Sky & Telescope - January 2022 - 11

NASA / JPL-CALTECH / SWRI / MSSS / KEVIN M. GILL
SOLAR SYSTEM
" Mushballs " Might Fall on
the Giant Planets
HAIL-LIKE, AMMONIA-RICH " mushballs "
were first inferred to exist on
Jupiter, plunging deep below the cloud
bank in the giant planet's atmosphere.
Now, new research suggests mushballs
might also fall on Uranus and Neptune.
Key evidence for mushballs' existence
on Jupiter came from NASA's Juno
spacecraft. First it found an odd circulation
pattern, with little ammonia anywhere
in the atmosphere except along
the equator (S&T: Dec. 2017, p. 14).
Then came Juno's dramatic close-ups of
violent thunderstorms, which showed
lightning flashing in the upper atmosphere.
Lightning requires the presence
of a liquid, so astronomers were baffled
to find it in a region where temperatures
are colder than −88°C (−126°F).
But what if water mixed with antifreeze?
Ammonia is " the best antifreeze
you can get, " explains Tristan Guillot
(Côte d'Azur University, France). Last
year, he and colleagues proposed that
pThis enhanced-color image from Juno shows the swirling cloud deck of Jupiter's southern
hemisphere.
drops of two parts water to one part
ammonia could remain slushy enough
to allow lightning. This mixture would
form hail-like mushballs as it fell, drawing
ammonia underneath the cloud
deck. This also explains why ammonia
is more abundant in Jupiter's equatorial
region, where there are fewer storms.
Recently, infrared observations have
confirmed that ammonia is also rare
on Uranus and Neptune, at least as far
down as we can see from afar. At the
Europlanet Science Congress 2021 in
September, Guillot showed that mushballs
could explain this phenomenon,
too. In fact, mushball formation could
be even more efficient on the ice giants
than on Jupiter.
However, David Stevenson (Caltech)
is cautious. He coined the term " mushballs "
and worked with Guillot on the
Jupiter research but not on the other
planets. " In the case of Uranus and
Neptune, " he says, " the data admit
alternatives. "
¢JEFF HECHT
sphere. The transfer module's three cameras,
generally used for monitoring the solar panels,
also glimpsed Mercury during the flyby.
¢EMILY LAKDAWALLA
Moon Landing Site Selected
for VIPER Mission
NASA has announced that its VIPER rover,
set for launch in 2023 (S&T: Jan. 2021, p.
34), will land just outside the western rim of
Nobile Crater near the lunar south pole. The
targeted study area covers 93 square kilometers
(36 square miles); VIPER is expected
to traverse 16 to 24 km during its primary,
100-day journey. NASA based the final site
selection on four criteria: Earth visibility, which
is necessary for direct line-of-sight communication;
access to sunlight to charge the
solar-powered rover; terrain that's suitable to
traverse; and evidence for the likely presence
of water ice. VIPER will search on and below
the surface in at least six permanently shadowed
regions, drilling and sampling in at least
three of these locations. The rover carries
three instruments to analyze volatiles (such as
water ice) and mineral composition, as well
as multiple cameras to capture the twilit landscape.
The rover will follow several other lunar
landers planned to launch in this time frame,
including two NASA-funded landers and one
each from Japan, Russia, and India.
¢DAVID DICKINSON
Sun-like Stars Eat Their Own
Some Sun-like stars that were born in the
same gas cloud as their companions show
unexpected differences - perhaps because
some stars eat their own planets. To
understand the chemical differences between
such stars, Lorenzo Spina (INAF Astronomical
Observatory of Padua, Italy) led a team in examining
the composition of pairs of Sun-like
stars. On August 30th in Nature Astronomy,
the researchers report that in 33 of 107 pairs,
one of the stars has more iron than expected.
(The other stellar pairs are all chemically
identical.) Iron is a refractory element that can
survive engulfment by a star, and it's readily
available in rocky planet cores. Furthermore,
the stars with higher iron abundances also
tended to have more lithium. Stars destroy
lithium in fusion reactions, but it's plentiful
in planets, so this finding also supports the
planet-engulfment scenario. Based on the
sample, Spina's team estimates that about a
quarter of Sun-like stars eat from their own
planetary buffet, speaking to the chaos -
and carnage - of planet formation.
¢MONICA YOUNG
White Dwarfs Still Burn
When stars near the end of their lives, they
stop burning. Most stars shed their outer layers
before collapsing into white dwarfs. With
no source of energy, these objects should
slowly cool and dim. But ultraviolet Hubble
Space Telescope observations of two globular
clusters (M3 and M13) show that some white
dwarfs still burn. While both clusters are both
about 13 billion years old, Jianxing Chen
(University of Bologna, Italy) and colleagues
find that M13 has extra bright white dwarfs.
That abundance, they show, originates in the
cluster's larger fraction of weensy stars, with
less than about half the Sun's mass. Even
after they collapse, these stars retain a hydrogen
envelope for later burning. About 70% of
the white dwarfs in M13 are of the slow-burn
variety. The finding upsets the notion of white
dwarfs as forever-cooling embers.
¢ MONICA YOUNG
sk yand tele scope .o r g * JANUARY 2022 11
http://skyandtelescope.org

Sky & Telescope - January 2022

Table of Contents for the Digital Edition of Sky & Telescope - January 2022

Contents
Sky & Telescope - January 2022 - Cover1
Sky & Telescope - January 2022 - Cover2
Sky & Telescope - January 2022 - 1
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Sky & Telescope - January 2022 - Cover3
Sky & Telescope - January 2022 - Cover4
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