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mapping the planet’s magnetic field. To achieve
the necessary accuracy and spatial resolution, these
measurements need to be made as close as possible to
the planet.

Atmosphere: Where’s the water?
The surprisingly clear ‘hot spot’ into which the
Galileo probe plunged was so dry that, even at
pressures four times greater than where we expected
to encounter the water clouds, water was still greatly
depleted. Images of Jupiter taken from Earth at the
time of the Galileo probe’s entry show that it dropped
into a region between the cloud bands in what was
apparently a fierce downdraft of dry gas from the
upper atmosphere.
Australian Sky & Telescope readers are familiar with
Jupiter’s white and tawny orange cloud bands and its
enigmatic, slowly shrinking Great Red Spot (AS&T:
Apr. 2016, p. 16). Visible-light spectra show that the
bright zones are clouds of ammonia droplets. But
the colouring agent of the darker belts and the GRS
has puzzled astronomers for many decades. Current
thinking is that irradiation of atmospheric gases
produces long-chain sulfur compounds that are mixed
in with ammonium hydrosulfide.
When we view Jupiter at longer, infrared
wavelengths, the energy we see is not light reflected
from the Sun but instead emitted from hotter gases
below the planet’s upper cloud decks. This shows
clearly in the 5-micron image at left (facing page): cold,
high-altitude ammonia clouds look black, and the
bright bands record infrared energy from deeper down
that’s escaping between the clouds. If we look at even
longer wavelengths in the microwave region, we probe
deeper into the atmosphere.
Juno’s microwave radiometers (MWR) will record
six bands at wavelengths from 1.37 to 50 cm, chosen
to detect water down to pressures of at least 100 bars
and to ensure that we obtain oxygen’s elemental
abundance. The MWR observations should also yield
the abundance of nitrogen (derived from ammonia).
These findings, added to results from the Galileo
probe, will be crucial to understanding how Jupiter
formed, how it acquired its atmosphere and how it
evolved over time.
While the radiometers map the abundance and
distribution of materials below the clouds, the
JunoCAM, UVS and JIRAM instruments will make
complementary observations of what’s happening
topside at visible, ultraviolet and infrared wavelengths,
respectively. Thanks to Juno’s highly inclined orbit,
we’ll get our first pictures looking directly down on the
poles of Jupiter. Does the alternating pattern of belts

JunoCAM and you

Although taking pretty pictures isn’t one of Juno’s main goals, the team

added JunoCAM to increase public involvement in the mission. Even

better: you can vote on which pictures you want it to take. The JunoCAM

website (missionjuno.swri.edu/junocam) also solicits images of Jupiter

you’ve taken with your own telescope, and hundreds are already posted.

These images will enable scientists studying Jupiter’s cloud structures

with Juno’s instruments to get the context of those observations. What’s

happening in those cloud bands? Where are the dark spots? Where are

the winds blowing strongest before, during and after each Juno orbit?

S&T:

LEAH TISCIONE

A sense of pressure comes from the

force of gravity you feel through the

area of your feet due to the weight

of your body. Stand on one foot and

you halve the area, doubling the

pressure. The pressure of the gas in

the atmosphere you are breathing

is equivalent to the force you’d

experience through your feet with four

people stacked on your shoulders.

This seems incredible, but we’re used

to it. The pressures at the centre of

Jupiter are some 50 million times

greater. That’s like 1,000 elephants

stacked up with the bottom elephant

standing on one foot — and balancing

on a stiletto heel!

and zones persist all the way to the poles? We’ll soon

have the answer!

Magnetosphere:

What drives Jupiter’s intense aurorae?

Jupiter’s powerful dynamo generates a huge

magnetosphere — it’s the largest object within the

Solar System. On the planet’s sunward side, this

magnetic bubble typically extends to distances of 3½ to

7 million km — 50 to 100 times Jupiter’s radius. On the

nightside, Jupiter’s magnetotail stretches beyond the

orbit of Saturn.

Astronomers first detected the magnetosphere

of Jupiter in 1954, four years before Explorer 1’s

discovery of the Van Allen radiation belts around

Earth, via bursts of radio emission. These early radio

measurements showed that Jupiter has a strong

magnetic field with an axis tilted about 10° from the

How much pressure?