Cassini-Huygens Space Probe: The Accidental Discovery of “Lakes”

Kaidree Elliott & Michylo Leask

 Introduction

Saturn is an extraordinary and interesting planet, encircled by sixty-two moons.  However, one particular moon has proven to be just as intriguing as the planet itself, showing many similarities to that of Earth.  Titan has many distinct characteristics for a moon, one of the most illustrious is the occupancies of lakes on its surface.  The investigation inquiry of this page focuses on the discovery behind these bodies of fluid through the Cassini-Huygens space probe, and the importance behind them.  Created to observe Saturn and explore the surface of Titan, the Cassini-Huygens mission was launched in 1997 on the 15^th of October, and holds responsibility behind the revelational discovery.  Without the various instruments equipped on the probe, the lakes on Titan would most likely still be undiscovered.  Not only were lakes identified on Titan, but an internal ocean, methane/ethane cycle, and rainstorms were also detected and investigated.  Through deeper examination, many of these aspects demonstrate parallels to Earth, demanding further examination to learn more about their importance.  Despite its importance in discovering more about Titan, as well as Saturn and the other moons, the Cassini mission has recently become discharged, opening doors for further research into future missions.  However, the data it did collect, has sparked new studies and thoughts for future resources.

The Cassini Huygens Space Probe

• The Initial Purpose of The Cassini-Huygens Space Probe

The Cassini-Huygens Satellite’s original mission was to orbit around Saturn to explore the planet and its moons. The satellite was launched on the 15^th of October 1997, which was a combination mission between NASA, ESA and ASI.¹ The Cassini-Huygens was the first space probe to land on a moon in the outer solar system; that moon being Titan. Cassini-Huygens was sent to answer many questions about Saturn and its moons. One question later being “are there any oceans on Titan?”¹ The Huygens probe was sent onto the surface of Titan on the 14^th of January 2005 and started to relay information back to Cassini via its instruments.

FIGURE 1. This is an image of the actual Cassini-Huygens space probe.  The Huygens probe was provided by the European Space Agency.  It entered Titan’s atmosphere and descended to the moon’s surface via parachute.  Credit to NASA.https://www.flickr.com/photos/nasacommons/9457930247/in/photostream/

Before the Huygens probe was launched to land on Titan, it made a few laps around Saturn’s orbit. During these laps around the orbit, it was taking pictures with its Imaging Science Subsystem, which was able to take photos under the cloudy haze and see the surface. Figure 2 is the film of the first photos taken of the moon’s surface. This led to the discovery of clouds, rain, and lakes on Titan.² The most important discovery was the finding of salt water oceans in the subsurface of the extraterrestrial world. The Hidden ocean was found through the research completed on the lakes, detecting large tides, implying a layer of water underneath the surface.³ With this finding, the Huygens probe was dropped to investigate further into Titan.  Figure 3 shows an artist’s depiction of the Huygens descent onto Titan’s surface.

FIGURE 2. Credit to ESA/NASA/JPL/University of Arizona. Titan_first_images_-_slideshow

 

(Video 1: Touchdown of Huygens on Saturn’s Moon Titan. https://www.youtube.com/watch?v=RJ8hvbjdqek)

• The Instruments on The Cassini-Huygens Space Probe

The Huygens-Cassini Space Probe was sent to Saturn’s moon, Titan, equipped with six instruments to aid in mapping out the conditions on Titan. Each instrument was meant to do a different job once it reached the surface of the astronomical identity.  The Descent Imager/ Spectral Radiometer (DISR) was one instrument which was advantageous for the gathering of more information about the lakes on Titan. The DISR took images during its descent on to Titan and made spectral measurements using sensors which covered a wide area.⁴ To do this, the DISR would turn on lamps a couple hundred meters before impact to receive a spectra of the surface material.⁵ When the probe was about to reach the surface it took a “spectra of the surface material”.⁴ This helped determine where the lakes may be in accordance with landing.  If the probe was able to move, it may have also showed depths of the lakes. Another instrument which proved useful in developing a better understanding of the lakes was the Doppler Wind Experiment (DWE). This instrument used radio signals to measure wind from the atmospheric properties and determined how high (or rough) the waves on the lakes could be. Through the wind it was able to measure a Doppler shift in the carrier signal.⁴ The last instrument which helped provide essential information was the Gas Chromatograph and Mass Spectrometer (GCMS).  The GCMS was a gas chemical analyzer which was used to register the chemicals in the atmosphere on Titan.⁵ A spectrometer is an instrument used to measure a property of light as its proportion to an electromagnetic spectrum.⁶  It is used with wavelengths, frequency and or energy to measure its intensity of light.⁶ They determined that the lakes are high in methane content. These three devices were the major instruments in discovery of the lakes on Titan.

