Saturn’s moon Enceladus has captivating scientists ever since the Voyager 2 mission passed through the system in 1981. The mystery has only deepened since the arrival of the Cassiniprobe in 2004, which included the discovery of four parallel, linear fissures around the southern polar region. These features were nicknamed “Tiger Stripes” because of their appearance and the way they stand out from the rest of the surface.
Since their discovery, scientists have attempted to answer what these are and what created them in the first place. Thankfully, new research led by the Carnegie Institute of Science has revealed the physics governing these fissures. This includes how they are related to the moon’s plume activity, why they appear around Enceladus’ south pole, and why other bodies don’t have similar features.
As soon as the Cassini-Huygens mission arrived the Saturn system in 2004, it began to send back a number of startling discoveries. One of the biggest was the discovery of plume activity around the southern polar region of Saturn’s moon Enceladus’, which appears to be the result of geothermal activity and an ocean in the moon’s interior. This naturally gave rise to a debate about whether or not this interior ocean could support life.
Since then, multiple studies have been conducted to get a better idea of just how likely it is that life exists inside Enceladus. The latest comes from the University of Washington’s Department of Earth and Space Sciences (ESS), which shows that concentrations of carbon dioxide, hydrogen and methane in Enceladus’ interior ocean (as well as its pH levels) are more conducive to life than previously thought.
We can’t seem to get enough of Saturn. It’s the most visually distinct object in our Solar System (other than the Sun, of course, but it’s kind of hard to gaze at). The Cassini mission to Saturn wrapped up about a year ago, and since then we’re relying on the venerable Hubble telescope to satisfy our appetite for images of the ringed planet.
The Cassini orbiter revealed many fascinating things about the Saturn system before its mission ended in September of 2017. In addition to revealing much about Saturn’s rings and the surface and atmosphere of Titan (Saturn’s largest moon), it was also responsible for the discovery of water plumes coming from Enceladus‘ southern polar region. The discovery of these plumes triggered a widespread debate about the possible existence of life in the moon’s interior.
This was based in part on evidence that the plumes extended all the way to the moon’s core/mantle boundary and contained elements essential to life. Thanks to a new study led by researchers from of the University of Heidelberg, Germany, it has now been confirmed that the plumes contain complex organic molecules. This is the first time that complex organics have been detected on a body other than Earth, and bolsters the case for the moon supporting life.
The existence of a liquid water ocean in Enceladus’ interior has been the subject of scientific debate since 2005, when Cassini first observed plumes containing water vapor spewing from the moon’s south polar surface through cracks in the surface (nicknamed “Tiger Stripes”). According to measurements made by the Cassini-Huygens probe, these emissions are composed mostly of water vapor and contain molecular nitrogen, carbon dioxide, methane and other hydrocarbons.
The combined analysis of imaging, mass spectrometry, and magnetospheric data also indicated that the observed southern polar plumes emanate from pressurized subsurface chambers. This was confirmed by the Cassini mission in 2014 when the probe conducted gravity measurements that indicated the existence of a south polar subsurface ocean of liquid water with a thickness of around 10 km.
Shortly before the probe plunged into Saturn’s atmosphere, the probe also obtained data that indicated that the interior ocean has existed for some time. Thanks to previous readings that indicated the presence of hydrothermal activity in the interior and simulations that modeled the interior, scientists concluded that if the core were porous enough, this activity could have provided enough heat to maintain an interior ocean for billions of years.
However, all the previous studies of Cassini data were only able to identify simple organic compounds in the plume material, with molecular masses mostly below 50 atomic mass units. For the sake of their study, the team observed evidence of complex macromolecular organic material in the plumes’ icy grains that had masses above 200 atomic mass units.
This constitutes the first-ever detection of complex organics on an extraterrestrial body. As Dr. Khawaja explained in a recent ESA press release:
“We found large molecular fragments that show structures typical for very complex organic molecules. These huge molecules contain a complex network often built from hundreds of atoms of carbon, hydrogen, oxygen and likely nitrogen that form ring-shaped and chain-like substructures.”
The molecules that were detected were the result of the ejected ice grains hitting the dust-analyzing instrument aboard Cassini at speeds of about 30,000 km/hour. However, the team believes that these were mere fragments of larger molecules contained beneath Enceladus’ icy surface. As they state in their study, the data suggests that there is a thin organic-rich film on top of the ocean.
