A solution to the mystery of lightning on Jupiter

Artist's concept of lightning distribution in Jupiter's northern hemisphere (Image NASA/JPL-Caltech/SwRI/JunoCam)
Artist’s concept of lightning distribution in Jupiter’s northern hemisphere (Image NASA/JPL-Caltech/SwRI/JunoCam)

Two articles, one published in the journal “Nature” and one published in “Nature Astronomy”, describe two researches on Jovian lightning. A team led by Shannon Brown of NASA’s JPL described the ways in which lightning strikes on the planet Jupiter are similar to those on Earth even if they’re someway the opposite. Another team led by Ivana Kolmašová of the Czech Academy of Sciences in Prague created the largest database of low-frequency radio emissions generated by lightning strikes on Jupiter, called in jargon whistlers. In both cases, the researchers used data collected by NASA’s Juno space probe.

The existence of lightning on the planet Jupiter was theorized a long time ago but only in 1979 the passage of NASA’s Voyager 1 allowed to verify it. Various lightnings were photographed and other instruments’ findings suggested that many others had insufficient intensity to be recorded by the Voyager 1’s camera. The data collected was enough to verify that, contrary to the theory, the radio signals associated with lightning don’t match the ones produced by lightning on Earth.

Shannon Brown explained that lightning strikes act like radio transmitters regardless of the planet but the space probes that studied Jupiter – Voyager 1, Voyager 2, Galileo and Cassini – detected the lightning signals only in their visible light emissions and in the kilohertz range of radio waves despite having looked for signals in the megahertz range.

The Juno space probe was designed over decades of experience in space missions and its instruments include the Microwave Radiometer Instrument (MWR), which records emissions from Jupiter across a wide spectrum of frequencies. After entering the gas giant’s orbit on July 4, 2016, during its first eight flybys it recorded thanks to MWR 377 lightning discharges. Their emissions were in the megahertz and even gigahertz range, which is what happens for the emissions of Earth’s lightning.

According to Shannon Brown, Juno had more luck than previous space probes because it flies much closer to lightning and the frequencies searched pass easily through Jupiter’s ionosphere. In short, Jovians lightning is after all similar to the Earth’s but its distribution is opposite in the two planets, in the sense that on Jupiter there’s a lot of activity near the poles but none close to the equator, the opposite of what happens on Earth.

According to the researchers, this difference is due to the fact that on Earth the heat comes mainly from solar emissions and is therefore at its maximum concentration around the equator while Jupiter receives much less heat from the Sun and produces much more of it inside it. Solar emissions still affect mainly the Jovian equator but just enough to create a stability in the upper atmosphere, inhibiting the rising of warm air from the inside. At the poles, on the other hand, convective currents can rise creating the conditions for the formation of lightning.

The team led by Ivana Kolmašová collected over 1,600 low-frequency lightning signals emitted on Jupiter. It’s an amount almost ten times higher than that detected by Voyager 1 with peaks of 4 lightning strikes per second, an amount similar to that observed in Earth’s storms and six times greater than the peaks detected by Voyager 1.

Scott Bolton, the Juno mission’s principal investigator, explained that these discoveries were possible only thanks to this space probe with its orbit and its specifically designed advanced instruments. This also makes it possible to detect weak signals coming from lightning within the chaos of Jupiter’s radio emission.

It’s for reasons such as these that in recent days NASA announced an extension of the Juno mission until July 2021. Originally 36 orbits were scheduled of 14 days each but some problems with the space probe’s main engine led to lengthening the orbits to a new duration of 53 days each.

The extension will allow to complete the 36 orbits and the good health of Juno’s instruments offers further hope for new discoveries. This was one of the critical factors of the mission because of the very harsh environment in which it works, filled by Jupiter’s powerful electromagnetic emissions, but for now they keep on working perfectly.

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