Titan on Target

Huygens’ probe will enter Titan’s thick atmosphere around the first of next year.
Credit: NASA/ESA

Cassini’s Huygens probe, now orbiting Saturn on board the spacecraft, is in good health and successfully passed its fifteenth ‘In-Flight Checkout’ on 14 September 2004.

Huygens will be the first spacecraft to land on a world in the outer Solar System. In January 2005, it will land on the surface of Titan, Saturn’s largest moon, and the only moon in the Solar System to possess a thick atmosphere. The gold-coloured foil blanket will help to control the probe’s temperature during the interplanetary cruise phase. The heat-resistant tiles covering the front shield are hidden underneath the foil and will provide protection against the very high temperatures that will be generated during the entry into Titan’s atmosphere.

This in-flight checkout procedure was the last but one planned before separation of the Huygens probe from Cassini in December this year, and it included some specific activities that were intended to prepare for the separation. The main difference in this procedure from previous checkouts was that there was a test of the Mission Timer Unit (MTU). Because Huygens will spend three weeks coasting towards Titan following separation from the Cassini orbiter, its systems and instruments are powered down.

Titan descent by Huygens probe leaving Cassini storage, Christmas 2004. Image Credit: JPL/Space Science Institute

The MTU is the ‘triple-redundant’ alarm-clock that has the most important job of waking up Huygens a few hours before its entry into Titan’s atmosphere.

The checkout also included some specific payload activities required to configure the Huygens instruments before separation.

The procedure was carried out live, with Cassini transmitting the data to Earth in real-time. However the data arrived on Earth with an 80-minute delay as this is the time taken for light, and therefore radio signals, to travel the distance between Saturn and Earth. The preliminary analysis of the real-time data received, performed within 12 hours after the test, indicates that the MTU test was successful, and that all instruments performed as expected.

Titan, the second largest moon in the solar system, is larger than the planet Mercury and is the only known moon with a thick atmosphere, actually 1.5 times more dense than Earth’s.

Much of the interest in Titan centers on its unusual chemistry. Compounds like organic polymers (plastics) found in Titan’s upper nitrogen-methane atmosphere are called "tholins", a Greek term referring to their muddy brown color in the laboratory. Titan’s tholins are created by ultraviolet sunlight and electrons streaming out from Saturn’s magnetic field. Experiments done 20 years ago show that dissolving tholins in liquid water produces amino acids, the building blocks of organic life. So given liquid water, there may be amino acids brewing in Titan’s version of primordial soup. Last year, Caitlin Griffith, of Arizona’s Lunar and Planetary Laboratory, discovered water ice on Titan’s surface. So planetary scientists would like to test if volcanoes or comet impacts might be temporary sources of liquid water, either from igneous heat or direct deposit of the solvents required to stir this strange primordial soup.

The haze of an atmospheric layer on Saturn’s moon, Titan. With an atmosphere thicker than Earth’s, and composed of many biochemically interesting molecules (methane, hydrogen and carbon), Titan’s rich chemistry will continue to interest astrobiologists as they look forward to landing a probe on its surface in 2004-5. Credit: Voyager Project, JPL, NASA

Better still, Titan’s water may not immediately freeze because it’s probably laced with enough ammonia (antifreeze) to remain liquid for about 1,000 years, noted Arizona scientists in a research paper published in last November’s issue of "Astrobiology." So although Titan is extremely cold — about 94 degrees Kelvin (minus 180 degrees Celsius or minus 300 degrees Fahrenheit) — water may briefly flow across the surface, supplying oxygen and a medium for chemistry, they conclude.

Cassini is conducting a four-year orbital mission, circling Saturn 77 times and cruising by more than 50 close encounters (and another dozen or so more-distant encounters) with the planet’s moons.








Related Web Pages

Saturn Edition, Astrobiology Magaz.
Saturn’s Rings in UV
Cassini Closes In on Saturn

Saturn– JPL Cassini Main Page
Lord of the Rings
Space Science Institute, Imaging Team Boulder, Colorado
Saturn: The Closest Pass