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Saturn's magnetic field lines
Saturn's magnetic field lines

Orbiting Saturn and its icy moons is like seafaring on a celestial scale -- it helps to have a compass that can "see" through celestial bodies. In Cassini's case, such a compass is provided by its onboard Dual Technique Magnetometer (MAG). The instrument measures the strength and direction of Saturn's magnetic field near the spacecraft.

"The coolest thing about the magnetometer is that it allows you to 'see' inside planets such as Saturn and moons such as Enceladus," says Marcia Burton, investigation scientist for the Cassini magnetometer and the Cassini Magnetospheric Discipline Scientist. "By measuring the magnetic field very accurately we can determine the size of Saturn's core."

Studying the dynamic interactions between different magnetic fields in the planetary environment allows scientists to better understand the complex Saturnian region.

Dr. Nick Achilleos
Dr. Nick Achilleos

"Our instrument is like a very sophisticated magnetic compass traveling through space," says Dr. Nick Achilleos, a science planner and operations engineer working on the magnetometer. "Measuring Saturn's internal magnetic field often shows signatures of the boundaries which separate Saturn's magnetosphere from the solar wind -- these hold information about how Saturn's magnetosphere is continually changing in size and shape."

Data from MAG could also shed light on various mysteries. For one, Saturn has a magnetic field similar to Earth's, but it has one characteristic that scientist consider to be very strange.

"We all know that when we take a compass reading on Earth, magnetic north is generally not in the same direction as geographic north. Saturn is the only planet on which these directions are exactly the same," Burton says. "This presents a real conundrum to scientists who work in this field. By the time the mission is over, we hope to be able to understand this."

Exactly how fast is Saturn rotating is another mystery.

"The rotation rate could be determined by measuring persistent features in the atmosphere such as clouds, but the rotation rate determined by tracking these features seems to vary with latitude," Burton says.

Since the early 1980s, it was thought that a radio signal emitted by Saturn at a roughly 10 1/2 hour period indicated the rate of the planet's rotation. But the period of that radio signal measured in the intervening years has slowed and would indicate that Saturn has slowed down a great deal, much greater than scientists think makes sense.

This artist concept shows current understanding of the interaction of Saturn's magnetic field with the atmosphere of Enceladus.
This artist concept shows current understanding of the interaction of Saturn's magnetic field with the atmosphere of Enceladus.

"We are not sure what is occurring, but we are using the magnetometer data to try to accurately determine the rotation period of Saturn," Burton said. "We see periodic features -- about 10.5 hours -- in the magnetic field data. Since the magnetic field is produced deep inside the planet, any periodicity in the magnetic field should indicate the rotation rate of the deep interior."

Data from MAG also help scientists understand the composition of the moons orbiting Saturn.

"The Enceladus flybys showed us that, very often, an important physical process such as mass loss from Enceladus can be detected magnetically before other instruments pick up any signatures," Achilleos says. "The 'distortion' of the magnetic field often seen near the icy satellites of Saturn can be used to infer what physical processes are operating at and beneath the atmospheres of these moons. In other words, whether or not they are losing mass, do they have a conducting atmosphere or subsurface ocean?"

With its ability to "see" inside celestial bodies, MAG is also contributing to the study of planets much farther away than Saturn.

"What we learn about the interior of the gas giant planets in our solar system is not only of interest to scientists who study Saturn, it is of interest to astronomers who study planets in other solar systems that have been discovered," Burton says.

The plot shows the type of data the magnetometer measures
The plot shows the type of data the magnetometer measures.

MAG includes both a flux gate magnetometer and a vector/scalar helium magnetometer. Because magnetometers are sensitive to electric currents and ferrous metal components, they are generally placed on an extended boom, as far from the spacecraft as possible. On Cassini, the flux gate magnetometer is located midway out on the 11-meter (36-foot) magnetometer boom extending out from the spacecraft, and the vector/scalar helium magnetometer is located at the end of the boom. The boom itself, composed of thin, nonmetallic rods, was folded during launch and deployed about two years after launch. The magnetometer electronics are located in a bay in the Cassini orbiter's spacecraft body.

The core of the team is based at Imperial College in London. It includes people from Britain, South Africa, Canada, the Netherlands, Argentina and Australia.

For more information, read the engineering technical write-up or visit the science team's Web site.

At a Glance

Dual Technique Magnetometer (MAG) diagram inboard
MAG Diagram (Inboard)

The Dual Technique Magnetometer (MAG) is a Direct Sensing Instrument (think smell or taste) that measures the strength and direction of the magnetic field around Saturn. Measuring the magnetic field is one of the ways to probe the core, even though it is far too hot and deep to actually visit. The instrument's goals are to develop a three-dimensional model of Saturn's magnetosphere, as well as determine the magnetic state of Titan and its atmosphere, and investigate the icy satellites and their role in the magnetosphere of Saturn.

  • Mass (current best estimate) = 3.00 kg
  • Average Operating Power (current best estimate) = 3.10 W
  • Average Data Rate (current best estimate) = 3.60 kilobits/s