Cassini’s View of Weird & Wonderful Saturn
Cassini has released the highest-resolution images to date of Saturn’s hexagon feature at its north pole. The hexagon is formed by a jet stream that loops around the pole. Credit: NASA/JPL-Caltech/SSI/Hampton University
Since Cassini entered orbit around Saturn in 2004, it has sent back a wealth of information about the planet and its moons. On Wednesday, December 4, NASA and the Jet Propulsion Laboratory (JPL) hosted a Google Hangout, ‘Weird and Wonderful Cassini’, to introduce some of the newest images captured by the spacecraft, including Saturn’s striking hexagon and the newest version of Carl Sagan’s Pale Blue Dot.
A polar hexagon
First on hand was Cassini imaging team associate Kunio Sayanagi of Hampton University in Virginia to speak about the striking hexagon around Saturn’s north pole. Formed by the polar jet stream, the image from Cassini shows the feature in its highest resolution yet.
The six-sided feature is not a new feature, but one that has been around for at least three decades.
"We’ve known that it existed since at least 1981," Sayanagi said.
When Cassini entered orbit in 2004, it quickly confirmed that the feature still existed. At the time, Saturn’s north pole was in winter darkness. Only recently, after the planet passed through its equinox, has the hexagon become clearer to see.
"Sunlight has been slowly starting to shine on the northern hemisphere during the earlier part of the mission," Sayanagi said.
On July 19, 2013, NASA’s Cassini spacecraft captured an image of Earth, as well three other planets and the moons of Saturn, while Saturn eclipsed the sun. Credit: NASA/JPL-Caltech/Space Science Institute
The image released is a composite of several photos taken over 2012 that were pieced together by the team. In addition to the glaring hexagon, it also features a fast vortex spinning counter-clockwise that Sayanagi referred to as a "polar hurricane."
The six-sided figure is not a fixed geological feature on the surface of Saturn, as the gas giant has no solid surface. An astronaut attempting to land would find themselves sinking through the atmosphere all the way to the planet’s core.
Instead, the jet stream in the atmosphere creates the hexagon and shapes the peculiar cloud pattern.
"The hexagon is not an optical illusion but an actual feature, a geometric pattern formed by the jet stream at that latitude," Sayanagi said.
"A jet stream can act as a transport barrier. It can prevent materials from going across the jet stream because it is blowing so fast."
Such a quick-moving airflow can be easy to move along but difficult to cross.
Although the feature may appear odd, Sayanagi pointed out that similar patterns exist on Earth. Earth’s jet stream wraps around the high altitudes of the planet, its meandering shape created by the mountain ranges and oceans it winds over.
"Earth is basically really messy," Sayanagi said. "Saturn is a gas giant; that’s why Saturn can maintain a steady geometric shape."
Pale Blue Dot, Take Two
A collage of about 1,600 images taken by members of the public as part of NASA Cassini mission’s ‘Wave at Saturn’ campaign. Credit: NASA/JPL-Caltech/SSI
In 1990, Voyager 1 took a photo of the six planets that included a pixel-sized photograph of Earth. The image, re-centered on our planet, became known as the Pale Blue Dot, and was addressed by Carl Sagan in his 1994 book of the same name.
"My attachment to the idea of taking a picture of the Earth goes all the way back to Voyager," Cassini imaging team lead Carolyn Porco, who worked with Sagan on the original Pale Blue Dot image on the Voyager mission, said.
Taken from a distance of 3.7 billion miles (6 billion kilometers), it was not the image itself that was astounding.
"It was what Carl had to say about the image, and the way that he romanced it and turned it into an allegory of the human condition that, ever since, has made the phrase ‘pale blue dot’ synonymous with the inspirational call to protect the environment, the call of planetary brotherhood," Porco, of the Space Science Institute in Colorado, said.
"Ever since the beginning of my tenure as the leader of the imaging team on Cassini, I have wanted to do that picture over again.….except make it better.”
On July 19, 2013, Saturn’s eclipse of the sun allowed Cassini to photograph Earth, Venus, and Mars along with the ringed planet. This time, Porco made sure that people around the world knew about the photograph before it happened as part of her "The Day the Earth Smiled" event. .
"We could tell people in advance, at this moment, your picture is going to be taken from the orbit of Saturn," Porco said, speaking of her desire to invitation to e them to take a few moments to "contemplate their cosmic whereabouts."
With the announcement made, people around the world were able to include the ringed planet in their daily events. Several sent JPL photos of themselves, which were combined into a photo montage as part of one of the activities of the day called, "Wave at Saturn".
