Today’s digital palette cleanser is bought to you courtesy of Cassini and a small icy moon filled with intrigue.
As we constantly check the news sites for updates on the minutia of our daily lives, refresh our social media feeds, and ponder the existential dread that seems to be flooding our immediate future with increasing volume, it’s nice to find little islands of tranquility that appear out of nowhere. Today, I found that island in a beautiful processed image of Saturn’s moon Enceladus by the incredibly talented Kevin Gill, who works at NASA’s Jet Propulsion Laboratory:
[NASA/JPL-Caltech/SSI/CICLOPS/Kevin M. Gill]
In his tweet, Kevin simply describes this view as “solitude” and that’s pretty damn near perfect. In this image, the beautifully back-lit plumes are visible with the tenuous E-ring of Saturn creating an atmospheric backdrop.
Enceladus is a fascinating moon. During the NASA Cassini mission, which ended its glorious 13-year reign in Saturn orbit in 2017, the spacecraft became intimately familiar with the icy moon and its famous geysers. After flying through the plumes of water vapor, it became clear to mission scientists that not only does this 313 mile wide icy marble have an extensive subsurface liquid water ocean, that ocean contains organic molecules that could hint at astrobiological possibilities.
It’s sometimes nice to escape to Saturn orbit every now and again, so be sure to check out Kevin’s awe-inspiring Flickr album for more.
Don’t forget your spacesuit: Complex lifeforms, such as humans, would not survive on many of the worlds we thought would be interstellar tropical getaways
[Pixabay]
Worlds like Earth may be even rarer than we thought.
We live on a planet that provides the perfect balance of ingredients to support a vast ecosystem. This amazing world orbits the Sun at just the right distance where water can exist in a liquid state—a substance that, as we all know, is an essential component for our biology to function. Earth is also an oddball in our solar system, being the only planet where these vast oceans of liquid water persist on its surface, all enshrouded in a thick atmosphere that provides the stage for a complex global interplay of chemical and biological cycles that, before we industrialized humans came along, has supported billions of years of uninterrupted evolution and biological diversity.
Humans, being the proud intelligent beings that we profess to be, are stress-testing this delicate balance by pumping an unending supply of carbon dioxide into the atmosphere. Being a potent greenhouse gas, we’re currently living through a new epoch in our planet’s biological history where an exponential increase in CO2 is being closely followed by an increase in global average temperatures. We are, in effect, altering Earth’s habitability. Well done, humans!
While this trend is a clear threat to the sustainability of our biosphere, spare a thought for other “habitable” worlds that may appear to have all the right stuff for complex lifeforms to evolve, but toxic levels of the very chemicals that keep these worlds habitable has curtailed the possibility of complex life from gaining a foothold.
Welcome to the Not-So-Habitable Zone
Habitable zone exoplanets are the Gold Standard for exoplanet-hunters and astrobiologists alike. Finding a distant alien world within this zone—a region surrounding any star where it’s not too hot and not too cold for water to exist on its surface, a region also known as the “Goldilocks Zone” for obvious reasons—spawns a host of questions that our most advanced telescopes in space and on the ground try to answer: Is that exoplanet Earth-sized? Does it have an atmosphere? What kind of star is it orbiting? Does its system possess a Jupiter-like gas giant? These questions are all trying to help us understand whether that world has the Earthly qualities that could support hypothetical extraterrestrial life.
(Of course, there’s the debate as to whether all life in the universe is Earth-life-like, but as we’re the only biological examples that we know of in the entire galaxy, it’s the best place to start when pondering what biological similarities extraterrestrial life may have to us.)
The habitable zone for exoplanets is a little more complicated than simply the distance at which they orbit their host stars, however. Greenhouse gases, such as carbon dioxide, can extend the area of a star’s habitable zone. For example: If an atmosphere-less planet orbits beyond the outermost edge of its habitable zone, the water it has on its surface will remain in a solid, frozen state. Now, give that planet an atmosphere laced with greenhouse gases and its surface may become warm enough to maintain the water in a liquid state, thereby boosting its habitable potential.
But how much is too much of a good thing? And how might this determination impact our hunt for truly habitable worlds beyond our own?
