Saturn occupies a special place in astronomy: it is one of the first- or possibly the very first- celestial body to be seen through a telescope. In the history of astronomy, there have been a few moments that have made a sudden and permanent change in the human concept of the universe. Before these moments, we were ignorant; after them, we suddenly knew more, and the difference was great enough to alter our whole idea of the world and our place in it. One such moment was the fateful night, often reenacted in TV documentaries, when Galileo, the famed Italian astronomer, first pointed his homemade telescope at Saturn. His hand-ground lenses were less than perfect; he thought he saw a planet with bulges on its sides, which he compared to ears. It was only later, when better telescopes were used, that it became apparent that this structure was a ring system circling the entire planet.

From those humble beginnings, things have come a long way. The Cassini-Huygens probe might be viewed as a culmination of the work started so long ago by Galileo and the other pioneers of Saturnian study. The name of the mission is a tribute to two others: Christian Huygens (1629-1695) who actually identified Saturn’s rings for what they were and discovered the moon Titan, and Jean-Dominique Cassini (1625-1712) who discovered Saturn’s moons Iapetus, Rhea, Tethys and Dione.

The Cassini spacecraft was designed and built by NASA, while the Huygens lander was a project of the ESA. The two were launched as a single unit aboard a Titan IVB/Centaur rocket from Cape Canaveral, Florida, on October 15, 1997. During its journey, the craft made four gravity-assist maneuvers, in which the gravity of a planet was used to alter the craft’s course. These were at Venus in 1998, Venus again in June 1999, Earth in August 1999 and Jupiter in December 2000. The spacecraft arrived at Saturn in July 2004. In December 2004, at the end of Cassini’s third orbit around Saturn, the Huygens lander was deployed. That craft landed on Titan on January 14, 2005.

The Huygens lander was 2.7 meters wide and consisted of two parts: the Entry Assembly Module and the Descent Module. The Entry Assembly Module contained the equipment to control Huygens after its separation from Cassini and included a bulbous heat shield to protect the delicate inner workings during the descent through Titan’s atmosphere. The Descent Module contained the scientific instruments for studying Titan. The probe used three parachutes in sequence during its descent.

The descent and landing of the Huygens probe was absolutely one of the most spectacular visual experiences in all space exploration. As the lander flew over the landscape, we saw what looked surprisingly like Earth: river systems, deserts, mountains and valleys spread out in an intricate panorama. The landing was successful and the probe survived for several hours, sending back a series of stunning images showing a foggy landscape dotted with rounded boulders.

The Earth-like appearance of Titan was deceptive. Those rivers and lakes, which looked so much like water from above, were actually liquid methane. The boulders that we saw around the lander were actually chunks of water ice, permanently frozen in the -180C cold. These substances behave exactly like the substances we are more familiar with here on Earth. Methane acts just like water does here; it is so plentiful that it condenses out of the atmosphere and forms the rivers and lakes. Hydrocarbon particles form dune-covered deserts just like silica (sand) does here on Earth. All of the oxygen is locked up in the water ice on the ground- which is good, because if there were any free oxygen, it could react with the methane to cause a Titan-wide explosion. There is growing evidence that there may be an ocean of liquid water and ammonia on Titan.

Titan is a big, varied world with much to study. We’ll be trying to figure it all out for a long time to come- and that’s not all. Cassini, still whizzing around Saturn, has returned some other science that has enormous implications in the search for extraterrestrial life. Scientists were electrified when geyser-like plumes were seen shooting from Saturn’s moon, Enceladus. These plumes were coming from thin, parallel cracks on the moon nicknamed “tiger stripes,” and spectroscopic analysis showed that they were rich in organic molecules and water vapor. Since then, there has been additional evidence that, as with Titan, there may be an ocean of liquid water under the surface of Enceladus. (See our article on Enceladus at this site.) Now that we know Enceladus has water and the chemical building blocks of biology, it will be a prime candidate in our search for extraterrestrial life.

Cassini has revealed that the rings of Saturn are not as smooth and serene as they appear. Actually, they are rough neighborhoods where particles large and small collide, and the activity in there is quite complex. This mission has allowed more detailed study of the rings than ever before. Having an observation post within the Saturnian system for a long period of time has allowed scientists to see the rings at different angles of sunlight, allowing unprecedented detail and perspective in the observations.

The moon Iapetus is a strange-looking object, and will bear further study. One side of the moon is white and the other side is black, and there is a monumental bulge around its equator. The reason for the unusual shape and color scheme is unknown, but it’s certainly got the scientists scratching their heads.

There may be faint rings around the moon Rhea; further observation may confirm their existence.