Pale Blue Dot, the second take. A zoomed-in image of Earth and its moon taken by Saturn and magnified five times. Earth is on the left, the moon on the right. Credit: NASA/JPL-Caltech/Space Science Institute
Speaking of "The Day the Earth Smiled", Porco said,"This image represents the best of what humanity has to offer."
She pointed out that the "war-like inhabitants" of Earth are also "the seekers and thinkers and explorers who took this picture."
"To be that small and reach so far is, in the end, what makes us the extraordinary citizens of planet Earth."
The moons of Saturn
Cassini’s mission is far from complete. Over the next few years, if Nasa’s budget allows the mission to continue, the spacecraft will continue to study the planet as the northern hemisphere moves toward summer, an event that won’t happen again for another three decades.
"The colors are reversing as we’re approaching winter in the southern hemisphere of Saturn," Cassini project scientist Linda Spilker of JPL said.
In addition to affecting the planet, the change in seasons will also affect Saturn’s moon Titan, a Mercury-sized planet with a thick nitrogen atmosphere and one of the few worlds where liquids—in Titan’s case, methane and ethane—pool at the surface.
"The next few years will be some of the most exciting times for Titan weather," Spilker said.
Titan is covered with lakes of methane that are visible as black splotches in the infrared. Most of these lakes are clustered around the moon’s north pole, with only a few exceptions.
"Not only is Titan a giant laboratory for how life might have started on the Earth, one also wonders, could there possibly be some kind of methane-based life that might be in the lakes?" Spilker asked.
Most of Titan’s lakes exist in the northern polar region of the moon, revealed here as dark patches. Credit: NASA/JPL-Caltech/University of Arizona/University of Idaho
As the weather shifts, she conjectured that the layout of the lakes might change as well. Some may evaporate completely, while other dry lake beds could be filled.
"The seasons are changing, we we’re really anxious to see what will happen when that sun shines down on the north pole of Titan," Spilker said.
Titan isn’t the only Saturn moon with potential. It also hosts the icy moon Enceladus, where the material from hundreds of geysers shoots from the south pole into the atmosphere. Describing the carbon dioxide, nitrogen, and organics present in the plumes, Spilker said, "We basically have the ingredients there for life, possibly in that liquid water reservoir in Enceladus’ south pole."
According to Porco, the moon is "the most accessible habitable zone in all the solar system."
"You can fly through those geysers and scoop up material," Porco said.
Although Cassini doesn’t have the instruments needed to scoop up pieces of Enceladus, the spacecraft will be traveling through those geysers at the end of 2015. Passing through the jets during their maximum emission for the first time, the craft will have a chance to make more detailed compositional measurements.
The shifting seasons may also provide more insight into the structure of the icy moon. As the north pole shifts into the sunlight, the team will search for evidence of ancient tiger stripes: fractures in the planet’s crust. They also hope to determine if Enceladus’ north pole was once as active as the south pole is today.
‘Shoot the pier’
Cassini program manager Earl Maize, also of JPL, spoke about Cassini’s capabilities over the remainder of its mission.
"We are 6 weeks into our 17th year of flight," he said. (Cassini launched from Earth on October 15,1997).
He went on to note that 96 percent of the propellant used to power the main engines had already been consumed, but that over 30 percent of the propellant used to control the smaller thrusters remained. These smaller thrusters allowed the team to make minute adjustments in the course of the craft that have allowed science to continue beyond the 4 years of its primary mission.
Over a hundred glaciers spit plumes of organics from the south pole of Saturn’s moon Enceladus. Credit: NASA/JPL-Caltech/Space Science Institute
"By carefully controlling how we fly by Titan, we can bend the trajectory," he said," noting that sometimes the immediate changes were only on the order of a few tens of millimeters per second, but resulted in significant trajectory shifts. These minor changes, made very meticulously by the flight team, allow for a large adjustments in Cassini’s flight path while using very little fuel.
As Cassini’s mission comes to a close in 2017, Maize described the spacecraft’s final maneuver.
"We’re going to shoot the pier," he said, referencing the risky surfing move of maneuvering a surfboard between the pilings of a pier.
There is a 1200-mile gap between Saturn’s innermost ring and its atmosphere.
"We’re going to use one last Titan flyby to push us into that gap," Maize said.
Cassini will zip up and down through that gap 22 times, sampling Saturn’s atmosphere and exploring new regions.
Then, on September 11, 2017, a final flyby from Titan will nudge the spacecraft into Saturn’s atmosphere for good.
"The spacecraft will be destroyed almost immediately, nearly four days after the flyby," Maize said.
"Thus the end to an incredibly glorious exploration of the Saturnian system, a legacy of scientific data and engineering achievement that I don’t believe will be duplicated for a very long time, and I think a wealth of information for generations to come."