Life on Titan and Enceladus
Alyssa Derdall, Leah Dueck, Matthew Gjevre, Maxine Paterson
Saturn is surrounded by 62 moons, Titan and Enceladus being two that have been recently explored by the Cassini mission orchestrated by NASA. This mission has revealed previously unseen events that have changed our understanding of how planetary systems form and what conditions might lead to habitats for earth-like life. With Enceladus’s global ocean, unique chemistry and internal heat, it has great potential to be one of the world’s where earth-like life could exist. Along with Enceladus, Titan contains molecular substances that have the ability to support life. The question we wish to answer with this project is: how do we know there is the possibility for life on Saturn’s moons, Titan and Enceladus?
Saturn, named after the Roman god of agriculture, is a gas giant predominantly comprised of hydrogen and helium. Saturn is the sixth planet from the Sun and is the second largest planet in the Solar System with a radius of nine times the size of Earth. Its core is made up of iron, nickel and rock and has a pale-yellow hue due to ammonia crystals in its upper atmosphere. Saturn has the largest and most visible ring system with 9 continuous main rings that are composed mostly of ice particles. The rings extend from 6,630 km to 120,700 km outward from Saturn’s equator averaging 20 meters in thickness.
Figure: 1 Map of the Saturn system, Wikimedia Commons Images, https://commons.wikimedia.org/wiki/File:Saturn-map.jpg, NASA
This is a brief look at some of the physical and chemical conditions that can impact the development of life.
In terms of acidity most bacteria exist within fairly neutral systems, however bacteria have been known to exist in environments ranging from 1 to 11 on the pH scale.1 Bacteria that exist in highly acid environments are called acidophilic. An example of an acidophilic bacterium is Helicobacter pylori. It can be active in the gastric acid inside of human stomachs which has a ph. ranging from 1 to 3.2 Bacteria that exist in highly alkaline environments are called alkalophilic. An example of this type of bacterium is the Natronomonas bacterium able to grow in a pH. of 11.3 Due to the extreme environments that bacteria are known to live in we can safely say that the acidity of celestial objects does not rule out the existence of life.
Bacteria have been known to survive and be active in temperatures ranging from -20 to 121 degrees Celsius. However, bacteria are able to survive in temperatures below -20 degrees without activity.1
This places a limit on the likelihood of life existing in certain environments. In our case, since Titan and Enceladus are relatively cold, the -20 degree limit would be the major point of contention.
Typically, it is thought that the most likely area for life is on planets within the “habitable zone” as this provides an ideal amount of heat for liquid water. However, if heat and energy are produced through some means apart from the Sun, such as tidal heating, this is not necessarily required.
Not all bacteria require oxygen to survive, meaning that it is not necessary to consider as an essential element for life.
In terms of an atmosphere; in general, it is not necessarily required for life if liquids exist. However, Enceladus’s liquids are under a deep sheet of ice and Titan does have an atmosphere meaning that no issues should arise pertaining to atmospheres either way.4
In all environments bacteria hold solutes in their cell envelopes and obtain liquid water through osmosis. Liquids also allow for certain chemical reactions resulting in life relying on the presences of liquids.5
Environments that contain large amounts of water give microbes the ability to grow extensively on the surface of materials in the form of biofilms. In arid conditions (including hot and cold deserts), however, bacteria exist primarily in soil or rock. This method also provides large amounts of minerals that the bacteria need for growth.1
This leads to the conclusion that, on celestial bodies with limited amounts of liquid, microorganisms may still be able to exist inside of rock formations.
Liquid water is a somewhat better contender for allowing the development of life due to its special properties such as expanding when it freezes which leads to solid water floating to the surface of liquid water rather than sinking to the bottom like other molecules do. This in turn allows for creatures living on seafloors and lakebeds to survive. However, it is conceivably possible for life to exist within another liquid besides water provided that osmosis can occur within that liquid.
