Saturn's magnetic field has no discernible tilt relative to the planet's rotational axis, according to data from NASA's Cassini spacecraft. This unexpected result means the exact length of a day on the planet is still unknown.

Cassini is currently undergoing its Grand Finale phase – 22 weekly orbits of Saturn that take the spacecraft between the planet and its rings. This stage of the Cassini's mission began on 26 April and it has completed 14 of the orbits. After 22, Cassini will perform its final act and plummet into the planet's atmosphere on September 15.

Aligned and challenging

Among the vast swathes of data sent back by Cassini, the spacecraft's magnetometer instrument has revealed that Saturn's magnetic field is closely aligned to its rotational axis. The tilt is in fact much smaller than the lower limit (0.06°) the magnetometer data indicated before the Grand Finale.

The observation challenges the current understanding of how a planet generates a magnetic field. It is thought that there must be some degree of tilt to sustain currents flowing through liquid metal within the planet. Without a tilt, the currents should subside, causing the magnetic field to disappear.

Unknown day

The result also means the true length of a day on Saturn is unknown, because it is measured by a daily "wobble" in the planet's interior caused by the misalignment of magnetic field and rotational axis. "We have not been able to resolve the length of day at Saturn so far, but we're still working on it," says Michele Dougherty, Cassini magnetometer investigation lead from Imperial College London in the UK.

There is the possibility, however, that the lack of tilt may be rectified with further data. Dougherty and colleagues believe an aspect of Saturn's atmosphere could be masking the true magnetic fields and Cassini's plunge into Saturn may reveal further clues.