Hubble Homes in on Hazy Worlds.
Astronomers looking for water in the atmo-
spheres of fi ve hot Jupiters found less water
than expected, suggesting these planets are
surrounded by a high-altitude haze. Water is
a common molecule and had already been
detected on the most studied of the fi ve, HD
209458b, which also hosts strong winds and
high-altitude clouds. But the researchers had
to include similar clouds in their atmospheric
models for the other four planets in order to
explain why their water signals are less clear,
as Drake Deming (University of Maryland) and
colleagues report in two papers in the Astro-
physical Journal.
■ MONICA YOUNGFirst Noble Gas Molecule in Space.
Infrared observations of the Crab Nebula (M1)
reveal the presence of argon hydride ions
(ArH++), Michael Barlow (University College
London) and colleagues report in the Decem-
ber 13th Science. The argon is a special type:
it’s the isotope argon-36, which astronomers
expected to form in supernovae like the one
that created the Crab Nebula. Observers
fi rst detected knots of ionized argon in the
supernova remnant two decades ago, but they
haven’t known which isotope it is. The new
observations show that the strongest emission
from both ionized argon and ArH+ come from
the same part of the remnant. On Earth, argon-
40 is the dominant isotope. It is released by the
radioactive decay of potassium in rocks.
■ CAMILLE M. CARLISLEPiggyback Radio Receivers. NRAO and
the Naval Research Laboratory have joined
forces to put a system of low-frequency radio
receivers on 10 antennas of the Karl G. Jansky
Very Large Array (VLA) in New Mexico. The
new system, called VLITE (VLA Ionospheric
and Transient Experiment), will watch over the
dishes’ proverbial shoulders during normal
research activities, monitoring Earth’s iono-
sphere and looking out for short-lived radio
bursts. Because VLITE will be at the whim of
where other observers point the antennas, it
won’t produce a methodical survey. Still, plan-
ners expect to see 25% of the observable sky
for 100 seconds or longer each year, and 10%
for 1 hour or more.
■ CAMILLE M. CARLISLEIN BRIEF
News NotesSOLAR SYSTEM I Plumes on Europa?
Astronomers have detected faint
emission above Europa’s south pole that
might be from spewing water vapor,
Lorenz Roth (Southwest Research Insti-
tute and University of Cologne, Ger-
many) and colleagues reported Decem-
ber 12th at the American Geophysical
Union meeting. The result also appears
in Science.
This detection is not the fi rst of poten-
tial activity. “The history of apparent
plumes on Europa has been somewhat
of a sordid one,” says planetary scientist
Robert Pappalardo (JPL). Both Voyager
and Galileo saw hints that were later
dismissed, as were thermal observations
suggesting an outburst. Roth and col-
leagues also saw an inconclusive signal
from the moon in 2009.
But with a liquid water ocean beneath
its icy crust, Europa could have plenty of
fuel for eruptions. Features on its surface
look akin to those from fi ssure erup-
tions on Earth. And there’s the parallel
with Saturn’s satellite Enceladus, which
coughs out enough water vapor to form
the planet’s E ring.
Roth’s team used the Hubble Space
Telescope to take ultraviolet spectra of
the moon’s tenuous gas envelope. They
focused on two important points in
Europa’s orbit: its closest and farthest
approaches to Jupiter, also known as
periapse and apoapse.
Activity on Enceladus suggests that
plume outbursts should be stronger at
apoapse. When an icy moon like Encela-
dus or Europa is close to its planet, the
tidal forces stretch and squish it, closing
up any cracks in its surface, explains
Francis Nimmo (University of California,
Santa Cruz). But when the moon moves
farther away, it becomes “unsquished,”
opening the cracks. These open cracks
could expose liquid water to the vacuum
of space, causing it to boil off as tempo-
rary plumes.
Roth’s team found that at apoapse,
emission from oxygen and hydrogen
atoms above Europa’s south pole was at
least three times brighter than the aver-
age elsewhere. The signals were absent
during two periapse observations.The result gives scientists a clear
scenario to test: if more observations con-
fi rm the uptick in hydrogen and oxygen
at apoapse, the detections will be solid
evidence for transient plumes on Europa.
Until then, both the team and other
scientists will remain cautious.
“I’ll sleep better knowing that it’s
been reproduced,” Pappalardo admits.
Seeing the signal in the same wave-
length would be convincing, but it’d be
even better if observers could catch infra-
red emission from warmed material, he
adds. “I guess I’m 70-ish percent sure
now, based on what I see. I bet if they did
it again I’d be 90% sure, and then I’d be
up at 100% if I saw the infrared.”
The team thinks the emission could
come from plumes 200 km tall, spitting
out 7 tons of material each second. That
rate is 35 times higher than Enceladus’s.
Even though eruption speeds might
be about 700 meters per second (1,
mph), Roth notes that the vapor won’t
reach escape velocity. Instead, it’ll rain
back down on the moon, freezing imme-
diately on the cracked, −150°C (−240°F)
surface. The whole process would take
about 20 minutes.
■ CAMILLE M. CARLISLEThis composite image combines smoothed
Hubble Space Telescope ultraviolet data (blue)
with a visible image of the leading hemisphere
of Europa. Emission from hydrogen and oxygen
suggests that plumes of water vapor are erupt-
ing from the moon’s south polar region.LORENZ ROTH / SWRI14 March 2014 sky & telescopeNN layout.indd 14 12/23/13 11:38 AM