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Titan’s Seasons Studied as Cassini Team Plans ‘Grand Finale’

Witnessing Titan’s ever-changing seasons has been a major payoff of the Cassini mission, whose end is now close enough (September, 2017) to cause us to reflect on its accomplishments. We now see winter settling in firmly in the southern hemisphere, along with a strong vortex now developing over the south pole. When Cassini arrived in 2004, we saw much the same thing, only in the northern hemisphere. Athena Coustenis (Observatoire de Paris) is presenting results on Titan’s climate at the ongoing joint meeting of the American Astronomical Society’s Division for Planetary Sciences and 11th European Planetary Science Congress.

“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,” says Dr. Coustenis. “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.”

The overall cycle of heat circulation on Titan is clearly defined. Warm gases rise at the summer pole as cold gases subside at its winter equivalent. The equinox occurred on Titan in 2009, and since then Cassini has observed a reversal of the system. A strong, revolving pattern of circulation, or vortex, has developed in the stratosphere over the south pole, one that is enriched in trace gases that are otherwise rarely found in Titan’s atmosphere. Cassini also revealed an atmospheric hot spot developing at high altitudes within months of the equinox, while its counterpart in the northern hemisphere had greatly diminished two years later.


Image: 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. This view looks towards the trailing hemisphere of Titan (5,150 kilometers across). North on Titan is up and rotated 17 degrees to the left. Images taken using red, green and blue spectral filters were combined to create this natural-color view. The image was taken with the Cassini spacecraft narrow-angle camera on July 30, 2013. Credit: NASA/JPL-Caltech/Space Science Institute.

Within the polar vortex over the south pole, trace gases are accumulating as sunlight diminishes. Here again the parallel is direct. We now see, according to this Europlanet news release, the appearance of complex hydrocarbons and nitriles like methylacetylene and benzene, which were before observed only at high northern latitudes. Coustenis again:

“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.”

While the onset of winter led to a swift temperature drop of 40 degrees Celsius in the stratosphere over the southern pole, the warming effects in the northern hemisphere as the seasons change have been much more gradual, with a 6-degree rise since 2014. In these northerly regions, Cassini has found trace gases that persist into the summer. Although these should eventually disappear, Coustenis says an area of depleted molecular gas and aerosols has emerged across the entire northern hemisphere at an altitude of 400-500 kilometers.

High altitudes on Titan are, in other words, complicated, and while we’re developing a consistent picture thanks to Cassini’s twelve years of observations, these complex effects bear further study. Remember that although we’re entering Cassini’s last year, we have a two-part endgame to go through that involves a final close flyby of Titan to reshape the spacecraft’s orbit. In its new trajectory, Cassini will make 22 passes through the gap between the rings and the planet.

The so-called Grand Finale begins in April of 2017 and takes us to a first dive through the ring/planet gap on April 27. It should be quite a ride, with the closest observations ever made of Saturn, including mapping the planet’s magnetic and gravity fields at high precision, along with samples of particles in the main rings and gases from Saturn’s outer atmosphere. In addition, we should get spectacular views of the rings when, in November of this year, Cassini begins a series of 20 passes just beyond the outer edge of the main rings. Cassini has not gotten this close to the rings since its arrival at Saturn in 2004; we’ll see the ring structure at high resolution. The spacecraft’s final dive into Saturn is planned for September 15, 2017.

“While it will be sad to say goodbye, Cassini’s final act is like getting a whole new mission in its own right,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, California. “The scientific value of the F ring and Grand Finale orbits is so compelling that you could imagine an entire mission to Saturn designed around what we’re about to do.”


Comments on this entry are closed.

  • DJ Kaplan October 21, 2016, 12:48

    Isn’t Titan’s year about 30 of our Earth years? I wouldn’t expect seasonal changes to be so abruptly noticeable over the few years that Cassini has been orbiting.

    • Randy McDonald October 21, 2016, 23:04

      Titan’s seasonality is strongly linked to the Saturn system’s orbit around the sun.

    • andy October 22, 2016, 4:08

      I just checked, Cassini’s been there for just over 12 years now (orbit insertion on July 1st, 2004), not quite half of a Saturnian year but definitely long enough for seasonal effects. That’s longer than I’d thought, time’s going by so fast and here I am getting old…

      • Paul Gilster October 22, 2016, 7:56

        You and me both. I swear it seems like Cassini just got there a few years back!

