Sky & Telescope - September 2022 - 53

FESUS ROBERT / SHUTTERSTOCK.COM
those taken by the Galileo spacecraft 20
years later reveals no feature that can be
attributed to an impact in 1983.
The brief duration of the flash is
hardly consistent with a volcanic eruption.
While Io's lavas glow brightly at
infrared wavelengths of 3 microns or
longer, they have no appreciable emission
in the violet region of the spectrum.
In 1993, Hammel and Nelson
speculated that Io may have acted like a
mirror, reflecting the momentary glare
of a large impact on Jupiter's averted
hemisphere. (Europa was on the Earthward
side of Jupiter when the anomalous
flash on Io occurred.) Theoretical
models suggest that the impact of an
object at least 5 kilometers in diameter
would be required for its reflected fireball
to increase the apparent brightness
of Io by 50%. At the time this seemed to
be a very plausible explanation.
During an unforgettable week in
July of 1994, the fragments of Comet
Shoemaker-Levy 9 struck Jupiter's
nightside just over the planet's morning
limb. At the urging of Hammel and
Nelson, astronomers in Italy, Spain,
Ukraine, Brazil, Japan, and Australia
monitored the brightness of the Galilean
satellites using sensitive high-speed
photometers in a concerted effort to
detect light echoes. The results were disappointing.
The reflections of even the
largest fireballs were so feeble that their
photometric signals barely rose above
the background noise.
If the flash on Io was the light echo
of a Jovian impact, the object must
have been considerably larger than
even the biggest comet pieces. Many
of the SL9 fragments produced dark
" bruises " that were by far the most
visually prominent features ever seen
on Jupiter. Their sooty particulates
quickly encircled Jupiter's globe and
remained aloft for months.
My review of the records of the
Association of Lunar and Planetary
Observers and the British Astronomical
Association failed to turn up any
reports of an unusual Jovian feature
in the summer or autumn of 1983. It's
hard to imagine that the scar of a large
impact would have eluded the scores
of observers monitoring the planet.
But Hammel and Nelson had cited one
other possible explanation, the reflection
of a " superbolt " of lightning in
Jupiter's atmosphere.
On Earth, a typical lightning
discharge measures about 10 kilometers
long, lasts for about one-fifth of a
second, and heats the surrounding air
to 20,000°C (36,000°F). A network of
satellites in geostationary orbits jointly
operated by NASA and the National
Oceanic and Atmospheric Administration
(NOAA) have recorded unusually
violent thunderstorms producing megaflashes
of incredible size and energy on
very rare occasions.
The current record is held by an April
2020 discharge over the southern United
States that traveled horizontally for 768
kilometers, comparable to the length
of the Florida peninsula. A June 2020
discharge over Uruguay and northern
Argentina lasting more than 17 seconds
holds the current record for duration.
Instruments aboard the Voyager and
Galileo spacecraft detected the radio
static generated by huge thunderstorms
in Jupiter's turbulent atmosphere that
routinely produce lightning discharges
hundreds of times more powerful than
their terrestrial counterparts. Until
recently these bolts were assumed to
occur only 45 to 65 kilometers beneath
the visible cloud deck of frozen ammonia
crystals, where temperatures are
warm enough for charge-transporting
droplets of liquid water to exist.
Three years ago, the navigation camera
aboard the Juno spacecraft detected
frequent flashes of " shallow lightning "
at the very top of the cloud canopy on
Jupiter's nightside. In these cold clouds,
ammonia gas dissolves in water and acts
like antifreeze, allowing droplets to exist
at temperatures as low as -90°C. This
unexpected discovery lends new credibility
to the notion that the 1983 flash
on Io may have been the reflection of a
Jovian megaflash and could be the key to
unraveling a four-decade-old mystery.
¢Contributing Editor TOM DOBBINS
has observed rare phenomena on many
bodies within our solar system, both real
and imaginary.
The longest lightning " megaflash " on Earth stretched for 768 kilometers. Two discharges
detected by satellites lasted more than 16 seconds. Could even larger, stronger bolts
occur high in Jupiter's turbulent cloud canopy?
sk yand tele scope .o r g * SEPTEMBER 2022 53
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Sky & Telescope - September 2022

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