Are we alone in the so­lar sys­tem?

As­tronomers and plan­e­tary sci­en­tists are racing to dis­cover whether alien life is wide­spread among the worlds in our cos­mic neigh­bour­hood

All About Space - - Contents - Writ­ten by Giles Spar­row

The NASA-led re­search that has pro­vided a surprising an­swer

It’s not too long ago that sci­en­tists as­sumed our planet was the only place in the So­lar Sys­tem with the right con­di­tions for life, but a se­ries of stun­ning dis­cov­er­ies have re­cently shown that’s far from the case. In­stead of hav­ing to search for tell­tale hints in the light of dis­tant planets or­bit­ing other stars, per­haps alien life (at least in the most sim­plest form) is wait­ing to be found on our cos­mic doorstep.

Ask a dozen bi­ol­o­gists for a def­i­ni­tion of life and you’re likely to get a dozen dif­fer­ent an­swers – life is one of those things that is hard to pin down, though you know it when you see it. Even the most open minded of bi­ol­o­gists, how­ever, tend to agree that two of the key re­quire­ments for life, which guide our chances of find­ing it else­where in the So­lar Sys­tem, are abun­dant car­bon and a plen­ti­ful solvent, most likely liq­uid wa­ter.

Car­bon is im­por­tant be­cause, of all the el­e­ments, it is the one best suited to build­ing the hugely com­plex, self-repli­cat­ing mol­e­cules required by most liv­ing pro­cesses. For­tu­nately it’s one of the most com­mon el­e­ments in our galaxy, gen­er­ated in huge quan­ti­ties by nuclear fu­sion pro­cesses in­side stars and scat­tered through in­ter­stel­lar space when they die, for in­cor­po­ra­tion into later gen­er­a­tions of stars and planets.

Wa­ter, mean­while, is needed for the most ba­sic of rea­sons: in or­der for the com­plex chem­i­cal pre­cur­sors of life to arise, sim­pler chem­i­cals must first en­counter one an­other and go through chem­i­cal re­ac­tions. This means they must be able to move around, some­thing that’s most likely to hap­pen when they’re dis­solved in a fluid solvent. The unique chem­istry of wa­ter makes it the most ef­fec­tive solvent among all liquids that com­monly oc­cur in na­ture, and once again we’re for­tu­nate that it seems to be wide­spread in our galaxy.

So what can the re­quire­ment for these two ba­sic in­gre­di­ents tell us about the pos­si­bil­i­ties for life in our So­lar Sys­tem? While car­bon is com­mon­place across all the So­lar Sys­tem’s planets and moons, liq­uid wa­ter seems at first glance to be a much trick­ier re­quire­ment – Earth is the only planet with abun­dant sur­face wa­ter, thanks to its po­si­tion in the So­lar Sys­tem’s ‘Goldilocks zone’, where tem­per­a­tures are nei­ther so hot that the oceans boil away into the at­mos­phere, nor so cold that they freeze solid. Up un­til the dawn of the space age, many as­tronomers sus­pected that our neigh­bour­ing planets, Venus and Mars, might also have liq­uid wa­ter on their sur­faces, but the first spaceprobe fly­bys put an end to these hopes, re­veal­ing Venus as a toxic, roast­ing hell­hole and Mars as a frozen, arid desert.

For­tu­nately it’s now clear that the Goldilocks zone isn’t the be-all and end-all of pos­si­bil­i­ties for life. Plan­e­tary sci­en­tists have dis­cov­ered ev­i­dence

for liq­uid wa­ter in surprising places across the

So­lar Sys­tem – for ex­am­ple hid­den be­neath the icy crusts of moons whose in­te­ri­ors are heated by the strong ti­dal forces of their par­ent planets, or per­haps kept liq­uid even at sub-zero tem­per­a­tures by the pres­ence of other chem­i­cals such as salts or am­mo­nia. Mean­while, in the past few decades, bi­ol­o­gists have also found that life on Earth is able to thrive in ex­tremes of acid, al­kali, heat, cold and dark­ness very dif­fer­ent from those we nor­mally ex­pe­ri­ence. The dis­cov­ery of these ‘ex­tremophile’ or­gan­isms has opened up a whole range of new habi­tats where life might ex­ist be­yond Earth.