Bacteria can assimilate organic material such as carbohydrates, amino acids, nucleic acids, hydrocarbons as well as some inorganic molecules. Both types can be used to fuel the construction of new biomass.1
Carbon is the most important element to consider in terms of nutrients since plays a very important role in the molecular composition of cells. This is because of carbons’ unique bonds it can form long chains of molecules allowing the formation of lipids and proteins and all other organic molecules which are the building blocks of life. Silicon is also hypothesized to work similarly, however since no known silicon life exists we will be looking just at carbon based lifeforms.6 If there is evidence of organic molecules existing on Enceladus and Titan then this requirement for life is fulfilled.
Life is less likely to thrive while under bombardment from meteorite impacts or other such hazards. Life can still exist in such environments but if they are lacking then there is a higher chance that life will be able to thrive.
Enceladus, named after a giant of Greek Mythology, is the sixth largest out of Saturn’s sixty-two moons, even though its five-hundred-kilometer diameter is small enough that it wouldn’t take up even half of our province’s surface area.7 It is also one of the most fascinating satellites in our Solar System due to the potential for life within, discovered by the unique features it presents including icy plumes full of elements that have been found to be shooting out from the global ocean beneath the moon’s icy exterior.
The moon was first discovered by the German-born, British astronomer, William Herschel on August 28th, 1789, who also discovered Uranus and other moons of the outer planets. At this time, he was the holder of the largest telescope in the world which significantly assisted with these findings.8 The moon was observed during a Saturnian Equinox, where the rings are directly in the Sun. This is the easiest time to observe Saturn’s structures, since it allows three dimensional views of objects surrounding and within the rings based on the shadows that arise due to the reduced glare of the rings.8 This only happens every fifteen years, so Enceladus was a lucky catch!
Figure: 2 Saturn-Taken less than a month after it’s Equinox. Image via
The next point in Enceladus’ history came with the images that were captured during the flybys of the Voyager space crafts. Launched in 1977, these probes were designed to study the outer planets of our Solar System and their features. Voyager 2 the third spacecraft to view Saturn and it’s features and was able to capture the first close-up image of Enceladus on August 25th, 1981.9 (Figure 3) Without these images, no one would have noticed the different surface features of the moon, and what they tell us about its internal activities.
Figure 3 (below): One of the first close-up images of Enceladus, taken by Voyager 2 in 1981. Image via: NASA/JPL/USGS https://photojournal.jpl.nasa.gov/catalog/PIA00347
One thing that these close-up images didn’t capture, was the icy plumes that were spraying out from beneath the icy surface on the moon’s southern side. Believe it or not, it was the pictures of Saturn by Voyager 1 that first featured this phenomenon. However, no one took a careful enough look at these images to realize the importance of what lay in the background. It was only in the past year that these images were brought back up for examination by Ted Stryk, who recently worked with the NASA New Horizons team.10 After Cassini first discovered these shooting plumes full of elements in 2005, scientists have been on their toes with new findings. However, Stryk had the first thought of looking back on images to determine whether these plumes could have been detected much longer ago. He stacked photos from the 1981 fly-by that featured Saturn with the moon in the background. Focusing on Enceladus during this process revealed faint lines shooting off the moon, which have now been found to be plumes. (Figure 4) Due to this, the discovery of a possibility for life to exist on Enceladus was pushed back 25 years.10
Figure 4: One of the original images of Saturn taken by Voyager 1 in 1980, along with the image that was created through stacking more of these first pictures. Image via NASA / JPL-Caltech / Ted Stryk http://gizmodo.com/an-exciting-discovery-may-be-lurking-in-this-voyager-ph-1792624903
What first tipped off that the moon may have geological activity came from studying the surface of it. Voyager 2 was the first to observe the details of the icy top layer, but these features were much more distinguishable with Cassini’s flyby images. Unlike most moons, Enceladus features five different types of terrain. Areas that have been recently resurfaced, appear smooth and unharmed. It was this observation that began the theory of a possible salty subsurface ocean. It appears that the youngest surface area on Enceladus consists of four fractures known as tiger stripes which were first observed by Cassini.11 Out of these stripes, appears to be plumes of water vapor and ice along with numerous other elements that will be discussed later on.
