Close encounters

THIS RECENT IMAGE taken from the Cassini spacecraft reveals a faint pinprick of light between Saturn's wing-like rings, upper left. That tiny dot is the earth. The image is the second ever taken of our world from deep space. - AP

The Cassini mission's two years with Saturn reveal new mysteries of the planet and its satellites.

THE National Aeronautics and Space Administration's (NASA) Cassini mission to Saturn has been called the last mission of the second golden age of space exploration. The first era saw the history-making voyages of the Pioneer, Mariner, Viking and Voyager probes. After a lull during the 1980s, space exploration got back on track in the 1990s with the launch of such missions as Galileo, the Mars rovers, Mars Global Surveyor, Mars Express and Cassini.

Cassini, the size of a double-decker bus and bristling with 11 individual scientific instruments, was launched on October 15,1997. After a seven-year voyage across interplanetary space, the spacecraft reached its destination in July 2004. Two years down the line, it has produced a mass of spectacular photographs, measurements and readings of Saturn, its rings and its moons. Many of the moons have deservedly hogged the limelight, from the water fountains of Enceladus to the smog-shrouded world of Titan, investigated by Huygens, the instrumented probe which rode piggyback on Cassini. But now it is the turn of Saturn, surely the most striking of all the planets with its gentle hues and delicate rings, to come into prominence. Cassini answered many of the questions about Saturn before it arrived and created many new questions that have mission scientists eager for new data.

The last stage of Cassini's journey came on July 1, 2004, when the spacecraft burned its engines for an hour and a half to go into orbit around Saturn. It has been nearly two years since the spacecraft slipped into the Saturnian system and started surveying the planet and its neighbourhood.

Cassini is a $3.2 billion effort of NASA, the European Space Agency (ESA) and the Italian Space Agency. It is arguably the most sophisticated and challenging mission that NASA's Jet Propulsion Laboratory (JPL) has ever undertaken. The Cassini mission originated in 1983 after data from the Voyager flybys roused the interest of both NASA and ESA planners in follow-up trips to Saturn. In order to exploit planetary alignments and reach Saturn in 2004 as planned, this mission was finally launched in 1997.

Cassini is the largest, most massive and most technologically sophisticated interplanetary spacecraft NASA has ever launched. It is more than 6.7 metres long and weighs some six tonnes. Many components of Cassini and most of the 350-kg Huygens probe were built by the ESA and the Italian Space Agency. Scientists from the Mullard Space Science Laboratory of Britain, built part of the Cassini Plasma Spectrometer, while the Radio and Plasma Wave Science Instrument was developed with the help of scientists from the University of Sheffield in the United Kingdom. Cassini's scientific teams include 122 European researchers.

Why this mission? Between 1979 and 1981, three unmanned probes flew by Saturn - Pioneer 11 and Voyagers 1 and 2. But none of these probes, all launched by NASA, gave scientists a chance to give Saturn a truly in-depth examination. They are counting on Cassini to remedy the situation.

For many scientists, Titan is the star of the show. Huygens descended into Titan's murky atmosphere on December 25, 2004, while the orbiter itself mapped the surface of the satellite using radar. Huygens landed on Titan on January 14, 2005.

One of 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 to document the motion of Saturn's clouds.

Scientists are also anxious to investigate Saturn's atmosphere in the vertical dimension. Cassini will not release a probe into Saturn's atmosphere, but it will use other methods. One of these takes advantage of the fact that Cassini's radio signals to the earth will pass through Saturn's atmosphere each time the orbiting craft disappears behind the planet and then reappears. By analysing variations in the radio signals, scientists can obtain data on temperatures, pressures and compositions of Saturn's upper atmosphere.

Many planetary scientists believe that heat from the interior is a major influence on Saturn's atmospheric circulation. The planet radiates about 80 per cent more heat than it receives from the sun, a fact that is as yet unexplained. Most of Saturn's gaseous bulk is composed of a mixture of hydrogen and helium and scientists have speculated that over the planet's lifetime (4.6billion years), heavier helium atoms may have migrated towards the planet's core, producing heat. If Cassini is able to confirm this, it will not only explain Saturn's excess heat but also help scientists understand better the evolution of the giant gas planets.

