's inner and outer edges. Note that these bright ripples also cast shadows as well. This image was acquired by the Cassini spacecraft on June 8, 2009, and many more such images can also be found at the CICLOPS website.

Welcome to Solar System Watch!

First post...welcome! This blog is devoted to the latest astronomy news, with a special emphasis on planetary science, which is also my field of study. And if you are wondering who is doing the blogging here, please have a look at my profile.

The blog's title graphic is extracted from this image of Saturn that was acquired by the Cassini spacecraft on May 4, 2005. Here we are looking obliquely onto the dark side of the ring plane. Saturn was in winter when this image was acquired, so the Sun is south of the rings, which also cast shadows onto Saturn's northern hemisphere.