Research on lightning discharge in planetary atmosphere
The importance of lightning-discharge activity is confirmed by research on the Earth. So what is the merit of research into the phenomenon on other planets? Lightning discharge has been confirmed or the possibility of it considered in several planets or their satellites in the solar system. The obvious evidence of lightning discharge on Jupiter, Saturn, Uranus and Neptune was caught by optical or radio-wave receivers onboard space probes such as Voyager. In addition, it is suggested that discharge phenomena occur on Mars, Venus, and Saturn's satellite Titan. Some researchers point out that, if the global circuit is present in such planets and satellites, it must affect their aerosol and cloud physics. Further, there is speculation that CH4 and C2H4 on Jupiter, NO on Venus, and CO on Uranus were produced by lightning discharges.
Other than the Earth, Jupiter is the only planet to show decisive evidence of lightning-discharge optical emission. Very strong lightning-discharge emissions at night were photographed by Voyager 1, 2 and the Galileo spacecraft. The scale of a single lightning-discharge emission is estimated to be 100 to 1,000 times that of average emissions on the Earth. By comparing with clouds observed during the daytime of Jupiter, it seems that the lightning discharges take place in the well-developed cumulonimbus regions similar to thunderclouds on Earth (Fig. 2). At the First Europlanet Congress organized in Berlin in September 2006, the author felt the great excitement in Europe celebrating the series of successful missions such as Mars Express, Venus Express, Cassini/Huygens. A session for “planetary lightning discharge” was held at the congress and an interesting report was presented. With observations by Cassini, it was found that lightning discharges occurred only around the Great White Spot on Saturn and they were all large-scale ones 100 to 1,000 times that of emissions on Earth. The argument as to whether lightning discharge occurs on Venus has continued over the past 20 years and is not yet concluded. However, the magnetometer onboard Venus Express recorded large variations that are estimated with high possibility to originate from lightning. We need decisive evidence by optical observation.
Japan will try to solve this enduring mystery by using the Lightning and Airglow Camera (LAC), which is designed to detect lightning-discharge emissions and launched onboard the Venus exploration project PLANET-C. Unlike the Earth, lightning-discharge emission and radio-wave radiation are very important as indices to accurately show convection activity on other planets where direct observation of wind is impossible and observation tools are very limited.