Liv­ing moons

Nat­u­ral satel­lites in our So­lar Sys­tem may be prime tar­gets in the search for life

How It Works - - CONTENTS - Words by Jonathan O’cal­laghan

The satel­lites in our own So­lar Sys­tem that could host alien life

The search for life in our So­lar Sys­tem has taken many twists and turns over the decades. Once, Mars was deemed the most plau­si­ble lo­ca­tion for past and present life, while even worlds like Venus bear the hall­marks of hav­ing been hab­it­able. In a change of course, much of the fo­cus to­day is on new lo­ca­tions that hold con­sid­er­able prom­ise – the moons of the outer plan­ets, where icy sur­faces and other fea­tures may hide life-har­bour­ing en­vi­ron­ments. NASA’S Pi­o­neer 10 and 11 space­craft in the 1970s were the first to re­turn close-up im­ages of Jupiter, Saturn and their moons, set­ting in mo­tion a thrilling sci­en­tific story that is mov­ing for­wards at full pace to­day. Whereas our own Moon seems mostly de­void of life, early space­craft like these, and the sub­se­quent Voy­ager space­craft, showed there were plenty of se­crets await­ing our dis­cov­ery. It wasn’t un­til NASA’S Galileo space­craft ar­rived at Jupiter in 1995, and their Cassini space­craft at Saturn in 2004, that ex­cite­ment re­ally started to ramp up. These probes showed those moons were far more ex­cit­ing than we could have imag­ined, with ev­i­dence mount­ing that some could har­bour oceans be­neath their sur­face. While none seemed to pos­sess any signs of life on their sur­face, un­der­ground – safe from ra­di­a­tion – sci­en­tists started to won­der what could be go­ing on. Fast-for­ward to to­day, and the moons of Jupiter and Saturn, and per­haps other moons too, like Nep­tune’s Triton, are look­ing like the best bet in the search for life in the So­lar Sys­tem. Us­ing radar and other im­ages, we have al­most con­clu­sively proven that lo­ca­tions like Europa and Ence­ladus house oceans be­neath their sur­face. These oceans are thought to be tens of kilo­me­tres un­der the ice of their re­spec­tive moons, too deep for us to reach with cur­rent tech­nol­ogy. But Europa and Ence­ladus in par­tic­u­lar seem to be fir­ing plumes from their oceans into space, with some of that ma­te­rial avail­able to study ei­ther in space or on the sur­face. The Cassini space­craft was ac­tu­ally able to fly through the plumes of Ence­ladus and thereby sam­ple its in­te­rior. When the mis­sion was de­signed, how­ever, these plumes were not known about, so the in­stru­ments avail­able to study them were lim­ited. Fu­ture mis­sions could in­ves­ti­gate these plumes even fur­ther and look for or­ganic com­pounds or

As these moons are pushed and pulled by the in­tense grav­ity of their host plan­ets they ex­pe­ri­ence tidal heat­ing, melt­ing the vast swathes of ice into the liq­uid we think re­sides there to­day. This liq­uid is thought to be boun­ti­ful – some of these liv­ing moons are be­lieved to con­tain more water than there is on Earth. This tidal heat­ing is in­ter­est­ing for an­other rea­son too. Life as we know it needs sev­eral key in­gre­di­ents to thrive, in­clud­ing liq­uid water, heat and en­ergy. It might be that on some of these moons this grav­i­ta­tional ef­fect heats the cores, form­ing hy­dro­ther­mal vents on the ocean floors. On Earth such vents pro­vide not only heat for life but en­ergy and sus­te­nance too. Could it be that some of these moons, deep in their in­te­ri­ors, are sim­i­lar to Earth?

It’s not just the oceans that are in­ter­est­ing, as Saturn’s moon Ti­tan is in­trigu­ing for a whole other rea­son. While we think this moon may have an ocean un­der­ground, it’s what is tak­ing place on the sur­face that has sci­en­tists talk­ing. Ti­tan is the only world other than Earth known to have bod­ies of liq­uid on its sur­face – here in the form of liq­uid meth­ane and eth­ane, creat­ing a jet fuel-like liq­uid. Add in its thick at­mos­phere and a climate sys­tem not too dis­sim­i­lar to our own, and Ti­tan starts to tick a lot of boxes.

If life does ex­ist on the sur­face of Ti­tan, it is likely life as we don’t know it, re­ly­ing on pro­cesses that we don’t yet un­der­stand. Much of the fo­cus on the ocean moons, mean­while, has been based on life as we do know it, which makes sense; we know life ex­ists on our planet in cer­tain con­di­tions, so why would we not look for those same con­di­tions else­where?

To get an­swers to these ques­tions and more, a num­ber of space­craft are now be­ing de­signed that could probe these moons like never be­fore. NASA’S Europa Clip­per, for ex­am­ple, will study the moon it is named af­ter in de­tail. Launch­ing in the mid-2020s, it will try to work out how thick Europa’s icy sheet is and whether there are signs of hab­it­abil­ity on it.

To com­ple­ment this mis­sion, NASA is look­ing to de­velop a Europa Lan­der that could land on the sur­face of this moon. If the plumes of Europa rain back down on the sur­face as we ex­pect, then it could be pos­si­ble to sam­ple this ocean without hav­ing to drill through tens of kilo­me­tres of ice. Oth­ers ar­gue Saturn’s moon Ence­ladus is a bet­ter bet for such a mis­sion, as its plumes are more con­stant, with more ma­te­rial avail­able to study. Bud­gets are lim­ited, how­ever, so for now Europa is in the lime­light.

An­other mis­sion, the Euro­pean Space Agency’s (ESA) Jupiter Icy Moons Explorer (JUICE), will also be in­ves­ti­gat­ing this sys­tem. Aside from just Europa, it will study Jupiter’s other in­ter­est­ing moons, in­clud­ing Ganymede and Cal­listo. The lat­ter may hide an an­cient ocean that has ex­isted long enough to al­low life to take hold, whereas the for­mer is the only moon in the So­lar Sys­tem known to have a mag­netic field. Earth’s mag­netic field pro­tects us from ra­di­a­tion; per­haps Ganymede’s makes it sim­i­larly suit­able for habi­ta­tion.

The search for life in the uni­verse is pro­gress­ing steadily, with some sci­en­tists favour­ing a closer look at Mars, which may have once been more like Earth with seas and oceans on its sur­face. Oth­ers favour study­ing worlds be­yond our So­lar Sys­tem, called ex­o­plan­ets, to look for some that may be sim­i­lar to our own. But the moons of the outer plan­ets are without doubt among the most promis­ing tar­gets at the present time.

We still know very lit­tle about their oceans and po­ten­tial hab­it­abil­ity, but in the next cou­ple of decades we might get closer than ever to find­ing out if life in our So­lar Sys­tem is lim­ited to our own rocky planet or is spread abun­dantly on worlds like and un­like Earth.

Fu­ture mis­sions could explore the oceans of Europa and else­where

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