April 11, 2021: How To Interpret The Absence Of Bottom Echo In Kraken Mare Determined From The Cassini Radar Altimeter Data ?
A recent study has revealed that Kraken Mare may be remarkably deep thanks to data obtained from the Cassini orbiter on the basis of the Radar Altimeter. In fact, Kraken Mare which represents the largest sea on Saturn's largest moon Titan may be several hundred meters deep or around 1,000 feet deep. A new analysis entitled "The Challenging Depths of Titan's Seas", published in the Journal of Geophysical Research: Planets on April 9, 2021 and proposed by Ralph D. Lorenz brings us key information to interpret the absence of bottom echo in Kraken Mare. Several hypotheses can be advanced to try to explain the phenomenon. In the first hypothesis, the absence of bottom echo implies that the sea appears too deep for the identification of a bottom echo. In the second hypothesis, the liquid of the lake contains a relatively significant fraction of ethane so that any radar signal will tend to be absorbed by the liquid. In the third hypothesis, the sea is very deep and contains a relatively significant fraction of ethane. Methane appears to be the main molecule of the seas, lakes or rivers on Titan but ethane and molecular nitrogen can also be present in relatively significant concentrations within the seas, lakes or rivers.
Planetologists believe that the seas, lakes or rivers on the Opaque Moon Titan contain a mixture of methane, ethane and molecular nitrogen. The composition of that mixture can vary depending on the latitude, the area or the season. Methane can be compared to water on Earth since it is the key molecule of the meteorological cycle of Titan. Like water on Earth, methane on Titan can evaporate, form clouds or fall as rain. Methane whose chemical composition is CH4 may be particularly volatile on Titan. Ethane whose chemical formula is C2H6 may be less volatile than methane and may be comparable to salt on Earth. Ethane on Titan must represent a substance that tends to remain in the bed of any sea, lake or river even if the evaporation process of methane is relatively strong. Some ancient seas on Earth have kept their salt over time. Ethane may follow the same logic on Titan as salt on the Blue Planet. The mysterious seas, lakes and rivers of Titan must be remarkably diverse. Researchers try to gather new clues regarding those liquid bodies on the basis of the huge amount of data acquired from the Huygens probe on January 14, 2005 and acquired from the Cassini spacecraft during its long mission in the Saturn System, from 2004 to 2017.
Seas, lakes and rivers on Titan tend to be found in the high latitudes of each hemisphere or in the polar regions. The radar data as well as the infrared or near-infrared data obtained from the Cassini spacecraft during its orbital dance in the Saturn System have clearly shown that the seas, lakes and rivers are mostly concentrated in the high latitudes of the northern hemisphere of the giant moon. How to explain that dichotomy or that asymmetry in the distribution of seas, lakes and rivers on Titan ? Why is the north polar region of Saturn's largest moon more humid than the south polar region of that world ? The first stable extraterrestrial pool of liquid was Ontario Lacus, a lake or sea located in the high latitudes of the southern hemisphere of Titan. Ontario Lacus is much smaller than Kraken Mare, the largest pool of liquid in the northern hemisphere. Is the dichotomy a coincidence ? Is it related to physical or orbital factors ? Is it related to seasonal factors ? Is it related to geographical differences in the thickness of the external crust ? Planetologists believe that the level, the size or the dynamics of Ontario Lacus, Kraken Mare, Ligeia Mare or Punga Mare can significantly vary depending on seasonal factors.
The scientists of the Cassini mission have clearly noticed, on the basis of radar data, that some lakes found in the high latitudes of the Orange Moon had partially or completely evaporated in a relatively recent past. Some researchers advance that some pools of liquid found in the north polar region of the giant moon may represent cryovolcanic lakes because they appear relatively round with relatively sharp rims. Some planetologists also advance that there may be a subsurface ocean, a subsurface sea or pockets dominated by liquid methane beneath the external crust of the north polar region. Thus, there may be internal sources to the relatively high concentration of methane identified in Titan's atmosphere. Data obtained from the Cassini orbiter have clearly shown that there may be an internal ocean beneath the external crust of Saturn's largest moon. The composition of that presumed internal ocean is unknown but the observations of geysers rich in water in the fractures of the south polar region of the tiny moon Enceladus demonstrate that an ocean dominated by liquid water is clearly a possibility inside the moons evolving around Saturn. Tethys, Dione, Rhea or Titan may contain a subsurface ocean dominated by liquid water.