In a new study published in the Astrophysical Journal, researchers have taken another look at the much-coveted habitable zone exoplanets to find that, while some of the atmospheric gases are essential to maintain a temperature balance, should there be too much of the stuff keeping some of those worlds at a habitable temperature, their toxicity could curtail any lifeforms more complex than a single-celled microbe from evolving.
“This is the first time the physiological limits of life on Earth have been considered to predict the distribution of complex life elsewhere in the universe,” said Timothy Lyons, of the University of California, Riverside, and director of the Alternative Earths Astrobiology Center.
“Imagine a ‘habitable zone for complex life’ defined as a safe zone where it would be plausible to support rich ecosystems like we find on Earth today,” he said in a statement. “Our results indicate that complex ecosystems like ours cannot exist in most regions of the habitable zone as traditionally defined.”
Toxic Limits
Carbon dioxide is an essential component of our ecosystem, particularly as it’s a greenhouse gas. Acting like an insulator, CO2 absorbs energy from the Sun and heats our atmosphere. When in balance, it stops too much energy from being radiated back out into space, thereby preventing our planet from being turned into a snowball. Levels of CO2 have ebbed and flowed throughout the biological history of our planet and it has always been a minor component of atmospheric gases, but its greenhouse effect (i.e. the atmospheric heating effect) is extremely potent and the human-driven 400+ppm levels are causing dramatic climate changes that modern biological systems haven’t experienced for millions of years. That said, the CO2 levels required to keep some “habitable” exoplanets in a warm enough state would need to be a lot more concentrated than the current terrestrial levels, potentially making their atmospheres toxic.
“To sustain liquid water at the outer edge of the conventional habitable zone, a planet would need tens of thousands of times more carbon dioxide than Earth has today,” said lead author Edward Schwieterman, of the NASA Astrobiology Institute. “That’s far beyond the levels known to be toxic to human and animal life on Earth.”
In the blue zone: some of the known exoplanets that fall within the habitable zones of their stars may have an overabundance of CO (yellow/brown), at a level that is toxic to human life. Likewise, the more CO2 (from blue to white) will become toxic at a certain point. The sweet-spot is where Earth sits, with Kepler 442b (if it has a habitable atmosphere) coming in second [Schwieterman et al., 2019. Link to paper]
From their computer simulations, to keep CO2 at acceptable non-toxic levels, while maintaining planetary habitability, the researchers realized that for simple animal life to survive, the habitable zone will shrink to no more than half of the traditional habitable zone. For more complex lifeforms—like humans—to survive, that zone will shrink even more, to less than one third. In other words, to strike the right balance between keeping a hypothetical planet warm enough, but not succumbing to CO2 toxicity, the more complex the lifeform, the more compact the habitable zone.
This issue doesn’t stop with CO2. Carbon monoxide (CO) doesn’t exist at toxic levels in Earth’s atmosphere as our hot and bright Sun drives chemical reactions that remove dangerous levels of the molecule. But for exoplanets orbiting cooler stars that emit lower levels of ultraviolet radiation, such as those that orbit red dwarf stars (re: Proxima Centauri and TRAPPIST-1), dangerous levels of this gas can accumulate. Interestingly, though CO is a very well-known toxic gas that prevents animal blood from carrying oxygen around the body, it is harmless to microbes on Earth. So it may be that habitable zone exoplanets orbiting red dwarfs could be a microbial heaven, but an asphyxiation hell for more complex lifeforms that have cardiovascular systems.
While it could be argued that life finds a way—extraterrestrial organisms may have evolved into more complex states after adapting to their environments, thereby circumventing the problems complex terrestrial life has with CO2 and CO—if we are to find a truly “Earth-like” habitable world that could support human biology, these factors need to be considered before declaring an exoplanet habitable. And, besides, we might want to make the interstellar journey to one of these alien destinations in the distant future; it would be nice to chill on an extraterrestrial beach without having to wear a spacesuit.
“Our discoveries provide one way to decide which of these myriad planets we should observe in more detail,” said Christopher Reinhard, of the Georgia Institute of Technology and co-leader of the Alternative Earths team. “We could identify otherwise habitable planets with carbon dioxide or carbon monoxide levels that are likely too high to support complex life.”
