Titan News 2006
October 28, 2006: Calderas and volcanic lakes present in Titan's northern polar region ?
The recent radar images of Titan's northern latitudes unveil a multitude of dark patches with well defined boundaries. Since the signal of the radar is absorbed by the surface, we deduce that the areas corresponding to the dark patches are very smooth. The most obvious conclusion is to say that they are likely lakes. In other words, as dark patches are a signature of lakes or seas, we can suggest that they are lakes with a high probability. Several clues tend to strengthen the hypothesis of lakes: dark channels connected to those dark patches suggest they represent drainage channels , the shape of the dark patches is generally irregular with clear boundaries or rims and a large cloud formation has been spotted around Titan's north pole.
The radar images acquired during the flyby of July 22, 2006 also reveal striking topographic features: dozens of rounded depressions with a uniform dark patch inside can be spotted. Their shape suggests they represent calderas, volcanic structures that we encounter on Earth. A caldera takes shape when the ground collapses after lava has drained out from under it in volcanic eruptions. Most of those round patches show steeply sloping sides, a characteristic of terrestrial calderas. They are surrounded by a circular bright material. Is it pure water ice or a mixture of water and ammonia expelled in volcanic eruptions ? This radar shape implies that they are likely collapsed slush domes. They are not simply lakes. Some circular bright domes seem to be on the verge of collapsing. Slight dark circles suggest this "scenario". The caldera structures are sometimes nested one within the other, a characteristic of calderas that occurs when successive eruptions lead to multiple collapses at the same spot. At first glance, the northern polar region is a place where volcanoes and lakes are common. There is probably a hot spot beneath the surface there, according to an analysis led by Charles Wood of Wheeling Jesuit University in Wheeling, West Virginia, US: " This is a persistent volcanic province that has heat flow escaping over a long period of time."
Rosaly Lopes of the Jet Propulsion Laboratory in Pasadena, California, US points out that if volcanoes are active on Titan, they might help explain why Titan has so much methane in its atmosphere. She adds that water slush could bring methane up from under the surface,"very similar to the way volcanoes on Earth bring sulphur to the surface". But the origin of the methane present in Titan's atmosphere is still not clearly understood: is it linked to a meteorological cycle or to cryovolcanism ? Maybe both. The volcanic hypothesis is reinforced by the fact that methane is constantly being broken down by sunlight which implies that it must be replenished somehow.
Kurt Mitchell of the Jet Propulsion Laboratory in Pasadena, California, US puts forward the hypothesis of life in those hot spots:" In magmatically active areas you expect heat, you expect gases to be released... What with the methane lakes, perhaps some type of exotic exobiology might not be completely out of the question."
Robert Brown, who heads Cassini's Visual and Infrared Mapping Spectrometer team questions the origin of the dark circular patches: he privileges the hypothesis that meteorite impacts could explain the rounded depressions seen, noting that there may be a gooey layer of soft hydrocarbons, such as benzene, on the moon's surface. He adds:" Take a layer of tar and drop a rock in it. What would that look like ? " However, it appears that most of the caldera-like structures are located in the northern polar region. So why would meteorites hit exclusively the northern polar region. That's what Charles Wood notes. Few volcanic signs have been identified up to now: a dome-shaped structure found in the first radar images of the surface taken in October 2004 around 30 degrees north of the equator is likely the result of volcanic material welling up beneath the surface.
The brightest spot 400 kilometres across revealed by infrared images ( VIMS instrument in July 2004 ) may be linked to a volcanic phenomenon. It has been seen brightening and growing. The hypothesis of the cloud is ruled out because it tends to break up within hours or days. Robert Nelson and Karl Mitchell of the JPL in Pasadena, US favours the volcanic hypothesis. Robert Brown notes that the apparent variations of the spot are likely just an illusion caused by the haze in Titan's atmosphere. This haze varies over time and is difficult to account for in observations.
Jason Barnes of the University of Arizona in Tucson, US specifies that eruptions of a mixture of liquid water and ammonia coming from underground to the surface are conceivable. Nevertheless, the bright spot, supposed to be an active volcano, doesn't show any extra heat. The temperature is about 94 K (-179 degrees Celsius or -290 degrees Fahrenheit ).
David Stevenson of the California Institute of Technology in Pasadena, US proposes that the brightening events could instead be the result of methane buried just beneath the surface getting heated and spurting above ground.
