(Sen) - One of space exploration's greatest successes came on January 14, 2005, when Europe's Huygens probe landed on Saturn's largest moon Titan.
It was a remarkable achievement early in the life of NASA's Cassini mission. That mothership, which left Earth 15 years ago today, is still orbiting the giant ringed planet and sending back valuable data and spectacular images.
Huygens descent by parachute to a soft landing on Titan told us a lot about this moon which, with a diameter of 5,150 km (3,200 miles), is bigger than the planet Mercury.
Now, more than seven years later, scientists have used a range of data to tell them exactly how Huygens landed, which in turn reveals much about the nature of the moon's surface.
They found that it bounced, slid and wobbled in the ten seconds before it came to rest in that strange alien landscape. There had been speculation that Titan's surface resembled a "crème brûlée" dessert - instead it is now being likened to soft, damp sand.
The researchers were able to reconstruct the landing by combining information from different instruments that were switched on during touchdown. Accelerometer data from the Huygens Atmospheric Structure Instrument and the Surface Science Package together with photometry data from the Descent Imager/Spectral Radiometer, were compared to results from computer simulations and a drop test using a model.
Huygens descended slowly through Titan's dense, methane-rich air - it is the only planetary satellite to have a significant atmosphere. Results show that when the probe finally hit the ground it dug a 12 cm (nearly 5 inch) deep hole, then bounced out onto a flat surface.
Huygens, tilted by about 10 degrees, then slid between 30 and 40 cm (12 to 16 inches) across Titan's surface before wobbling five times to a standstill.
Dr Stefan Schröder of the Max Planck Institute for Solar System Research, in Germany, is lead author of a paper describing the findings in the journal Planetary and Space Science.
This points out that the surface could not have been wet like mud or the probe would simply have landed with a splat. It must have been soft enough for the probe to make a hole but hard enough to allow it to rock back and forth.
Video shows the landing at normal speed and in slow motion. Credit: ESA/C. Carreau
"A spike in the acceleration data suggests that during the first wobble, the probe likely encountered a pebble protruding by around 2 cm from the surface of Titan, and may have even pushed it into the ground, suggesting that the surface had a consistency of soft, damp sand.
"We also see in the Huygens landing data evidence of a 'fluffy' dust-like material – most likely organic aerosols that are known to drizzle out of the Titan atmosphere – being thrown up and suspended for around four seconds after the impact."
Huygens, which survived on the surface for around 90 minutes before it ran out of power, carried a Surface-Science Package designed by UK scientists under the leadership of Professor John Zarnecki of the Open University.
Professor Zarnecki told Sen last night: "It is wonderful to revisit the landing. We thought Huygens had landed with a crunch. There was indirect evidence that the surface was damp but these results suggest the surface was dryish. Otherwise it would have tended to have landed with a squelch."
The Cassini-Huygens mission is a collaborative project between NASA, the European Space Agency and the Italian Space Agency.