Earth: Unique, Precious
Like many astronomical and astrobiological studies, our ongoing quest to explore strange, new (and habitable) worlds has inevitably led back to our home and the relationship we have with our delicate ecosystem.
“I think showing how rare and special our planet is only enhances the case for protecting it,” Schwieterman said. “As far as we know, Earth is the only planet in the universe that can sustain human life.”
So, before we test the breaking point of our atmosphere’s sustainability, perhaps we should consider our own existential habitability before its too late to repair the damage of carbon dioxide emissions. That’s the only way that we, as complex (and allegedly intelligent) lifeforms, can continue to ask the biggest questions of our rich and mysterious universe.
I, for one, welcome our new Mars desert-dwelling overlords.
Just some random (terrestrial) microbes doing microbial things [MSU]
It’s a question I’ve been pondering for some time: if we discover microbes eking out an existence on Mars, what might they be called? At first, I presumed it would be a variation on how we designate microbial names on Earth. Something like Staphylococcus aureus but swap out the “aureus” for “ares” (Greek for “Mars”, the god of war) or … something.
As you can see, biology isn’t my strong suit and butchering Latin and Greek is all in a day’s work. So, feeling out of my depth, I decided to leave that thought alone and file the idea under “Interesting, But Needs More Research.” That’s where the topic stayed for a while; I wanted to wait for a related piece of science to appear in a journal that could be a catalyst for my question. And last week, that research surfaced. I saw my opportunity.
Searching for Martians on Earth
The Atacama Desert is an amazing place. Having visited the ESO’s Paranal Observatory and the Atacama Large Millimeter/submillimeter Array in 2016 as a lucky member of the #MeetESO team, I have first-hand experience of that extreme and breathtaking region. While driving between sites, we’d often go for hours without seeing any vegetation or life of any kind. Atacama is the driest place on Earth; its salty, parched soil is bombarded by ultraviolet radiation, and the core of the desert doesn’t receive rain for decades. But just because life isn’t obvious in the arid ‘scapes, that doesn’t mean it’s not there.
The flora and fauna that does call Atacama their home are very specialized in finding ways to thrive. On the smallest life scales, for some microbes that means living underground, which makes them very interesting organisms indeed.
In a new study, published in Frontiers in Microbiology, the results of a mock-Mars-life-hunting rover campaign in the Atacama Desert’s core have been revealed.
The research was driven, in part, to develop techniques for robotic missions to the Red Planet that will seek out alien bacteria that may be holed up in an underground colony. Remember, Mars has the same land area as Earth, so there’s a lot of real estate to search for microscopic lifeforms. Sure, scientists are smart and can narrow down potentially-habitable regions that they can drop a life-seeking robot on, but once landed on that toxic soil, what kind of methodology should they use to look for these hypothetical bacteria? The Atacama Desert makes for a decent analog of Mars; it’s very dry and its soil is laced with toxic perchlorate salts, so if microbes on Mars bear any resemblance to the nature of microbes in the Atacama, scientists can take a stab at predicting their behavior and guide their Mars rovers to the most likely places where they might be hiding.
Researchers already know that bacterial life occupies even the harshest Atacama regions, but according to team leader Stephen Pointing, a professor at Yale-NUS College in Singapore, the microbes we are familiar with are common species that live on the surface, using sunlight for energy. But Pointing isn’t so interested in what’s on the surface; his rover is fitted with a drill and extraction system that can take samples of soil from underground. During the campaign, Pointing’s team made some compelling discoveries.
“We saw that with increasing depth the bacterial community became dominated by bacteria that can thrive in the extremely salty and alkaline soils,” he told me. “They in turn were replaced at depths down to 80 centimeters by a single specific group of bacteria that survive by metabolizing methane.”
These subsurface microbes are known to science — they have been found in deep mine shafts and other subterranean environments — but they’ve never been found living under the surface of the world’s most arid region. They’ve also fine-tuned their evolution to specifically adapt to this harsh environment. “The communities of bacteria that we discovered were remarkably lacking in complexity, and this likely reflects the extreme stress under which they develop,” said Pointing.
