THE YEAR 2005 will be remembered in the history of space exploration for the first landing of a probe on a surface in the outer solar system - on 14 January, the Huygens probe landed on the surface of the mysterious moon Titan. The landing of Huygens on Titan was a remarkable achievement. For comparison, let's consider landing on Mars, whose distance from Earth is only 55m to 401m km, a planet explored by numerous spacecraft, starting with Mariner 4 in 1965. We successfully landed two Viking probes on Mars in the 1970s, followed by Pathfinder, Spirit, and Opportunity. We have studied Mars with numerous orbiters and fly-bys since Mariner 4, yet we still consider a successful landing on the Red Planet to be no small feat. Now consider Titan, about 1,300m km away from the Earth. The Huygens probe was built before Titan's surface had ever been mapped, before any spacecraft had ever orbited this planet-sized moon to make measurements considered necessary for safe descent and landing. Mars has a very tenuous atmosphere, in contrast, Titan's atmosphere extends 10 times further into space than the Earth's and the pressure at the surface is 50% higher. A body's atmosphere has a significant effect on probe entry and descent, yet Huygens had to make its way using scarce knowledge of atmospheric conditions. Surface conditions were also virtually unknown. Given the possibility of pools of liquids, Huygens could have landed (or crashed) into either liquid or solid. Great engineering and science and undoubtedly some luck, ensured that the probe reached its destination, landed softly and obtained remarkable measurements both during descent and while on the ground.

What did we know about Titan before Cassini and Huygens? Saturn's largest moon was discovered in 1655 by Dutch astronomer Christiaan Huygens. At 5,150km in diameter, Titan is larger than the planets Mercury and Pluto, and is the second largest known moon (only Ganymede is larger). Its atmosphere is the second densest of the solid bodies in the solar system (Venus has the densest), and its surface, shrouded by a thick atmosphere, is extremely cold, about 95° K (-178°C). Titan's thick atmosphere is about 95% nitrogen, with a few percent methane. Titan is of special interest to planetary scientists for several reasons, one of which is the many interesting organic compounds created by ultraviolet light from the Sun and energetic electrons from Saturn's radiation belts causing dissociation of nitrogen and methane in the upper atmosphere. The resulting radicals combine to form complex hydrocarbons such as ethane and benzene, ni-triles, and Titan's orange haze, which obscures the surface. When the Voyager 1 spacecraft flew by Titan in 1981, its cameras showed an orange ball, much to the disappointment of planetary geologists (1,2). The Cassini orbiter instruments were designed to penetrate the haze and image the surface (3). Specifically, the Titan Radar Mapper (4) is part of the payload, it is able to reveal the surface in unprecedented detail. Two other instruments have contributed to our current knowledge of Titan's surface: the Imaging Science Subsystem (ISS, 5) can use filters that cut through some of the haze, and the Visible and Infrared Mapping Spectrometer (VIMS, 6) can make use of methane spectral "windows" to see down to the surface. Other orbiter instruments contribute to our knowledge of Titan's atmosphere and interaction with the solar wind and with Saturn's magnetosphere. The Cassini mission and its payload of instruments are reviewed in Russell et al. (3) and Harland (7). The knowledge of Titan before Cassini and Huygens is well summarized in the books by Coustenis and Taylor (1) and Lorenz and Mitton (2).

The Cassini-Huygens mission

Cassini-Huygens is a collaborative mission between NASA, the European Space Agency and the Italian Space Agency. It has the participation of 17 countries, making it the most international planetary mission ever flown. The orbiter carries 12 instruments and makes repeated fly-bys of Titan, as well as of other moons. Cassini was launched on 15 October 1997, and inserted into orbit around Saturn on 1 July 2004. The Huygens probe, carried by the orbiter, was released on 24 December 2004, and landed on Titan's surface on 14 January 2005 (8). The orbiter's first fly-by of Titan was on 2 July, 2004, at a distance of over 300,000km. Closer fly-bys followed, some as low as 1,150km, using different instruments, and Titan begun to yield its secrets. No doubt many more will be revealed

I Above: Fig I: Cassini has found Titan's upper atmosphere to consist ofa surprising number oflayers ofhaze, as shown in this ultraviolet image ofTitan's night side limb, colourized to look like true colour. The many fine haze layers extend several hundred kilometres above the surface. Although this is a night side view, w/th only a thin crescent receiving direct sunlight, the haze layers are bright from light scattered through the atmosphere. Image courtesy NASA/JPL/Space Science Institute.

the Cassini-Huygens mission is to understand Titan's atmospheric dynamics and chemistry.

