14 AUSTRALIAN SKY & TELESCOPE JULY 2016
J
upiter reigns supreme among the worlds of our
Solar System: it’s the largest, most massive,
fastest rotating, most strongly magnetised, and
has the greatest number of known satellites (67).
Moreover, Jupiter’s importance extends far beyond
theSun’srealm.It’sthearchetypeforhundredsof
similarly massive planets found around other stars.
Yet despite an exterior of pretty, swirling clouds
that get all the public’s attention, Jupiter harbours
many secrets. Some of the prime issues of planetary
science are hiding inside this planet. Does Jupiter
have a core? How do the stirrings inside generate
such a strong magnetic field? And perhaps the most
important question: how much water lies hidden
below those colourful clouds?
We took our first step to gain firsthand answers on
December 7, 1995, when NASA’s Galileo probe — a
339-kilogram, 1.3-metre-wide craft — hurtled into
Jupiter’s atmosphere at 48 km per second. Slowed by
parachutes and protected by a heat shield, the probe
and its seven scientific instruments measured the
properties of the atmosphere for a little under an hour
as the ensemble descended nearly 160 km to pressures
exceeding 23 Earth atmospheres.Revealing
Jupiter’s
inner Secrets
NASA scientists hope their Juno orbiter will get the
‘inside story’ on our Solar System’s largest planet.FRAN BAGENALEARLIER EXPLORATIONS
Juno will be the 10th spacecraft to study Jupiter at close range. The others
were: Pioneer 10 (1973), Pioneer 11 (1974), Voyagers 1 and 2 (1979), Ulysses
(1992), Galileo probe and orbiter (1995–2003), Cassini (2000) and New
Horizons (2007).Juno at Jupiter
Everyone expected the probe to pass through three
dense, distinct layers of clouds: ammonia (NH 3 ) at the
top, ammonia hydrosulfide (NH 4 SH) in the middle,
and water clouds (H 2 O) below. The temperatures,
pressures and winds recorded weren’t far from
scientists’ predictions, but the big surprise was a lack
of clouds — especially the water layer. Did the probe
just hit a dry, cloudless spot? Or is there really much
less water than expected? Jupiter’s apparently dry
weather had enormous implications.Why care about water?
Conventional thinking holds that the giant planets
began as small grains of rock, metal and ice. As
they bumped into each other, the grains grew bigger
— first to boulders and then to kilometre-sized
planetesimals — eventually forming Earth-sized
planetary embryos.
Since the giant planets formed beyond the ‘snow
line,’ a hypothetical boundary in the primordial solar
nebula beyond which water turned to ice, the snowball
embryos could continue to grow until they reached
10 to 15 Earth masses. These became the cores of the
giant planets.
Such large cores then had enough gravity to pull
in the most volatile gases — mainly hydrogen, plus
helium and neon — from the surrounding nebula.
Most of Jupiter’s 318-Earth-mass bulk comprises these
light gases, with the remaining ‘heavy elements’ —
astrophysicists’ shorthand for any atoms more massive
than helium — dominated by oxygen, likely in the
form of water.