Lightning
is important for the chemistry and dynamics of the atmosphere. The
study of whether lightning occurs on Venus provides important information
on the planet's atmosphere. Previous Venus missions (Venera, Pioneer
Venus, Vega, and Galileo) had provided ambiguous or contradictory
results, but now, detailed analysis by researchers-from the CNRS Paris
Observatory (DESPA, Département de recherche spatiale), the
University of Iowa, and NASA (Goddard Space Flight Center)-of radio
data gathered by the Cassini space probe has demonstrated the absence
of terrestrial-like lightning on Venus.
Lightning is accompanied by characteristic radio impulses, which cover
a large spectral band-of at least several megahertz-and their intensity
generally decreases with the inverse square of the frequency and of
the distance from the observer. The Cassini space probe, which flew
past Venus in April 1998 and June 1999, passed the Earth in August
1999, and is now en route to Saturn, carried radio equipment programmed
for the optimal detection of these characteristic radio impulses.
As the probe flew past the Earth, more than a thousand discharges
characteristic of lightning bolts were detected, whereas the rare
impulses detected close to Venus did not possess any of the characteristics
of terrestrial lightning.
If lightning occurs at all on Venus, it must therefore be 100-1000
times weaker, rarer, or briefer than terrestrial lightning. Discharges
inside high altitude clouds, or between such clouds and the ionosphere
could have these characteristics. The absence of terrestrial-like
lightning on Venus suggests that vertical convection in the atmosphere,
which produces electrification and charge separation on Earth, could
be inhibited by the rapid horizontal circulation, or "super-rotation"
of the Venusian atmosphere.
