Geology

Oceanography in space! Using a satellite to profile an extraterrestrial lake

Lis Henderson

Mastrogiuseppe, Marco, Valerio Poggiali, Alexander Hayes, Ralph Lorenz, Jonathan Lunine, Giovanni Picardi, Roberto Seu et al. “The bathymetry of a Titan sea.” Geophysical Research Letters 41, no. 5 (2014): 1432-1437.

 Saturn’s largest moon, Titan, contains several hundreds of lakes.  Ligeia Mare is defined as a sea by the International Astronomical Union although, most would consider it more akin to a lake.  This group of researchers took advantage of an opportunity via the Cassini satellite orbiting Saturn to calculate the bathymetry of Ligeia Mare for the first ever bathymetry sounding of an extraterrestrial sea.

Figure 1. An image generated by Cassini flybys to show portions of Titan’s surface (NASA). As you can see, the image is not complete because we can only see the portions of the moon that Cassini flew over. Four flybys were used to create this image. Figure 2. Diagram of satellite coverage with mapping coordinates used to create figure 1 (Lorenz 2008).
Figure 1. An image generated by Cassini flybys to show portions of Titan’s surface (NASA). As you can see, the image is not complete because we can only see the portions of the moon that Cassini flew over. Four flybys were used to create this image.
Figure 2. Diagram of satellite coverage with mapping coordinates used to create figure 1 (Lorenz 2008).
Figure 2. Diagram of satellite coverage with mapping coordinates used to create figure 1 (Lorenz 2008).

The Cassini-Huygens unmanned spacecraft was launched from Cape Canaveral on October 15, 1997.  Seven years later, the craft reached Saturn.  The craft was composed of two parts.  The Cassini is a Saturn orbiter while the Hyugens is a probe designed to land on Titan.  This study used data collected by the Cassini spacecraft during a flyby of Titan on May 23, 2013. 

This particular flyby was used because in order for Cassini RADAR to map the topography and the bathymetry of the lake, the wavelengths must propagate directly down from the craft.  For normal image mapping (Fig. 1; Fig. 2), the satellite collects images from an angle.  The echo soundings produce altimeter profiles (Fig. 3). These profiles indicate the echoes received by the satellite.  The first strong echo indicates contact with the surface of Ligeia Mare; the second, smaller peak indicates contact with the bottom of the sea (Fig. 4).

In order to tease out other variability in the sounding profiles, several processing methods were applied.  The end result is a profile of the surface and bottom of Ligeia Mare along the Cassini flyby transect (Fig. 5).  This sound profile can be converted to a depth profile (Fig. 5) based on the speed of the echo sounding waves and the time lapsed between when the pulse is emitted from the satellite and the echo is received.  This is a slight over-simplification; the National Geophysical Data Center provides a more detailed explanation.

Figure 3. Some of the Cassini altimeter data used to calculate lake depth. The first large pulse denotes the surface of the lake; the second, smaller pulse denotes the bottom depth.
Figure 3. Some of the Cassini altimeter data used to calculate lake depth. The first large pulse denotes the surface of the lake; the second, smaller pulse denotes the bottom depth.
Figure 4. Satellite altimetry schematic (General Bathymetric Chart of the Oceans, GEBCO).
Figure 4. Satellite altimetry schematic (General Bathymetric Chart of the Oceans, GEBCO). 
Figure 5. Upper: Satellite image of Ligeia Mare with superimposed satellite-sounding track; red circles indicate start and end of track. Lower: Radargram obtained from processing echo soundings. Note the straight, horizontal red line suggests the sea surface; the fainter red line below suggests the bathymetry.
Figure 5. Upper: Satellite image of Ligeia Mare with superimposed satellite-sounding track; red circles indicate start and end of track. Lower: Radargram obtained from processing echo soundings. Note the straight, horizontal red line suggests the sea surface; the fainter red line below suggests the bathymetry.

The greatest depth found in this profile was 160m.  As seen in figure 6, the northern shore has a gentler slope than the southern.  This matches previous conceptions of higher sedimentation in the north.

So is the bathymetry of this extraterrestrial sea different from what we might see on Earth?  Potentially.  Maximum depth in the Ligeia Mare probably does not exceed 160m significantly.  Under this assumption, the sea is fairly shallow relative to those on earth.  Another study by Lorenz et al. (2008) suggests that most large bodies of water the same width on Earth would reach about 350m.  Although, that estimation allows variability that would not exclude 160m depth.

Figure 6. Bathymetric profile of Ligeia Mare (solid) with smooth polynomial (dashed) fitted to within an error of about 14m.
Figure 6. Bathymetric profile of Ligeia Mare (solid) with smooth polynomial (dashed) fitted to within an error of about 14m.

Recall that this is just one transect of the Ligeia Mare.  But, the researchers estimate that this profile is representative of the entire sea.  Using this assumption and satellite derived sea surface area (126,000km2), the mean depth is estimated to be around 70m and the volume is about 10,000km3.  This also yields an estimate of about 5000 gigatons (GT) of carbon (nearly 100 times the known terrestrial oil and gas reserves of Earth).  I don’t think we have to worry about mining energy from Titan anytime soon.  The 7-year trip each way seems hardly economical.

 

Cited

Lorenz, Ralph D., et al. “Titan’s inventory of organic surface materials.” Geophysical Research Letters 35.2 (2008).

 

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