The biggest discovery made during this research was that the subsurface colonies of bacteria were very patchy, said Pointing, a factor that will have ramifications for the search for their Martian cousins. “The patchy nature of the colonization suggest that a rover would be faced with a ‘needle in a haystack’ scenario in the search for Martian bacteria,” he said.
Desert Planet Survivor
This research is a fascinating glimpse into how Earth-based environments are being used to better understand how alien bacteria may evolve in their native environments. But the desert-thriving, methane-munching bacteria of the Atacama may also inspire their name — should they be discovered one day.
Pointing explained: “The way we assign Latin names to bacteria is based on their evolutionary relationship to each other and we measure this using their genetic code. The naming of Martian bacteria would require a completely new set of Latin names at the highest level if Martian bacteria were a completely separate evolutionary lineage — that is they evolved from a different common ancestor to Earth bacteria in a “second genesis” event [and not related to Earth life via panspermia]. If we find truly “native” Martian bacteria I would love to name one, and call it Planeta-desertum superstes, which translates in Latin to ‘survivor on the desert planet.'”
So there we have it, an answer to my question about what our Martian neighbors might be called, if we find them: Planeta-desertum superstes,the desert planet survivor.
New models trying to infer the geology of potentially habitable moons orbiting Jupiter and Saturn hint at surprisingly cool, geologically inactive worlds, the opposite of what a diverse alien ecosystem would need
[NASA/JPL-Caltech]
Imagine a spaceship finally landing on Europa and slowly drilling into the ice. After weeks of very careful progress, it pierces the moon’s frozen shell and releases a small semi-autonomous submarine connected to the probe with an umbilical to ensure constant communication and a human taking over in case of an emergency. Much of the time, it will chart a course of its own since piloting it with an hour long delay between command and response would be less than ideal. It navigates through the salty ocean, shining its light on structures never before seen by a human eye, making its way deeper and further into the alien environment to find absolutely… nothing at all.
That’s the sad scenario proposed by a team of geologists who crunched the numbers on the four leading contenders to host alien life in our outer solar system: Europa, Ganymede, Titan, and Enceladus. According to their models, looking at gravity, the weight of water and ice on the rocks underneath, and the hardness of the rocks themselves, these moons would be more or less geologically dead. Without volcanoes or sulfur vents, there would be very little in terms of nutrient exchange and therefore, very little food and fuel for an alien ecosystem more complex than microbe colonies.
Of course, these results are a pretty serious departure from the hypotheses commonly held by planetary scientists that the gravity of gas giants cause tidal kneading inside their moons, citing Io as an example. According to the researchers’ model, only Enceladus would be a promising world to look for life, as evidenced by the plumes breaking through its icy crust, spraying organic material into space. The reason why the numbers are different, they say, is because its core is likely to be porous, meaning its ocean would be heated deep inside the moon, fueling geysers and churning organic matter while effectively making the little world a ball of soggy slush.
Since these findings are so different from what’s implied by observations, the researchers aren’t in a rush to publish them are are soliciting other scientists’ opinions to make sure they have a complete picture, and lead investigator Paul Byrne grumbled about his disappointment with what the models indicate. That said, while he’s hoping to be proven wrong, we shouldn’t forget that these are alien worlds and while we’ve spent decades studying them, our knowledge came in bursts. Simply put, we might know a fair bit but far from everything and disappointing surprises may lurk under their icy surfaces and subterranean oceans.
A view from the Viking 1 deck, showing trenches its robotic arm dug out to acquire samples for testing [NASA/JPL-Caltech/Roel van der Hoorn]
When rains came to one of the driest places on Earth, an unprecedented mass extinction ensued.
The assumption was that this rainfall would turn this remote region of the Atacama Desert in Chile into a wondrous, floral haven — dormant seeds hidden in the parched landscape would suddenly awake, triggered by the “life-giving” substance they hadn’t seen for centuries — but it instead decimated over three quarters of the native bacterial life, microbes that shun water in favor of the nitrogen-rich compounds the region has locked in its dry soil.
In other words, death fell from the skies.
“We were hoping for majestic blooms and deserts springing to life. Instead, we learned the contrary, as we found that rain in the hyperarid core of the Atacama Desert caused a massive extinction of most of the indigenous microbial species there,” said astrobiologist Alberto Fairen, who works at Cornell Cornell University and the Centro de Astrobiología, Madrid. Fairien is co-author of a new study published in Nature’s Scientific Reports.
