After we discovered the geysers on Enceladus, why did it take all of this time to find their source?
Other than to look for plumes coming off Enceladus, which we first saw in February 2005 in a set of images designed to look for [these ejections], we had no other plans in the nominal four-year mission for specific observations of geysering activity. How could we? We weren’t sure [the ocean] existed. So once we found them, we altered the nominal mission as much as we could to get a closer look and, of course, we designed many observations in the extended mission to develop a much more complete picture about the phenomenon. This included a set of flybys to measure the mass distribution within the outer layers of the moon in the south polar region, as well as very high-resolution images of the geyser field and Enceladus’ surface.
It wasn’t until 2013, at which point the gravity results were adequately analysed, that we had confirmation of a liquid layer about ten kilometres (6.21 miles) thick under the south polar terrain. We realised that it was at least as wide as the south pole region, but a global ocean couldn’t be ruled out. Under certain reasonable assumptions, some analyses suggested it should be global. But that conclusion didn’t carry a high degree of confidence. As we saw, it wasn’t until last month that my team members finally finished their work utilising our high-resolution images of the surface. We managed to tease out a small wobble in the rotation of Enceladus that is best explained by a global ocean.
These analyses are all, at this stage in the mission, very sophisticated, requiring more than just a look/ see engagement with an image, but a lot of work. And good, careful work takes time.
Could you tell us a bit more about the techniques used to find the ocean?
The first confirmation that there was a liquid layer about 35 kilometres (21.75 miles) below the south polar terrain came from the gravity signature. The distribution of mass under the south polar terrain was detectable by carefully tracking the motion of the Cassini spacecraft as it flew closely over Enceladus’ south pole. Doing three such flybys allowed the gravity team to figure out that the water layer had to be about 35 kilometres (21.75 miles) beneath the surface and about ten kilometres (6.21 miles) thick.
The confirmation that the ocean is global came from noticing how the rotation of Enceladus was not uniform but sometimes slightly faster and sometimes slightly slower. To do this, my team members were using features on the surface as benchmarks – kind of like painting a white stripe on your car tyre – and comparing the positions of the features at various times to see how they compared to where they would have been if there were no wobble.
And the magnitude of the wobble indicates something about the interior – in particular, whether the ice shell moves independently of the core or is attached to it. In this particular case, they found that the ice shell has to be moving independently and that can only be the case if it is floating on a liquid layer.
What do you think the ocean is likely to be made up of?
We know from those instruments on Cassini that have sampled the Enceladus plume that the source of the ocean is mostly water, with trace amounts of salt and organic compounds.
Does the confirmation of an ocean make Enceladus an additional target for life?
Once we knew Enceladus had any liquid at all, but especially that it’s liquid is in contact with a core, we knew it was a target for searching for extraterrestrial biology. So we’ve known that for quite a while and for about ten years. This new observation, however, makes the story richer, and may tell us that there is more energy being injected into Enceladus over the course of time than we had previously thought. So if it does anything, it may mean that any living organisms there may have had a longer time to evolve than we originally suspected. Maybe when we get back there to look closely, instead of finding micro-organisms, we’ll find lobster and sushi!
If you could hazard a guess, what do you think life on Enceladus would be like?
It’s fair to say we all have micro-organisms on the brain. I’d fall off my chair, assuming I’m still able to sit on chairs when the time comes, if there were anything as complex as shrimp or sea horses, wow, that would be a stunner.
Similarly, we’re interested in Jupiter’s moon Europa for its oceans. Is there now going to be difficulty in deciding which moon we should send a dedicated-mission to first?
In my mind, Enceladus was the clear winner if the goal was to sample an extraterrestrial habitable zone – a candidate environment for life – as soon and as easily as possible. Why? The operative word with Enceladus is accessibility. You don’t have to land, scratch, drill or bunker your spacecraft in lead [to protect it from radiation] in order to search for evidence of life. However, other factors like political and technical momentum won and a mission to Europa was chosen and is now under way. So I say: great! Now that we have that mission out the door, the next one in line needs to go to Enceladus. A lot of curious minds want to know if there are microbes snowing at the south pole on that lovely little ice ball.
Would a mission to Enceladus be similar to the Europa Clipper?
The next mission, the Europa Clipper, will essentially bring our knowledge of Europa up to the same level as that of Enceladus, which we have been studying closely now for 11 years. We need to complete the mapping of [Europa’s] surface in visible and near- infrared wavelengths to see if there is any thermal emission coming from the surface among other things – all of the things we’ve already done at Enceladus with Cassini.
If Europa has plumes, the Clipper will carry instrumentation to sample them, though at the moment it seems there are no plumes so it’s now not clear how effective that will be.
