Titan, the largest moon of Saturn, is one of the most fascinating objects in the solar system. We got our first glimpses from space of Titan and its thick atmosphere on the Voyager spacecrafts’ trips through the Saturn system in 1980 to 1981. With the latest mission — Cassini-Huygens — actually landing on Titan, some mysteries of the strange moon are being answered, while others remain.

This natural-color composite was taken during the Cassini spacecraft’s April 16, 2005, flyby of Titan. It is a combination of images taken through three filters that are sensitive to red, green and violet light. It shows approximately what Titan would look like to the human eye: a hazy orange globe surrounded by a tenuous, bluish haze. The orange color is due to the hydrocarbon particles that make up Titan’s atmospheric haze. North on Titan is up and tilted 30 degrees to the right. Courtesy of ESA/NASA/JPL.

First look

The Saturnian moon is one of the two bodies in the solar system (the other is Earth) with a nitrogen-rich atmosphere. Titan’s atmosphere, also rich in methane, may resemble the hypothesized early Earth. Earth’s now oxygen-rich atmosphere has evolved partly under the influence of life, but Titan’s much colder atmosphere remains more primitive. When Voyagers I and II passed through Saturn’s moon system, they saw Titan’s hazy orange atmosphere. The distinct appearance comes from the presence of methane in the atmosphere, which gives rise to a system with complex organic chemistry.

One of the great mysteries surrounding Titan, which formed about 4.5 billion years ago, is the origin of the methane: With a lifetime of 20 million years, it should regularly be resupplied in the atmosphere to be as abundant as it is today. We think that ethane and other complex hydrocarbons and carbon-nitrogen-based compounds also form high in the atmosphere of Titan, irradiated by solar ultraviolet rays and bombarded by energetic particles from Saturn’s space environment.

Most of the hydrocarbon and organic compounds slowly descend in the atmosphere and condense at a chilly minus 70 degrees Celsius (minus 158 degrees Fahrenheit) at an altitude of 45 kilometers. Those condensate organic particles grow and form the aerosols that give the well-known orange color to Titan’s atmosphere. The aerosols eventually rain down to the surface and cover it with an organic layer that may be tens or even hundreds of meters thick.

The surface of Titan remained hidden from Voyagers’ cameras, which led to speculations as to what it may actually look like. The surface pressure on Titan is 1.5 times that on Earth and the surface temperature is minus 180 degrees Celsius, only slightly above the point at which methane can be both a liquid and gas. We thus expected to find liquid methane on Titan’s surface or in underground reservoirs. The methane would likely convert to ethane, acetylene and ethylene. When combined with nitrogen, it would convert to hydrogen cyanide — a building block of amino acids, which form the basis of life on Earth. Although the images returned by the Voyagers were almost featureless, the richness of the detected organic compounds confirmed that Titan was indeed a very unique object that deserved to be revisited and explored in much more detail.

Now, as the Voyager I spacecraft reaches the edge of the solar system, more than 20 years later, we are getting a second, more close-up look at Titan with the Cassini-Huygens mission. The spacecraft arrived in orbit around Saturn on July 1, 2004, after daringly crossing the rings twice. The Huygens probe was released from the mother ship during the third orbit around Saturn on Dec. 25, 2004. Huygens landed successfully on the surface of Titan on Jan. 14. During its two-and-a-half hour descent by parachute, it collected a wealth of data about the physical properties and the chemical composition of the methane- and nitrogen-rich atmosphere of Titan and captured several hundred images of the surface of a world that appeared at first glance to be much like Earth.

Winds blew the probe across an icy landscape laced with what appears to be drainage channels and rivers in which liquid methane had flowed in the past. When Huygens landed, liquid methane evaporated from the surface, hinting at the wonders to come.

An international affair

Although ideas to send a spacecraft to Saturn with a Galileo-like mission — which sent a probe and orbiter to Jupiter in 1995 after years of planning — had been talked about since the late 1970s, the then-called Cassini mission (after Giovani-Domenico Cassini, the discoverer of the gap in Saturn’s main ring) was not born until 1982.

This artist’s conception shows Titan’s surface with Saturn appearing dimly in the background through Titan’s thick atmosphere of mostly nitrogen and methane. The Cassini spacecraft flies overhead with its high-gain antenna pointed at the Huygens probe as it nears the surface. The Huygens probe successfully landed on Titan in January and found dried riverbeds and channels that likely once held liquid methane. Image by Craig Attebery, courtesy of ESA/NASA/JPL.

After seeing the NASA Voyagers’ observations of Titan in the early 1980s, the idea of a Titan probe mission was submitted by a team of European and U.S. scientists to the European Space Agency (ESA) for collaboration with NASA. Soon, the concept of a Saturn orbiter and Titan probe mission emerged as the way to go, with a joint ESA-NASA study of the Cassini mission begun in 1984.

