Enceladus: a place for life?
forget Mars, this frozen member of Saturn's family of moons could hold the key to organisms beyond theconfines of earth
Forget Mars, the hunt begins on this icy moon
Enceladus is Saturn's sixth-largest moon and a world that is seven-times smaller than our natural satellite. It orbits Saturn at around 1.2 billion kilometres (746 million miles) away from Earth and reflects nearly 100 per cent of the sunlight that hits its surface. Very little was known about this icy world but that changed when NASA’s Cassini spacecraft performed its first flyby of Enceladus and imaged its south pole. This revealed plumes of ice and water vapour erupting into space, and Cassini returned about 2.5 years later on 12 March 2008.
This was when scientists realised that this moon has a hidden, rich and surprising interior that could make it a keen target for astrobiological study.
Now it is considered one of the – if not the – most exciting prospects for extraterrestrial life and a potentially habitable environment elsewhere in our Solar System. “As the evidence continues to grow, Enceladus becomes a more and more attractive candidate to look for life. For me, Enceladus is the most likely place to look for life beyond Earth,” Dr Linda Spilker, the project scientist for Cassini, explains to all about space.
over a decade later, scientists have recently been reanalysing the data collected by the cassini spacecraft. This study has discovered that the molecules that constitute these erupting space plumes are more complex and exciting than previously thought. courtesy of cassini’s two scientific instruments, the cosmic Dust Analyzer (cDA) and the ion and neutral Mass Spectrometer (inMS), the identification of these complex, carbonrich organic molecules now tick another box when it comes to the criteria of the requirements for life.
“enceladus checks all the boxes for habitability: liquid water, organic material for use as building blocks and a source of chemical energy. furthermore, the material vented into space from its ocean provides an easy means to investigate the ocean below,” Dr Hunter waite of the Southwest research institute in San Antonio, Texas, United States and also principal investigator of cassini’s inMS instrument, tells all about space. “europa [Jupiter’s fourth largest moon] may be just as interesting from an astrobiological perspective, or more so, but we do not have the data to draw that conclusion yet. Given our present state of knowledge, enceladus is the most practical place to look for extant life outside of earth.” for over a decade, cassini obtained unrivalled observations of enceladus that explain why this moon of Saturn is one of the brightest objects in the Solar System, reflecting almost all of the sunlight that hits its surface. There was also an unknown connection between the moon and Saturn’s e-ring which needed to be answered as well. when cassini arrived at the scene in 2005, its suite of finely tuned instruments and equipment had the ability to resolve the unexpected icy water jets spewing out from the ‘tiger-stripes’ surface features at the south pole of enceladus from afar. cassini also had the ability to fly through these plumes, gathering unique data about them and revealing a mixture of volatile gases, water vapour, carbon dioxide and carbon monoxide, as well as organic materials. The density of organic materials was about 20-times denser than expected.
Astronomers back on earth have been thoroughly examining the data over the course of this mission. early analysis of enceladus’ plumes revealed the presence of a subsurface ocean about ten kilometres (six miles) deep beneath an ice shell about 30 to 40 kilometres (19 to 25 miles) thick. There was also evidence for the presence of hydrothermal vents, which are cracks on the planet’s core releasing heat, residing at the bottom of its ocean. Scientists are certain this is due to the intense gravitational pull of Saturn acting on enceladus.
in the same way the earth exerts a gravitational pull on the Moon to keep it in orbit, the Moon also exerts a gravitational pull on earth. The gravitational pull of the Moon is what provides us with tides, and as enceladus is much smaller than the Moon and Saturn is much more massive than earth, Saturn’s gravitational effect on enceladus is much more dominant. The effect is actually so intense that enceladus’ interior is heated up through the friction of the continuous pulling and pushing on its interior due to Saturn’s gravity. This effect is known as ‘tidal heating’ and it is the cause of the moon’s interior hydrothermal vents and what stops the ocean turning into a frozen, barren world.
now this new study has discovered another fascinating fact about enceladus which has ramped up the excitement surrounding it, even after cassini’s demise in September 2017. A recent study led by Dr frank postberg of the University
“enceladus checks all the boxes for habitability: liquid water, organic material and a source of chemical energy” Hunter Waite
of Heidelberg, Germany, has identified incredibly complex organic molecules with masses over
200 atomic mass units, which is more than tentimes heavier than methane. “This is the first ever detection of such complex organics coming from an extraterrestrial water-world, but we do not know if the organics we found are of biogenic origin or in any way connected to life,” explains postberg. “indeed, complex organics do not necessarily provide a life-friendly environment. on the other hand any life as we know it, and even any prebiotic chemistry, requires complex organic molecules.”
This discovery comes as a result of two of cassini’s 12 on-board scientific instruments. The cDA and inMS are both mass spectrometers that were hit by ice grains at speeds of around 30,000 kilometres (18,641 miles) per hour. with each impact the ice grains were broken up and the instruments were able to take a look at the fragments that remained. “we can then analyse these fragments and compare them to related laboratory experiments to draw conclusions about the chemical nature of these organics. How the molecules break apart provides information on the elements present and the nature of the molecular structure,” says waite.
it seems like enceladus is quite hydra-headed in terms of that when one question gets answered, three more enter in its place. Although scientists have been able to identify the complexity of molecules present at the distant water-world, now there are questions over how they are formed, what this tells us about the moon’s potential habitability and if there are any forms of life residing there at this present time.