A group of us have planned a small Discovery-class mission to return to Enceladus. We aim to do one thing and one thing only: fly through the plume repeatedly with only a few instruments to collect samples and look for chemical evidence of life. If that mission is chosen, that’s what the next mission to Enceladus will look like. Assuming we haven’t found the place to be as dead as a door, the only thing that makes sense would be a more sophisticated mission to do both in-situ sampling and to return a sample to Earth. I hope I live to see the day!
Baada ya kugundua geysers kwenye Enceladus, kwa nini ilichukua muda mrefu sana kupata chanzo chao? "Kulingana na uchunguzi wa Enceladus, ambayo tuliona kwa mara ya kwanza mnamo Februari 2005 katika seti ya picha zilizotengenezwa kutafuta ""kuchukuliwa"" kwa Enceladus, hatukuwa na mipango mingine katika ujumbe wa miaka minne wa uchunguzi maalum wa shughuli za geysers." Tungewezaje kufanya hivyo? Hatukuwa na uhakika kwamba [bahari] ilikuwako. """Kwa hivyo mara tu tulipowapata, tulibadilisha ujumbe wa jina kadiri tulivyoweza ili kupata kuangalia kwa karibu na, bila shaka, tulibuni uchunguzi mwingi katika ujumbe uliopanuliwa ili kuendeleza picha kamili zaidi juu ya jambo hili." Hii ilijumuisha seti ya flybys kupima usambazaji wa wingi ndani ya tabaka za nje za mwezi katika mkoa wa kusini wa polar, pamoja na picha za azimio la juu sana la uwanja wa geyser na uso wa Enceladus. Ilikuwa ni hadi mwaka 2013 ambapo matokeo ya uvutano yalichambuliwa kwa kutosha, kwamba tulikuwa na uthibitisho wa safu ya kioevu yenye unene wa kilomita kumi chini ya eneo la polar ya kusini. Tuligundua kwamba ilikuwa angalau pana kama mkoa wa ncha ya kusini, lakini bahari ya kimataifa haiwezi kuondolewa. Chini ya dhana fulani za busara, uchambuzi fulani ulidokeza kwamba inapaswa kuwa ya ulimwenguni pote. Lakini hitimisho hilo halikuwa na uhakika mkubwa. Kama tulivyoona, haikuwa hadi mwezi uliopita kwamba wanachama wa timu yangu hatimaye kumaliza kazi yao kwa kutumia picha zetu za azimio la juu ya uso. "Kama matokeo, tulipata ""kutofautiana kidogo"" katika mzunguko wa Enceladus, ambayo inaelezwa vizuri na bahari ya dunia." Uchambuzi huu wote, katika hatua hii ya utume, ni tata sana, unahitaji zaidi ya kuangalia tu, kuona ushiriki na picha, lakini kazi nyingi. Na kazi nzuri, yenye uangalifu huchukua wakati. Je, unaweza kutuambia zaidi kuhusu mbinu zilizotumiwa kupata bahari? Uhakikisho wa kwanza kwamba kulikuwa na safu ya kioevu karibu kilomita 35 chini ya eneo la polar kusini ulikuja kutoka kwa saini ya mvuto. Usambazaji wa wingi chini ya eneo la kusini la barafu uligunduliwa kwa kufuatilia kwa uangalifu mwendo wa chombo cha angani cha Cassini wakati kiliporuka karibu na ncha ya kusini ya Enceladus. Kwa kufanya flybys tatu kama hizo, timu ya mvuto iliruhusu kugundua kwamba safu ya maji ilikuwa karibu kilomita 35 chini ya uso na karibu kilomita kumi unene. "Kuthibitisha kwamba bahari ni ya kimataifa ilitokana na kugundua jinsi mzunguko wa Enceladus haukuwa sawa lakini wakati mwingine ulikuwa wa haraka kidogo na wakati mwingine ulikuwa wa polepole kidogo. """ Ili kufanya hivyo, wanachama wa timu yangu walikuwa wakitumia vipengele kwenye uso kama vigezo - aina kama kupaka rangi mstari mweupe kwenye tairi yako ya gari - na kulinganisha nafasi za vipengele wakati mbalimbali ili kuona jinsi walivyolinganishwa na ambapo wangekuwa kama hakukuwa na kutetemeka. Na ukubwa wa kutetemeka huonyesha kitu kuhusu mambo ya ndani - hasa, kama ganda la barafu husonga bila kujali kiini au