Sky & Telescope - May 2019 - 10

NEWS NOTES

Astronomers Identify
Jupiter Weather Cycle
PLANETARY SCIENTISTS HAVE

realized that the upper cloud layer at
the gas giant's equator clears out in a
predictable cycle. What's more, professionals and amateurs alike conﬁrm that
the planet is now undergoing another
cloud-clearing event.
In the October 29, 2018, Geophysical Research Letters, a team of scientists
using data from NASA's Infrared Telescope Facility published evidence of a
recurring cycle: Every six or seven years
the planet's Equatorial Zone, typically
dark at infrared wavelengths, becomes
bright. Comparing the infrared cycle to
a large database spanning more than
four decades of observations, the scientists found it corresponds to a visible
shift in cloud cover.
"Jupiter's equator is normally com-

TRANSIENTS

Celestial Event Dubbed
"The Cow" Puzzles
Astronomers
ASTRONOMERS AT THE WINTER

meeting of the American Astronomical
Society proposed contrasting scenarios
to explain a strange celestial blast spotted last year. Did a medium-mass black
hole tear a white dwarf apart, or are we
seeing a new type of supernova?
The robotic Asteroid Terrestrialimpact Last Alert System (ATLAS)
survey picked up the event on June 16,
2018, and automatically designated it
AT2018cow, but astronomers quickly

AT2018cow

"The Cow"

M AY 2 0 1 9 * S K Y & T E L E S C O P E

pletely clouded over, appearing dark in
the infrared because those cold ammonia clouds appear in silhouette against
Jupiter's warm internal glow," notes
Arrate Antuñano (University of Leicester, UK), who led the study. "Those
thick clouds make the equator look
white through a visible telescope." But
when the equator brightens at infrared
wavelengths, the equatorial zone turns
distinctly yellowish-brown. As the white
ammonia clouds dissipate, they reveal
darker, lower layers of atmosphere.
The scientists saw the disturbance in
archival images from 1973, 1979, 1992,
1999, and 2006; each event lasted 12 to
18 months. While the team expected to
see similar infrared-brightening events
in 1985 and 2013, the clouds didn't
clear completely at those times.
u Infrared (top) and visible-light (bottom) images show a 6- to 7-year cycle in the cloud
cover in the North and South Equatorial Belts
(NEB, SEB) and the Equatorial Zone (EZ).

dubbed it "The Cow." The ﬂash, which
appeared in a spiral arm of a starforming galaxy 200 million light-years
away, took only two days to climb to
a peak luminosity of 100 billion Suns
- at least 10 times brighter than a
typical supernova. Even stranger, the
explosion's aftermath showed only
the chemical ﬁngerprints of hydrogen
and helium, rather than the heavier
elements characteristic of "ordinary"
supernovae. And follow-up observations
showed that the source remained hot
for weeks after its discovery, rather than
cooling down as supernovae typically
do. These and other observations suggest that some sort of "engine" kept the
explosion going for weeks.
One option is that a star came
too near a black hole and was torn
to shreds, said Amy Lien (University
of Maryland). The Cow appeared so
suddenly that the shredded star would
have had to be small, such as a helium
white dwarf, which would lack heavier
elements. The black hole would have to
be relatively small, too - 100,000 to
1 million times the mass of the Sun,
making it a member of the elusive class

Now, the event is under way once
again. Dedicated Jupiter observers,
including Christopher Go, conﬁrmed
that the brown color of the equatorial
zone, ﬁrst spotted in late 2018, continued into January. Glenn Orton (Jet Propulsion Laboratory), a coauthor on the
study, says the current event is indeed a
full-blown clearing.
■ SEAN WALKER

NEB
EZ
SEB

March 3, 2007

Jan 3, 2016

NEB
EZ
SEB

March 2, 2007

NEB
EZ
SEB

Jan 2, 2016

of intermediate-mass black holes. However, such black holes are thought to
form in globular clusters, so its location
in a spiral arm would be unexpected.
Another possibility is a new kind of
supernova. The event is clearly atypical
of ordinary exploding stars, but Raffaella Margutti (Northwestern University) argues that, while astronomers
typically see the glow of supernova
ejecta, The Cow offered a clear view of
the birth of a new black hole or neutron
star at the blast's center.
Two lines of evidence support this
picture. X-ray spectra of the blast from
NASA's NuSTAR and the European
Space Agency's INTEGRAL observatories
show the signature of cold, dense material around the compact object. And
longer-wavelength emission, seen by
the Submillimeter Array on Mauna Kea,
Hawai'i, could come from a shockwave
that's plowing into the ejecta around
the newly formed compact object.
Ongoing observations of the fading
remnant may eventually tell astronomers what, if anything, remains from
this mysterious explosion.
■ ELIZABETH HOWELL

JUPITER: TOP: N ASA / IRTF, BOT TO M LEF T: A N THON Y WESLE Y, BOT TO M RIG H T: TIZIA NO
OLIV E T TI; G A L A X Y: R. M A RGU T TI / W. M. K ECK OBSERVATORY

SOLAR SYSTEM

Sky & Telescope - May 2019

Table of Contents for the Digital Edition of Sky & Telescope - May 2019