NASA probe finds Saturn ring gap emptier than predicted

This unprocessed image shows features in Saturn’s atmosphere from closer than ever before. The view of Saturn’s polar vortex was captured by NASA’s Cassini spacecraft during its first “Grand Finale” dive past the planet on April 26, 2017. Credit: NASA/JPL-Caltech/Space Science Institute

NASA’s Cassini spacecraft sped through a gap between Saturn and its rings for the second time Tuesday after data from the probe’s first perilous passage through the unexplored region last week found it to contain fewer potentially hazardous dust particles than expected.

The finding is one of several results from Cassini’s first trip through the ring gap that has puzzled scientists. Engineers in charge of keeping Cassini safe, on the other hand, are pleased that the space between Saturn and its rings harbours fewer dangers.

“The region between the rings and Saturn is ‘the big empty,’ apparently,” said Earl Maize, Cassini project manager at NASA’s Jet Propulsion Laboratory in Pasadena, California. “Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected.”

Cassini radioed ground controllers April 27 that it safely made the first-ever flight through the 1,500-mile (2,400-kilometre) ring gap, coming closer to Saturn than any spacecraft in history.

The orbiter used its last flyby of Saturn’s largest moon Titan on April 22 to reshape its path around the planet, plunging Cassini on an orbit that will take it inside the rings once every week until Sept. 15, when it will dive into the ringed world’s hydrogen-helium atmosphere to end the mission.

Cassini made its second journey inside the rings Tuesday, and mission control at JPL received confirmation from the spacecraft around 1530 GMT (11:30 a.m. EDT) that it survived the encounter.

This image of Saturn’s north polar region was taken April 26 as Cassini approached the planet for its first passage inside the rings. Credit: NASA/JPL-Caltech/Space Science Institute

During last week’s flyby, Cassini turned to use its its 13-foot (4-metre) high-gain dish antenna as a shield to protect the spacecraft’s sensitive components, like computers and scientific instruments, from the bombardment of any microscopic dust grains in its path.

Scientists crunching data captured last week said the passage produced far fewer dust impacts than predicted.

Models of the dust environment suggested Cassini would sail through the ring gap unscathed, so officials were not too concerned going into the first flyby. Nevertheless, recordings of the dust strikes were quieter than scientists expected.

The craft’s radio and plasma wave science instrument detected hundreds of dust hits per second when Cassini was passing just outside Saturn’s rings over the last few months, but only registered a few impacts inside the ring gap.

Scientists converted the raw radio and plasma wave data into an audio format, NASA said, to listen for debris striking Cassini’s antenna.

“Dust particles hitting the instrument’s antennas sound like pops and cracks, covering up the usual whistles and squeaks of waves in the charged particle environment that the instrument is designed to detect,” NASA said in a press release. “The RPWS team expected to hear a lot of pops and cracks on crossing the ring plane inside the gap, but instead, the whistles and squeaks came through surprisingly clearly on April 26.”

“It was a bit disorienting — we weren’t hearing what we expected to hear,” said William Kurth, radio and plasma wave science team lead at the University of Iowa. “I’ve listened to our data from the first dive several times and I can probably count on my hands the number of dust particle impacts I hear.”

Cassini made the trip through the ring gap at a relative velocity of about 77,000 mph (124,000 kilometres per hour), fast enough to travel from New York to Los Angeles in less than two minutes.

The video posted below includes the audio recording from Cassini’s radio and plasma wave science instrument during the April 26 flyby.

The grains that hit Cassini were likely no bigger than a particle of smoke, or about 1 micron in size, according to NASA.

Cassini’s swing inside Saturn’s rings Tuesday occurred without using the craft’s antenna as a shield. Mission managers decided such a precaution was no longer necessary after sampling the dust during the first flyby.

But four of the 20 remaining ring gap passages will place Cassini closer to the inner edge of Saturn’s D ring, where scientists expect more dust particles. During those orbits, which begin in late May, the spacecraft will again turn its high-gain antenna into a shield.

Imagery from Cassini’s approach to Saturn on April 26 revealed the closest-ever views of the planet’s clouds and a bizarre six-sided polar vortex scientists had only studied from afar before.

“These images are shocking,” said Kevin Baines, an atmospheric scientist on the Cassini team at JPL. “We didn’t expect to get anything nearly as beautiful as these images. All the different structures we see on them are phenomenal. We predicted we’d see fogs and something pretty boring, but we’re seeing lots of great features — a lot of activity going on on Saturn.”

Baines called the hexagonal storm swirling at Saturn’s north pole the planet’s “belly button.”

“This is a hole in the pole that is very deep, and we can tell that from looking at different colors of light,” Baines said Friday in a Facebook Live event, comparing its structure to the behaviour of water in a flushing toilet. “This is about 2,000 kilometres (1,200 miles) across.”

Winds whip around the storm at up to 180 mph, or 300 kilometres per hour, Baines said. Like a hurricane on Earth, the wind speeds die down farther from the center of circulation, where individual storm clouds appear to move around Saturn in the planet’s jet stream.

Cassini took this image of Saturn and part of the planet’s ring system April 29. Credit: NASA/JPL-Caltech/Space Science Institute

“Now we see structure,” Baines said. “You see the curly cues on here, all sorts of strange features that we’re trying to understand … Now we’re seeing little tiny circular clouds that really have their own individual characters.

“They might (have) convective upwelling from below, so we’re looking for lightning and other things to see if we can really confirm that,” Baines said.

“(For) this first dive, we’re focusing on looking at Saturn,” said Linda Spilker, Cassini’s project scientist at JPL. “We got a series of images from the pole to the equator. We have other data as well, spectra in the infrared, the far-infrared and ultraviolet that will help us put together the puzzle of what we’re seeing.”

During the mission’s second orbit through the ring gap, Cassini’s cameras were programmed to take pictures of Saturn’s rings backlit by the sun, a viewing geometry that allows the instruments to see faint ringlets and other fine structures.

Future encounters will focus on studying Saturn’s interior, magnetic field and taking the first measurement of the mass of the planet’s rings, which will tell scientists about their age and origin.

The video posted below condenses one hour of observations into an animated movie showing a series of Cassini images taken April 26.

The movie shows Cassini’s view of Saturn starting from an altitude of 45,000 miles to just 4,200 miles (72,400 kilometers to 6,700 kilometers) above the planet’s cloud tops.

“I was surprised to see so many sharp edges along the hexagon’s outer boundary and the eye-wall of the polar vortex,” said Kunio Sayanagi, an associate of the Cassini imaging team based at Hampton University in Virginia, who helped produce the new movie. “Something must be keeping different latitudes from mixing to maintain those edges.”

“The images from the first pass were great, but we were conservative with the camera settings. We plan to make updates to our observations for a similar opportunity on June 28 that we think will result in even better views,” said Andrew Ingersoll, a member of the Cassini imaging team based at Caltech.

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