Cassini’s expedition to the Saturn system was undoubtedly one of NASA’s most spectacular missions yet. For 13 years, the probe had studied the gas giant and its nearby moons, collected vast amounts of data, and shot countless shots that led to many new discoveries. For example, the five-ton instrument panel discovered an ocean under the ice cap of the moon, Enceladus . It studied lakes of liquid methane on the Titan, showed us the wonders of the complex ring system , and even sent a snapshot of Earth 1.4 billion kilometers away from home.
As impressive as Cassini’s mission was, everything, we know, has an end: On September 15, 2017, Cassini burned its last reserves of fuel during a kamikaze flight into the atmosphere of the gigantic Ring Planet. With this suicide maneuver during which the probe passed on the inside of the rings, NASA wanted to prevent Cassini from shattering on one of Saturn’s moons, thereby contaminating it with traces of life from Earth. No images were taken during the last moments of the probe, the camera system had been switched off a day earlier. But even in blind flight Cassini could still collect data: The radio link remained active until the last second.
The evaluation of these measurements during the “Grand Finale” , as NASA refers to call the last hours of the Cassini mission, is still not completed, but some of it has already culminated in a series of studies, which are now published in the journal “Science”.
Some of these studies have so far confirmed what earlier Cassini observations had already suggested: the rings of Saturn are much more diverse than thought. And they evidently have a significant impact on the composition of the uppermost atmospheric layers of the gas giant.
The ring system, which is between ten and 100 meters thick and has a width of almost one million kilometers, is apparently more chemically complex than assumed. How long it will last though, is unclear. “Two of our findings have particularly surprised me,” says Thomas Cravens of the University of Kansas, who, together with his team, studied the interactions between the innermost D-ring and Saturn itself. “One is the chemical complexity of that ‘rain’ of small particles from that ring, the second is the sheer volume of material that is continuously falling onto Saturn.” So far, it has been assumed that the particles consist almost entirely of water ice – this opinion should now be revised.
On the other hand, the INMS mass spectrometer on board Cassini discovered that at least the inner rings are contaminated with methane, ammonia, carbon monoxide, molecular nitrogen and carbon dioxide when passing through the gap between the rings and Saturn.
This continued rainfall is also likely to have long-term effects on the gas envelope of Saturn. “What we’ve found is that the material changes the outer layers of the Saturn’s atmosphere around the equator, and we detected both dust and water ice grains with admixtures of organic substances, but what happens is going in there is not completely understandable, even with our new data,” says Cravens.
The unexpectedly high loss of material from the D-ring could mean that the lifespan of the ring system may be shorter than expected. “At least ten times faster than expected, it trickles out of the ring towards Saturn, becoming these particles not somehow replaced, we have to assume that the rings have an expiry date,” the researchers assume, not least because
Jupiter once possessed a considerably larger ring system, of which only a sparse rudiment exists today. “Apparently, these ring systems come and go,” Cravens says, “but it will not be possible to say for how long Saturn’s famous ring will last.”