The organic compounds that are photochem-ically produced in the atmosphere eventually condense and rain down to the surface. Titan has methane rain or possibly, as Cassini scientist Ralph Lorenz suggested, methane monsoons (9), a term first coined by Arthur C. Clarke in his book "Imperial Earth". One of the mysteries of Titan is the amount of methane in the atmosphere. Because methane forms the basis of many photochemical reactions on Titan, it should have been depleted over time, but this has not happened. What is the mechanism re-supplying the methane? Planetary scientist Jonathan Lunine proposed an ocean of liquid hydrocarbons (10), but this has not been observed. Current results suggest that surface liquids are not plentiful enough, so there must be some other process going on. Yolcanism is a possibility, as several possible volcanic features have been seen on the surface (4, 10, 11), though there is not evidence that they are still active.

Cassini carries a mass spectrometer that can sample Titan's surface during close fly-bys and determine molecular masses of the atmospheric constituents. During the first close fly-by in October 2004, a complex array of short and long-chain hydrocarbons and nitriles was detected (12). One of the most significant findings is that Titan's atmosphere is enriched in the heavy isotope of nitrogen (15N) relative to the more abundant 14N (12). The heavier isotope is much more abundant on Titan, relative to the lighter one, than it is on Earth. What is causing this enrichment? Another of Cassini's instruments, the Magnetospheric Imaging Instrument (MIMI) detected a gas torus around Saturn during orbit insertion (13). The torus circles Saturn along with Titan, implying that Titan is the source. Titan's atmosphere is continuously bombarded with energetic electrons from Saturn's radiation belts. The collisions cause gas from Titan's atmosphere to be ejected; the lighter isotopes can move faster and escape more easily from Titan's gravitational pull. It is thought that vast

in the next few years. The Cassini mission is scheduled to last until at least mid-2008, making a total of 44 fly-bys of Titan.

Titan's atmosphere: initial results from the Cassini orbiter

The photochemical processes in Titan's atmosphere are thought to be similar to those in the early Earth and it is likely that understanding the atmospheric processes on Titan will contribute to knowledge of the Earth prior to the evolution of life. One of the key objectives of

amounts of Titan's atmosphere must have been lost to result in the 15N enrichment we see today. According to Hunter Waite and colleagues (12), the atmosphere was, in the past, between 1.6 and 100 times more massive than today.

Titan's atmosphere (Fig. 1) is dynamic, perhaps even stormy. ISS and VIMS images showed methane clouds near the south pole (5, Fig. 2). Cassini observed Titan while it was summer in the southern hemisphere, so the relative abundance of clouds in the south polar region may be a result of evaporation of surface methane. The clouds have been seen to move between different fly-bys, or even different images in the same fly-by. On some occasions they are not present at all. Other clouds have been detected, not only from Cassini but also from ground-based observations. Mid-latitude clouds seem to last only a few hours and are fainter than those at the south pole. There are some mysterious east-west clouds that are hundreds of kilometres long, streaky in appearance, and seem to originate from fixed positions. These have been speculated to be a result of venting of gases from the interior, which are then carried along by winds (14).

Titan's surface:

the view from the orbiter

Cassini's first views of Titan's surface came on 2 July 2004. Images from ISS and VIMS showed distinct light and dark regions (5, 9), which are suspected of having different compositions. One possibility is that darker areas at visible wavelengths have more organic material, while brighter areas may have more water ice. The largest bright area, Xanadu, had been known from Earth-based infrared observations prior to Cassini. This area is thousands of kilometres wide and has often been thought of as being of higher elevation, but observations have not yet confirmed this. Radar observations obtained in four fly-bys to date (October 2004 and February, September, and October 2005) have shown the surface geology in detail over about 4% of Titan's

I Above left: Fig. 2: Moso/c ofimages ofTitan's south polar region acquired as Cassini passed by at a range of339,000km on 2_/u/y 2004. These images were acquired through special filters designed to see through the thick haze and atmosphere. The bright spots near the bottom represent a ft eld ofclouds near the south pole. Image courtesy NASA/JPL/Space Science Institute. I Opposite: Fig. 3:This image taken by Cassini's visual and infrared mapping spectrometer (VIMS) is a composite offalse-colour images taken at three infrared wavelengths: 2 microns (blue); 21 microns (red); and 5 microns (green). Surface features can be seen, as well as a methane cloud at the south pole (bottom ofimage). This picture wos obtained as Cassini flew by Titan at altitudes ranging from 100,000km to 140,000km. The inset picture shows the area ofthe landing site ofCassini's Huygens probe. Image courtesy NASA/JPL/University ofArizona. I Above right: Fig. 4: This radar image shows one ofthe two impact craters detected on Titan to date. This crater, approximately 80km in diameter, on the very eastern end ofthe radar image strip taken by the Cassini orbiter on its third close flyby ofTitan on 15February2005. The appearance ofthe craterand the extremely bright (hence rough) blanket ofmaterial surrounding it is indicative ofan origin by impact, in which a hypervelocity comet or asteroid, in this case, roughly 5-10km in size, slammed into the surface of Titan. The bright surrounding blanket is debris, or ejecta, thrown out ofthe crater. Image courtesy NASA/JPL.

surface (4, 15). Because the Synthetic Aperture Radar mode can only be used when the spacecraft is relatively close to the surface, the area imaged in each fly-by is small, around 1%.

Titan's surface has shown itself to be remarkably varied and, apparently, quite young in planetary terms. Only two impact craters have been found, far less than would be expected in comparison with other Saturn satellites. This indicates that craters must be destroyed by recently or currently active geologic processes. The major geologic processes shaping all planetary surfaces are volcanism, tectonism, impact cra-tering and erosion. Since the scars of impacts are being erased on a large scale, one or more of the other processes must be responsible. The first radar images of Titan (October 2004) showed

I Above: Fig. 5: Synthetic aperture radar image ofa possible cryovolcanic flow on the surface of Titan, where water-rich liquid likely welled up from Titan's interior. The image was acquired on 26 October 2004, when the Cassini spacecraft f/ew approximately 2,500km above the surface and acquired radar data for the first time. The radar illumination wos from the south: dark regions may represent areas that are smooth, made ofradar-absorbing materials, or are sloped away from the direction ofillumination. The bright flow-like feature stretches from upper left to lower right across this image, w/th connected 'arms' to the east. The fact that the lower (southern) edges ofthe features are brighter is consistent with the structure being raised above the relatively featureless darker background. The image covers an area about 150km2. Image courtesy NASA/JPL.

M Above right: Fig. 6: The Cassini radar system imaged this area during the spacecraft's third close flyby ofTitan on 15 February 2005. The bright //nes are interpreted as channels in which fluid flowed toward the bright area in the upper right. Areas that appear bright at radar wavelengths may be rough or inclined toward the direction ofillumination. The bright area in this image could have received outflows ofdebris from the channels, making the surface appear radar bright. In this sense, the area may resemble the rubble strewn plains in the region where the Huygens probe landed (see Fig. 9). The fluid carrying the debris was most likely liquid methane. The longest channel in the feature is approximately 200km long. The seams running across the image are an effect ofthe matching ofthe different radar beams to assemble the full image. Image courtesy NASA/JPL.

evidence of volcanism (Fig. 5) in the form of a dome and extensive flows. Ridges that may be tectonic in nature were shown in radar images obtained a year later. In the meantime, other radar images showed alluvial deposits (Fig. 6) and fields of dunes (Fig. 7), indicating erosion by liquids and wind on a large scale. Titan appears to be a dynamic world, in some ways remarkably Earth-like in its geology. Few well-preserved impact craters are seen on Earth, because our planet's active geology has erased these features formed relatively early in the history of the Solar System. In general, the older a surface has remained unmodified, the more craters it will display. Therefore, our own Moon, a largely dead world, shows a plethora of impact scars. In comparison, Titan looks far from dead.

However, when looked at in detail, Titan and Earth are not at all similar. While water carves river valleys on Earth, the liquid on Titan is likely to be methane. Liquid methane may exist in lakes or pools, such as in some dark areas imaged by Cassini. Methane may also come down in the form of rain and we know from Cassini orbiter images that methane is present in clouds near the south pole. Rivers of methane may carve dendritic channels such as those seen in radar images. Volcanism on Titan is also nothing like Earth's. The flows and dome seen in radar images were formed by cryovolcanism (4, 11), where the "magma" is not molten rock but liquid water coming from below the frozen surface, most likely mixed with other components such as ammonia.

One of the most remarkable surprises from the orbiter's radar images was the discovery of "sand seas" (16), that is, large areas of the surface covered by dunes that appear similar to longitudinal dunes on Earth, such as those in Namibia. This is further proof that Titan's surface and atmosphere are indeed dynamic and that weather on Titan has a great influence on the appearance of the surface.

Huygens landing

The wolc-shaped Huygens probe carried six instruments to study Titan's surface and atmosphere in-situ (8, 17). During the 2.5hr descent, instruments made measurements of atmospheric conditions, including the direction and speed of winds. At 120km altitude, Titan's winds, blowing mostly in the direction Titan is rotating, reached 120m/s, which is faster than Titan rotates. Titan's atmosphere is therefore referred to as "super-rotating". Although this had been predicted (18), Cassini observations confirmed the prediction. In contrast, winds at the surface were very weak, about lm/s. Can these light winds account for the formation of dunes on the surface, or are there stronger winds at times? The question remains open.

The composition of the atmosphere was also analyzed during descent and preliminary results indicate the presence of organic compounds containing nitrogen, which may include amino, imino and nitrale compounds. These aerosols are thought to fall steadily on the surface as "organic rain", depositing a global layer that may be as thick as 1km. This

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