Saturn in Full Colour

Image credit: NASA/JPL/Space Sciences
Saturn and its rings completely fill the field of view of Cassini’s narrow angle camera in this natural color image taken on March 27, 2004. This is the last single `eyeful’ of Saturn and its rings achievable with the narrow angle camera on approach to the planet. From now until orbit insertion, the rings will be larger than the camera’s field of view. The image is a composite of three exposures in red, green, and blue, taken when the spacecraft was 47.7 million kilometers (29.7 million miles) from the planet. The image scale is 286 kilometers (178 miles) per pixel.

Color variations between atmospheric bands and features in the southern hemisphere of the planet, as well as subtle color differences across Saturn’s middle B ring, are now more distinct than ever. Color variations generally imply different compositions. The nature and causes of any compositional differences in both the atmosphere and the rings are major questions to be investigated by Cassini scientists as the mission progresses.

The bright blue sliver of light in the northern hemisphere is sunlight passing through the Cassini Division in Saturn’s rings and being scattered by the cloud-free upper atmosphere.

Two faint dark spots are visible in the southern hemisphere. These spots are close to the latitude where Cassini saw two storms merging in mid-March. The fate of the storms visible here is unclear. They are getting close and will eventually merge or squeeze past each other. Further analysis of such dynamic systems in Saturn’s atmosphere will help scientists understand their origins and complex interactions.

Moons visible in this image are (clockwise from top right): Enceladus (499 kilometers, 310 miles across), Mimas (398 kilometers, 247 miles across), Tethys (1060 kilometers, 659 miles across), and Epimetheus (116 kilometers, 72 miles across). Epimetheus is dim and appears just above the left edge of the rings. Brightnesses have been exaggerated to aid visibility.

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 in Four Wavelengths

Image credit: NASA/JPL/Space Science Institute
A montage of Cassini images, taken in four different regions of the electromagnetic spectrum from the ultraviolet to the near-infrared, demonstrates that there is more to Saturn than meets the eye.

The pictures show the effects of absorption and scattering of light at different wavelengths by both atmospheric gas and clouds of differing heights and thicknesses. They also show absorption of light by colored particles mixed with white ammonia clouds in the planet’s atmosphere. Contrast has been enhanced to aid visibility of the atmosphere.

Cassini’s narrow-angle camera took these four images over a period of 20 minutes on April 3, 2004, when the spacecraft was 44.5 million kilometers (27.7 million miles) from the planet. The image scale is approximately 267 kilometers (166 miles) per pixel. All four images show the same face of Saturn.

In the upper left image, Saturn is seen in ultraviolet wavelengths (298 nanometers); at upper right, in visible blue wavelengths (440 nanometers); at lower left, in far red wavelengths just beyond the visible-light spectrum (727 nanometers); and at lower right, in near-infrared wavelengths (930 nanometers).

All gases scatter sunlight efficiently at short wavelengths. That’s why the sky on Earth is blue. The effect is more pronounced in the ultraviolet than in the visible. On Saturn, helium and molecular hydrogen gases scatter ultraviolet light strongly, making the atmosphere appear bright. Only high altitude cloud particles, which tend to absorb ultraviolet light, appear dark against the bright background, explaining the dark equatorial band in the upper left ultraviolet image. The contrast is reversed in the lower left image taken in a spectral region where light is absorbed by methane gas but scattered by high clouds. The equatorial zone in this image is bright because the high clouds there reflect this long wavelength light back to space before much of it can be absorbed by methane.

Scattering by atmospheric gases is less pronounced at visible blue wavelengths than it is in the ultraviolet. Hence, in the top right image, the sunlight can make its way down to deeper cloud layers and back to the observer, and the high equatorial cloud particles, which are reflective at visible wavelengths, also are apparent. This view is closest to what the human eye would see. At bottom right, in the near-infrared, some methane absorption is present but to a much lesser degree than at 727 nanometers. Scientists are not certain whether the contrasts here are produced mainly by colored particles or by latitude differences in altitude and cloud thickness. Data from Cassini should help answer this question.

The sliver of light seen in the northern hemisphere appears bright in the ultraviolet and blue (top images) and is nearly invisible at longer wavelengths (bottom images). The clouds in this part of the northern hemisphere are deep, and sunlight is illuminating only the cloud-free upper atmosphere. The shorter wavelengths are consequently scattered by the gas and make the illuminated atmosphere bright at these wavelengths, while the longer wavelengths are absorbed by methane.

Saturn’s rings also appear noticeably different from image to image, whose exposure times range from two to 46 seconds. The rings appear dark in the 46-second ultraviolet image because they inherently reflect little light at these wavelengths. The differences at other wavelengths are mostly due to the differences in exposure times.

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

A Movie of Titan’s Hazy Atmosphere

Image credit: Keck
As the Cassini-Huygens spacecraft approaches a July encounter with Saturn and its moon Titan, a team of University of California, Berkeley, astronomers has produced a detailed look at the moon’s cloud cover and what the Huygens probe will see as it dives through the atmosphere of Titan to land on the surface.

