Radar details large Titan crater

Radar has no problem piercing the photochemical smog that shrouds Titan Enlarge Image

The Cassini spacecraft's latest flyby of Titan has produced a radar image of a 440km-wide (273 miles) crater.

Such features are rare on the Saturnian satellite as its surface, unlike many in the Solar System, is being reshaped.

Tuesday was Cassini's fourth flyby of the moon, but the first on which the probe's radar and imaging camera have overlapped in their coverage.

This overlap should provide more information about surface features than either technique alone, scientists say.

Cassini's first close approach to Enceladus Enlarge Image

A second radar image released by the US space agency (Nasa) shows parallel linear features that have been nicknamed "cat scratches".

These may be formed by winds, like sand dunes, or by other geological processes.

On Thursday, Cassini conducted its first close flyby of Saturn's icy moon Enceladus, at a distance of approximately 1,180km (730 miles).

Enceladus is one of the most reflective objects in the Solar System, so bright that its surface resembles freshly fallen snow.

Bright lights

Cassini has also been involved in a collaborative effort with the Hubble Space Telescope to study Saturn's auroras - bright lights created above the planet when charged particles from the Sun hit molecules in its atmosphere.

Over several weeks Hubble snapped ultraviolet images of the atmospheric lights, whilst Cassini's radio and plasma wave science instrument recorded radio emissions from the same regions, and the probe's plasma spectrometer and magnetometer instruments measured the solar wind.

All measurements were combined to give the most accurate glimpse yet of Saturn's auroras and the role of the solar wind in generating them.

Combined ultraviolet and visible images of Saturn's polar lights Enlarge Image

And the observations show the light phenomenon behaves in unexpected ways.

It can be seen that Saturn's auroras vary from day to day, as they do on Earth, moving around on some days and remaining stationary on others. But compared with Earth, where dramatic brightening of the auroras lasts only 10 minutes, Saturn's can last for days.

Professor Michele Dougherty, from Imperial College, UK, is Principal Investigator for the magnetometer instrument on Cassini.

She said: "These latest findings show that the Sun's magnetic field and the solar wind play a much greater role in Saturn's auroras than previously thought.

"Cassini measurements show that the auroras sometimes move along with Saturn as it spins on its axis whilst the Hubble images show that some displays remain still while the planet rotates beneath.

"This difference indicates that Saturn's auroras are driven in an unexpected manner by the Sun's magnetic field and the solar wind, and in particular one big surprise is the fact that the magnetic field embedded in the solar wind plays a much smaller role at driving the auroras than we expected it to."

The findings are published this week in the journal Nature.

The Cassini-Huygens mission to Saturn is a joint venture between Nasa, the European Space Agency and the Italian Space Agency.