Sky and Telescope - July 2016 - 19

NASA / JPL

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.
Theoretical models give a range of Jupiter's combined
heavy-element abundance, somewhere between 3% and
13% by mass. This is a huge uncertainty, and frankly to
have up to 20 Earth masses of oxygen unaccounted for
seems a bit of an embarrassment. Pinning down the
Jovian water abundance has major compositional implications not just for Jupiter but also for all solar systems.

BIG AND BRAVE Three giant solar-cell panels, each 9 m (29½ feet)
long, power NASA's Juno spacecraft. Once it begins orbiting Jupiter on
July 4th, Juno will have a limited lifetime before its electronics absorb a
lethal dose of high-energy charged particles.

Befuddled by the Galileo probe's dry descent, planetary
scientists initially clamored to have NASA send more
probes into Jupiter's atmosphere to find out where the
water could be hiding. Scott Bolton and colleagues at the
Jet Propulsion Laboratory had a better idea. Microwaves
are absorbed by water (that's how a mug of tea gets heated
in a microwave oven), and Jupiter emits microwave radiation from its hot, deep interior. So, Bolton thought, why
not just fly a microwave receiver above the clouds of Jupiter to map the distribution of water? Flying close enough
to Jupiter to do that would also enable detailed mapping of
the planet's gravitational and magnetic fields.
But there was a catch: the dense equatorial belt of
Sk yandTelescope.com July 2016

19



http://www.SkyandTelescope.com

Sky and Telescope - July 2016

Table of Contents for the Digital Edition of Sky and Telescope - July 2016

Contents
Sky and Telescope - July 2016 - Cover1
Sky and Telescope - July 2016 - Cover2
Sky and Telescope - July 2016 - 1
Sky and Telescope - July 2016 - Contents
Sky and Telescope - July 2016 - 3
Sky and Telescope - July 2016 - A
Sky and Telescope - July 2016 - B
Sky and Telescope - July 2016 - 4
Sky and Telescope - July 2016 - 5
Sky and Telescope - July 2016 - 6
Sky and Telescope - July 2016 - 7
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Sky and Telescope - July 2016 - Cover3
Sky and Telescope - July 2016 - Cover4
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