When it comes to the ba­sic ma­te­ri­als for life in the So­lar Sys­tem, it now seems that all bets are off – so where should we look, and what might we find?

At first glance, Mars re­mains the most ob­vi­ous can­di­date as an en­vi­ron­ment for life. Since those early dis­ap­point­ments, pho­to­graphs and other data from or­bit­ing spaceprobes, along with soil anal­y­sis by sur­face rovers, have re­vealed there’s much more to the Red Planet than arid desert. The sur­face soil is mixed with large amounts of ice to form per­mafrost, and in some places even flows to cre­ate glacier-like fea­tures. An­cient fea­tures also show that liq­uid wa­ter flowed freely on the sur­face in the dis­tant past, when the Mar­tian at­mos­phere was thicker and its or­bit was per­haps dif­fer­ent. Mars al­most cer­tainly had the right con­di­tions for life to gain a foothold bil­lions of years ago – but is there any chance it could still cling on to­day?

So far the only ex­per­i­ments to de­lib­er­ately search for life on the Mar­tian sur­face were car­ried aboard the Vik­ing mis­sions of the 1970s. These robot lan­ders ex­posed soil sam­ples to a se­ries of chem­i­cal re­ac­tions and looked for signs of liv­ing meta­bolic pro­cesses. They pro­duced in­con­clu­sive re­sults and have never been prop­erly re­peated – the Bri­tish-built Bea­gle 2 Lan­der, de­signed to con­tinue the di­rect search for life, sadly ended up wrecked on the Mar­tian sur­face dur­ing its 2003 land­ing.

Per­haps the most con­tro­ver­sial ev­i­dence for life, how­ever, comes from a me­te­orite called ALH84001 – a frag­ment of 4.5-bil­lion-year-old Mar­tian rock that was blasted off the planet in a me­te­orite im­pact and fell to Earth in Antarc­tica about 13,000 years ago. In 1996, a team of NASA sci­en­tists claimed to have dis­cov­ered chem­i­cal biomark­ers (mol­e­cules cre­ated by bi­o­log­i­cal ac­tiv­ity) and mi­cro­scopic fos­sil-like struc­tures within it. They sus­pected the ac­tion of prim­i­tive ‘nanobac­te­ria’, sim­i­lar to (though much

“Dis­cov­ery of these ‘ex­tremophile’ or­gan­isms has opened up a whole range of new habi­tats where life might ex­ist”

smaller than) some of Earth’s own ‘ex­tremophile’ bac­te­ria. The claim re­mains hugely con­tro­ver­sial, how­ever – other sci­en­tists have pro­posed ways for the mol­e­cules and ‘fos­sils’ to have arisen with­out the need for life, and the mat­ter prob­a­bly won’t be set­tled for good un­til sci­en­tists have more sam­ples of Mar­tian rock to ex­am­ine.

But is to­day’s Mars suit­able for life? Con­clu­sive ev­i­dence of liq­uid wa­ter on the sur­face to­day (per­haps seep­ing from un­der­ground wa­ter ta­bles) re­mains frus­trat­ingly elu­sive, and while a lack of liq­uid wa­ter on Mars to­day wouldn’t en­tirely rule out spe­cially adapted mi­crobes, it would seem to make it far less likely.

Bal­anced against this, the most in­trigu­ing ev­i­dence for pos­si­ble Mar­tian life so far comes from the de­tec­tion of meth­ane gas. The first traces of meth­ane (a few parts per bil­lion in the at­mos­phere) were dis­cov­ered from Earth-based tele­scopes and or­bit­ing spaceprobes in the early 2000s, and have since been con­firmed by rovers such as NASA’s Cu­rios­ity. The gas is puz­zling be­cause it is un­sta­ble in Mar­tian con­di­tions – fierce ul­tra­vi­o­let ra­di­a­tion should rapidly break its mol­e­cules apart – so for meth­ane to per­sist, some­thing must be con­stantly pro­duc­ing it.

