November 19, 2019 : Planetologists Have Finalized And Unveiled The First Global Geologic Map Of The Giant Moon Titan

A new global geologic map of Saturn's largest moon Titan has been unveiled on the basis of the large amount of data obtained from the Cassini spacecraft during its long mission in the Saturn System from 2004 to 2017. One can notice, in this map, the remarkable diversity of the landscape. Titan is clearly an active world where the surface and the dense, deep, opaque or hazy atmosphere are interacting to engender captivating landscape features. Most moons of the Solar System are devoid of any atmosphere. As a result, their surface is dominated by impact craters, from tiny impact craters to giant impact craters. Titan is in a completely different category of worlds due to the action of its thick atmosphere which engenders significant erosional processes. The new global map only reveals a few apparent craters. Those observations demonstrate that Titan's surface is relatively active or young. The meteorology of our planet is dominated by liquid water. Water is frozen or appears as ice on Titan's surface due to the extremely low environmental temperatures. Therefore, the meteorology of the Opaque Moon is not based on liquid water or water vapor.

However, a parallel can be drawn between the meteorological cycle of the Earth and the meteorological cycle of Titan since methane on Titan plays a relatively similar role to that of water on Earth. There are evaporation processes, condensation processes and precipitation processes on Titan like on Earth but the molecule involved on Titan is methane instead of water. Methane or ethane which represent gases in the environment of the Earth can appear in their liquid form on the surface of Saturn's largest moon because the environmental temperature is particularly low, around minus 179 degrees Celsius, minus 290 degrees Fahrenheit or 94 Kelvin. Rosaly Lopes who is a planetary geologist at NASA's Jet Propulsion Laboratory in Pasadena, California and who is lead author of a new study mobilized to generate the global geologic map of Titan pointed out :  Titan has an active methane-based hydrologic cycle that has shaped a complex geologic landscape, making its surface one of most geologically diverse in the solar system.  The Earth and Titan are the only known worlds in the Solar System where pools of stable liquid can be found on the surface.

Researchers are particularly interested in the potential interactions between the atmosphere, the soil and the crust or the underground of the giant moon. During the Cassini-Huygens mission in the Saturn System, we have been in a position to acquire a huge amount of information regarding the meteorology or the hydrology of Titan. Thanks to radar data acquired from the Cassini orbiter, we have clearly identified lakes, seas and rivers in the high latitudes or in the polar regions of the Hazy Moon. We have also identified polar cyclones, polar vortices as well as clouds of methane or ethane in the polar areas and in the equatorial or tropical areas. Thus, one can encounter rainfall events or even snowfall events on Titan like on Earth. Is there a strong tectonic activity on Titan ? We have identified topographic signs which suggest that fractures can take shape on the surface. The Orange Moon unveils hills, mountains, canyons or dunes. The presence of cryovolcanoes is not ruled out. Planetologists have identified topographic features which may represent cryovolcanoes spewing water molecules. If that's the case, one can easily imagine a subsurface ocean dominated by liquid water.

The new map clearly reveals the diversity of Titan's landscape. The polar areas are dominated by lakes, seas or rivers and the most humid area appears to be the north polar region. The dark areas found at a relatively low latitude and which unveil a clear contrast with bright areas tend to be dominated by linear and parallel dunes extending over long distances. Those exotic dunes are probably rich in organics or hydrocarbons. The sand of those dunes may be closely linked to the organic haze of the atmosphere. Tholins may be widespread on Titan ! During its atmospheric descent performed on January 14, 2005, the Huygens probe had obtained remarkable panoramic images of Titan's landscape which demonstrate its diversity. We had observed bright hills composed of a network of sinuous channels and a dark or brown plain which looked like a sea from the parachuted probe. Did the dark sinuous channels represent fractures, the outcome of cryovolcanoes or geysers or simply drainage channels related to the meteorological cycle of Titan ? The Huygens probe landed onto the dark plain and the famous color view from the surface, unveiling eroded stones or pebbles, suggests that the probe had landed onto an ancient brook or river.

