Titan News 2012

 

April 26, 2012: The Lake Ontario Lacus on Titan May be Drier than Expected and Shows Similarities with the Etosha Mudflat in Namibia

Planetary scientists have long suspected the existence of oceans of hydrocarbons such as methane, ethane or propane on Saturn's largest moon Titan. The near-infrared views obtained with the "Visual and Infrared Mapping Spectrometer" of the Cassini spacecraft which arrived in the Saturn System in mid-2004 clearly revealed dark or low-albedo regions in the low latitudes of the Orange Moon and bright or highly reflective regions at higher latitudes.

This landscape contrast had led scientists to advance the hypothesis of oceans of methane or ethane in the low latitudes of the Opaque Moon where dark areas dominate. This hypothesis was shared, for instance, by the French astronomer André Masclet. However, on January 14, 2005, the Huygens probe which plunged into the atmosphere to land into the Shangri-La/Adiri region unveiled a relatively dry landscape with dried-up drainage channels on the bright hills of Adiri.

The images taken from the landing site, located in the low latitudes of the moon, revealed what might be a dried-up river made of pebbles or eroded stones. Later, in 2005, the radar images obtained with the Radar Mapper of the Cassini probe revealed strange linear and parallel features in the dark regions of the low latitudes of Titan. Planetary scientists called it "Cat Scratches".

Thanks to the radar images, we know, now, that the dark regions of the low latitudes of the Orange Moon are dominated by Seif dunes or linear and parallel dunes which extend over long distances. Those Seif dunes appear to be shaped by prevailing winds. They can be deflected by topographic obstacles or bright features. They are reminiscent of the Seif dunes in the Namib desert in Namibia.

The first pool of liquids clearly identified on Saturn's largest moon was found in the high latitudes of the southern hemisphere, on the basis of a near-infrared image taken in 2005. This kidney-shaped lake was named Ontario Lacus. Later, the radar images brought clearer data on the shape and the characteristics of the lake.

A radar image of the lake was captured, for instance, on January 12, 2010 with the radar instrument of the Cassini spacecraft. This image shows that Ontario Lacus covers an area approximately 140 by 47 miles or 230 by 75 kilometers. Some dark drainage channels can be noticed and the depth of the lake seems to vary.

The southern portion of the dark Ontario Lacus appears to be less humid than the northern portion of the lake since reflective areas can be well discerned in the southern portion of the depression. As planetary scientists suggest, the southern boundary of the depression unveils strong evidence for long-standing channels carving the lake bed.

A new study led by Thomas Cornet of the Université de Nantes, France, a Cassini associate confirms the idea that the dark Ontario Lacus may not be completely filled with liquid hydrocarbons such as ethane and methane. The study entitled "Geomorphological significance of Ontario Lacus on Titan: Integrated Interpretation of Cassini VIMS, ISS and RADAR data and comparison with the Etosha Pan (Namibia)" is presented in the volume 218, issue 2 of Icarus in April 2012.

According to Thomas Cornet and his team, Ontario Lacus lies in an arid or semi-arid zone, in a flat basin similar to the Etosha Basin, where the Etosha pans are found and it is only partially covered by liquids, contrary to what had been previously thought. The liquids of the lake may engender dissolution processes within the depression which may lead to an enlargement of the lake.

The area may be saturated with liquid hydrocarbons such as ethane, methane or even propane and the depression may drain and refill from below, exposing liquid portions ringed by materials like saturated sand or mudflats. Thomas Cornet claimed: "We conclude that the solid floor of Ontario Lacus is most probably exposed in those areas."

The team of Thomas Cornet draws a parallel between Ontario Lacus and the Etosha salt pan on Earth, in Namibia which is mostly dry. The Etosha salt pan is a lake bed that can be fed, during the rainy season with a shallow layer of water that emerges from the ground or the underground. The evaporation processes of this layer of water leave sediments like tide marks revealing the previous extent of the lake.

Bonnie Buratti, a co-author of the paper and a Cassini team member based at NASA's Jet Propulsion Laboratory in Pasadena, California advanced: "Some of the things we see happening in our backyard are right there on Titan to study and learn from."..."On Earth, salt pans tend to form in deserts where liquids can suddenly accumulate, so it appears the same thing is happening on Titan."

Saturn's largest moon reveals a complex cycle of hydrocarbons which may be intimately linked to seasonal factors. The radar, infrared or near-infrared data have allowed us to determine  that the liquid bodies are mostly found in the high latitudes of the northern hemisphere and the southern hemisphere.

Dynamic cloud systems have been observed in the area of Ontario Lacus implying precipitation and evaporation processes. A giant ethane cloud system engulfing the north polar region was also found in 2006. The largest concentration of lakes, seas and rivers appears in the high latitudes of the northern hemisphere.

This asymmetry in the distribution of liquid bodies between the northern hemisphere and the southern hemisphere may be explained by orbital parameters and seasonal factors which influence the amount of solar energy received by each hemisphere. A Titanian year lasts approximately 30 Terrestrial years and each season lasts about 7 Terrestrial years.

The Summer season in the southern hemisphere ended in August 2009 with the equinox which marked the start of the Autumn season. This equinox marked the end of the Winter season and the beginning of the Spring season in the northern hemisphere. Will the level of the liquid of Ontario Lacus rise in the coming years as the Winter season in the southern hemisphere is approaching?