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. To scientists, Saturn's ring system is the most puzzling. Cassini will provide the best chance yet to investigate the composition of the rings. Cassini's sensors, with their greater range of spectral response, will allow for more detailed analysis than Voyager allowed. 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 for Cassini is that it will also help 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?

Of Saturn's 31 known satellites, Titan is to receive the most attention from Cassini. With a diameter of 5,150 km, Titan is the second largest moon in the solar system after Jupiter's Ganymede and 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. Huygens may throw some light on the mystery of how the transition from chemistry to biology occurs.

In addition to 45 planned flybys of Titan, Cassini will have approximately six flybys of Saturn's medium-sized iced satellites at altitudes between 500 km 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 Iapetus and Enceladus. Half of Iapetus's surface is covered with bright ice and the other half is as dark as asphalt. Cassini's images should help determine whether the dark substance is from Iapetus's interior or from an outside surface. Cassini is also to determine whether active ice volcanoes now exist on Enceladus.

The Huygens probe, soon after its release from Cassini, spent about two and a half hours boring through the atmosphere before finally making contact with Titan. When the probe slowed down enough, it deployed a parachute and 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. A fancy little chemistry laboratory within Huygens sprang to life once the probe hit the atmosphere. The laboratory includes a Gas Chromatograph and a Mass Spectrometer in a capsule only nine feet (2.7 metres) across. With these instruments, the probe should be able to identify the chemicals it runs into.

Huygens also has an aerosol collector and a pyrolyzer (a high-tech oven). When the probe gets low enough, it expects to find aerosol particles in the air for the collector to grab. It would then feed the aerosols into the oven, which would cook them and forward the resulting gas to the chromatograph and spectrometer for final analysis. Huygens would transmit its findings back to Cassini, which in turn would beam the data back to the earth. In addition, Huygens is to capture the images of the atmosphere and surface, measure temperatures and reveal a thing or two about Titan's winds.

The Cassini spacecraft passed closest to Phoebe, one of Saturn's moons, on June 30, 2004, before entering the planet's orbit. Cassini found dark material like dirt on Phoebe. This heavily cratered moon appears to be mainly consisting of ice, with water ice, water-bearing minerals, carbon dioxide, possible clays and primitive organic chemicals detected in patches on the surface forming an overall dark crust.

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 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.

Cassini has succeeded in penetrating the dense atmosphere of Titan and in identifying the physical features on its surface. Scientists speculate that Titan may preserve in deep-freeze many chemical compounds that preceded life on the earth. Cassini gathered data before and during a distant flyby of Titan on July 1, 2004. Cassini's Visible and Infrared Mapping Spectrometer pierced the smog surrounding Titan. This instrument, capable of mapping mineral and chemical features of the moon, reveals an exotic surface bearing a variety of materials in the south and a circular feature that may be a crater in the north.

This is the first time scientists have been able to map the mineralogy of Titan. "The top of Titan's atmosphere is being bombarded by highly energetic particles in Saturn's radiation belts, and that is knocking away this neutral gas. In effect, Titan is gradually losing material from the top of its atmosphere, and that material is being dragged around Saturn," said Dr. Stamatios Krimigis of Johns Hopkins Applied Physics Laboratory, principal investigator for the magnetospheric imager. The study of Titan is one of the major goals of the Cassini mission. The July 1 flyby at the closest distance of 339,000 km provided Cassini's best view of Titan so far, but over the entire period of four years the orbiter is to execute 45 Titan flybys as close as 950 km.

With eyes sharper than any that have peered at Saturn before, the Cassini spacecraft has already discovered two new moons, which may be the smallest bodies so far seen around the ringed planet. The moons are approximately 3 km and 4 km across. The moons, located 194,000 km and 211,000 km from the planet's centre, are between the orbits of two other Saturnian moons, Mimas and Enceladus.