Figure 5: Tiger Stripes. Taken in 2005. Image via NASA/JPL/Space Science Institute https://upload.wikimedia.org/wikipedia/commons/thumb/0/09/Enceladus_Tiger_Stripes_Up_Close_PIA06247.jpg/240px-Enceladus_Tiger_Stripes_Up_Close_PIA06247.jpg
Enceladus also features many craters of different sizes, with some being up to thirty-five kilometers in diameter. These craters come in different levels of wear and tear, suggesting that the surface has been reformed many times in the past. The moon’s surface also presents different tectonics including troughs, scarps, rifts, and belts of grooves and ridges.12 Smooth plains, and linear cracks also add to the texture of this icy satellite.
Figure 6: Close up of Enceladus’ surface, taken in 2009 by Cassini. Image via NASA/PL/Space Science Institute https://www.jpl.nasa.gov/news/news.php?feature=1890
Although the ocean on Enceladus is covered by surface ice, it’s geysers are able to breakthrough it’s subsurface by means of travelling through the ice cracks on the moon. On October 28, 2015, Cassini travelled through the moon’s geysers which consisted of plumes of gas and icy particles. Through this particular fly-through, the spacecraft was able to pick up on a large amount of molecular hydrogen. As discussed above, Cassini scientists have been able to confirm that the causes of these large amounts of molecular hydrogen is from the hydrothermal reactions taking place on the moon’s very own ocean floor. These reactions could very well be comparable to Earth’s hydrothermal vents and the hydrogen-producing reactions occurring in these vents.13
Planet Earth is host to many hydrothermal vents, which can be described as fissures in the Earth’s ocean crust. It is through the ocean crust that geothermally heated water flows out. This is where the water interacts with the Earth’s magma. Different types of rich bacteria are hosted by these hydrothermal vents and we see this in geographical locations such as Grand Prismatic Spring, located in Yellowstone National Park.13
Figure 7: The above photograph created by Cassini with the use of back-light, provides an image of Enceladus’s geysers erupting into space which provides evidence for Cassini scientists of molecular hydrogen which could have been created through the hydrothermal processes on Enceladus’s ocean floor. Image via NASA, https://saturn.jpl.nasa.gov/science/enceladus/
Figure 8: Image via NASA, https://solarsystem.nasa.gov/planets/enceladus.
At this same time, in 2015, Cassini tested these plumes and concluded that almost 98 percent of the gas is water and approximately only 1 percent is hydrogen. Along with these elements is a combination of carbon dioxide, methane, and ammonia.14
Cassini used its Ion and Neutral Mass Spectrometer (INMS) instrument to measure the plume’s gases and their chemical compositions. The INMS detects these gases by “sniffing” them to enable it to dissect their chemical make-up. This instrument was developed specifically to test and analyze the upper atmosphere of Titan. However, because of the 2005 discoveries of Enceladus’s plumes spewing icy substance from its fissures (located close to the south pole), the attention was then focused on Enceladus and the instrument was then focused on gathering samples from these unexpected plumes.14
Figure 9: Image Via Wikimedia, https://commons.wikimedia.org/wiki/File:Cassini_spacecraft_instruments_1_ukr.png: Ion and Neutral Mass Spectrometer (INMS) within Cassini used to measure the gases and chemical compositions within the plumes.
Because of these reactions, living microbes could indeed be supported on Enceladus because of its possibility for a source of chemical energy.13
Figure 10: This picture displays the “tiger stripes” and the geysers of Enceladus. Photographed by the Cassini-Huygens probe in its October 2015 pass-through. Image via NASA, https://www.jpl.nasa.gov/news/news.php?feature=4755
Even though it appears to be one of the brightest objects seen in our solar system from a standard distance, since it’s thick and clean icy surfaces reflects almost all the sunlight that hits it, the temperature of Enceladus’ surface is still chilly at negative two-hundred degrees Celsius.15 This created a bit of confusion when it was discovered that the interior of the moon is much warmer, and questions arose regarding how this planet could possibly hold a global ocean beneath the cold surface.