The radar map of Titan is still incomplete but the radar sections reveal that the northern latitudes apparently harbour a higher amount of liquid areas, lakes, seas, rivers or lava lakes than in lower latitudes. Titan's north pole is currently in the winter period. As a result, it is believed that the presence of this pleiade of assumed lakes is linked to the harsh climate conditions dominating the winter hemisphere. Furthermore, under those conditions, ultraviolet radiations are less likely to break down methane into other molecules.
Probably that methane condenses easier at northern latitudes than at Titan equator. That's what proposes Marc Lafferre. He makes a parallel between empirical thermal conditions on Earth and those on Titan. He takes into account two different spots on Earth and on Titan at similar latitudes and carries out an extrapolation from terrestrial thermal differences to determine roughly the potential temperature at the latitude where Titan lakes were identified. But he insists on the fact that it is a rough extrapolation. The obliquity of the rotation axis of planet Earth is less high ( 23.15 degrees compared to 26 degrees for Titan ) which implies that the seasons are less marked, basically. The atmosphere of Titan is also deeper and denser which is likely to increase thermal evolutions from one season to another. Both factors must play a key role in thermal changes on Titan. That's one of the main reasons why the following comparison is a rough evaluation.
Here is the reasoning: the surface temperature at Titan's landing site ( 10.2 degrees south, 192.4 degrees west ) was -179.5 degrees Celsius ( -291 degrees Fahrenheit or 93.5 Kelvin ). So what can be the surface temperature where the assumed lakes were spotted at 78 degrees North latitude ? He takes two different sites on Earth at roughly similar latitudes: Luanda ( Angola ) at 8.5 degrees South and 13 degrees East and Svalbard at 78 degrees North latitude and 15 degrees East longitude. The mean air temperature in Luanda in January is 26.7 degrees Celsius. In Svalbard, at the same period of the year, the mean air temperature is -15.3 degrees Celsius. As a result, the solar energy received in Luanda is around 80% higher than that received in Svalbard. By extrapolating this thermal difference to similar latitudes on Titan, we obtain a surface temperature of -193 degrees Celsius ( -315.5 degrees Fahrenheit or 80 Kelvin ) at 78 degrees North latitude on Titan ( a 14 degree difference ) which is close to the boiling point of nitrogen.
This radar image was captured on July 22, 2006 by the Radar mapper onboard the Cassini spacecraft. It shows Titan's northern latitudes. The irregular dark patches seem to be lakes while circular dark patches surrounded by a bright material are likely calderas with a volcanic dome. An apparent drainage channel is clearly visible top right of the image. Image source: NASA, JPL
- To get further information on that news, go to: http://www.newscientistspace.com/article/dn10304-slushy-volcanoes-might-support-life
September 20, 2006: Vast polar ethane cloud spotted on Titan
Four radar images taken at different perods by the VIMS onboard the Cassini spacecraft reveal a huge ethane cloud formation just north of 50 degrees latitude. The infrared images show the reflection of sunlight on Titan's atmosphere at 2.8 microns, longer wavelengths than human eyes can detect. The images appear in false color so that the highest reflection appears as a reddish hue. A reddish hue is clearly visible north of 50 degrees latitude which makes planetologists conclude that the north polor region, currently in the winter period, is covered with a large cloud system. This area of reflective particles is about 40 kilometres above the surface, around the altitude where the atmosphere is at its coldest. It seems that the detected cloud doesn't consist of methane because the particles are smaller than expected for condensed methane. They range from 1 to 3 microns across.
The cloud system would rather be made up of ethane particles due to the size of particles. Ethane is generated in Titan's atmosphere by the breakdown of methane , present in large quantities, under the action of ultraviolet radiations. Before NASA's Cassini spacecraft reached Titan, scientists suspected oceans of liquid ethane mixed with liquid methane. The amount expected would have covered the entire moon in a layer of ethane 300 metres deep. We now know that most of Titan's surface is dry. Nevertheless, it appears that the high latitudes in the northern hemisphere are made of a multitude of lakes or seas. The flyby of July 22, 2006 has allowed the Radar mapper onboard the Cassini spacecraft to unveil the most significant clues of lakes, identifying dark patches with clear boundaries or rims, a radar signature of liquid areas on Earth. The assumed liquid areas are located in latitudes higher than 75 degrees north. It appears that there is a link between the lakes or seas and the cloud system. Titan would rather be governed by an ethane cycle rather than a methane cycle. The Titan atmosphere probably presents reamarkable similarities with Earth's atmosphere: a parallel between the hydrosphere of planet Earth and the assumed ethanosphere of Titan will likely bring us significant elements in understanding the mechanism by which the climate functions and evolves on Earth, and hence to better understand the process of global warming. As Marc Lafferre notes, Oxygen is almost absent in Titan's atmosphere but there is another molecule which might have relatively similar physical implications: this molecule is methane. It is the basis in generating another molecule which likely appears as a liquid on Titan's surface, ethane.
