The European Space Agency's Venus Express is shedding new light on the mysterious holes in the Venusian atmosphere.  Back in 1978, the NASA Pioneer Venus Orbiter detected a hole in the planet's ionosphere.  It discovered that in this area, the density dropped out.  Scientists were baffled.  Today, Venus Express has been in polar orbit around Venus for the past eight years and has spotted these same holes.

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Venus Express has discovered that the ionosphere of Venus is in a shape resembling a comet's tail during times of low solar wind pressure.  The solar wind blows the ionosphere around the planet and most of the time the plasma particles from the solar wind form channels on the nightside of the planet.  But during low solar pressure, the ionosphere ballooned on the nightside.  The cause of this is Venus' weak magnetic field.  On Earth there is a strong magnetic field which keeps the ionosphere in a stable shape.  But Venus does not have this magnetic field and thus the flow of plasma particles increase as the channels widen due to the lower pressure,  which in turn causes the balloon effect.

Pioneer Venus Orbiter only saw the holes in the ionosphere during solar max.  But Venus Express has confirmed the holes exist also at solar min.  The holes also extend much further in the atmosphere than previously thought.

In July, Venus Express made a daring plunge into the Venusian atmosphere and then climbed back to orbit.  As it flew through the holes it recorded a jump in the field strength along with very cold particles flowing in and out of the holes.  It detected two holes in the shape of cylinders extending out from the planet's nightside into space.  The holes contain low density materials with the highly charged plasma particles having been squeezed out of them.  Some sort of magnetic effect must be the cause.

As the magnetic field lines from the sun hit Venus, they wrap around the planet creating two long straight magnetic field lines on the other side of the planet.  These lines could squeeze the plasma out and form the holes.  But that would place the holes starting on each side of the planet, not directly behind it as the spacecraft have observed.

One theory is that the magnetic field lines hit the planet and instead of bending around, go through the ionosphere and through the outer mantle of the planet itself.  They would then hit the planet core that would make them emerge on the other side coming out from the planet.  This would require that Venus' interior is made up much like Earth's.

Another theory is that the magnetic fields bend around the planet and collide with other plasma already there on the other side and this interaction forms the holes.

Venus Express is now in a new orbit after the aerobraking maneuver in the Venusian atmosphere.  After eight years its propellant is running low and scientists tried and succeeded with an experiment to send the spacecraft surfing through the atmosphere and then plunging it up into a new orbit.  This maneuver extended the mission for at least another six months and also provided scientists with a new detailed look at the Venusian atmosphere.  Venus Express does not have enough fuel to perform the maneuver again and will eventually completely run out of fuel and drop into the atmosphere for good, ending its mission sometime around December.