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by Bill Steigerwald
June 20, 2016
from
PHYS Website
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The space
environment around a planet plays a key role
in determining what molecules exist in the
atmosphere - and whether the planet is
habitable for life.
New NASA research shows
that the electric fields around Venus helped
strip its atmosphere of the components
needed to make water.
Credit:
NASA/Conceptual Image Lab.
Venus has an "electric wind" strong enough to remove the components
of water from its upper atmosphere, which may have played a
significant role in stripping Earth's twin planet of its oceans,
according to new results from ESA's (European Space Agency)
Venus
Express mission by NASA-funded researchers.
"It's amazing, shocking," said Glyn
Collinson, a scientist at NASA's Goddard Space Flight Center in
Greenbelt, Maryland.
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"We never dreamt an electric wind
could be so powerful that it can suck oxygen right out of an
atmosphere into space. This is something that has to be on the
checklist when we go looking for habitable planets around other
stars."
Glyn Collinson is lead author of
a paper (The
Electric Wind of Venus - A Global and Persistent 'Polar Wind' like
Ambipolar Electric Field sufficient for the Direct Escape of Heavy
Ionospheric Ions) about this research published June 20, 2016, in the journal
Geophysical Research Letters.
Venus is in many ways the most like Earth in terms of its size and
gravity, and there's evidence that it once had oceans worth of water
in its distant past.
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However, with surface temperatures around 860�F
(460�C), any oceans would have long since boiled away to steam and
Venus is uninhabitable today. Yet Venus' thick atmosphere, about 100
times the pressure of Earth's, has 10,000 to 100,000 times less
water than Earth's atmosphere.
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Something had to remove all that
steam, and the current thinking is that much of the early steam
dissociated to hydrogen and oxygen:
the light hydrogen escaped,
while the oxygen oxidized rocks over billions of years.
Also the
solar wind - a
million-mile-per-hour stream of electrically conducting gas blowing
from the sun - could have slowly but surely eroded the remainder of
an ocean's worth of oxygen and water from Venus' upper atmosphere.
"We found that the electric wind,
which people thought was just one small cog in a big machine, is
in fact this big monster that's capable of sucking the water
from Venus by itself," said Collinson.
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The space
environment around a planet plays a key role in
determining what molecules exist in the
atmosphere - and whether the planet is habitable
for life.
New NASA
research shows that the electric fields around
Venus helped strip its atmosphere of the
components needed to make water.
Credit:
NASA�s Goddard Space Flight Center, Genna
Duberstein
Just as every planet has a gravity field, it is believed that every
planet with an atmosphere is also surrounded by a weak electric
field.
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While the force of gravity is trying to hold the atmosphere
on the planet, the electric force (the same force that sticks
laundry together in a drier and pushes electricity through wires)
can help to push the upper layers of the atmosphere off into space.
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At Venus, the much faster hydrogen
escapes easily, but this electric field is so strong that it can
accelerate even the heavier electrically charged component of water
- oxygen ions - to speeds fast enough to escape the planet's
gravity.
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When water molecules rise into the upper
atmosphere, sunlight breaks the water into hydrogen and oxygen ions,
which are then carried away by the electric field.
"If you were unfortunate enough to
be an oxygen ion in the upper atmosphere of Venus then you have
won a terrible, terrible lottery," said Collinson.
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"You and all
your ion friends will be dragged off kicking and screaming into
space by an invisible hand, and nothing can save you."
Venus and
Earth are similar sizes and have similar gravity
- but Venus is bone dry and more than 10 times
as hot as our home planet. Recent NASA research
describes a key process that removes water from
the Venusian atmosphere.
Credit:
NASA/Conceptual Image Lab.
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The team discovered Venus' electric
field using the electron spectrometer, a component of the
ASPERA-4
instrument, aboard the ESA Venus Express.
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They were monitoring electrons flowing
out of the upper atmosphere when it was noticed that these electrons
were not escaping at their expected speeds. The team realized that
these electrons had been tugged on by Venus' potent electric field.
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By measuring the change in speed, the
team was able to measure the strength of the field, finding it to be
much stronger than anyone had expected, and at least five times more
powerful than at Earth.
"We don't really know why it is so
much stronger at Venus than Earth," said Collinson.
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"But, we
think it might have something to do with Venus being closer to
the sun, and the ultraviolet sunlight being twice as bright.
It's a challenging thing to measure and even at Earth to date
all we have are upper limits on how strong it might be."
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New
research suggests that the electric field around
Venus may be a key factor in shaping what
molecules exist in the Venusian atmosphere -
including its lack of the molecules needed to
make water.
Credit:
NASA/Conceptual Image Lab.
Such information also helps us understand other worlds around the
solar system.
"We've been studying the electrons
flowing away from
Titan [a moon of Saturn] and
Mars as well as
from Venus, and the ions they drag away to space," said Andrew
Coates, who leads the electron spectrometer team at University
College London in the U.K.
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"The new result here shows that the
electric field powering this escape is surprisingly strong at
Venus compared to the other objects. This will help us
understand how this universal process works."
Another planet where the electric wind
may play an important role is Mars.
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NASA's
MAVEN mission is currently
orbiting Mars to determine what caused the Red Planet to lose much
of its atmosphere and water.
"We are actively hunting for Mars'
electric wind with MAVEN's full arsenal of scientific
instruments," said Collinson.
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"MAVEN is a robotic detective on
this four-billion-year-old mystery of where the atmosphere and
oceans went, and the electric wind has long been a prime
suspect."
Taking the electric wind into account
will also help astronomers improve estimates of the size and
location of habitable zones around other stars.
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These are areas where the temperature
could allow liquid water to exist on the surface of alien worlds,
making them places where life might be found.
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Some stars emit more ultraviolet light
than the sun, so if this creates stronger electric winds in any
planets orbiting them, the habitable zone around such stars may be
farther away and narrower than thought.
"Even a weak electric wind could
still play a role in water and atmospheric loss at any planet,"
said Alex Glocer of NASA Goddard, a co-author on the paper.
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"It could act like a conveyor belt,
moving ions higher in the ionosphere where other effects from
the solar wind could carry them away."
ESA's Venus Express was launched on Nov.
9, 2005, to study the complex atmosphere of Venus.
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The electron spectrometer was built by
the Southwest Research Institute in San Antonio, Texas, and is led
by University College London. The spacecraft orbited Venus between
2006 and December 2014.
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After a successful mission that far
exceeded its planned life, the spacecraft exhausted its fuel supply
and burned up upon entry into Venus' dense atmosphere.
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The research was funded by,
More information at "The
Electric Wind of Venus - A Global and Persistent 'Polar Wind' like
Ambipolar Electric Field sufficient for the Direct Escape of Heavy
Ionospheric Ions" - G. Collinson et al. - Geophysical
Research Letters 20 June 2016, DOI: 10.1002/2016GL068327.
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