Build­ing a planet in the lab

Meet the sci­en­tists who are cre­at­ing ex­otic worlds

All About Space - - Contents -

In Rochester, New York, United States, there is a lab­o­ra­tory that is home to the sec­ond most pow­er­ful laser in the world. This room can pro­duce tem­per­a­tures com­pa­ra­ble to the core of the Earth, all in the name of re­search. This sanc­tu­ary of sci­en­tific re­search is the Lab­o­ra­tory of Laser En­er­get­ics (LLE), part of the Univer­sity of Rochester’s south­ern cam­pus.

Within the LLE is the pièce de ré­sis­tance, the OMEGA laser, which sci­en­tists are us­ing to man­u­fac­ture a new world. Within its tar­get cham­ber, sur­rounded by 60 lasers, sci­en­tists are peer­ing into the in­te­rior of gas gi­ant plan­ets such as Jupiter, Saturn and dis­tant exoplanets that are pri­mar­ily com­posed of the most abun­dant and sim­plest el­e­ment in the uni­verse: hy­dro­gen.

Hy­dro­gen, made up of one pro­ton and one elec­tron, makes up 74 per cent of the nor­mal mat­ter in our uni­verse, with sec­ond place go­ing to he­lium, which which makes up 24 per cent. Th­ese per­cent­ages are sim­i­lar to our Sun, but the Jo­vian plan­ets be­yond the as­ter­oid belt are also known to be made up of mostly hy­dro­gen. Jupiter and Saturn both con­sist of about 90 per cent hy­dro­gen, with the heav­ier el­e­ments sink­ing to­wards their re­spec­tive cores and hy­dro­gen dom­i­nat­ing the resid­ual lay­ers, span­ning tens of thou­sands of kilo­me­tres in ra­dius.

On the face of each planet the layer of hy­dro­gen that ama­teur as­tronomers can ob­serve with a good tele­scope is hy­dro­gen in its gaseous state, also known as molec­u­lar hy­dro­gen (H2). Molec­u­lar hy­dro­gen is hy­dro­gen in its most sta­ble state and ex­ists in a di­atomic form; this is the state that sci­en­tists are most fa­mil­iar with on Earth.

When you make your way to­wards the cen­tre of a gas gi­ant such as Jupiter, hy­dro­gen turns into a more ex­otic state – me­tal­lic hy­dro­gen. This is the state of hy­dro­gen sci­en­tists are trying to cre­ate at the LLE, as Dr Mo­hamed Zaghoo, a re­search as­so­ciate at the LLE, tells All About Space:

“This is in­deed a very ex­cit­ing area of re­search where lab­o­ra­tory data and space ob­ser­va­tion are equally valu­able to build a more ac­cu­rate pic­ture of hy­dro­gen-rich plan­ets.”

Much like how wa­ter changes state based on tem­per­a­ture, melt­ing ice into liq­uid wa­ter and fur­ther evap­o­rat­ing into wa­ter vapour with a con­tin­ual in­crease in tem­per­a­ture, gaseous hy­dro­gen will morph into its me­tal­lic hy­dro­gen state with an in­crease in tem­per­a­ture and pres­sure. Th­ese con­di­tions pro­vide enough en­ergy to pull the elec­trons away from their pro­tons and form a sea of pro­tons, which can also be thought of as hy­dro­gen ions, and freeflow­ing elec­trons. As there are many free elec­trons shift­ing through this state of mat­ter it is thought to be su­per­con­duc­tive.

Me­tal­lic hy­dro­gen is the­o­rised and heav­ily sup­ported – but not yet proven – to be the con­di­tion present in the in­ner lay­ers of Jupiter and Saturn. It is also thought that by learn­ing more about the me­tal­lic hy­dro­gen state we can learn about a gas gi­ant’s mag­ne­to­sphere and dy­namo ef­fect. A mag­ne­to­sphere is the area oc­cu­pied by a planet’s mag­netic field and the dy­namo ef­fect is the churn­ing of con­duc­tive in­ter­nal ma­te­rial pow­er­ing the mag­ne­to­sphere. Due to me­tal­lic hy­dro­gen’s su­per­con­duc­tive na­ture, it would ex­plain why Jupiter has the most pow­er­ful mag­ne­to­sphere.

