Cassini Sees Shepherding Moons

Image credit: NASA/JPL/Space Science Institute
Cassini has sighted Prometheus and Pandora, the two F-ring-shepherding moons whose unpredictable orbits both fascinate scientists and wreak havoc on the F ring.

Prometheus (102 kilometers, or 63 miles across) is visible left of center in the image, inside the F ring. Pandora (84 kilometers, or 52 miles across) appears above center, outside the ring. The dark shadow cast by the planet stretches more than halfway across the A ring, the outermost main ring. The mottled pattern appearing in the dark regions of the image is ‘noise’ in the signal recorded by the camera system, which has subsequently been magnified by the image processing.

The F ring is a narrow, ribbon-like structure, with a width seen in this geometry equivalent to a few kilometers. The two small, irregularly shaped moons exert a gravitational influence on particles that make up the F ring, confining it and possibly leading to the formation of clumps, strands and other structures observed there. Pandora prevents the F ring from spreading outward and Prometheus prevents it from spreading inward. However, their interaction with the ring is complex and not fully understood. The shepherds are also known to be responsible for many of the observed structures in Saturn’s A ring.

The moons, which were discovered in images returned by the Voyager 1 spacecraft in 1980, are in chaotic orbits–their orbits can change unpredictably when the moons get very close to each other. This strange behavior was first noticed in ground-based and Hubble Space Telescope observations in 1995, when the rings were seen nearly edge-on from Earth and the usual glare of the rings was reduced, making the satellites more readily visible than usual. The positions of both satellites at that time were different than expected based on Voyager data.

One of the goals for the Cassini-Huygens mission is to derive more precise orbits for Prometheus and Pandora. Seeing how their orbits change over the duration of the mission will help to determine their masses, which in turn will help constrain models of their interiors and provide a more complete understanding of their effect on the rings.

This narrow angle camera image was snapped through the broadband green spectral filter, centered at 568 nanometers, on March 10, 2004, when the spacecraft was 55.5 million kilometers (34.5 million miles) from the planet. Image scale is approximately 333 kilometers (207 miles) per pixel. Contrast has been greatly enhanced, and the image has been magnified to aid visibility of the moons as well as structure in the rings.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Office of Space Science, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colorado.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org.

Original Source: CICLOPS News Release

Best Image Ever Taken of Titan’s Surface

Image credit: ESO
Titan, the largest Saturnian moon and the second largest moon of the solar system (only Jupiter’s Ganymede is slightly larger), is the only satellite known with a substantial atmosphere. It is composed mainly of nitrogen (like that of the Earth) and also contains significant amounts of methane. Opaque orange hazes and clouds of complex organic molecules effectively shield the solid surface from view, cf. e.g. the Voyager images.

Recent spectroscopic and radar observations suggest that there are huge surface reservoirs of liquid hydrocarbonates and a methane-based meteorological cycle similar to Earth’s hydrological cycle. This makes Titan the only known object with rainfall and potential surface oceans other than the Earth and thus a tantalizing research object for the study of pre-biotic chemistry and the origin of life on Earth.

The Huygens probe launched from the NASA/ESA Cassini-Huygens mission will enter Titan’s atmosphere in early 2005 to make measurements of the physical and chemical conditions, hopefully surviving the descent to document the surface as well.

Coordinated ground-based observations will provide essential support for the scientific return of the Cassini-Huygens encounter. However, only 8-10 m class telescopes with adaptive optics imaging systems or space-borne instruments can achieve sufficient image sharpness to attain a useful level of detail.

The new map of a large part of Titan’s surface, shown in PR Photo 11a/04, represents an important contribution in this direction.

A question of atmospheric windows
The first intriguing views of Titan’s surface were obtained by the Hubble Space Telescope (HST) in the 1990’s. From the ground, images were obtained in 2001-2 with the Keck II and Gemini North telescopes and more recently with the ESO Very Large Telescope (VLT), cf. ESO PR Photos 08a-c/04. All of these observations were made through a single narrow-band filter at a time.

The wavelengths used for such observations are critical for the amount of surface detail captured on the images. Optimally, one would look for a spectral band in which the atmosphere is completely transparent; a number of such “windows” are known to exist. But although the above observations were made in wavebands roughly matching atmospheric windows and do show surface features, they also include the light from different atmospheric layers. In a sense, they therefore correspond to viewing Titan’s surface through a somewhat opaque screen or, more poetically, the sight by an ancient sailor, catching for the first time a glimpse of an unknown continent through the coastal haze.

