What Cassini found as it plunged into Saturn a year ago
A year after Cassini ended its 20-year mission to study Saturn, the last data the spacecraft recorded before plunging into the planet’s atmosphere are revealing its long-held secrets.
Cassini’s sensors were recording until the last second during its “death dive,” and all of the data were transmitted to NASA scientists on Earth before the spacecraft disintegrated.
Over the past year, scientists have been analyzing the data.
Saturn’s magnetic field has long remained elusive, but Cassini’s instruments allowed scientists to measure the planet’s internal and external fields. However, they are no closer to understanding how it formed.
Their detections revealed a radiation belt, acting like an electric current, trapped between Saturn and its rings. This suggests that there is a lot of structure within the dust environment around Saturn. And when Cassini flew through the magnetosphere, they were able to pick up on the radio emissions the planet gives off. Understanding the radio emissions could help in the search for exoplanets, the researchers said.
The rest of the results, published Thursday in the journal Science, include an unprecedented look at the planet’s upper atmosphere and its rings.
Cassini’s up-close encounter with Saturn has shed light on “a new element of how our solar system works,” said Thomas Cravens, co-author of one of the studies and a University of Kansas professor of physics and astronomy.
Saturn’s ‘ring rain’
During the final plunge, Cassini’s Ion and Neutral Mass Spectrometer acted as the “nose” of the spacecraft, directly sampling the composition and structure of the atmosphere. It’s something that can’t be done from orbit, said Hunter Waite, principal investigator for the spectrometer.
This was in the hopes of investigating the “ring rain” phenomenon discovered by NASA’s Voyager mission in the early 1980s, in which it appeared that the rings were raining material onto the planet and causing changes in the atmosphere. The spectrometer could determine what material is from the rings and what is part of the atmosphere.
According to the spectrometer team, Cassini’s nose hit the “jackpot” as it sniffed out the unknown region between the planet and its closest rings. This is key because Saturn’s upper atmosphere extends almost to the rings.
In a new study, researchers determined that complex organic compounds are raining a chemical cocktail of dust grains from the closest ring, D ring, into the upper atmosphere. The spectrometer revealed the rings to be composed of water, methane, ammonia, carbon monoxide, molecular nitrogen and carbon dioxide.
“Turns out, ring rain is more like a ring downpour,” said Waite, lead study author. “While [the spectrometer] was designed to investigate gases, we were able to measure the ring particles because they hit the spacecraft at such high velocities they vaporized. Water ice, along with the newly discovered organic compounds, is falling out of the rings way faster than anyone thought — as much as 10,000 kilograms of material per second.”
Water ice was expected, but the other organics were not.
“Molecular hydrogen was, as expected, the most abundant atmospheric constituent,” said Kelly Miller, study co-author and Southwest Research Institute scientist. “But the downpour coming from the rings included plenty of water as well as molecules like butane and propane — the kind of chemicals you might use for a grill or camping stove.”
Recovering and examining the data from the last functioning instrument on Cassini wasn’t easy, and the process took months.
“It was worth it,” Waite said. “The large mass of infalling material has implications for ring evolution, hinting that material from the C ring repeatedly replenishes the neighboring D ring. This infalling material likely affects the atmospheric chemistry and the carbon content of Saturn’s ionosphere and atmosphere.”
Because the ring is spinning faster than the planet’s atmosphere, the inflow of materials could actually change the carbon and oxygen content of the atmosphere over time, the researchers said. The chemicals actually convert the ions in Saturn’s upper atmosphere, making them heavier. The team is still studying what those impacts might be, but they believe this is heating the upper atmosphere and changing its composition.
“We saw it was happening even though it’s not fully understood,” Cravens said. “What we saw is this material, including some benzine, was altering the uppermost atmosphere of Saturn in the equatorial region. There were both grains and dust that were contaminated.”
But how does the material fall out of the rings in the first place? One of the studies in the suite of research released Thursday determined that grains of silicate and water ice and atoms in the rings collide, which allows them to fall out of orbit and plunge into the atmosphere.
If the rings are losing this much material to the planet, that could shorten their “life span,” so to speak.
“We know that it’s bumping material out of the rings at least 10 times faster than we thought,” Cravens said. “If it’s not being replenished, the rings aren’t going to last — you’ve got a hole in your bucket. Jupiter probably had a ring that evolved into the current wispy ring, and it could be for similar reasons. Rings do come and go. At some point they gradually drain away unless somehow they’re getting new material.”