Revolutionary science emerges from Saturn's ultra-close orbits

Illustration: NASA's Cassini probe orbiting Saturn. Credit: NASA / JPL-Caltech Credit: NASA / JPL-Caltech

New research emerging from the final orbits of NASA's Cassini spacecraft represents a significant advance in our understanding of Saturn's system – particularly the mysterious, never-before-seen region between the planet and its rings. Some preconceptions turn out to be false as new issues come up.

Six teams of researchers publish their work on October 5 in the journal Science, based on the conclusions of the grand finale of Cassini. It was at this point that, while the spacecraft ran out of fuel, the mission team flew Cassini dramatically near Saturn on 22 orbits before deliberately evaporating it during a crash. final dive into the atmosphere in September 2017.

Illustration: NASA's Cassini spacecraft plunges between Saturn and its innermost rings, as part of the grand finale of the mission. Credit: NASA / JPL-Caltech

Knowing that Cassini's days were numbered, his mission team headed for gold. The spaceship flew where it was never designed to fly. For the first time, he surveyed the magnetized environment of Saturn, flew over frozen and rocky ring particles and sniffed the atmosphere in the gap between the rings and the tops of the clouds. Not only did the flight path push the spacecraft to its limits, but new discoveries show how powerful and agile the instruments were.

Many other scientific results from the grand finale are forthcoming, but here are some of today's highlights:

• Complex organic compounds encased in water nanograins pour out from the rings of Saturn in the upper atmosphere. The scientists saw water and silicates, but they were surprised to see also methane, ammonia, carbon monoxide, nitrogen and carbon dioxide. The composition of the organic compounds is different from that found on the Enceladus moon – and also different from that of the Titan moon, which means that there are at least three distinct reservoirs of organic molecules in the Saturn system.

• For the first time, Cassini closely examined the interactions of the rings with the planet and observed particles and gases from the inner ring falling directly into the atmosphere. Some particles take electrical charges and spiral along magnetic field lines, falling into Saturn at higher latitudes – a phenomenon known as "circular rain". But scientists were surprised to see that others are quickly drawn into Saturn at the equator. And all of this falls off the rings faster than scientists thought – up to 22,000 pounds (10,000 kilograms) of material per second.

• Scientists were surprised to see what the material looked like in the space between the rings and Saturn's atmosphere. They knew that the particles in the rings were large to small. But sampling in space has mostly revealed tiny particles of nanoscale, such as smoke, suggesting that a yet unknown process involves grinding particles.

• Saturn and its rings are even more interconnected than scientists thought. Cassini revealed a previously unknown electrical current system that connected the rings to the top of Saturn's atmosphere.

• Scientists have discovered a new radiation belt around Saturn, close to the planet and composed of energetic particles. They found that, if the belt actually intersected with the innermost ring, the ring was so tenuous that it did not block the formation of the belt.

• Unlike all other planets with a magnetic field in our solar system, Saturn's magnetic field is almost completely aligned with its axis of rotation. The new data indicates a tilt of the magnetic field less than 0.0095 degrees. (The earth's magnetic field is tilted 11 degrees from its axis of rotation.) According to all that scientists know about how planetary magnetic fields are generated, Saturn should not have any. It is a mystery that physicists will try to solve.

• Cassini flew over the magnetic poles of Saturn, directly sampling the regions where radio broadcasts are generated. The results more than doubled the number of direct measurements of radio sources on the planet, one of the few non-terrestrial locations where scientists have been able to study a radio generation mechanism that is supposed to work in the universe as a whole.

Some results from Cassini's direct sampling: complex organic matter escapes from the rings of Saturn; The particles of the inner ring take up electric charges and move along the magnetic field lines; recently discovered electric current system and radiation belt; and a closer measurement of the tilt of the near-zero magnetic field of Saturn. Credit: NASA / JPL-Caltech

For the Cassini mission, the science deployed from the orbits of the Grand Finale justifies more than the calculated risk of diving into space: fly over the upper atmosphere and bypass the edge of the inner rings, said Linda Spilker, Cassini Project Manager.

"Almost everything that was happening in this area has turned out to be a surprise," Spilker said. "It was important to go there, to explore a place we had never been before. And the expedition has really paid off – the data is extremely exciting. "

The analysis of Cassini data from spacecraft instruments will continue in the coming years, which will help to paint a clearer picture of Saturn.

"Many mysteries remain, as we assemble the pieces of the puzzle," Spilker said. "The results of the latest Cassini orbits have proved more interesting than we could have imagined."

This illustration shows NASA's view of Cassini's probe during one of her last dives between Saturn and her innermost rings, as part of the Mission's Grand Final. Credit: NASA / JPL-Caltech

Articles published in Science are:

• "Chemical interactions between the atmosphere of Saturn and its rings", by J. Hunter Waite, et.al.
• "The D-Ring dust falling in the equatorial ionosphere and the upper atmosphere of Saturn", by Donald Mitchell, et.al.
• "In situ collection of dust grains falling from the rings of Saturn into its atmosphere", by Hsiang-Wen Hsu, et al.
• "An energy proton radiation belt located between Saturn and its rings",by Elias Roussos, Peter Kollmann, et al.
• "The magnetic field of Saturn revealed by the Cassini Grande Final", by Michele Dougherty, et al.
• "The low-frequency source of Saturn's kilometric radiation (SKR)", by Laurent Lamy, et.al.

On October 4, during the lifting of the embargo in the Science publication, articles describing the research complementary to these findings will be published online in Geophysical Research Letters (GRL), a journal of the American Geophysical Union (AGU).