Author: Fraser Cain

Mosaic of Huygens’ Titan images showing its landing spot. Image credit: ESA. Click to enlarge.
As the large amount of data collected by the ESA Huygens probe during its descent onto Titan is being processed, new views of this fascinating world become available.

The Descent Imager Spectral Radiometer (DISR) team have now produced the first complete ?stereographic? and ?gnomonic? mosaic images. Using special image projection techniques, the team combined a series of images captured by Huygens while rotating on its axis at an altitude of about 20 kilometres.

The DISR on board Huygens took its series of photographs of the ever-approaching surface in sets of three, or ?triplets?, as it dropped through Titan?s atmosphere on 14 January this year. The images sent back to Earth partially overlap, due to the probe?s rotation during the descent and due to the overlap between the fields of view of the different cameras.

DISR scientists are studying these images for similarities, such as physical features common to more than one image, and are constructing ?mosaics?, like jigsaw puzzles.

There are many different ways of rendering three-dimensional objects into two dimensions. Different kinds of projections for maps or photographs are able to represent realistically things like size, areas, distances and perspective. One particular kind of projection used for spheres in two dimensions (for example on some maps of Earth or the celestial sphere) is ?stereographic? projection.

A ?gnomonic? projection has also been produced, and this tends to make the surface appear as if it was flat. This type of projection is often found on maps used by navigators and aviators in determining the shortest distance between two points. However there is a lot of distortion of scale at the outer edges of gnomonic projections.

On the stereographic view, like that through a ?fish-eye? lens, the bright area to the north (top of the image) and west is higher than the rest of the terrain, and covered in dark lines that appear to be drainage channels. These lead down to what appears to be a shoreline with river deltas and sand bars.

The current interpretation of these lines is that they are cut by flowing liquid methane. Some of them may have been produced by precipitation run-off, producing a dense network of narrow channels and features with sharp branching angles. Some others may have been produced by sapping or sub-surface flows, giving shape to short stubby channels that join at 90 degree angles.

The largest run off channel starts at about the 12 o?clock position from an inlet on the shoreline and stretches to the left. The largest sapping channel starts at the 9 o? clock position and goes in a straight line up and left. The dark wide corridor to the west just below the sapping channel appears to be a major flow channel that empties into the mud flats of the lakebed.

The bright shapes to the north-east and east look to be ridges of ice gravel that are slightly higher than the flats around them, and the probe landing is believed to be just south-west of the semi-circular shape. The light and dark areas to the south are still of unknown nature.

On the gnomonic projection, the landing site is approaching and the surface features become sharper. North is at the top of the image. From lower left to upper right appears to be a ridge of ice boulders projecting through the darker lakebed material.

They are thought to slow the major flow from the west and cause the fluid to pond on the north-west side of the image, causing sedimentation of the dark material. Seepage between the boulders cuts the sediment into channels as the fluid continues to the south-east.

The members of the Huygens DISR instrument team are based throughout the USA and Europe, with the largest contributing groups from the University of Arizona, USA, the Max Planck Institute, Germany, and the Paris Observatory, Meudon, France.

Original Source: ESA News Release

Image of Titan’s thick hazy atmosphere which is surprisingly similar to the early Earth. Image credit: NASA/JPL/SSI. Click to enlarge.
Titan’s atmospheric winds, temperature and mixing have been revealed by new observations from the Cassini spacecraft. The thick atmosphere of Saturn’s giant moon is rich in organic compounds, whose chemistry may be similar to that which occurred on Earth before the emergence of life.

“Titan is not just a dot in the sky; these new observations show that Titan is a rich, complex world much like the Earth in some ways,” said Dr. Michael Flasar of NASA’s Goddard Space Flight Center, Greenbelt, Md., Composite Infrared Spectrometer instrument (CIRS) principal investigator. Flasar is lead author of a paper on this research published May 13 in the Journal Science.

The CIRS science team found evidence for an isolated polar vortex similar to one that occurs on Earth. CIRS’ observations indicated that strong winds circulating around Titan’s north pole isolate the atmosphere there during the polar night. Mixing of the polar region with the lower latitude regions of the atmosphere is inhibited during this time. On Earth, the south polar atmosphere is isolated for months during the long Antarctic winter allowing the formation of polar stratospheric clouds. Normally inert chlorine compounds (such as chlorine nitrate) undergo chemical reactions on the cloud crystals that free molecular chlorine. In the spring, sunlight decomposes the molecular chlorine, leading to the famous annual Antarctic ?ozone hole?. Titan’s atmosphere contains no ozone; however the CIRS results show that a large part of its atmosphere is isolated during the polar night, and that could allow unusual and complex chemistry to occur.