A study entitled "Shoreline retreat at Titan's Ontario Lacus and Arrakis Planitia from Cassini Imaging Science Subsystem observations", proposed by Elizabeth P. Turtle and her team (J.E. Perry, A.G. Hayes and A.S. McEwen) and appearing in the Volume 212, Issue 2 of Icarus in April 2011 showed that observations from the Imaging Science Subsystem of the Cassini spacecraft reveal that a portion of the shoreline of Ontario Lacus had retreated by several kilometers.

On that basis, one may argue that the area may have undergone less precipitation than evaporation between the observations from the Cassini spacecraft. However, there may be absorption processes through the floor of Ontario Lacus similar to those found, for instance, in the plateau of Calern near the village of Caussols in the south east of France.

Therefore, the hydrocarbon cycle based on methane, ethane and even propane may be the outcome of a complex interaction between the atmosphere, the surface and the subsurface. What are the properties of the floor which covers the Ontario Lacus depression? Is it porous? Is the subsurface saturated with ethane or methane? Are there dissolution processes?

A comparative analysis between lakes, humid areas or landscape features on Earth and on Titan allows us to put forward strong hypotheses on the characteristics of the topographic features we observe on Titan via the VIMS or radar data.

Nicolas Altobelli, ESA's Cassini-Huygens project scientist pointed out: "These results emphasize the importance of comparative planetology in modern planetary sciences: finding familiar geological features on alien worlds like Titan allows us to test the theories explaining their formation."

Titan remains a relatively dry world. Very few clouds have been identified in the equatorial and tropical regions of the Opaque Moon and the largest concentrations of clouds have been found in the polar regions, especially in the high latitudes of the northern hemisphere. The dark low-latitudes seem to be devoid of lakes, seas or rivers and are dominated by linear and parallel dunes extending over long distances.

The cloud cover of the Earth generally exceeds 50% but on Titan, this cloud cover likely represents less than 1% as the VIMS observations suggest. Ontario Lacus is probably located in one of the most humid areas on Titan but it may lie in a relatively dry area in terms of Terrestrial standards.

As a result, a parallel is drawn between the Etosha Pan located in the Kalahari Basin in the north of Namibia  and Ontario Lacus. The Etosha Pan is a mudflat representing a surface area of about 75 by 40 miles or 120 by 65 kilometers. Ontario Lacus has probably closer characteristics to the Etosha Pan, the Dead Sea, Lake Chad or the Aral Sea than Lake Ontario in North America, in terms of depth and precipitation.

 

- To get further information on that news, go to: http://saturn.jpl.nasa.gov/news/cassinifeatures/feature20120419, http://www.sciencedirect.com/science/article/pii/S0019103512000280 and http://www.sciencedirect.com/science/article/pii/S0019103511000546.

 

 

March 31, 2012: Cassini Observations Suggest Very Infrequent Rainfall on Titan

Since the arrival of the Cassini/Huygens spacecraft in the Saturn System in mid-2004, the landscape, the climate and the meteorology of the Opaque Moon have been intensively studied on the basis of multiple instruments of the Cassini probe and the Huygens probe.

The radar data allows us to observe the topography of Titan. The near-infrared data allows us to identify landscape features as well as clouds. The observations reveal surprising similarities between the environment of Saturn's largest moon and the environment of the Earth.

The Huygens probe which landed in the Shangri-La/Adiri area on January 14, 2005 revealed dark drainage channels on bright hills during its atmospheric descent. The probe probably landed on a dried-up river. The images obtained from the surface unveiled pebbles or eroded stones around the probe.

The VIMS (Visual and Infrared Mapping Spectrometer) data as well as the radar data acquired with the radar mapper of the Cassini spacecraft have clearly revealed mountains, rivers, lakes, seas, islands or canyons. It appears that the liquid bodies are concentrated in the high latitudes or the polar regions of the moon.

Ontario Lacus was the first lake or sea clearly identified on Titan in 2005 thanks to the near-infrared data of the VIMS onboard the Cassini spacecraft. Later, in 2006, the radar data showed a pleiade of lakes, seas and rivers in the high latitudes of the northern hemisphere.

Kraken Mare, Ligeia Mare and Punga Mare appear to be the most famous bodies of liquids identified in the high latitudes of the northern hemisphere. Likewise, the cloud systems are mostly found in the polar regions.

Dynamic cloud systems have been found in the area of Ontario Lacus during the Summer season in the southern hemisphere. A giant ethane cloud system engulfing the north polar region was also found in 2006 during the end of the Winter season in the northern hemisphere.

The meteorology of the Orange Moon appears to be dominated by methane and ethane. The atmosphere of Titan is mostly composed of molecular nitrogen which represents approximately 98% of its composition and methane can reach up to 5% of its composition. There is a methane cycle between the ground and the atmosphere which shows striking similarities with the water cycle observed on the "Blue Planet".

There are rainfalls, evaporation processes, clouds, ponds, lakes, rivers or seas. Those processes are closely related to a limited amount of energy on the soil and in the atmosphere. The Huygens probe recorded a surface temperature of about -179 degrees Celsius, -290 degrees Fahrenheit or 94 Kelvin. At those temperatures, water will appear in its solid form, as water ice, and methane and ethane can be found in their liquid form.

The distribution of lakes, seas and rivers on Titan may be closely related to seasonal factors. As the Winter season approaches in the southern hemisphere, the amount of clouds may increase engendering more precipitation and the level of the lakes, seas and rivers may increase.

At the moment, the north polar region appears to be the most humid area on Titan with the biggest known body of liquid, Kraken Mare. This asymmetry in the distribution of lakes and seas on the Orange Moon may also be explained by orbital parameters such as the inclination of the rotation axis and the variation of the distance of the moon to the Sun. These factors will strongly influence the amount of energy reaching the soil.