  • Alex Tolley October 21, 2016, 14:21

    I look forward to reading some papers when they are published. I see that Athena Coustenis has a nice slide deck (undated) introducing the questions about Titan’s atmosphere here.

    It would be really nice if sometime in the future we have many permanent satellites around all the interesting bodies in our solar system sending back images and data, just as we have for Earth. A problem for our descendants may be collisions between satellites around these bodies.

    • David Cummings October 23, 2016, 13:01

      I agree, Alex. Permanent satellites around all important solar system bodies will be a first step to an integrated human-solar-system infrastructure.

      • DJ Kaplan October 24, 2016, 12:30

        Indeed, it would be nice if the Huygens probe was as long-lived as Opportunity and Curiosity.

    • ljk October 27, 2016, 9:56

      That is why you spread CubeSats throughout the Sol system. Not only can they cover more ground and do so far more cheaply than one or two big probes (no offense, Cassini), but if you lose a few the mission does not come to a grinding halt.

      Think small and numerous, that should be the new motto of planetary and interstellar space exploration.

  • Andrew Palfreyman October 24, 2016, 17:20

    Cassini – my goodness! I have lost count of all the discoveries, both major and minor, he has made over the years. There ought to be some sort of prize :)

  • ljk October 27, 2016, 9:54

    Making the most of the time that is left. Good advice across the board.


    To quote:

    On April 22, 2017, the spacecraft will begin its final flyby of Titan, a Mercury-size moon. Titan’s gravity will help curve the spacecraft’s orbit and send it plunging through a 1,200-mile (2,000 kilometers) gap in the rings on April 26, according to Spilker. The gravitational boost from Titan “gives us an equivalent of about a third of the amount of fuel on board,” Spilker said.

    The Cassini team will spend the next day anticipating the “all clear” signal from the spacecraft. Although the gap in the rings is thought to be cleared out and fairly safe, nothing is certain, Spilker told Space.com.

    “It’s still an unknown place,” she said.

    Are they talking about the Cassini gap in the rings? Was it not determined back with Pioneer 11 in 1979 that the gap is really just darker ring particles?

    Not saying it wouldn’t be cool to go through the rings, because it would, but I can also see Cassini coming to an end there before its plunge into Saturn. I once had an idea for placing small probes designed like soccer balls in both shape and sizer to be spread throughout the rings, where they could gently bounce around touching and sampling particles as they drifted through the ring system. They could then transmit their data and images to the “mother ship” that brought them there, which would be in Saturn orbit, for transfer to Earth.

  • ljk November 8, 2016, 13:52

    A beautiful mosaic of Saturn taken by Cassini on October 28, 2016:


  • ljk November 22, 2016, 18:48

    JPL News | November 22, 2016

    NASA Saturn Mission Prepares for ‘Ring-Grazing Orbits’

    First Phase in Dramatic Endgame for Long-Lived Cassini Spacecraft

    A thrilling ride is about to begin for NASA’s Cassini spacecraft. Engineers have been pumping up the spacecraft’s orbit around Saturn this year to increase its tilt with respect to the planet’s equator and rings. And on Nov. 30, following a gravitational nudge from Saturn’s moon Titan, Cassini will enter the first phase of the mission’s dramatic endgame.

    Full article here:


    To quote:

    “We’re calling this phase of the mission Cassini’s Ring-Grazing Orbits, because we’ll be skimming past the outer edge of the rings,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, California. “In addition, we have two instruments that can sample particles and gases as we cross the ringplane, so in a sense Cassini is also ‘grazing’ on the rings.”

    On many of these passes, Cassini’s instruments will attempt to directly sample ring particles and molecules of faint gases that are found close to the rings. During the first two orbits, the spacecraft will pass directly through an extremely faint ring produced by tiny meteors striking the two small moons Janus and Epimetheus. Ring crossings in March and April will send the spacecraft through the dusty outer reaches of the F ring.

    “Even though we’re flying closer to the F ring than we ever have, we’ll still be more than 4,850 miles (7,800 kilometers) distant. There’s very little concern over dust hazard at that range,” said Earl Maize, Cassini project manager at JPL.