The best theory thus far for this interior heat, includes a more outer moon, Dione and its gravitational tug. Right now, Enceladus is in a 2:1 mean-motion orbital resonance with the Dione, which means that Enceladus goes around Saturn twice as fast as Dione does. This motion maintains the eccentricity of Enceladus’ orbit, and results in tidal deformation which leads to the heating of its interior. This aids with the idea that the ocean gets hotter towards the rocky core.16
Another contributing factor is the wobble of the moon. This wobble could only exist if there were a layer of water beneath the surface moving around as the moon rotates and gets pulled about. In September of 2015, the results of this discovery were published, after scientists brought together data from Cassini dated from January 2005 to April 2012.17 They looked at any changes in rotation, and analyzed how different interiors would affect this wobble, finding that the only way for the movement to be possible would be due to a global ocean. A mathematical layout of the simulations that were conducted is available here. Scientists then looked into whether or not this ocean model could survive for long periods of time, but this would depend on, “Enceladus’s eccentricity history, its initial thermal state, the amount of tidal energy dissipated in Saturn, mechanisms for forming melted (or soft) portions that enhance tidal energy dissipation, a range of plausible physical characteristics of the shell and core, and the power currently being produced.”17 It has been determined that Enceladus’ icy surface varies anywhere from less than five kilometres thick to around twenty kilometres thick, with the global ocean below having a depth of around ten kilometres.”18
It may be hard to believe, but winter on Enceladus in the south has also assisted scientists with their observations of the heated moon. This is because the Sun is no longer a heat contributor at this time, and Cassini can catch the heat from the moon much easier. No pictures of the constantly changing surface are able to be taken while this season is occurring, but this data is allowing scientists to learn more about the ocean that lies beneath.19
The week of April 15, 2017 is perhaps one of the most important travels for the Cassini space mission thus far. The very thought of Saturn’s moons having the ability to support life, although possible given Titan and Enceladus’ surface liquid, seemed too intricate to prove with very little time left for Cassini’s travels. However, on this day Cassini scientists discovered that Enceladus’s ocean floor could indeed support life due to the ocean floor’s habitable area.20 In 2005 Cassini discovered Enceladus’s fissures erupting rapid streams of water ice and vapor which served as evidence that Enceladus contains a salty ocean below its composed icy surface.20 It has more recently been discovered that these streams contain hydrogen gas proving that there is current reactions taking place on the ocean floor.20 This same chemistry is what supports microbial life on Earth which suggests that Enceladus may be in the lead for providing life outside of Earth that being in our very own solar system! In past travels through Enceladus’s ocean floor Cassini discovered methane and formaldehyde organic molecules and in October 2015 ample amounts of Hydrogen gas (H2) was found 48 km above its surface. It is believed that the H2 is produced by the ocean floors’ hydrothermal activity which could possibly be similar to Earth’s seafloor vents that eject H2 (which sustains the well-nourished microbial existence).20 Some researchers are left wondering if this hydrothermal activity is being created by the reaction of water and iron or perhaps other reactions producing rudimentary organic molecules which are stuck within its rocky core. Methanogenesis is a chemical reaction where burning hydrogen and carbon dioxide are both dissolved in the ocean creating methane and water within the ocean. The microbes that could be hosted by Enceladus would possibly be able to produce their own energy through methanogenesis. This is the same reaction supporting the creation of life on Earth.20
Again, in 2015 glass beads were found on Saturn’s E rings which led to Cassini scientists studying the particles. Their conclusion was that these glass beads came from Enceladus’s plumes which provides concrete evidence of seafloor activity.20 These discoveries of Enceladus’s ocean floor have left scientists in awe that life could possibly exist on this beautiful, unfathomable moon of one of the most spectacular planets of our solar system.
Watch this short video on Enceladus’s plumes and its favorable conditions to host and sustain life.
Although currently life has not been discovered below Enceladus’s ice covered crust, the chemistry needed to host and sustain such life is indeed on this moon.13
Lead author of the Enceladus study, Hunter Waite, stated the following about the moon itself: “Although we can’t detect life, we’ve found that there’s a food source there for it. It would be like a candy store for microbes.”13
This statement brings excitement and optimism that we, in our lifetime, may possibly still discover that Enceladus may actually be host to living organisms pushing its fame even further ahead than planets such as Mars.13
Scientists are more hopeful than ever, and the search for life goes on with missions being continually discussed. Proposed in 2015, the “Enceladus Life Finder” mission may have a launch data of early 2020s. The “ELF” mission would involve a “search for biosignature and biomolecules in the geysers of Enceladus”.21 The orbiter would fly over the moon approximately eight to ten times through the jets that are spraying out water full of elements, and later be tested for evidence of microbial life.21 The three objectives of the mission are:
“1. To measure abundances of a carefully selected set of neutral species, some of which were detected by Cassini, to ascertain whether the organics and volatiles coming from Enceladus have been thermally altered over time.