Rosaly Lopes advanced :  Despite the different materials, temperatures and gravity fields between Earth and Titan, many surface features are similar between the two worlds and can be interpreted as being products of the same geologic processes. The map shows that the different geologic terrains have a clear distribution with latitude, globally, and that some terrains cover far more area than others.  The mean density of Saturn's largest moon is much lower than that of the Earth which implies a higher concentration of lighter molecules. In the Saturn System, water ice is widespread. Moons like Tethys or Enceladus are particularly rich in water ice. Rosaly Lopes collaborated with several researchers like Michael Malaska from JPL and the planetary geologist David Williams of the School of Earth and Space Exploration at Arizona State University in Tempe to develop new hypotheses or theories regarding Titan. The team was in a position to determine the relative age of the geologic terrains of the giant moon. The research work was recently published in the journal Nature Astronomy.

The planetologists have been in a position to mobilize the impressive amount of data acquired from the Cassini orbiter proposed by NASA during its long journey in the Saturn System. Radar data acquired from the Radar Mapper and data acquired with the Visual and Infrared Mapping Spectrometer have been particularly useful to produce the final map of Titan's geology. Images obtained in the visible spectrum only reveal the opaque atmosphere which prevents us from discerning surface features. The Titanian atmosphere is completely opaque like the Venusian atmosphere. However, Titan's atmosphere is dominated by molecular nitrogen whereas the atmosphere of Venus is dominated by carbon dioxide. The Cassini orbiter has performed more than 120 flybys of Titan during its mission allowing us to progressively build the puzzle of Titan's environment or geology. Surface features on Titan can be clearly discerned in the infrared or near-infrared spectrum or thanks to radar data. The eyes of the Cassini spacecraft have been in a position to see what the eyes of the Pioneer 11 spacecraft, the Voyager 1 spacecraft or the Voyager 2 spacecraft had not been in a position to see.

Rosaly Lopes pointed out :  This study is an example of using combined datasets and instruments.  She added :  Although we did not have global coverage with synthetic aperture radar [SAR], we used data from other instruments and other modes from radar to correlate characteristics of the different terrain units so we could infer what the terrains are even in areas where we don't have SAR coverage.  David Williams brought his expertise to the JPL collaborators to find, for the first step, what geologic features could be well characterized on the basis of radar images. The next step was to extrapolate those geologic features to the regions which have not been covered by the Radar Mapper. David Williams mobilized his knowledge, his experience in the analysis of radar images of Venus obtained from NASA's Magellan orbiter as well as his experience in the development of a previous regional geologic map of Saturn's largest moon in order to perform his interpretations on surface features on that exotic moon. The resolution in the images of the terrain or topography is not always very high. That's why the analysis of Titan's geology is far from being obvious.

David Williams advanced :  The Cassini mission revealed that Titan is a geologically active world, where hydrocarbons like methane and ethane take the role that water has on Earth.  He added :  These hydrocarbons rain down on the surface, flow in streams and rivers, accumulate in lakes and seas, and evaporate into the atmosphere. It's quite an astounding world !  Seasonal factors must play a key role in the chemistry and in the dynamics of Titan's atmosphere as the data acquired from the Cassini orbiter reveal. The Titanian year is very long since it lasts almost 30 Terrestrial years and each season on Titan represents about 7 Terrestrial years. The Cassini orbiter has captured data for 3 Titanian seasons, from the Summer season in the southern hemisphere to the Winter season in the southern hemisphere. However, we have not acquired atmospheric data for a full Titanian year. Planetologists must clearly determine why the lakes and seas are mostly found in the north polar region or in the high latitudes of the northern hemisphere. They must also determine whether there are internal sources for the methane present in Titan's atmosphere. Are some lakes or pools fed by reservoirs or pockets of liquid methane hidden beneath the external crust ?

The image above corresponds to the first global geologic map of the Opaque Moon. The map which was recently unveiled was produced on the basis of infrared or near-infrared data and radar data acquired from the Cassini orbiter during its long mission in the system of the Ringed Planet Saturn. One can notice in particular the land of lakes and seas in the high latitudes of the northern hemisphere where Kraken Mare, the largest body of surface liquids on Titan, can be found. Image credit: NASA/JPL-Caltech/ASU.

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