Table 4 Summary of hypotheses regarding Venus lightning

There is no electrical activity on Venus

• Sensitive Cassini search yielded null result

• Various optical searches have yielded null results

• A priori theoretical difficulty of charge separation

• Requires alternative mechanisms (e.g., local plasma noise) for all orbital electromagnetic evidence (Pioneer OEFD, VEx magnetometer)

• Also requires alternative mechanism for atmosphere/surface electromagnetic evidence (Veneras 11–14)

• Implies Venera 9 optical detection spurious (not improbable per the present paper)

• Implies ground-based optical detection spurious

Unlikely

Sporadic electrical discharges occur with characteristics different from Earth

• Consistent with electromagnetic signatures

• Occasional optical detections

• If optical signals are present at all, perhaps they are transient luminous events (TLE’s, i.e., sprites or similar)

• Cassini radio nondetection was “unlucky”—ionospheric conditions?

Not excluded

Frequent lightning similar to Earth

• Venera 9 original interpretation

• Sensitive Cassini search yielded null result, but detected ample activity at Earth with similar observation

• Orbital electromagnetic signatures imply lower rates

• Various optical searches have yielded null results or low flash rates

• A priori theoretical difficulty of charge separation

Unlikely