Discovery Guides Areas


The Cassini-Huygens Mission: Follow-up File
(Released March 2005)

  by Salvatore Vittorio  


Key Citations

Web Sites



Review Article Go To Lifting the Veil: Review
Go To Cassini visits Enceladus

"We have it? We have it!" - Cassini-Huygens mission team member upon seeing the first transmitted Titan data from the Huygens space probe on January 14, 2005

A Happy New Year's landing

The Huygens European probe, a component of the Cassini-Huygens Mission to the planet Saturn and its moons, has landed on Saturn's moon Titan, a mysterious satellite that has perplexed astronomers for decades. The Cassini-Huygens Mission, a project of NASA, ESA and the Italian space agency, was launched Oct. 15, 1997, from Cape Canaveral, Florida, to study Saturn, its rings, and its many moons.

Built by the European Space Agency (ESA), the 705-pound (320-kilogram) Huygens probe landed on Titan between 7:45-7:46 a.m. EST (1245-1246 GMT) on January 14, 2005, as scheduled, and apparently began beaming at least some data to NASA's Cassini orbiter for later transmission to Earth. [8]

Huygens lander Huygens lander
Pilot parachute pulling off the aft cover of the Huygens probe.
An artist's impression shows the Huygens landing unit
separating from the Cassini orbiter.

Expecting pictures and atmospheric measurements, ESA science director David Southwood confirmed the Huygens probe sent data up to the Cassini spacecraft, which transmitted the information back to antennas on Earth. Applause erupted at mission control in Darmstadt, western Germany at the news. The data are expected to shed light on the composition of Titan's atmosphere and surface, and possibly on the origins of life on Earth. Early data showed that one of Huygens' experiments, designed to measure the Titanic winds, had begun to work, said Jean-Pierre Lebreton, ESA mission manager. Officials hoped that usable scientific data, including pictures, would follow hours later. ''We clearly have an engineering success,'' Lebreton said. ''We are going to work very hard to convert this into a scientific success.''

Mission officials had waited seven years for Huygens to reach its destination, and had tears in their eyes as the first signal was picked up, indicating that the probe was transmitting to its mother ship, the international Cassini spacecraft. Science director Southwood said the mission had passed a difficult and critical step. Huygens was spun off from Cassini on December 24, 2004, to begin its free-fall toward Titan, the first moon other than the Earth's to be explored by a spacecraft. Named after Titan's discoverer, the 17th century Dutch astronomer Christiaan Huygens, the probe carries instruments to explore what Titan's atmosphere is made of and to find out whether it has the cold seas of liquid methane and ethane that scientists have theorized .

Timers inside the probe awakened it just before it entered Titan's atmosphere. Huygens is shaped like a wok and covered with a shield to survive the intense heat of entry. On the descent, it was to shed its heat shield and collect information on wind speeds and the makeup of Titan's atmosphere using a special camera and instruments. The data transmitted back to Cassini will, in turn, be relayed to NASA's Deep Space Network in Pasadena, California, and on to ESA controllers in Darmstadt.

Titan is the only moon in the solar system known to have a significant atmosphere. Rich in nitrogen and containing about 6 percent methane, its atmosphere is believed to be 1½ times thicker than Earth's. Alphonso Diaz, science administrator for NASA, thinks Titan may offer hints about the conditions under which life first arose on Earth. Diaz describes Titan as a time machine that will provide us the opportunity to study conditions that may well have existed on Earth in the beginning. It may have preserved in a deep freeze many chemical compounds that set the stage for life on Earth. Part of a $3.3 billion international mission to study the Saturn system, Huygens is also equipped with instruments to study Titan's surface upon landing. Scientists were not sure exactly what it would hit when it landed at about 22 mph. The probe floats and can survive a landing in methane or ethane, which exist in liquid form due to temperatures of 292 below zero F. One hazard would be landing on a solid slope in a position that doesn't permit a strong signal back to Cassini. Engineers at ESA were counting on the probe having at least three minutes to transmit information and images from Titan's surface, before its battery ran out or Cassini traveled out of range. [9]