Despite all these observations and analysis, the debate over the nature of the liquid is still open: Jonathan Lunine, a professor of planetary sciences at the University of Arizona favours the hypothesis of methane lakes rather than ethane lakes. He insists on the dynamic gas-liquid system that parallels Earth's hydrosphere. He says: "Half a century after Gerard Kuiper first found methane on Titan, the Cassini-Huygens mission has discovered a methanosphere on Titan". Caitlin Griffith of the University of Arizona in Tucson, US who unveiled the ethane clouds in Titan's northern high latitudes points out: "We think that ethane is raining or, if temperatures are cool enough, snowing on the north pole right now. When the seasons switch , we expect ethane to condense at the south pole during its winter." She also notes that if ethane moves towards the poles in this way, it might explain why so little ethane has been found closer to the equator. The accumulation of ethane via the breakdown of methane can tell scientists how long methane has been present in Titan's atmosphere, she adds. If this process has been at work since Titan formed, a layer of ethane 2 kilometres thick should have formed at the poles. She insists: "But if we look at the south pole we don't see evidence for the amount of ethane that we would need to have if methane was in the atmosphere over the lifetime of Titan."Paradoxically, the methane could instead be a relatively recent addition to the moon's atmosphere she says."You have to speculate that maybe Titan is volcanically active and still burping out methane."
Up to now, the high latitudes of the north polar region have been visited, but the north pole is still unexplored. Most of the northern polar region is in winter's shadow and won't be fully illuminated until 2010, Griffith noted. We expect a harsher temperature in the north polar region. The lower temperatures might explain the presence of the lakes observed in radar images of Titan's northern latitude. Griffith added:"By the end of next year Cassini will have recorded the first polar temperature profile of Titan, which will tell us how cold conditions are at the pole."
The poles appear to be a special place for the development of clouds. The south pole has unveiled a large cloud formation hundreds of km across. It was associated with higher temperatures since the south pole is currently in the summer period. But the north pole, currently in the winter period, also reveals huge cloud formations as well as a multitude of lakes or seas. Moreover, the flyby of July 22, 2006 has enabled the VIMS of the Cassini spacecraft to spot clouds spreading out along the 40-degree-south latitude line.Before the Cassini /Huygens spacecraft arrived at Saturn, there were some doubts that significant masses of clouds were present on Titan. Now, we can say that Titan is not devoid of clouds. But the weather is probably not as active as on Earth: thunderstorms are rather located at high latitudes where large cloud systems appear. As Ralph Lorenz mentionned, we can expect monsoon events. Clouds, Thunderstorms, flashes, lightning, tornadoes, all those meteorological phenomena are not a particularity of planet Earth.
The four infrared images above were captured by the VIMS instrument onboard the Cassini spacecraft at 2.8 microns, at different periods. Image ( A ) was taken on Dec.13, 2004; image ( B ) on Aug.22, 2005; image ( C ) on Aug.21, 2005; and image ( D ) on Sept.7, 2005. The reddish and yellow band just north of 50 degrees latitude correspond to the vast ethane cloud. Image source: NASA, JPL
This infrared image was acquired during the Titan flyby of July 22, 2006 by the VIMS instrument onboard the Cassini probe. The reddish band in the lower part of the moon corresponds to clouds spreading out along the 40-degree-south latitude line. Image source: NASA, JPL
- To get further information on that news, go to: http://www.newscientistspace.com./channel/solar-system/dn10073-huge-ethane-cloud-discovered-on-titan.html or http://www.spaceflightnow.com/cassini/060914titancloud.html
July 29, 2006: Strong evidence for lakes around Titan's north pole
For the first time, we've captured radar images of the high latitudes in the Titan's north polar region where it is currently winter. The Titan flyby of July 21, 2006 has allowed the Cassini spacecraft to spot a "myriad" of very dark patches up to 100 km across with well defined boundaries or rims. The lake-like features are apparently uniformly dark, have irregular shapes which means that the hypothesis that they result from impact craters is unlikely. Furthermore, they show clear rims with a radical contrast in their edges in many cases.
The earlier images had unveiled dark patches, but they were less dark than these high latitude dark patches. Enrico Flamini of the Italian Space Agency in Rome, a member of the radar instrument team pointed out: " In this case, it is much clearer. The contrast is so great that there are few doubts that the surface is a liquid one." Anyway, the radar beam hit a very smooth surface, probably liquid methane or ethane.