These large molecules would be the result of by complex chemical processes, which could be those related to life. Alternately, they may be derived from primordial material similar to what has been found in some meteorites or (as the team suspects) that is generated by hydrothermal activity. As Dr. Postberg explained:
“In my opinion the fragments we found are of hydrothermal origin, having been processed inside the hydrothermally active core of Enceladus: in the high pressures and warm temperatures we expect there, it is possible that complex organic molecules can arise.”
As noted, recent simulations have shown the moon could be generating enough heat through hydrothermal activity for its interior ocean to have existed for billions of years. This study follows up on that scenario by showing how organic material could be injected into the ocean by hydrothermal vents. This is similar to what happens on Earth, a process that scientists believe may have played a vital role in the origins of life on our planet.
On Earth, organic substances are able to accumulate on the walls of rising air bubbles created by hydrothermal vents, which then rise to the surface and are dispersed by sea spray and the bubbles bursting. Scientists believe a similar process is happening on Enceladus, where bubbles of gas rising through the ocean could be bringing organic materiel up from the core-mantle boundary to the icy surface.
When these bubbles burst at the surface, it helps disperse some of the organics which then become part of the salty spray coming through the tiger cracks. This spray then freezes into icy particles as it reaches space, sending organic material and ice throughout the Saturn System, where it has now been detected. If this study is correct, then another fundamental ingredient for life is present in Enceladus’ interior, making the case for life there that much stronger.
This is just the latest in a long-line of discoveries made by Cassini, many of which point to the potential existence of life on or in some of Saturn’s moons. In addition to confirming the first organic molecules in an “ocean world” of our Solar System, Cassini also found compelling evidence of a rich probiotic environment and organic chemistry on Titan.
In the future, multiple missions are expected to return to these moons to gather more evidence of potential life, picking up where the venerable Cassini left off. So long Cassini, and thanks for blazing a trail!
For decades, ever since the Pioneer and Voyager missions passed through the outer Solar System, scientists have speculated that life might exist within icy bodies like Jupiter’s moon Europa. However, thanks the Cassinimission, scientists now believe that other moons in the outer Solar System – such as Saturn’s moon Enceladus – could possibly harbor life as well.
For instance, Cassini observed plume activity coming from Enceladus’ southern polar region that indicated the presence of hydrothermal activity inside. What’s more, these plumes contained organic molecules and hydrated minerals, which are potential indications of life. To see if life could thrive inside this moon, a team of scientists conducted a test where strains of Earth bacteria were subjected to conditions similar to what is found inside Enceladus.
For the sake of their study, the team chose to work with three strains of methanogenic archaea known as methanothermococcus okinawensis. This type of microorganism thrives in low-oxygen environments and consumes chemical products known to exist on Enceladus – such as methane (CH4), carbon dioxide (CO2) and molecular hydrogen (H2) – and emit methane as a metabolic byproduct. As they state:
“To investigate growth of methanogens under Enceladus-like conditions, three thermophilic and methanogenic strains, Methanothermococcus okinawensis (65 °C), Methanothermobacter marburgensis (65 °C), and Methanococcus villosus (80 °C), all able to fix carbon and gain energy through the reduction of CO2 with H2 to form CH4, were investigated regarding growth and biological CH4 production under different headspace gas compositions…”
These strains were selected because of their ability to grow in a temperature range that is characteristic of the vicinity around hydrothermal vents, in a chemically defined medium, and at low partial pressures of molecular hydrogen. This is consistent with what has been observed in Enceladus’ plumes and what is believed to exist within the moon’s interior.
These types of archaea can still be found on Earth today, lingering in deep-see fissures and around hydrothermal vents. In particular, the strain of M. okinawensis has been determined to exist in only one location around the deep-sea hydrothermal vent field at Iheya Ridge in the Okinawa Trough near Japan. Since this vent is located at a depth of 972 m (3189 ft) below sea level, this suggests that this strain has a tolerance toward high pressure.
For many years, scientists have suspected that Earth’s hydrothermal vents played a vital role in the emergence of life, and that similar vents could exist within the interior of moons like Europa, Ganymede, Titan, Enceladus, and other bodies in the outer Solar System. As a result, the research team believed that methanogenic archaea could also exist within these bodies.