Background: Unveiling the mystery of Titan

The Cassini-Huygens Mission arrived at the ringed planet Saturn and maneuvered into orbit on July 1, 2004. Cassini will study the Saturn system from orbit for at least four years. This orbiter-probe combination is the first human spacecraft to go into orbit about Saturn. All prior missions simply snapped their amazing images of Saturn and its rings and moons as they flew by. None stuck around long enough to delve into the mystery of Titan, the only moon in our solar system shrouded by an atmosphere so dense we can't see through it at visible wavelengths. Simply, Titan looks like a featureless yellowish-orange globe in orbit about Saturn. [1]

On October 26th, 2004, Cassini swooped by Titan and peeked through its hazy atmosphere with radar to take the first surface images we've ever seen of this alien terrain. The October 26th flyby marked the first time Cassini's imaging radar was used to observe Titan. The radar instrument works by bouncing radio signals off Titan's surface and timing their return, a process similar to timing the returning echo of one's voice across a canyon to tell its width. Radio waves can penetrate the thick veil of haze surrounding Titan. The radar instrument team of Dr. Charles Elachi, director of NASA's Jet Propulsion Laboratory (JPL) and leader of the radar instrument team so far has mapped about 1% of Titan's surface, and already there are surprises. [1,44]

"Titan is a dynamic place with complex geologic processes that may be shaping its surface. Its surface may well be covered with organic materials, but we still don't know how much of the surface is liquid or solid. The fact that we have seen few craters tells us that Titan's surface is young," stated Dr. Jonathan Lunine of the University of Arizona. [1, 4]

Instruments aboard the Cassini spacecraft have sent back the most detailed images ever captured of Titan's surface. They've also presented scientists with a major mystery: Scientists expected the a huge cloud formation over the moon's south pole, spanning 620 miles (1000 kilometers) at its widest, but they also expected it to be made of methane, and it isn't. Titan's atmosphere contains an abundance of methane, which was detected many years ago by spacecraft that flew past the planet. When recent images from the Keck Observatory in Hawaii showed clouds at Titan's south pole, scientists assumed they were methane. According to their model, Titan is so cold that methane can form liquid pools on its surface. When this liquid is heated by the sun, it evaporates, forming methane clouds. Water clouds on Earth work the same way. The sun heats the ocean; water evaporates, and clouds form in the atmosphere. Because Titan's south pole is currently pointed toward the sun, it is receiving the greatest amount of heat right now. So a massive cloud form over the south pole fits the model perfectly - if the cloud is made of methane. [2]

However, according to data gathered by Cassini, the particles that make up the cloud are too big to be methane. "I don't believe it," says Chris McKay, a planetary scientist with the NASA Ames Research Center in Moffett Field, California. "It would be like flying over Earth and saying the clouds are not water. If those clouds are really not methane, then a lot of the things we think about Titan are wrong. A lot of things we think about those clouds are wrong - the whole explanation of why they're there." The Cassini science team has not yet had time to fully analyze the data sent back by the spacecraft, so they don't yet know what the clouds are made of. One possibility, McKay speculates, is ethane, except that ethane is a photochemical product that is produced in the upper atmosphere and rains downward. So it wouldn't make sense that a massive ethane cloud would appear at the south pole of Titan. [2]

Another possibility, according to McKay, is that it could be some sort of organic polymer, essentially plastic particles. But, as with ethane, these would form from above. There's no known reason why a massive cloud of them should exist at Titan's south pole. One plausible scenario is that the cloud particles could start out as methane, produced as expected on the ground and carried aloft. Once airborne, however, they become coated with some other substance, perhaps ethane. Further study of the spectral data collected by Cassini, together with laboratory-based experiments should enable scientists to unravel the mystery, but it will take some time for them to do so. [2]