On Earth, meth­ane is pro­duced by liv­ing or­gan­isms or ge­o­log­i­cal ac­tiv­ity such as ac­tive vol­ca­noes. Vol­can­ism or other pro­cesses can’t yet be ruled out, but the an­nounce­ment in early 2018 of a sea­sonal cy­cle in which meth­ane lev­els in the Mar­tian north­ern hemi­sphere rise to a peak in later sum­mer adds to the mys­tery – could it be that meth­ane-pro­duc­ing mi­crobes are stirred into ac­tiv­ity by the sum­mer sun­shine? The Eu­ro­pean Space Agency's and Roscos­mos' Ex­oMars Trace Gas Or­biter, due to start work in or­bit around the Red Planet, may shed more light on the mys­tery.

Fur­ther out in the So­lar Sys­tem, a good hand­ful of worlds of­fer tan­ta­lis­ing prospects for life. Solid worlds be­yond the mid­dle of the as­ter­oid belt – such as dwarf planets, moons, as­ter­oids and comets – are made from rock and ice mixed in vary­ing amounts, and it’s now clear that ti­dal forces raised on satel­lites or­bit­ing gi­ant planets, or sim­ply the ad­di­tion of chem­i­cals that lower the freez­ing point of wa­ter, can be enough to cre­ate a deep liq­uid ocean layer be­neath a solid outer crust.

The best known ex­am­ples of such hid­den oceans are Jupiter’s satel­lite Europa and Saturn’s moon

“Europa and Ence­ladus are prob­a­bly the So­lar Sys­tem’s most likely habi­tats for life – and not just sin­gle-celled mi­crobes”

Ence­ladus. On Europa, a 25-kilo­me­tre- (15-mile) thick outer crust of jostling ice plates slowly drifts and re­ar­ranges it­self on top of a global ocean about 160-kilo­me­tres (100-miles) deep. The ocean on Ence­ladus is shal­lower, but closer to the sur­face, with the crust just five-kilo­me­tres (three-miles) thick in places. On Ence­ladus at least, sea-floor hy­dro­ther­mal vents belch out gas and min­er­als from deep within the crust. The en­vi­ron­ment around these vents could pro­vide an ideal oa­sis for life to arise (in­deed, many bi­ol­o­gists now sus­pect that life on Earth got started around sim­i­lar vents).

Although it’s cur­rently im­pos­si­ble to in­ves­ti­gate these hid­den oceans di­rectly, both moons re­lease plumes of vapour into space which we can study. The jets above Ence­ladus have al­ready pro­vided clues to con­di­tions in its ocean – mol­e­cules of hy­dro­gen within them have been linked to ac­tive un­der­sea vents. Europa’s vapour plumes are thin­ner and more in­ter­mit­tent – they may not es­cape di­rectly from the ocean, but might in­stead be knocked off the moon’s icy sur­face by ra­di­a­tion from the Sun and Jupiter. But since Europa’s sur­face ice is it­self made up of so­lid­i­fied and re­cy­cled ocean ice, even this could of­fer im­por­tant clues. NASA’s Europa Clip­per mis­sion, planned to launch in the mid-2020s, will aim to find out more.

To­gether, Europa and Ence­ladus are prob­a­bly the So­lar Sys­tem’s most likely habi­tats for alien life – and per­haps not just sin­gle-celled mi­crobes, but more ad­vanced crea­tures that have evolved to suit their en­vi­ron­ment. Such or­gan­isms might have stream­lined shapes sim­i­lar to fish, or flex­i­ble forms that take ad­van­tage of buoy­ant con­di­tions, like squid and other cephalopods. But then again, it’s worth bearing in mind that sin­gle-celled life on Earth ex­isted for at least 3 bil­lion years be­fore blos­som­ing into var­ied, mul­ti­cel­lu­lar or­gan­isms (for rea­sons that we don’t yet fully un­der­stand).