limeunganishwa nacho. Katika kesi hii, waligundua kwamba barafu lazima iwe na mwendo wa kujitegemea, na inaweza kuwa hivyo tu ikiwa inawinda kwenye tabaka la kioevu. Unafikiri bahari hiyo inaweza kutengenezwa kwa nini? Vifaa vya Cassini vilivyopima mviringo wa Enceladus vinasema kwamba bahari hiyo ina maji na kiasi kidogo cha chumvi na kemikali. Je, uthibitisho wa kuwapo kwa bahari hufanya Enceladus iwe shabaha ya ziada ya uhai? Enceladus ina maji mengi, lakini kwa sababu ya kuwa na kiini cha maji, inaonekana kuwa na uwezo wa kugundua viumbe wa nje ya dunia. Kwa hiyo tumekuwa tukijua hilo kwa muda mrefu na kwa miaka kumi hivi. "Hata hivyo, uchunguzi huu mpya unafanya hadithi hiyo iwe na utajiri zaidi, na inaweza kutuambia kwamba kuna nishati zaidi inayopigwa katika Enceladus kwa muda kuliko tulivyofikiri hapo awali.""" Kwa hiyo, ikiwa kuna kitu kinachofanya hivyo, huenda ikawa kwamba viumbe hai vyovyote vilivyokuwa huko vilikuwa na wakati mrefu zaidi wa kubadilika kuliko tulivyofikiri mwanzoni. Labda tunaporudi huko kutazama kwa karibu, badala ya kupata viumbe vidogo-vidogo, tutapata kamba-mti na sushi! Ikiwa ungeweza kukisia, unafikiri maisha kwenye Enceladus yangekuwaje? Ni haki kusema sisi sote tuna viumbe vidogo-vidogo katika ubongo. Ningekuwa kuanguka kutoka kiti changu, kudhani mimi bado nina uwezo wa kukaa juu ya viti wakati wakati inakuja, kama kulikuwa na kitu kama tata kama udongo au farasi wa bahari, wow, kwamba itakuwa stunner. Vivyo hivyo, tunapendezwa na mwezi wa Jupita, Europa, kwa sababu ya bahari zake. Je, sasa kutakuwa na ugumu wa kuamua ni mwezi gani ambao tunapaswa kutuma utume wa kujitolea kwanza? Katika akili yangu, Enceladus alikuwa mshindi wazi kama lengo lilikuwa sampuli ya eneo la nje ya dunia habitable - mazingira mgombea kwa ajili ya maisha - haraka na kwa urahisi iwezekanavyo. Kwa nini? Neno muhimu kwa Enceladus ni upatikanaji. Huna haja ya kutua, scratch, drill au bunker chombo chako cha angani katika risasi [kuilinda kutoka mnururisho] ili kutafuta ushahidi wa maisha. Hata hivyo, mambo mengine kama vile msukumo wa kisiasa na kiufundi alishinda na utume wa Europa alichaguliwa na sasa ni katika njia. Kwa hiyo nasema: nzuri! Sasa kwa kuwa tumefanya kazi hiyo, yule anayefuata katika mstari anapaswa kwenda Enceladus. "Watu wengi wanaotaka kujua kama kuna ""microbes"" theluji katika barafu ya kusini ya dunia." Je, safari ya kwenda Enceladus ingekuwa sawa na ile ya Europa Clipper? Mfano wa kwanza ni Europa Clipper, ambayo inatarajiwa kuweza kuunganisha elimu ya Ulaya na Enceladus, ambayo imekuwa ikijaribiwa kwa miaka 11 sasa. """Tunahitaji kukamilisha ramani ya uso wa Europa katika urefu wa mawimbi yanayoonekana na karibu na infrared ili kuona ikiwa kuna mtiririko wowote wa joto kutoka kwa uso, kati ya mambo mengine, mambo yote ambayo tayari tumefanya kwa Enceladus na Cassini." Kama Europa ina plumes, Clipper itabeba vyombo vya kuchukua sampuli yao, ingawa kwa sasa inaonekana hakuna plumes hivyo ni sasa si wazi jinsi ufanisi kwamba itakuwa. Kikundi chetu kimepanga safari ndogo ya darasa la Discovery kurudi Enceladus. "Tunakusudia kufanya kitu kimoja tu: kuruka kupitia plume mara kwa mara na vyombo vichache tu kukusanya sampuli na kutafuta ushahidi wa kemikali wa maisha. """ Ikiwa utume huo utachaguliwa, ndivyo utume ujao wa Enceladus utakavyoonekana. Kwa kudhani hatujapata mahali hapo kuwa amekufa kama mlango, jambo pekee ambalo lina maana itakuwa ni utume wa kisasa zaidi kufanya sampuli za in-situ na kurudisha sampuli duniani. Natumaini nitaishi ili kuona siku hiyo!