One narrow “window” is available in the near-infrared spectral region near wavelength 1.575 ?m. In February 2004, an international research team [1] working at the ESO VLT at the Paranal Observatory (Chile) obtained images of Titan’s surface through this spectral window with unprecedented spatial resolution and with the lowest contamination of atmospheric condensates to date.

They accomplished this during six nights (February 2, 3, 5, 6, 7 and 8, 2004) at the time of the commissioning phase of a novel high-contrast imaging mode for the NACO adaptive optics instrument on the 8.2-m VLT YEPUN telescope, using the Simultaneous Differential Imager (SDI) [2]. This novel optical device provides four simultaneous high-resolution images (PR Photo 11b/04) at three wavelengths around a near-infrared atmospheric methane absorption feature.

The main application of the SDI is high-contrast imaging for the search for substellar companions with methane in their atmosphere, e.g. brown dwarfs and giant exoplanets, near other stars. However, as the present photos demonstrate, it is also superbly suited for Titan imaging.

Simultaneous Views of Titan’s Surface and Atmosphere
Titan is tidally-locked to Saturn, and hence always presents the same face towards the planet. To image all sides of Titan (from the Earth) therefore requires observations during almost one entire orbital period, 16 days. Still, the present week-long observing campaign enabled the team to map approximately three-quarters of the surface of Titan.

A new map of the surface of Titan (in cylindrical projection and covering most, but not all of the area imaged during these observations) was created. For this, the simultaneous “atmospheric” images (at waveband 1.625 ?m) were “subtracted” from the “surface” images (1.575 and 1.600 ?m) in order to remove any residual atmospheric features present in the latter. The ability to subtract simultaneous images is unique to the SDI camera [2].

This truly unique map shows the fraction of sunlight reflected from the surface – bright areas reflect more light than the darker ones. The amount of reflection (in astronomical terms: the “albedo”) depends on the composition and structure of the surface layer and it is not possible with this single-wavelength (“monochromatic”) map alone to elucidate the true nature of those features.

Nevertheless, recent radar observations with the Arecibo antenna have provided evidence for liquid surfaces on Titan, and the low-reflection areas could indicate the locations of those suspected reservoirs of liquid hydrocarbonates. They also provide a possible source for the replenishment of methane that is continuously lost in the atmosphere because of decomposition by the sunlight.

Presumably, the bright, highly reflective regions are ice-covered highlands.

Provisional names of the new features
A comparison with an earlier NACO image obtained through another filter is useful. It demonstrates the importance of employing a filter that precisely fits the atmospheric window and hence the gain of clarity with the present observations. It also provides independent confirmation of the reality of the gross features, since the observations are separated by 15 months in time.

Over the range of longitudes which have been mapped during the present observations (PR Photo 11a/04), it is obvious that the southern hemisphere of Titan is dominated by a single bright region centered at approximately 15? longitude. (Note that this is not the so-called “bright feature” seen in the HST images at longitude 80? – 130?, an area that was not covered during the present observations).

The equatorial area displays the above mentioned, well-defined dark (low-reflection) structures. In order to facilitate their identification, the team decided to give these dark features provisional names – official names will be assigned at a later moment by the Working Group on Planetary System Nomenclature of the International Astronomical Union (IAU WGPSN). From left to right, the SDI team [1] has referred to these features informally as: the “lying H”, the “dog” chasing a “ball”, and the “dragon’s head”.

Original Source: ESO News Release

Cassini Sees Merging Storms on Saturn

Image credit: NASA/JPL/Space Science Institute
Only a month and a half into its long approach to Saturn, the Cassini spacecraft captured two storms, each a swirling mass of clouds and gas, in the act of merging. With diameters close to 1000 kilometers (621 miles), both storms, which appear as spots in the southern hemisphere, were seen moving westward, relative to the rotation of Saturn’s interior, for about a month before they merged on Mar. 19-20, 2004.

Merging is one of the distinct features of storms in the giant planet atmospheres. On Earth, storms last for a week or so and usually fade away when they enter the mature phase and can no longer extract energy from their surroundings. On Saturn and the other giant planets, storms last for months, years, or even centuries, and instead of simply fading away, many storms on the giant planets end their lives by merging. How they form is still uncertain.