“Lab­o­ra­tory data and space ob­ser­va­tion are equally valu­able to build a more ac­cu­rate pic­ture of hy­dro­gen-rich plan­ets”

Dr Zaghoo

Me­tal­lic hy­dro­gen is not easy to cre­ate, how­ever. Pres­sure needs to be raised to be­tween 1.4 and 1.7 megabar, and tem­per­a­tures be­tween 1,500 and 2,400 de­grees Cel­sius (2,700 and 4,400 de­grees Fahrenheit). This is over a thou­sand-times the pres­sure of Earth’s av­er­age at­mo­spheric pres­sure and th­ese tem­per­a­tures are ca­pa­ble of melt­ing lead. Th­ese un­earthly con­di­tions are ex­tremely dif­fi­cult to man­u­fac­ture, hence why this re­search is be­ing con­ducted in one of the world’s most unique lab­o­ra­to­ries.

In early 2017, Pro­fes­sor of Nat­u­ral Sciences Isaac Sil­vera and post­doc­toral fel­low Dr Ranga Dias, both of Har­vard Univer­sity in Cam­bridge, Mas­sachusetts, were ac­tu­ally able to cre­ate me­tal­lic hy­dro­gen us­ing a nifty but ex­pen­sive piece of equip­ment called a di­a­mond anvil cell (DAC). This is what was fit­ted at the cen­tre of OMEGA’s tar­get cham­ber con­tain­ing the hy­dro­gen sam­ple. “The di­a­mond anvil cells gen­er­ate the pres­sures stat­i­cally, while the lasers [OMEGA] gen­er­ate the tem­per­a­tures,” ex­plains Zaghoo.

A DAC is a high-pres­sure de­vice that has two op­pos­ing di­a­monds fixed to­gether via rhe­nium gas­kets. The di­a­mond tips are pol­ished to en­sure there will be min­i­mal cracks or dam­age that will lessen the pres­sure within the cen­tral sam­ple re­gion, which mea­sures less than a mil­lime­tre. It is in­cred­i­ble to think that in a lab­o­ra­tory that stands ten me­tres (32 feet) tall and is ap­prox­i­mately 100 me­tres (328 feet) in length, the cul­mi­na­tion of the sci­en­tists’ work all comes from a sam­ple that is un­per­ceiv­able to the hu­man eye.

This sam­ple is fit­ted within the DAC, which is ap­prox­i­mately the size of a coke can, and then fit­ted in­side the OMEGA tar­get cham­ber. In the same way that peo­ple stamp on a coke can to squash it down, sci­en­tists and en­gi­neers have to squeeze the di­a­monds to­gether and ramp up the pres­sure with each mil­lime­tre of com­pres­sion. When this ap­pa­ra­tus is fixed into po­si­tion and the sam­ple is un­der pres­sures com­pa­ra­ble to the in­te­rior of Jupiter, re­searchers fire the lasers.

As the ex­per­i­ment room can rise to un­com­fort­able tem­per­a­tures the sci­en­tists are sit­u­ated in the LLE’s con­trol room when the heat­ing be­gins. The OMEGA laser driv­ers are fired up, ini­ti­at­ing the process by cre­at­ing shaped seed pulses via a 60-beam ul­tra­vi­o­let neodymium glass laser, which are in­ten­si­fied with stage A am­pli­fiers. By the time the lasers are con­cen­trated on the tar­get cham­ber OMEGA is ca­pa­ble of pro­duc­ing 30 kilo­joules of en­ergy and 60 ter­awatts of power in just one-bil­lionth of a sec­ond. When com­pared with the fact that a stan­dard oven can use up to 5,000 watts, the OMEGA pro­duces 12 bil­lion-times more en­ergy in a frac­tion of the time.

