How water escapes from Saturn: Magnetic reconnection

UM Professor Daniel Reisenfeld is a member of the Cassini research team. Cassini is a NASA-managed probe that studies Saturn. It has been in orbit continuously collecting data since 2004.

One of the instruments on Cassini measures the planet's magnetosphere -- the charged particles, known as plasma, that are trapped in the space surrounding Saturn by its magnetic field. One of Cassini's past discoveries is that Saturn's plasma comprises water ions, which are derived from Saturn's moon Enceladus, which spews water vapors from its Yellowstone-like geysers. Knowing that the water ions would not be able to accumulate indefinitely, the team of researchers set out to explain how the water ions escape from Saturn's magnetosphere.

The answers to this phenomenon were published by Nature Physics in an article titled "Cassini in situ observations of long-duration magnetic reconnection in Saturn's magnetotail."

In the paper, the authors explain that the plasma found a place to exhaust out of the magnetosphere at a reconnection point -- basically where magnetic fields from one environment disconnect and reconnect with magnetic fields from another environment. In the case of Saturn, researchers discovered the reconnection point was located at the back of the planet, where the magnetotail was connecting with the solar winds' magnetic field.

Reisenfeld likens the situation to a rotary or a traffic circle. Once you get into the rotary you have limited exit points.

"If you can't find the exit, you keep going around in circles," he said. "So, the plasma around Saturn is basically trapped to go around the rotary. We assumed it had to escape somehow and somewhere, but actually finding the jettison point is pretty cool."

Saturn is a very rapidly rotating planet. This discovery will help scientists understand the physics of how other rapid rotators such as Jupiter, stars and pulsars expel their materials and the details of how it works. "It's very exciting to have discovered this reconnection location because reconnection is one of the holy grails of plasma physics," Reisenfeld said.