The alleged methane pools are likely the outcome of rainfall, perhaps seasonal storms. Some of the patches are apparently fed by drainage channels. Some dark channels, presumably drainage channels, are connected to dark patches. It contrasts with the channels, identified in the Xanadu mountains, which appeared bright in radar images. In other words, the Xanadu channels, carved by a liquid, are likely empty. Comparisons can be made between dark patches of the high latitude radar image and terrestrial landscapes: small interconnected lakes resemble parts of Finland and Canada.
How to explain this pleiade of dark patches across the north pole? It is probably linked to the winter season: there is less sunlight, the temperature is a little colder. As a result, methane is less likely to evaporate, there is no convection and liquid methane is more stable at lower temperatures. Nevertheless, the temperature difference is slight between the pole and the equator. Enrico Flamini insists:"When we have more coverage of the equator, we could see lakes there, too. We've only covered a few per cent of the surface so far." Cassini radar scientist Ralph Lorenz of the University of Arizona, US said:"We could hope to see sea-surface textures due to waves diffracting around islands or vortices in the wake of islands." This sort of features would make liquids come alive."
We have, now, significant clues that Titan is covered with lakes, seas and rivers. One of the most significant elements is the landscape view of Titan during the Huygens descent on January 14, 2005. It shows a network of channels on white hills ending their way in a dark reddish area.The image of the soil reveals small pebbles, probably eroded by liquid methane. Unexpected radio reflection from the surface of Titan has allowed scientists to deduce the average size of stones and pebbles close to the Huygens' landing site. Miguel Pérez-Ayucar explains that part of the radio signal leaked downward and hit the surface of Titan before being reflected back up to Cassini. On its way up, it interfered with the direct beam. Pérez began running computer models and saw not only could he reproduce the received signal, but also it was sensitive to the size of pebbles on the surface of Titan. With his team, they discovered that the surface swath must be relatively flat and covered mostly in stones of around 5 centimeters to 10 centimeters in diameter.
Huygens data also show strong evidence that liquid methane drizzles from the atmosphere of Titan onto the moon's surface. It is shown that there is a lower, barely visible, liquid methane-nitrogen cloud that drops rain to the surface of Titan. At higher altitudes, one encounters an upper methane ice cloud. The downward flow of methane might be balanced by upward transport of methane gas.
Christopher McKay of NASA' Ames Research Center in California pointed out:" The rain on titan is just a slight drizzle, but it rains all the time, day in, day out. It makes the ground wet and muddy with liquid methane." He added:" This is why the Huygens probe landed with a splat. It landed in methane mud." Hence, there is a methane cycle, a meteorology associated with liquid methane which engenders those fluvial or river-like features. The computer models reveal that thin liquid methane clouds cover about half of Titan. Methane abundance decreases with latitude. Chris McKay explains:" We determined that the rain on Titan is equal to about two inches ( about 5 centimeters ) a year. This is about as much as Death Valley ( receives). The difference is on Titan, this rain is spread out evenly over the entire year." As a result, the erosion is quite limited, but sufficient to wet the surface material.
Image source: NASA, JPL; The radar image above was taken during the flyby of the Cassini spacecraft on July 21, 2006. The dark patch is likely a lake, around 47 km long ( 29 miles ) and 25 km wide ( 16 miles ). A large drainage channel, at least 5 km long ( 3 miles ) is clearly visible on the right side of the dark patch.
- To get further information on that news , go to: http://www.newscientistspace.com/article/dn9612-titan-may-be-a-land-of-lakes-after-all, http://www.saturndaily.com/reports/Evidence_Strong_That_It_Rains_On_Titan_999.html and http://www.saturndaily.com/reports/Huygens_Establishes_Pebble_Sizes_On_Titan_999.html
July 22, 2006: Xanadu is dominated by hills, high mountains, channels and probably caves
The radar images taken by the Cassini spacecraft during the Titan flyby of April 30, 2006 reveal that Xanadu, a bright region in infrared wavelengths, is made of hills, high mountains and river channels. Xanadu is an Australia-size area, measuring about 4,000 kilometers ( 2,485 miles ) east to west and 2,000 kilometers ( 1,243 miles ) north to south. It is surrounded by darker terrain.
Mountains roughly the height of the Appalachian Mountains crisscross Xanadu. Mountain chains can exceed a kilometre high. It contradicts the expectations of most scientists who thought that Titan harboured hills no higher than 300 meters. Ralph Lorenz, radar team member of the University of Arizona in Tucson, US, says:" These are the highest mountains measured on Titan so far." The rest of the moon appears relatively flat.
