January 20, 2018 : Researchers Produce A New Global Topographic Map Of Titan Revealing That The Main Seas Have The Same Height And May Be Interconnected With A Subsurface Network Of Liquids

Some new studies unveiled in Geophysical Review Letters on December 2, 2017 reveal that the main seas located in the high latitudes of the northern hemisphere on Saturn's largest moon Titan have the same level similar to our sea level on Earth. This finding implies that the three seas may be interconnected via a subsurface network of liquid hydrocarbons or that the drainage channels between the pools redistribute the liquids so that the level of each sea is similar to the level of the other seas. The new analyses were published in two papers of Geophysical Research Letters. The first paper entitled  Titan's Topography and Shape at the End of the Cassini Mission  was proposed by Paul Corlies, the first author of the paper. The second paper entitled  Topographic Constraints on the Evolution and Connectivity of Titan's Lacustrine Basins  was proposed by Alex Hayes, assistant professor of astronomy.

Researchers managed to produce a new global topographic map of the Opaque Moon which unveils new features such as new mountains that don't exceed the height of 700 meters. The topographic map clearly shows variations in altitude of surface features such as hills or depressions on the entire globe of the Orange Moon. Planetologists were in a position to determine that there are two areas representing depressions in the equatorial region. The depressions may correspond to ancient seas or dried seas or they may represent cryovolcanic flows. The global map also shows that the Hazy Moon is a little bit flatter or more oblate than previously thought. That configuration implies that the thickness of the crust may vary to a larger extent than previously believed. The second paper reveals that the three seas unveil a common equipotential surface that is to say the equivalent of the common sea level on Earth.

There may be a subsurface reservoir of liquid hydrocarbons in the area of Kraken Mare and Ligeia Mare or the channels connecting the seas may balance the level of each sea. The well known seas of Titan's northern hemisphere may communicate or interact via the presumed subsurface reservoir. The analytical work reveals that most lakes represent sharp-edged depressions. Some high ridges can be observed in some locations. Is the environment of Titan's lakes and seas similar to the environment of karstic terrain or limestone areas on Earth ? Are there caves engendered by dissolution processes or erosion ? Liquid water is absent on the surface of Titan due to extremely harsh conditions. The lakes and seas of Titan may be mainly composed of methane and ethane. So, what are their properties or interactions with the soil or crust of the giant moon ? Why do the lakes studied unveil a process of uniform scarp retreat ?

Paul Corlies who is a doctoral student and who worked in the study regarding Titan's topography explained that it took about a year to produce the map. The global map was generated on the basis of various sources. The resolution obtained from the Cassini probe during its exploration campaign can vary from one location to the other location. Only approximately 9 percent of the topography of the Opaque Moon has been scanned in relatively high resolution and 25 to 30 percent of Titan's topography has been scanned in lower resolution. Therefore, for the other areas, the mapping process was performed on the basis of an interpolation algorithm and of a global minimization process which limited some errors like the errors related to the probe location. Paul Corlies pointed out :  The main point of the work was to create a map for use by the scientific community.

Very quickly after the online release of the data set, he started to collect instructions on how to use the new amount of data. The topographic data related to pure observations as well as the topographic data incorporating interpolated data which are not based on pure observations can be downloaded. The topographic map may play a key role in several research fields such as in the development of simulations of Titan's climate, in the study of the shape and gravity of the Hazy Moon or in the modeling of Titan's interior or structure. The topographic map may also be helpful in the analysis of land forms, surface features or morphology on Saturn's largest moon. The work on topography also involved the senior author from Cornell Alex Hayes, doctoral candidate Samuel Birch as well as research associate Valerio Poggiali.

