May 1st 2019 : A New Study Reveals A Giant Belt Of Ice-Rich Bedrock In The Tropical Area Of Titan Implying A Potential Tectonic Activity
A new study entitled « A corridor of exposed ice-rich bedrock across Titan's tropical region », recently published in the journal Nature Astronomy and led by Caitlin Griffith, a professor in the UA Lunar and Planetary Laboratory, unveils a giant corridor of ice-rich bedrock in the Tropical area of Saturn's largest moon Titan. That belt extending over long distances may potentially be the outcome of a significant tectonic activity in the long history of Titan's geology. The discovery of that enigmatic corridor was performed thanks to a research work upon the origin of the methane observed on Titan. Methane whose chemical formula is CH4 can be found in the atmosphere of the Opaque Moon as well as on its surface. Titan's atmosphere is completely opaque in the visible spectrum from outer space but thanks to several instruments of the Cassini orbiter and thanks to the Huygens probe which landed onto Titan's surface on January 14, 2005, we have been in a position to gather major clues regarding the Titanian geology or the nature of Titan's surface.
At low or mid-latitudes, in the infrared or near-infrared spectrum, a significant contrast between relatively low-albedo areas and relatively high-albedo areas can be discerned. The panoramic views obtained from the Huygens probe during its unforgettable atmospheric descent in the hazy atmosphere clearly revealed a major contrast between bright hills and a dark or brown plain. Those bright hills found at a relatively low latitude close to the Equator may be dominated by water ice. The brown or dark areas may be rich in methane, hydrocarbons or organics. The radar data acquired from the Cassini orbiter, during the Cassini mission in the Saturn System from 2004 to 2017, have clearly shown that the dark areas found at low or mid-latitudes and which mark a sharp contrast with bright areas are dominated by linear and parallel dunes extending over long distances. Those dunes are reminiscent of the Seif dunes found in the Namib Desert for instance. By contrast, the dark and uniform areas located in the high latitudes or in the polar regions of the Orange Moon represent pools of liquid.
The lakes or seas of the north polar region tend to be dominated by methane. Several hydrocarbons can be present in their liquid form on Titan's surface. The environmental temperature of Titan is around minus 180 degrees Celsius, minus 292 degrees Fahrenheit or 93 Kelvin which allows the molecules of methane, ethane and propane to be present in their liquid form on the surface of the giant moon. Ontario Lacus, the giant pool of liquid found in the high latitudes of the southern hemisphere, may contain methane with a relatively significant concentration of ethane. The composition of the pools, lakes or seas may vary geographically or seasonally on Saturn's largest moon. The pools of the north polar region may be richer in methane than the pools of the south polar region at the present time. The observations of clouds, lakes and seas as well as the identification of rainfall events have led researchers to focus their attention on the hydrological cycle or on the methane cycle of Titan and on the origin of methane on that enigmatic world.
Methane as well as other hydrocarbons or organics can be generated via photochemistry in the upper atmosphere of Titan under the particular influence of ultraviolet light from the Sun. Larger molecules go down due to gravity and a haze of organics takes shape and develops. Snowfall events involving hydrocarbons or organics are likely to occur regularly or from time to time. Over time, sediments of organics or hydrocarbons are likely to accumulate on the surface and can form linear and parallel dunes at low or mid-latitudes under the influence of prevailing winds. Logically, methane may be progressively absorbed by the surface of Titan. The organic veneer may be composed of the molecules or compounds of past atmospheres. Planetologists have noticed that there are evaporation processes and condensation processes with cloud formation related to the pools of liquid found in the high latitudes of the Orange Moon. So the lakes or seas represent a major source of the methane identified in Titan's atmosphere. Let's point out, however, that they contain only one-third of the atmospheric methane. Over geological time scales, that molecule should progressively disappear.
Therefore, is the origin of methane related to cryovolcanism or subsurface reservoirs of methane ? Is there a layer of liquid methane beneath the presumed icy crust of the giant moon ? The hypothesis of cryo-volcanoes involving methane is seriously considered by scientists. Some studies focusing on surface features or on the topography of the Opaque Moon have revealed the presence of a particular region which may correspond to a cryovolcanic area. That region is known as Sotra. Sotra unveils icy flow features. That's why we believe that Sotra may potentially represent a cryo-volcano. Caitlin Griffith and her team undertook a search for potential cryo-volcanoes. So, the group of planetologists analyzed the composition of half of the surface of the Hazy Moon. The scientists didn't identify any convincing features related to potential cryo-volcanoes. However, they noticed that Sotra unveiled the strongest ice features. Sotra clearly appeared to be the best candidate for the identification of a potential cryo-volcano.