“in principle there is a wide range of possibilities [for how these molecules formed]. Such large molecules can only be created by complex chemical processes – including those related to life,” explains postberg. “Alternatively they could come from primordial material as found in some meteorites or, more likely, be generated by hydrothermal activity.”
There are three main possible explanations as to how these carbon-rich molecules have come about, and all three are positive signs for life on enceladus in their own way. one scenario is that these are primordial pieces of carbonaceous material that have been released due to the thermal evolution of the moon. The other is that these complex organics are the combination of simpler carbon materials that are formed on the aforementioned hydrothermal vents, or it is even possible that this is the refuse of living organisms. The first two abiotic scenarios are positive for a sign of life, as it means that there is a fuel source being created for any potential organisms. The last scenario, however, could mean that there is, or has previously been, a form of life at enceladus. when asked about what type of life form could be at enceladus, waite replies that “life on enceladus would likely be single-celled microbes, similar to the most primitive forms of life on earth, such as methanogens.”
in order to get to the bottom of how these organics turn up in such an environment, particularly as there is no spacecraft there at the moment, scientists require much more extensive theoretical models and laboratory experiments.
How they're formed is relatively unknown, but how they find their way to the cassini instruments is by hitching a ride on the surface of bubbles.
“After entering the ocean the organics can be transported upwards to the ocean surface on the walls of rising bubbles of gas,” says postberg. “when reaching the ocean surface, the organics form a layer or film there.”
However, even with all the questions that have surfaced courtesy of this detection of complex molecules, it has been more promising than
Mars when it comes finding organic compounds. postberg makes a point that “The most intriguing difference is how easily one finds complex organics on enceladus compared to Mars. if you consider the enormous efforts put into the search for organics on Mars since the 1970s without success, it is just amazing how easily we found a complex and
diverse organic chemistry on enceladus just with a spacecraft flying by.”
Mars has had numerous amounts of orbiters, landers and rovers searching for signs of life since the Mariner 9 became the first spacecraft to orbit Mars in 1971. The search for life on Mars is an idea that has been the driving force behind many Martian exploration missions, and scientists have since been finding evidence of ancient organic compounds. The most recent discovery is nASA’s curiosity rover drilling into a 3-billion-year-old sedimentary rock at the Gale crater and finding ‘tough’ organic compounds. curiosity’s Sample Analysis at Mars (SAM) instrument found large, carbon-based, organic compounds that also include sulphur. Sulphur was not discovered at enceladus, but this is because the instruments are not sensitive enough to detect sulphur. it could be the case that sulphur is present at enceladus but it hasn’t been detected, meaning the dry red planet and distant water-world moon could be very similar in terms of organic compounds. Mars is now considered an inhospitable world with no atmosphere, meaning that the Sun’s unfiltered rays can rain down on the unprotected surface and evaporate any water. This makes it extremely hard for any form of life to survive in this difficult terrain. now evidence has come to light of organic compounds at a more favourable environment, scientists will need to start looking at moons with interior oceans such as enceladus, europa, Ganymede and callisto more extensively.
Unfortunately, the future exploration of enceladus is looking bleak, as there is no spacecraft commissioned to go back to the Saturn system, and in particular enceladus. when you reflect on the amazing discoveries that cassini made with a collection of instruments that weren’t designed for examining this unique world, as they were built before scientists knew about enceladus’ activity, imagine what astrobiologists and planetary scientists could learn from a spacecraft that was specifically made for studying the moon.
“There will probably be new enceladus mission proposals for the next nASA new frontiers call. A future enceladus mission could consist of a series of flybys of enceladus, similar to the europa clipper mission, or perhaps the spacecraft will go into orbit around enceladus,” says Spilker. “Any future enceladus missions will probably carry instruments to search for evidence of life and to better characterise the habitability of the ocean.”
with the advancements in technology and the innovative new techniques astronomers come up with each day, a pioneering orbiter – which could even include a lander to grace the surface – would unveil unbelievable results about Saturn’s shining moon. it appears that the major space organisations such as nASA and the european Space Agency are convinced that europa would be the better target, as it also exhibits signs of an interior ocean. However, with the data cassini collected and the new discoveries that we continue to learn from it, a return to enceladus is imminent.
Cassini completed 23 flybys of Enceladus and several deep dives into its plumes
dr Hunter Waite is the principal investigator of the InMs instrument
Future missions such as nasa’s Europa Clipper will explore a similar icy moon - Europa
Nothing on the surface Looking for life on Mars' surface is futile, as it is hit by extreme radiation that makes it uninhabitable. Hydrothermal vents have been observed at the bed of Earth’soceans, and are thought to also residein Enceladus Could there be more?At lower levels there is much less exposure to radiation; this could mean there are genuine organic, carbon-based molecules. Digging a bit deeper Slightly deeper into the surface there are remnants of ancient organic compounds hidden in the sedimentary rock. Crossing the lineWhen these bubbles reach space they explode and cover the organics in an ice blanket, preserving them and ejecting on one of Enceladus’ plumes. Formation in the oceanScientists are not sure whether the organics are created via thermal evolution, forged on vents or are the debris of living organisms. Bubble transportation It is believed that the organics latch on to the bubbles of gas from hydrothermal vents and are carried to the fractures in the ice crust.