The moons Tethys and Dione are spewing great steams of matter into space, indicating possible volcanic activity. Here again, further investigation is needed.

The original Cassini mission came to an end in 2008, but since the probe is still in fine working order, its tour of duty has been extended through the end of 2010. After that, there may be further extensions. It looks like this is one of those space missions that keeps on and on, long after its projected end. We can expect a lot more from Cassini, and when it is finally done with its work, we will know where the future missions need to go.

Cassini has already provided us with a few signposts for further research. Titan will definitely be visited again, and possibly colonized eventually. Those tiger stripes on Enceladus are getting a lot of scrutiny, and will certainly be a target for landings. Tethys and Dione sound interesting, too; if they have the heat for volcanic activity, there may be liquid water inside them, too. There may be life on or in some of these moons; there surely will be many fascinating things to see. For the time being, Cassini will help us to see them.

If the ghosts of Galileo, Cassini and Huygens can look down on us now, they must be proud.

Sources:
“Cassini-Huygens: Fifth Anniversary of the Landing on Titan” at the website of the European Space Agency: esa.int/SPECIALS/Cassini-Huygens/index.html

“Science and Technology: Cassini-Huygens” at website of the European Space Agency: sci.esa.int/science-e/www/area/index.cfm?fareaid=12

“Cassini Equinox Mission” at NASA website: saturn.jpl.nasa.gov/index.cfm

“Cassini Equinox Mission- News and Features: Cassini Finds Saturn Moons Are Active” at NASA website: saturn.jpl.nasa.gov/news/newsreleases/newsrelease20070614/

“ESA Cassini-Huygens: Titan Virtual Tour” at website of the European Space Agency: esa.int/SPECIALS/Cassini-Huygens/index.html

The Cassini project was set up and executed by three agencies these being the National Aeronautics and Space Administration (NASA), the European Space Agency and the Italian Space Agency. The spacecraft was launched in 1997 with its mission being to explore the planet Saturn and its system. The spacecraft took 7 years to reach Saturn and once there its initial mission to explore the planet took a further 4 years to complete. However this raised many further questions about Saturn and with the spacecraft remaining healthy it is now into overtime in its mission. It is currently being used to try and answer some of the questions raised during its 4 year mission

The spacecraft made a pass over the northern polar region of Saturn in October 2009 and infrared instruments detected a stunning light display. The aurora is the largest identified in our solar system to date and towers a massive 1,200 kilometers above the northern hemisphere of the planet. Although scientists have long suspected the aurora was present, the images sent back by Cassini in November are the first time they have actually been viewed in high resolution and provided conclusive evidence of their existence.

Auroras generally form in the high latitudes around a planet’s magnet poles. They are known to occur on Earth, Saturn and Jupiter and are caused by charged particles plunging into a planet’s atmosphere along magnetic lines. This causes the atmosphere to glow and produces the stunning light shows typical of the aurora borealis on Earth.

The video images of the huge aurora above Saturn should help scientists better understand the processes involved in generating them. The moving images was put together from around 500 still pictures which were taken over an 81 hour period between the 5^th and 8^th of October 2009. The finished video showed that there are very rapid changes in the shape and brightness of the aurora as it gyrates above the planet. The height of the aurora should also provide some clues into the amount of energy that is required to light it up.

The Cassini mission is currently scheduled to continue until September 2010 and it will continue to send back data regarding Saturn and its surrounding system until this time. With 2009 drawing to a close the highlights of the year for the project have included multiple flybys of Titan and Enceladus. These are two of Saturn’s many moons and further flybys of these are planned for the remainder of the mission. Data ring scientists also paid close attention in August when Saturn went through the Solar Equinox with the sun crossing from the southern to the northern hemisphere and this provided excellent conditions to view the rings.

In the 12 years since its launch the Cassini spacecraft has proved to be a stunning success and the data and images it has sent back about the planet Saturn and its moons has greatly increased our understanding of the system. Hopefully further discoveries will be made in the coming weeks and months until the completion of the mission and that these will help scientists in their quest to understand our solar system.

The special interest in this little moon began in 2005, when Cassini observed huge jets of vapor and particles coming from parallel cracks on Enceladus’ surface.  Cassini obtained a spectrogram of the material in one of the plumes by sampling the light from a star as it passed behind the plume.  To the amazement of scientists, the plume was loaded with water vapor and other chemicals necessary to produce life.   Not only that, but the cracks from which the geysers were coming, nicknamed “tiger stripes,” were actually deep valleys providing protected areas where nature could perform the delicate chemistry of forming life.  As you can imagine, Enceladus has been the object of intense scrutiny ever since.

One of the main questions that are being asked is, where’s the water coming from?  Since the initial discovery, scientists have been tossing around the interesting idea that there may be large amounts of water beneath the surface of Enceladus.  If this turns out to be true, there are further questions: just how much water, and how deep is it?  There is even the tantalizing possibility that Enceladus may have an internal ocean, which would be an even better place for life to start than the tiger stripes.

The Cassini probe is still in operation, and has continued to study Enceladus as well as the rest of Saturn’s moons.  Enceladus has not proven disappointing; in fact, the more that is learned about it, the more intriguing it becomes.  To date, Cassini has made eight flybys of Enceladus, as well as conducting observations of Saturn’s E ring, which is thought to be replenished by particles coming from Enceladus.  While interpretations of the data are still quite preliminary- and controversial, in some cases- the evidence is mounting that the moon has large amounts of water beneath its surface, possibly even the internal ocean that theorists have envisioned.

One piece of supporting evidence is the detection of sodium in the material of the E ring.  Sascha Kempf of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, has reported that Cassini’s cosmic dust detector found sodium in the E ring in concentrations of about one part in 100,000.  Jonathan Lunine of the University of Arizona in Tuscon, a Cassini researcher who was not a part of Kempf’s team, says that an ocean beneath the surface of Enceladus is the best explanation for the sodium.

“A liquid water layer or pocket in contact with the rock, which is deep below Enceladus’ surface, will acquire sodium from the rock, essentially leaching the rock,” Lunine says.  The flowing water would transport the water to the jets, which would spray it into space.  Water containing dissolved sodium would freeze into ice particles, which would be added to Saturn’s E ring.

The idea is controversial.  Roger Yelle of the University of Arizona points out that the sodium could be coming from the surface of Enceladus rather than from deep underground, and therefore would not imply the existence of an ocean.  And any interpretation of the new data is complicated by the fact that observations by Earth-based telescopes, such as the Keck Observatory at Mauna Kea, Hawaii, have not found any indication of sodium in the E ring.

But Kempf points out that such observations could only detect sodium in its gaseous state, not frozen in ice particles.  Cassini’s cosmic dust detector, on the other hand, is designed to study solid particles- and it did find sodium.

Unfortunately, Cassini’s neutral mass spectrometer did not find sodium when it flew through the plumes in March, though it did identify several organic compounds that could support life.

Other interesting information was gained when Cassini sampled Enceladus’ plumes in March and October.   Ammonia was detected in the plume material, which is interesting because ammonia is water soluble, and keeps water liquid at a lower temperature than would otherwise be possible.

Another substance found in the plume material was argon-40.  Like the sodium mentioned earlier, argon-40 would probably form in rock deep within the moon, and would stay there unless dissolved and transported to the geysers by water.  Again we have the tantalizing possibility of large quantities of water moving beneath the surface.

The evidence is inconclusive, but it’s certainly intriguing.  Enceladus, as well as the rest of Saturn’s moons, will provide us with great pictures and data for years to come.  Maybe that ocean really is there, waiting for us.  Maybe someday we’ll have submarines in there, looking for life and perhaps finding it.

Sources:

Cowen, Ron: Saturn’s Moon May Host an Ocean in Science News magazine, June 24, 2009 www.sciencenews.org/view/generic/id/44975/title/Saturn’s_moon_may_host_an_ocean

Cowen, Ron: Evidence Mounts For Liquid Interior of a Saturn Moon in Science News magazine, July 22, 2009 www.sciencenews.org/view/generic/id/45823/title/Evidence_mounts_for_liquid_interior_of_a_Saturn_moon

Website for CICLOPS (Cassini Imaging Central Laboratory for Operations) ciclops.org/view/6005/Enceladus_Rev_121_Flyby_Raw_Preview_1

Cassini: Equinox Mission at JPL website: saturn.jpl.nasa.gov/science/moons/enceladus/

The art/science of studying the stars was engaged by several ancient populations like Maya, Incas, Egyptians and Greeks, and soon grew in importance to the point where those who practiced it were highly regarded and respected in their own society. The reason for this is evident: this science could provide, even from its first, rudimentary structure, an explanation to phenomena strictly connected to their life, such as the alternating of day and night or the cycle of seasons, and provided an essential instrument for activities such as agriculture and navigation.

The history of Astronomy is in part — from its origins to the invention of the telescope by Galileo Galilei, in 1610 — also the history of astrology. In prehistoric ages, the most advanced tribes were familiar with the motion and trajectories of just a few, visible objects like the sun, the moon and some of the brightest stars. The most commonly cited example of such knowledge is the Stonehenge complex, residing in Great Britain, which is thought to have served as a monumental calendar.

Chinese astronomy was born before 2000 BC and is still cited nowadays for its great tradition of carefully, a-critic documentation. From the documents in our possession we know that in their time they were already aware of events such as the passage of comets, or even the explosion of a Supernova star.

A few civilizations in central America also reached astonishing results, but unfortunately they didn’t manage to share they knowledge with other populations. Maya and Inca tribes would often build pyramids and temples, which were devoted to the Gods of the Sky. Their religion was strictly related to the planet Venus and, based on estimations of its motions, they managed to create an incredibly precise astronomical calendar, finding out, among other things, that the planet would accomplish five complete orbital revolutions in the time span of exactly 584 days.

Babylonians soon showed exceptional knowledge in the matter of astronomy, which would later be inherited by Egyptians and Indians. In their case, the desire to perfect this science at all costs came, rather than an actual need, from skeptical reasons that linked the motion of stars and planets to good or bad luck (solar and lunar eclipses were thought to bring extremely bad luck, and this conception would persist until relatively recent times). Even lacking any sort of precise instrumentation, Babylonians managed to find out many things about the apparent motion of planets, basing their observation on the position of a few bright stars on the sky: they therefore discovered the orbital revolution time of many planets, among which Venus, Mars, Jupiter and Saturn, only mistaking by a few days, and reporting the results of their calculations on special tables, most of which are well-preserved and can still be seen now in astronomical museums.

Egyptians’ immense and astonishing knowledge regarding astronomy relies, once more, on their ability to forge precise calendars describing the motion of stars and planets. As their life cycle was strictly linked to that of the Nile river, astronomy was given a central role by this society from the very beginning. Around 3000 BC, Egyptians were already used to dividing their day and night time in regular intervals of 12 parts each: day time would be measured by sundials, while night time would be measured by observing the relative position of 24 bright stars. Measuring this way, their ‘hours’ would have a different duration depending on the season, but still averaging 60 minutes each.

It was only with Greek astronomy, though, that a stress was posed on developing theories that would explain the birth of the Universe and its mechanics: Anaximander thought the planet Earth was a cylinder at the center of the Universe, while the stars would rotate around it in all directions; Plato had at a first time a theory, extremely advanced for its time, that put the Sun at the center of the Universe, but he later withdrew it to favor an Earth-centric theory similar to that of Anaximander; Eudoxus of Cnidus, finally, advanced a theory that was later approved by Aristotle, according to which the Universe was made of concentric spheres, rotating one inside another, where the Earth would be in the center.

The Aristotle conception of the structure of the universe was meant to last, with minimal variations, until the year 1500 AD, when Nicolaus Copernicus — which many consider the father of modern astronomy — advanced a theory that put the Earth orbiting in perfect circles around the Sun, together with all the other planets: this approach could in fact solve many of the contradictions that those who supported Aristotle had to face. A few decades later, John Kepler refused yet another innovative model of the universe from his mentor Tycho Brahe, and later became famous for formulating the three laws of star mechanics that were named after him, which are considered valid still nowadays.

In 1610, Galileo Galilei invented the telescope, after a long period of research and experimentation. As soon as he pointed it at the stars, a never seen before universe appeared in front of his eyes: the Moon had a surface full of craters, Jupiter was surrounded by four satellites, while the Milky Way suddenly appeared as nothing but a huge mass of countless stars. In 1632, after publishing his book ‘Dialogo sopra i due massimi sistemi del mondo’ [On the main two models of the Universe] in which he was openly exposing the results of his observations, he was forced by the Catholic Church to abjure not having made those discoveries.

A few decades later, while researching innovative techniques to build more and more powerful telescopes, an important debate took place between the scientist Huygens and Newton over the nature of light: the first said it was a wave, while the second thought it was made of physical ‘atoms’ (photons). The debate that was destined to be solved once and for all just a few decades ago (light is, indeed, both a wave and a physical object). Huygens studied advanced optics as well, and managed to build a telescope that could noticeably minimize the chromatic aberration in observations, which led him to discover Saturn’s rings and its moon, Titanus.

Just a few years later, Cassini and Romer found out that phenomena such as solar eclipses would happen just several minutes after they were expected: this led them to think that light could actually travel at a finite although extremely high speed, rather than to an infinite speed: their estimation put the speed of light at 230,000 km per hour (the actual speed of light is 300,000 km/h).

Starting from the 19th century, following the Industrial Revolution, the continuous development of innovative techniques and instruments for the observation of the sky led to a series of discoveries that quickly contributed to our knowledge. Nowadays, the main purpose of astronomy is to study the life cycle of stars and galaxies, the origin and future of the Universe, obscure objects like pulsars and black holes, and methods to measure interstellar distances with increased precision.