ORLANDO, June 17, 2019 - Even though NASA's Cassini spacecraft's mission to Saturn ended in 2017, scientists are still poring over the copious amounts of data it transmitted.
Now, in a new paper that appeared in Science on Friday and includes two University of Central Florida co-authors, researchers are offering glimpses into the nature and composition of the mighty planet's legendary rings by using data from some of the closest observations ever made of the main rings.
The paper is a big picture and detailed look at the planet's rings and includes an analysis of Cassini's "grand finale" observations made before the spacecraft's planned crash into the planet on Sept. 15, 2017.
The study reports the rings, which are comprised of icy particles ranging from the size of a marble to the size of a car, have three distinct textures - clumpy, smooth and streaky - and that tiny moons exist within the rings and interact with surrounding particles.
Josh Colwell, a UCF physics professor and study co-author, has been a part of the Cassini mission since some of its earliest planning stages in 1990, including the design and observation planning for the Ultraviolet Imaging Spectrograph, or UVIS, on the multi-instrument spacecraft.
In the paper, Colwell and his former student Richard Jerousek, a researcher with UCF's Florida Space Institute and a study co-author, measured and described the structure of Saturn's largest innermost ring, the C Ring, using UVIS data recorded by Cassini.
Using the UVIS's photometer, which measured the brightness of starlight shining through the rings, and by having Cassini take observations from multiple different angles, the researchers were able to create a three-dimensional map of the ring.
They did this by having the photometer focus on a star from a particular angle and then measure the star's brightness as the spacecraft looked at the star through the ring.
Areas where more light passed through indicated areas with less material or gaps in the ring, while areas with less light shining through indicated a denser area where more material was present.
"You can think of it like a friend running through the woods at night with a flashlight pointed at you," Colwell said. "You would see the flashlight flicker because of trees blocking the light."
"So, we did a similar thing with the rings and the flickering of the star tells us something about how many ring particles there are, how big they are and how they are clumped together," Colwell said. "We did many of these observations called stellar occultations."
Colwell and Jerousek found streaky textures in the ring, which seem to be big holes created by the gravity of large boulders that are significantly larger than most ring particles.
They found that vertical thickness of the ring in the locations of these holes is only about 20 feet, while the rings themselves span hundreds of thousands of miles across.
Colwell said it's somewhat odd that there appears to be a larger proportion of the large, boulder-like objects at certain locations in the ring, because they could be made from smaller particles that have run into each other and are sticking together, a process known as accretion. This would be intriguing as the tidal force from Saturn tends to pull objects apart in the rings. The large boulder-like objects also could be fragments of something that's broken apart.
"Both of those possibilities are interesting, and it ties into questions about the early stages of how planets form because the same kinds of processes that form planets could be going on in Saturn's rings today," Colwell said.
Colwell developed the computer model of the mapping procedure to originally examine clumps in the rings, while Jerousek flipped the model to measure the holes.
"From this work we were able to constrain the widths and number of these very narrow gaps or holes as well as the vertical extent of these regions of the rings," Jerousek said.
"These properties are helping us to understand more about the icy boulders that open these gaps and may provide an excellent analog to the early stages of planet formation."
"Understanding these small moonlets and the gaps and textures they create in Saturn's rings provide a snapshot into the early solar system and the conditions in the protoplanetary disk from which planets formed," he said. "Since the details of planet formation are still poorly understood, we're really lucky to have a ring system like Saturn's in our astronomical backyard to help us work out the kinks in our understanding."
Colwell said the end of the mission has been sad, as it has been a part of his life for more than 25 years, and he's made many personal and professional connections as well as had many great experiences. However, he said there is much data still to be analyzed from the mission, and it is something he and his students plan to be working on for years to come.
"We still have so much excellent data, and there's still a tremendous amount to learn," he said.
And while there are no immediate plans to go back to Saturn, there are mission proposals developed. However, Colwell said considering the expense and time it takes to get to Saturn, it could be decades before there is a return.
That means as of now, Cassini is the only spacecraft to make an extensive visit to Saturn. Previous visits were only the flybys made by Voyager 2 in 1981, Voyager 1 in 1980 and Pioneer 11 in 1979. Cassini was launched from Cape Canaveral Air Force Station on Oct. 15, 1997. It arrived at Saturn on June 30, 2004.
Colwell is a Pegasus Professor of physics and assistant director of the Florida Space Institute. He received his doctorate in astrophysical, planetary and atmospheric sciences from the University of Colorado at Boulder and his bachelor of science in physics from Stetson University. He joined UCF in 2006.
Jerousek is a professor of physics and astronomy at Valencia College and joined the Florida Space Institute as a researcher in 2018. He received his doctorate, master's and bachelor's degrees in physics from UCF.