Three other instruments proved importance in learning about Titan as a whole, but not specifically the lakes. Every instrument has unique and useful properties in exploring if there is a future for human colonization on the distant world. The Surface Science Package (SSP) for example, would help in obtaining knowledge of any mineral properties on Titan which would be a suitable harvesting possibility. Along with Huygens probe, the Cassini satellite has instruments which would assist in gaining more information about Titan as well.

Although the Cassini Satellite was mainly built to study Saturn, it was instrumental in the data collected about Titan. Some of the instruments on the Cassini Satellite were developed specifically for the moon.  A simple, but important instrument, is the Imaging Science Subsystem (ISS). This instrument captured pictures in “visible, near-ultraviolet and near-infrared light.”⁴ This device provided the images of Titan that we have today. A vital component on the Cassini Satellite was the Cassini Radar (RADAR). This instrument mapped out the entire surface of Titan using radar images. These devices were essential in discovering the lake bodies on Titan, detecting its internal water supply, and where to land for future missions.  Without the instruments, the satellite would be impractical, as it would not be able to collect information used for research. It took all the instruments to work and map out the surface of Titan, the lakes of Titan, and the ocean of Titan.  The different components of the space probe are illustrated in Figure 4.

(Video 2: Every Instrument of the Cassini Spacecraft. https://www.youtube.com/watch?v=2qdlBaIHfjA)

• The Detection of the “Hidden” Ocean on Titan

When the Cassini-Huygens mission was launched to study Saturn, it also recorded images of Saturn’s moons, in this specific case Titan.  As the moon orbited Saturn, researchers saw stretching and squeezing occurring to Titan.⁷ If Titan was in fact entirely solid rock, the gravitational attraction of Saturn would create bulges of only three feet in height on the surface of the moon.⁸  However, the bulges were measured at a height of thirty feet according to the data provided by the spacecraft.⁸ This leads scientists to believe that Titan is not solid rock, but must contain a liquid material under its crust.  Since Titan only takes sixteen days to orbit Saturn, this allowed scientists to study the shape of the moon at different parts of its orbit.⁸ Titan is slightly elongated, so when it is closer to the planet, its axis grows closer towards it.⁸ When farther away from the planet, Titan became more rounded.  The Cassini probe measured the gravitational effect, creating the “squeezing and pulling.”  After examining six close fly-bys of Titan, researchers were able to conclude the interior of the moon contained a “hidden ocean” through data returned to NASA’s Deep Space Network (DSN), which used measurements from the variations in the gravitational pull of Titan.⁸  Figure 5 shows the different layers of Titan, illustrating an internal ocean.

Titan’s lakes

• The formations of the lakes

Before lakes, the indentations to host the liquid substance had to be formed.  Through immense research, the lake depressions are believed to have formed through processes similar to those on Earth.  Data from the joint NASA and the European Space Agency (ESA) Cassini mission proposes that the lake beds on Titan initially formed through processes that create sinkholes on Earth.⁹  Sinkholes occur in what is referred to as “Karst terrain,” meaning terrain where soluble bedrock can be dissolved, on Earth, by water.¹⁰  The bedrock then becomes exposed, worn down, creating holes that will eventually fill with liquid.¹⁰  Titan’s lakes show parallels to these karstic landforms on Earth.  A crew led by Thomas Cornet of the European Space Agency performed calculations to determine the length of time it would take for the patches on the moon’s surface to dissolve and create these impressions.⁹  This team accounted for present day models of the moon’s climate, presumed Titan’s surface was covered in solid organic matter and concluded that the dissolving agent was liquid hydrocarbons.  Scientists have also calculated the amount of years it would take to create the indentations; taking approximately fifty million years to produce the massive 300ft depression located near Titan’s moderately showery polar regions.⁹  This dissolution process occurs on the moon of Saturn thirty times slower than that of Earth, for two distinct reasons: Titan’s duration of a year is far more extensive than that of Earth, and Titan’s minimal rainfall interval (it only rains in the summer season).⁹  As stated by ESA’s Cassini project scientist, Nicolas Altobelli, “by comparing Titan’s surface features with examples on earth and applying a few simple calculations, we have found similar land-shaping processes that could be operating under very different climate and chemical regimes.”⁹