“The hyperdry soils before the rains were inhabited by up to 16 different, ancient microbe species. After it rained, there were only two to four microbe species found in the lagoons,” he added in a statement. “The extinction event was massive.”
El Valle de la Luna (Valley of the Moon) near San Pedro de Atacama looks very Mars-like [photo taken during #MeetESO in 2016, Ian O’Neill]
Climate models suggest that these rains shouldn’t hit the core regions of Atacama more than once every century, though there is little evidence of rainfall for at least 500 years. Because of the changing climate over the Pacific Ocean, however, modern weather patterns have shifted, causing the weird rain events of March 25 and Aug. 9, 2015. It also rained more recently, on June 7, 2017. Besides being yet another reminder about how climate change impacts some of the most delicate ecosystems on our planet, this new research could have some surprise implications for our search for life on Mars.
Over forty years ago, NASA carried out a profound experiment on the Martian surface: the Viking 1 and 2 landers had instruments on board that would explicitly search for life. After scooping Mars regolith samples into their chemical labs and adding a nutrient-rich water mix, one test detected a sudden release of carbon dioxide laced with carbon-14, a radioisotope that was added to the mix. This result alone pointed to signs that Martian microbes in the regolith could be metabolizing the mixture, belching out the CO2.
Alas, the result couldn’t be replicated and other tests threw negative results for biological activity. Scientists have suggested that this false positive was caused by inorganic reactions, especially as, in 2008, NASA’s Phoenix Mars lander discovered toxic and highly reactive perchlorates is likely common all over Mars. Since Viking, no other mission has attempted a direct search for life on Mars and the missions since have focused on seeking out water and past habitable environments rather than directly testing for Mars germs living on modern Mars.
With this in mind, the new Atacama microbe study could shed some light on the Viking tests. Though the out-gassing result was likely a false positive, even if all the samples collected by the two landers contained microscopic Martians, the addition of the liquid mix may well have sterilized the samples — the sudden addition of a large quantity of water is no friend to microbial life that has adapted to such an arid environment.
“Our results show for the first time that providing suddenly large amounts of water to microorganisms — exquisitely adapted to extract meager and elusive moisture from the most hyperdry environments — will kill them from osmotic shock,” said Fairen.
Another interesting twist to this research is that NASA’s Mars rover Curiosity discovered nitrate-rich deposits in the ancient lakebed in Gale Crater. These deposits might provide sustenance to Mars bacteria (and may be a byproduct of their metabolic activity), like their interplanetary alien cousins in Atacama.
As water-loving organisms, humans have traditionally assumed life elsewhere will bare similar traits to life as we know it. But as this study shows, some life on Earth can appear quite alien; the mass extinction event in the high deserts of Chile could teach us about how to (and how not to) seek out microbes on other planets.
Fortunately for life on Earth, our planet has an ozone layer. This high-altitude gas performs an invaluable service to biology, acting as a kind of global “sunscreen” that blocks the most damaging forms of ultraviolet radiation. Early in the evolution of terrestrial life, if there were no ozone layer, life would have found it difficult to gain a foothold.
So, in our effort to seek out exoplanets that are suitable for life, future telescopes will seek out so-called “biosignatures” in the atmospheres of alien worlds. Astrobiologists would be excited to find ozone in particular — not only for its biology-friendly, UV-blocking abilities, but also because the molecule’s building blocks (three oxygen atoms) can originate from biological activity on the planet’s surface.
But in a new study published Wednesday (Nov. 29) in the journal Monthly Notices of the Royal Astronomical Society, researchers modeling atmospheric dynamics on tidally-locked “habitable zone” exoplanets have concluded that finding ozone in these exo-atmospheres may be a lot more challenging than we thought.
Red Dwarf Hellholes
Recently, two exoplanets have taken the science news cycle by storm. The first, Proxima b, is touted as the closest temperate exoplanet beyond our solar system. Located a mere 4.22 light-years from Earth, this (presumably) rocky world orbits its star, Proxima Centauri, at just the right distance within the habitable zone. Should this world possess an atmosphere, it would receive just the right amount of energy for any water on its surface to exist in a liquid state. As liquid water is essential for life on Earth, logic dictates that life may be possible there too.