The environment of Titan is extremely harsh with ambient temperatures which evolve around 94 Kelvin, -179 degrees Celsius or -290 degrees Fahrenheit at the level of the equator. In the polar regions, the ambient temperature is even lower but the difference in the ambient temperature between the equatorial region and the polar region is very limited compared to the same difference on Earth. At the level of the poles of Titan, the environmental temperature is only 2 or 3 degrees Kelvin lower than at the level of the equator. The equatorial or tropical regions appeared relatively dry during the Cassini-Huygens mission in the Saturn System. But the aerial views obtained from the Huygens probe during its parachuted descent toward the surface of Titan have clearly revealed bright hills containing a network of dark channels which may represent drainage channels related to seasonal factors. In the low or mid-latitudes of Saturn's largest moon, strong rainfall events are likely to take shape from time to time during the long Titanian year. In 2010, the infrared or near-infrared views taken from the Cassini spacecraft had clearly revealed a large system of clouds at a low or mid-latitude of Titan. That system of clouds apparently related to seasonal factors may have generated heavy rainfall events.
Is liquid methane less stable or more volatile in the low or mid-latitudes of Titan than in its high latitudes where the seas, lakes or rivers are found ? The radar data obtained from the Cassini orbiter have shown that the dark areas of the low or mid-latitudes of the giant moon tend to be dominated by linear and parallel dunes extending over long distances. Are those relatively dark areas rich in dunes which mark a sharp contrast with relatively bright areas ancient seas or oceans ? That's a possibility because some deserts composed of dunes on Earth also represent ancient seas or oceans. During the long Titanian year which lasts approximately 30 Terrestrial years, heavy rainfall events can occur in the low or mid-latitudes potentially engendering transient seas or oceans. The climate of Titan may also have significantly evolved over geological time scales. The Opaque Moon may have been much more humid in the distant past. The dunes found in the relatively dark areas of the low or mid-latitudes, which are reminiscent of the Seif Dunes found in the Namib Desert on Earth for instance and which may be rich in hydrocarbons or organics, may have taken shape in ancient seas or oceans.
The radar or altimetry data have revealed that Ontario Lacus, the first lake or sea identified on Titan, was particularly smooth. Ontario Lacus could be compared to a mirror. Prior to the Cassini-Huygens mission in the Saturn System, some planetologists had imagined or anticipated that relatively strong waves could be encountered in the oceans, seas, or lakes of Titan. Their logic was that the gravity on Titan is much weaker than on Earth and that the air of the Opaque Moon at sea level is much denser than on Earth. Relatively strong winds engendering strong waves within the lakes, seas or oceans could have been imagined. In reality, the radar data captured from the Cassini orbiter have unveiled a particularly quiet lake or sea in the high latitudes of the southern hemisphere of Saturn's largest moon. Are the lakes or seas present in the high latitudes of the northern hemisphere less quiet or more active ? Seasonal factors must play a key role in the dynamics of the lakes, seas or rivers in the polar regions of Titan. The Cassini spacecraft has acquired data regarding the climate or the meteorology of Titan during less than a Titan year so that the meteorological puzzle or the climate puzzle is incomplete.
Researchers regularly find new clues regarding the geology, the climate or the meteorology of Saturn's largest moon. They have a huge amount of data at their disposal thanks to the Cassini-Huygens mission in the Saturn System. The seas, lakes and rivers of that exotic world are really fascinating and intriguing for the planetologists or the general public. We have now a good idea upon the profile of the major seas or lakes on Titan. Ontario Lacus may contain about 38 percent of ethane and may be about 50 meters deep. Ligeia Mare may contain about 11 percent of ethane and may be about 160 meters deep. The maximum depth of Kraken Mare is unknown. Kraken Mare may contain about 11 percent of ethane. Punga Mare may contain about 8 percent of ethane and may be about 110 meters deep. Surprisingly, Moray Sinus which represents an estuary at the northern margin of Kraken Mare may be about 85 meters deep. The contrast between Kraken Mare, the largest pool of liquid in the northern hemisphere, and Ontario Lacus, the largest pool of liquid in the southern hemisphere, can appear quite remarkable in terms of composition. That demonstrates the remarkable diversity or complexity of Saturn's largest moon. What types of chemical processes are likely to take shape in the pools of Titan ?
The image above represents a portion of a radar swath obtained on February 22, 2007 from the Cassini orbiter during the T25 Flyby. Each side of the image represents 100 km. The view unveils a portion of Kraken Mare where Moray Sinus can be found in particular. One can clearly discern a well-defined coastline which appears quite irregular. Credit for the original radar view: NASA/JPL/Cassini RADAR Team/Jason Perry.
- To get further information on that news, go to: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020JE006786 .