Washington: During a recent excursion out of Saturn’s equatorial plane, NASA’s Cassini spacecraft has detected a concentration of high-altitude haze and a vortex materializing at the south pole of planet’s largest moon Titan, signs that the seasons are turning on Saturn’s largest moon.
Saturn, Jupiter, Neptune, Venus and Earth also have polar vortices.
Images captured by Cassini’s imaging team during a flyby on June 27 show the polar vortex, or a mass of swirling gas around the pole in the atmosphere of the moon, as it appears to execute one full rotation in about nine hours -- faster than the moon’s rotation period.
“We’ve been watching this vortex become more developed in the last three to four months, and now, captured in exquisite detail, we’re seeing finer scale features within the detached haze than have been seen to date,” said Jason Perry, an imaging team associate at the University of Arizona, Tucson.
Since Cassini arrived in the Saturn system in 2004, Titan has had a visible “hood” high above the north pole. It was northern winter during this period, and much of the high northern latitudes were in darkness. But the hood, or an area of denser, high altitude haze compared to the rest of the moon’s atmosphere, was high enough to still be illuminated by sunlight.
The seasons have been changing since Saturn’s August 2009 equinox signalled the beginning of spring in the northern hemisphere and fall in the southern hemisphere for the planet and its many moons. Now the high southern latitudes are moving into darkness. The formation of the vortex at Titan’s south pole may be related to the coming southern winter and the start of what will be a south polar hood.
The massing of clouds around the south pole was seen as early as May 22 in infrared wavelengths by Cassini’s visual and infrared mapping spectrometer. And Cassini’s visible light cameras have seen a concentration of yellowish haze in the detached haze layer at the south pole since at least March 27.
These new, more detailed images are only possible because of Cassini’s newly inclined orbits, which are the next phase of Cassini Solstice Mission. Previously, Cassini was orbiting in the equatorial plane of the planet, and the imaging team’s images of the polar vortex between late March and mid-May were taken from a location over Titan’s equator.
At that time, images showed a brightening or yellowing of the detached haze layer on the limb, or edge of the visible disk of the moon, over the south polar region.
“I believe we are seeing some fascinating events on the way to the formation of the south polar vortex,” said Bob West, an imaging team member and an atmospheric scientist at the Jet Propulsion Lab in Pasadena, Calif.
“Future observations of this feature will provide good tests of dynamical models of the Titan circulation, chemistry, cloud and aerosol processes in the upper atmosphere,” he added.
On Saturn, Jupiter, Neptune, Venus and Earth, similar vortices are centered on the north or south pole. However, the polar vortices on these planets each differ from each other in some ways. Titan’s south polar vortex also appears extraordinary in that it contains what appears to be a cloud of condensing organic material, or material that contains carbon, hydrogen and nitrogen. The puffy structures in this cloud are especially intriguing.
Scientists interpret these new images to show open cell convection. In open cells, air sinks in the center of the cell and rises at the edge, forming clouds at cell edges. However, because the scientists can’t see underneath the layer visible in these new images, they don’t know what mechanisms may be at work.
“The structure inside the polar vortex is reminiscent of the open cellular convection that is often seen over Earth’s oceans,” said Tony Del Genio, a member of the Cassini imaging team at NASA’s Goddard Institute for Space Studies, N.Y.
“But unlike on Earth, where such layers are just above the surface, this one is at very high altitude, maybe a response of Titan’s stratosphere to radiative cooling as southern winter approaches. But so soon in the game, we’re really not sure,” Del Genio added.