The Cassini spacecraft has sent back images of Saturn's moon Titan giving scientists the closest views yet of the mysterious satellite.
The shots were beamed back to a Nasa antenna based in Madrid, Spain, on Wednesday, at 0225 BST.
The probe went within 1,200km (746 miles) of the moon, 300 times closer than its first flyby in July.
But, so far, there is no sign of the oceans of ethane and methane thought to exist on the moon's surface.
"That is in itself a surprise because we thought they would jump out at us," said Dr Kevin Grazier, Cassini imaging scientist.
The photos reveal more detail on the surface than ever before, showing sharply defined areas of light and dark.
"We are still mystified, and we are not quite sure what we are looking at," Dr Carolyn Porco, Cassini imaging team leader, told a news conference.
One theory is that the dark patches are icy landmasses, while lighter areas are the oceans of liquid methane and ethane.
The pictures reveal there are few impact craters on Titan, suggesting the moon has an active surface that is constantly being resurfaced. Hydrocarbon rain or snow could fill in the craters or volcanic activity could be responsible.
"Rough" and "smooth" areas can also be seen. These could indicate differences in composition, age or porosity. And there are areas that look "slushy", said Dr Grazier.
Later this year, the piggybacked Huygens probe will be released from Cassini and enter Titan's atmosphere.
It will transmit data during its parachute-assisted descent, and carry out science tasks on the surface - if it survives.
Titan is thought to have oceans of liquid methane and ethane on its frozen surface
The first images were the clearest yet of the moon's surface, and scientists said they expected better images later.
Attempts to view Titan's icy surface are difficult because of the thick, orange haze that envelops the moon. But scientists had hoped Tuesday's pass would be close enough for Cassini's instruments to penetrate the satellite's dense smog.
Whole new world
Cassini used its cameras, an instrument called the Visual Infrared Mapping Spectrometer (Vims) and its imaging radar to penetrate the moon's thick haze.
Radar data can be processed to show highly detailed surface images resembling black-and-white photographs showing topographical data. Radar findings are expected to be released on Thursday. Vims is investigating atmospheric phenomena and geological features on Titan.
Mass spectrometers took measurments of the composition of Titan's atmosphere during the pass.
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
Conditions on Titan - which is the second biggest moon in the Solar System - are thought to be very similar to those on Earth 4.6 billion years ago. Temperatures rarely venture above -179C (-290F) and the atmosphere is dominated by nitrogen and carbon-based compounds.
So mission scientists think the moon might have something to teach us about the conditions that were necessary for the origin of life on our planet.
Cassini's flyby - one of 45 planned for its tour of Saturn - is expected to give a taster of what Huygens can expect when it enters Titan's atmosphere.
Cassini entered into orbit around Saturn in July, on its four-year mission to explore the ringed planet and its moons. It is a cooperative project between Nasa, the European Space Agency (Esa) and the Italian Space Agency