Category: Saturn

Image credit: NASA/JPL/Space Science
Cassini continues its ground-breaking observations of Saturn’s mysterious moon Titan, stealing another early peek at its haze-enshrouded surface.

The spacecraft was 29.3 million kilometers (18.2 million miles) from Titan on May 5, 2004 when the image on the left was taken through one of the narrow angle camera’s spectral filters (centered at 938 nanometers) specifically designed to penetrate the moon’s thick atmosphere. The image scale is 176 kilometers (109 miles) per pixel, an improvement in resolution of 30% over the images released on May 6. Cassini’s view of Titan now surpasses Earth-based observations in its ability to show detail.

The image has been magnified 10 times using a procedure which smoothly interpolates between pixels to create intermediate pixel values, and has been enhanced in contrast to bring out details. The mottled pattern is an artifact of the processing. The larger scale brightness variations are real. No further processing to remove the effects of the overlying atmosphere has been performed.

The superimposed coordinate system grid in the accompanying image on the right illustrates the geographical regions of the moon that are illuminated and visible, as well as the orientation of Titan — north is up and rotated 25 degrees to the left. The yellow curve marks the position of the boundary between day and night on Titan.

This image shows about one quarter of Titan’s surface, from 180 to 250 degrees West longitude, and overlaps part of the surface shown in the previous Cassini image release (PIA 05390). (That release also included a map of relative surface brightness variations on Titan as measured from images taken with the Hubble Space Telescope.) The dark northwest-southeast trending southern hemisphere feature extending from 210 degrees to 250 degrees West longitude, and the bright region to the east (right) and southeast of it at -50 degrees latitude and 180 to 230 degrees West longitude on the Hubble map, are visible again in today’s release.

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

Image credit: UA
While the Cassini spacecraft has been flying toward Saturn, chemists on Earth have been making plastic pollution like that raining through the atmosphere of Saturn’s moon, Titan.

Scientists suspect that organic solids have been falling from Titan’s sky for billions of years and might be compounds that set the stage for the next chemical step toward life. They collaborate in University of Arizona laboratory experiments that will help Cassini scientists interpret Titan data and plan a future mission that would deploy an organic chemistry lab to Titan’s surface.

Chemists in Mark A. Smith’s laboratory at the University of Arizona create compounds like those condensing from Titan’s sky by bombarding an analog of Titan’s atmosphere with electrons. This produces “tholins” ? organic polymers (plastics) found in Titan’s upper nitrogen-methane atmosphere. Titan’s tholins are created by ultraviolet sunlight and electrons streaming out from Saturn’s magnetic field.

Tholins must dissolve to produce amino acids that are the basic building blocks of life. But chemists know that tholins won’t dissolve in Titan’s ethane/methane lakes or oceans.

However, they readily dissolve in water or ammonia. And experiments done 20 years ago show that dissolving tholins in liquid water produces amino acids. So given liquid water, there may be amino acids brewing in Titan’s version of primordial soup.

Oxygen is the other essential for life on Earth. But there is almost no oxygen in Titan?s atmosphere.

Last year, however, Caitlin Griffith, of UA?s Lunar and Planetary Laboratory, discovered water ice on Titan?s surface. (See Titan Reveals a Surface Dominated by Icy Bedrock.) UA planetary scientist Jonathan Lunine and others theorize that when volcanoes erupt on Titan, some of this ice could melt and flow across the landscape. Similar flows could result when comets and asteroids slam into Titan.

Better still, Titan?s water may not immediately freeze because it’s probably laced with enough ammonia (antifreeze) to remain liquid for about 1,000 years, Smith and Lunine noted in a research paper published in last November’s issue of “Astrobiology.”

So although Titan is extremely cold — about 94 degrees kelvin (minus 180 degrees Celsius or minus 300 degrees Fahrenheit) — water may briefly flow across the surface, supplying oxygen and a medium for chemistry, they conclude.

To further understand how all this might work together, Smith’s group is generating tholins in the lab, analyzing their spectroscopic properties, and trying to understand their chemistry.

?We?re trying to learn how the compounds will react with molten water on Titan?s surface, what compounds they?ll make, and, therefore, what we should really be looking for,” Smith explained. “We?re not just looking for atmospheric plastic sitting on the surface, but the result of time and energy input over billions of years.