The series of eight images shown here was taken between Feb. 22 and Mar. 22, 2004; the image scale ranges from 381 kilometers (237 miles) to 300 kilometers (186 miles) per pixel. All images have been processed to enhance visibility. The top four frames, spanning 26 days, are portions of narrow angle camera images that were taken through a filter accepting light in the near-IR region of the spectrum centered at 619 nanometers, and show two spots approaching each other. Both storms are within half a degree of 36 degrees south latitude and sit in an anti-cyclonic shear zone, which means that the flow to the north is westward relative to the flow to the south. Consequently, the northern storm moves westward at a slightly greater rate than the southern one: 11 vs. 6 meters per second (25 and 13 miles per hour), respectively. The storms drift with these currents and engage in a counterclockwise dance before merging with each other.

The bottom four frames are from images taken on Mar. 19, 20, 21, and 22, respectively, in a region of the spectrum visible to the human eye and illustrate the storms’ evolution. Just after the merger, on Mar. 20, the new feature is elongated in the north-south direction, with bright clouds on either end. Two days later on Mar. 22, it has settled into a more circular shape and the bright clouds have spread around the circumference to form a halo. Whether the bright clouds are particles of a different composition or particles at a different altitude is uncertain.

The new storm is a few tenths of a degree farther south than either of its progenitors. There, its westward velocity is weaker and it is almost stationary relative to the planet’s rotation. Although these particular storms move slowly westward, storms at Saturn’s equator move eastward at speeds up to 450 meters per second (1000 mph), which is ~10 times the speed of the Earth’s jet streams and ~ three times greater than the equatorial winds on Jupiter. Saturn is the windiest planet in the solar system, which is another mystery of the ringed giant.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Office of Space Science, Washington, D.C. The imaging team is based at the Space Science Institute, Boulder, Colorado.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org.

Original Source: NASA/JPL News Release

New Images of Titan

Image credit: ESO
Titan, the largest moon of Saturn was discovered by Dutch astronomer Christian Huygens in 1655 and certainly deserves its name. With a diameter of no less than 5,150 km, it is larger than Mercury and twice as large as Pluto. It is unique in having a hazy atmosphere of nitrogen, methane and oily hydrocarbons. Although it was explored in some detail by the NASA Voyager missions, many aspects of the atmosphere and surface still remain unknown. Thus, the existence of seasonal or diurnal phenomena, the presence of clouds, the surface composition and topography are still under debate. There have even been speculations that some kind of primitive life (now possibly extinct) may be found on Titan.

Titan is the main target of the NASA/ESA Cassini/Huygens mission, launched in 1997 and scheduled to arrive at Saturn on July 1, 2004. The ESA Huygens probe is designed to enter the atmosphere of Titan, and to descend by parachute to the surface.

Ground-based observations are essential to optimize the return of this space mission, because they will complement the information gained from space and add confidence to the interpretation of the data. Hence, the advent of the adaptive optics system NAOS-CONICA (NACO) [1] in combination with ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile now offers a unique opportunity to study the resolved disc of Titan with high sensitivity and increased spatial resolution.

Adaptive Optics (AO) systems work by means of a computer-controlled deformable mirror that counteracts the image distortion induced by atmospheric turbulence. It is based on real-time optical corrections computed from image data obtained by a special camera at very high speed, many hundreds of times each second.

A team of French astronomers [2] have recently used the NACO state-of-the-art adaptive optics system on the fourth 8.2-m VLT unit telescope, Yepun, to map the surface of Titan by means of near-infrared images and to search for changes in the dense atmosphere.

These extraordinary images have a nominal resolution of 1/30th arcsec and show details of the order of 200 km on the surface of Titan. To provide the best possible views, the raw data from the instrument were subjected to deconvolution (image sharpening).

Images of Titan were obtained through 9 narrow-band filters, sampling near-infrared wavelengths with large variations in methane opacity. This permits sounding of different altitudes ranging from the stratosphere to the surface.

Titan harbours at 1.24 and 2.12 ?m a “southern smile”, that is a north-south asymmetry, while the opposite situation is observed with filters probing higher altitudes, such as 1.64, 1.75 and 2.17 ?m.

A high-contrast bright feature is observed at the South Pole and is apparently caused by a phenomenon in the atmosphere, at an altitude below 140 km or so. This feature was found to change its location on the images from one side of the south polar axis to the other during the week of observations.

Original Source: ESO News Release

What Would Titan’s Oceans Look Like?

