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Conditions on Titan

Long hailed as a natural astrobiology laboratory, Saturn’s largest moon Titan is certainly on the minds of many scientists today. This moon is the second largest in the solar system (Jupiter’s Ganymede is the largest); its diameter is about 3200 miles (larger than the planet Mercury), or approximately the distance across the United States. Titan is interesting because it is unique: it is the only moon in the solar system known to have a thick atmosphere. The composition of the atmosphere is largely nitrogen (about 90%). By comparison, Earth’s atmosphere has a nitrogen content of about 78%. There are also smaller amounts of ethane and methane in Titan’s atmosphere; these molecules are created when energetic ultraviolet light from the sun reacts with gasses (e.g. ammonia) released from its interior. Due to the very cold temperatures (about -290 degrees Fahrenheit!), scientists believe that these molecules could exist in liquid form and could thus drizzle out of the atmosphere onto the surface, creating perhaps large lakes of methane and ethane.

Since Titan is so cold, it is devoid of the liquid water that makes life possible here. Thus, scientists are skeptical that we will find life there, but they are just the same optimistic about learning more about the pre-biotic chemistry that might have occurred on our own planet. Titan can be regarded as a time machine: it will allow researchers to study the photochemistry and chemical reactions that were possibly taking place on early Earth.

Early Observations

Titan was first discovered by Christian Huygens in 1655, and since that time, scientists have learned about its composition and structure from measurements at a distance. Ground based observations, coupled with measurements from Voyager and the Hubble Space Telescope, have already told us something about the nature of this moon. Scientists are confident of our knowledge about some aspects of Titan, such as its size and average density. However, they are less certain about the exact nature of the surface underneath the hazy shroud. The thick atmosphere (about 1.5 times the pressure of Earth’s) precluded any direct images of the surface during the Voyager mission, although subsequent infrared imaging by the Hubble revealed lighter and darker areas on the moon.

Getting a Closer Look

Although there is still much that scientists don’t know about Titan, many of the outstanding questions may soon become in reach. The Huygens Probe, an integral part of the Cassini mission to Saturn, will plunge through Titan’s atmosphere early in 2005. Even though it took a seven-year voyage while piggybacked on the Cassini orbiter, it will only have a functional lifetime of about three hours. During this time, the probe will be very busy simultaneously collecting aerosols for chemical analyses, making spectral measurements, and measuring physical and electrical properties of the atmosphere. But before and after the demise of the probe, the Cassini orbiter will undertake several close fly-bys of the moon. With onboard radar and various spectrometers, the orbiter will give insights about the moon from a more global perspective. Some of the astrobiology-related science objectives for the Cassini-Huygens mission include:

—Determine the relative amounts of different components of the atmosphere.

—Observe vertical and horizontal distributions of trace gases; search for complex molecules; investigate energy sources for atmospheric chemistry; study formation and composition of aerosols .

—Determine the physical state, topography, and composition of Titan’s surface; characterize its internal structure.

(taken from the JPL Cassini-Huygens webpage)

NAI’s Role

Both during and after Cassini-Huygens, scientists will be pouring over the data from the mission. Many of these scientists will be also involved with the NASA Astrobiology Institute’sTitan Focus Group, a collection of about 30 members from all over the country. This group, chaired by Jonathan Lunine of the University of Arizona, will have a functional lifetime of about three years. The Titan Focus Group’s core objective is to “ensure the conceptualization and initial study of appropriate techniques for advanced organic analysis on Titan, after Cassini-Huygens.” The members are especially interested in helping to shape future requirements for subsequent Titan surface missions.

Many important questions remain about Titan and what it can tell us about pre-biotic chemistry that occurred during early Earth evolution. It will take a close-up look and collaborations from several different scientists to effectively answer them. You can learn more about the mission and actively monitor the updates at JPL’s Cassini-Huygens’ homepage.