After subjecting the strains to Enceladus-like temperature, pressure and chemical conditions in a laboratory environment, they found that one of the three strains was able to flourish and produce methane. The strain even managed to survive after the team introduced harsh chemicals that are present on Enceladus, and which are known to inhibit the growth of microbes. As they conclude in their study:
“In this study, we show that the methanogenic strain M. okinawensis is able to propagate and/or to produce CH4 under putative Enceladus-like conditions. M. okinawensis was cultivated under high-pressure (up to 50 bar) conditions in defined growth medium and gas phase, including several potential inhibitors that were detected in Enceladus’ plume.”
From this, they determined that some of the methane found in Enceladus’ plumes were likely produced by the presence of methanogenic microbes. As Simon Rittmann, a microbiologist at the University of Vienna and lead author of the study, explained in an interview with The Verge. “It’s likely this organism could be living on other planetary bodies,” he said. “And it could be really interesting to investigate in future missions.”
In the coming decades, NASA and other space agencies plan to send multiple mission to the Jupiter and Saturn systems to investigate their “ocean worlds” for potential signs of life. In the case of Enceladus, this will most likely involve a lander that will set down around the southern polar region and collect samples from the surface to determine the presence of biosignatures.
Alternately, an orbiter mission may be developed that will fly through Enceladus’ plumes and collect bioreadings directly from the moon’s ejecta, thus picking up where Cassini left off. Whatever form the mission takes, the discoveries are expected to be a major breakthrough. At long last, we may finally have proof that Earth is not the only place in the Solar System where live can exist.
Be sure to check out John Michael Godier’s video titled “Encedalus and the Conditions for Life” as well:
When the Cassini mission arrived in the Saturn system in 2004, it discovered something rather unexpected in Enceladus’ southern hemisphere. From hundreds of fissures located in the polar region, plumes of water and organic molecules were spotted periodically spewing forth. This was the first indication that Saturn’s moon may have an interior ocean caused by hydrothermal activity near the core-mantle boundary.
According to a new study based on Cassini data, which it obtained before diving into Saturn’s atmosphere on September 15th, this activity may have been going on for some time. In fact, the study team concluded that if the moon’s core is porous enough, it could have generated enough heat to maintain an interior ocean for billions of years. This study is the most encouraging indication yet that the interior of Enceladus could support life.
Prior to the Cassini mission’s many flybys of Enceladus, scientists believed this moon’s surface was composed of solid ice. It was only after noticing the plume activity that they came to realize that it had water jets that extended all the way down to a warm-water ocean in its interior. From the data obtained by Cassini, scientists were even able to make educated guesses of where this internal ocean lay.
All told, Enceladus is a relatively small moon, measuring some 500 km (311 mi) in diameter. Based on gravity measurements performed by Cassini, its interior ocean is believed to lie beneath an icy outer surface at depths of 20 to 25 km (12.4 to 15.5 mi). However, this surface ice thins to about 1 to 5 km (0.6 to 3.1 mi) over the southern polar region, where the jets of water and icy particles jet through fissures.
Based on the way Enceladus orbits Saturn with a certain wobble (aka. libration), scientists have been able to make estimates of the ocean’s depth, which they place at 26 to 31 km (16 to 19 mi). All of this surrounds a core which is believed to be composed of silicate minerals and metal, but which is also porous. Despite all these findings, the source of the interior heat has remained something of an open question.
This mechanism would have to be active when the moon formed billions of years ago and is still active today (as evidenced by the current plume activity). As Dr. Choblet explained in an ESA press statement:
“Where Enceladus gets the sustained power to remain active has always been a bit of mystery, but we’ve now considered in greater detail how the structure and composition of the moon’s rocky core could play a key role in generating the necessary energy.”
For years, scientists have speculated that tidal forces caused by Saturn’s gravitational influence are responsible for Enceladus’ internal heating. The way Saturn pushes and pulls the moon as it follows an elliptical path around the planet is also believed to be what causes Enceladus’ icy shell to deform, causing the fissures around the southern polar region. These same mechanisms are believed to be what is responsible for Europa’s interior warm-water ocean.