Image credit: ESA
When the European Huygens probe on the Cassini space mission parachutes down through the opaque smoggy atmosphere of Saturn’s moon Titan early next year, it may find itself splashing into a sea of liquid hydrocarbons. In what is probably the first piece of “extraterrestrial oceanography” ever carried out, Dr Nadeem Ghafoor of Surrey Satellite Technology and Professor John Zarnecki of the Open University, with Drs Meric Srokecz and Peter Challenor of the Southampton Oceanography Centre, calculated how any seas on Titan would compare with Earth’s oceans. Their results predict that waves driven by the wind would be up to 7 times higher but would move more slowly and be much farther apart. Dr Ghafoor will present their findings at the RAS National Astronomy Meeting at the Open University on Wednesday 31 March.

The team worked with a computer simulation, or ‘model’, that predicts how wind-driven waves on the surface of the sea are generated on Earth, but they changed all the basic inputs, such as the local gravity, and the properties of the liquid, to values they might expect on Titan.

Arguments about the nature of Titan’s surface have raged for a number of years. Following the flyby of the Voyager 1 spacecraft in 1980, some researchers suggested that Titan’s concealed surface might be at least partly covered by a sea of liquid methane and ethane. But there are several other theories, ranging from a hard icy surface at one extreme to a near-global hydrocarbon ocean at the other. Other variants include the notion of hydrocarbon ‘sludge’ overlying an icy surface. Planetary scientists hope that the Cassini/Huygens mission will provide an answer to this question, with observations from Cassini during several flybys of Titan and from Huygens, which will land (or ‘splash’) on 14 January 2005.

The idea that Titan has significant bodies of surface liquid has recently been reinforced by the announcement that radar reflections from Titan have been detected using the giant Arecibo radio dish in Puerto Rico. Importantly, the returned signals in 12 out the 16 attempts made contained reflections of the kind expected from a polished surface, like a mirror. (This is similar to seeing a blinding patch of light on the surface of the sea where the Sun is being reflected.) The radar researchers concluded that 75% of Titan’s surface may be covered by ‘open bodies of liquid hydrocarbons’ – in other words, seas.

The exact nature of the reflected radar signal can be used to determine how smooth or choppy the liquid surface is. This interpretation says that the slope of the waves is typically less than 4 degrees, which is consistent with the predictions of the British scientists, who showed that the maximum possible slope of waves generated by wind speeds up to 7 mph would be 11 degrees.

“Hopefully ESA’s Huygens probe will end the speculation” says Dr Ghafoor. “Not only will this be by far the most remote soft landing of a spacecraft ever attempted but Huygens might become the first extraterrestrial boat if it does indeed land on a hydrocarbon lake or sea.” Although not designed specifically to survive landing or to float, the chances it will do so are reasonable. However, the link back to Earth from Huygens via Cassini, which will be flying past Titan and acting as a relay, will only last for a maximum of 2 hours. During this time, if the probe is floating on a sea, one of the 6 instruments Huygens is carrying, the Surface Science Package experiment, which is led by John Zarnecki, will be making oceanography measurements. Among the 9 sensors that it carries are ones that will measure the height and frequency of the waves and also the depth of the sea using sonar. It will also attempt to determine the composition of the sea.

What would the sea look like? “Huygens does carry a camera so it is possible we shall have some direct images,” says Professor Zarnecki, “but let’s try to imagine that we are sitting onboard the probe after it has landed in a Titan ocean. What would we see? Well, the waves would be more widely dispersed than on Earth but they will be very much higher – mostly as a result of the fact that Titan gravity is only about 15% of that on Earth. So the surface around us would probably appear flat and deceptively calm, but in the distance we might see a rather tall, slow-moving wave advancing towards us – a wave that could overwhelm or sink us.”

Original Source: RAS News Release

Cassini’s New Saturn Movie

Image credit: NASA/JPL
Wind-blown clouds and hazes high in Saturn’s atmosphere are captured in a movie made from images taken by the Cassini narrow angle camera between Feb. 15 and Feb. 19, 2004. The images were made using a filter sensitive to a narrow range of wavelengths centered at 889 nanometers where methane in Saturn’s atmosphere absorbs sunlight. Cassini was 65.6 million kilometers (40.7 million miles) from Saturn when the images, reduced in size by a factor of two onboard the spacecraft, were taken. The resulting image scale is approximately 786 kilometers (420 miles) per pixel.

