Titan: Stranger to the Familiar

Surface orange pebbles from Titan. Click image for larger view. Credit: ESA

Interpreting aerial photographs is a matter of training and a good sense of scale. Images that look similar may mask the underlying landscape if the scales are different. The images returned from Titan two billion miles away can look like the familiar aerial picture of a terrestrial shoreline, depending on the local weather, scale of measurement and even the eye of the beholder.

What the Huygens probe discovered however had nothing to do with lakes of liquid water, since any surface pools would freeze solid instantly.

Instead of a base chemistry defined by hydrogen and oxygen like Earth’s, Titan offers a eroded landscape shaped by the chemistry of hydrogen and carbon. This new hydrocarbon world is frozen, choking and wind-blown.

After a 4 billion kilometer (2.4 billion mile) journey through the Solar System that lasted almost seven years, the tiny Huygens probe plunged into the hazy atmosphere of Titan in the early morning hours on January 14th and landed safely on its frozen ground at 7:45 EST.

The probe continued transmitting from the surface for several hours, even after the Cassini orbiter dropped below the horizon and stopped recording the data to relay them towards Earth. The mothership circling overhead served as the communications relay for the probe far below the clouds. Cassini received excellent data from the surface of Titan for 1 hour and 12 minutes.

More than 474 megabits of data (approx. 60 megabytes) were received in 3 hours 44 minutes from Huygens, including some 350 pictures collected during the descent and on the ground.

Once communications reached Earth, the data could be written to a single compact disk for transfer among scientists stationed around the world.

What happened to Titan’s nitrogen? Comparison of Titan with Mars, Venus, Jupiter and Earth. Click image for larger view. Image Credit: NASA/JPL

Among the surprises from those early pictures, the Titan lander revealed a landscape apparently modelled by erosion. Titan had an intricate maze of drainage channels, shoreline-like features and even pebble-shaped objects on the surface.

The temperature measured at ground level was indeed supercold, about minus 180 degrees Celsius (or minus 292 Fahrenheit). There is no place on Earth even remotely like Titan’s winter. The lowest temperature ever recorded on earth (minus 129 °F, minus 90 Celsius) was at Vostok, Antarctica on July 21, 1983.

Titan’s atmosphere was probed and sampled for analysis at altitudes from 160 km (96 miles) to the ground, revealing a uniform mix of methane with nitrogen in the stratosphere. Methane concentration increased steadily in the troposphere down to the surface.

Clouds of methane at about 20 km (12 miles) altitude and methane or ethane fog near the surface were detected.

The probe’s signal, monitored by a global network of radio telescopes on Earth, will help reconstruct its actual trajectory with an accuracy of 1 km (0.6 miles) and will provide data on Titan’s winds.

Early analysis of the received signal indicate that Huygens was still transmitting after three hours on the surface. Later recordings are being analyzed to see how long Huygens kept transmitting from the surface.

Titan’s changing face as dark and light patches rotate in circulation. Image Credit: JPL/Space Science Institute

The initial signal sent to terrestrial radio telescopes from Titan had little more strength than the power of an average cellphone.

Samples of aerosols were also collected at altitudes between 125 and 20 km (12-100 miles) and analyzed on board.

During the descent, sounds were recorded in order to detect possible distant thunder from lightning, providing an exciting acoustic backdrop to Huygens’ descent.

As the probe touched down at about 4.5 meters per second (10 miles per hour), a whole series of instruments provided a large amount of data on the texture of the surface. At ground level, Titan resembles wet sand or clay with a thin solid crust, and its composition as mainly a mix of dirty water ice and hydrocarbon ice, resulting in a darker soil than expected.

Scientist will continue to test the image and chemical data for clues to whether Titan’s surprising landscape might ever have been wet or warm enough for more interesting biochemistry to arise.

Listen to sounds from the microphone onboard the Huygens during its descent (wav file format, approx. 600 kB each):

Related Web Pages

Rendezvous with Titan
Huygens, Phone Home
Saturn– JPL Cassini Main Page
Space Science Institute
Prebiotic Laboratory
Planet Wannabe
Where is Cassini Now?
Did Fluid Once Flow on Titan?