Cassini Detected ‘Ring Rain’ During Plunge into Saturn

It has been more than a year since the iconic Cassini probe ended its mission by diving into the atmosphere of Saturn, the planet it studied for more than a decade. However, Cassini is the mission that just keeps on giving. Scientists are still analyzing the data collected by Cassini during its final days, and they say the spacecraft encountered an unexpected phenomenon near Saturn: ring rain.

Cassini arrived in orbit of Saturn in 2004, but it was unable to detect this newly discovered process from its perspective high above the planet. The probe had to stay far away from the planet because of the intense radiation and ring debris. NASA threw caution to the wind in 2017 as the spacecraft ran low on fuel. Without the ability to control Cassini, NASA worried it might one day impact one of the planet’s moons and contaminate a potential ecosystem. Therefore, NASA crashed Cassini into Saturn.

On the way to annihilation, NASA executed what it called Cassini’s “Grand Finale,” a series of sweeping passes between Saturn and its rings. That’s how the probe got into position to spot the ring rain. During the final few orbits of Saturn, Cassini’s Ion and Neutral Mass Spectrometer (INMS) detected water ice and complex organic molecules falling from the rings into the clouds of Saturn. Scientists long expected some transfer of material from the rings to the planet, but they were not prepared for the volume — it was a downpour.

Cassini’s data shows that 22,000 pounds (10,000 kilograms) of material rains down on Saturn every second. The INMS also characterized what was in the rain. It was molecular hydrogen and helium, but 3.4 percent of it was more complex. There was water, of course, but also organic (carbon-containing) compounds. That included methane, carbon monoxide, and even “complex” organics like butane and propane.

Cassini was only able to gather so much data on the rain because it was moving so fast through the ring system. Rain particles hit the INMS antenna and shattered, which made the data difficult to interpret and some molecules in the readout might be fragments of larger molecules. Still, the probe reported the rainfall varied during its various passes, and that could suggest the chemistry of the D-ring varies based on location. Scientists want to take a closer look at the ring rain with proper instruments in the future.

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