From the very beginning, it was clear that, if the mission were to take place, ESA would lead the Titan probe and NASA the Saturn orbiter. It took five years to study the mission and to work out all the political agreements that had to be in place for such a venture to proceed. In 1988, ESA, as expected, chose to head up the Titan probe and named it Huygens, after Christiaan Huygens, who discovered Titan in 1655 (see sidebar, page 25). One year later, NASA got the budget and the authorization to start developing Cassini. The Italian Space Agency reached a bilateral agreement with NASA in 1992 to provide major hardware for the orbiter and became the third major partner of the mission.

The 12-instrument payload of the orbiter and the six-instrument payload of the probe were developed by a U.S.-European consortium of universities, research laboratories and industries — making it an example of international collaboration in space exploration.

A challenging trip

ESA and NASA worked for seven challenging years to get the Cassini-Huygens craft ready for launch. It took another seven years to get to Saturn. The journey required the “gravity assist” of Venus (in April 1998 and June 1999), Earth (in August 1999) and Jupiter (in December 2000) to get to Saturn. In their gravity assists, the planets deflected the trajectory of the spacecraft and changed its velocity by using a slingshot effect when it flew through their gravity fields.

During the trip, Huygens was essentially dormant, well taken care of by the orbiter that continuously monitored its internal temperature, to assess its health and to control the internal heat dissipation when it was switched on for a regular health check. Biannual activations for a few hours allowed us to verify the good health of Huygens and to determine its payload fit for the job at Titan.

In 2000, we discovered that Huygens would not be heard properly by its radio receivers aboard Cassini during its descent because of what became known as the Huygens Doppler problem: As Huygens descended through Titan’s atmosphere, the Cassini orbiter would approach Titan, causing the frequency of Huygens’ radio signals to shift as it moved closer to Earth. The Huygens radio receiver bandwidth was too narrow to cope with this Doppler Effect. It took three years to find, design and implement a solution, which included redesigning the first two orbits of Cassini into three orbits and releasing the probe onto Titan on the third orbit (it was initially planned for the first orbit).

From flybys of Titan made by Cassini between July and December 2004, we were able to verify that the atmosphere of Titan was well within the predicted “engineering envelope” for the probe mission to be carried out safely. And on Dec. 25, Cassini released Huygens to encounter Titan three weeks later.

During the two days after release, Cassini took images of the probe with the star field in the background to refine its position and trajectory with Titan. An alarm clock was set to wake up the probe 4 hours and 23 minutes before it reached Titan. It did so within a second of the programmed wake-up time. This early wake- up (initially the probe was programmed to awaken just 23 minutes before arrival) was one of the modifications implemented in the probe sequence to cope with the Doppler problem. It allowed additional time to warm up the electronics on board Huygens and to tune the data-stream frequency for the Huygens receivers to decode it flawlessly on board Cassini.

Huygens reached the upper layers of Titan’s atmosphere at a speed of 6 kilometers per second (some 13,000 miles per hour) and started to detect the atmospheric drag 1,500 kilometers above the surface. Maximum deceleration (more than 120 meters per square second) was reached after three minutes. Less than a minute later, it had decelerated to 400 meters per second (a mere 895 miles per hour). This was the trigger for the parachute deployment sequence.

A mortar expelled a so-called pilot parachute, which removed the back cover and deployed the main 8-meter-diameter parachute. The front heat shield was discarded 30 seconds later. The main parachute was released after 15 minutes and a smaller chute, 3 meters in diameter, deployed to bring the probe down to the surface in less than two-and-a-half hours. Both parachutes were designed to operate at supersonic speeds, around Mach 1.5, corresponding to 400 meters per second in Titan’s upper atmosphere. Huygens reached the surface at a velocity of about 4.7 meters per second (about 11 miles per hour) after approximately two-and-a-half hours of descent.

After a soft landing, the probe continued to function for several hours. Data started to be relayed to the Cassini orbiter via two channels soon after main parachute deployment. The orbiter received data from the surface for 72 minutes until it dipped below the horizon. It relayed the data to Earth about six hours later. It turned out that one of the two Huygens radio receivers on board Cassini was not configured properly, and data that were not redundant were lost. Fortunately, however, we had an Earth-based radio observation program in place.

An array of 17 radio telescopes on Earth was set up to listen to one of the two direct Huygens radio transmissions with the main goal of reconstructing the angular position of the probe during its descent with kilometer-accuracy from more than a billion kilometers away. A real-time Doppler tracking signal was detected at the Green Bank Telescope in West Virginia and at the Parkes Telescope in Australia. The probe radio transmission was switched on at 9:11 Universal Time on Titan. The first direct Huygens radio signal was expected to reach Earth 67 minutes later. It was detected within a few seconds of the expected time and continually tracked at Green Bank and later at Parkes. The successful observation allowed us to fully recover the Huygens radio experiment that gave us wind and other data (see Geotimes, April 2005).