Like Earth, Titan’s axis of rotation is tilted, so its poles also experience a long night during winter. The polar winter on Titan is many earth years long, because Saturn orbits the sun once in almost 30 years. Currently it is early winter in Titan’s northern hemisphere. The CIRS team found significant temperature differences between Titan’s north pole and the equator. The team used this observation to derive the speed of circumpolar winds around the north pole. The team believes these winds are isolating the atmosphere around Titan’s north pole because the CIRS data showed that the concentration of several heavy organic (carbon-containing) molecules is highest there.

Heavy organic molecules form naturally in Titan’s atmosphere, blanketing the moon with an orange haze. Titan?s atmosphere consists of about 98 percent nitrogen with most of the remainder being methane. When these molecules rise to the upper atmosphere, they are broken apart by sunlight and the fragments form heavier organic molecules like propane, ethane, acetylene, hydrogen cyanide, and even more complex molecules. Because the stratospheric air over the winter pole is cold, it sinks and brings down the heavy organic compounds that formed higher up. If the air over Titan’s north pole is isolated during the winter, the heavy organics should build up in the stratosphere over the season. This is just what the CIRS team is seeing.

“We don’t know if there are even more similarities to Earth’s ozone hole process, like polar clouds that react with molecules in the atmosphere, simply because we haven’t seen them yet,” said Flasar. “But we wouldn’t be surprised to discover them, nor would we be surprised to find that Titan has some unique twists of its own. This is what makes science so exciting. Nature is too rich for us to predict exactly what we will find when we go exploring.”

The research was funded by NASA and the European Space Agency. 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 Science Mission Directorate, Washington, D.C.

Original Source: NASA News Release

Saturn’s moon Iapetus and its strange “rindge”. Image credit: NASA/JPL/SSI. Click to enlarge.
Is there any more mysterious and beautiful planet for the observer than Saturn? While all four gas giants in our solar system have a ring system, only Saturn’s can be seen from Earth. Backyard astronomers have long been thrilled to witness its two bright rings and the dark Cassini division, while observatory telescopes have identified many separate rings and gaps. Not until the beginning of the 1980’s when Voyager made it’s “fly-by” were we aware of more than a thousand individual rings bound by the gravity of Saturn and its many small moons. The rings themselves are nothing more than icy particles ranging in size from dust motes to boulders. Joining in this intricate dance are the satellites – from Mercury-sized atmospheric Titan to tumbling, eccentrically orbiting Hyperion. Since the late 18th century we have known of Titan, Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus. Our studies have revealed that four of the moons play a key role in shaping Saturn’s ring system – Pan, Atas, Pandora and Prometheus. We know that Enceladus’ highly reflective surface is comprised of ice and that Iapetus is much brighter one one side than the other…

And may have collected a ring as it swept through orbital changes.

From the time of its discovery in 1672, we have been aware the leading hemisphere of Iapetus is fully a magnitude darker than the trailing side. Thanks to the Cassini mission’s images taken in December 2004, the presence of a large equatorial ridge has been discovered on Iapetus’ dark side.

According to a Geophysical Research Letter submitted on the 29th of April by Paulo C.C. Freire of Arecibo Observatory, “…this ridge and the dark coating of the hemisphere on which it lies are intimately interlinked and are the result of a collision with the edge of a primordial Saturnian ring, ultimately caused by a sudden change in the orbit of Iapetus”. Says Freire, “Because of its unique nature, we will henceforth refer to the equatorial ridge of Iapetus simply as ‘the Rindge’ to mean that this feature is not a ridge in the usual sense of the term; i.e., a mountain chain caused by tectonic process. This model naturally explains all of the unique features of this satellite; and is probably the solution to one of the oldest mysteries in solar system astronomy.”

One of the scientific goals of the Cassini fly-by imaging was to shed some light on Iapetus’ dark side, called Cassini Regio. To the surprise of the researchers, it revealed a great equatorial ridge unlike anything else found in the solar system – a ridge so symmetrical relative to the Cassini Regio that the two features must be linked, as acknowledged previously by Carolyn Porco – head of the Cassini Imaging Team. The majority of the clues point to how the ring system and the forming moons once orbited Saturn itself.

Current understanding of the formation of the solar system (and, in a smaller scale, the Saturnian system) indicates that many planetoids (and proto-satellites) may have once started in orbits that later became unstable. They could have collided with each other, or been ejected from their system by close encounters with others. In the case of Saturn, it is possible they could have been tidally disrupted when approaching Saturn’s gravity and formed ring systems. Closer to the planet, in an area known as the “Roche Zone”, the tidal pull of Saturn prevents proto-satellite formation from ring particles. In order for the ring collision theory to match what Cassini has imaged, Iapetus had to have been one of these moons with unstable orbits.