• General Information About the Lakes

The lakes on Titan are a liquid hydrocarbon mix of methane and ethane.¹¹  This is the only other body in the universe known to have bodies of liquid on it’s surface.⁹  The space probe detected the dark spots on the moon, and the liquid was distinguished by the way infrared light, from one of the instruments on Cassini, was reflected and absorbed by the chemically different material.¹² The lake, Ontario Lacus, was the first lake detected by the Cassini-Huygens space probe, which is located in the south polar region and is larger than 20,000km² in size.⁷ Radar maps taken by the Cassini probe indicate that the extraterrestrial moon is dotted with lakes, some being as large as the Great Lakes here on Earth.¹¹  Along with containing pools of methane and ethane on its surface, Titan is also believed to harbor a layer of liquid water beneath its ice shell.⁸  A recent study, headed by researchers from Cornell University, used data collected from the space probe to prove that Titan, much like Earth, also encompasses a common sea elevation, or a “sea level.”¹² They did so through creating a topographic map of the astronomical feature.  They were also able to determine that the liquid substance is transferred between the lakes, indicating liquid flows in the subsurface of Titan.¹² Having lakes, a sea level, and the transfer of liquid between lakes are all parallels to Earth.  Titan, though very different from earth, also demonstrates many comparisons to our home planet.  These constant similarities between the Earth and Titan are extremely interesting, as we can learn more about Titan by looking at Earth and possibly vice versa (Titan could hold answers to the production of prehistoric life on Earth).

• Rain Storms on Titan

Titan has a hydrological cycle, similar to Earth’s water cycle.  It has been theorized that the methane rain and possible ethane snow enforce a hydrological cycle, that eventually drains into the lakes located on the moon’s surface.  Observations have also suggested that evaporation of the lakes occurs, resulting in the lakes being ringed by dark beaches.⁷

Another similarity of Titan to that of Earth, is the occurrence of rain storms, resulting form hydrocarbon clouds.  These rainstorms on Titan can become rather extreme. A study led by Saun P. Faulk, a former graduate student of UCLA’s Department f Earth, Planetary, and Space Sciences, determined how extreme weather conditions on Titan have shaped the moon’s surface through simulations of the celestial body.¹² It was discovered, that like Earth, extreme methane rain storms imprint the moon’s icy surface, paralleling the effects of rain storms on Earth.¹² Sand dunes were found to dominate the lower latitudes of Titan, while large lakes dominated the higher grounds, drawing yet more parallels to Earth with water transporting sediment into lower regions.¹²  Titan’s rainstorms occur sporadically, only occurring less than once per Titan year (approximately twenty-nine and a half Earth years).¹²  These extreme weather conditions are presumed to be caused by variations in temperature; wetter air from higher latitudes and drier air form lower latitudes cause the intense rainfalls.¹²

(Video 3: Take a Trip to Titan. https://www.youtube.com/watch?v=jfiE5AVM7Zc)

• The “Disappearing Islands”

During fly-bys of Titan, the Cassini Huygens space probe noticed a feature that was not there on previous cycles.  This feature continues to disappear and reappear in images taken by the space probe.  These features would soon be known as the “disappearing islands.”  Initially, this astronomical discovery was proposed to be a vanishing island, rising and sinking in the liquid on the surface of Titan.¹²  Another assumption was the influence of waves since the “islands” would occasionally double in size.¹²  Through further observation, it was speculated that the alterations in the object could be influenced by seasonal changes on the moon.¹²  A study was directed by Michael Malaska of NASA’s Jet Propulsion Laboratory (JPL), where they mimicked the cold temperatures on Titan and the affects it could have on the moon’s nitrogen atmosphere.  The rain on Titan is composed of extremely cold methane, that when it falls, absorbs nitrogen in a substantial amount, which then collects in the pools.¹²  As they altered temperature, pressure and the ethane/methane composition, nitrogen would bubble out of the lake solution, creating these island forms that were detected by the Cassini-Huygen Space probe.¹² Exsolution occurs when nitrogen is released from something and was found to occur during seasonal changes and the warming of seas on Titan.¹²  The Cassini space probe was able to detect these nitrogen bubbles due to their reflective property.¹²  This is important because it is yet another aspect of the lakes, whose presence was once unknown, which was detected through the fly-bys of the Cassini-probe, and it also demonstrates more similarities Titan has with Earth; seasonal changes, and the influence these alterations have on the distant world.

