* Astronomy

An unexpected radio reflection from the surface of Titan has allowed ESA scientists to deduce the average size of stones and pebbles close to the Huygens’ landing site. The technique could be used on other lander missions to analyse planetary surfaces for free.

When Huygens came to rest on the surface of Titan on 14 January 2005, it survived the impact and continued to transmit to the Cassini mothership, orbiting above. Part of that radio signal 'leaked' downwards and hit the surface of Titan before being reflected back up to Cassini. On its way up, it interfered with the direct beam.
As Miguel Pérez-Ayúcar, a member of the Huygens Team at ESA’s European Space Research and Technology Centre (ESTEC) in The Netherlands, and his colleagues watched the signal coming back, they were initially puzzled to see the power of the signal rising and falling in a repetitive manner.

"Huygens had not been designed to necessarily survive impact so we had never thought about what the signal would look like from the surface" - Miguel Pérez-Ayúcar.

After making a joke that aliens must be dragging the craft along the surface, Pérez and the team began work at once to understand the signal.



The clue was the repetitive oscillation of the power. It made Pérez think about the interaction of the direct signal with that reflecting from the surface of Titan. As Cassini travelled away from the Huygens landing site, the angle between it and Huygens changed. This altered the way in which the interference between the reflected and direct beams was detected, perhaps causing the variation in power.
He began running computer models and saw that not only could he reproduce the received signal but also it was sensitive to the size of pebbles on the surface of Titan.
Cassini collected data for 71 minutes after Huygens landed. After that time, the spacecraft’s motion took it below the horizon as seen from Huygens' landing site. Until then, it soaked up radio signals that encoded information about a swathe of Titan’s surface from 1 metre to 2 kilometres to the west of the landed probe.
To accurately mirror the true signal, Pérez and his team discovered that the surface swathe must be relatively flat and covered mostly in stones of around 5-10 centimetres in diameter.
This unique result complements the data taken by the Descent Imager and Spectral Radiometer (DISR) instrument. When Huygens came to rest on the surface of Titan, DISR was pointing due south. Its images show stones and terrain in good agreement with the newly deduced western facing radio data.

"This is a real bonus to the mission. It requires no special equipment, just the usual communications subsystem" - Miguel Pérez-Ayúcar.

Now that the scientists have understood the process using the unexpected Huygens data, the technique could be implemented on future lander missions.

"This experience can be inherited by any future lander. All that will be needed is a few refinements and it will become a powerful technique" - Miguel Pérez-Ayúcar.

By subtly altering the properties of the radio beam for instance, the radio transmitter and receiver can be optimised to help deduce the chemical composition of the planetary surface.

Source

This image of Titan was taken by the Cassini space probe on July 22, 2006, when it was approximately 200,417 kilometres away


Credit NASA/JPL

The image was taken using the IRP0 and CB3 filters.

The highlands of Titan may be riddled with caves, according to the latest images of Saturn's giant moon.

On 30 April, the Cassini spacecraft flew over a large bright region called Xanadu that spans about 4000 kilometres across.
Xanadu was already thought to be a highland area, where bright hills of ice poke up above Titan's dark sooty plains. A new picture made with the spacecraft's haze-penetrating radar confirms that.
In fact, the interior of the region is crossed by mountain chains that tower more than a kilometre high.

"These are the highest mountains measured on Titan so far" - Ralph Lorenz, radar team member of the University of Arizona in Tucson, US.

But it seems that the mountains are not solid. The radio waves bouncing off Xanadu reveal that it has peculiar electrical properties – specifically a low dielectric constant.

"The only reasonable material makeup that could have a very low dielectric constant and still hold together enough to form the structures that we see would be some sort of porous stuff – most likely porous water ice" - Steve Wall, radar team member of NASA's Jet Propulsion Laboratory in Pasadena, California, US.

He suggests the region might be filled with caverns, presumably carved out by the methane rain that is though to fall on Titan.
That rain would also have created the long river valleys that meander among the hills of Xanadu. Cassini scientists speculate that these rivers could carry ice grains down to the plains to form the dunes seen on much of Titan's surface.
The highlands are also marked by small, dark patches that may be methane lakes – although, as elsewhere on the moon, there is no strong evidence of liquid still present on the surface.
That could change this weekend, when Cassini makes its first pass above the wintry north pole of Titan. The poles are the most likely places for lakes or seas to survive because they are cold enough to prevent any liquid methane on the surface from evaporating quickly.

But what if there are no lakes near the pole?

"If we don't find them there, then I will begin to wonder if they are visible on Titan at all" - Ralph Lorenz.

Source

This image of Titan was acquired on April 30, 2006, by Cassini's radar instrument in synthetic-aperture mode over the continent-sized region called Xanadu.


Credit NASA/JPL/Cassini team

Xanadu is one of the brightest areas on Titan, measuring about 4,000 kilometres east to west and 2,000 kilometres north to south. The radar coverage shown ranges from 220 to 490 kilometres from top to bottom, and is about 4,850 kilometres wide. Smallest details in this image are about 400 meters across.

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