A tour of Saturn

The most detailed view ever of Saturn's rings, obtained from the Cassini spacecraft. - REUTERS

Titan's rich organic chemistry makes it a planetary-scale laboratory for studying the pre-biotic processes that may have led to the origin of life on the earth. Hence the special interest in this satellite of Saturn as the Cassini-Huygens mission takes a close look at it.

THE Cassini-Huygens probe was launched from Cape Canaveral in the United States on October 15, 1997,and for seven arduous years the spacecraft has travelled 3.3 billion kilometres through interplanetary space, conducting fly-bys of Venus (twice), the earth and Jupiter during its voyage towards its final destination, Saturn. The last stage of the probe's journey began on July 1, 2004, when the spacecraft burned its engines for an hour and a half to go into orbit around Saturn. And for the next four years, Cassini-Huygens will explore the planet, its rings and its moons as never before.

Cassini-Huygens, a twin-spacecraft, is a $3.2 billion cooperative project of the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA) and the Italian Space Agency (ISA). Cassini-Huygens is arguably the most sophisticated and challenging mission that NASA's jet propulsion laboratory (JPL) has ever undertaken. Cassini is the largest, most massive and most technologically sophisticated interplanetary spacecraft the NASA has ever launched. It is more than 6.7 metres long and weighs some six tonnes and is very much an international effort. Many components of the orbiter Cassini and most of the 350 kg Huygens probe were built by the ESA and the ISA. Scientists from the Mullard Space Science laboratory of the United Kingdom built part of the Cassini Plasma Spectrometer, while the Radio and Plasma Wave Science instrument was developed with the aid of scientists from Sheffield University. Cassini's scientific teams include 122 European researchers.

Cassini's journey to Saturn has been a circuitous one, including two fly-bys of Venus (April 1998 and June 1999), one of the earth (August 1999) and one of Jupiter (December 2000). The planets' gravitational pulls acted as slingshots to accelerate the spacecraft's journey toward Saturn. After firing its braking engine to go into orbit around Saturn on July 1, 2004, Cassini has already employed a dozen instruments in its scientific reconnaissance. Then on December 25, the Huygens lander separated from its mother ship for a three-week-long coast to the large, cloud-enshrouded moon Titan. On January 14, 2005, Huygens slammed into Titan's thick atmosphere at 22,500 km an hour and parachuted through the haze and the clouds, perhaps splashing down in a sea of liquid methane. Cassini will orbit Saturn 76 times during its nominal four-year science mission and will also have 52 close encounters with seven of its 31 known moons. A spacecraft like Cassini can study time variations and the interactions between diverse phenomena in a manner completely impossible for smaller spacecraft.

IN early 1655, the Dutch mathematician, physicist and astronomer Christian Huygens had his first look at Saturn. Huygens' telescope was much better than those of many of his contemporaries. From telescopic observations, Huygens discovered the largest satellite of Saturn, Titan. In 1675, Giovanni Domenico Cassini, an Italian astronomer who became the first Director of the Paris Observatory, discovered a dark gap in the rings of Saturn, now known as Cassini's division. That is why this mission has been given the name Cassini-Huygens.

Why this mission? Between 1979 and 1981, three unmanned probes flew by Saturn - Pioneer 11, Voyagers 1 and 2. All three were launched by NASA. All these just flew by and it was a real quick look. As a result, scientists have never had the chance to give Saturn a truly in-depth examination. Now scientists are counting on Cassini to remedy the situation. After going into orbit around Saturn, Cassini is to begin a four-year tour of Saturn, its rings and its satellites.

For many scientists, Titan is the star of the show. Cassini delivered Huygens to descend into its murky atmosphere and the orbiter itself is to map the satellite's surface using radar. Cassini will stage a thorough investigation of Saturn itself.

Among the mysteries that Cassini might resolve is what creates the so-called zonal jets, horizontal bands that cross Saturn's cloudy upper atmosphere. Saturn's winds reach speeds as high as 1,760 km an hour. But what drives these winds? To answer this question, Cassini is outfitted with a powerful array of instruments, including a high-resolution camera that is to be used to document the motion of Saturn's clouds.

Saturn's ring system is the most beautiful object in the sky. Other planets have rings, as have been detected by space probes. But none can compare with the glory of Saturn's system. To scientists, Saturn's ring system is the most puzzling of these. Cassini will also provide the best chance yet to investigate the rings' composition. Cassini's sensors, with their greater range of spectral response, will allow for a more detailed analysis than that was facilitated by Voyager. Probing the rings at far-infrared and microwave wavelengths, Cassini's instruments should even be able to detect any rocky material that may lie beneath the ring particles' icy surfaces. One of the biggest expectations from Cassini is that it will also help to determine where the rings came from. Was the rings' parent world one of Saturn's satellites or was it an interloper from another region of the solar system? This uncertainty may be solved by Cassini's findings.