The equatorial and tropical regions, dominated by Seif dunes or parallel dunes extending over long distances appear particularly dry. No lakes or rivers have been clearly identified in the low latitudes even if multiple fractures or dried-up channels have been observed in the radar images of the low-latitude region.

Doctor Ralph Lorenz and Elizabeth P. Turtle from the Johns Hopkins Applied Physics Laboratory  in Laurel, Maryland demonstrated, during the 43rd Lunar and Planetary Science Conference in 2012 that the mean cloud cover on Titan is particularly weak and that precipitation may be extremely rare in some parts of the Orange Moon such as in the equatorial and tropical regions.

In the low latitudes of Titan, it may periodically rain for brief periods of time but it may be similar to monsoon events on Earth with very strong rainfall which can feed or engender deep lakes or rivers.

Large cloud systems have been recently observed in the low latitudes of Titan in 2010, a little after the Spring Equinox in the northern hemisphere. Those cloud systems may have generated rainfalls of methane or ethane as the near-infrared images of the areas suggest. Some regions appeared darker or brighter after the storm. Those cloud systems forming and moving in the low latitudes may be explained by seasonal changes.

The drier part of Titan may receive rainfall every 1,000 years, roughly speaking. If the typical interval between rain storms on Earth is around one week, the typical interval between rain storms on the Orange Moon may be between 100 and 1000 years. The average cloud cover of our Blue Planet is comprised between 50% and 65% whereas the cloud cover of Titan is approximately 0.3% on the basis of a ground-based spectroscopic monitoring campaign studying the atmosphere on 138 nights over 2.2 years.

Observations from the Keck telescope on 16 nights during the 2001-2003 period revealed a cloud coverage ranging from 0.2 to 0.6%. In fact, it rains only between 0.01% and 0.1% of the time. But the probability of rain in the south polar region and in the north polar region may be significantly higher than in the equatorial and tropical region.

A new mission to Saturn's largest moon is envisaged by scientists for the near future. A mission called TiME or Titan Mare Explorer corresponding to the category of NASA Discovery missions would be launched for a landing, in 2023, in the north polar lake Ligeia Mare. The parachuted probe would likely spend approximately 1 hour descending through the troposphere of the moon.

The probability of encountering a rainstorm for the probe would be lower than 0.2%. The north polar region will be in the Summer period in 2023 at the time of the expected landing.

The TiME probe, generated by nuclear power, would spend 96 days or 6 Titan days on Ligeia Mare. The probe would benefit from a full diurnal cycle at Ligeia Mare since the Sun will be positioned between +20 degrees and -6 degrees of elevation.

Will the probe detect or see rain? The probability of rain hitting the lander may be around 50% thanks to the relatively long duration of the mission. The probe may operate during 2500 hours or more than 100 Terrestrial days. The camera of the probe could identify nearby rainfall approximately 5 times during the campaign. The spacecraft will be able to see cloud formation, rain shafts and even methane rainbows.

If we assume that the rain shafts are 10 km wide and that they are observable at distances of about 20 km, then the area observable is a circle of approximately 1,200 km². That's why the chance of finding rainfalls is relatively high. The estimates unveiled by Ralph Lorenz and Elizabeth Turtle  suggest that regions near the poles receive rainfall between 10 and 100 hours every Titanian year which corresponds to almost 30 Earth years.

How volatile are the lakes, seas and rivers in the north polar region? As the Spring season advances in the northern hemisphere, the great lakes or seas Kraken Mare and Ligeia Mare come to light allowing near-infrared observations which enable Cassini scientists to monitor the size and the evolution of the liquid bodies as well as the dynamics of cloud systems in the area. Kraken Mare and Ligeia Mare may appear smaller or see their level diminish during the next Summer season in the northern hemisphere of Titan.

Due to the complexity of the climate and meteorology of Titan and due to the length of a Titanian season (around 7 Terrestrial years) or a Titanian year (around 30 Terrestrial years), scientists need to continue gathering meteorological or atmospheric data in the coming years to improve their climate and meteorological models of the Orange Moon. The destination of the future lander of Titan may change depending on the evolution of the meteorological processes in the coming years.

The near-infrared view above, taken with the VIMS instrument of the Cassini probe on September 14, 2011,  reveals some surface features of Saturn's largest moon Titan. The low latitudes tend to be dominated by Seif dunes or linear and parallel dunes extending over long distances in the low-albedo areas. The dark area of the north polar sea Kraken Mare can be observed in the upper part of the disc. The bright areas are likely devoid of any liquid bodies. The equatorial and tropical regions may be relatively dry due to very infrequent rainfall. Image Credit: NASA, JPL-Caltech/Space Science Institute

The artistic view above represents the probe of a project, called Titan Mare Explorer or TiME, in the north polar lake Ligeia Mare of the Orange Moon. This probe could be sent for an expected landing in 2023 into this enigmatic lake. Image Credit: TSSM NASA/ESA joint summary report

The image above corresponds to an artistic view of a hypothetical probe floating in one of the largest bodies of liquids in the north polar region of Titan, Punga Mare. One can notice a foggy environment and a relatively quiet lake with small waves. The depth of the lake, likely made of hydrocarbons such as methane and ethane is very low here. One can clearly discern the bottom of the lake in this view, close to the shoreline. The probe emits a powerful light to explore the area which receives a relatively low amount of solar radiation during this period of autumn. Image Credit: Marc Lafferre, 2012.

- To get further information on that news, go to: http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2472.pdf, http://www.universetoday.com/94321/rare-rain-on-titan-once-every-1000-years and http://www.astrobio.net/pressrelease/4651/rare-rains-on-titan.