Evidence points to the fact that something changed Iapetus’ orbit before colliding with ring material. Had this not happened, the ring would have adjusted to Iapetus’ gravity as evidenced by satellites currently imbedded within the rings. In the case of these satellites – no collision scenario can occur. In Iapetus’ circumstance, its orbit was necessarily eccentric, or no velocity differences would exist between Iapetus and the ring particles and again – no collisions would occur.

An impact with a ring also suggests this changed orbit had a perisaturnium at the outer edge of the Roche Zone, where rings can exist for longer periods of time. This is a clue that Iapetus was quite probably much closer to Saturn than its present orbit. “The existence of the rindge suggests the orbit of Iapetus at the time of the collision was equatorial” says Freire, “otherwise, with its present inclination a collision with a ring would not produce a sharp edge, but something more like a wispy dark coating of the leading hemisphere.” In conclusion, a satellite with an equatorial and eccentric orbit has a very large probability of interacting further with other satellites – providing the means to change yet again to a different orbit.

Now that we have set the stage, how do the images taken of this unique rindge support the theory? According to Freire, “The ring collisions scenario naturally produces a linear feature exactly at the equator: this is the geometric intersection of a ring plane and the surface of a moon with a (previously) equatorial orbit.” Very careful consideration has been given to tectonics, but such a perfectly linear formation – located exactly at the equator – is unlikely to result from tectonic processes and Iapetus shows no signs of volcanic activity.

“Another key feature of the rindge is that its height varies extremely slowly with longitude,” says Freire, “This can be expected from deposition of material from a ring, but such a constant height has never been observed for any tectonic feature. If the origin of the rindge was tectonic and preceded the dark coating, then it should not necessarily be confined to Cassini Regio. If it postdated the coating, then the rindge being built from an upwelling from the interior of Iapetus should be much brighter than the surrounding surface.”

Considerable analysis has been given to the information that Cassini imaging has provided. The longitudinal length of the ridge is less than 180 degrees, which suggests that Iapetus was never fully inside the ring region – indicating that it just collided with a ring edge. Celestial mechanics considerations indicate that a collision with a ring edge should have caused an eastward motion of the particle’s impacts relative to the surface of the satellite. “This accounts for an important observed fact: although Cassini Regio is symmetrical relative to the rindge in the north/south direction, it is not so in the east/west direction.” This collision model suggests that the rindge would be taller on the western side where the impacts were closer to vertical and then would slowly digress moving eastward – a fact supported by the images. With millions of impact craters being formed every second along a line, this pattern would become unmistakable. The sublimation of the ices contained in the impacting particles would produce a transient atmosphere, with a strong pressure gradient away from the rindge. This gradient would produce fast winds capable of carrying fine dust. Says Freire, “In our hypothesis, the dust deposited by such winds is the dark coating of the region known today as Cassini Regio.” Such a scenario is supported by other evidence: “The dark streaks observed at the edge of Cassini Regio indicate that it was a wind blowing from the equator that deposited the ‘dust’. We can be certain of this because Cassini imagery shows clearly that the dust is deposited downward from the crater rims.” This can’t be accounted for by ballistic flight of the particles from the equator, as suggested by the leader of the Cassini Imaging Team, Carolyn Porco. It can’t be produced in present-day Iapetus, since it has no atmosphere. The conclusion that a transient atmosphere once existed becomes inescapable.

Could these exciting findings truly be from an earlier impact with one of Saturn’s rings? The clues certainly seem to make the pieces of the puzzle fit together neatly. Thanks to work done by researchers like Paulo Freire, we may have solved a 333 year-old solar system mystery.

Written by Tammy Plotner, with many thanks to Paulo Freire for his contributions.

Saturn’s moons Dione and Tethys poised above Saturn’s rings. Image credit: NASA/JPL/SSI. Click to enlarge.
Saturn’s moon Dione occults part of Saturn’s distant rings while Tethys hovers below. Dione is 1,118 kilometers (695 miles) across, while Tethys is 1,071 kilometers, 665 miles) across.

This image offers excellent contrast with a previously released view (see Sister Moons) that showed the bright, wispy markings on Dione’s trailing hemisphere. The huge impact structure Odysseus (450 kilometers, or 280 miles across) is near the limb of Tethys. Compared with the battered surface of Tethys, Dione appears much smoother from this distance.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on March 19, 2005, at a distance of approximately 2.7 million kilometers (1.7 million miles) from Saturn. The image scale is approximately 15 kilometers (9 miles) per pixel.

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 mission for NASA’s Science Mission Directorate, 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, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . For additional images visit the Cassini imaging team homepage http://ciclops.org .

Original Source: NASA/JPL/SSI News Release