2. To determine the details of the interior marine environment — pH, oxidation state, available chemical energy, and temperature — that permit characterization of the life-carrying capacity of the interior.
3. To look for indications that organics are the result of biological processes through three independent types of chemical measurements that are widely recognized as diagnostic of life.”21
For life to exist on Earth there must be water, an energy source and a combination of carbon, nitrogen, oxygen, phosphorus, and sulphur. These same elements excluding phosphorus and sulfur exist on Enceladus as well. Perhaps this is where life will begin to take shape if it hasn’t already.13
Life on Titan
Until the Cassini mission, there had been very little information retrieved about Saturn’s moon Titan. It has a dense atmosphere of orange haze composed of nitrogen and methane that prevented us from knowing anything about the surface properties of the moon. Titan had been observed as the only known world that had a nitrogen rich atmosphere, much the same as Earth. The Cassini mission enlightened our views and knowledge about Titan and taught us about its Earth-like properties that have the potential to sustain life. The Cassini mission unveiled the lakes and seas of methane and ethane that are replenished by hydrocarbon clouds. This system of lakes, seas and rivers of liquid are playing the same role on Titan as water does on Earth.
Figure 11: The Colorful Globe of Saturn’s Largest Moon, Titan, Passes in Front of The Planet and Its Rings in This True Color Snapshot from NASA’s Cassini Spacecraft. https://saturn.jpl.nasa.gov/science/titan/ Web. 8 June 2017
Titan’s atmosphere contains approximately 1.4% methane in its stratosphere. This is the second most abundant constituent in Titans atmosphere with Nitrogen being the most common.22 Titan’s atmosphere also contains a noticeable amount of molecular hydrogen and traces of hydrocarbons besides methane including ethane and acetylene. Traces of cyanogen and carbon dioxide were also detected.23 The Cassini-Huygens mission determined this composition. Methane is an organic molecule which shows that organic molecules are present on Titan. Organic molecules are the building blocks for life as we know it and are required for life to develop. Their presence on Titan does support the possibility that life could exist on Titan however it by no means guarantees this. Certain bacteria are known to consume Methane. On Earths ocean floor bacteria that do not require oxygen thrive on methane being released from sediments.24 This shows that life forms can process Methane as a source of nutrients.
Figure 12: Cassini and Huygen revealing Titan as never seen before using powerful instruments to peer through the persisting cloud layers. NASA/JPL/University of Arizona/University of Idaho. 2015-12-4.
Titan has an average surface temperature of -179 degrees Celsius.25 Since the coldest known temperature in which life is able to grow is approximately -20 degrees Celsius this acts against the likelihood of life existing on Titan.1 It is not impossible that alien life could evolve to be resistant to these temperatures though. Low temperatures imply very low rates of reactions. Since the temperature on Titan is unfavorably cold this would imply a problem for the development of life. However, by the use of catalysts, life can speed up any thermodynamically favorable reaction.26 If some type of catalyst exists to increase the ability for chemicals to react the temperature of Titan may not prevent life from being active.