Dr Steve Wall, the Cassini radar team's deputy leader at NASA's Jet Propulsion Laboratory, Pasadena, California insists:" This land is heavily tortured, convoluted and filled with hills and mountains." Jonathan Lunine, Cassini interdisciplinary scientist at the University of Arizona, Tucson, adds:" Although Titan gets far less sunlight and is much smaller and colder than Earth, Xanadu is no longer just a mere bright spot, but a land where rivers flow down to a sunless sea."
What are those mountain chains made of ? It is likely that the mountains are not solid. The radio waves bouncing off Xanadu imply that it has peculiar electrical properties - specifically a low dielectric constant. Steve Wall points out:" The only reasonable material makeup that could have a very low dielectric constant and still hold together enough to form the structures that we see would be some sort of porous stuff - most likely porous water ice." He specifies:" There appear to be faults, deeply cut channels and valleys. Also, it appears to be the only vast area not covered by organic dirt. Xanadu has been washed clean. What is left underneath looks like very porous water ice, maybe filled with caverns."
The drainage channels, clearly visible on radar images, are likely carved by liquid methane. The origin of those river networks is undetermined but 2 possibilities are put forward: rain and cryovolcanism. In other words, liquid methane might fall as rain or trickle from springs or geysers. Cassini scientists speculate that these rivers could carry ice grains down to the plains to form the dunes seen on much of Titan's surface. One can note that more channels snake through the eastern part of Xanadu, ending on a dark plain where dunes, abundant elsewhere, seem absent.
Steve Wall said:" In the 1980's, it took the Shuttle Imaging Radar to discover subsurface rivers in the Sahara. Similarly, if it hadn't been for the Cassini radar, we would have missed all of this. We have a newly discovered continent to explore, just like the early explorers of America."
Marc Lafferre concludes:" The highlands of Xanadu unveil a landscape probably comparable to mountain chains on Earth with rivers, caverns, caves or lakes. But it is not made of silicate rocks like on Earth but probably composed of porous water ice, mountains of water ice. And for the rivers, liquid water is likely replaced by hydrocarbon molecules ( methane, ethane...). It is really a weird world, an exotic environment . The mountains are not grey, rather white and bright. And yet, we are not in the fourth dimension, we are on Titan."
Image source: NASA, JPL; The Cassini radar image above was acquired during the flyby of April 30, 2006. It shows the bright region of Xanadu with its highlands, drainage channels and dark patches.
- To get further information on that news, go to: http://www.newscientistspace.com/article/dn9588-titan-may-be-riddled-with-caves.html or http://www.spaceflightnow.com/cassini/060719titan.html
May 6, 2006: Seas of sand and blankets of ice
The infrared images of Titan taken by the VIMS ( Visual and Infrared Mapping Spectrometer ) onboard the Cassini Spacecraft show a clear contrast between a bright area and a dark area on the globe. Planetologists have long speculated that the dark patches might represent an ocean of methane or ethane. But the radar images and the panoramic images of the Huygens probe during its plunge into Titan's atmosphere have gathered data that translate another reality. No lake or sea. The dark area is apparently composed of wet sand.
Several radar images show a multitude of parallel linear strreaks in the dark area ( in infrared ). Radar images taken during the Titan flyby of October 2005 show dunes 330 feet ( 100 meters ) high that run parallel to each other for hundreds of miles at Titan's equator. One dune field runs more than 930 miles ( 1500 km ) long, pointed out Ralph Lorenz of UA's Lunar and Planetary Laboratory. He added:" It's bizarre. These images from a moon of Saturn look just like radar images of Namibia or Arabia. Titan's atmosphere is thicker than Earth's, its gravity is lower, its sand is certainly different - everything is different except for the physical process that forms the dunes and resulting landscapes."
Sand dunes hadn't been expected on Titan because the wind speed was believed to be low. The solar-driven surface winds couldn't be powerful compared to that of Earth due to the little solar energy received. And if the wind is weak, how can he sculpt sand dunes? That's true that a weak wind will have a limited effect in sculpting the landscape but Titan is governed by physical conditions which favour the formation of sand dunes: the gravity is weak, the surface material has a low density and the atmosphere is denser than on Earth. Moreover, Saturn's high gravity creates powerful tides in Titan's atmosphere. Saturn's tidal effect on Titan is around 400 times greater than our moon's tidal pull on Earth.
