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ONE of our neighbouring planets can still pack a few surprises, it seems.
Using satellite data, an international team of researchers has found that Venus
sports a giant, ion-packed tail that stretches almost far enough to tickle the
Earth when the two planets are in line with the Sun.
鈥淚 didn鈥檛 expect to find it,鈥� says team member Marcia Neugebauer of the Jet
Propulsion Laboratory in Pasadena, California. 鈥淚t鈥檚 a really strong signal, and
there鈥檚 no doubt it鈥檚 real.鈥�
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NASA鈥檚 Pioneer Venus Orbiter first found the tail in the late 1970s. Around
70 000 kilometres from the planet, the spacecraft detected bursts of hot,
energetic ions, or plasma. The tail exists because ions in Venus鈥檚 upper
atmosphere are bombarded by the solar wind, a stream of plasma that blows out
from the Sun.
But now Europe鈥檚 Solar and Heliospheric Observatory (SOHO), a project partly
sponsored by NASA, has shown that the tail stretches some 45 million kilometres
into space, more than 600 times as far as anyone realised. This satellite, which
sits about 1.5 million kilometres away from the Earth, passed through the tail
last July, when it was roughly in line with Venus and the Sun.
Over a period of five hours, SOHO detected three unexpected bursts of between
35 and 60 oxygen and carbon ions. Each burst lasted less than 45 seconds. In the
latest issue of Geophysical Research Letters (vol 24, p 1163), the team
concludes that the satellite may have passed through three separate streams in
the ion tail. Alternatively, it may have been a single filament that was
鈥渇lapping鈥� in the solar wind. 鈥淲e don鈥檛 know if we saw the same ray three times,
or three different ones,鈥� Neugebauer says.
Neugebauer suspects the tail is 鈥渁 lot of little stringy things鈥� like those
of some comets, which can have several ion tails. If so, says Neugebauer, 鈥渢he
theorists are going to have fun trying to explain why they鈥檙e as narrow as we
saw them鈥�. Standard physics says that narrow plasma streams are unstable and
should dissipate fast. No one can yet explain how they hold together over tens
of millions of kilometres.
The Earth and Jupiter are well shielded from the solar wind because they have
magnetics fields, which deflect the ions. But because Venus has no magnetic
field, the solar wind may have stripped away a significant amount of the ions in
the planet鈥檚 upper atmosphere over its lifetime of about 4.5 billion years.
Janet Luhmann of the University of California at Berkeley says that this effect
would have been strongest early in the life of the Solar System, when the Sun
was more active. 鈥淚t鈥檚 likely the escape rate was much higher,鈥� she says.
杏吧原创s believe that interactions between sunlight and the surface of
Venus were most important in shaping the composition of the planet鈥檚 corrosive
atmosphere, which is laden with sulphuric acid. Luhmann now speculates that the
ion loss may also have played a role.
