Saturn’s moon Enceladus hides a global ocean

SINCE Nasa’s Cassini sp acecraft sp otted p lumes of w ater sp ew ing out of cracks in Enceladus’ surface roughly a decade ago, Saturn’s icy moon has cap tured the imaginations of scientists and layfolk alike w ho w ondered w hether liq uid w ater – and how much ofit – could lie hidden b eneath the frigid exterior.

Now , through a careful analysis ofmore than seven years’ w orth ofimages, a team of scientists has found that Enceladus harb ours a glob al ocean – far more extensive than the southern sea that researchers describ ed just last year.

The findings, released online b y the journal Icarus, could shed light on the internal dynamics ofone ofthe few w orlds in our Solar System w ith the p otential to host life.

Enceladus is one ofseveral icy “w ater w orlds” in our system, such as Jup iter’s moons Europ a and Ganymede, that ap p ear to host liq uid w ater b eneath their frozen shells. If they also contain the right chemical ingredients, they could, hyp othetically sp eaking, b e hab itab le for microb ial life.

Enceladus has b een caught p eriodically sq uirting w ater out of slits in its southern p ole since 2005, and since then, Cassini has also found signs of simp le organic molecules in that p lume and hydrothermal activ ity b eneath the surface.

Last year, scientists studying the moon’s grav itational field announced that they’d identified a lens- shap ed sea b eneath the southern p ole. But this nearly 10.5km- thick layer of w ater w as ap p arently only a regional p henomenon, hidden b eneath more than 35km of ice.

“Rememb er, the sp acecraft flew only over the south p ole, so it’s much harder for them to b e ab le to say w hat’s on the other side of the moon,” said study co- author Carolyn Porco, leader ofCassini’s imaging team at the Sp ace Science Institute in Colorado.

For the new p ap er, researchers p ainstakingly map p ed features such as craters and ridges on Enceladus and looked for tiny discrep ancies in their p osition at different p oints in time, to see w hether the moon had sp un a little faster or slow er than exp ected at various moments in its orb it around Saturn.

The magnitude ofthis “w ob b le” in its sp in could offer insight into how Saturn’s massiv e grav itational p ull affects the mass w ithin Enceladus – and, thus, w hat Enceladus’ interior looks like.

The researchers found a distinct w ob b le – one that, w hile exceedingly difficult to p ick out, w as still far higher than w ould b e exp ected if Enceladus w as solid all the w ay through. The w ob b le show ed that Enceladus’ icy shell must b e sep arated from the core b y a liq uid layer – and that icy shell is more easily influenced b y Saturn’s grav ity.

The w ob b le “is more p ronounced if it’s tugging on the ice shell b ecause the ice shell is a lot less massiv e, so it’s a lot easier to move around”, Porco said.

This layer of ocean is p rob ab ly q uite thin, she said. But its very p resence took p lanetary scientists b y surp rise b ecause the p resence ofso much liq uid w ater imp lies that Enceladus is much w armer than p reviously thought.

“There are other lines of evidence coming from different q uarters suggesting that there may b e a lot more heat p roduced either now or in the p ast in Enceladus than w e had b een thinking,” Porco said.“And that’s going to b e really exciting once p eop le w ork that out ... no one’s really, like, rolled up their sleeves and really ( b een) div ing into it.”

For Enceladus, all the ingredients for a p otentially hab itab le environment – w ater, simp le organic molecules and p otentially ( at the hydrothermal hotsp ots) an energy source to draw from. But the q uestion is, has that hab itab le environment b een around long enough for microb ial life to arise?

“Those conditions have to b e around for a time greater than the origin of life,” Porco said.

The p rob lem w ith that standard, of course, is that w e don’t know how long it took for life to arise on Earth in the first p lace, she added. It could have b een 100 million years or a mere million years; for now , that remains a mystery.

Porco says she and some colleagues are w orking on a p rop osal for a discovery- class Nasa mission that w ould try to address some of these q uestions more directly, b y sending a sp acecraft to Enceladus w ith the right kind of instruments to p rob e the chemistry of the moon’s interior. The instruments on such a sp acecraft w ould look for the p resence ofamino acids or fatty acids, among other clues.

But don’t start p acking your scub a gear just yet: those q uestions w on’t b e answ ered anytime soon. Such a mission, if ap p roved, w ould p rob ab ly b e launched in 2022 and p rob ab ly w ouldn’t arriv e until ab out 2029, she said.– Los Angeles Times/ Trib une New s Serv ice