The dunes unveiled by the radar images of the Cassini orbiter are a particular linear or longitudinal type that is characteristic of dunes formed by winds blowing from different directions. When the tidal wind that moves toward the equator combines with Titan's west-to-east zonal wind, it forms dunes aligned nearly west-east except near mountains that change local wind direction. Ralph Lorenz said:"If you look at the dunes, you see tidal winds might be blowing sand around the moon several times and working it into dunes at the equator. It's possible that tidal winds are carrying dark sediments from higher latitudes to the equator, forming Titan's dark belt."
Regarding the action of the wind on the topography, he added:" Even though this is a very gentle wind, this is enough to blow grains along the ground in Titan's thick atmosphere and low gravity."Even if the composition of Titan's sand is undetermined, we think that the grains of Titan's sand are bigger and less dense than on Earth or Mars. Lorenz pointed out:"These grains might resemble coffee grounds."
Surface wind speed averages around one mile per hour ( a half meter per second ) which is enough on Titan to form sand dunes. The sand dune composition may be determined thanks to the Cassini's Visual and Infrared Mapping Spectrometer. Some scientists speculate it is made of organic solids or water ice. The origin of the sand is unclear. But an hypothesis is put forward: sand may have formed when liquid methane rain eroded particles from ice bedrock. Yet, clouds are rare. So, any rainfall is not frequent. Lorenz said that when it does rain on Titan, it rains in very energetic events, just as it does in the Arizona desert. And he added that energetic rain that triggers flash floods may be a mechanism for making sand. Another possibility is that the sand may come from organic solids produced by photochemical reactions in Titan's atmosphere.
Other hypothesis are also considered and notably the following one: Titan would be covered with a bright and white blanket except in the dark areas close to the equator where the winds are stronger and the temperatures higher. As a result, the west to east winds would have eroded, displaced or removed the ice blanket or "ice-pack".The relatively high temperatures are likely to make the ice evaporate, generating clouds and rain which also acts on the topography. The dark area would be nothing but the wet sand under the ice-pack, as Marc Lafferre notes. The panoramic images captured by the Huygens probe clearly show a contrast of appearance between a dark, reddish area and a bright and white elevated terrain with dark branching channels ( fractures, gullies or rivers ). If the bright area which covers most part of the moon consists of ice, one can wonder for instance if the nature of the ice is the same as that of the surface of Enceladus.
Certain topographic phenomena are consistent with the hypothesis of the ice blanket: bright patches in darks areas ( infrared and radar images ) tend to be stretched in the sense of the wind which means they are displaced by winds. Some circular features, likely impact craters, in bright areas appear dark at their center ( VIMS images ). That involves the hypothesis that the thin ice layer would have been removed by the impact revealing the subsurface layer made of sand.
Lorenz notes:"It's exciting that the radar, which is mainly to study the surface of Titan, is telling us so much about how winds on Titan work. This will be important information for when we return to Titan in the future, perhaps with a balloon."
© NASA/JPL, upper photo and NASA, lower photo; The image above shows two kinds of landscape: in the upper part of the image, one can observe a radar image of Titan with its dark streaks, linear and parallel. The lower part of the images corresponds to the Namibian sand dunes on Earth.
- To get further information on that news, go to: http://uanews.org/cgi-bin/WebObjects/UANews.woa/16/wa/MainStoryDetails?ArticleID=12614
March 4, 2006: An underground source for the atmospheric methane gas is very likely
Data from the ESA's Huygens probe and the Cassini spacecraft have allowed Gabriel Tobie ( Laboratoire de Planetologie et Geodynamique, Université de Nantes, France and LPL University of Arizona, Tucson, USA ), Jonathan I. Lunine ( LPL, University of Arizona, Tucson, USA and IWAF, Rome, Italy ), Christophe Sotin ( Laboratoire de Planetologie et Geodynamique, Université de Nantes, France ) and their teams to build a new model of the evolution of Titan, focusing on the source of Titan's atmospheric methane.
The model suggests that most of the methane supply derives from internal sources. In other words, it's not the remnant of the primitive atmosphere. Gabriel Tobie points out:" This model is in agreement with the observations made so far by both the Huygens probe that landed on Titan on 14 January 2005 and the remote sensing instruments on board the Cassini spacecraft."
Methane is a key compound of Titan's atmosphere, representing up to 5% of its composition. When the Huygens probe landed, the instruments detected a sharp increase in the number of methane molecules. It indicates that methane is concentrated on the soil.
The river-like features identified by the Huygens module as well as the flow-like features unveiled by the VIMS and the radar mapper onboard the Cassini spacecraft reveal what might be the sign of a very active cryovolcanism. Unlike volcanism on Earth which is a silicate volcanism, volcanism or cryovolcanism on Titan would involve ice melting and ice degassing.