Whether or not Proxima b has the right orbit about its star is academic; there are many other factors to consider before calling it “Earth-like.” For starters, habitable zone exoplanets around red dwarfs will be “tidally locked.” Tidal locking occurs because red dwarf habitable zones are very close to the cool star; so to receive the same amount of heating as our (obviously) habitable Earth, habitable exoplanets around red dwarfs need to cuddle up close. And because they are so close, the same hemisphere will always face the star, while the other hemisphere will always face away. These strange worlds are anything but “Earth-like.”
Also, Proxima Centauri is an angry little star, blasting its locale with regular flares, irradiating its interplanetary space with X-rays, UV and high-energy particles — things that will strip atmospheres from planets and drench planetary surfaces with biology-wrecking radiation. As I’ve previously written, Proxima b is likely a hellhole. And things don’t bode well for that other “habitable” exoplanet TRAPPIST-1d, either.
It’s a Trap
But let’s just say, for astrobiology-sake, that a tidally-locked world orbiting a red dwarf does host an atmosphere and an alien biosphere has managed to evolve despite these stellar challenges. This biosphere is also pretty Earth-like in that oxygen-producing lifeforms are there and the planetary atmosphere has its own ozone layer. As previously mentioned, ozone would be a pretty awesome molecule to find (in conjunction with other biosignatures). But what if no ozone is detected? Well, according to Ludmila Carone, of the Max Planck Institute for Astronomy in Germany, and her team, not finding detecting ozone doesn’t necessarily mean it’s not there, it’s just that the atmospheric dynamics of tidally-locked worlds are very different to Earth’s.
“Absence of traces of ozone in future observations does not have to mean there is no oxygen at all,” said Carone in a statement. “It might be found in different places than on Earth, or it might be very well hidden.”
Earth’s ozone is predominantly produced at the equator where sun-driven chemical reactions occur high in the atmosphere. Atmospheric flows then transport chemicals like ozone toward the poles, giving our planet a global distribution. When carrying out simulations of tidally-locked worlds, however, Carone’s team found that atmospheric flows may operate in reverse, where atmospheric flows travel from the poles to the equator. Therefore, any ozone produced at the equator will become trapped there, greatly reducing our ability to detect it.
“In principle, an exoplanet with an ozone layer that covers only the equatorial region may still be habitable,” added Carone. “Proxima b and TRAPPIST-1d orbit red dwarfs, reddish stars that emit very little harmful UV light to begin with. On the other hand, these stars can be very temperamental, and prone to violent outbursts of harmful radiation including UV.”
So the upshot is, until we have observatories powerful enough to study these hypothetical exoplanetary atmospheres — such as NASA’s James Webb Space Telescope (JWST) or the ESO’s Extremely Large Telescope (ELT) — we won’t know. But modelling the hypothetical atmospheres of these very alien worlds will help us understand what we will, or won’t, see in the not-so-distant future.
“We all knew from the beginning that the hunt for alien life will be a challenge,” said Carone. “As it turns out, we are only just scratching the surface of how difficult it really will be.”
NASA’s Cassini mission captured this view of icy moon Enceladus on March 29, 2017. The crescent is lit by the sun, but the near-side green hue is reflected sunlight bouncing off Saturn’s atmosphere — a.k.a. “Saturn glow” (NASA/JPL-Caltech/Space Science Institute)
The day before Cassini plunged into Saturn’s atmosphere, dramatically ending 13 years of Saturn exploration (and nearly two decades in space), I was sitting on a bench outside the Von Karman Visitor Center on the NASA Jet Propulsion Laboratory campus in La Cañada Flintridge with Linda Spilker, who served as the mission’s project scientist since before Cassini was launched.
“I feel very fortunate to be involved with Cassini since the very beginning … and just to be there, to be one of the first to see SOI [Saturn Orbital Insertion] with those first incredible ring pictures,” she told me. “I love being an explorer. I worked on the Voyager mission during the flybys of Jupiter, Saturn, Uranus and Neptune; that sort of whet my appetite and made me want more, to become an explorer to go to the Saturn system.”
