Today we conclude another year in our excursion through the Saturnian environment ... a year that brought Cassini's prime mission to successful completion and saw the start of its 2-year `Equinox' mission around the ringed planet.
During the past twelve months and the 43 revolutions we have traveled around Saturn during that time, we have continued to map, with increasing detail, the surfaces of Saturn's moons. We have tracked a powerful electrical storm roiling the planet's southern atmosphere, observed ring arcs of icy material accompanying the tiny moons Anthe and Methone in orbit between Mimas and Enceladus, and eyed the vast yawning gulf of Saturn's biggest cyclone.
We've watched as the shadows of the planet's rings have slipped southward across its face, and noted the changing character of the eclipses of its moons as Saturn's shadow, cast into space, has slowly angled closer to the equatorial plane with the northward motion of the sun. We have also kept keen vigil as the famed `spokes' in Saturn's middle B ring put in an increasingly frequent and robust appearance ... an observation in and of itself that is key to understanding their origins.
But most exciting of all, four times during this past year we maneuvered closer than ever before over the small moon Enceladus, in some cases to scoop up samples of its plume for on-board compositional analysis, in others to measure the surface temperatures and record fine-scale geological structures near the sources of its dozen jets. These exploits have yielded progressively clear indications of a liquid-water reservoir, laced with comparatively complex organic compounds, as the subsurface source of the jets. They also provided insights into the possible time variability of the jetting activity and convincing evidence of past tectonic spreading accompanied by the creation of new surface within the terrain capping the south pole of the moon.
Our objectives for the next year and a half of Cassini's Equinox Mission are unchanged from before: to observe, monitor, catalogue, map, and measure the bodies and phenomena we find within this magnificent planetary system, only now informed by careful examination and appreciation of the previous half-decade's worth of remarkable finds.
At the top of the list is continued observance of the geological activity of Enceladus and the geographical diversity of Saturn's largest moon, Titan. Repeated close investigation of Titan is virtually guaranteed by the need to use it, during close flybys, for gravity assistance in making major alterations to the trajectory of the spacecraft. Of immense interest is the evolution of the bodies of liquid hydrocarbons, first imaged in June 2005 in the south polar region and in February 2007 in the north ... observations subsequently supported by data gathered by other Cassini instruments. Will the fluids filling these basins evaporate with the change of the season and, carried by atmospheric circulation, fall as rain elsewhere on Titan? The next few years may tell.
And the presence of a habitable zone beneath the south polar terrain of Enceladus, and all that that implies, is so captivating that several exquisitely close flybys of this fascinating moon have been designed into Cassini's Equinox Mission, with three of them already behind us.
Even focusing our attention on these two worlds will leave plenty of opportunity for prolonged study of Saturn's atmosphere, rings and other moons over the course of time. We have not nearly exhausted all the possibilities for further exploration of this great sector of our solar system. As northern winter draws to an end and spring is nearly upon it, there will be much left to learn.
In the meantime, we on Cassini are pleased to celebrate today the close of another glorious year around Saturn with a host of heavenly sights, gifted to us by our faithful servant and explorer from clear across the solar system.
Happy New Year to all!
Carolyn Porco Cassini Imaging Team Leader CICLOPS Boulder, CO