However, the energy produced by tidal friction in the ice is too weak to counterbalance the heat loss seen from the ocean. At the rate Enceladus’ ocean is losing energy to space, the entire moon would freeze solid within 30 million years. Similarly, the natural decay of radioactive elements within the core (which has been suggested for other moons as well) is also about 100 times too weak to explain Enceladus interior and plume activity.
To address this, Dr. Choblet and his team conducted simulations of Enceladus’ core to determine what kind of conditions could allow for tidal heating over billions of years. As they state in their study:
“In absence of direct constraints on the mechanical properties of Enceladus’ core, we consider a wide range of parameters to characterize the rate of tidal friction and the efficiency of water transport by porous flow. The unconsolidated core of Enceladus can be viewed as a highly granular/fragmented material, in which tidal deformation is likely to be associated with intergranular friction during fragment rearrangements.”
What they found was that in order for the Cassini observations to be borne out, Enceladus’ core would need to be made of unconsolidated, easily deformable, porous rock. This core could be easily permeated by liquid water, which would seep into the core and gradually heated through tidal friction between sliding rock fragments. Once this water was sufficiently heated, it would rise upwards because of temperature differences with its surroundings.
This process ultimately transfers heat to the interior ocean in narrow plumes which rise to the meet Enceladus’ icy shell. Once there, it causes the surface ice to melt and forming fissures through which jets reach into space, spewing water, ice particles and hydrated minerals that replenish Saturn’s E-Ring. All of this is consistent with the observations made by Cassini, and is sustainable from a geophysical point of view.
In other words, this study is able to show that action in Enceladus’ core could produce the necessary heating to maintain a global ocean and produce plume activity. Since this action is a result of the core’s structure and tidal interaction with Saturn, it is perfectly logical that it has been taking place for billions of years. So beyond providing the first coherent explanation for Enceladus’ plume activity, this study is also a strong indication of habitability.
As scientists have come to understand, life takes a long time to get going. On Earth, it is estimated that the first microorganisms arose after 500 million years, and hydrothermal vents are believed to have played a key role in that process. It took another 2.5 billion years for the first multi-cellular life to evolve, and land-based plants and animals have only been around for the past 500 million years.
Knowing that moons like Enceladus – which has the necessary chemistry to support for life – has also had the necessary energy for billions of years is therefore very encouraging. One can only imagine what we will find once future missions begin inspecting its plumes more closely!
Ever since the Cassini mission entered the Saturn system and began studying its moons, Enceladus has become a major source of interest. Once the probe detected plumes of water and organic molecules erupting from the moon’s southern polar region, scientists began to speculate that Enceladus may possess a warm-water ocean in its interior – much like Jupiter’s moon Europa and other bodies in our Solar System.
In the future, NASA hopes to send another mission to this system to further explore these plumes and the interior of Enceladus. This mission will likely include a new instrument that was recently announced by NASA, known as the Submillimeter Enceladus Life Fundamentals Instrument (SELFI). This instrument, which was proposed by a team from the NASA Goddard Space Flight Center, recently received support for further development.
Prior to the Cassini mission, scientists thought that the surface of Enceladus was frozen solid. However, Cassini data revealed a slight wobble in the moon’s orbit that suggested the presence of an interior ocean. Much like Europa, this is caused by tidal forces that cause flexing in the core, which generates enough heat to hold liquid water in the interior. Around the southern pole, this results in the ice cracking open and forming fissures.
The Cassini mission also discovered plumes emanating from about 100 different fissures which continuously spew icy particles, water vapor, carbon dioxide, methane, and other gases into space. To study these more closely, NASA has been developing some ambitious instruments that will rely on millimeter-wave or radio frequency (RF) waves to determine their composition and learn more about Enceladus’ interior ocean.
According to SELFI Principal Investigator Gordon Chin, SELFI represents a significant improving over existing submillimeter-wavelenght devices. Once deployed, it will measure traces of chemicals in the plumes of water and icy parties that periodically emanated from Enceladus’ southern fissures, also known as “Tiger Stripes“. In addition to revealing the chemical composition of the ocean, this instrument will also indicate it’s potential for supporting life.