Meanwhile, down on Titan's surface, another mystery is unfolding. Recent images have revealed stunning surface detail, never before seen. But, according to Caroline Porco, team leader for the Imaging Science Subsystem (ISS) aboard Cassini-Huygens, "we don't know exactly what we're looking at." The "don't know" part applies both to the moon's surface composition and to its topography. Figuring out what the moon is made of should be relatively easy, using data from Cassini's Visual and Infrared Mapping Spectrometer (VIMS) instrument. Different materials reflect light at different frequencies. Each pixel captured by the VIMS records the strength of the light reflected at each of 352 distinct frequencies. Some of these frequencies are visible to human eyes; others are in the infrared region of the electromagnetic spectrum. By analyzing this spectral data, scientists should be able to figure out Titan's surface composition. The analysis requires intensive computation, matching the observed spectra to standards stored in spectral libraries here on Earth. [2]

Surface mysteries

Titan's topography will be even tougher to figure out. According to Porco, because of the global haze layer, we do not see shadows on the surface of Titan, and because we don't see shadows, we can't look at an image and immediately deduce what's up and what's down. There could be massive mountains and deep valleys there, or the surface could be completely flat. At this point, there's no way to tell.

Titan is unlike the planet Mars and the icy moons of the planet Jupiter, which have little or no atmosphere. So the techniques that have been used for interpreting airless bodies, all those methods of examining solid surfaces from planetary spacecraft that have been learned over the last half century, don't apply, according to Porco. Because Titan is a very different environment, what scientists can use is a combination of data from several different Cassini instruments, collected over a long period of time. During its four-year mission, Cassini will make many more close passes by Titan, 45 in all, during which it will collect both radar data and stereo images. By integrating these two types of data, the Cassini science team will be able slowly to build up a picture of Titan's topography. That process, however, will take months - or even years. [2]

radar image
In this radar image, brighter areas may correspond to rougher terrains and darker areas are thought to be smoother. The Cassini team has nicknamed the darker terrain here "Si-Si the Cat" after a team member's daughter, who pointed out its cat-like appearance
Source: NASA/JPL

"Titan is not an ice ball"

An important new finding made by Cassini's Italian-built imaging radar system, in its radiometry mode, is that "Titan is not an ice ball," according to Ralph Lorenz, an imaging radar team member from the University of Arizona at Tucson. "Titan emits heat more effectively than ice balls" more typical for solid bodies in the outer solar system, he said. This new thermal finding will be significant in later calculations on the complex interaction of organics on the surface and in the atmosphere, according to Lorenz.

But imagery from the radar, along with Cassini's ISS and VIMS, provided the most striking findings of this first Titan flyby. About 25 of the flybys will also involve the radar, developed starting 14 years ago by Alenia and five other Italian contractors in connection with the JPL. Non-radar-imaging data shows that the known continent-sized feature in the southern hemisphere, dubbed Xanadu, appears white with acoastline-like appearance where it transitions into dark material also on a continental scale. The nature of either terrain type is unknown, although areas farther north indicate a higher level of organics. [3]

The October 26th flyby, which was the first, was the only one that imaged the landing area for the Huygens probe before landing on Titan's surface. The flyby took 500-1000 images and detailed chemistry measurements. Infrared imagery showed the landing site to be about a 50-50 mixture of light and dark terrain (if the dark area is terrain and not a liquid). The Huygens team had no prior imagery with which to pick exactly where the probe would land. Huygens project scientist, Jean-Pierre Zebreton, said that the team was delighted by the site's diversity once it saw the probe's projects landing site. [3]

The radar portion of the October 26th flyby lasted only about five minutes, but it was an intense five minutes of precise pointing activity for the spacecraft, according to Dr. Charles Elachi. The initial radar pass covered an approximately 75-mile-wide by 1250-mile-long strip well north of the area imaged by the Cassini ISS and the VIMS. The radar and imaging data will eventually be overlapped, but, because of design limitations, the imaging instruments and 13-ft. radar dish cannot be pointed at the same area simultaneously. [3]

