T
he cloudscape of Jupiter is divided
into alternating bright zones and
dusky belts that run parallel to its
equator, giving the planet its char-
acteristic striped appearance. The
contrast between the zones and belts
arises from localized differences in the
vertical motion and temperature of the
planet’s upper troposphere. Jupiter’s
upper troposphere is cooler in zones and
warmer in belts. Zones are sites where
atmospheric upwelling of ammonia-
rich air expands and cools, forming
a dense layer of brilliant white clouds
of frozen ammonia crystals at high
altitudes. Within the belts air warms
as it descends, causing the ammonia
clouds to evaporate and exposing lower
cloud layers that display a rich palette of
warm colors imparted by compounds ofphosphorus, sulfur, and carbon.
Late last year, a team led by Arrate
Antuñano (University of Leicester, U.K.)
published a study that revealed a cycle
of large-scale meteorological changes in
Jupiter’s Equatorial Zone, or EZ (S&T:
May 2019, p. 10). Based on more than
three decades of data acquired using
NASA’s 3-meter infrared telescope atop
Mauna Kea in Hawai‘i, the work sheds
new light on a recurring phenomenon
that has captivated Jupiter observers for
a century and a half.
The broad EZ, which extends from
latitudes 7°S to 7°N, occupies almost a
third of Jupiter’s visible disk. For visual
observers it is usually the brightest and
whitest part of the planet. However, at
wavelengths of several microns (μm)
in the thermal-infrared region of the52 JUNE 2019 • SKY & TELESCOPE
spectrum, the EZ normally appears
dark because its overlying canopy of
cold ammonia ice clouds insulates and
obscures Jupiter’s warm internal glow.
Every six or seven years the ammonia
clouds that top the EZ suddenly dissi-
pate for a period of a year or more, caus-
ing the entire region to appear bright in
thermal infrared but dark and colorful
in visible light. As Jupiter emerged from
solar conjunction in the closing days
of 2018, dedicated predawn observers
reported signs that the clearing was
already underway.
John Rogers, director of the Brit-
ish Astronomical Association’s Jupiter
Section, coined the term “EZ coloration
events” to describe these episodes.
His summary of their appearance and
evolution is an excellent guide to theOld Wine in New Bottles
A recurring change in Jupiter’s Equatorial Zone brings color to the historical record.
JUNE 2019 OBSERVING
Exploring the Solar System by Thomas A. DobbinsChristopher Go’s images capture the normal pearly white
Equatorial Zone in May 29, 2017 (left) and its intensifying color
this year on February 21st. South is up in both photos.