Aside from these two icy show­cases, it’s now clear that many other So­lar Sys­tem bodies could have hid­den oceans deep be­neath their sur­faces. On Jupiter’s gi­ant moons Ganymede and Cal­listo, that wa­ter is sealed off hun­dreds of kilo­me­tres be­low the sur­face, and de­tectable only through in­ter­ac­tions with Jupiter’s mag­netic field. NASA’s Dawn as­ter­oid probe has re­vealed signs of a hid­den ocean on the largest as­ter­oid, Ceres, and there could even be liq­uid wa­ter on dis­tant Pluto. Dur­ing its 2015 flyby, the New Hori­zons mis­sion pho­tographed ex­tra­or­di­nary fea­tures that many be­lieve could only have been cre­ated by the ef­fects of a fluid man­tle just be­neath the crust. Any of these worlds could have their own hy­dro­ther­mal vents, and po­ten­tially their own life, though it would be far harder for us to de­tect and in­ves­ti­gate.

How­ever, if there’s a prize for the most un­likely po­ten­tial out­post for life, it must surely go to Ti­tan. Cloaked in an opaque, ni­tro­gen-rich at­mos­phere, Saturn’s largest moon is one of the cold­est worlds in the So­lar Sys­tem, al­low­ing hy­dro­car­bons such as meth­ane to con­dense into liq­uid and play a sim­i­lar role to that of wa­ter on Earth. Meth­ane rains from clouds, erodes a land­scape cov­ered in oily hy­dro­car­bon ices and, amaz­ingly, col­lects in lakes near the poles.

In many ways, Ti­tan is a low-tem­per­a­ture ver­sion of Earth – so if it has its own sur­face liq­uid, could it also have its own life? There’s car­bon aplenty, and in­deed hy­dro­car­bon mol­e­cules are an im­por­tant first step on the way to­wards com­plex bio­chem­istry. But while liq­uid meth­ane is a less ef­fi­cient solvent than wa­ter, once chem­i­cals are dis­solved, they are more likely to per­sist and re­main sta­ble for longer, giv­ing greater po­ten­tial for life­giv­ing re­ac­tions to arise.

Re­gard­less of the form it takes, if life is found to be wide­spread across the So­lar Sys­tem, it could have huge im­pli­ca­tions for our un­der­stand­ing of the wider uni­verse. If, for ex­am­ple, life on other worlds turned out to share ba­sic bi­o­log­i­cal fea­tures, it would sup­port the so-called ‘pansper­mia’ the­ory that life is car­ried be­tween worlds, and per­haps even be­tween stars, on comets and me­te­orites. If dif­fer­ent strains of life prove to be en­tirely in­de­pen­dent, it would sug­gest that life arises nat­u­rally wher­ever con­di­tions are even re­motely suit­able. In ei­ther case, we could ex­pect ba­sic life in our galaxy to be equally com­mon­place – per­haps even giv­ing rise to in­tel­li­gent aliens that we might one day con­tact.

the pansper­mia the­ory sug­gests that comets have seeded many of our So­lar Sys­tem’s worlds with life

naSa’s europa Clip­per is a ded­i­cated probe to in­ves­ti­gate the icy moon in a se­ries of close fly­bys due to launch in the 2020s

Strange forms of life such as these gi­ant 'tube worms’ flour­ish around deep-sea vents on earth – could the same go for ence­ladus?

elec­tron mi­croscopy of mar­tian as­ter­oid alh84001 re­vealed tiny mi­crobe-like ‘fos­sils’ whose true na­ture is still con­tro­ver­sial

any fu­ture mis­sion to land on europa would need to be ster­ilised in or­der to pre­vent con­tam­i­na­tion of the moon’s en­vi­ron­ment

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