The radar and other key instrument findings include:

  1. A potential methane lake: Dr. Elachi said a portion of the radar coverage shows a feature, somewhat like the silhouette of a cat's head, which is some of the darkest terrain ever seen in radar coverage. Imaging radar returns from extremely flat surfaces, like liquids, show up as such returns, leading to the idea that this might be a liquid area. The area is about half the size of Lake Champlain.
  2. Geologic features: The geology on Titan, instead of involving typical rocks, could involve frozen water ice being acted upon by tectonic forces, or cryo-volcanism where ammonia-water solutions with a lower freezing point are forced up and act upon the surface in connection with high winds or wave action. The radar imagery is rich in complex features, including one that looks like an uplifted dome, as well as fan-like patterns similar to those seen on the planet Venus. There are also long linear features.
  3. Atmospheric data: Titan is a "super-rotator," where mid-level winds are faster than those that can be caused by planetary rotation alone. An image of the backlit atmosphere shows it extends nearly 250 miles high and has complex layering. [3]

First images of Titan's surface from Huygens

early Titan image
An early image of Titan as captured by Huygens
Source: NASA/ESA
Titan close-up
Titan as seen from 8 km up by Huygens
Source: NASA/ESA

The first pictures revealing the surface of Saturn's moon, Titan, were shown from the Huygens probe during its landing on January 14, 2005. The images reveal what look like drainage channels on the surface of what, until that moment, was a planet totally hidden from view. The image unveiling marked the end of a successful journey for the hardy Huygens probe and the culmination of 25 years of work by mission managers, scientists, engineers and supporters.

Huygens' first image, taken from an altitude of 16 kilometers, had a ground resolution of about 40 meters, according to Martin Tomasko, principal investigator for Huygens' Descent Imager/Spectral Radiometer (DISR). Tomasko said on January 14th that Huygens research teams had about 350 pictures to work with. The image appears to show ravines that could have been carved by the liquid hydrocarbons thought to cover much of Titan's surface. The ravines, stubby drainage-like channels, appeared to funnel toward what appeared to be a shoreline, researchers reported during their initial reactions to the image.

"If it's not a sea, it appears to be a lake of tar-like material," explains John Zarnecky, principal investigator for the Huygens' Surface Science Package, which is taking data from the surface of Titan. Zarnecky says that the 350 images taken by Huygens of Titan's surface were only about half the anticipated photographic harvest researchers were expecting from Huygens 2.5-hour descent to the Titan surface. Unfortunately, one of the two communications channels on the satellite apparently malfunctioned, significantly cutting the number of images received by NASA's orbiting Cassini satellite and relayed to mission control in Darmstadt, Germany.

image of Titan
Huygens first color image of Titan
Source: ESA/NASA/University of Arizona

The communications channel glitch was the only Huygens mishap that mission managers have reported. While the redundant transmission channel is not working properly, only one of the probe's six instruments - a Doppler tool to study Titan's winds - is dependent solely on that channel and may be compensated for by data from ground-based observations, said mission scientists. "This is a historic event," ESA Science Director David Southwood says. "The torch has now been passed from the engineers who delivered the probe and got the data sent to Cassini to the scientists who will evaluate the data." [9]

Titan's wind speeds

A preliminary estimate of the wind variations with altitude from about 110 kilometers (68 miles) down to the surface has been recovered by a joint team of researchers from NASA's Jet Propulsion Laboratory, Pasadena, California, collaborating with the Huygens Doppler wind experiment team led by Dr. Michael Bird in Bonn, Germany, and with the ground-based European Very Long Baseline Interferometry team led by Dr. Leonid Gurvits.