New Research Being Done

• The Termination of the Cassini-Huygens Space Probe

Though it provided researchers with immense amounts of data about Titan, as well as Saturn and other moon’s orbiting the planet, the Cassini-Huygens space probe mission has been terminated.  There are two reasons as to why the probe has been destroyed.  The first motive being that the spacecraft was running out of fuel.¹³ Only 61 of the original 6,565 pounds of propellant fuel remained in the Cassini craft.¹³ Some argue that the remaining fuel could have been used to drive the probe into the orbit around Saturn, allowing for it to still collect data.¹³  However, the primary reason the mission was ended, dealt with the international treaty; to not contaminate potentially-habitable worlds.¹³  In other words, it was destroyed in order to “preserve our ability to study other worlds as they exist in their natural states; to avoid contamination that would obscure our ability to find life elsewhere (if it exists), and to ensure that we take precautions to protect Earth’s biosphere in case it does.”¹³  Since, Cassini has also studied the moon Enceladus, and discovered the moon contains plumes as well as an undersea saltwater ocean, contact with the pristine entity would prove to be a risk.¹³  If we were to propel Cassini into Saturn’s orbit, Cassini could collide with Enceladus, contaminating the extraterrestrial world; radioactive pollution being one of the worst-case scenarios.¹³  Other cases include depositing Earth passengers in the situation of crashing into Enceladus.¹³ It has been proven that some organisms can survive the harsh vacuum of space; certain bacteria have been discovered to survive up to a year to a year and a half on the space station, as well as microscopic aquatic animals, such as tardigrades (see FIGURE 8), which have survived for at least ten days in space.¹³  If invasive species were to inhabit Enceladus after a collision, results could be catastrophic.¹³  For the reasons listed, the Cassini-Huygens space mission was discontinued, but new missions are being explored.

• Measuring Waves

Since the Cassini probe’s mission was recently terminated due to the probe crashing into Saturn’s atmosphere, new ideas for future projects to explore Titan are eminent.  In order to further investigate the lakes on Titan, researchers are designing new probes to land on the lakes.  However, in order for future missions to safely land on the lakes, teams need to investigate roughness of the waves.  A study, titled “Surface Roughness of Titan’s Hydrocarbon Seas,” which was led by the University of Texas under the command of Cyril Grima, a research associate at the University of Texas for Geophysics (UTIG), concluded that the waves did not appear to be too aggressive.¹²  The primary purpose of their research was to verify the activeness of lakes in the northern polar region of Titan.¹²  The team used data obtained from the Cassini mission, which included measurements of Titan’s northern lakes.¹²  Grim and his team, with the help from sources such as the Cassini RADAR Team, researchers from Cornell University, the Johs Hopkins University Applied Physics Laboratory, and NASA’s Jet Propulsion Laboratory, developed a technique known as radar statistical reconnaissance.¹²  This technique allowed the researchers to measure snow density and surface roughness.  From their findings using this incredible system, Grim and colleagues were able to determine the waves on Titan are calm enough for future missions to land and investigate the conditions of Titan.  Since Titan has a rich organic chemical environment, it could be explored for further research into the prebiotic chemistry necessary for life to function.¹² This is an interest for planetary researchers, as it could result in research promoting ideas as to how life on Earth began.

• Methods for Future Missions

There have been many different methods proposed for future missions to investigate more deeply into Titan.  Building on the success of the Cassini-Huygens space probe, it has been advocated for a future mission to use another aerial explorer equipped with a lander.¹²  This would entail a balloon to explore the moon at a lower altitude, while the lander would explore the surface.

Other ideas proposed fall into two categories: Lighter-Than Air crafts (LTA) and Heavier-Than-Air (HTA) crafts, both of which would be suited for Titan’s dense atmosphere.¹²  The moon’s dense atmosphere allows for LTA crafts, such as balloons, to fly better, while Titan’s low gravity is beneficial for HTA crafts, which would weigh less on a world with lower gravity.¹²  Other beneficial factors for the two types of vehicles include energy usage and accuracy.  LTA technologies are more buoyant, therefore needing less energy to stay aloft, and thus allowing for more energy to be directed towards science instruments and communication.¹²  HTA objects are better for hitting targets quickly and with accuracy since winds have less of an effect on them.¹²  Some designs for future missions include the rotorcraft, fixed wing vehicles, the TSSM Montgolfiere Balloon, Titan Helium Balloon, each with varying constructive qualities.¹² The Montgolfere Balloon and Titan Aerial Daughtercraft (TAD) are depicted in Figure 9 and Figure 10.  These are just illustrations of possible new crafts that will further explore Titan.

(Video 4: What About a Mission to Titan? https://www.youtube.com/watch?v=rzg78ySwIn4)

What Could This Mean for Future Resources?