          

 

February 25, 2012: A New Study Shows North Polar Clouds On Titan Thinning Out From Winter To Spring

Planetary scientists focus their attention on the study of the geology, climate, meteorology and atmosphere of Saturn's largest moon Titan because Titan is a complex world revealing striking similarities with the Blue Planet.

We've identified mountains, canyons, lakes, seas, rivers and massive clouds on the Opaque Moon. The methane cycle on Titan is reminiscent of the water cycle on the Earth. The atmosphere of Titan is reminiscent of the atmosphere of the Early Earth. The chemistry of organics and hydrocarbons on Titan is intriguing because it can lead to the formation of complex molecules.

Therefore, the Orange Moon Titan is of astrobiological interest. The radar and near-infrared data have revealed lakes, seas and rivers in the high latitudes of the northern hemisphere and the southern hemisphere.

Dynamic clouds have been clearly identified in the south polar region in the area of Ontario Lacus, the largest lake unveiled in the high latitudes of the southern hemisphere. Likewise, a high concentration of ethane clouds has been found in the high latitudes of the north polar region as soon as December 2006.

The polar regions may undergo strong evaporation and precipitation processes, regularly. Can the wet polar regions harbor a lifeform based on methane, ethane or hydrocarbons?

A series of papers relating to the characteristics of the Orange Moon has just been released in the journal Planetary and Space Science in a special issue titled "Titan through Time: Formation, evolution and fate". The special issue focusing on Titan was presented by Ralph D. Lorenz, a Cassini team scientist based at the Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland and Conor A. Nixon, a Cassini team scientist at the NASA Goddard Space Flight Center, Greenbelt, Maryland.

Conor A. Nixon advanced: "As a whole, these papers give us some new pieces in the jigsaw puzzle that is Titan."..." They show us in detail how Titan's atmosphere and surface behave like Earth's -with clouds, rainfall, river valleys and lakes. They show us that the seasons change, too, on Titan, although in unexpected ways."

In the special issue on Titan which is published in the volume 60, issue 1 of the January 2012 "Planetary and Space Science", Dominic Fortes, an outside researcher based at University College London, England proposes a model of the internal structure of Titan in a paper entitled "Titan's internal structure and the evolutionary consequences".

Valeria Cottini, a Cassini associate based at Goddard, in collaboration with Conor Nixon and other colleagues, proposes a study on the spatial and temporal variations in the surface temperature of Saturn's largest moon in a paper entitled "Space ans temporal variations in Titan's surface temperatures from Cassini CIRS observations".

Stéphane Le Mouelic, a Cassini team associate at the French National Center for Scientific Research (CNRS) at the University of Nantes, in collaboration with Pascal Rannou, Christophe Sotin and other colleagues shows, on the basis of infrared or near-infrared images, that the cloud system in the north polar region vanished progressively as the moon approached equinox in 2009 in a paper entitled "Dissipation of Titan's north polar cloud at northern spring equinox."

A giant ethane cloud system engulfing the north polar region was observed from the Visual and Infrared Mapping Spectrometer of the Cassini spacecraft as soon as December 2006. At that time, the north polar region was partly illuminated and the cloud system appeared to cover the north polar region completely down to approximately 55 degrees north latitude.

The subsequent infrared or near-infrared images of the area showed that the cloud system progressively vanished as the Orange Moon approached equinox in 2009. The cloud system was first identified during the Winter period in the northern hemisphere and the equinox which marked the end of the Winter period and the start of the Spring season in the northern hemisphere occured in August 2009. As the clouds dissipated or thinned out, planetary scientists were able to observe the surface of the area including the largest body of liquids identified so far, Kraken Mare.

The global circulation model of Rannou et al. had predicted a decrease of cloud coverage in northern latitudes in the same period. The study of Stéphane Le Mouélic shows that the clouds are observed  at an altitude between 30 and 65 km, on the basis of a radiative transfer model in spherical geometry.

Stéphane Le Mouélic pointed out: "Snapshot by snapshot, these images give Cassini scientists concrete evidence that Titan's atmosphere changes with the seasons."..."We can't wait to see more of the surface, in particular in the northern land of lakes and seas."

Valeria Cottini reveals, in her study, that there are variations in surface temperature over time and that the surface temperature is closely related to latitude and the time of the Titanian day. The temperatures are determined on the basis of the radiance through a spectral window in the thermal infrared at 19 µm characterized by lower atmospheric opacity.

The team modeled the Cassini Composite Infrared Spectrometer (CIRS) far infrared spectra obtained in the period 2004-2010 - The team determined a systematic decrease from the equator toward the poles- The decrease is of about 1K at 60 degrees south and of about 3K at 60 degrees north.

The asymmetric decrease between the northern hemisphere and the southern hemisphere is related to differences in seasons between the two hemispheres. From 2004 to August 2009, the northern hemisphere was experiencing the Winter period whereas the southern hemisphere was experiencing the Summer period. The scientists were able to determine small seasonal changes of up to 2K at 60 degrees North from Winter to early northern Spring.

The scientists also determined diurnal changes in the surface temperature around the equatorial regions of approximately 1.5K. The temperatures tend to increase from the morning to the early afternoon and to decrease during the night. Let's recall that the Titanian day lasts approximately 16 Earth days and that the Huygens probe which landed in the Adiri/Shangri-La region on January 14, 2005 recorded a surface temperature of about 94K, -179.15 degrees Celsius or -290 degrees Fahrenheit and a surface pressure of about 1,467 hPa.

