The joint NASA/ESA Cassini-Huygens mission revealed some amazing things about Saturn and its system of moons. In the thirteen years that it spent studying the system – before it plunged into Saturn’s atmosphere on September 15th, 2017 – it delivered the most compelling evidence to date of extra-terrestrial life. And years later, scientists are still poring over the data it gathered.
For instance, a team of German scientists recently examined data gathered by the Cassini orbiter around Enceladus’ southern polar region, where plume activity regularly sends jets of icy particles into space. What they found was evidence of organic signatures that could be the building blocks for amino acids, the very thing that life is made of! This latest evidence shows that life really could exist beneath Enceladus’ icy crust.
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.
Since the 1970s, when the Voyager probes captured images of Europa’s icy surface, scientists have suspected that life could exist in interior oceans of moons in the outer Solar System. Since then, other evidence has emerged that has bolstered this theory, ranging from icy plumes on Europa and Enceladus, interior models of hydrothermal activity, and even the groundbreaking discovery of complex organic molecules in Enceladus’ plumes.
However, in some locations in the outer Solar System, conditions are very cold and water is only able to exist in liquid form because of the presence of toxic antifreeze chemicals. However, according to a new study by an international team of researchers, it is possible that bacteria could survive in these briny environments. This is good news for those hoping to find evidence of life in extreme environments of the Solar System.
Basically, on bodies like Ceres, Callisto, Triton, and Pluto – which are either far from the Sun or do not have interior heating mechanisms – interior oceans are believed to exist because of the presence of certain chemicals and salts (such as ammonia). These “antifreeze” compounds ensure that their oceans have lower freezing points, but create an environment that would be too cold and toxic to life as we know it.
For the sake of their study, the team sought to determine if microbes could indeed survive in these environments by conducting tests with Planococcus halocryophilus, a bacteria found in the Arctic permafrost. They then subjected this bacteria to solutions of sodium, magnesium and calcium chloride as well as perchlorate, a chemical compound that was found by the Phoenix lander on Mars.
They then subjected the solutions to temperatures ranging from +25°C to -30°C through multiple freeze and thaw cycles. What they found was that the bacteria’s survival rates depended on the solution and temperatures involved. For instance, bacteria suspended in chloride-containing (saline) samples had better chances of survival compared to those in perchlorate-containing samples – though survival rates increased the more the temperatures were lowered.
For instance, the team found that bacteria in a sodium chloride (NaCl) solution died within two weeks at room temperature. But when temperatures were lowered to 4 °C (39 °F), survivability began to increase and almost all the bacteria survived by the time temperatures reached -15 °C (5 °F). Meanwhile, bacteria in the magnesium and calcium-chloride solutions had high survival rates at –30 °C (-22 °F).
The results also varied for the three saline solvents depending on the temperature. Bacteria in calcium chloride (CaCl2) had significantly lower survival rates than those in sodium chloride (NaCl) and magnesium chloride (MgCl2)between 4 and 25 °C (39 and 77 °F), but lower temperatures boosted survival in all three. The survival rates in perchlorate solution were far lower than in other solutions.
However, this was generally in solutions where perchlorate constituted 50% of the mass of the total solution (which was necessary for the water to remain liquid at lower temperatures), which would be significantly toxic. At concentrations of 10%, bacteria was still able to grow. This is semi-good news for Mars, where the soil contains less than one weight percent of perchlorate.
However, Heinz also pointed out that salt concentrations in soil are different than those in a solution. Still, this could be still be good news where Mars is concerned, since temperatures and precipitation levels there are very similar to parts of Earth – the Atacama Desert and parts of Antarctica. The fact that bacteria have can survive such environments on Earth indicates they could survive on Mars too.
In general, the research indicated that colder temperatures boost microbial survivability, but this depends on the type of microbe and the composition of the chemical solution. As Heinz told Astrobiology Magazine:
“[A]ll reactions, including those that kill cells, are slower at lower temperatures, but bacterial survivability didn’t increase much at lower temperatures in the perchlorate solution, whereas lower temperatures in calcium chloride solutions yielded a marked increase in survivability.”
The team also found that bacteria did better in saltier solutions when it came to freezing and thawing cycles. In the end, the results indicate that survivability all comes down to a careful balance. Whereas lower concentrations of chemical salts meant that bacteria could survive and even grow, the temperatures at which water would remain in a liquid state would be reduced. It also indicated that salty solutions improve bacteria survival rates when it comes to freezing and thawing cycles.
Of course, the team emphasized that just because bacteria can subsist in certain conditions doesn’t mean they will thrive there. AsTheresa Fisher, a PhD student at Arizona State University’s School of Earth and Space Exploration and a co-author on the study, explained:
“Survival versus growth is a really important distinction, but life still manages to surprise us. Some bacteria can not only survive in low temperatures, but require them to metabolize and thrive. We should try to be unbiased in assuming what’s necessary for an organism to thrive, not just survive.”
As such, Heinz and his colleagues are currently working on another study to determine how different concentrations of salts across different temperatures affect bacterial propagation. In the meantime, this study and other like it are able to provide some unique insight into the possibilities for extraterrestrial life by placing constraints on the kinds of conditions that they can survive and grow in.
These studies also allow help when it comes to the search for extraterrestrial life, since knowing where life can exist allows us to focus our search efforts. In the coming years, missions to Europa, Enceladus, Titan and other locations in the Solar System will be looking for biosignatures that indicate the presence of life on or within these bodies. Knowing that life can survive in cold, briny environments opens up additional possibilities.
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: