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PHOTO: NASA/JPL-CALTECH/SETI INSTITUTE
for a slightly different technique: watching
Europa as it transits Jupiter’s face and its
thin atmosphere is lit up from behind. He
says he has “intriguing” images from early
2014 that could show a plume but so far
hasn’t confirmed them.
Part of the problem is that, in Europa’s
stronger gravity, plumes will never spurt as
high as they do at Saturn’s moon Enceladus,
where water escapes the moon’s gravity altogether and helps form one of Saturn’s rings.
As a result, plumes at Europa would be
more difficult both to observe remotely and
to sample. There are also reasons to think
the plumes might come and go, says Alyssa
Rhoden, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory
in Laurel, Maryland. Europa has a small
inclination in its orbital axis that wobbles
with time. Rhoden’s models suggest that
this wobble causes the stress states in the ice
to wander, too, opening and closing cracks.
She, too, doesn’t want too much instrumental capability centered on plume sampling.
“Planning a mission around one data point
would be a very dangerous proposition.”
The jets themselves may seal cracks in
the ice shell, says Sascha Kempf, a planetary
scientist at the University of Colorado, Boulder. As water vapor rises through fissures
in the ice, it can condense along the fissure
walls and weld them shut, he says. In case
the plumes exist—or even just for picking
up traces of Europa’s wispy atmosphere—
Kempf thinks the Clipper should carry instruments similar to two spectrometers on
NASA’s Cassini Saturn probe, which is using
them to identify the composition of both
solid particles and gases in the Enceladus
plume. (The spectrometers have identified
a variety of organic molecules at Enceladus,
but not the fragile, complex ones associated
with life, such as amino acids.)
But Peter Willis, an astrobiologist at JPL,
says such Cassini-style instruments probably
won’t help much with life detection if the
Clipper flies through a plume at high speeds
that destroy organic molecules. He wants to
cushion the impact by capturing plume material with an aerogel—a kind of lightweight
silica foam—like the one used to capture cosmic dust in the NASA Stardust mission. The
material could then be dissolved in water or
some other solvent before being fed into a
spectrometer for detection. Willis would
look for two things: the types of amino acids
present, and also their chirality, or handedness. The 70 or so amino acids so far discovered in space show no preference for left- or
right-handedness, but the 20 amino acids
that make up proteins on Earth are exclusively left-handed.
McKay says the Clipper could also drop
small independent satellites to sample a
plume closer to the source, at slower speeds.
Cassini did something similar when it released a probe called Huygens, contributed
by the European Space Agency (ESA), which
landed on Saturn’s moon Titan in 2005.
In a talk to planetary scientists after the
Ames meeting, NASA’s planetary division
director, James Green, hinted that he was
in discussion with ESA about another mission contribution. Green also suggested that
the Europa payload could be changed later
on if the plume science firms up. “Perhaps
some additional instruments may need to be
brought on,” he says.
McKay isn’t holding his breath. “I’m imagining they’ll say ‘life search’ is the next mission,” he says. ■
Fissures and smooth plains indicate that an ocean sometimes erupts to resurface Europa.
SCIENCE sciencemag.org
SEISMIC RISK
New jitters over
megaquakes in
Himalayas
Large earthquakes in
teeming region could strike
anywhere, anytime
By Priyanka Pulla, in Bangalore, India
S
eismologists worried about the prospect of a massive earthquake in the
shadow of the Himalayas, where it
could devastate cities such as Kathmandu and Delhi, have long cast a
wary glance at an eerily calm region
called the central seismic gap (CSG). A massive earthquake in southwestern Tibet in
1505 C.E., researchers proposed a decade
ago, relieved enough strain to quiet that
stretch of the restive Himalayas. But new
findings now suggest that the 1505 temblor
was smaller than thought and was just one of
a cluster of potent quakes to rattle the region
within a few centuries. If so, major quakes
in the Himalayas, unlike in many other seismic hot spots, may not relieve enough strain
to forestall later quakes—meaning that authorities must gird for a megaearthquake
anywhere at any time.
Thrust up by the continuing collision
of the Indian subcontinent with Asia, the
Himalayas are frequently rattled by major
earthquakes. But for several centuries, the
CSG, a 600-kilometer-long region extending northeast of Delhi, has been quiet,
even though it straddles major faults. In
2003, the late Greek geologist Nicholas
Ambraseys and Roger Bilham, a geophysicist at the University of Colorado, Boulder,
proposed that a large earthquake on 6 June
1505, known from Tibetan annals and the
Akbarnama, a chronicle of the 16th century
Mughal emperor Akbar’s reign, could have
relieved some of the strain building up at
the CSG. Based on severe structural damage to Tibetan monasteries located nearly
700 kilometers apart, Ambraseys and
Bilham estimated that the 1505 quake
would have registered at 8.2 or so on the
moment magnitude scale, which measures
the energy released during an earthquake.
The geological smoking gun for the quake
seemed to materialize a few years later. After
digging six trenches at points along a 250kilometer stretch of the CSG in 2006, ge27 FEBRUARY 2015 • VOL 347 ISSUE 6225
Published by AAAS
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Artist’s impression of Jupiter as viewed from the icy
surface of the moon Europa.