This is the first movie ever made showing Saturn in these near-infrared wavelengths. The movie, consisting of 30 stacked images, spans five days and captures five complete but non-consecutive Saturn rotations. The direction of motion is prograde, or left to right. Each 10.6 hour Saturn rotation is evenly sampled by six images. In `movie time’, there is 0.25 second between each of the six images in an individual rotation, and one second between rotations. After each rotation sequence, the planet can be seen to grow slightly in the field of view.

Cassini has three filters designed to sense different heights of clouds and hazes in Saturn’s atmosphere. Any light detected by cameras using the 889 nanometer filter is reflected very high in the atmosphere, before the light is absorbed. Thus, the bright areas in these images represent high hazes and clouds near the top of Saturn’s troposphere.

In the movie, atmospheric motions can be seen most clearly in the equatorial region and at other southern latitudes as well. Saturn’s equatorial region seems disturbed in the same way that it has been for the past decade, as revealed by observations from the Hubble Space Telescope. Researchers have speculated that the bright cloud patterns there are associated with water-moist convection arising from a deeper atmospheric level where water condenses on Saturn, and rising to levels at or above the visible cloud tops. Close analysis of future images by scientists on the Cassini-Huygens mission should help determine if this is the case.

Saturn’s rings are extremely overexposed in these images. Because the range of wavelengths for this spectral filter is narrow, and because most of this light is absorbed by Saturn, the disk of Saturn is inherently faint and the exposures required are quite long (22 seconds). The rings do not strongly absorb at these wavelengths, and so reflect more light and are overexposed compared to the atmosphere. Orbiting moons in the images were manually removed during processing.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Office of Space Science, Washington, D.C. The imaging team is based at the Space Science Institute, Boulder, Colorado.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org.

Original Source: CICLOPS News Release

Saturn With Cassini’s Blue Filter

Image credit: NASA/JPL
Bands and spots in Saturn’s atmosphere, including a dark band south of the equator with a scalloped border, are visible in this image from the Cassini-Huygens spacecraft.

The narrow-angle camera took the image in blue light on Feb. 29, 2004. The distance to Saturn was 59.9 million kilometers (37.2 million miles). The image scale is 359 kilometers (223 miles) per pixel.

Three of Saturn’s moons are seen in the image: Enceladus (499 kilometers, or 310 miles across) at left; Mimas (398 kilometers, or 247 miles across) left of Saturn’s south pole; and Rhea (1,528 kilometers, or 949 miles across) at lower right. The imaging team enhanced the brightness of the moons to aid visibility.

The BL1 broadband spectral filter (centered at 451 nanometers) allows Cassini to “see” light in a part of the spectrum visible as the color blue to human eyes. Scientist can combine images made with this filter with those taken with red and green filters to create full-color composites.

In this image, everything on the planet is a cloud, and the contrast between bright and dark features is determined by the different blue-light absorbing properties of the particles that comprise the clouds. White regions contain material reflecting in the blue; dark regions contain material absorbing in the blue. This reflecting/absorbing behavior is controlled by the composition of the cloud’s colored material, which is still a mystery — one which may be answered by Cassini. The differing concentrations of this material across the planet are responsible for its banded appearance in the visible region of the electromagnetic spectrum.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Office of Space Science, Washington, D.C. The imaging team is based at the Space Science Institute, Boulder, Colo.

For more information about the Cassini-Huygens mission visit, http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org .

Original Source: CICLOPS News Release

Cassini Sees Clumps in Saturn’s Rings

Image credit: NASA/JPL
Clumps seemingly embedded within Saturn’s narrow, outermost F ring can be seen in these two Cassini narrow angle camera images taken on Feb. 23, 2004 from a distance of 62.9 million kilometers (39 million miles). The images were taken nearly two hours apart using the camera’s broadband green filter, centered at 568 nanometers. Image scale is 377 kilometers (234 miles) per pixel.

The core of the F ring is about 50 kilometers (31 miles) wide, and from Cassini’s current distance, is not fully resolvable. Contrast has been greatly enhanced, and the images have been magnified, to aid visibility of the F Ring and the clump features.

The images show clumps as they revolve about the planet. Like all particles in Saturn’s ring system, these features orbit the planet in the same direction in which the planet rotates. This direction is clockwise as seen from Cassini’s southern vantage point below the ring plane. Two clumps in particular, one of them extended, can be seen in the upper part of the F ring in the image on the left, and in the lower part of the ring in the image on the right. Other knot-like irregularities in the ring’s brightness can also be seen in the right hand image.