Ob­ser­va­tions are the most es­sen­tial part of any ex­per­i­ment, but it is im­pos­si­ble to ob­serve any change with the naked eye. Be­cause of this, re­searchers such as Zaghoo use an in­ge­nious dat­a­col­lec­tion method. “The data is col­lected us­ing fast op­ti­cal de­tec­tors that record the re­flectance of a laser light off the hy­dro­gen sam­ples. The premise of the mea­sure­ments is sim­ple: me­tal­lic sub­stances re­flect light, while in­su­lat­ing ones don’t. Thus we mea­sure how much light the com­pressed and hot hy­dro­gen re­flects,” ex­plains Zaghoo. “At low pres­sures and tem­per­a­tures hy­dro­gen is trans­par­ent liq­uid, but at suf­fi­ciently high pres­sures the molec­u­lar form of hy­dro­gen breaks down un­der

“At suf­fi­ciently high pres­sures the molec­u­lar form of hy­dro­gen breaks down and gives away its elec­trons”

the crush­ing pres­sure and gives away its elec­trons, en­abling con­duc­tion and light re­flec­tion.”

When the light is re­flected back into their de­tec­tors sci­en­tists are over­joyed, yet there is still work to do. They have had a glimpse into the in­te­rior of gas gi­ant plan­ets – and in par­tic­u­lar to this study, Jupiter – and af­ter de­cod­ing the data they can un­cover the mys­tery of their in­tense mag­ne­to­spheres and dy­namos. “By vary­ing the fi­nal pres­sures and tem­per­a­tures we were able to build this ‘con­duc­tiv­ity pro­file’ or map of me­tal­lic hy­dro­gen con­duc­tiv­ity at dif­fer­ent depths along Jupiter’s in­te­rior,” says Zaghoo. The con­duc­tiv­ity map cre­ated in this in­stance has showed that Jupiter’s dy­namo orig­i­nates closer to the sur­face than Earth’s dy­namo.

The re­search doesn’t end there though. In sci­ence col­lab­o­ra­tion is key, and there are dif­fer­ent mis­sions with dif­fer­ent per­spec­tives that need to be ac­counted for. By in­cor­po­rat­ing th­ese re­sults into sim­u­la­tions that also in­clude up-close ob­ser­va­tions from space­craft like NASA’s Juno mis­sion – the space probe cur­rently in or­bit around Jupiter, gath­er­ing vi­tal in­for­ma­tion about its mag­ne­to­sphere and cloud top com­po­si­tion – sci­en­tists are con­tin­u­ing to un­der­stand the in­ner work­ings of Jupiter to a level that is sim­ply in­cred­i­ble.

“One of the great in­sights that Juno very re­cently pro­vided is that the thun­der­ous winds, or bands, that char­ac­ter­is­ti­cally dis­tin­guish the planet’s sur­face ex­tend much deeper into the plan­ets in­te­rior, to al­most 3,000 kilo­me­tres (1,900 miles) in depth,” says Zaghoo. “By combining this data with our ex­per­i­men­tal con­duc­tiv­ity pro­file we are able to fur­ther con­strain the depth of the dy­namo process and the in­ter­ac­tion be­tween the strong swirling winds with the con­duc­tive fluid deep in­side.”

Dr Mo­hamed Zaghoo

The tar­get cham­ber is a two-me­tre (six-foot) stain­less steel sphere

Juno is sit­u­ated at Jupiter, collecting vi­tal data about the planet’s mag­ne­to­sphere

The OMEGA laser sys­tem is alsoused to ini­ti­ate nu­clear fu­sion

Dr Mo­hamed Zaghoo is a re­search as­sosi­ate at the LLE

Th­ese stud­ies al­low sci­en­tists to peak be­hind the plan­e­tary cur­tains of gas gi­ants

Di­a­mond anvil cells con­tain the hy­dro­gen sam­ple that is less than a mil­lime­tre long

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