• A Potential Fuel Source

Titan has many resources that are well suitable to harvest for energy, such as Methane, which is available on the surface.¹⁴ Because of the abundance of methane on Titan, it could also be used as a fueling center for further space travel. As there is a vast ocean underneath the surface, harnessing the energy from the water on Titan may prove to be valuable in the future.

• Creating A Colony on Titan

Plans for the future of Titan are still being conducted on how we can properly use Titan’s resources.  There are many resources on Titan that future people could use for energy such as methane and hydropower, both mentioned previously. Presently, Titan is one of the places that warrants serious consideration for colonization, second only to Mars.¹⁴ Although there are many lakes on Titan, they are not however on equal ground, which indicates that Hydro Power would create challenging engineering obstacles (though still possible).¹⁴ One suggestion is to construct Hydro Turbines, due to the large tides that occur on Titan, to create a suitable living colony.¹⁴ Even though there are no immediate proposals for colonization at this time, perhaps future generations will be able to call Titan their home.

(Video 5: Titan: Saturn’s Largest Moon- An Alternate Earth. https://www.youtube.com/watch?v=jiD-NMq0pRE)

Conclusion

The Cassini-Huygens space probe was the key to discovering the lakes on Titan, and their resource potential. Even though the mission of the Cassini-Huygens Space probe was to take photos of Saturn and investigate Titan’s dense atmosphere, it’s images of the planet’s moons, in this case Titan, lead to the discovery of the lakes on the extraterrestrial rock; proving to be of importance as it is the only other planetary mass found with water in it.  Although the Cassini-Huygens mission has been terminated, new missions to explore the foreign world are being investigated with new concepts of transport being procured.  Future missions will allow for us to learn more about the moon and can then in turn lead to additional future possibilities.  Though the data collected has brought on many conjectures and theories (exploring the origins of life, potential resources, and the possibility of colonization), one day these inferences can be proven true or can be applied and put into practice.

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References

¹ ESA. Cassini-Huygens Mission Facts. https://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Cassini-Huygens_mission_facts (Accessed 27 February 2019)

² NASA Science. Solar System Exploration. https://solarsystem.nasa.gov/moons/saturn-moons/titan/exploration/?page=0&per_page=10&order=launch_date+desc%2Ctitle+asc&search=&tags=Saturn&category=33#dutch-astronomer-christiaan-huygens-discovers-titan (Accessed 27 February 2019)

³ ESA. Titan’s Tides Point to Hidden Ocean. https://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Titan_s_tides_point_to_hidden_ocean (Accessed 27 February 2019)

⁴ ESA. Cassini Instruments. HTTPS://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Cassini_instruments (Accessed 27 February 2019)

⁵ ESA. Cassini-Huygens. https://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Huygens_instruments (Accessed 14 March 2019)

⁶ Spie. Spectrometer, Spectroscope, and Spectrograph. http://www.spie.org/publications/fg08_p02_spectrometerspectroscopespectrograph?SSO=1 (Accessed 13 March 2019)

⁷ ABC Science. Liquid Flowing on the Surface of Titan. http://www.abc.net.au/science/articles/2008/07/31/2320322.htm (Accessed 24 January 2019)

⁸ Astrobiology Magazine. Titan’s Subsurface Ocean. https://www.astrobio.net/titan/titans-subsurface-ocean/ (Accessed 21 January 2019)

⁹ NASA. The Mysterious “Lakes” on Saturn’s Moon Titan. https://www.nasa.gov/feature/the-mysterious-lakes-on-saturns-moon-titan (Accessed 20 January 2019)

¹⁰ J. Vincent. Independent. What are Sinkholes, How are They Formed, and Why are We Seeing So Many. Retrieved from https://www.independent.co.uk/news/science/sinkholes-what-are-they-how-do-they-form-and-why-are-we-seeing-so-many-9136235.html (Accessed 28 February 2019).

¹¹ M. Schirber. Studying Titan’s Lakes on Earth. https://phys.org/news/2010-01-titan-lakes-earth.html (Accessed 21 January 2019)

¹² Universe Today. Methane Lakes on Titan. https://www.universetoday.com/tag/methane-lakes-on-titan/ (Accessed 22 January 2019)

¹³ F. Kossakovski. PBS News Hour. Analysis: Why NASA’s Cassini Probe had to be Destroyed. https://www.pbs.org/newshour/science/analysis-nasas-cassini-probe-must-destroyed (Accessed 14 March 2019)

¹⁴ NewScientist. Titan’s conditions could be just right to power US-sized colony. https://www.newscientist.com/article/2140048-titans-conditions-could-be-just-right-to-power-us-sized-colony/ (Accessed 27 February 2019)