“We want to know what sorts of molecules have evolved, and whether they’ve evolved along pathways that might provide insights into how biological molecules developed on primordial Earth,? he said.

Mark A. Smith, professor and head of UA’s chemistry department

?Some of what we?ve learned so far in our experiments is that these materials are gross mixtures of incredibly complex molecules,? Smith added. ?Carl Sagan spent the last 10 years of his life studying these compounds in experiments like ours. What we?ve found complements his work. We see the same spectroscopic signatures.”

But Smith’s group also has found that there is a component of these molecules that is very reactive and could easily, within a reasonable time frame, react on the surface of Titan to yield oxygenated compounds.

“And that?s what we?re just starting to unravel now,? Smith said.

?Our work will get much more interesting this fall, in our experiments at the Advanced Light Source of the Lawrence Berkeley Lab,” he added. “We?ll be using a synchrotron to create tholins photochemically, using very energetic photons to break up this Titan gas by vacuum ultraviolet radiation.?

Vacuum ultraviolet radiation hits nitrogen and methane molecules in Titan’s upper atmosphere and blasts them apart. Scientists don’t know if this produces the same kinds of polymers that are formed from an electrical discharge.

?When you can crack nitrogen and methane molecules with light, you might get polymers similar to those formed when an electrical discharge cracks them apart,” Smith said. “Or you may get different polymers. The chemistry is quite complex, and we just don’t know the answers to so many of the simplest questions. But that’s one of the reasons we’ll conduct the experiments at Berkeley.?

The work going on in Smith’s lab is important to scientists on NASA’s Cassini Mission and possible follow-up missions to Saturn. The Cassini orbiter was launched in 1997 and is to launch a probe into Titan’s atmosphere in December. This Huygens probe will float to Titan’s surface next January.

?Titan?s thick orange aerosol haze layer is basically a bunch of organic plastics ? polymers of carbon, hydrogen and nitrogen,” said Smith, head of UA’s chemistry department. “The particulates eventually settle on Titan?s surface, where they produce the organic feedstock for any organic chemistry going on.”

Cassini’s Huygens probe will be the first instrument to actually sample this aerosol. It will give scientists some rudimentary chemical information on this material. But the probe won’t tell them much about organic chemistry at Titan’s surface.

A follow-up mission to Titan that includes a robotic organic chemistry laboratory will give scientists a much more detailed look at the surface. The experiment is being designed by Lunine and Smith in collaboration with researchers from Caltech and NASA’s Jet Propulsion Laboratory.

Lunine leads NASA?s Astrobiology Institute focus group on Titan and is one of three interdisciplinary Cassini mission scientists for the Huygens probe.

?We don?t really know how life formed on the Earth, or on whatever planet it formed,? Lunine said. ?There are no traces left of how it happened on Earth, because all of Earth?s organic molecules have been processed biochemically by now. Titan is our best chance to study organic chemistry in a planetary environment that has remained lifeless over billions of years.?

Original Source: UA News Release

Image credit: NASA/JPL/Space Science Institute
As Cassini nears its rendezvous with Saturn, new detail in the banded clouds of the planet’s atmosphere are becoming visible. Cassini took this narrow angle camera image on April 16, 2004 when it was 38.5 million kilometers (23.9 million miles) from Saturn. The image scale is approximately 231 kilometers (144 miles) per pixel. Contrast has been enhanced to aid visibility of features in the atmosphere.

This image was taken using a filter sensitive to light near 727 nanometers, which is one of the near-infrared absorption bands of methane gas, one of the constituents of Saturn’s atmosphere. Dark locales are generally areas of strong methane absorption, relatively free of high clouds. The bright areas are places with high, thick clouds which shield the methane below.

The clouded bands follow lines of constant latitude, and reflect the dominant effect of the planet’s rotation on the dynamics of its atmosphere. Bands move at different speeds, and the irregularities at their edges may be due to either the differential motion between them or to disturbances originating below the visible cloud layer. Such disturbances might be powered by the planet’s internal heat: Saturn radiates more energy than it receives from the Sun.

The dark spot at the south pole is remarkable because it is so small and well-centered. The spot could be affected by Saturn’s magnetic field, which is nearly aligned with the planet’s rotation axis, unlike the magnetic fields of Jupiter and Earth. From south to north, other notable features are the two white spots at roughly the same longitude but different latitudes, and the large dark oblong-shaped feature that extends into the bright equatorial band. The darker band beneath the bright equatorial region has begun to show a lacy pattern of lighter-colored, high altitude clouds, indicative of turbulent atmospheric conditions.