While Titan does not have large oceans like Earth or Enceladus it does contain lakes composed of methane. We know this because the Cassini ISS system detected a large dark lake like area on the surface of Titan.27 The Cassini Imaging Science System (ISS) uses spectral filters and imaging capabilities.28 These allowed it to gather the information regarding the large dark lake like area on Titan which is how we know this lake exists. Additionally, hundreds of “radar dark” areas were detected by the Cassini RADAR system.27 The Cassini RADAR system sends out radar transmissions towards targets and captures blackbody radiation and reflected radar signals from targets.29 These radar dark patches are thought to be smaller lakes. The Gas Chromatograph Mass Spectrometer on the Huygens probe has allowed for the chemical composition of lakes to be estimated, the results have shown that the lakes are composed of hydrocarbons.27
One may wonder what the environmental limits consist of when it comes to developing life without water. Titan is so cold, the amount of energy available for building biochemical structures is assumed to be limited, although we do not have a lot of experience with temperatures these drastically low.30 On Titan, the extreme cold temperatures deny the planet of liquid water, although the presence of liquid hydrocarbons are present behaving much like the hydrological cycle.30 Cassini-Huygens mission has revealed rivers and lakes of methane and ethane, which evaporate to form clouds, and hydrocarbons rain down back to the moon’s surface.30 An issue with these hydrocarbons is the low solubility of organic substances in liquid methane. Because water is so soluble, we have no experience of how life would adapt with a low solubility26. Active transport and organisms with large surfaces to volume ratios has potential to mitigate this problem.26 Methane and ethane are also simple hydrocarbon molecules, but they are versatile. They can assemble themselves into complex structures and have the potential to form life30. The real question is, can hydrocarbons on Titan become the “life as we do not know it”?
There are five definite craters caused by the impact of asteroids on Titan’s surface as well as another 44 unconfirmed craters that have been identified on the 22% of Titan’s surface which the Cassini mission has imaged. 31 This shows that Titan is apparently not very well protected from asteroid impacts. While asteroid impacts may be infrequent, since these craters could have been accumulated over the course of many years, this does at least show that Titan is not as well protected from large meteor impacts as the Earth is. Additionally, the hydrocarbon lakes are thought to evaporate rapidly enough that the shorelines of Titan’s lakes could move significant distances per year.32 Since the most likely area for life is in these lakes the fact that they experience this much evaporation could affect the development of life. While life can still exist with this going on the fact that this occurs shows that the climate of Titan can be unstable. Based upon this it seems that Titan is not as calm an environment as would be preferred for the development of life.
If life was based from liquid methane on Titan, it could be widespread on the surface of the moon just as water is distributed on Earth. Sunlight on Titan produces complex hydrocarbons that could be a source of energy when reacted with atmospheric hydrogen. Similar to the process on earth, the presence of chemical energy in the form of organics on Titan is another reason why there is speculation about earth-like life on this moon. Life on the present-day Titan is plausible with the ethane providing the best energy source.26
In conclusion Titan plays host to organic molecules, in the form of hydrocarbons, which could act as a source of nutrients and facilitate the development of life. Titan is very cold (-179 degrees Celsius) which acts against life being present but the existence of catalysts could potentially allow life to exist. Titan has lakes of liquid hydrocarbons which, since it is a liquid, is an advantage for the development of life however, since the liquid is hydrocarbons it is difficult to fully predict whether life could survive in these lakes. Titan also seems to have a somewhat perilous environment which may act against the development of life. Based upon this information we can see that some of the requirements for life are fulfilled but there is not enough information to conclude whether life does exist, or even if it could exist, on Titan.
The Cassini mission that NASA coordinated was successful at revealing unseen footage of both Titan and Enceladus. This mission has changed our understanding of what different conditions may lead to habitat earth-like life. Current life has not yet been discovered on either of the moons, although there is evidence that both moons have the chemical capability of hosting life. On Enceladus, the Cassini mission has proven that there is potential for life, as there is a salty ocean underneath the ice-covered crust. Energy sources such as carbon, nitrogen and oxygen exist on Enceladus, which are very important components of life here on earth. The discovery of large amounts of molecular hydrogen caused by the hydrothermal reaction on Enceladus’ ocean floor is very similar to Earth’s hydrothermal vents and the hydrogen-producing reactions taking place in them. The similarities of the hydrothermal vents and the chemicals they produce is a sign that life could exist on Enceladus. On Earth, the coldest known temperature for active life is at -20 degrees Celsius. The temperatures on Titan at -179 degrees Celsius, and Enceladus at -200 degrees Celsius is a common burden both moons have that will inhibit growth of earth-like life. On titan, the chemical composition is quite a bit different consisting of lakes of methane, opposed to large oceans like Enceladus and Earth.
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