As dusk sets on NASA’s Cassini mission, Saturn still providing surprises
After twenty years in space and thirteen years directly observing Saturn and its system of hypnotic rings and moons, the Cassini spacecraft is continuing to tease out tantalizing data from the mysterious ringed beauty about every six days.
Currently in its sixteenth of twenty-two Grand Finale orbits that will culminate in the spacecraft’s plunge into Saturn’s atmosphere on September 15, 2017, Cassini keeps sending back consistently stunning images as well as jaw dropping data.
“Cassini is performing beautifully in the final leg of its long journey,” said Earl Maize, Cassini Project Scientist at NASA’s Jet Propulsion Laboratory (JPL), Pasadena, California. “Its observations continue to surprise and delight as we squeeze out every last bit of science that we can get.”
The Imaging Science Subsystem (ISS) was given priority to observe Titan for two periods, each lasting several hours, to view the atmosphere as well as the surface, and in hopes of observing formation and changes to clouds on Titan. In addition to the ISS, the Composite Infrared Spectrometer (CIRS) and the Visible and Infrared Mapping Spectrometer (VIMS) were active in this pursuit.
As Cassini approached and swooped past Saturn, the Ultraviolet Imaging Spectrograph (UVIS) observed both the northern and southern auroral zones, with the southern zone in darkness and the northern in sunlight, and a stunning image mosaic of this pass was made.
When Cassini began its Grand Finale orbits back in April 2017, one of the hopes of scientists studying the rings was to be able to closely observe the ring particles and measure their mass to get a better handle on their age and composition.
On June 29, 2017, during the third of four of Cassini’s close approaches to the innermost D-ring, Cassini’s scientists and engineers decided to take a chance and turn the spacecraft so that the cosmic dust analyzer (CDA) instrument could directly sample the nanometer size particles. This strategic alignment allowed the spacecraft to be able to take a sample of some of the super fine particles as it passed just 3,040 miles (4,890 kilometers) from the inner edge of the D-ring while using the CDA.
Cassini spies ring ‘plateaus’
Ring science researchers are excited to receive CDA results in coordination with some of the best high-resolution images ever received of the rings, including the C-ring with its bright bands and streaky textured appearance referred to as “plateaus”.
The central feature in this image, called Plateau P1, is found approximately 47,300 miles (76,200 kilometers) from Saturn’s center. It is situated amid some undulating structure that characterizes this region of the C ring. None of this structure is well understood. This image, especially the enhanced version (right), reveals three different textures with different kinds of structure. Images & Caption Credit: NASA / JPL-Caltech / Space Science Institute
Unlike geologic plateaus, Saturn’s ring plateaus aren’t necessarily higher in elevation, but rather they are an area of higher particle density which appears as brightness. When these regions are compared to the surrounding ring region, the non-plateau areas seem to lack any apparent structure whereas the plateaus are approximately five times denser.
“The data we are seeing from Cassini’s Grand Finale are every bit as exciting as we hoped, although we are still deep in the process of working out what they are telling us about Saturn and its rings,” said Cassini Project Scientist Linda Spilker at JPL.
The new level of detail in the images combined with the results from the CDA data should shed some light on the questions of why and how they were created, and what the makes them different from other regions of the rings.
The central feature in this image, called Plateau P5, is found approximately 52,700 miles (84,800 kilometers) from Saturn’s center. It is situated amid some undulating structure that characterizes this region of the C ring. None of this structure is well understood. This image, especially the enhanced version (right), reveals that the plateau itself is shot through with elongated streaks. Images & Caption Credit: NASA / JPL-Caltech / Space Science Institute
It wasn’t only the rings that received Cassini’s attention. The ion and neutral mass spectrometer (INMS) was also able to take samples of the planet’s exosphere – the atmosphere’s outermost layer – just 1,750 miles (2,810 kilometers) above Saturn’s cloud tops. Cassini’s imaging cameras were able to get some of the highest resolution images ever taken of Saturn’s clouds. These images include two new image mosaics and a movie sequence.