With a diameter of 5,150 km, Titan is the second largest moon in the solar system after Jupiter's Ganymede and it is the only planetary satellite enveloped by a thick atmosphere. Scientists are interested in Titan mostly because of its resemblance to the earth. In terms of atmospheric composition and surface pressure, Titan is more similar to the earth than any other body in the solar system. The atmospheres of both are dominated by nitrogen (77 per cent for the earth, 90 to 97 per cent for Titan), and Titan's atmosphere produces a surface pressure 50 per cent greater than the earth's at sea level. Adding to the intrigue, Titan's rich organic chemistry makes it a planetary-scale laboratory for studying pre-biotic processes that may have led to the origin of life on the earth. The question of how life formed on the earth makes Titan a particularly attractive place. The chemistry of Titan's atmosphere appears to be much closer to the early earth's and that is why it is a much more promising place to search for how the transition from chemistry to biology occurs. Huygens may throw some light on this mystery.

In addition to 45 planned fly-bys of Titan, Cassini will conduct approximately six fly-bys of Saturn's medium-sized iced satellites at altitudes between 500 and 2,000 km. These encounters should produce remarkably detailed images. For example, Cassini should be able to map nearly the entire surface of both Lapetus and Enceladus. One half of Lapetus' surface is covered with bright ice and the other half is as dark as asphalt. Cassini's images should help to determine whether the dark substance comes from Lapetus' interior or from an outside surface. Cassini will also try to determine whether active ice volcanoes now exist on Enceladus.

Saturn's magnetic field is very powerful, as its total magnetic energy is some 540 times stronger than the earth's. Scientists are eager to learn more about the structure of Saturn's magnetic field and its interaction with the rings, satellites and the stream of charged particles emanating from the sun, known as solar wind. Cassini will employ several instruments to study the magnetosphere, including a magnetometer and a plasma spectrometer to study the highly ionised gas within Saturn's magnetosphere.

The Cassini-Huygens mission has already entered the orbit around Saturn, passing deftly through the rings of the planet. The probe is already in action, and within hours of entering the orbit sent back new images of the rings. The Cassini orbiter, which carries 12 science instruments, passed closest to Phoebe, one of Saturn's moons, on June 30, 2004 before entering Saturn's orbit. Cassini found dark material like dirt on Phoebe. This heavily cratered moon appears to be mainly ice, with water ice, water-bearing minerals, carbondioxide and primitive organic chemicals detected in patches on the surface, forming an overall dark crust. One of the large impact craters (around 50 km across) on Phoebe reveals dark and light layers near the surface of the moon and a lighter interior.

Cassini has confirmed that the rings of Saturn are mainly boulder-sized lumps of water ice, although the ice is purer than expected. A surprise came from the analysis of the size of the particles making up these lumps, using Cassini's Visual and Infrared Mapping Spectrometer - the grain size gets bigger and the water ice purer farther away from the planet. But ice is not the only component of the rings. There is also something known as "dirt" or dark material. What is interesting is its distribution in the rings - there is proportionately more dirt in the thin, dark parts of the ring system such as the Cassini division, and much less in the light parts, which are mainly ice. This suggests some unknown sorting mechanism in the rings.

Scientists of NASA, the ESA and the ISA have started examining critically the images and data from the spacecraft following its entry into a Saturn orbit and its October 26 and December 13 close fly-bys of Titan. The studies have yielded some important findings. 1. The confirmation that major storm systems in the atmosphere of Saturn spark lightning. Radio signals from these discharges are a million times more powerful than those from terrestrial lightning. 2. The discovery of two tenuous rings. The first shares the same orbit as the small moon Atlas. The second ring is located just inside the orbit of the moon Prometheus. Several new ringlets have also been found. 3. The discovery of a new moon has been confirmed. It is only about 5 km across. 4. Images of the moon Iapetus show several white peaks poking high above dark terrain. Analysis suggests that some of these peaks might be 10 to 20 km high, which could rival Mars' giant volcanoes for being the highest mountains in the solar system. Scientists may have to wait until the September 2007 close Iapetus fly-by to nail down the altitudes.

