INDICADORES DE RENTABILIDAD

Lightning is often observed during volcanic eruptions on Earth, caused by particles

becom ing electrically charged in the rising ash column (Figure 1.11). Several spacecraft

and Earth-based instruments have indicated the presence o f lightning in the venusian

atmosphere and these observations have been used to suggest a link with volcanism. The

Venera Landers (11 - 14) detected impulsive electromagnetic signals in the very low

frequency (VLF) range associated with terrestrial lightning (Ksanfomality at al., 1983;

Donahue and Russel, 1997), PVO detected VLF signals in the ionosphere which could have

been propagating through from the atmosphere below (Taylor et al., 1979; Scarf and

R ussell, 1983), the Galileo plasma wave receiver observed electrom agnetic radiation

attributable to lightning at about 1 MHz (Gumett et al., 1991), and similar optical signals

have been observed in recent years by Earth-based telescopes (Hansell et al., 1995).

The location o f many o f the VLF bursts detected by PVO appeared to be in areas o f

high altitude. Since some volcanoes are related to highland areas, it was interpreted that the

VLF signals were related to volcanic plumes stimulating lightning in the cloud tops (Scarf

and Russel, 1988; Prinn, 1985). These conclusions have been used to suggest evidence for

recent, ongoing volcanism . Taylor et al. (1985, 1987) however demonstrated that the

preference o f bursts over the highlands was an artefact o f PV O ’s orbital coverage, which

over-sam pled the mountainous regions. Natural occurring lightning was originally

dismissed by some workers on the grounds o f prohibitive atmospheric conditions on Venus

(Esposito et al., 1983; Levin et al, 1983). Taylor and Cloutier (1986) however attribute the

disturbances to plasma instabilities in the venusian ionosphere.

Although many workers have totally refuted the claim that any bursts o f radiation are

attributable to volcanism (Taylor and Cloutier, 1986, 1992; Grebowsky et al., 1997), there

remains a persistence o f lightning-volcano scenarios in much recent literature (Taylor and

Cloutier, 1992). Two recent flybys o f Venus by the Cassini spacecraft were used to search

for high frequency radio signals from lightning. Several electrical pulses were detected, but

these did not match observed pulses created from terrestrial lightning (Gumett et al., 2001).

I N T R O D U C T I O N 4 5

e x t r e m e l y rare or very different from that seen on Earth. Also, since cl ouds on V e n u s are at ver y high altitudes ( - 4 0 k m or more) , they c o n c l u d e that the si gnal s m a y c o m e f r om very w e a k c l o u d - t o - c l o u d l i gh tni ng, s i m i l a r to the c l o u d - t o - i o n o s p h e r e d i s c h a r g e s r e ce n tl y d i s c o v e r e d on Ear th ( S e n t m a n et al., 1995; W e s c o t t et al., 1995). A full a c c o u n t o f v e n u s i a n l ightning det ec t ed by spac ecr af t and their i ns t ru m e n t s is g i ven by G r e b o w s k y et al. ( 1997) (Fi gure 1.12).

A;

F i g u r e 1.11. V e s u v i u s in e r u p t i o n on 18^*^ J u n e 1794, d e p i c t e d in a c o n t e m p o r a r y e n g r a v i n g. L i g ht n in g Hickers in the ash c o l u mn whi ls t a l ava t l o w d e s c e n d s to the sea on the right side. F ro m Francis (1996).

I N T R O D U C T I O N 4 6 G a lile o ^ Beyond Ionosphere P io n e e r V e n u s Orbiter W HISTLERS VHP WAVES Ionosphere Ionosphere Peak ' VHP \W A V E S T DAMPED / W HISTLERS HP \ WAVES WAVES ^ CLOUDS Lightning Venera 11,12 F i gu r e 1.12. A d i a g r a m d e m o n s t r a t i n g the e s c a p e / n o n e s c a p e o t ' e l e c t r o m a g n e t i c w a v e s from lightning on Venus. T h e u p pe r grey region r epresent s the i on os ph e r e layer. Whist lers are re ll ect ed p l a s m a d is t ur ba nc e s c a u s ed by li ghtni ng d i s c h a r g e . Af t e r G r e b o w s k y et al. ( 1997).

1.10.2 Levels o jS O

in the Venusian Atmosphere

P V O s h o w e d there to be fl uct ua ti ons in the o b s er v e d l evels o f SO2 in the ven u si an a t mo s ph e r e . Th i s has bee n c o n f i r m e d m o r e recent ly f r om o b s e r v a t i o n s by l UE ( Na et al., 1990) a nd H S T ( N a a nd E s pos it o, 1996). T h e P V O U V s p e c t r a a p p e a r e d to s h o w a d e c re a s e in the SO2 level o f 2 to 5 t i me s o ve r the fo ll owi ng six yea rs w h e n m e a s u r e m e n t s w e r e t a k e n ( E s p o s i t o et al., 1988). Th is w a s a t t ri bu te d to be t he a f t e r m a t h o f a large i nj ect ion o f SO2 into the v en u s i an st rat ospher e, and the s u b s e q u e n t r a pi d p h o t o c h e m i c a l c on v e r s i o n o f SO2 into H2SO4 aerosol. Esposito ( 19 84 ) w e n t on to s ugge s t that the source o f the SO2 w a s a v o l c an i c erupti on. F o l l o w i n g c a l cu la t i o ns o f t he scale o f the v ol cani c e v e n t n e e d e d to inject a p l u m e o f SO2 into the v e n u s i a n c l o u d t ops, E s p o s i t o ( 19 84 )

INTRODUCTION 47

concluded an eruption as energetic as Krakatoa in 1883 would have had to occur. In this

case, large scale volcanism would almost inevitably saturate the lower atmosphere with

SO] having implications for atmospheric evolution. Since the SO2 would not be washed

out by rain, as on Earth, and only removed slow ly by weathering reactions with surface

materials (Robinson and Wood, 1993), Espostio (1984) suggests that an episodic increase

in SO2 in the venusian cloud tops is due to an unknown phenomenon that m ixes SO2 from

the lower atmosphere across a stable layer at 40 - 50 km altitude into the upper atmosphere.