Clumps such as these were first seen when the two Voyager spacecraft flew past Saturn in 1980 and 1981. It is not certain what causes these features, though several theories have been proposed, including meteoroid bombardment and inter-particle collisions in the F ring.

The Voyager data suggest that while the clumps change very little and can be tracked as they orbit for 30 days or more, no identified clump survived from the Voyager 1 flyby to the Voyager 2 flyby nine months later. Thus, scientists have only a rough idea of the lifetime of clumps in Saturn’s rings – a mystery that Cassini may help to answer.

The small dot at center right in the second image is one of Saturn’s small moons, Janus (181 kilometers, 112 miles across). Janus was discovered by ground-based astronomers in 1966, and was first resolved by the Voyager 1 spacecraft in 1980. The moon shares almost the same orbit with another small satellite, Epimetheus. Janus and Epimetheus, both thought to consist mostly of porous ices, play a role in maintaining the outer edge of Saturn’s A ring.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Office of Space Science, Washington, D.C. The imaging team is based at the Space Science Institute, Boulder, Colorado.

For information about the Cassini-Huygens mission, http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org.

Original Source: NASA/JPL News Release

Simulating Titan’s Atmosphere in the Lab

Image credit: ESA
It takes at least three elements to harbor life as we know it: water, energy and an atmosphere. Among Mars and the moons around both Jupiter and Saturn, there is evidence of one or two of these three elements, but less is known if a complete set is available. Only Saturn’s moon, Titan, has an atmosphere comparable to Earth’s in pressure, and is much thicker than the martian one (1% of Earth’s sea level pressure).

The most interesting point about simulations of Titan’s hydrocarbon haze is that this smoggy component contains molecules called tholins (from the Greek word, muddy) that can form the foundations of the building blocks of life. For example, amino acids, one of the building blocks of terrestrial life, form when these red-brown smog-like particles are placed in water. As Carl Sagan pointed out, Titan may be regarded as a broad parallel to the early terrestrial atmosphere with respect to its chemistry and in this way, it is certainly relevant to the origins of life.

This summer, NASA’s Cassini spacecraft, launched in 1997, is scheduled to go into orbit around Saturn and its moons for four years. In early 2005, the piggybacking Huygens probe is scheduled to plunge into the hazy Titan atmosphere and land on the moon’s surface. There are 12 instruments onboard the Cassini Spacecraft orbiter, and 6 instruments onboard the Huygens Probe. The Huygens probe is geared primarily towards sampling the atmosphere. The probe is equipped to take measurements and record images for up to a half an hour on the surface. But the probe has no legs, so when it sets down on Titan’s surface its orientation will be random. And its landing may not be by a site bearing organics. Images of where Cassini is in its current orbit are continuously updated and available for view as the mission progresses.

Astrobiology Magazine had an opportunity to talk with research scientist, Jean-Michel Bernard of the University of Paris, about how to simulate Titan’s complex chemistry in a terrestrial test tube. His simulations of Titan’s environment build on the classic prebiotic soup, first pioneered fifty years ago by University of Chicago researchers, Harold Urey and Stanley Miller.

Astrobiology Magazine (AM): What first stimulated your interest in the atmospheric chemistry of Titan?

Jean-Michel Bernard (JB): How do two simple molecules (nitrogen and methane) create a very complex chemistry? Does chemistry become biochemistry? The recent discoveries of life in extreme conditions on Earth (bacteria in the South Pole at -40?C and archaea at more than +110?C in the vicinity of hydrothermal sources) allow to suppose that life could be present on other worlds and other conditions.

Titan has astrobiological interest because it is the only satellite in the solar system with a dense atmosphere. Titan’s atmosphere is made of nitrogen and methane. The energetic particles coming from the Sun and Saturn’s environment allow complex chemistry, such as formation of hydrocarbons and nitriles. The particles also generate a permanent haze around the satellite, rains of methane, winds, seasons Recently, lakes of hydrocarbons seem to have been detected on Titan’s surface. I think that this discovery, if it is confirmed by the Cassini-Huygens mission, will be of great interest.

It would make Titan an analog to the Earth, since it would have an atmosphere (gas), lakes (liquid), haze and soil (solid), the three necessary environments for the appearance of life.

