A Simon and Garfunkel song comes to mind—and that has scientists scratching their heads as the spacecraft heads today for a second dive.

As the Cassini spacecraft swooped between Saturn and its innermost ring on April 26th, one of its instruments listened for the sounds of its passage through the heretofore unexplored region. What it heard was of great interest to engineers planning for the second dive, and as well as to scientists who study Saturn’s rings.

The engineers were hoping to hear the sound of silence, specifically nothing indicative of lots of dust hitting the spacecraft. If the gaps between Saturn and its rings turned out to be very dusty, on future dives — including the next one, scheduled for today, May 2nd — the spacecraft would once again have to use its 13-foot-wide antenna to shield its delicate instruments.

The ring scientists, though, were expecting to hear the sound of lots of particles hitting the spacecraft — because they thought this gap would be fairly dusty.

As it turned out, the engineers came away delighted, and the scientists puzzled.

“The region between the rings and Saturn is ‘the big empty,’ apparently,” said Cassini Project Manager Earl Maize of NASA’s Jet Propulsion Laboratory, quoted in a press release. “Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected.”

Let’s listen to what the spacecraft heard:

This video presents data collected by the Radio and Plasma Wave Science instrument, or RPWS, on NASA’s Cassini spacecraft. RPWS tunes in, so to speak, to radio waves, as well as plasma waves, around Saturn. In the sound accompanying the video, the whistles and squeaks you hear come from waves in the charged particle environment.

These data provide scientists with valuable information beyond what Cassini’s imaging instruments offer. As NASA puts it:

By studying radio and plasma waves around Saturn, scientists can better understand Saturn’s relationship with its moons and rings, as well as how the planet interacts with the solar wind. For example, Saturn’s auroras emit radio waves in approximately the same frequency range as AM radio stations on Earth.

But the instrument also can hear any dust that’s hitting it. Not quite like a microphone on a recorder, though. Instead, data on particle strikes are converted to an audio format, producing pops and cracks.

In the audio track above, there is virtually no detectable peak in pops and cracks from dust particles striking the spacecraft as it shoots through the gap, labeled as “Ring Plane Crossing.”

“It was a bit disorienting — we weren’t hearing what we expected to hear,” said William Kurth, RPWS team lead at the University of Iowa, quoted in the NASA press release. “I’ve listened to our data from the first dive several times and I can probably count on my hands the number of dust particle impacts I hear.”

In stark contrast, here’s what it sounded like when Cassini passed through a faint, dusty ring on Dec. 18, 2016:

That peak in pops and cracks toward the middle is the sound of a very dusty environment. And it is obviously very different from what Cassini heard as it shot through the gap on April 26th.

That dive, and the second one scheduled for today, are part of Cassini’s ‘Grand Finale.’ This final chapter in Cassini’s epic journey will include 22 dives between the rings and the planet. It will all culminate on September 15, 2017, when Cassini is scheduled to dive into Saturn’s atmosphere, putting an end to the mission.