Valeria Cottini advanced: "while the temperature difference -1.5 Kelvins- is smaller than what we're used to on Earth, the finding still shows that Titan's surface behaves in ways familiar to us earthlings."..."We now see how the long Titan day (about 16 Earth days) reveals itself through the clouds."

Dominic Fortes constructed an array of models on the internal structure of the Orange Moon. He compared the models with the gravity data from the magnetometer and from the radio science experiment of the Cassini probe.

It appears that the estimated moment of inertia of Titan is 0.342 on the basis of the quadrupole gravity field measured by the Cassini probe. It implies a partially differentiated internal mass distribution or a fully differentiated internal mass distribution. The hypothesis of a metallic core is ruled out. The interior may be relatively wet and cool. The densest portion of the moon is the core which may be less dense than expected.

The core may be composed of mixtures of silicon and water forming minerals of relatively low density. The core may be made of hydrous silicates or a mixture of ice with anhydrous silicates. The core may be relatively cool with a temperature ranging from 500K to 800K. The density of the core may be between 2460 kg m-3 and 2570 kg m-3. The radius of the core may be between 1980 and 2120 km.

The core may be covered  by a high-pressure ice. A global subsurface ocean may be found between the high-pressure ice and the crust or the outer shell. The surface may be rich in organics. The subsurface ocean may be composed of liquid water. Let's recall that scientists also predict a subsurface ocean of water inside the icy moon Enceladus. Jonathan Lunine had hypothesized that the subsurface ocean may incorporate liquid ammonia.

The presence of methane and argon-40 in the atmosphere of the Opaque Moon is quite hard to explain because methane tends to be broken down over time under the action of UV light from the Sun. Is there an internal source for the atmospheric methane? No clear cryovolcanic sign of methane eruptions has been identified so far. Methane and Argon-40 don't seem to be able to escape from the core which strengthens the paradox of their presence in the Titanian atmosphere.

The mosaic of false-color images above based on VIMS data from 2006 to 2009 reveals the evolution of the cloud system in the north polar region of Saturn's largest moon Titan. The cloud systems appeared to be thinner or to dissipate in the transition period between the Winter season in the northern hemisphere and the the Spring season in the northern hemisphere. The Spring Equinox occured in August 2009. Image credit: NASA/JPL-Caltech/University of Arizona/CNRS/LPGNantes

The mosaic above unveils pairs of images of the cloud system in the north polar region of the Orange Moon, obtained from 2006 to 2009 with the VIMS of the Cassini probe. One can notice that the clouds seem to dissipate over time as the Spring season in the northern hemisphere approaches. Image credit: NASA/JPL-Caltech/University of Arizona/CNRS/LPGNantes/SSI

The artistic view above describes the internal structure of the Orange Moon on the basis of data obtained from the Cassini spacecraft (Radar data, VIMS data, magnetometer data...) and on the basis of the work of Dominic Fortes who developed a series of models to account for the internal structure of Titan. The core may be composed of hydrous silicate and an internal ocean of liquid water may exist between the crust and the high-pressure ice beneath it. Image credit: A.D. Fortes/UCL/STFC.

- To get further information on that news, go to: http://saturn.jpl.nasa.gov/news/cassinifeatures/feature20120223. 

 

 

February 2, 2012: The Size of Titan's Dunes is Intimately Related to Latitude and Altitude Parameters

Since the arrival of the Cassini/Huygens spacecraft in the Saturn System in 2004, the Radar Mapper and the Visual and Infrared Mapping Spectrometer of the Cassini probe have allowed planetary scientists to discern and analyze landscape features on the surface of the "Opaque Moon" Titan.

A contrast between bright and dark areas has been well identified by planetologists on the basis of near-infrared images obtained with the VIMS instrument of the Cassini spacecraft.

It appears that the dark areas are concentrated in the low latitudes of the Orange Moon. The bright areas dominate the high latitudes. Icy materials such as water ice or frozen carbon dioxide may be widespread in the bright areas. As on Earth, mountain chains can be found in the bright regions such as the Continent-sized Xanadu which can undergo tectonic movements or quakes.

Bright areas are less abundant in the low latitudes. The bright Adiri of the equatorial and tropical region was imaged from the Huygens probe during its atmospheric descent into the Shangri-La/Adiri region on January 14, 2005. It revealed topographic fractures or drainage channels which seemed dried up suggesting occasional rainfall.

The Huygens probe apparently landed close to the bright Adiri in a dried-up river in the Shangri-La region. The soil and the sky appeared orange and eroded stones or pebbles appeared in the foreground.

The radar instrument of the Cassini spacecraft has allowed scientists to gather crucial information on the dominant topographic structures of the dark areas and on meteorological phenomena such as the direction of prevailing winds.

The radar images clearly reveal the famous "Cat Scratches" which correspond to Seif Dunes or linear and parallel dunes extending over long distances. Those dune fields are found in the dark areas of the low latitudes. Thus, the dark Shangri-La, the dark Fensal/Aztlan or the dark Belet are dominated by Seif Dunes or parallel and linear dunes shaped by prevailing winds.

A new study led by Alice Le Gall, of LATMOS-UVSQ, Paris and NASA-JPL, California reveals that the size of the dunes and the structure of the dune fields are related to altitude and latitude factors.

An image comparison was performed between dunes in the Belet region, dunes in the Fensal region, dunes in Oman on Earth and the Kalahari dunes. The size and spacing of the dunes in the Belet region appear different from the size and spacing of the dunes in the Fensal area. The Fensal area is made of narrower dunes, lower altitude dunes with a higher spacing between the dunes. The gaps between the dunes are brighter in the Fensal area implying a thinner covering of sand.

