Thursday, November 7, 2019

Catching a Big Ring Wave Ride Courtesy a Tiny, Amazing Moonlet Called Daphnis -OR- Saturn: Forever Our Awesome Lord of the Rings

I'd like to dedicate this entry to the Planet Saturn

… In particular, to one of its many, many moons (82 at last count) …

No, not the major moons we all know and love, but rather to one of its tiny moons -- a "moonlet" -- that plays a small but fascinating role in Saturn's awesome ring system …

The moonlet is named Daphnis and it is a "wavemaker" moonlet as it eternally rides through Saturn's ring material, specifically, in the 42-km-wide Keeler Gap of the A Ring. Indeed, the Keeler Gap exists precisely because of Daphnis, which keeps that zone clear of ring material.

Daphnis itself is an irregular shaped object about 8-km in diameter and with 14.258-hour synchronous orbit that is slightly inclined to the ring plane -- and its passage through that plane creates literal waves ahead of and behind it. These "waves" reach a "height" of about 1,500 meters above the ring plane.

Colorized image of Daphnis as it "plows" the Keeler Gap. This was Nov 3, 2019 Astronomy Picture of the Day. Image Credit: NASA, JPL-Caltech, Space Science Institute, Cassini

Before continuing, let's take a detour into rings themselves.

To begin with, here is a schematic diagram of the ring system with each ring (A through E) labeled along with major gaps plus where the Cassini probe crossed the ring plane:

Click on image for larger version.


The rings of Saturn are composed almost entirely of water ice (with trace amounts of rock) -- with a barely measurable "atmosphere" due to the dissociation of water molecules by solar ultraviolet radiation and related photochemical processes.

What's more, for as awesome as the rings appear from a distance, they are remarkably thin -- with an estimated thickness of only about 10 meters (30 feet).

Enceladus (the dot) appears to be perfectly "balanced" on the almost invisibly thin rings. But note the thick, complex pattern of shadows the rings are casting on the cloud tops of Saturn.

The amazing, wonderful Cassini probe gave us a good estimate of the mass of the rings of Saturn: (1.54 ± 0.49) × 10^19 kilograms. That is, they have a mass of 15.4 quintillion (plus or minus 4.9 quintillion) kilograms.

15.4 quintillion kilograms is equivalent to 15.4 million gigatons (where 1 gigaton equals 1 trillion kilograms).

As enormous as this sounds, it is only a bit over half the estimated mass of the Antarctic ice sheet, which is put at 26.5 million gigatons -- but spread over an area 80 times larger than Earth. Also, this mass indicates the rings are very young compared to Saturn -- only 10 million to 100 million years old.

What Saturn in the daytime sky of Earth would look like if it were at the distance of the Moon. (The setting is, I believe, a random place in Alabama.)

Of course, this is not a situation that could happen -- and still have a habitable Earth.


And given the "ring rain" rate estimated by Cassini, the rings may vanish in under 100 million years. In other words, if that's the case, then human beings happen to exist on Earth right at the time that Saturn possesses rings.

What the rings of Saturn might look like from high in the Saturnian atmosphere -- just about cloud top level with daytime Rayleigh scattering-induced blue sky from more distant sunlight. 


Whatever the case, and whatever their origin, what do the rings of Saturn actually look like up close? Below is an artist's conception of Saturn's rings -- showing them as pebble-to-boulder-sized, ever-shifting agglomerations of water ice.

Caption: This is an artist concept of a close-up view of Saturn's ring particles. The planet Saturn is seen in the background (yellow and brown). The particles (blue) are composed mostly of ice, but are not uniform. They clump together to form elongated, curved aggregates, continually forming and dispersing.

Here is portion of the full large, high-resolution version of that image:

Caption continued: The space between the clumps is mostly empty. The largest individual particles shown are a few meters (yards) across. Image by M. Peterson, based on a 1984 image by William K. Hartmann. Hartmann's image illustrated early research by Stuart Weidenschilling and co-workers at the Planetary Science Institute, on dynamical ephemeral bodies in Saturn's rings.

Source NASA Cassini website.


Another artist's conception of Saturn's rings as seen from within them. 


Animation of what Saturn's rings look like as the ring material orbits the planet.


Returning to Daphnis, it and other moonlets -- some too small to detect -- create waves and even "propeller-like" formations in the ring material.

Here is the original NASA image. It was taken by Cassini on Jan 16, 2017 from a distance of 17,000 miles (28,000 km) way during one of its numerous "Grand Finale" orbits. The resolution is 551 feet (168 meters). Daphnis itself appears to have ridges that are thought to be accumulated ring particles.

Daphnis itself was discovered in Cassini images taken in 2005. In subsequent images from 2009, during Saturn's equinox, the ring waves it raises were so high that they cast notable shadows.

Here is the 2009 image in question:

I've rotated it sideways so that the direction of motion of Daphnis is "up" and I am including a paraphrased version of the Wikipedia image page caption:

Vertical structures created by the tiny moon Daphnis cast long shadows across the rings in this startling image taken as Saturn approaches its mid-August 2009 equinox. Daphnis, 8 km (5 miles) across, occupies an inclined orbit within the 42-km (26-mile) wide Keeler Gap in Saturn's outer A ring. The moon's gravitational pull perturbs the orbits of the particles forming the gap's edge and sculpts the edge into waves having both horizontal and vertical components. Daphnis itself can be seen casting a shadow onto the nearby ring. This view looks toward the sunlit side of the rings from about 57 degrees below the ringplane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 24, 2009. The view was acquired at a distance of approximately 826,000 km (513,000 miles) from Daphnis and at a Sun-Daphnis-spacecraft, or phase, angle of 121 degrees. Image scale is 5 km (3 miles) per pixel.

Above and below are two additional artist's renditions -- stylized -- of what Daphnis looks like as it "plows" through the ring material, helping to keep "clear' the Keeler Gap within the A Ring even while creating "waves" ahead and behind it due to its gravitational effects …


Here is a picture of the moonlet of Pan orbiting inside the 325-km wide Encke Gap of the A Ring.

[The Encke Gap] is caused by the presence of the small moon Pan, which orbits within it. Images from the Cassini probe have shown that there are at least three thin, knotted ringlets within the gap.

Spiral density waves visible on both sides of it are induced by resonances with nearby moons exterior to the rings, while Pan induces an additional set of spiraling wakes (see image at left).

And with that, I will sign off this entry, except for one more picture of Saturn in all its ringed glory …

This is one of the iconic Voyager 2 images. We, the Children of Earth, love you, Saturn.


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