Spilker especially loved studying Saturn’s rings, not only from a scientific perspective, but also because they are so beautiful, she continued. “It’s been a heartwarming experience,” she said.
Before Cassini crashed into Saturn’s atmosphere, it took a series of observations that created this mosaic of Saturn and its rings. Cassini plunged into the Saturnian atmosphere on Sept. 15 (NASA/JPL-Caltech/Space Science Institute/Mindaugas Macijauskas)
But Cassini’s “legacy discovery,” said Spilker, was the revelation that the tiny icy moon of Enceladus is active, venting water vapor into space from powerful geysers emerging from the moon’s “tiger stripes” — four long fissures in the moon’s south pole. After multiple observations of these geysers and direct sampling of the water particles during flybys, Cassini deduced that the icy space marble hides a warm, salty ocean.
“What Cassini will be remembered for — its legacy discovery — will be the geysers coming from Enceladus with the ocean with the potential for life. It’s a paradigm shift.” — Linda J. Spilker, Cassini project scientist, NASA Jet Propulsion Laboratory (JPL), Sept. 14, 2017.
Alongside Jupiter’s moon Europa, Enceladus has become a prime destination for future explorations of life beyond Earth. Its subsurface ocean contains all the ingredients for life as we know it and Cassini was the mission that inadvertently discovered its biological potential. So now we know about this potential, Spilker is keen to see a dedicated life-hunting mission that could go to Enceladus, perhaps even landing on the surface to return samples to Earth.
Artist impression of Cassini flying through Enceladus’ water plumes venting from the moon’s south pole (NASA/JPL-Caltech)
As Enceladus is much smaller and less massive than Europa, its gravity is lower, meaning that landing on the surface is an easier task. Also, the radiation surrounding Saturn is much less aggressive than Jupiter’s radiation belts, meaning less radiation shielding is needed for spacecraft going to Saturn’s moons.
But if we ever send a surface mission to Enceladus (or any of the icy moons in the outer solar system), the planetary protection requirements will be extreme.
“If any life were found on these moons, it would be microbial,” said Larry Soderblom, an interdisciplinary scientist on the Cassini mission. “Some [terrestrial] bacteria are very resilient and can survive in hot acid-reducing environments. They can be tenacious. We have to make sure we don’t leave any of these kinds of Earthly bacteria on these promising moons.”
Soderblom has a unique perspective on solar system exploration. His career spans a huge number of NASA missions since the 1960’s, including Mariner 6, 7, 9, Viking, Voyager, Galileo, Magellan, Mars Pathfinder, the Mars Exploration Rovers, Deep Space 1, to name a few. While chatting to me under the shade of a tree on the JPL campus, he pointed out that the outer solar system was seen as a very different place over half a century ago.
“When I started to explore the solar system as a young guy just out of graduate school, our minds-eye view of the outer solar system was pretty bleak,” he remembered. “We expected lifeless, dead, battered moons with no geologic activity.”
After being involved with many outer solar system missions, this view has radically changed. Not only have we discovered entire oceans on Enceladus and Europa, there’s active volcanoes on Jupiter’s tortured moon Io, an atmosphere on Titan sporting its own methane cycle and surface lakes of methane and ethane. Other moons show hints of extensive subsurface oceans too, including distant Triton, a moon of Neptune. When NASA’s New Horizons flew past Pluto in 2015, the robotic spacecraft didn’t see a barren, dull rock as all the artistic impressions that came before seemed to suggest. The dwarf planet is a surprisingly dynamic place with a rich geologic history.
With a diameter of only 313 miles, tiny Enceladus is a surprising powerhouse of internal activity. Subsurface oceans are heated through tidal interactions with Saturn (and, possibly, radioactivity in its rocky core), forcing water through its south pole fissures (NASA/JPL-Caltech)
Sending our robotic emissaries to these distant and unforgiving places has revolutionized our understanding of the solar system and our place in it. Rather than the gas and ice giant moons being dull, barren and static, our exploration has revealed a rich bounty of geologic variety. Not only that, we’re almost spoilt for choices for our next giant leap of scientific discovery.
