Aug 20, 2015: Cassini's Final Close Views of Dione - A pockmarked, icy landscape looms beneath NASA's Cassini spacecraft in new images of Saturn's moon Dione taken during the mission's last close approach to the small, icy world. (Image Advisory can be found here.)
September 21, 2009
It is a drama as ancient as the sun, as unflinching as time ... a never-ending whirl of celestial movements, scripted and precise, in a silent show of cosmic force, played out in light and shadow. It is a drama called equinox.
The scene opens on regal Saturn, resplendently wreathed in a vast garland of icy rubble perpetually in motion, arrayed in rings, and slicing knife-like across the sky directly above the planet's equator.
Here, the distant sun shines only weakly on the brightest of summer days. And now, in late northern winter, we find its faint star-like glow hanging low over the rings, its light spread thin and dim on this broad icy plane. The planet itself is hurling through space, but so far from the sun its swift and fearsome motion can bring only gradual changes in the sun's position aloft.
A nearby robotic wanderer has been navigating the reaches of this planetary system throughout northern winter, dispassionately observing the scene. To its operators at significant remove, a billion miles away, it has been a long and gripping wait for this special season about to unfold, a season between seasons called equinox, when the sun passing overhead from south to north begins to set on the rings.
Now, the lowering sun slips the shadows of saturnian moons off the planet onto the enormous disk surrounding it. Equinox season has begun. What follows is a gradual transfiguration of this normally brilliant, preternaturally flat stretch of white icy fragments ... boulders, cobbles and pebbles ... into a dark and forbidding realm. Never-before-seen mountainous waves of rising and falling rubble, looming like colossi over the panorama below, are thrown into stark relief. Small moonlets, only 400 meters across, forty times larger than the largest ring fragments yet twenty times smaller than the smallest ring-moon, protrude above the surroundings and throw long shadows across the rings.
And this is only the beginning. To all onlookers, human and machine, the awaited moment is yet to come.
The front edge of the sun's tiny disc, with its easy northerly procession, eventually intercepts the plane of the rings, its slanted rays grow more so, and extreme darkness falls across the scene. It will take four days for the sun to traverse its own width in the sky. At the exact moment of equinox, the ring plane will precisely bisect the sun's golden face.
During this four day period, the faintest light shines on both sides of the rings, north and south. With the sun's rays at their shallowest, even small bumps betray their presence with discernible shadows. A gentle yet inexplicable spiral vertical corrugation, shocking in its 11,000-mile extent across the inner rings, comes into clear view. A protruding, shadow-casting 80-mile-long propeller-shaped disturbance, bright as it catches the direct rays of the sun and created by the agitation of nearby ring particles by an embedded moonlet, is unmistakable against the dark rings. Sunlit clouds of tiny particles, elevated above the rings, are seen in several places ... the telltale signs of meteoroid impacts into the rings. And many, many places across the plane where the horizontal paths of ring particles are predictably squeezed by the gravitational forcing of moons, near and far -- either into narrow dense lanes that spiral all the way around the planet, or into much smaller scale clumps of fragments forced into linear shapes by compression -- now surprisingly appear with notable vertical relief.
Like the seas of Earth, this wide icy expanse has settled into a mathematically precise cast that, here and there, froths and churns, not by wind but by the convulsive forces of saturnian moons. This famous adornment, impressed deep in the human mind for four centuries as a pure, two-dimensional form, has now, as if by trickery, sprung into the third dimension.
From high above, the silent explorer captures one sweeping view of the scene below: planet and all its darkened rings, at sunset ... one of the most enchanting spectacles this solar system has to offer.
Before too long, the sun will rise again, lavishing its brilliance everywhere. To the vehicle orbiting above and its remote companions, all hints of shadows and hidden dimension will magically vanish.
But blessed memory of this occasion will linger on. For we, fragile and ephemeral spectators in a grand cosmic theatre, have borne witness to a solemn celestial phenomenon no human has beheld before ... not like this: a primeval baptism of light, a sacred rite of passage that, until this moment, had lived only in the imagination.
Now, the show is over, the curtain falls. But far, far away, in the northern reaches of that ringed planet we have come so intimately to know, spring, bringer of warmth and light, has at last arrived.
Carolyn Porco Cassini Imaging Team Leader Director, CICLOPS Boulder, CO