Titan probe's pebble 'bash-down'

By Jonathan Amos
BBC News science reporter, in Birmingham

More than 100 test drops have been run

An ice pebble was almost certainly the first thing the Huygens probe struck as it landed on Titan.

Open University scientists have been running experiments to try to simulate the data returned by a spike that protruded from the lander's underside.

This penetrometer was the first part of Huygens to touch the Saturnian moon and drove about 10cm into the surface.

More than 100 tests by the team have now provided the clearest indication of what that material might be.

Martin Towner and John Zarnecki presented some of the OU's findings here at the UK National Astronomy Meeting.

In the immediate hours and days after the landing, the researchers spoke of Huygens hitting a surface that had a thin crust with a softer, uniform material underneath - something akin to crème brûlée, was the humorous analogy used.

They did a drop into a crème brûlée - it had a nice crust and, horrifyingly, they got a signal that wasn't a million miles from the real one from Titan Prof John Zarnecki

"A crust is still a possibility, but we now think it's most likely we hit one of those water-ice pebbles you see in the ground image; the biggest you see is about 15cm," Professor Zarnecki told the BBC News website.

"We probably pushed the pebble to one side and then ploughed into the stuff underneath which we are pretty convinced is a 'sand' made of ice," the principal investigator on the Huygens surface science package explained.

The OU has a test rig at its Milton Keynes base where an exact copy of the penetrometer can be dropped into a variety of materials to simulate the mechanical properties of Titan's surface.

The largest pebble is about 15cm across

Glass beads of different sizes have been put in the rig, as well as a range sands and gravels. The penetrometer strikes the test sample at roughly 4-5 m/s, producing a signal that can be matched to the real data returned by Huygens.

Some of the drops have been done at angles to cover the likelihood that Huygens was swinging from side to side under its parachute as it came down.

The results favour a pebble impact.

"A crust and a pebble will give you an initial peak but the match looks better with a pebble and if we're seeing lots of them in the ground image it's hardly fanciful that we've bashed one of them," Professor Zarnecki said.

The sand and gravel Huygens landed in - ranging in size up to 8mm - is not the eroded silicate material one finds on Earth, but water-ice particles mixed with a significant amount of hydrocarbon ice.

The surface temperature gets down to as low as about -180C.

And all this fits with the picture that is emerging of a moon which probably experiences methane rain that runs in channels and rivers, with this fluid flow then depositing "silt" downstream.

Huygens swung from side to side as it came down

"If liquid flows through a solid, it erodes the surface away. If that's made of ice then we'll get ice-sand deposited in the lowlands and that's what we've hit," Professor Zarnecki said.

Future work by the OU team will look at whether the sand is "wet" or gooey because it still holds the rain.

And what of the crème brûlée analogy? Yes, the OU team has tested it.

"I have to stress this was the students; it was all their idea and they did it in their spare time, not in the university's time," Professor Zarnecki admitted.

"They did a drop into a crème brûlée. It had a nice crust and, horrifyingly, they got a signal that wasn't a million miles from the real one from Titan. So maybe the moon is a crème brûlée in the sky after all," the researcher joked.

Huygens landed on Titan, the largest moon of Saturn, on 14 January.

The robotic lab was the major European component in the joint US-Europe Cassini mission that is spending four years investigating Saturn and its satellites.

The Royal Astronomical Society's National Astronomy Meeting has been held at the University of Birmingham.