Astronomer Imke de Pater and her UC Berkeley colleagues used adaptive optics on the Keck Telescope in Hawaii to image the hydrocarbon haze that envelops the moon, taking snapshots at various altitudes from 150-200 kilometers down to the surface. They assembled the pictures into a movie that shows what Huygens will encounter when it descends to the surface in January 2005, six months after the Cassini spacecraft enters orbit around Saturn.

“Before, we could see each component of the haze but didn’t know where exactly it was in the stratosphere or the troposphere. These are the first detailed pictures of the distribution of haze with altitude,” said atmospheric chemist Mate Adamkovics, a graduate student in UC Berkeley’s College of Chemistry. “It’s the difference between an X-ray of the atmosphere and an MRI.”

“This shows what can be done with the new instruments on the Keck Telescope,” added de Pater, referring to the Near Infrared Spectrometer (NIRSPEC) mounted with the adaptive optics system. “This is the first time a movie has been made, which can help us understand the meteorology on Titan.”

Adamkovics and de Pater note than even after Cassini reaches Saturn this year, ground-based observations can provide important information on how Titan’s atmosphere changes with time, and how circulation couples with the atmospheric chemistry to create aerosols in Titan’s atmosphere. This will become even easier next year when OSIRIS (OH-Suppressing Infra-Red Imaging Spectrograph) comes on-line at the Keck telescopes, de Pater said. OSIRIS is a near-infrared integral field spectrograph designed for the Keck’s adaptive optics system that can sample a small rectangular patch of sky, unlike NIRSPEC, which samples a slit and must scan a patch of sky.

De Pater will present the results and the movie on Thursday, April 15, at an international conference in The Netherlands on the occasion of the 375th birthday of the Dutch scientist Christiaan Huygens. Huygens was the first “scientific director” of the Acad?mie Fran?aise and the discoverer of Titan, Saturn’s largest moon, in 1655. The four-day conference, which started April 13, is taking place at the European Space & Technology Centre in Noordwijk.

The Cassini-Huygens mission is an international collaboration between three space agencies – the National Aeronautics and Space Administration, the European Space Agency and the Italian Space agency – involving contributions from 17 nations. It was launched from Kennedy Space Center on Oct. 15, 1997. The spacecraft will arrive at Saturn in July, with the Cassini orbiter expected to send back data on the planet and its moons for at least four years. The orbiter also will relay data from the Huygens probe as it plunges through Titan’s atmosphere and after it lands on the surface next year.

What makes Titan so interesting is its seeming resemblance to a young Earth, an age when life presumably arose and before oxygen changed our planet’s chemistry. The atmospheres of both Titan and the early Earth were dominated by nearly the same amount of nitrogen.

The atmosphere of Titan has a significant amount of methane gas, which is chemically altered by ultraviolet light in the upper atmosphere, or stratosphere, to form long-chain hydrocarbons, which condense into particulates that create a dense haze. These hydrocarbons, which could be like oil or gasoline, eventually settle to the surface. Radar observations indicate flat areas on the moon’s surface that could be pools or lakes of propane or butane, Adamkovics said.

Astronomers have been able to pierce the hydrocarbon haze to look at the surface using ground-based telescopes with adaptive optics or speckle interferometry, and with the Hubble Space Telescope, always with filters that allow the telescopes to see through “windows” in the haze where methane doesn’t absorb.

Imaging the haze itself hasn’t been as easy, primarily because people have had to observe at different wavelengths to see it at specific altitudes.

“Until now, what we knew about the distribution of haze came from separate groups using different techniques, different filters,” Adamkovics said. “We get all that in one go: the 3-D distribution of haze on Titan, how much at each place on the planet and how high in the atmosphere, in one observation.”

The NIRSPEC instrument on the Keck telescope measures the intensity of a band of near-infrared wavelengths at once as it scans about 10 slices along Titan’s surface. This technique allows reconstruction of haze versus altitude because specific wavelengths must come from specific altitudes or they wouldn’t be visible at all because of absorption.

The movie Adamkovics and de Pater put together shows a haze distribution similar to what had been observed before, but more complete and assembled in a more user-friendly way. For example, haze in the atmosphere over the South Pole is very evident, at an altitude of between 30 and 50 kilometers. This haze is known to form seasonally and dissipate during the Titan “year,” which is about 29 1/2 Earth years.

Stratospheric haze at about 150 kilometers is visible over a large area in the northern hemisphere but not the southern hemisphere, an asymmetry observed previously.

At the southern hemisphere’s tropopause, the border between the lower atmosphere and the stratosphere at about 42 kilometers altitude, cirrus haze is visible, analogous to cirrus haze on Earth.

The observations were made on Feb. 19, 20 and 22, 2001, by de Pater and colleague Henry G. Roe of the California Institute of Technology, and analyzed by Adamkovics using models made by Caitlin A. Griffith of the University of Arizona, with co-author S. G. Gibbard of Lawrence Livermore National Laboratory.

The work was sponsored in part by the National Science Foundation and the Technology Center for Adaptive Optics.

Original Source: UC Berkeley News Release

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