On Earth, hydrothermal vents are home to thriving ecosystems, and are even suspected to be the place where life first emerged on Earth. Hence why scientists are so eager to study hydrothermal activity on moons like Enceladus, since these could represent the most likely place to find extra-terrestrial life in our Solar System. As Chin indicated in a NASA press statement:
“Submillimeter wavelengths, which are in the range of very high-frequency radio, give us a way to measure the quantity of many different kinds of molecules in a cold gas. We can scan through all the plumes to see what’s coming out from Enceladus. Water vapor and other molecules can reveal some of the ocean’s chemistry and guide a spacecraft onto the best path to fly through the plumes to make other measurements directly.”
Molecules like water, carbon dioxide and other elements broadcast specific radio frequencies, which submillimeter spectrometers are sensitive to. The spectral lines are very discrete, and the intensity at which they broadcast can be used to quantify their existence. In other words, instruments like SELFI will not only be able to determine the chemical composition of Enceladus’ interior ocean, but also the abundance of those chemicals.
For decades, spectrometers have been used in space sciences to measure the chemical compositions of planets, stars, comets and other targets. Most recently, scientists have been attempting to obtain spectra from distant planets in order to determine the chemical compositions of their atmospheres. This is crucial when it comes to finding potentially-habitable exoplanets, since water vapor, nitrogen and oxygen gas are all required for life as we know it.
Performing scans in the submillimeter band is a relatively new process, though, since submillimeter-sensitive instruments are complex and difficult to build. But with help of NASA research-and-development funding, Chin and his colleagues are increasing the instrument’s sensitivity using an amplifier that will boost the signal to around 557 GHz. This will allow SELFI to detect even minute traces of water and gases coming from the surface of Enceladus.
Other improvements include a more energy-efficient and flexible radio frequency data-processing system, as well as a sophisticated digital spectrometer for the RF signal. This latter improvement will employ high-speed programmable circuitry to convert RF data into digital signals that can be analyzed to measure gas quantities, temperatures, and velocities from Enceladus’ plumes.
These enhancements will allow SELFI to simultaneously detect and analyze 13 different types of molecules, which include various isotopes of water, methanol, ammonia, ozone, hydrogen peroxide, sulfur dioxide, and sodium chloride (aka. salt). Beyond Enceladus, Chin believes the team can sufficiently improve the instrument for proposed future missions. “SELFI is really new,”he said. “This is one of the most ambitious submillimeter instruments ever built.”
For instance, in recent years, scientists have spotted plume activity coming from the surface of Europa. Here too, this activity is believed to be the result of geothermal activity, which sends warm water plumes from the moon’s interior ocean to the surface. Already, NASA hopes to examine these plumes and those on Enceladus using the James Webb Space Telescope, which will be deploying in 2019.
Another possibility would be to equip the proposed Europa Clipper – which is set to launch between 2022 and 2025 – with an instrument like SELFI. The instrument package for this probe already calls for a spectrometer, but an improved submillimeter-wave and RF device could allow for a more detailed look at Europa’s plumes. This data could in turn resolve the decades-old debate as to whether or not Europa’s interior is capable of supporting life.
In the coming decades, one of the greatest priorities of space exploration is to investigate the Solar System’s “Ocean Worlds” for signs of life. To see this through, NASA and other space agencies are busy developing the necessary tools to sniff out all the chemical and biological indicators. Within a decade, with any luck, we might just find that life on Earth is not the exception, but part of a larger norm.
In October of 2018, the James Webb Space Telescope (JWST) will be launched into orbit. As part of NASA’s Next Generation Space Telescope program, the JWST will spend the coming years studying every phase of cosmic history. This will involve probing the first light of the Universe (caused by the Big Bang), the first galaxies to form, and extra-solar planets in nearby star systems.
In addition to all of that, the JWST will also be dedicated to studying our Solar System. As NASA recently announced, the telescope will use its infrared capabilities to study two “Ocean Worlds” in our Solar System – Jupiter’s moon Europa and Saturn’s moon Enceladus. In so doing, it will add to observations previously made by NASA’s Galileo and Cassini orbiters and help guide future missions to these icy moons.
The moons were chosen by scientist who helped to develop the telescope (aka. guaranteed time observers) and are therefore given the privilege of being among the first to use it. Europa and Enceladus were added to the telescope’s list of targets since one of the primary goals of the telescope is to study the origins of life in the Universe. In addition to looking for habitable exoplanets, NASA also wants to study objects within our own Solar System.