As the Huygens probe descended through Titan's upper atmosphere strong westerly winds of up to 400 kilometers (250 miles) per hour buffeted the probe. Near the surface of Titan, however, the winds eased to a mild breeze. A network of radio telescope facilities, located around the world, received the radio signals transmitted by the Huygens probe to the Cassini orbiter during the probe's descent and landing on Titan. The information from the radio telescopes was originally intended to supplement similar wind data received from the Huygens Doppler wind experiment. However, because the onboard experiment failed to return data, the "ground-based work salvaged the Doppler wind experiment," according to Sami Asmar, a JPL co-investigator on the Huygens Doppler wind experiment. He had reported detecting the signal on the ground from the Green Bank Telescope facility in West Virginia: "The signal from the Huygens probe was not designed to be detected on Earth. Sometimes it pays to eavesdrop," explains Asmar. The winds were determined by the Doppler shift of the signal. [10]

Meanwhile, back on Saturn…

Colorful new images of Saturn have been taken by the Cassini spacecraft. In the first image, the icy moon Mimas is set against a dazzling and dramatic portrait of Saturn's azure northern hemisphere and the shadows of its rings. A second image shows that Saturn's northern polar region is a dim blue.

The moon Mimas in its orbit against the azure backdrop of Saturn's northern latitudes. The long, dark lines are shadows of the planet's rings.
Source: NASA/JPL/Space Science Institute

The blue color of Saturn's northern latitudes may to be linked to the apparently cloud-free nature of the upper atmosphere there. A precise understanding of the phenomenon may come from further study by Cassini imaging scientists. In the first of these colorful views, Mimas moves in its orbit against the blue backdrop of Saturn's atmosphere, which is draped by sweeping shadows cast by the rings. A few large craters are visible on Mimas, giving the icy moon a dimpled appearance. The second view shows Saturn's northern polar region, where shadows cast by the rings surrounding the pole appear as dark bands. The ring shadows at higher latitudes correspond to locations on the ring plane that are farther from the planet - in other words, the northernmost ring shadow in this view is cast by the outer edge of Saturn's A ring. Spots of bright clouds also are visible throughout the region.

 Saturn's northern hemisphere
Saturn's northern hemisphere is presently a serene blue, more befitting of Uranus or Neptune, as seen in this natural color image from Cassini.
Source: NASA/JPL/Space Science Institute

The view of Saturn and Mimas was taken by the Cassini spacecraft's narrow angle camera on January 18, 2005, at a distance of approximately 1.4 million kilometers (870,000 miles) from Saturn. The view of Saturn's northern polar region was taken with Cassini's wide angle camera on December 14, 2004, at a distance of 719,200 kilometers (446,900 miles) from Saturn. [11]

Fingerprints of an unseen moon

A few relatively small moons, clearly visible in major gaps between Saturn's rings, had been discovered in Voyager data. With diameters as little as 12 miles (20 kilometers), these moons are known to gravitationally shepherd ring particles as well as maintain gaps between the rings. But whether or not every one of the dozen or so gaps in Saturn's rings have embedded moons is an important question the Cassini spacecraft is designed to answer. Scientists also do not know how Saturn's rings originally formed or exactly what they're made of.

Carolyn Porco of the Space Science Institute in Boulder, Colorado led a study of Cassini images suggesting one possible new moonlet. Porco has said that she has "circumstantial evidence" for a small moon in the Keeler gap, near the outer edge of Saturn's main rings. The evidence is in the form of "spiky wisps" of material that protrude into the Keeler gap from its outer edge. The features resemble those generated on the inner edge of the F ring by the shepherding moon Prometheus, Porco says. The moon has not been seen yet, and Porco suspects it's no bigger than about 3 miles (5 kilometers) or so in diameter. She stressed that the size estimate is just a guess, based on the size of the gap - just 22 miles (35 kilometers) wide - the dynamics involved, and the fact that the moon has never been seen. [12]

New Cassini data from the inner rings has yielded more reliable approximations of the masses of two of Saturn's ring-bound moons, Atlas and Pan. The research implies the moons are very porous - similar to moons orbiting just outside Saturn's rings. The low densities suggest that all the inner moons and satellites were smaller bits and pieces that were gravitationally pulled together; basically orbiting rubble piles.

