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

All About Space - - Contents - Re­ported by Lee Cavendish

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.

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