One of the main focuses will be on the plumes of water that have been observed breaking through the icy surfaces of Enceladus and Europa. Since 2005, scientists have known that Enceladus has plumes that periodically erupt from its southern polar region, spewing water and organic chemicals that replenish Saturn’s E-Ring. It has since discovered that these plumes reach all the way into the interior ocean that exists beneath Enceladus’ icy surface.
In 2012, astronomers using the Hubble Space Telescope detected similar plumes coming from Europa. These plumes were spotted coming from the moon’s southern hemisphere, and were estimated to reach up to 200 km (125 miles) into space. Subsequent studies indicated that these plumes were intermittent, and presumably rained water and organic materials from the interior back onto the surface.
These observations were especially intriguing since they bolstered the case for Europa and Enceladus having interior, warm-water oceans that could harbor life. These oceans are believed to be the result of geological activity in the interior that is caused by tidal flexing. Based on the evidence gathered by the Galileo and Cassini orbiters, scientists have theorized that these surface plumes are the result of these same geological processes.
The presence of this activity could also means that these moons have hydrothermal vents located at their core-mantle boundaries. On Earth, hydrothermal vents (located on the ocean floor) are believed to have played a major role in the emergence of life. As such, their existence on other bodies within the Solar System is viewed as a possible indication of extra-terrestrial life.
The effort to study these “Ocean Worlds” will be led by Geronimo Villanueva, a planetary scientist at NASA’s Goddard Space Flight Center. As he explained in a recent NASA press statement, he and his team will be addressing certain fundamental questions:
“Are they made of water ice? Is hot water vapor being released? What is the temperature of the active regions and the emitted water? Webb telescope’s measurements will allow us to address these questions with unprecedented accuracy and precision.”
Villanueva’s team is part of a larger effort to study the Solar System, which is being led by Heidi Hammel – the executive VP of the Association of Universities for Research in Astronomy (AURA). As she described the JWST’s “Ocean World” campaign to Universe Today via email:
“We will be seeking signatures of plume activity on these ocean worlds as well as active spots. With the near-infrared camera of NIRCAM, we will have just enough spatial resolution to distinguish general regions of the moons that could be “active” (creating plumes). We will also use spectroscopy (examining specific colors of light) to sense the presence of water, methane and several other organic species in plume material.”
For Enceladus, the team will be analyze the molecular composition of its plumes and perform a broad analysis of its surface features. Due to its small size, high-resolution of the surface will not be possible, but this should not be a problem since the Cassini orbiter already mapped much of its surface terrain. All told, Cassini has spent the past 13 years studying the Saturn system and will conclude the “Grande Finale” phase of its mission this September 15th.
These surveys, it is hoped, will find evidence of organic signatures in the plumes, such as methane, ethanol and ethane. To be fair, there are no guarantees that the JWST’s observations will coincide with plumes coming from these moons, or that the emissions will have enough organic molecules in them to be detectable. Moreover, these indicators could also be caused by geological processes.
Nevertheless, the JWST is sure to provide evidence that will allow scientists to better characterize the active regions of these moons. It is also anticipated that it will be able to pinpoint locations that will be of interest for future missions, such as NASA’s Europa Clipper mission. Consisting of an orbiter and lander, this mission – which is expected to launch sometime in the 2020s – will attempt to determine if Europa is habitable.
As Dr. Hammel explained, the study of these two “Ocean Moons” is also intended to advance our understanding about the origins of life in the Universe:
“These two ocean moons are thought to provide environments that may harbor water-based life as we know it. At this point, the issue of life elsewhere is completely unknown, though there is much speculation. JWST can move us closer to understanding these potentially habitable environments, complementing robotic spacecraft missions that are currently in development (Europa Clipper) and may be planned for the future. At the same time, JWST will be examining the far more distant potentially habitable environments of planets around other stars. These two lines of exploration – local and distant – allow us to make significant advances in the search for life elsewhere.”
Once deployed, the JWST will be the most powerful space telescope ever built, relying on eighteen segmented mirrors and a suite of instruments to study the infrared Universe. While it is not meant to replace the Hubble Space Telescope, it is in many ways the natural heir to this historic mission. And it is certainly expected to expand on many of Hubble’s greatest discoveries, not the least of which are here in the Solar System.