Cassini investigates Saturn’s Magnetic field
As interesting as all of that is, some of the most intriguing results have come from the gravitational and magnetic field data. When Saturn was first visited by the Voyager probes in 1980 and 1981, they noted that Saturn’s magnetic tilt was very well aligned with its axial tilt, which made calculating the exact length of a Saturnian sidereal day impossible. Its sidereal day is currently estimated at 10 hours, 47 minutes.
When Cassini arrived 13 years ago, it found much the same thing, but the spacecraft had come prepared with far more sensitive equipment. Not only does Cassini have a more sensitive magnetometer (MAG), it has two – the vector/scalar helium magnetometer located at the far end of the 36-foot (11-meter) boom, and the fluxgate magnetometer positioned half-way out along the boom.
Both instruments can measure strength and direction of magnetic fields, but they also have individual abilities as well. The vector/scalar helium magnetometer can also detect the strength of fields alone, whereas the fluxgate magnetometer can detect a range of strength three times greater than the vector/scalar magnetometer.
Why is this important? Because measuring and mapping the magnetic fields on a planet should give you an understanding of how those magnetic fields are generated. However, this is where things get a bit weird on Saturn.
Until now, we believed that electromagnetic fields in planets, called dynamos, were created and sustained by liquid metals surrounding and moving around a solid metal core deep inside of a planet. The greater the planet’s mass and more movement within the liquid core, the larger and stronger the magnetic field. Our understanding of how the liquid metal core spins is based on the difference in the axial rotation or the tilt of the planet as it spins or rotates in space.
The Earth has a 23.5-degree axial tilt, and Jupiter has an axial tilt of just 3.13 degrees. Saturn, on the other hand, has an axial tilt of less than 0.06 degrees, and that is only an estimate because it’s the lowest measurement the equipment is capable measuring down to.
The comparison of the gravitational field data and magnetic field data has come back with more than a small number of discrepancies from the expected models. These discrepancies suggest that there is something quite strange going on deep inside of the planet and that Saturn’s deep atmosphere could be masking how and where the internal magnetic field is being generated.
Everything that we believe thus far suggests that a planet with virtually no axial tilt, such as Saturn, would be incapable of sustaining a dynamo, let alone such a powerful dynamo as it possesses. Is there a link to the magnetic field generation somewhere in the deep narrow atmospheric lanes and zones? Is it generated by an as yet unidentified substance? Are Saturn’s thread-like convective cells contributing to it?
It will be interesting to see what researchers and scientists are able to piece together based on the data from these next (and last) seven passes. Once Cassini is lost after it plunges into Saturn on September 15, 2017, any additional data to answer these questions could take decades to obtain as there are no new missions currently being planned that extend beyond the orbit of Jupiter.
The Cassini-Huygens mission is a cooperative project of NASA, European Space Agency (ESA), and the Italian Space Agency. NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the mission for NASA’s Science Mission Directorate, Washington. JPL designed, developed, and assembled the Cassini orbiter.
The sounds and colorful spectrogram in this still image and video represent data collected by the Radio and Plasma Wave Science (RPWS) instrument on NASA’s Cassini spacecraft, as it crossed through Saturn’s D ring on May 28, 2017. Image/Audio & Caption Credit: NASA / JPL-Caltech / University of Iowa
A native of the Greater Los Angeles area, Ocean McIntyre's writing is focused primarily on science (STEM and STEAM) education and public outreach. McIntyre is a NASA/JPL Solar System Ambassador as well as holding memberships with The Planetary Society, Los Angeles Astronomical Society, and is a founding member of SafePlaceForSpace.org. McIntyre is currently studying astrophysics and planetary science with additional interests in astrobiology, cosmology and directed energy propulsion technology. With SpaceFlight Insider seeking to expand the amount of science articles it produces, McIntyre was a welcomed addition to our growing team.