Missions like Cassini are essential for science. Before that spacecraft entered Saturn orbit 13 years ago, we had a very limited understanding of what the Saturnian system was all about. Now we can confidently say that there’s a tiny moon there with incredible biological potential — Enceladus truly is Cassini’s legacy discovery that will keep our imaginations alive until we land on the ice to explore its alien ocean.
For more on my trip to JPL, read my two HowStuffWorks articles:
Artist’s impression of Titan’s surface and atmosphere (credit: Benjamin de Bivort, debivort.org / CC BY-SA 3.0)
Titan is a very strange moon.
Orbiting the ringed gas giant Saturn, Titan is the only moon in the solar system that sports a thick atmosphere. Although the moon is extremely cold, its atmosphere is very dynamic; exhibiting seasons, precipitation and even creating vast seas.
Although this may sound very much like Earth’s atmosphere — where water evaporates from the oceans, condenses as clouds and precipitates as rain, forming rivers that flow back into the oceans — Titan’s atmosphere is dominated by a methane cycle, not a water cycle.
This may sound like the antithesis of Earth’s life-giving chemistry, but astrobiologists have been gradually finding clues to Titan’s habitable potential and today (July 28) scientists have announced the confirmation of a key molecule that could be the proverbial backbone to a weird kind of “alternative” alien life on Titan — based not on liquid water, but on liquid methane.
“The presence of vinyl cyanide in an environment with liquid methane suggests the intriguing possibility of chemical processes that are analogous to those important for life on Earth,” said astrochemistry researcher Maureen Palmer, of NASA’s Goddard Space Flight Center in Greenbelt, Md.
Palmer is lead author of a study published in Science Advances describing the detection of vinyl cyanide (also known as acrylonitrile) at Titan using the awesome power of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.
B. Saxton (NRAO/AUI/NSF); NASA
Previous observations of Titan’s atmosphere by NASA’s Cassini mission and chemical modeling of the moon’s surface have hinted that it is the ideal environment for vinyl cyanide to form. But it was only when analysis of archived data collected by ALMA between February to May 2014 was carried out that its presence was confirmed. And there appears to be a lot of the stuff.
So what is vinyl cyanide and why is it so important?
The molecule (C2H3CN) has the ability to form membranes and, if found in liquid pools of hydrocarbons on Titan’s surface, it could form a kind of lipid-based cell membrane analog of living organisms on Earth. In other words, this molecule could stew in primordial pools of hydrocarbons and arrange itself in such a way to create a “protocell” that is “stable and flexible in liquid methane,” said Jonathan Lunine (Cornell University) who, in 2015, was a member of the team who modeled vinyl cyanide and found that it might form cell membranes.
“This is a step forward in understanding whether Titan’s methane seas might host an exotic form of life,” Lunine, who wasn’t a member part of the team that announced today’s results, said in a statement.
Life As We Don’t Know It
When studying Titan’s nitrogen-rich atmosphere, ALMA detected three unambiguous millimeter-wavelength signals produced by vinyl cyanide that originated from 200 kilometers above Titan’s surface. It is well known that the moon’s atmosphere is a vast chemical factory; the energy of the sun and particles from space convert simple organic molecules into more complex chemistry. These chemicals then cycle down to Titans rich hydrocarbon surface.
But speculating about life on Titan is a hard task. The moon’s atmosphere is often compared with that of early Earth’s, but there are some huge differences. Titan is crazy-cold, averaging around 95 Kelvin (that’s an incredible -178 degrees Celsius or -288 degrees Fahrenheit); at no time in history has Earth’s atmosphere been that cold. Also, it’s thought that early Earth had large quantities of carbon dioxide in its atmosphere, Titan does not. As for water? Frozen. Oxygen? Forget about it.
So this research underpins our quest to find the chemistry of life as we DON’T know it, using the building blocks that follow the pattern of life that we do know, but swapping out key components (like water) to see if an analog of life’s chemistry can under very alien conditions.
“Saturn’s moon, Enceladus is the place to search for life like us, life that depends on — and exists in — liquid water,” said Lunine. “Titan, on the other hand, is the place to go to seek the outer limits of life — can some exotic type of life begin and evolve in a truly alien environment, that of liquid methane?”
Perhaps it’s time for a return mission to Titan’s extreme surface.
astroengine.com 