The Fensal region is higher in altitude and in latitude than the Belet area. One can infer, from the radar data, that the amount of sand grains is lower at higher latitudes and higher altitudes and that the dunes tend to become narrower, lower and more widely separated at higher latitudes and altitudes, because there is a lower amount of sediments available in those regions.

The size, the shape and the direction of the dunes provide us with significant clues on the geology of the area and on the climate and meteorology of the region. One can observe in the radar views that prevailing winds have a strong power and that they shape the topography, generating linear and parallel dunes which can be deflected by bright topographic obstacles made of a different material.

Scientists have determined that dune fields represent approximately 13% of the surface of Saturn's largest moon, stretching over an area of 10 million square kilometers or 4 million square miles. That corresponds to about the equivalent of the surface area of the United-States, Canada or China.

The Titanian dunes which resemble the Terrestrial dunes found in Namibia (Namib Desert) or in the Arabian Peninsula are on average 1 to 2 kilometers wide or 0.6 to 1.2 miles wide, hundreds of kilometers or miles long and approximately 100 meters or 300 feet high, their size and spacing varying as a function of the altitude and the latitude.

The chemical composition of the Titanian sand appears exotic. On Earth, the dunes are dominated by silicates (SiO2...) but on Titan, the dunes may be dominated by solid hydrocarbons that precipitate out of the atmosphere. They may incorporate the famous tholins imagined by the astronomer Carl Sagan. The exact nature of the hydrocarbons is unknown but the dunes may be made of molecules such as acetylene and benzene. The hydrocarbons from the atmosphere may have aggregated into millimetre-sized grains or grains 0.04 inch in size.

The dunes appear to be confined to its equatorial area, in a band between 30 degrees south and 30 degrees north. The higher latitudes appear to be less dry or to have a higher level of moisture. The VIMS instrument of the Cassini probe has revealed, via near-infrared images, that the high latitudes of the southern hemisphere can reveal dynamic cloud formations which are likely to engender methane/ethane rain and that the high latitudes of the southern hemisphere can harbor lakes or seas. The dark "Ontario Lacus" was the first body of liquids clearly identified on Titan. Ontario Lacus is located in the high latitudes of the southern hemisphere.

Likewise, the high latitudes of the northern hemisphere host a myriad of lakes, seas and rivers. The pools of liquids appear to be mostly concentrated in the high latitudes or in the polar region of Titan's northern hemisphere. The dark Kraken Mare is so big that it can be regarded as a sea. Ligeia Mare and Punga Mare are also well known lakes or seas found in the high latitudes of the northern hemisphere.

No dune fields have been identified in the polar areas of Titan. The soil may be too wet in those areas due to relatively frequent storms or rainfall events which fuel the lakes, seas and rivers of methane or ethane. As a result, the sand particles will be heavier and less mobile. The drier the sand grains, the more easily they can be displaced or transported by the winds to generate dunes.

Doctor Alice Le Gall advanced: "As one goes to the north, the soil moisture probably increases, making the sand particles less mobile and, as a consequence, the development of dunes more difficult."

Nicolas Altobelli, ESA's Cassini-Huygens project scientist pointed out: "Understanding how the dunes form as well as explaining their shape, size and distribution on Titan's surface is of great importance to understanding Titan's climate and geology."

The formation of clouds in the low latitudes of the Orange Moon appears particularly rare on the basis of the near-infrared images taken with the VIMS instrument of the Cassini spacecraft. However, unusual, large cloud systems were identified in 2010 a little after the equinox which occured in August 2009. The variations in the amount of solar energy reaching the soil, related to seasonal changes and orbital distances may have a strong impact on the meteorology of the equatorial or tropical regions.

The low-latitude regions may have recently received precipitations or rainfall of hydrocarbons (methane or ethane). Alternating periods of precipitation and dryness implying evaporation and erosional processes may have favoured the development of sand dunes in the dark areas of the low latitudes over time.

The climate and meteorology of Titan are relatively complex because the obliquity of the moon is relatively high with a tilt of the rotation axis of about 27 degrees relative to the normal of the orbital plane around the Sun. Moreover, the orbit of the Saturn System around the Sun is elliptical with an aphelion which is 12% farther from the Sun than the perihelion which implies that the potential amount of energy reaching the soil can be 20% weaker at aphelion than at perihelion.

The Titanian year is roughly 29.5 years long with seasons that last for about 7 Earth years. The Summers in the southern hemisphere are shorter and more intense than Summers in the northern hemisphere due to the fact that the Orange Moon moves closer and faster to the Sun during the Summer period in the southern hemisphere. That may explain the asymmetry in the distribution of lakes and seas between the north polar region and the south polar region.

As the Winter period approaches in the southern hemisphere, there may be more and more precipitation in the high latitudes of the south polar region in the coming years which may engender new lakes, seas and rivers.

Further analyses are needed to correctly determine the methane/ethane cycle in the low latitudes of the Opaque Moon. Will there be monsoon events or storms in the area? Will the Seif dunes of the low latitudes disappear under the action of strong methane or ethane precipitation?

The Cassini spacecraft has examined the geology, the climate and the meteorology of the moon for about seven Earth years which is only the equivalent of a Titanian season. But scientists are now in a position to improve their climate models and to make predictions.

Several parallels or similarities can be established between the geological features on Earth and on Titan and between the methane/ethane cycle and the water cycle on Earth. As on Earth, there are precipitation and evaporation processes, mountains, rivers, lakes and seas and the dune fields are reminiscent of dune fields in the Namib Desert.