Early in March, the Cassini orbiter will swing past the moon Enceladus, whose surface is relatively young, about 100 million years, and has many interesting features. The ice that coats it is especially clean, and gives the moon the highest albedo, a measure of light reflectivity, of any body in the solar system. Enceladus has an albedo greater than 90 percent, whereas the Earth and its moon have albedos of 38 percent and 12 percent, respectively. [13]

Molecular oxygen detection

On the last week of February 2005 several new images and data from the mission were published in the journal Science. Among the findings is the detection of molecular oxygen around Saturn's A ring. (The ring system names begin from the inside out in this order: D, C, B, A, F, G, E.) Molecular oxygen forms when two oxygen atoms bond together. It is rarely seen beyond Earth, where it is created continuously as a byproduct of photosynthesis in plants. On Saturn, where there is no plant life, molecular oxygen must be formed in a different way: through a chemical reaction between the Sun's radiation and the icy particles that comprise Saturn's rings, according to scientists.

Scientists studying radiation in the rings of Saturn also found evidence of oxygen. Doug Hamilton of the University of Maryland said the main radiation belts in the rings are comprised mostly of oxygen and water. According to Hamilton, this is most likely the result of the bombardment of the planet's rings and icy moons by the radiation trapped in Saturn's magnetic field. Molecular oxygen has also been found above the icy Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto).

Working in coordination with Hamilton's group, a group of scientists at the Applied Physics Laboratory at Johns Hopkins University found an unexpected radiation belt in the D ring, and they learned that the radiation belts of Saturn are more intense on the night-side of the planet. [13]

Undulating kinks and theft by Prometheus

New images from the Cassini spacecraft released on Friday, December 3, 2004, reveal undulating kinks in one of the Saturn rings and the theft of ring dust by a moon of Saturn. The kinks are seen in greater detail than ever before and the "thievery" has been photographed for the first time. The Saturn moon Prometheus is the likely culprit.

The material in Saturn's ring - ice and rock from the size of dust to mountains - orbits the planet in a flat plane divided into several rings. In the F ring, kinks travel along the orbital path, looking a bit like moving bumps in a road. Scientists have speculated in the past that Saturn's moon Prometheus creates the kinks by pulling material from the F ring and temporarily putting batches of the stuff on a slightly different path. That view is bolstered by an image that appears to show a faint strand of material running from the F ring to Prometheus, presumably lured there by the moon's gravity. The potato-shaped satellite is about 63 miles (102 kilometers) long. The kinks in the F ring are seen in an animation with 44 panels taken at 3-minute intervals. The rings appear to tilt during the sequence because of the spacecraft's movement south and away from the planet during the roughly 2-hour photography session. [14]


The pulling of material from the F ring would not be the first time Prometheus has stolen something. In Greek mythology, Prometheus stole fire from the Gods and gave it to the mortals. [14] Unlike the mythological Prometheus, however, the very real Cassini-Huygens Mission has not been "punished" for giving us its gifts of knowledge. The severe conditions of interplanetary space or those of the gargantuan Saturnian system have been no match for the perseverance of the Cassini-Huygens spacecraft in its quest to continue the mission of Prometheus in enlightening humanity. This mission is expected to continue until at least 2008, when Cassini's four-year journey around Saturn is expected to come to a triumphant end.

Prometheus & F-ring material
Saturn's moon Prometheus appears to steal material from the planet's F-ring, which itself is divided into several ringlets.
Source: NASA/JPL/Space Science Institute
kinks in F-ring
Kinks ripple through Saturn's F-ring. They may be induced by Prometheus' gravitational tug.
Source: NASA/JPL/Space Science Institute

© Copyright 2005, All Rights Reserved, CSA