Be sure to check out this video on the kinds of spectrographic data the JWST will provide in the coming years, courtesy of NASA:
NASA has announced the discovery of hydrogen in the plumes on Enceladus. This is huge news, and Cassini scientists have looked forward to this day. What it means is that there is a potential source of energy for microbes in the oceans of Enceladus, and that energy from the Sun is not required to support life.
We’ve known about the plumes on Enceladus for a while now, and Cassini has even flown through those plumes to determine their content. But hydrogen was never discovered until now. What it means is that there is a geochemical source for hydrogen in Enceladus’ ocean, coming from the interaction between warm water and rocks.
“This is the closest we’ve come, so far, to identifying a place with some of the ingredients needed for a habitable environment.” – Thomas Zurbuchen, NASA.
This is a capstone finding, according to NASA. As far as we know, life needs three things to exist: water, energy, and the right chemicals. We know it has the necessary chemicals, we know it has water, and we now know it has a source of energy.
On Earth, hydrothermal vents deep in the ocean floor provide the energy for a web of life reliant on those vents. Bacteria live there, forming the base of a food chain that can include tube worms, shrimp, and other life forms. This discovery points to the possibility that similar communities might exist in the sub-surface ocean of Enceladus.
“This is the closest we’ve come, so far, to identifying a place with some of the ingredients needed for a habitable environment,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate at Headquarters in Washington.
Microbes in Enceladus’ ocean could use the hydrogen in a process called methanogenesis. They obtain energy by combining hydrogen with dissolved carbon dioxide in the water. This process produces a methane by-product. Methanogenesis is a bedrock process at the root of life here on Earth.
“Confirmation that the chemical energy for life exists within the ocean of a small moon of Saturn is an important milestone in our search for habitable worlds beyond Earth,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
Hubble Confirms Plumes On Europa
NASA has also announced that the Hubble Space Telescope has confirmed the presence of plumes on another of our Solar System’s icy moons, Europa.
These plumes were first seen by the Hubble in 2014, but were never seen again. Since repeatability is key in science, those findings were put on the back burner. But in 2016, NASA announced today, Hubble spotted them again, in the same place. This is the same spot that the Galileo probe noticed a thermal hot spot.
We don’t know if Europa has hydrogen in its oceans, but it’s easy to see where this is going. NASA’s excitement is palpable.
NASA’s Europa Clipper mission will visit Europa and determine the thickness of its ice layer, as well as the depth and salinity of its ocean. It will also analyze the atmosphere and the composition of the plumes. Europa Clipper will fill in a lot of gaps in our understanding.
Europa Clipper will be launched around 2022, but a mission to Enceladus will have to wait a little longer. One mission under consideration in NASA’s Discovery program is ELF, Enceladus Life Finder. ELF would fly through Enceladus’ plumes 8 or 10 times, taking more detailed samples of their content.
The discovery of hydrogen in the plumes of Enceladus is huge news any way you look at it. But that discovery begs the question: Are we doing it all wrong? Are we looking for life in the wrong places?
The search for life elsewhere in the Universe, so far, has mostly revolved around exoplanets. And then refining that search to identify exoplanets that are in the habitable zones of their stars. We’re searching for other Earths, basically.
But maybe we should be changing our focus. Maybe it’s the ice worlds, including icy exomoons, that are the most likely targets for our search. This new evidence from NASA’s Cassini mission, and from the Hubble Space Telescope, suggests that in our Solar System at least, they are the best place to search.
One Final Ingredient Needed?
There’s a fourth ingredient needed for life. Once there is water, energy, and the necessary chemicals, life needs time to get going. How much time, we’re not exactly certain. But this is where Enceladus and Europa are different.
Europa is about 4 billion years old, or so we think. That’s only half a billion years younger than Earth, and we think life started on Earth about 3.5 billion years ago. This hints that, if conditions on Europa are favorable, life has had a long time to get going. Of course, that doesn’t mean it has.
On the other hand, Enceladus is probably much younger. A study of the orbits of Saturn’s moons suggests that Enceladus may only be 100 million years old. If that’s true, it’s not very much time for life to get going.
The hydrogen discovery is huge news. There are still a lot of questions, of course, and lots to be debated. But confirming a source of energy on Enceladus builds the case for the same type of hydrothermal vent life that we see on Earth.