Nicolas Altobelli explained: "As their material is made out of frozen atmospheric hydrocarbons, the dunes might provide us with important clues on the still puzzling methane/ethane cycle on Titan, comparable in many aspects with the water cycle on Earth."

The dunes of hydrocarbons of the "Paradise of Hydrocarbons" may have undoubtedly represented a gold mine for our oil companies if it had taken shape on our Blue Planet because the resources seem almost illimited up there.

 

The mosaic above shows some similarities between dune fields on Earth and on Titan. One can notice that the dunes in the Fensal area appear narrower and less high than the dunes in the Belet area which is found at a lower latitude and where there is a higher amount of sand. The Oman dunes and the Kalahari dunes also reveal linear and parallel structures or Seif dunes. Image source: NASA/JPL-Caltech, and NASA/GSFC/METI/ERSDAC/JAROS and U.S./Japan ASTER Science Team

- To get further information on that news, go to: http://www.esa.int/esaCP/SEMX5NH8RXG_index_0.html and http://astrobio.net/pressrelease/4504/titans-giant-dunes.

    

 

January 21, 2012: Meteorological Phenomena on Titan Explained by a New Three-Dimensional Atmospheric Model which Brings Predictions

The "Orange Moon" Titan is an exotic world which presents striking similarities with our "Blue Planet". Like the Earth, Titan is covered with a significant atmosphere comprising dynamic clouds. However, Titan orbits farther than the Earth from the Sun receiving less solar energy than the Earth. The Huygens probe which landed in the Shangri-La/Adiri region on January 14, 2005 recorded a surface temperature of approximately -179 degrees Celsius, -290 degrees Fahrenheit or 94 Kelvin.

The atmospheric pressure on the surface of Saturn's largest moon is largely higher than that on the surface of the Earth at sea level. The Huygens probe recorded an atmospheric pressure of 1467 hPa or mb on the ground of the Opaque Moon on January 14, 2005.

The composition of Titan's atmosphere is different from that of the Earth: Molecular nitrogen is the most abundant molecule in the atmosphere of the Earth and Titan. It represents approximately 78% of the composition of the atmosphere of the Earth and about 94% of the composition of the atmosphere of Titan.

Contrary to the atmosphere of the Earth, that is made of 21% oxygen, the atmosphere of Titan is devoid of any significant amount of oxygen. The second most abundant molecule in the atmosphere of the Orange Moon is methane which can represent up to 5% of the composition of the atmosphere.

The meteorology of Titan is intimately linked to methane which is involved in a methane cycle comparable to the water cycle of the Terrestrial meteorology.

The air above the soil of Titan is largely denser than the air of the Earth at seas level. Moreover,  due to the relatively weak density of the Titanian sphere and the smaller size of Titan which is about 5150 km or 3200 miles in diameter, the gravity on the surface of Titan is about seven times weaker than the gravity on the surface of the Earth.

Like the Earth, the landscape of Titan is made of lakes, seas, rivers and the atmosphere of Titan incorporates clouds of methane and ethane which can generate methane or ethane rain as well as snow. Thus, there are processes of evaporation and precipitation on Titan like in the atmosphere of the Earth.

But the dynamics of cloud formation appears different on Titan due to seasons and orbital distances. Like on Earth, the seasons are well marked due to the relatively high obliquity of Titan whose rotation axis is tilted by about 27 degrees to the normal of the orbital plane compared to 23.4 degrees for the obliquity of the Earth.

Planetary scientists are trying to reproduce the dynamics of the meteorology of Titan on the basis of the physical and meteorological characteristics of the opaque, thick and deep atmosphere of the Orange Moon. What are the key meteorological observations which must be reproduced by computer models of atmospheric circulation?

Since the arrival of the Cassini spacecraft in the Saturn system in 2004, a high concentration of bright clouds has been observed in the high latitudes of the southern hemisphere during its summer period. Dynamic cloud systems have been noticed in the area of the radar-dark Ontario Lacus, the first body of liquid identified in the south polar region of the moon.

A large ethane cloud system, reminiscent of a giant cyclone or hurricane was observed in 2006 above the north polar region with the VIMS instrument of the Cassini probe. The observation was performed during the winter period in the northern hemisphere.

Radar images obtained from the Cassini probe have also shown a pleiade of bodies of liquids in the high latitudes of the northern hemisphere during the Winter period. Kraken Mare, Ligeia Mare and Punga Mare are the most famous lakes or seas in the north polar region. A relationship has been made between cloud formation and the presence of seas, lakes and rivers.

Paradoxically, the equatorial and tropical region where the Huygens probe landed on January 14, 2005 appears particularly dry, dominated by dark Seif Dunes or parallel dune fields extending over long distances. No lakes or seas have been identified in the low latitudes of Titan and a few sporadic cloud systems have been observed at mid-latitudes.

Yet, an unusual concentration of bright cloud systems was recently observed in the low latitudes of Titan during the transition between the Winter period in the northern hemisphere and the Spring season in the northern hemisphere. The vernal equinox occured in August 2009 which marked the beginning of the Autumn season in the southern hemisphere and the Spring season in the northern hemisphere. Each season lasts approximately 7 Earth years and a Titanian year lasts about 29.5 years.

The higher amount of energy reaching the soil in the low latitudes during the period of the equinox may have engendered a higher amount of evaporation and massive cloud systems bringing precipitations in the areas of the dark Belet, Senkyo, the bright Adiri, the dark Shangri-La or the dark Fensal/Aztlan.