NEWS | I N D E P T H
quakes in 1255 C.E. and 1344 C.E. that are
known from historical records, the team
reports in an article posted online last
month in the Journal of Geophysical Research: Solid Earth. Yet the trench showed
no sign of a later, large earthquake.
R. Jayangondaperumal, a geologist at the
Wadia Institute of Himalayan Geology in
Dehradun and a member of Kumar’s team
in 2006, says a reanalysis he has conducted
on their 2006 trenches also suggests a pair
of earthquakes rather than a single one.
The finding not only casts doubt on the extent of the strain-relieving quake in 1505
but also “confirms an irregular cycle for the
earthquakes in Himalaya,” says Jean-Louis
Mugnier, a geologist at the University of Savoy, Bourget-du-Lac, in France who wasn’t
part of the study.
Bilham declined to comment on the new
findings. But Laurent Bollinger, a geologist
at France’s Alternative Energies and Atomic
Energy Commission, argues that the evidence so far is not persuasive enough to verify the clustering hypothesis or rule out the
big 1505 quake. “There is a risk that there
are some very big earthquakes that are being
missed in the historical chronicles,” he says.
What’s needed, he says, are more data from
more trenches.
In the meantime, Mugnier says, disaster
management authorities need to recognize
that a massive temblor can strike anywhere
in the Himalayas, at any time. “The level of
risk is stable: always high.” In 2013, India’s
National Disaster Management Authority estimated that an earthquake of magnitude 8
or greater just about anywhere in the rapidly
urbanizing Himalayas would kill, on average, about 800,000 people. ■
An illusory calm
A 1505 earthquake thought to have relieved strain and lowered earthquake risk in the central seismic
gap may have been smaller than thought and just one of a cluster of potent quakes to rattle the gap
within a few centuries.
AFGHANISTAN
Earthquake
Central seismic gap
CHINA
Quake regions
Disputed area
PAKISTAN
Ramnagar
1505
BHUTAN
New Delhi
Agra
MYANMAR
INDIA
934
NEPAL
BANGLADESH
BIOSECURITY
As new botulism
threat implodes,
more questions
Secrecy around “toxin
H” hampered research,
government scientists say
By Martin Enserink
I
t appeared to be a serious new threat
to biosecurity that justified an unusual
level of scientific secrecy—until, suddenly, it wasn’t. In 2013, Stephen Arnon
of the California Department of Public
Health (CDPH) reported finding a novel
type of botulinum toxin against which no existing antitoxins offered protection, leaving
society defenseless against bioterrorists who
might manage to produce the compound
and spread it through food or air. To protect
against the threat, Arnon decided not to reveal the genetic sequence of the microbe that
produced the toxin in his papers, a move
that attracted considerable media attention.
But late last year, U.S. government researchers concluded that the secrecy was
unnecessary because the toxin poses no special threat at all. They went on to post the
entire sequence in GenBank. Their as-yetunpublished findings were met with a sigh
of relief in biodefense circles. But even today,
many in the small field of botulinum research
wonder how two labs could arrive at such
radically different conclusions. And many
say the episode could have ended much earlier—or been prevented altogether—if Arnon
had been willing to share the strain of Clostridium botulinum with other labs sooner.
sciencemag.org SCIENCE
27 FEBRUARY 2015 • VOL 347 ISSUE 6225
Published by AAAS
PHOTO: EYE OF SCIENCE/SCIENCE SOURCE
ologist Senthil Kumar, who was then at the
University of Nevada, Reno, and colleagues
uncovered faults that could be radiocarbon dated using charcoal in the sediments.
They attributed the faults to an earthquake
between 1400 and 1422 C.E. But written records do not mention a major quake during this period, and because the charcoal
dating’s error bars encompassed 1505 C.E.,
Kumar’s group chalked up the findings to
the 1505 Tibetan quake. Also cited as evidence for that quake was a trench in western
Nepal described only in an abstract in 2006.
Other experts, however, have doubted
the 1505 earthquake’s potency. Many medieval monasteries in the region are built
from rock masonry without mortar, making
them vulnerable to even moderate earthquakes, says Chittenipattu Rajendran, a
paleoseismologist at the Jawaharlal Nehru
Centre for Advanced Scientific Research
here. Such damage, therefore, “is not realistic to use as an indicator of magnitude,”
says Rajendran, who adds that there is no
historical record of extensive damage in Indian cities like Agra and Delhi in 1505.
To get a fresh perspective on the CSG’s
seismic history, Rajendran’s group dug a
new trench in the Uttarakhand district of
Ramnagar, adjacent to a trench Kumar’s
team had excavated. They got lucky, uncovering a colluvial wedge of sediment—
the geological signature of any quake that
ruptures the earth’s surface. The rupture
briefly pushes up a scarp, which crumbles
to form a wedge. Rajendran’s team found
that the material in the wedge had been
ruptured not once, but twice, which could
only mean two earthquakes, they say. Carbon dating linked both ruptures to earth-