There is a methane meteorology on Titan while there is a hydrological meteorology on Earth but most of methane gas is released from underground sources. The problem regarding the methane cycle on Titan is that methane gas is destroyed by ultraviolet light over a timescale of tens of millions of years. Methane should have disappeared from the atmosphere long ago. That implies that something is replenishing it, notably via cryovolcanoes.
On that point, the new model distinguishes three episodes: the first episode followed the accretion and differentiation period in which Titan formed its dense rock core and water mantle beneath an ice crust. Jonathan Lunine explained that the first release of methane was aided by ammonia acting as an anti-freeze, heat leftover from formation, and heat from radioactive elements. He added that much of the methane in the first release might have been reabsorbed into Titan's interior. But whatever was left in the atmosphere had to have been photochemically destroyed during the first billion years.
The second episode of methane release occured around 2 billion years ago when convection started in the silicate core. Jonathan Lunine explained:" The core, made of rock, continued to heat up because it contains natural radioactive elements, like uranium, potassium and thorium. He added, on Earth, these elements are concentrated in the crust , but on Titan, they'd be deep down in the rock. So the core gets hotter and hotter, until finally it's soft enough for convection to start." Convection is the mechanical turnover of material to remove heat. During this second era of methane release, a burst of convection heat was injected into Titan's overlying mantle, causing the ice crust to thin and methane to pour through the ice to the surface.
The last episode which is a geologically recent time occured about 500 million years ago due to enhanced cooling of the moon by solid-state convection in the outer crust.
Titan's methane supply may be stored in a kind of methane-rich ice that scientists call a "clathrate hydrate". That methane rich ice would form a crust above an ocean of liquid water mixed with ammonia. Lunine advances:"We are now in an era where there's enough outgassing to add methane to the atmosphere, but not enough for widespread seas of methane."
Gabriel Tobie points out:"According to our model, during the last outgassing episode, the dissociation of the methane clathrate and hence release of methane are induced by thermal anomalies within the icy crust, which are generated by crystallisation in the internal ocean."He adds:"As this crystallisation started only relatively recently ( 500 to 1000 million years ago ), we expect that the ammonia-water ocean is still present few tens of kilometres below the surface and that methane outgassing is still operating. Even though the outgassing rate is expected to decline now ( it peaked about 500 million years ago ), release of methane through cryovolcanic eruptions should still occur on Titan."
Lunine notes:"Methane outgassing will cease within the next few hundred million years. Then photochemistry will destroy the surface methane and Titan will indeed dry up. The atmosphere will clear of haze, and Titan will look very different."
The model is presented in the 2 March issue of Nature. The article is entitled "Episodic outgassing as the origin of atmospheric methane on Titan".
- To get further information on that news, go to: http://www.spaceflightnow.com/cassini/060301methane.html , http://www.spacedaily.com/reports/Titans_Methane_Mystery_May_Be_Solved.html , http://www.esa.int/esaCP/SEM4BQMVGJE_index_0.html or http://foxtrot.nature.com/nature/journal/v440/n7080/abs/nature04497.html
February 13, 2006: Changing weather in Titan's south polar region !
The Cassini Huygens mission has allowed scientists to make some major breakthroughs in understanding the properties of Titan's enigmatic atmosphere. We now know the pressure and temperature profile of this opaque haze layer thanks to the Huygens probe which plunged into Titan's atmosphere on January 14, 2005. We also know its composition: methane is present in large proportions in the atmosphere, covering around 5% of its composition.
But Titan shows striking similarities with the Earth's atmosphere and notably the primitive atmosphere of planet Earth. It is primariliy composed of nitrogen ( 94% of its composition ) and it presents a significant amount of argon ( around 1% of its composition ). Complex hydrocarbon molecules are found in the upper atmosphere. The ultraviolet radiations break up methane molecules and the new compounds recombine to form other molecules. Hence, one can encounter molecules containing up to 8 carbon atoms. But where does the methane come from ? no incontestable answer, only speculations.
It appears that Titan's atmosphere is governed by a meteorology. This meteorology is unique : it doesn't involve water. It involves another molecule, probably methane ( CH4 ) or ethane ( C2H6 ). The Huygens spacecraft might have encountered a cloud during the 2.5 hour-descent into Titan's atmosphere. And lightning might have been detected. The VIMS ( Visual and Infrared Mapping Spectrometer ) onboard the Cassini spacecraft and Earth-based telescopes have confirmed the presence of sporadic clouds at temperate regions near 40 degrees South. But most clouds are concentrated in the south pole, currently in the summer hemisphere.