The composition of Titan’s haze is unknown. Only optical data are available and they are difficult to analyze due to the complexity of this carbonaceous material. Many experiments have been carried out in order to mimic the chemistry of Titan’s atmosphere, most notably the aerosols analogs named “tholins” by Carl Sagan’s group. It seems that tholins could be involved in the origin of life. Indeed, hydrolysis of these Titan aerosol analogs gives rise to the formation of amino acids, the precursors of life.

AM: Can you describe your experimental simulation for extending the Miller-Urey experiments in a way that is customized for Titan’s low temperatures and unique chemistry?

JB: Since the Miller-Urey experiments, many experimental simulations of supposed prebiotic system have been carried out. But after the retrieval of Voyager’s data, it appeared necessary to come back to this approach to simulate Titan’s atmosphere. Then several scientists carried out such simulation experiments by introducing a nitrogen-methane mixture in a system like Miller’s apparatus. But a problem became obvious due to the difference between the experimental conditions and Titan’s conditions. The pressure and temperature were not representative of Titan’s environment. Then we decided to carry out experiments which reproduce the pressure and the temperature of Titan’s stratosphere: a gas mixture of 2% of methane in nitrogen, a low pressure (about 1 mbar) and a cryogenic system in order to have a low temperature. Furthermore, our system is placed in a glove box containing pure nitrogen in order to avoid contamination by ambient air of the solid products.

AM: What do you consider the best energy source for triggering Titan’s synthetic chemistry: the magnetosphere of Saturnian particles, solar radiation, or something else?

JB: Scientists debate about what energy source would best simulate the energy sources in Titan’s atmosphere. Ultraviolet (UV) radiation? Cosmic rays? Electrons and other energetic particles coming from Saturn’s magnetosphere? All these sources are involved, but their occurence depends of the altitude: extreme ultraviolet radiation and electrons in the ionosphere, UV light in the stratosphere, while cosmic rays occur in the troposphere.

I think the appropriate question should be: What is the experimental goal? If it is to understand the hydrogen cyanide (HCN) chemistry in Titan’s stratosphere, a simulation with UV radiation of HCN is appropriate. If the goal is to determine the effects of electric fields generated by galactic cosmic rays in the troposphere, a corona discharge of a simulated Titan-atmosphere is preferable.

In studying Titan’s stratospheric conditions, we chose to use an electric discharge in our simulation. This choice is contested by a minority of scientists because the main energy source in Titan’s stratosphere is UV radiation. But our results validated our experiment. We detected all the organic species observed on Titan. We predicted the presence of CH3CN (acetonitrile) before its observation. We detected for the first time dicyanoacetylene, C4N2, an unstable molecule at room temperature that has also been detected in Titan’s atmosphere. The middle infrared signature of the solid products created in our experiment was in line with Titan observations.

AM: How are your results part of the planned atmospheric testing for the Cassini-Huygens probe?

JB: After collaborating with a team from the Observatoire Astronomique de Bordeaux in France, we determined the dielectric constants of aerosol analogs. This will allow us to estimate how Titan’s atmosphere and surface properties could affect the performance of the Cassini-Huygens radar experiments. The altimeter onboard the Huygens probe could be affected by the aerosol properties, but complementary experiments must be carried out to confirm this result.

Two years ago, we introduced a gas mixture, N2/CH4/CO (98/1.99/0.01). The goal was to determine the impact of carbon monoxide, the most abundant oxygenated compound on Titan. Surprisingly, we detected oxirane in the gaseous phase as the major oxygenated product. This unstable molecule was discovered in the interstellar medium but theoretical models do not predict it for Titan’s chemistry. Yet maybe this molecule is present on Titan.

Currently, we are analyzing the first molecules, radicals, atoms and ions (or ‘species’) created inside our experimental reactor. We are using infrared spectrometry and UV-visible emission to study excited species like CN, CH, NH, C2, HCN, C2H2. Next, we will observe the correlation between the abundance of these species and the structures of the solid products. . Coupling these experimental results with a theoretical model developed in collaboration with the University of Porto in Portugal, we will have a better understanding about the chemistry occurring into the experimental reactor. This will allow us to analyze the Cassini-Huygens data and Titan’s haze formation.