Early results

Initial processing is already revealing startling insights into the moon’s atmosphere and geology. With the Green Bank and Parkes radio telescopes, we were able to analyze the wind patterns on Titan. Together with the orbiter measurements, the data will establish a global map of the temperature field of Titan and its variability over several years — a sort of weather map that will allow us to investigate Earth-like meteorology features in a different environment.

We found winds on Titan to be blowing in the direction of Titan’s rotation (from west to east) at nearly all altitudes. Maximum wind speed above 100 meters per second (224 miles per hour) was measured about 10 minutes after the start of the descent, at an altitude of about 120 kilometers. The winds are weak near the surface and increase slowly with altitude up to about 60 kilometers. This pattern does not continue at altitudes above 60 kilometers, where a low-speed wind layer was observed at around 70 kilometers altitude.

This stereographic projection of Titan from the Huygens probe combines 60 images in 31 triplets, projected from a height of 3,000 meters (9,843 feet) above the black “lakebed” surface. The bright area to the north (top of the image) and west is higher than the rest of the terrain, and covered in dark lines that appear to be drainage channels. Courtesy of ESA/NASA/JPL/University of Arizona.

With this weather information, we were able to see that like Venus, Titan possesses an atmosphere that rotates much faster than its surface does. (Titan has a 16-day rotation period, with a slow surface rotation of 12 meters per second.) Several of the wind features already observed by both the probe and the orbiter were not well-predicted by global atmospheric circulation models in part derived from models of Earth. Titan data will definitely add more understanding to general circulation in the terrestrial atmosphere and will thus contribute to improving models of Earth.

In addition to better understanding Titan’s weather, we were able to get a glimpse of Titan’s surface. The images captured by Huygens’ camera reveal that Titan has extraordinarily Earth-like geology. Images have shown a complex network of narrow drainage channels running from brighter highlands to lower, flatter dark regions. These channels merge into river systems running into lakebeds, featuring offshore “islands” and “shoals” that look remarkably similar to those on Earth. Data provided in part by the Gas Chromatograph and Mass Spectrometer (GCMS) and the Surface Science Package (SSP) support these observations.

The data provide strong evidence for liquid methane flowing on Titan, but Titan’s drainage channels, rivers and lakes appeared dry at the time of the Huygens landing. Deceleration and penetration data provided by the SSP indicate that the material beneath the surface’s crust has the consistency of loose sand, possibly the result of methane rain falling on the surface over eons, or the wicking of liquids from below toward the surface. Heat generated by Huygens warmed the soil beneath the probe, and both the GCMS and SSP detected bursts of methane gas that boiled out of surface material, reinforcing methane’s principal role in Titan’s geology and atmospheric meteorology — forming clouds and precipitation that erodes and abrades the surface.

Surface images also show small rounded pebbles in a dry riverbed. Spectra measurements are consistent with a composition of dirty water ice rather than silicate rocks, as on Earth. However, these ice pebbles are rock-like solids at Titan’s temperatures.

Titan’s soil appears to consist at least in part of precipitated deposits of the hydrocarbon haze that shrouds the planet. This dark material settles out of the atmosphere. When washed off high elevations by methane rain, it concentrates at the bottom of the drainage channels and riverbeds, contributing to the dark areas seen in the images (see image above).

Thus, while many of Earth’s familiar geophysical processes occur on Titan, the chemistry involved is quite different. Instead of liquid water, Titan has liquid methane. Instead of silicate rocks, Titan has frozen water ice. Instead of dirt, Titan has hydrocarbon particles settling out of the atmosphere, and instead of molten rock, volcanoes spew very cold ice. Titan is an extraordinary world that has Earth-like geophysical processes operating on exotic materials in very alien conditions.

The ground truth

The high-resolution Huygens measurements will provide a unique field check at one surface location and on the flight path in the atmosphere for the Cassini orbiter observations, which will be made over more than 40 close flybys of Titan. Kilometer-resolution images of parts of the surface have been obtained by the orbiter camera and the visual and infrared mapping spectrometer. About 1 percent of Titan’s surface is imaged by the Cassini radar each time it takes its turn to probe the surface. High-resolution images of the Huygens landing site are expected in the coming months.

The exploration of Titan will continue for the next three years and possibly more, as it is expected that the mission will be extended beyond its nominal four-year lifespan at Saturn. The Huygens dataset and the first dataset from the Cassini orbiter are already providing new views of a fascinating world. The intensive exploration of Titan has just begun.

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