The international Cassini-Huygens probe has had its first opportunity to fly by Saturn's biggest moon, Titan.
A true-colour image of Titan taken by Cassini in June from 13 million km (Nasa/JPL/SSI)
Already the spacecraft has managed to detect large linear features on Titan's surface which are obscured from Earth telescopes by its thick atmosphere.
Imaging specialists said these could be tectonic structures - areas of crust which had been shaped by movement.
Cassini flew to within 350,000km of the moon, the first of more than 40 visits it will make in the next four years.
Not only is Titan a primary science target for the mission, but the sheer size of the moon means navigators can use its gravitational attraction to make major changes in the spacecraft's orbital paths.
This means Cassini can move to different parts of the Saturnian system without using too much fuel.
Scientists expect to release more data from the Titan flyby - which occurred at 1054GMT - over the weekend.
On approach, Cassini produced a map of a segment of the moon's surface more than 10,000km wide and stretching from just above the equator to below 60 degrees south.
SURFACE OF TITAN
The map made on approach shows some features with straight boundaries
Using various optical tricks, Cassini managed to pick out details that are new to science.
"The character of the Titan surface is becoming clearer to us," said Dr Carolyn Porco, who leads the Cassini imaging team.
"You see dark markings that seem to be linear; they cross each other. We could have seen everything being roughly circular, which we could have speculated were large impact basins.
CASSINI'S KEY PARTS
1. Antennas enabling communication with Earth
2. Boom carrying instrument to measure magnetic fields
3. Two cameras will take 300,000 pictures of the planet
4. Infra-red spectrometer analyses Saturn's temperature and composition
5. Radioisotope thermoelectric generators supply 750W of power
6. Cassini has two engines - one is a back-up
7. Thrusters used for small changes of direction or speed
8. Huygens probe will land on Saturn's largest moon, Titan
9. Plasma spectrometer measures charged particles and solar winds
"But we're not seeing that. We're seeing things criss-cross. And everywhere - whether it's on the Earth or on the Galilean (Jupiter) satellites - in the Solar System where you see linear features that generally means tectonics.
"You have pieces of the surface [slipping past or over each other]."
When the detailed images from the flyby come through, Dr Porco and her colleagues hope to see evidence of clouds in the atmosphere and bodies of liquid on the surface.
"That would be tremendously exciting," she said. Scientists think Titan may have seas of methane and ethane.
Confirming this would be a major achievement for the multi-billion-dollar mission, which will despatch the small Huygens probe on the side of Cassini to investigate the moon at close quarters in January.
The joint US-European venture arrived at Saturn on Thursday.
During the manoeuvre to insert the spacecraft in orbit, Cassini took remarkable images of Saturn's rings.
Some pictures showed patterned density waves in the rings, resembling stripes of varying width. Another revealed a ring's "scalloped" edge.