Our team is involved at the mission science level as well, as one of the scientists of the mission is also in our group at the Laboratoire Inter-Universitaire des Syst?mes Atmosph?riques, LISA). Our laboratory tholins will be used as guides to calibrate several of the instruments on the Huygens probe and the Cassini orbiter.

There are 18 instruments on board the probe and orbiter. Calibration tests are needed for gas chromatography and mass spectroscopy [GC-MS]. The GC-MS will identify and measure chemicals in Titan’s atmosphere.

Calibration tests are also needed for the Aerosol Collector and Pyrolyser (ACP). This experiment will draw in aerosol particles from the atmosphere through filters, then heat the trapped samples in ovens to vaporize volatiles and decompose the complex organic materials.

The Composite Infrared Spectrometer (CIRS), a thermal measuring instrument on the orbiter, also needs to be calibrated. Compared to previous deep space missions, the spectrometer onboard Cassini-Huygens is a significant improvement, with a spectral resolution ten times higher than the Voyager spacecraft’s spectrometer.

AM: Do you have future plans for this research?

JB: Our next step is an experiment developed by Marie-Claire Gazeau, called “SETUP”. The experiment has two parts: a cold plasma in order to dissociate nitrogen, and a photochemical reactor in order to photodissociate methane. This will give us a better global simulation of Titan’s condition.

Original Source: NASA Astrobiology Magazine

Titan Could Help the Study of Oceanography

Image credit: Mark Robertson-Tessi
After a 7-year interplanetary voyage, NASA?s Cassini spacecraft will reach Saturn this July and begin what promises to be one of the most exciting missions in planetary exploration history.

After years of work, scientists have just completed plans for Cassini?s observations of Saturn?s largest moon, Titan.

“Of course, no battle plan survives contact with the enemy,” said Ralph Lorenz, an assistant research scientist at the University of Arizona?s Lunar and Planetary Laboratory in Tucson.

The spacecraft will deploy the European Space Agency?s Huygens probe to Titan for a January 2005 landing. Nearly half the size of Earth, frigid Titan is the only moon in the solar system with a thick atmosphere. Smog has prevented scientists from getting more than a tantalizing hint of what may be on the moon?s amazing surface.

“Titan is a completely new world to us, and what we learn early on will likely make us want to adjust our plans. But we have 44 flybys of Titan in only four years, so we have to have a basic plan to work to.”

Scientists have long thought that, given the abundant methane in Titan’s atmosphere, there might be liquid hydrocarbons on Titan. Infrared maps taken by the Hubble Space Telescope and ground-based telescopes show bright and dark regions on Titan’s surface. The maps indicate the dark regions are literally pitch-black, suggesting liquid ethane and methane.

Last year, data from the Arecibo telescope showed there are many regions on Titan that are both fairly radar-dark and very smooth. One explanation is that these areas are seas of methane and ethane. These two compounds, present in natural gas on Earth, are liquid at Titan’s frigid surface temperature, 94 degrees Kelvin (minus 179 degrees Celsius).

Titan will be an outstanding laboratory for oceanography and meteorology, Lorenz predicts.

“Many important oceanographical processes, like the transport of heat from low to high latitudes by ocean currents, or the generation of waves by wind, are known only empirically on Earth,” Lorenz said. “If you want to know how big waves get for a given windspeed, you just go out and measure both of them, get a lot of datapoints, and fit a line through them.

“But that’s not the same as understanding the underlying physics and being able to predict how things will be different if circumstances change. By giving us a whole new set of parameters, Titan will really open our understanding of how oceans and climates work.”

Cassini/Huygens will answer many questions, among them:

Are the winds strong enough to whip up waves that will cut cliffs in the lakesides? Will they form steep beaches, or will the strong tides caused by Saturn’s gravity be a bigger effect, forming wide, shallow tidal flats?

How deep are Titan’s seas? This question bears on the history of Titan’s atmosphere, which is the only other significant nitrogen atmosphere in the solar system, apart from the one you’re breathing now.

And do the oceans have the same composition everywhere? Just as there are salty seas and freshwater lakes on Earth, some seas on Titan may be more ethane-rich than others.

Lorenz began working on the Huygens project as an engineer for the European Space Agency in 1990, then earned his doctorate from the University of Kent at Canterbury, England, while building one of the probe’s experiments. He joined the University of Arizona in 1994 where he started work on Cassini’s Radar investigation. He is a co-author of the book, “Lifting Titan’s Veil” published in 2002 by Cambridge University Press.

Original Source: UA News Release