How Huygens Landed on Titan
Scientists reconstructed the chain of events by analyzing data from a variety of instruments that were active during the impact, in particular changes in the acceleration experienced by the probe. The probe was supplied by the European Space Agency and named after the Dutch 17th century astronomer Christiaan Huygens.
The analysis reveals that, on first contact with Titan's surface, Huygens dug a hole about 4 1/2 inches (close to 12 centimeters) deep, before bouncing out onto a flat surface. The Huygens probe had a mass of 450 pounds (204 kilograms). It hit the ground with an impact speed that was similar to dropping a ball on Earth from a height of 3 1/2 feet (1 meter). Due to the lower gravity on Titan, the probe weighed only 60 pounds or 28 kilograms on Titan.
The probe, tilted by about 10 degrees in the direction of motion, then slid 12 to 16 inches (30 to 40 centimeters) across the surface. It slowed due to friction with the surface and, upon coming to its final resting place, wobbled back and forth five times, with each wobble about half as large as the previous one. Huygens' sensors continued to detect weak vibrations for another two seconds, until motion subsided nearly 10 seconds after touchdown.
"Huygens landed in what looks like a big flood plain," explained Erich Karkoschka, a senior staff scientist at the UA Lunar and Planetary Laboratory, who co-authored the study, which is published in the journal Planetary and Space Science. "There was a slight breeze in the lower atmosphere, causing the probe to parachute down at an angle. Because of the impulse, it bounced off sideways before it came to rest."
Like detectives poring over the scattered evidence of a crime scene, Karkoschka and his collaborators, Stefan Schrader and Ralph Lorenz, pieced together data Huygens sent to Earth during its descent through the hazy atmosphere of Saturn's largest moon in 2005, which lasted about two and a half hours. They compared the instrument data with results from computer simulations and a drop test using a model of Huygens designed to replicate the landing.
"A spike in the acceleration data suggests that during the first wobble, the probe likely encountered a pebble protruding by almost an inch [2 centimeters] 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," said Stefan Schrader of the Max Planck Institute for Solar System Research, lead author of the paper reporting the results.
Previous work measured how soft Titan's surface was at the point of Huygens' impact; those results found the surface to be quite soft. The new work goes one step farther to demonstrate that if something put little pressure on the surface, the surface was hard, but if an object put more pressure on the surface, it sank in significantly.
"It is like snow that has frozen on top," Karkoschka said. "If you walk carefully, you can walk as on a solid surface, but if you step on the snow a little too hard, you break in very deeply."
Read the full story at UANews.org.
