A (6)

16 AUSTRALIAN SKY & TELESCOPE JULY 2016

Core:What’s stirring inside?

Despite close-up explorations of Jupiter by several

previous spacecraft, the planet’s interior remains a

mystery. Not only do we not know what quantities of

heavy elements it contains, we also don’t know if these

elements remain concentrated in a central, solid core.

For decades textbooks have shown Jupiter with

asmallcentralcoreofsolidrockandiceandan

outermost layer of molecular hydrogen. In the large,

central volume in between, the pressure is sufficient to

allow the protons and electrons of atomic hydrogen to

move past each other freely, enabling strong electrical

currents to flow. Circulating flows within this large

volume of so-calledmetallic hydrogengive rise to the

planet’s strong magnetic field.

Jupiter has the second most powerful magnetic

dynamo in the Solar System (after the Sun’s). It

probably bears little more than superficial resemblance

to the one operating in Earth’s small, iron core, but is it

necessarily like the Sun’s dynamo?

Computer modeling can simulate swirling flows

driven by primordial internal heat and wrapped up in

Jupiter’s rapid rotation (the planet’s spin period is just

shyof10hours).Butsuchmodelsdependonknowing

how an unknown mix of hydrogen and heavy elements

behaves at the ultrahigh pressures (some 50 million

atmospheres), high temperatures (20,000 Kelvin —

four times hotter than the Sun’s photosphere), and

high density (somewhere between those of rock and

lead) present in the metallic-hydrogen layer.

Scientists gain insight into the properties of

hydrogen compressed to such an extreme state by

zapping it with lasers in laboratory experiments,

as well as from sophisticated quantum-mechanical

models. Recent studies suggest that the heavy

elements could be completely dissolved in the metallic

hydrogen and stirred up into a relatively uniform

mixture. So, contrary to the standard textbook picture,

Jupiter might not have a distinct core after all.

But these lab studies and computer models can

onlytakeussofar.Ultimately,ourknowledgeof

Jupiter’s interior will require three key measurements:

(1) a determination of the bulk abundance of heavy

elements, (2) mapping the planet’s gravity field, and (3)

Molecular

hydrogen

Metallic

hydrogen

Heavy elements

dissolved in

metallic hydrogen

Rock-metal

inner core

Core? No Core?

Volatile

outer core

TAKE YOUR

PICK Planetary

scientists hope

Juno will enable

them finally

to determine

whether Jupiter

has a solid core

of metal, rock and

ice (left cutaway)

or essentially no

core at all (right

cutaway).

S&T:

GREGG DINDERMAN

THERMAL PROBELeft:By recording Jupiter’s emissions at six microwave wavelength ranges, Juno will map the global abundances of water and ammonia
in and below its three distinct cloud layers. Right: When viewed at the infrared wavelength of 5 microns, Jupiter’s familiar belts and zones are replaced with a
pattern of heat escaping from deep within its cloud layers. Astronomers Thomas Momary and Glenn Orton recorded this view on April 4, 2016.

North

Equatorial Belt

South

Equatorial

Belt

Great Red Spot

LEFT:

SOURCE: NASA / JPL / M. JANSSEN,

RIGHT:

NASA / JPL / INFRARED TELESCOPE FACILITY

Channel sensitivity Kelvin

Depth below tropopause (km)

Pressure (bars)

0.20.0 0.4 0.6 0.8 1.0 200

50.0 cm

NH 3 cloud

NH 4 SH cloud

H 2 O cloud

Microwave

ranges Temperature

24.

11.

5.

3.

1.

400 600 800 1000

100

10

1

0.1^0

50

100

150

200

1000

Juno at Jupiter