The south polar region draws all the attention at the moment: a large cloud formation moving has clearly been identified as well as what might be a lake, hundreds of km wide, not so far away from the cloud blanket. By comparing infrared mosaics of the three latest flybys, one can observe a major evolution of the weather in the south pole. During the flyby of December 26, 2005, the mosaic shows a bright patch in the south pole as well as in the north pole. The bright patch in the south pole is linked to these cloud formations.
On the other hand, the infrared mosaics of the other flybys ( October 28, 2005 and January 15, 2006 ), which unveil roughly the opposite hemisphere from the flyby in December, reveal that the bright spots in the south pole and in the north pole are barely visible. As a result, it is tempting to say that the weather in the south polar region is very dynamic.
The infrared composite images of Titan, above, were acquired during the three latest flybys. The left mosaic corresponds to the flyby of October 28, 2005. The middle mosaic represents the flyby of December 26, 2005 and the right image corresponds to the flyby of January 15, 2006. The mosaic of the December flyby clearly shows a bright patch in the south pole and in the north pole. The other mosaics representing the opposite hemisphere don't show this bright patch. It has been shown that the bright patch in the south pole is associated with cloud formations.
Image source: NASA, JPL
A European team, led by Pascal Rannou of the Service d'Aeronomie, IPSL Université de Versailles-St Quentin, France, has developed a general circulation model to account for the meteorological phenomena observed on this orange moon and to predict their evolution. Their general circulation model couples dynamics, haze and cloud physics to study Titan climate. The model enables scientists to predict the cloud distribution for the complete Titan year ( 30 terrestrial years ). The Voyager missions of the early 1980's had provided indications that methane or ethane molecules were likely to form clouds. It was assumed that the organic molecules assembled in the upper atmosphere through the action of ultraviolet light would sink and condense into clouds. The evaporated methane from the surface would also condense at certain altitudes to form clouds. The temperatures on Titan make the formation of methane clouds possible: the temperature at the surface is close to the triple point of methane ( -180°C or -292°F ) and at an altitude of 40 km, the temperature can drop as low as -200°C ( -328°F ). So, methane molecules are likely to cool and condense while rising into the sky during the evaporation process.
Since the Voyager missions, several one-dimensional models of Titan's atmosphere including sophisticated microphysics models were constructed to predict the formation of drops of ethane and methane. The methane cycle had also been studied separately in a circulation model, but without cloud microphysics. It turned out that methane clouds could appear when air parcels cooled while moving upward or from equator to pole.
Rannou's team, thanks to their general circulation model, can now identify and explain the formation of several types of ethane and methane clouds, including the south polar and sporadic clouds in the temperate regions, especially at 40 degrees South in the summer hemisphere. The physical properties of the clouds in their model are consistent with recent observations. They predict ascending air motions where methane clouds have been observed. The observed south polar cloud formations appear at the top of a particular "Hadley cell", or mass of vertically circulating air, exactly where predicted at the south pole at an altitude of around 20-30 km. The recurrent large zonal ( longitudinal direction ) clouds at 40 degrees South and the linear and discrete clouds appearing in the lower latitudes are also related to Hadley cell in the troposphere, while smaller clouds at low latitudes are rather produced by mixing processes. Pascal Rannou points out: "Consistently, our model produces clouds at places where clouds are actually observed, but it also predicts clouds that have not, or not yet, been observed." The intriguing clouds at 40 degrees South are generated by the ascending branch of a Hadley cell, exactly like tropical clouds are in the Intertropical Convergence Zone ( ITCZ ).
Polar clouds engendered by polar cells are identical with those generated at mid-latitudes on Earth. Clouds only take shape at some longitudes. It may be due to a Saturn tidal effect. Cloudiness prediction for the coming years will be compared to observations made by Cassini and ground-based telescopes. The "General Circulation Model" of Pascal Rannou and his team is presented in the January 2006 issue of Science, titled "The latitudinal distribution of clouds on Titan".
The question of the origin of Titan's clouds implies the question regarding the origin of Titan's atmosphere. Some scientists suggest that it might be linked to cryovolcanism or ice volcanoes. Two bright patches at mid-latitudes examined in the latest flybys might represent surface deposits resulting from eruptions. The December flyby data reveal that the western margins of Tui Reggio, the biggest bright patch, have a complex flow-like character, probably deriving from an icy volcano. Does it produce the observed clouds? Does it regenerate the atmosphere with nitrogen, methane or ammonia?
- To get further information on that news, go to: http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=1994 and http://www.spaceflightnow.com/cassini/060123titanclouds.html
To have access to Titan news of 2005 and 2004, click on the link below: