Cassini’s 20-year mission culminated in a series of wild orbits. First, it grazed the outer rim of Saturn’s rings and then, in the Grand Finale phase, the spacecraft dove through the narrow gap between the gas giant and its icy rings before plunging into and disintegrating into the planet’s upper atmosphere. In a special issue of the journal Science, six articles provide new observations and insights into this previously unexplored region of the Saturn system.
NASA’s Cassini spacecraft in orbit around Saturn. Image credit: NASA / JPL-Caltech.
Knowing Cassini’s days were numbered, its mission team went for gold. The spacecraft flew where it was never designed to fly.
For the first time, Cassini probed Saturn’s magnetized environment, flew through icy, rocky ring particles and sniffed the atmosphere in the 1,200-mile (2,000 km) wide gap between the rings and the cloud tops.
Not only did the flight path push the spacecraft to its limits, the new findings illustrate how powerful and agile the instruments were.
Many more Grand Finale science results are to come, but here are some of them:
(i) Cassini saw up close how Saturn’s rings interact with the planet and observed inner-ring particles and gases falling directly into the atmosphere; some particles take on electric charges and spiral along magnetic-field lines, falling into the planet at higher latitudes — a phenomenon known as ‘ring rain;’ but the Cassini scientists were surprised to see that others are dragged quickly into Saturn at the equator; and it’s all falling out of the rings faster than the researchers thought — as much as 10 tons of material per second;
(ii) complex organic compounds embedded in water nanograins rain down from Saturn’s rings into its 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 organics is different from that found on Enceladus — and also different from that on Titan, meaning there are at least three distinct reservoirs of organic molecules in the Saturn system;
(iii) Cassini scientists were surprised to see what the material looks like in the gap between Saturn’s rings and the gas giant’s atmosphere; they knew that the particles throughout the rings ranged from large to small; but the sampling in the gap showed mostly tiny, nanoparticles, like smoke, suggesting that some yet-unknown process is grinding up particles;
(iv) Saturn and its rings are even more interconnected than researchers thought; Cassini revealed a previously unknown electric-current system that connects the rings to the top of Saturn’s atmosphere;
(v) Cassini scientists discovered a new radiation belt around Saturn, close to the planet and composed of energetic particles; they found that while the belt actually intersects with the innermost ring, the ring is so tenuous that it doesn’t block the belt from forming;
(vi) unlike every other planet with a magnetic field in our Solar System, Saturn’s magnetic field is almost completely aligned with its spin axis; the new data from Cassini show a magnetic-field tilt of less than 0.0095 degrees; according to everything scientists know about how planetary magnetic fields are generated, Saturn should not have one; it’s a mystery that planetary researchers will be working to solve;
(vii) Cassini flew above Saturn’s magnetic poles, directly sampling regions where radio emissions are generated; the findings more than doubled the number of direct measurements of radio sources from the planet, one of the few non-terrestrial locations where scientists have been able to study a radio-generation mechanism that is believed to operate throughout the Universe.
A few of the findings from Cassini’s direct sampling: complex organics rain down from Saturn’s rings; inner-ring particles take on electric charges and travel along magnetic-field lines; newly revealed electric-current system and radiation belt; and up-close measurement of Saturn’s near-zero magnetic-field tilt. Image credit: NASA / JPL-Caltech.
“For the Cassini mission, the science rolling out from Grand Finale orbits more than justifies the calculated risk of diving into the gap — skimming the upper atmosphere and skirting the edge of the inner rings,” said Cassini project scientist Dr. Linda Spilker, of NASA’s Jet Propulsion Laboratory.
“Almost everything going on in that region turned out to be a surprise. That was the importance of going there, to explore a place we’d never been before. And the expedition really paid off — the data is tremendously exciting.”
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E. Roussos et al. 2018. A radiation belt of energetic protons located between Saturn and its rings. Science 362 (6410); doi: 10.1126/science.aat1962
L. Lamy et al. 2018. The low-frequency source of Saturn’s kilometric radiation. Science 362 (6410); doi: 10.1126/science.aat2027
D.G. Mitchell et al. 2018. Dust grains fall from Saturn’s D-ring into its equatorial upper atmosphere. Science 362 (6410); doi: 10.1126/science.aat2236
J.H. Waite, Jr et al. 2018. Chemical interactions between Saturn’s atmosphere and its rings. Science 362 (6410); doi: 10.1126/science.aat2382
Hsiang-Wen Hsu et al. 2018. In situ collection of dust grains falling from Saturn’s rings into its atmosphere. Science 362 (6410); doi: 10.1126/science.aat3185
Michele K. Dougherty et al. 2018. Saturn’s magnetic field revealed by the Cassini Grand Finale. Science 362 (6410); doi: 10.1126/science.aat5434
This article is based on text provided by the National Aeronautics and Space Administration.
