31 May 1997
From New Scientist Print Edition.
by Jeff Hecht
The giant ion tail of Venus.
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.
"I didn't expect to find it," says team member Marcia Neugebauer of the Jet Propulsion Laboratory in Pasadena, California. "It's a really strong signal, and there's no doubt it's real."
NASA's 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's upper atmosphere are bombarded by the solar wind, a stream of plasma that blows out from the Sun.
But now Europe's 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 "flapping" in the solar wind. "We don't know if we saw the same ray three times, or three different ones," Neugebauer says.
Neugebauer suspects the tail is "a lot of little stringy things" like those of some comets, which can have several ion tails. If so, says Neugebauer, "the theorists are going to have fun trying to explain why they're as narrow as we saw them". 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's 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. "It's likely the escape rate was much higher," she says.
Scientists believe that interactions between sunlight and the surface of Venus were most important in shaping the composition of the planet's corrosive atmosphere, which is laden with sulphuric acid. Luhmann now speculates that the ion loss may also have played a role.
I would like to point out that it seems to me that every where you turn in the solar system you find "surprises". For as long as I can remember, every time a spacecraft made an initial pass of a body, or we get significantly better telemetry from a probe than we'd had before, they find new and wonderful things that no one had expected. I'm amazed that scientists speak with the kind of certainty they do when speaking of what we actually know about space and our own little solar system considering they don't expect a heck of a lot what they find.