The Huygens probe separated from the orbiter on December 25, during Cassini's third orbit of Saturn. On January 14, Huygens entered Titan's cloudy atmosphere for a two-and-a-half-hour descent. When the probe slowed enough, it deployed a parachute. As it descended into the atmosphere, it made a variety of measurements of the atmosphere's physical and chemical properties. Huygens can make measurements if it lands on liquid or solid ground. After spending seven long years between the earth and Titan, a fancy little chemistry laboratory within Huygens sprang to life after the probe did hit the atmosphere. The laboratory includes a Gas Chromatograph and a Mass Spectrometer (GCMS) in a capsule only nine feet across. With these instruments, the probe was able to identify the chemicals it ran into.

Huygens also has an aerosol collector and pyrolyzer. When the probe got low enough, it expected to find aerosol particles in the air for the collector to grab. It then fed the aerosols to the pyrolyzer - a high-tech oven, which cooked them and forwarded the resulting gas to the chromatograph and the spectrometer for final analysis. Huygens ultimately transmitted its findings back to Cassini, which in turn beamed the data back to the earth. Earth-bound scientists have thus been able to get a much clearer picture of just what kind of chemistry is going on on Titan. In addition, Huygens took images of the atmosphere and the surface, measured temperatures and revealed the secrets about Titan's winds as probes recorded their effect on its descent.

On January 14, a new chapter in the history of the solar system exploration was written when the Huygens probe plunged into the atmosphere of Titan. Almost exactly 350 years after Christian Huygens discovered Saturn's giant moon, a spacecraft named after him finally revealed the true nature of this remarkable world.

The Voyager 1 spacecraft was targeted to fly past Titan in November 1980. The encounter was a great success, revealing a nitrogen-rich atmosphere with a surface pressure 1.5 times that on the earth but, to the dismay of scientists, no glimpse of Titan's surface was availanle through the all-embracing haze of hydrocarbons. Since then, astronomers have realised that Titan's cloak of haze is transparent to infrared light, enabling them to unveil an abstract pattern of dark and light patches. Are these icy continents and hydrocarbon seas? This ignorance is about to evaporate after the Huygens probe has plunged into the atmosphere of Titan.

The arrival of Huygens was almost flawless, and the instruments worked to perfection. Huygens unfurled a series of parachutes as it slowly drifted toward Titan's surface. The probe descended for two hours 27 minutes before touching down at a gentle five metres per second (11 miles, or about 17.6 km per hour). Cassini received the data transmitted by Huygens from the surface for an hour and 12 minutes and radio telescopes on the earth detected Huygens's broadcasts for more than four hours after touchdown.

The first instrument to touch Titan's surface was the penetrometer, which is essentially a stick fixed to the bottom of the probe, designed to measure the force of impact and thus the strength of the material impacted. As soon as the sensor touched the ground, it saw a rather high force and the closest match was detected to be sand on clay along with something else. Next, the GCMS started analysing its new surroundings. This instrument, by design, can detect only gases.

The existence of liquid methane has been detected. The ground on Titan is very cold and as such methane stays in liquid form on the surface and acts very much as water does on the earth. It evaporates, condenses, forms clouds and rains back down onto Titan, where it creates lakes, streams, creeks and springs. The GCMS measurements did record the signature of Argon-40, a byproduct of the radioactive decay of potassium. But Huygens found no evidence for earth-like potassium-bearing rocks on the surface.

Descent Imager and Spectral Radiometer (DISR) team leader Martin G. Tomasko (University of Arizona) said in a press conference: "The place where Huygens landed was examined to be dry at the moment, but there was liquid within a few centimetres of the surface indicating that it must have rained not very long ago." When the DISR peered down onto Titan's surface, the images it saw matched perfectly with the putative hydrologic cycle suggested by the GCMS data. There were vast dark and light regions, 100-metre-tall ridges with bright spots atop them, dark elliptical arcs, and what appeared to be rivers, streams and deltas. As for the dark elliptical regions, they are most likely evaporated lakes.

Planetary scientists continue to analyse the data from Huygens' six instruments, but they are wrestling with a few unanswered questions: One, where is the ethane? Two, where are the global oceans? Three, where are the primordial noble gases? And four, where are the amino acids? No doubt, methane rain, water-ice terrain, and hints of cryvolcanism make Titan one of the most dynamic places in the solar system. No doubt, there are earth-like processes on Titan, though the ingredients are somewhat different. But the mysteries remain to be solved, as scientists dig deeper into the trove of measurements returned by Huygens.

Amalendu Bandopadhyay is a senior scientist with the M.P. Birla Institute of Fundamental Research in Kolkata.