One of the biggest surprises from the Cassini mission to Saturn has been the discovery of active geysers at the south pole of the moon Enceladus. At only about 500 km (310 miles) in diameter, the bright and ice-covered moon should be too small and too far from the Sun to be active. Instead, this little moon is one of the most geothermally active places in the Solar System.
Now, a new study from Cassini data shows that the south polar region of Enceladus is even warmer than expected just a few feet below its icy surface. While previous studies have confirmed an ocean of liquid water inside Enceladus which fuels the geysers, this new study shows the ocean is likely closer to the surface than previously thought. Additionally – and most enticing – there has to be a source of heat inside the moon that is not completely understood.
“These observations provide a unique insight into what is going on beneath the surface,” said Alice Le Gall, who is part of the Cassini RADAR instrument team, from Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), and Université Versailles Saint-Quentin (UVSQ), France. “They show that the first few meters below the surface of the area that we investigated, although at a glacial 50-60 K, are much warmer than we had expected: likely up to 20 K warmer in some places. This cannot be explained only as a result of the Sun’s illumination and, to a lesser extent, Saturn’s heating so there must be an additional source of heat.”
Microwave data taken during a close flyby in 2011 shows there is excess heat at three fractures in the surface of Enceladus. While similar to the so-called “tiger-stripe” features on this moon that are actively venting ice and water molecules into space, these three fractures don’t appear to be active, at least not in 2011.
Scientists say the seemingly dormant fractures lying above the moon’s warm, underground sea point to the dynamic character of Enceladus’ geology, suggesting the moon might have experienced several episodes of activity, in different places on its surface.
The 2011 flyby provided the first – and unfortunately, the only — high-resolution observations of Enceladus’ south pole at microwave wavelengths.
It looked at a narrow, arc-shaped swathe of the southern polar region, about 25 km (15 miles) wide, and located just 30 km to 50 km (18-30 miles) north of the tiger-stripe fractures.
The heat that was detected appears to be lying under a much colder layer of frost.
Because of operational constraints of the 2011 flyby, it was not possible to obtain microwave observations of the active fractures themselves. But this allowed the scientists to observe that the thermally anomalous terrains of Enceladus extend well beyond the tiger stripes.
Their findings show it is likely that the entire south pole region is warm underneath, meaning Enceladus’ ocean could be just 2 km under the moon’s icy surface in that area. The finding agrees with a 2016 study, led by another Cassini team member, Ondrej Cadek, which estimated the thickness of the crust on Enceladus’ south pole to be less than the rest of the moon. That study estimated the depth of the ice shell to be less than 5 km (1.2 miles) at the south pole, while average depth on other areas of Enceladus is between 18–22 km (11-13 miles).
What generates the internal heat at Enceladus? The main source of heat remains a mystery, but scientists think gravitational forces between Enceladus, Saturn, and another moon, Dione pull and flex Enceladus’ interior. Known as tidal forces, the tugging causes the moon’s interior to rub, creating friction and heat. It also creates stress compressions and deformations on the crust, leading to the formation of faults and fractures. This in turn creates more heat in the sub-surface layers. In this scenario, the thinner icy crust in the south pole region is subject to a larger tidal deformation that means more heat being created to help keep the underground water warm.
Since the geysers weren’t known until Cassini’s arrival at Saturn, the spacecraft didn’t have a specific payload to study them, but scientists used the instruments at their disposal to make the best observations they could, flying the spacecraft to within 49 km (30 miles) of the surface. To fully study the tidal heating — or to determine if there is another source of heat — scientists will continue to study the data already taken by various Cassini instruments. But since the mission will be ending in September 2017, it may require another mission to this intriguing moon to fully figure out this mystery.
“This discovery opens new perspectives to investigate the emergence of habitable conditions on the icy moons of the gas giant planets,” says Nicolas Altobelli, ESA’s Project Scientist for Cassini–Huygens. “If Enceladus’ underground sea is really as close to the surface as this study indicates, then a future mission to this moon carrying an ice-penetrating radar sounding instrument might be able to detect it.”
“Finding temperatures near these three inactive fractures that are unexpectedly higher than those outside them adds to the intrigue of Enceladus,” said Cassini Project Scientist Linda Spilker at the Jet Propulsion Laboratory. “What is the warm underground ocean really like and could life have evolved there? These questions remain to be answered by future missions to this ocean world.”
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