Previous atmospheric models failed to explain all those meteorological phenomena. Endogenic processes such as cryovolcanoes spewing methane or ethane have been incorporated into the atmospheric or meteorological models to account for the observations.

A new computer model accounts for all the observations of the Titanian atmosphere. The model is a Three-dimensional atmospheric model based on the fundamental principles of atmospheric circulation. The model takes into account a dynamic surface reservoir of methane. The model is presented by Tapio Schneider, Sonja Graves, Emily Schaller and Mike Brown in the January 5 issue of the journal Nature in a paper entitled "Polar methane accumulation and rainstorms on Titan from simulations of the methane cycle."

Tapio Schneider, the Frank J. Gilloon Professor of Environmental Science and Engineering advanced: "We have a unified explanation for many of the observed features."..."It doesn't require cryovolcanoes or anything esoteric."

The simulation reproduces quite well the atmospheric observations and it generates the distribution of lakes. Why are the lakes and seas concentrated in the polar regions? It appears that the amount of solar energy reaching the polar regions is lower than the amount of solar energy reaching the low latitudes. Therefore, methane will condense and accumulate more easily in the polar regions, forming lakes, seas and rivers.

Why is there an imbalance between the distribution of liquid bodies in the south polar region and the north polar region? The higher concentration of liquid bodies in the north polar region may be closely related to the elongated orbit of Saturn and Titan around the Sun. The Summer period appears to be the rainy period and the Orange Moon is farther  from the Sun when it is Summer in the northern hemisphere. As a result, on the basis of Kepler's second law, the Summer period in the northern hemisphere is longer than the southern Summer because  the moon orbits more slowly at aphelion than at perihelion.

Yet, the south polar region receives a more intense amount of solar energy during the Summer period because it moves closer to the Sun. A significant process of evaporation and precipitation may take shape in the area. Since the Winter season is approaching in the southern hemisphere, the lakes and seas of the south polar region may lose a portion of their volume in the coming years. The opposite may occur in the north polar region with net precipitation of methane and ethane as the Summer season of the northern hemisphere approaches.

The net precipitation in polar regions may be accompanied by slow along-surface methane transport towards mid-latitudes and subsequent evaporation processes. Rare and intense storms take shape around the equinoxes, generating enough precipitation to engender drainage channels or rivers. The clouds which bring methane rain are found in the Troposphere and they form primarily in middle and high latitudes of the Summer hemisphere. Tapio Schneider pointed out: "It rains very rarely at low latitudes."..."But when it rains, it pours."

The three-dimensional model of atmospheric circulation simulates the atmosphere of the Orange Moon for 135 Titan years or 3,000 Terrestrial years which is enough to reach a steady state. At aphelion, Titan evolves about 12% farther from the Sun than at perihelion, since  it orbits at a distance of approximately 1,513,325,783 km compared to about 1,353,572,956 km at perihelion.

Marc Lafferre, a specialist of Titan calculates that the amount of energy  received by Titan from the Sun at aphelion is about 20% weaker than at perihelion. At aphelion, the energy received is 13.50 Watt/m² compared to 16.88 Watt/m² at perihelion. As a result, the potential temperature, at aphelion, is -185.31 degrees Celsius compared to -180.27 degrees Celsius at perihelion which results in a potential surface temperature difference of 5.04 degrees Celsius between aphelion and perihelion.

It appears that the meteorology of Titan is influenced by the variation in the orbital distance as well as by the inclination of the rotation axis. The sporadic cloud systems identified in the low latitudes of Titan in 2009 by Emily Schaller, Mike Brown, Tapio Schneider and Henry Roe demonstrate that the low latitudes of Titan can encounter monsoon events or strong rainfall periodically. That may explain the presence of drainage channels in the bright Adiri as the images taken from the Huygens probe had revealed during its descent throughout the atmosphere of Titan.

Tapio Schneider, the Frank J. Gilloon Professor of Environmental Science and Engineering, Sonja Graves, a Caltech graduate student, Emily Schaller, a former Caltech graduate student and Mike Brown, the Richard and Barbara Rosenberg Professor and professor of planetary astronomy predict, on the basis of the simulations, that the lake levels in the north polar region will rise over the next fifteen years due to the changing seasons and that prominent clouds will form in the north polar region in the next two Terrestrial years. Tapio Schneider concluded: "In a few years, we'll know how right or wrong they are."           

 

 

The simulated views above, generated with the software Celestia, show the Orange Moon Titan in the foreground in the period of the Equinox on August 17, 2009. The texture of Titan, in this view, was produced by Marc Lafferre. One can notice a bright cloud formation in the low latitudes of Saturn's largest moon. Saturn and some of its moons such as Dione, Tethys and Enceladus can be identified in the background on the same plane as the equator of Saturn and the rings of Saturn, roughly speaking. The low latitude areas such as Shangri-La, Adiri or Fensal/Aztlan are thought to receive high amounts of rain of hydrocarbons (ethane, methane...) around the Equinox due to a higher amount of solar energy reaching the soil of the low latitudes in this period.

The image above corresponds to a near-infrared view of the disc of Titan revealing an unusual concentration of cloud systems at low latitudes on October 18, 2010. The climate model of the Opaque Moon predicts rainfall at low latitudes around the Equinox. The Equinox occured in August 2009. Large cloud formations can be identified over the Fensal/Aztlan region, here. Image credit: NASA/JPL/Space Science Institute

 

- To get further information on that news, go to: http://media.caltech.edu/press_releases/13484 and http://www.nature.com/nature/journal/v481/n7379/full/nature10666.html.      

  

 

 

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