By searching for ice features related to potential cryo-volcanoes, the group of Caitlin Griffith identified a huge ice feature, a large-scale feature which was completely unexpected. That landscape feature represents a linear corridor of ice that covers approximately 40 percent of the circumference of the Opaque Moon. Caitlin Griffith pointed out : « This icy corridor is puzzling, because it doesn't correlate with any surface features nor measurements of the subsurface. » She argued : « Given that our study and past work indicate that Titan is currently not volcanically active, the trace of the corridor is likely a vestige of the past. We detect this feature on steep slopes, but not on all slopes. This suggests that the icy corridor is currently eroding, potentially unveiling presence of ice and organic strata. » Is that corridor related to a significant tectonic activity ? Can that belt dominated by ice bring key information regarding the structure of Titan's crust ? Is the surface of Titan stable ? Are there movements of plates on Titan like on Earth ?
The study of the group of researchers also reveals a diversity of organic material in certain areas of the giant moon. The surface deposits observed by the planetologists draw their whole attention because laboratory simulations of the complex atmosphere of the Opaque Moon generate biologically interesting molecules or compounds such as amino acids which are the building blocks of proteins. Can tholins engender prebiotic molecules ? The group of Caitlin Griffith studied tens of thousands of spectral views of the topmost layer of Titan's surface with the Visible and Infrared Mapping Spectrometer or VIMS of the Cassini orbiter, on the basis of a method that allows the team to detect weak surface features. In their research work, the planetologists applied a technique called the Principal Components Analysis or PCA. The PCA allowed them to identify subtle features related to ice and organic sediments in the ocean of data related to the global haze and to various surface features. The main surface features often prevent us from discerning smaller features which are likely to tell us a lot scientifically speaking.
The Principal Components Analysis or PCA, which was applied to over 13,000 Cassini/VIMS spectra, uses all of the pixels to identify the main and more subtle signatures instead of measuring the landscape features individually for each pixel in a view. The group of Caitlin Griffith compared its outcome with previous analyses including the studies related to the data acquired from the Huygens probe which had revealed bright hills with dark drainage channels or fractures as well as a brown or dark plain during its breathtaking atmospheric descent on January 14, 2005. The comparison carried out by the planetologists validated both the technique and the outcome. The researchers plan to resort to this technique to explore the polar areas where methane lakes, seas and rivers can be found. Caitlin Griffith pointed out : « Both Titan and Earth followed different evolutionary paths, and both ended up with unique organic-rich atmospheres and surfaces. » She added : « But it is not clear whether Titan and Earth are common blueprints of the organic-rich of bodies or two among many possible organic-rich worlds. »
Titan is clearly a dynamic world with a complex atmosphere dominated by molecular nitrogen like the atmosphere of our planet. A parallel can be drawn between the methane cycle of that enigmatic world and the water cycle of our planet. Researchers are looking for clear signs of cryovolcanism or tectonic activity on Titan today. They want to know whether there is a layer of liquid methane or whether there are pockets of liquid methane beneath the presumed icy crust of the giant moon. The corridor of exposed ice-rich bedrock may be the outcome of a major tectonic event or the outcome of a major cryovolcanic event in the geologic history of Titan. Saturn's largest moon is unique in the Solar System due to its thick and deep atmosphere and researchers want to know whether that atmosphere is stable over geologic time scales. Is Titan's atmosphere regularly fuelled by internal sources via cryovolcanism ? The study was partly funded by space grants from NASA.
The image above reveals a global view of Titan based on data acquired with the Radar Mapper and with the Visual and Infrared Mapping Spectrometer (VIMS) of the Cassini orbiter. The view was released on December 14, 2010. The artificial globe is composed of several radar swaths. The white arrow indicates the region of Sotra Facula, a potential cryo-volcano identified on Titan. The area of Sotra Facula can be found at about 15 degrees south latitude and 40 degrees west longitude. Image credit: NASA/JPL-Caltech/ASI/USGS/University of Arizona.
- To get further information on that news, go to: https://uanews.arizona.edu/story/researchers-find-ice-feature-saturn-s-giant-moon and https://www.nature.com/articles/s41550-019-0756-5.