Dramatic seasonal changes on Titan captured by Cassini
Slipping into shadow, the south polar vortex at Saturn’s moon Titan still stands out against the orange and blue haze layers that are characteristic of Titan’s atmosphere. Images like this, from NASA’s Cassini spacecraft, lead scientists to conclude that the polar vortex clouds form at a much higher altitude – where sunlight can still reach – than the lower-altitude surrounding haze. Image & Caption Credit: NASA / JPL-Caltech / Space Science Institute
As those in the Northern Hemisphere mark the seasonal changes in the glorious colors of the leaves changing, NASA’s Cassini orbiter is also seeing changes on a world far removed from our own. The spacecraft, in orbit around the Saturnian system since 2004, has observed the planet and its moons long enough to capture seasonal changes on Saturn’s largest moon, Titan.
Cassini arrived as Titan’s northern hemisphere was experiencing winter. The orbiter observed major changes on both hemispheres during and after the moon’s 2009 equinox.
Titan’s south polar vortex. (Click to enlarge) Image Credit: NASA / JPL-Caltech / Space Science Institute.
Its most recent images show a vortex or whirling pattern of atmospheric circulation in the upper atmosphere above Titan’s south pole, which is now approaching winter solstice.
The clouds that comprise the vortex are visible because they formed at a high enough altitude for sunlight to reach them. At lower altitudes, the clouds would be obscured by Titan’s layers of blue and orange haze.
Data from Cassini indicates the vortex contains trace gases that are rarely seen in Titan’s atmosphere because they accumulate only when ultraviolet sunlight is absent.
Among these gases are complex hydrocarbons, such as methylacetylene and benzene, and nitrogen-bearing molecules called nitriles.
Over the last few years, as the southern hemisphere has headed toward winter, the temperature of its polar stratosphere has dropped 72 degrees Fahrenheit (40 degrees Celsius).
The stratospheric vortex appeared just months after the equinox, along with high-altitude atmospheric “hot spots”.
Features now visible above Titan’s south pole were seen over its north pole during its winter, including a similar vortex containing the same trace gases.
By 2011, as the northern hemisphere headed toward spring, these features had largely disappeared from Titan’s north polar region, which has since experienced gradual warming.
The warming of the northern hemisphere has been much more gradual than the cooling of the southern hemisphere. Temperatures in the northern hemisphere have increased by just six degrees Fahrenheit (3.3 degrees Celsius) since 2014.
Titan’s south polar vortex in motion. Credit: NASA / JPL-Caltech / Space Science Institute.
Although the trace gases are still present there as that hemisphere heads toward summer, scientists expect them to be broken up by a slow process of photochemical destruction. This process will take different periods of time for the different gases.
That process has already begun, as earlier this year, Cassini observed a region above the northern hemisphere at an altitude of 400–500 km depleted of molecular gas and aerosols.
Scientists believe this finding is evidence of complex dynamics in the high altitudes of Titan’s atmosphere.
Titan’s poles play a major role in the way heat circulates on the large moon. Hot gases well up at the summer pole while cold gases subside at the winter pole.
“Cassini’s long mission and frequent visits to Titan have allowed us to observe the pattern of seasonal changes on Titan, in exquisite detail, for the first time,” said Dr. Athena Coustenis of the Observatoire de Paris and member of Cassini’s Composite Infrared Spectrometer team.
Coustenis continued: “We arrived at the northern mid-winter and have now had the opportunity to monitor Titan’s atmospheric response through two full seasons. Since the equinox, where both hemispheres received equal heating from the Sun, we have seen rapid changes.
“We’ve had the chance to witness the onset of winter from the beginning and are approaching the peak time for these gas-production processes in the southern hemisphere. We are now looking for new molecules in the atmosphere above Titan’s south polar region that have been predicted by our computer models. Making these detections will help us understand the photochemistry going on.”
She presented the findings at the 48th meeting of the American Astronomical Society’s (AAS) Division for Planetary Sciences (DPS) and 11th European Planetary Science Congress (EPSC).
This article was amended at 20:18 EDT, Oct. 24, to correct an ambiguity regarding the term “nitriles”.
Laurel Kornfeld
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.
