Saturn’s icy rings are among the most iconic features in the solar system. But they’re raining so much water onto the planet that in 300 million years they could rain themselves nearly out of existence, leaving Saturn startlingly ringless.

“What we’re seeing is something on the order of about a ton and a half per second,” said James O’Donoghue of NASA’s Goddard Space Flight Center, in Greenbelt, Md., who reports the conclusions Monday in the journal Icarus.

“The rings of Saturn haven’t been around forever,” he said. “And they’re going to disappear someday.”

It’s difficult to imagine a solar system without Saturn’s rings, but it turns out they are an ephemeral, if majestic, phenomenon. Scientists have long debated the age and expected life span of the rings, and last year, some published findings suggesting that they are not ancient relics from the birth of the solar system, 4.5 billion years ago. Instead, they were fastened onto Saturn within the last few hundred million years — during the time of the dinosaurs. And now, multiple lines of evidence are converging to suggest that the rings come with an expiration date.