Huygens Pays a Visit to Titan

The centerpiece of Cassini’s symphonic mission, a brief aria of great excitement and beauty, was the descent of the Huygens probe to the surface of Titan in 2005.40 Titan is the most Earth-like world in the Solar System. It has weather, erosion, active geology, and a complex topography of lakes and rivers and flood plains. Three billion years ago, when the Sun was dimmer and no oxygen had been pumped into Earth’s atmosphere by microbes, the two cold worlds had strong similarities.

Huygens represented the main contribution of the European Space Agency to the Cassini mission and even though it returned a modest amount of data, it was not a disappointment. On Decem­ber 25, 2004, the probe separated from the main craft and began its perilous descent. Buffeted by winds in the upper atmosphere and unable to get a navigational lock on the Sun due to the thick smog, it slowed by parachute and landed on January 14, 2005, on what appeared to be a flood plain, scattered with cobbles of water ice (plate 8). Since the surface conditions of Titan were unknown, Huygens was never designed to be a lander. Rather, it was designed to survive landing on any surface from rocks to ocean, and trans­mit a small amount of data before expiring. This was dictated by the limitations of the batteries, which only had three hours of life, much of which was taken up by the descent. There was one minor disaster in the mission, when a software error prevented some of the lander’s images from being uploaded. While 350 images were returned, a similar number were lost. There was also a major di­saster averted. Long after launch, some dedicated and persistent engineers discovered that Cassini’s communication equipment had a major design flaw which would have caused the loss of all Huy­gens’s data. The probe had to send its data by radio to Cassini’s 4- meter antenna and then on to Earth. However, the acceleration of the probe would have Doppler-shifted its data out of range of the Cassini hardware, and the hardware could not be reprogrammed. To salvage this situation, flight engineers changed the landing and flyby trajectory so that the Doppler shift was greatly reduced.

Huygens weighed 700 pounds and carried six scientific instru­ments, most of which had been designed to study the atmosphere.

A microphone on one instrument captured the first sounds ever recorded on any planetary body apart from Earth. Another instru­ment mapped wind speeds at all elevations down to the ground. A third carried a lamp to illuminate the surface, which was useful, since Titan is a very murky moon. Huygens team member Martin Tomasko recalled: “We had great difficulty obtaining these pic­tures. We had only one percent of the illumination from the Sun, we’re going into a very thick atmosphere with lots of haze that blocks light from penetrating to low levels, and we’re taking pic­tures of an asphalt parking lot at dusk.”41 A fourth instrument did the clever trick of heating itself up just before the impact so it could analyze the vapor that came off the surface. The surface was a frigid -290°F or -179°C, cold enough that ice is brittle and methane is a liquid. Scientists clustered around monitors in the mission control room got to stare at a bleak and remarkable scene for several hours, longer than the expected thirty minutes, before the batteries finally died.