The moon Mimas (396 kilometers, 245 miles across) is visible to the left of the south pole. Saturn currently has 31 known moons, and Cassini scientists hope to discover new ones, perhaps embedded within the planet’s magnificent 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 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

Image credit: NASA/JPL/Space Sciences
The veils of Saturn’s most mysterious moon have begun to lift in Cassini’s eagerly awaited, first glimpse of the surface of Titan, a world where scientists believe organic matter rains from hazy skies and seas of liquid hydrocarbons dot a frigid surface.

Surface features previously observed only from Earth-based telescopes are now visible in images of Titan taken in mid-April through one of the narrow angle camera’s spectral filters specifically designed to penetrate the thick atmosphere. The image scale is 230 kilometers (143 miles) per pixel, and rivals the best Earth-based images.

The two narrow angle camea images displayed here show Titan from a vantage point 17 degrees below its equator, yielding a view from approximately 50 degrees north latitude all the way to its south pole. The image on the left was taken four days after the image on the right. Titan rotated 90 degrees in that time. The two images combined cover a region extending halfway around the moon. The observed brightness variations suggest a heterogenous surface, with variations in average reflectivity on scales of a couple hundred kilometers.

The images were taken through a narrow filter centered at 938 nanometers, a spectral region in which the only obstacle to the transmission of light through the molecular nitrogen atmosphere is the ubiquitous carbon-based, organic haze. Despite the rather long 38-second exposure times, there is no perceptible smear due to spacecraft motion. The images have been magnified 10 times using a procedure which smoothly interpolates between pixels to create intermediate pixel values, and have been enhanced in contrast to bring out details. No further processing to remove the effects of the overlying atmosphere has been performed.

The superimposed coordinate system grid in the accompanying images illustrates the geographical regions of the moon that are illuminated and visible, as well as the orientation of Titan — north is up and rotated 25 degrees to the left. The yellow curve marks the position of the terminator, the boundary between day and night on Titan. The enhanced image contrast makes the sunlit region within 20 degrees of the terminator darker than usual. The Sun illuminates Titan from the right at a phase (ie, Sun-Titan-Cassini) angle of 66 degrees. Because the Sun is in the southern hemisphere as seen from Titan, the north pole is canted relative to the terminator by 25 degrees.

Also shown here is a map of relative surface brightness variations on Titan as measured in images taken in the 1080-nanometer spectral region in 1997 and 1998 by the Near Infrared Camera (NICMOS) on Hubble Space Telescope (Meier, Smith, Owen and Terrile, Icarus 145: 462-473, 2000). NICMOS images have scales of about 300 kilometers (186 miles) per pixel. The map colors indicate different surface reflectivities. From darkest to brightest, the color progression is: deep blue (darkest), light blue, green, yellow, red, and deep red (brightest). The large, continent-sized, red feature extending from 60 degrees to 150 degrees West longitude is called Xanadu. It is unclear whether Xanadu is a mountain range, giant basin, smooth plain, or a combination of all three. It may be dotted with hydrocarbon lakes, but that is also unknown. All that is presently known is that in Earth-based images, it is the brightest region on Titan.

A comparison between the Cassini images and the Hubble map indicates that Xanadu is visible as a bright region in the Cassini image on the right. The dark blue northwest-southeast trending feature from 210 degrees to 250 degrees West longitude, and the bright yellow/green region to the east (right) and southeast of it at -50 degrees latitude and 180 to 230 degrees West longitude on the Hubble map, can both be seen in the image on the left.

It is noteworthy that the surface is visible to Cassini from its present approach geometry, which is not the most favorable for surface viewing. The success of these early Cassini observations portends success for upcoming imaging sequences of Titan in which the resolution improves by a factor of five over the next two months. These results are also encouraging for future, in-orbit observations of Titan that will be acquired from lower, more favorable phase angles.

The first opportunity to view small-scale features (2 kilometers or 1.2 miles) on the surface comes during a 350,000 kilometer (217,500 mile) flyby over Titan’s south pole on July 2, 2004, only 30 hours after Cassini’s insertion into orbit around the ringed planet.

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

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

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

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

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

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

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