The research work from the second paper unveils three major findings based on the new topographic map. Kraken Mare, Ligeia Mare or Punga Mare have a sea level probably related to a subsurface reservoir of methane or ethane or to active drainage channels between the seas. Alex Hayes advanced :  We're measuring the elevation of a liquid surface on another body 10 astronomical units away from the sun to an accuracy of roughly 40 centimeters. Because we have such amazing accuracy we were able to see that between these two seas the elevation varied smoothly about 11 meters, relative to the center of mass of Titan, consistent with the expected change in the gravitational potential. We are measuring Titan's geoid. This is the shape that the surface would take under the influence of gravity and rotation alone, which is the same shape that dominates Earth's oceans.  The planetologists had already noticed that the pools of liquids found in the high latitudes of the northern hemisphere apparently unveil a remarkably smooth surface without any strong waves.

The second major outcome of the second paper is the potential presence of a subsurface reservoir fueling the major seas of Titan's northern hemisphere. In other words, the seas or lakes tend to communicate with each other via the subsurface network of liquids which are likely composed of methane or ethane. The finding is in line with the hypothesis that Alex Hayes had put forward in his first paper in graduate school. Alex Hayes and his collaborators had determined the elevation of the lakes that contain liquid and the elevation of the lakes which are empty or dry. They concluded that lakes are present hundreds of meters above sea level and that within a section of study, the floors of the empty pools are all at higher elevations than the filled lakes of the same region. Alex Hayes pointed out :  We don't see any empty lakes that are below the local filled lakes because, if they did go below that level, they would be filled themselves. This suggests that there's flow in the subsurface and that they are communicating with each other. He added :  It's also telling us that there is liquid hydrocarbon stored on the subsurface of Titan. Methane, ethane and propane can appear in their liquid form on the surface of Titan. The exact composition of the surface liquids is a major topic of research today.

The third major outcome of the second paper is related to morphology, the properties of the liquid and the properties of the soil or crust. Most lakes on Saturn's largest moon are located in sharp-edged depressions that  literally look like you took a cookie cutter and cut out holes in Titan's surface  Alex Hayes explained. Around the pools of liquids, high ridges which can be several hundred meters high can be observed in some areas. A parallel can be drawn between the lakes on Titan and the karstic landforms on Earth. The Florida Everglades come to mind since strong dissolution processes take shape there beneath the surface which can collapse to form holes. Do the lakes of the Hazy Moon follow the same logic with surface materials which have the same properties as limestone on Earth for instance ? The lakes of the Opaque Moon appear isolated from other liquid sources since they are not connected to any drainage channel. Let's note however that the typical karst on our planet does not reveal sharp, raised rims.

The morphology of the pools of liquids implies a process known as uniform scarp retreat. That process is characterized by the fact that the rims of the lake are expanding at the same rate or proportionally each time. For instance, the biggest lake in the south seems to be composed of a series of smaller empty pools that have coalesced or merged into a bigger feature or pool. Alex Hayes pointed out :  But if these things do grow outward, does that mean you're destroying and recreating the rims all the time and that the rims are moving outward with it ? Understanding these things is in my opinion the lynchpin to understanding the evolution of the polar basins on Titan. Everybody has in mind the transient phenomena of the Magic Island, a radar-bright feature in Ligeia Mare which had suddenly appeared, which had changed shape and which had rapidly disappeared. The second paper was proposed by Alex Hayes, Paul Corlies, Samuel Birch, Valerio Poggiali, research associate Marco Mastrogiuseppe and Roger Michaelides'15. The study benefited from grants from NASA and the Italian Space Agency.

The image above represents a colorized radar map of Titan's northern hemisphere unveiling, in particular, the famous seas or lakes Kraken Mare and Ligeia Mare. The bodies of surface liquid in this view appear blue or dark. There may be a subsurface reservoir of methane or ethane fueling the seas or lakes found in the high latitudes of the northern hemisphere of the Hazy Moon. Image Credit: NASA/JPL-Caltech/ASI/USGS.

- To get further information on that news, go to: https://news.cornell.edu/stories/2018/01/saturns-moon-titan-sports-earth-features and https://saturn.jpl.nasa.gov/news/3137/cassini-finds-saturn-moon-has-sea-level-like-earth.


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