From the inside out, Saturn is believed to have a large rocky core, squeezed and heated to some 12,000o. This core may measure around 25,000 km across and be as massive as perhaps 15 earths combined. Beyond this deep core lies the bulk of the planet, consisting mainly of liquid hydrogen and helium. In a zone stretching to around 12,000 km above the core, the hydrogen behaves rather strangely. The immense pressure equivalent to more than two million earth atmospheres, coupled with temperatures around 6,000o, causes the hydrogen to behave more like a liquid metal. It is fluid motion within this `liquid metallic hydrogen' that generates the planet's powerful magnetic field. Moving outwards, the next shell is primarily liquid hydrogen, which stretches around 30,000 km all the way out to the highest extent of the atmosphere, gradually becoming gaseous somewhere in between. The cloud deck visible from space is light brown and is composed of ammonia. Beneath that are two separate layers, the first made up chiefly of ammonium hydrosulphide and the second primarily of water.

Its average distance of 1.4 billion km from the sun means that Saturn does not get a lot of heat from the sun. Yet, Saturn radiates a lot of heat, more than can be accounted for from what it receives. How is it generating this extra heat? Saturn's atmosphere has proportionally less helium than Jupiter's. It has been inferred that because of Saturn's lower temperatures it becomes less easy for the helium to mix with the hydrogen and so it has been gradually percolating and sinking through the lighter hydrogen towards the core. This helium `rain' has been slowly converting energy in Saturn's atmosphere into heat, and it is this process that is believed to be largely responsible for the excess heat the planet generates, when compared to what it absorbs from the sun.

One striking atmospheric feature that has been tracked by Cassini since 2004 is the `Dragon Storm', so named because of its appearance. This 3,500 km-wide electrical phenomenon is the largest lightning storm ever detected on Saturn. It has been linked to bursts of radio waves detected by Cassini and is likely to be a thunderstorm deep within the atmosphere, periodically flaring up to become visible at the cloud tops.

Another remarkable discovery relates to the effect of Saturn's rings upon the atmosphere, which is actually altering the appearance of the planet. The planet's axial tilt means that, during its 29.5-year-orbit, the northern and southern hemispheres alternate between being partially cloaked in the shadows of the rings and fully illuminated by the sun. Currently, the northern hemisphere is in partial shadow, and the consequent cooling has had a much more dramatic effect than expected. The characteristic beige/tan clouds appear to have sunk to lower depths, leaving the hemisphere with a blue tint, not dissimilar to the appearance of the atmospheres of Uranus and Neptune . This is adding yet another complication to the understanding of the planet as a whole.

One of the most successful results so far has involved observations of Saturn's deep interior by Cassini's magnetometer. Being a gas giant, Saturn does not rotate in precisely the same way as the earth, as it is not a rigid structure. Its rotation period has also been more difficult to pin down precisely. Cassini has detected that the planet has a rotation period of 10 hours and 47 minutes. This is a very important result, as it will allow scientists to constrain the amount of rotational energy the planet has and therefore make it easier to modify the models of how much internal heating is required to make the planet behave as we see it.

The volcanoes on Titan are thought to eject icy substances such as ammonia and methane. While they are gases on the earth, both flow as liquids on the cold surface of Saturn's moon and it is thought that the volcanoes themselves are made of water ice, which is believed to form the bedrock of Titan. This phenomenon of ice volcanoes is known as cryovolcanism and was first proposed to take place on Titan by Carl Sagan, the famous American astronomer.

During its flyby of Titan on July 22, 2006, Cassini observed dozens of what appear to be calm, smooth-surfaced lakes north of the moon's Arctic Circle. At 180oC, Titan's lakes probably consist of liquid methane and ethane - organic compounds that probably flow into the lakes via the many river channels that Cassini observed winding through the area.

The most intriguing discoveries have come from flyby encounters with Saturn's icy moons. On November 27, 2005, Cassini captured ice volcanoes erupting from the southern hemisphere of Enceladus. Enceladus has suddenly become a much better prospect in the search for life than any other satellite in the solar system. Hyperion, another moon of Saturn, is prominently pitted by features known as sun cups. In them, dark dust absorbs sunlight and melts the ice around it. The same thing happens in high-altitude frozen spots on the earth.

After a highly successful two years at Saturn, and with a further two years of the Cassini mission remaining, the details of how this giant planet works should unfold gradually. The Cassini mission represents a once-in-a-lifetime opportunity to solve Saturn's mysteries. No doubt, Cassini is going to be remembered as one of the greatest space missions.

Professor Amalendu Bandyopadhyay is a senior scientist at the M.P. Birla Institute of Fundamental Research, M.P. Birla Planetarium, Kolkata.